Physical Geology - H5P Edition

SK_badlands

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Figure 1.1 Badlands in southern Saskatchewan. Erosion has exposed layers of rock going back more than 65 million years. Created by Karla Panchuk (2017, CC BY-SA 4.0) Components: Badlands photograph: Karla Panchuk (2005) CC BY 4.0 Canada map: Lokal_Profil (2007) CC BY-SA 2.5 view source

Mars conglomerate NASA photo annotated

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Rearguard

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Figure 1.3 Rearguard Mountain and Robson Glacier in Mount Robson Provincial Park, BC. Left: Created by Karla Panchuk (2017, CC BY-SA 4.0) Components: Robson Glacier photo: Steven Earle (2015) CC BY 4.0 view source Canada map: Lokal_Profil (2007) CC BY-SA 2.5 view source Right: Photograph by A.P. Coleman (c. 1908) Public Domain. Photograph is in the Arthur P. Coleman Collection at Victoria University Library. Visit collection information

Svalbard

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La_Soufriere_JE1021

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What-are-scientific-methods

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timescale

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Proterozoic_1879

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Plates_map_small

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earth_and_limb_m1199291564l_color_2stretch_mask_0

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big-bang-and-cmb-JE2121

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Duck pond red shift ducklings JE2421a

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Created by Karla Panchuk (2021, CC BY 4.0). Updated from Karla Panchuk (2015, CC BY 4.0). View original. Ducklings on the duck pond at Dalmuir Park: Mark Harkin, 2013, CC BY 2.0. View source.

Duck pond red shift spectra JE2421

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Created by Karla Panchuk (2021, CC BY-SA 4.0). Updated from Karla Panchuk (2018, CC BY-SA 4.0). View original. Components: Red shift spectra: Georg Wiora (2011) CC BY-SA 2.5. View source. Galaxy UGC 2885 detail (background image): NASA, ESA and B. Holwerda (University of Louisville) (2020), Public Domain. View source. Lily pad: Public Domain Vectors (2020) Public Domain. View source.

Periodic table of some elements JE2521a

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Pillars

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dust bunnies JE2721

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Created by Karla Panchuk (2021) CC BY 4.0. Updated from Karla Panchuk (2018) CC BY 4.0. View original. Components: Planets inspired by NASA/JPL Planetary Missions and Near-Earth Objects graphic (2008) Public Domain. View original. Note: This link may not be stable. Background image: Hubble Peers into the Mouth of Leo A, ESA/Hubble & NASA; Acknowledgment: Judy Schmidt (2017). Public Domain. View source.

protoplanetary disk JE2021a

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Created by Karla Panchuk (2021) CC BY 4.0. Components: Protoplanetary disk artists' concept: NASA/JPL-Caltech (2008) Public Domain. View source. HL Tauri protoplanetary disk: ALMA (ESO/NAOJ/NRAO) (2014) CC BY 4.0. View source. Saturn photograph: NASA, ESA, J. Clarke (Boston University), and Z. Levay (STScI) (2005) Public Domain. View source. Ring detail: NASA/JPL/Space Science Institute (2005), Public Domain. View source.

Planet types JE2021

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Created by Karla Panchuk (2021, CC BY-NC-SA 4.0). Updated from Karla Panchuk (2015, CC BY 4.0). View original. Components: Terrestrial planets diagram: NASA (2003) Public Domain View source. Gas and ice giant planets: Lunar and Planetary Institute (2003) Public Domain View source. Uranus interior: FrancescoA (2011) Public Domain View source. Milky way background: R@PP (2017) CC BY-NC-SA 2.0 View source.

Solar_system_v2

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Figure 2.9 Our solar system. Top: The solar system shown with distances to scale. Distances are in astronomical units (AU), where 1 AU is the average distance from Earth to the sun. The edge of the Kuiper belt extends to 50 AU (7.5 billion km), but this distance is minuscule compared to the size of the solar system as a whole, which extends to the edge of the Oort cloud, thought to be 15 trillion km away. Bottom: Solar system with the sun and planets to scale. The gas giants are the largest planets, followed by the ice giants, and then the terrestrial planets. Note that the planets in this diagram likely do not reflect the entire population of planets in our solar system because evidence suggests that large planets are present beyond the Kuiper belt. Created by Karla Panchuk (2018, CC BY-NC-SA 4.0) Components: Milky Way photo: R@PP (2017) CC BY-NC-SA 2.0 view source Earth: NASA Goddard Space Flight Center Image by Reto Stöckli (land surface, shallow water, clouds). Enhancements by Robert Simmon (ocean color, compositing, 3D globes, animation). Data and technical support: MODIS Land Group; MODIS Science Data Support Team; MODIS Atmosphere Group; MODIS Ocean Group Additional data: USGS EROS Data Center (topography); USGS Terrestrial Remote Sensing Flagstaff Field Center (Antarctica); Defense Meteorological Satellite Program (city lights) (2002) Public Domain view source Mercury: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington (2011) Public Domain view source Venus: NASA (n.d.) Public Domain view source Mars: NASA/JPL-Caltech (2013) Public Domain view source Jupiter: NASA/JPL-Caltech (2000) Public Domain view source Saturn: NASA/JPL/Space Science Institute (2004) Public Domain view source Uranus: NASA/Space Telescope Science Institute (2009) Public Domain view source Neptune: NASA (2014) Public Domain view source Sun: SOHO-EIT Consortium (ESA/NASA) (1997) Public Domain view source

Pluto

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Hadean

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754px-Seymchan

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Berg et al

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22.11

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Deformation-aided_segregation_of_Fe-S_liquid_from_

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Transit method

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Created by Karla Panchuk (2021, CC BY-NC-SA 4.0). Modified after NASA/HESARC/TESS (2021), Public Domain. View source. Components: Sun: NASA's Goddard Space Flight Center/SDO (2020) Public Domain. View source. Milky way background: R@PP (2017) CC BY-NC-SA 2.0 View source.

Exoplanet travel bureau

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Tablelands JY2021

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Locator map created by Karla Panchuk (2018, CC BY-SA 4.0) with Canada map by Lokal_Profil (2007) CC BY-SA 2.5 view source Valley photograph: Leos Kral (2008) CC BY-NC-SA 2.0 view source Tablelands terrain: Tara Joyce (2013) CC BY-SA 2.0 view source

Tablelands ophiolite

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earth_interior

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Image created by Karla Panchuk (2018, CC BY 4.0). Earth photo: NASA (n.d.) Public Domain. View source.

JSt_John_eclogite

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Seismic ray JY2021

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moho_SE

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Shadow zones JY2021

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Seismic velocity profile JY2021

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Cocos_slab

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radio_heat_SE

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geothermal_gradient

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convection_soup_pot

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mantle_convection

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magnetic_field

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polarity reversal

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Created by Karla Panchuk (2021, CC By 4.0). Component images are output from the geodynamo model of Glatzmaier and Roberts (1995), and are released by NASA to the Public Domain. Note that labels of the Wikimedia Commons images for the before and after reversal images are reversed compared to Gary Glatzmaier's website describing the images. Component images: Between reversals. Geodynamo before reversal Geodynamo in reversal After reversal Glatzmaier, G. A., & Roberts, P.H. (1995). A three-dimensional self-consistent computer simulation of a geomagnetic field reversal. Nature, 377, 203-209.

paleomag_reversals_SE

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isostasy_raft_SE

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isostasy_rebound

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glacial_rebound_greenland

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rebound_rates_SE

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Skiff_sillyputty

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Thingvellir_rift

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Figure 4.1 Iceland is known for its volcanoes, which are present because Iceland is located on the Mid-Atlantic Ridge, where the Atlantic Ocean is spreading apart and new crust is forming. In fact, Iceland exists because that volcanic activity has built up the island from the ocean floor. Iceland is cut by rift zones (white lines on the map at left) where the island is splitting apart along with the rest of the Atlantic Ocean. Rift zones are marked by belts of young volcanic rocks (dark green). You can stand on a rift zone if you visit Thingvellir National Park (right). Rifting has produced a valley where the crust has settled downward. The margins of the North American and Eurasian tectonic plates are visible as ridges on either side of the valley. The photographer was standing on a ridge on the North American side. Created by Karla Panchuk (2018, CC BY-SA 4.0) Components Photograph: Ruth Hartnup (2005) CC BY 2.0 view source Globe: Ninrouter (2012) CC BY-SA 3.0 view source Blank Iceland map: Derivative of PinPin (2008) CC BY-SA 3.0 view source Map of volcanic rocks and rift zones: Derivative of Psiĥedelisto and Chris.urs-o (2017) CC BY-SA 3.0 view source (based on Thordarson & Larsen, 2007). Thordarson, T., and Larsen, G. (2007) Volcanism in Iceland in historical time: Volcano types, eruption styles and eruptive history. Journal of Geodynamics 43, 118–152. Full text

Wegener_Expedition-1930_008

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Snider-Pellegrini_Wegener_fossil_map

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Wegener_rocktypes

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Figure 4.4 Diagram from Alfred Wegener's book Die Entstehung der Kontinente und Ozeane comparing rock types on Canadian Arctic Islands and Greenland. Original figure caption:

Geologische Karte des Smith-Sundes u. des Robeson-Kanals, nach der Geologic Map of North America. 1 Trias, 2 Devon, 3 Silur, 4 Karbon, 5 Gneis, 6 Vorkambrium, 7 Spättertiär, 8 Kambrium und Unter-Ordovicium.

Created by Karla Panchuk (2018, CC BY 4.0) Components: Geological map: Derivative of Alfred Wegener (1920) Public Domain view source Blank north pole locator map: Flappiefh (2013), derivative of Reisio (2005), Public Domain. view source Wegener, A. (1920). Die Entstehung der Kontinente und Ozeane. Braunschweig, Germany: Friedr. Vieweg & Sohn. Full text at Project Gutenberg

Scotese_KarooGlaciation

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Figure 4.5 Carboniferous and Permian Karoo Glaciation in the southern hemisphere. Paleogeographic reconstruction for 306 million years ago. Cropped by Karla Panchuk from C. R. Scotese, PALEOMAP Project (www.scotese.com) Terms of use (from the website):

These maps may be used or modified in any manner for personal use, teaching, research or in scientific publications as long as appropriate credit is given to the author (see below). These maps may not be copied, resold, used or modified in any manner for commercial purposes, such as consulting reports, trade journals or the popular press, textbooks, videos, educational CD-ROMS, computer animations, museum exhibits, web sites on the Internet or for any other commercial use, without the express written consent of the author. Links may be made from any web site on the internet to the PALEOMAP website, www.scotese.com.

image0092

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Kuenen_geosyncline

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image0151

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image0131

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image0171

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bathymetry

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Figure 4.9 Ocean floor bathymetry (and continental topography). Inset (a): the mid-Atlantic ridge, (b): the Newfoundland continental shelf, (c): the Nazca trench adjacent to South America, and (d): the Hawaiian Island chain. [SE after NOAA, http://bit.ly/1OtRMc0]

sedthick9

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image023_2

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image0251

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image0271

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image0291

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image0312

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image035

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tranforms-2

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Transform-model-1024x405

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image0411

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Plate_tectonics_map

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image045_2

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divergent-boundary-processes

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rift-formation-e1439319064564

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image0551

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ocean-continent

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image0591

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image061

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image063

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Pangea-breakup-507x1024

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Figure 4.33 Sequence of paleogeographic reconstructions showing the breakup of Pangea. Created by Karla Panchuk (2017, CC BY-NC-SA 4.0). Maps from C. R. Scotese, PALEOMAP Project (www.scotese.com) Terms of use (from the website):

These maps may be used or modified in any manner for personal use, teaching, research or in scientific publications as long as appropriate credit is given to the author (see below). These maps may not be copied, resold, used or modified in any manner for commercial purposes, such as consulting reports, trade journals or the popular press, textbooks, videos, educational CD-ROMS, computer animations, museum exhibits, web sites on the Internet or for any other commercial use, without the express written consent of the author. Links may be made from any web site on the internet to the PALEOMAP website, www.scotese.com.

Links to maps: Early Jurassic (195 Ma) Late Jurassic (152 Ma) Late Cretaceous (94 Ma) End Cretaceous (66 Ma)

image067

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Pangea-ultima-494x1024

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Figure 4.35 Sequence of reconstructions showing the possible future configuration of land masses on Earth at 50, 150, and 250 million years from now. Movements culminate in the formation of a new supercontinent called Pangea Ultima. Created by Karla Panchuk (2017, CC BY-NC-SA 4.0). Maps from C. R. Scotese, PALEOMAP Project (www.scotese.com) Terms of use (from the website):

These maps may be used or modified in any manner for personal use, teaching, research or in scientific publications as long as appropriate credit is given to the author (see below). These maps may not be copied, resold, used or modified in any manner for commercial purposes, such as consulting reports, trade journals or the popular press, textbooks, videos, educational CD-ROMS, computer animations, museum exhibits, web sites on the Internet or for any other commercial use, without the express written consent of the author. Links may be made from any web site on the internet to the PALEOMAP website, www.scotese.com.

Links to maps: Modern World Future World (+50 Ma) Future World (+150 Ma) Future World (+250 Ma)

Wilson-cycle

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image081

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Naica_mine

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Figure 5.1 Giant crystals of gypsum in the Naica Mine in Mexico. The crystals formed in volcanically heated water, and became accessible when the cave was drained as part of mining activities. The cave was very hot, making it fatal for visitors to enter without cooling equipment and respirators. When mining activities ceased, caverns were allowed to flood again. Image assembled by Karla Panchuk (2018, CC BY-NC-SA 4.0) Naica Mine Photo: Paul Williams (2009) CC BY-NC 2.0 view source Blank locator map: Flappiefh (2013), derivative of Reisio (2005), Public Domain. view source

billes de mercure natif, cristaux de cinabre : New Idria Mine (New Idria group ; New Hope vein), Idria (New Idria), New Idria District, Diablo Range, San Benito Co., California, USA

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lattice-structure-and-composition-of-the-mineral-halite

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opal

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Blausen_0342_ElectronEnergyLevels_H_He

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Blausen_0342_ElectronEnergyLevels_LiNe

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NaCl_SE

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two-chlorine-atoms_SE

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carbon_covalent_SE

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diamond_graphite

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hydrogen_bonding

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metallic_bonding

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oxides_3rd

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Figure 5.14 Oxide minerals include metal ore minerals, industrial minerals, and gemstones. Created by Karla Panchuk (2018, CC BY-NC-SA 4.0) Component photographs by Roger Weller/Cochise College.

Photos are copyright free for non-commercial educational uses. Just credit photos to R.Weller/Cochise College.

Hematite Magnetite Limonite Bauxite Common corundum Gemstone corundum

sulfides_3rd

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Figure 5.15 Sulphide minerals often have a metallic lustre and include metal ores. Created by Karla Panchuk (2018, CC BY-NC-SA 4.0) Component photographs by Roger Weller/Cochise College.

Photos are copyright free for non-commercial educational uses. Just credit photos to R.Weller/Cochise College.

Galena Chalcopyrite Sphalerite Molybdenite Pyrite Bornite Arsenopyrite Stibnite

sulphate_3rd

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Figure 5.16 Sulphate minerals. Created by Karla Panchuk (2018, CC BY-NC-SA 4.0) Component photographs: Anhydrite: Rock Currier (2009) CC BY 3.0 view source Barite: Sam Wise (2009) CC BY-NC-SA 2.0 view source Images by Roger Weller/Cochise College:

Photos are copyright free for non-commercial educational uses. Just credit photos to R.Weller/Cochise College.

Gypsum Celestite

halides_3rd

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Figure 5.17 Halide minerals. Created by Karla Panchuk (2018, CC BY-NC-SA 4.0) Component photographs: Cryolite: James St. John (2017) CC BY 2.0 view source Images by Roger Weller/Cochise College:

Photos are copyright free for non-commercial educational uses. Just credit photos to R.Weller/Cochise College.

Fluorite Halite

carbonates_3rd

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Figure 5.18 Carbonate minerals. Created by Karla Panchuk (2018, CC BY-SA 4.0) Component photographs by Rob Lavinsky, iRocks.com, CC BY-SA 3.0. Links to source files are below. Calcite Dolomite/ Siderite Magnesite Malachite Azurite

phosphates_3rd

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Figure 5.19 Phosphate minerals. Created by Karla Panchuk (2018, CC BY-NC-SA 4.0) Component photographs by Roger Weller/Cochise College.

Photos are copyright free for non-commercial educational uses. Just credit photos to R.Weller/Cochise College.

Light green apatite Dark green apatite Blue apatite Turquoise

native_minerals

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Figure 5.20 Native element minerals are made up of a single element. Created by Karla Panchuk (2018, CC BY-SA 4.0) Component photographs: Gold: James St. John (2006) CC BY 2.0 view source Copper and silver: Rob Lavinsky, iRocks.com (n.d.) CC BY-SA 3.0 view source Sulphur: H. Zell (2009) CC BY-SA 3.0 view source

silicate

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tetrahedron

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isolated

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Created by Karla Panchuk (2021, CC BY-SA 4.0). Updated from Karla Panchuk (2018, CC BY-SA 4.0). View original. Component images: Tetrahedra: Modified after Steven Earle (2015), CC BY 4.0. View source. Olivine crystals: Psammophile (2011) CC BY-SA 3.0 view source/ visit website

atomic radii

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single chain

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Created by Karla Panchuk (2021, CC BY-SA 4.0). Updated from Karla Panchuk (2018, CC BY-SA 4.0). View original. End-on view modified after Klein, C. & Hurlbut, C. S., Jr. (1993). Manual of Mineralogy (after J. D. Dana). New York, NY: John Wiley & Sons, Inc. Component images: Tetrahedra: Modified after Steven Earle (2015), CC BY 4.0. View source. Aegirine (Acmite): R. Weller/ Cochise College (2015) view source Permission for R. Weller photographs:

Photos are copyright free for non-commercial educational uses. Just credit photos to R.Weller/Cochise College.

double chain

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Created by Karla Panchuk (2021, CC BY-SA 4.0). Updated from Karla Panchuk (2018, CC BY-SA 4.0). View original. End-on view modified after Klein, C. & Hurlbut, C. S., Jr. (1993). Manual of Mineralogy (after J. D. Dana). New York, NY: John Wiley & Sons, Inc. Component images: Tetrahedra: Modified after Steven Earle (2015), CC BY 4.0. View source. Hornblende: R. Weller/ Cochise College (2010) view source Permission for R. Weller photographs:

Photos are copyright free for non-commercial educational uses. Just credit photos to R.Weller/Cochise College.

sheet_silicate_3rd

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Figure 5.27 Micas are sheet silicates and split easily into thin layers along planes parallel to the sheets. Biotite mica (lower left) is has Fe and Mg cations. Muscovite mica (lower right) has Al and K instead. The muscovite mica shows how thin layers can split away in a sheet silicate. Created by K. Panchuk (2018, CC BY-NC-SA 4.0) Components: Sheet silicate structure: Modified after Steven Earle (2015) CC BY 4.0 view source Biotite: R. Weller/ Cochise College (2010) view source Muscovite: R. Weller/ Cochise College (2010) view source Permission for R. Weller images:

Photos are copyright free for non-commercial educational uses. Just credit photos to R.Weller/Cochise College.

feldspar ternary JY2021

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Get the ternary diagram without minerals photos superimposed. Created by Karla Panchuk (2018, CC BY-SA 4.0) Components: Orthoclase: Rob Lavinksy, iRocks.com (2010) CC BY-SA 3.0 view source Microcline: Rob Lavinksy, iRocks.com (2010) CC BY-SA 3.0 view source Microcline (Amazonite): Rob Lavinksy, iRocks.com (2010) CC BY-SA 3.0 view source Albite: Rock Currier (n.d.) CC BY 3.0 view source Labradorite: University College London Geology Collections (2012) CC BY 2.0 view source Anorthite: Rob Lavinksy, iRocks.com (n.d.) CC BY-SA 3.0 view source

Qtz_3rd

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Figure 5.29 Quartz is another silicate mineral with a three-dimensional framework of silica tetrahedra. Sometimes quartz occurs as well-developed crystals (left), but it is also present in common rocks such as granite (right). In addition to quartz, the granite contains potassium feldspar, albite, and amphibole. Created by Karla Panchuk (2018, CC BY-NC-SA 4.0). Photos by R. Weller/ Cochise College: Quartz Granite Permission for R. Weller images:

Photos are copyright free for non-commercial educational uses. Just credit photos to R.Weller/Cochise College.

Silicate structure activity

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Printable flashcards to practice silicate structures

colour_sulf_hem_3rd

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Figure 5.30 Colour is a useful diagnostic property for sulphur (left) and for some types of hematite (right) because the yellow and dark red colours are unique to those minerals. In contrast, silvery metallic forms of hematite are similar in appearance to many other minerals. Created by Karla Panchuk (2018, CC BY-NC-SA 4.0) Photos by R. Weller/ Cochise College: Small sulphur crystals Large sulphur crystals Red hematite Metallic hematite Permission for R. Weller images:

Photos are copyright free for non-commercial educational uses. Just credit photos to R.Weller/Cochise College.

qtz_colour_3rd

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Figure 5.31 The many colours of quartz. Created by Karla Panchuk (2018, CC BY-NC-SA 4.0) Photos by R. Weller/ Cochise College Colourless quartz Milky quartz Smoky quartz Amethyst Citrine Rose quartz Permission for R. Weller images:

Photos are copyright free for non-commercial educational uses. Just credit photos to R.Weller/Cochise College.

Hematite_streak_plate

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metallic_minerals_streak_plate

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Mohs hardness

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xtl_habit_3rd

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Figure 5.35 Hexagonal prisms of quartz (left), cubic crystals of pyrite (centre), and 24-sided crystals of garnet (right). Created by Karla Panchuk (2018, CC BY-NC-SA 4.0) Photos by R. Weller/ Cochise College: Quartz Pyrite Garnet Permission for R. Weller images:

Photos are copyright free for non-commercial educational uses. Just credit photos to R.Weller/Cochise College.

1_cleavage_3rd

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2_cleavage_3rd

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3_cleavage_3rd

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Wisconsin quartzite ripples

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Created by Karla Panchuk (2017, CC BY-SA 4.0). Updated from Karla Panchuk (2017, CC BY-SA 4.0). View original. Components: Close-up ripple photograph: Karla Panchuk (2016) CC BY-SA 4.0 Cliff photograph: Karla Panchuk (2016) CC BY-SA 2.0 view source Blank map of North America: Lokal_Profil (2007) CC BY-SA 2.0 view source

rock vs mineral

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Created by Karla Panchuk (2021, CC BY-NC-SA 4.0). Component photographs: Granite: James St. John (2019), CC BY 2.0. View source. Biotite schist: James St. John (2015), CC BY 2.0. View source. Quartz crystal: R. Weller/ Cochise College (2011). View source. Potassium feldspar: R. Weller/ Cochise College (2016). View source. Quartzite: R. Weller/ Cochise College (2017). View source. Terms of use for images by R. Weller:

Photo is copyright free for non-commercial educational uses. Just credit photo to R.Weller/Cochise College.

Rock Cycle

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Figure 6.3 The rock cycle describes processes that form the three types of rock: igneous, sedimentary, and metamorphic. These same processes can turn one type of rock into another. Created by Karla Panchuk (2017, CC BY-SA 4.0) Modified after Steven Earle (2015) CC BY 4.0 view original Components Cliff: A. Perkins (2010) CC BY 2.0 view source Sand: Mezza (2009) CC BY 3.0 view source Conglomerate: Daniel Mayer (n.d.) CC BY-SA 3.0 view source James St. John (2014) CC BY 2.0 view source Halema'uma'u Crater (Kīlauea, Hawaii): U. S. Geological Survey (2013) Public Domain view source Granite: Khruner (2007) CC BY-SA 4.0 view source Rhyolite: Amcyrus (2015) CC BY-SA 4.0 view source Arizona Mountains: Darkest Tree (2009) CC BY-SA 3.0 view source

USGS_pahoehoe

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Weathering_freeze_thaw_action,_Spain

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Okotoks_dust_storm

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SouthSaskRiver

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Fossil_Ripples_-_geograph.org.uk_-_831746

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QuadraIsland_ls

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Nyiragongo

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Figure 7.1 Lava lake of Mount Nyiragongo, a volcano in the Democratic Republic of Congo. Igneous rocks form when melted rock freezes. Created by Karla Panchuk (2018, CC BY-NC-SA 4.0) Components: Lava lake photo: Baron Reznik (2015) CC BY-NC-SA 2.0 view source Locator globe: Andreyyshore (2016) CC0 1.0 view source

Avg composition

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partial melt JY2021

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melting_triggers

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Compositions JY2021

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Bowen-reaction2

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Norman-Bowen-236x300

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Figure 7.6 Norman Bowen in his laboratory. Credit: University of Chicago Photographic Archive, apf1-00841, Special Collections Research Center, University of Chicago Library. View image source University of Chicago Photographic Archive rights and permissions:

Images in the University of Chicago Photographic Archive may be used for educational and scholarly purposes, but any such use requires that a credit line be included with any image used. Any use of images from the Chicago Maroon, the independent student newspaper of the University of Chicago, requires a separate grant of copyright permission from the Chicago Maroon (see below). Credit Line: University of Chicago Photographic Archive, [apf digital item number, e.g., apf12345], Special Collections Research Center, University of Chicago Library.

plagioklaz-NX-sm

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bowen

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xenolith

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magma-chamber2

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magma-chambers

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igneous rock classification JY2021

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Figure 7.13 Classification diagram for igneous rocks. Igneous rocks are classified according to the relative abundances of minerals they contain. A given rock is represented by a vertical line in the diagram. In the mafic field, the arrows represent a rock containing 48% pyroxene and 52% plagioclase feldspar. The name an igneous rock gets depends not only on composition, but on whether it is intrusive or extrusive. Created by Karla Panchuk (2018, CC BY-NC-SA 4.0) Component photographs by Roger Weller/Cochise College:

Photos are copyright free for non-commercial educational uses. Just credit photos to R.Weller/Cochise College.

Granite Rhyolite Diorite Andesite Gabbro Basalt Peridotite References Hall, A. (1996). Igneous Petrology (2nd ed.). Essex: Longman Group Limited. Klein, C., & Hurlbut, C. S. Jr. (1993). Manual of Mineralogy after J. D. Dana (21st ed.). John Wiley & Sons, Inc.

porphyry_3rd

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classification simplified_revised

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fraction_scale

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volcanic_rock_ID_3rd

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Figure 7.19 In volcanic igneous rocks, individual crystals are not visible. Colours change from light to dark as the composition of the rocks go from felsic to mafic. Vesicles and amygdules are common characteristics of basalt. Created by Karla Panchuk (2018, CC BY-NC-SA 4.0) Component photographs by R. Weller/Cochise College:

Photos are copyright free for non-commercial educational uses. Just credit photos to R.Weller/Cochise College.

Rhyolite Andesite Vesicular basalt Amygdaloidal basalt

glassy_rx_3rd

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Figure 7.20 Glassy volcanic rocks. Obsidian has a glassy lustre, but scoria and pumice are highly vesicular. Created by Karla Panchuk (2018, CC BY-NC-SA 4.0) Component photographs by R. Weller/Cochise College:

Photos are copyright free for non-commercial educational uses. Just credit photos to R.Weller/Cochise College.

Obsidian Scoria Pumice

xenoliths2

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Intrusive bodies_3rd

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batholith_4th

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mafic-dyke2

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hoodoos-2

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GeologicalExfoliationOfGraniteRock_small

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Coquihalla_exfoliation_SE

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Ice wedging

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talus_slope_SE

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tafoni_SE

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root_wedging_2018

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Dissolution_rxn_2018

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Mosul_Dam_sinkhole

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hydrolysis_granite

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pillow lava Banco de Imagenes

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granite_limonite_SE

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Mt.-Washington-Mine

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differential_weathering

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clasts-3

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Grain size chart

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Course grained Boulder: larger than 256 mm Cobble: 64 mm to 256 mm Pebble: 2 mm to 64 mm Medium grained (sand) Coarse sand: 500 microns to 2 mm Medium sand: 250 microns to 500 microns Fine sand: 63 microns to 250 microns Fine grained Silt: 2 microns to 63 microns Clay: smaller than 2 microns

IMG_4497-1024x871

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IODP-sorting-and-rounding

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soil_texture_USDA

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loess_SE

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soil horizons

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photo 2-2-1_l

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Figure 8.24 Soil erosion by rain and unchanneled runoff in a field in Alberta. The source of the materials is http://www.agriculture.alberta.ca. The use of these materials by Physical Geology, 3rd Adapted Edition is done without any affiliation with or endorsement by the Government of Alberta. Reliance upon use by Physical Geology, 3rd Adapted Edition is at the risk of the end user. Source: Alberta Agriculture and Forestry view source/ view context View Terms of Use From the website:

Terms of Use — Non-commercial or Educational Reproduction All information on this site has been posted to provide Albertans with direct access to information about the programs and services offered by the Government of Alberta. The information has been posted with the intent that it be readily available for personal and public non-commercial or educational use and except where otherwise prohibited, may be reproduced, in part or in whole and by any means, without charge or further permission from Alberta Agriculture and Forestry. We ask that: The materials not be modified. Users exercise due diligence in ensuring the accuracy of the materials before the use of the materials. The user identify the Government of Alberta as the source of the materials with the following statement: “The source of the materials is http://www.agriculture.alberta.ca. The use of these materials by [insert user’s name] is done without any affiliation with or endorsement by the Government of Alberta. Reliance upon [insert user’s name]’s use of these materials is at the risk of the end user.”

photo 2-2-2 (erosion)_l

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Figure 8.25 Soil erosion by wind in Alberta. The source of the materials is http://www.agriculture.alberta.ca. The use of these materials by Physical Geology, 3rd Adapted Edition is done without any affiliation with or endorsement by the Government of Alberta. Reliance upon use by Physical Geology, 3rd Adapted Edition is at the risk of the end user. Source: Alberta Agriculture and Forestry view source/ view context View Terms of Use From the website:

Terms of Use — Non-commercial or Educational Reproduction All information on this site has been posted to provide Albertans with direct access to information about the programs and services offered by the Government of Alberta. The information has been posted with the intent that it be readily available for personal and public non-commercial or educational use and except where otherwise prohibited, may be reproduced, in part or in whole and by any means, without charge or further permission from Alberta Agriculture and Forestry. We ask that: The materials not be modified. Users exercise due diligence in ensuring the accuracy of the materials before the use of the materials. The user identify the Government of Alberta as the source of the materials with the following statement: “The source of the materials is http://www.agriculture.alberta.ca. The use of these materials by [insert user’s name] is done without any affiliation with or endorsement by the Government of Alberta. Reliance upon [insert user’s name]’s use of these materials is at the risk of the end user.”

soil_map_interactive

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Figure 8.26 Distribution of soil orders in Canada. Source: Agriculture and Agri-food Canada (Go to map) Contains information licensed under the Open Government License-Canada. From the website (visit):

Open Government Licence - Canada You are encouraged to use the Information that is available under this licence with only a few conditions. Using Information under this licence Use of any Information indicates your acceptance of the terms below. The Information Provider grants you a worldwide, royalty-free, perpetual, non-exclusive licence to use the Information, including for commercial purposes, subject to the terms below. You are free to: Copy, modify, publish, translate, adapt, distribute or otherwise use the Information in any medium, mode or format for any lawful purpose. You must, where you do any of the above: Acknowledge the source of the Information by including any attribution statement specified by the Information Provider(s) and, where possible, provide a link to this licence. If the Information Provider does not provide a specific attribution statement, or if you are using Information from several information providers and multiple attributions are not practical for your product or application, you must use the following attribution statement: Contains information licensed under the Open Government Licence – Canada. The terms of this licence are important, and if you fail to comply with any of them, the rights granted to you under this licence, or any similar licence granted by the Information Provider, will end automatically.

image057

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Drumheller roadside

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The-rock-cycle

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lithification-1024x409

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cement

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clastic_table

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Figure 9.5 Types of clastic sedimentary rocks. Created by Karla Panchuk (2018, CC BY-NC-SA 4.0) Component photographs: Conglomerate: James St. John (2017) CC BY 2.0 view source Images by Roger Weller/Cochise College:

Photos are copyright free for non-commercial educational uses. Just credit photos to R.Weller/Cochise College.

Breccia Quartz arenite Feldspathic arenite (arkose) Wacke Shale Mudstone

arenite-sandstones

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sandstones-2

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corals

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tropical-reef

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Carbonate sediments

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1024px-Mono_lake_tufa

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Stalagmite,_stalactite_de_grotte_de_NEFZA

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Chert

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1280px-Black-band_ironstone_(aka)

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Spotted-Lake

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6093812704_4120139143_k

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Formation-of-coal

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coal

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Figure 9.19 The formation of coal begins when plant matter is prevented from decaying by accumulating in low-oxygen, acidic water. A layer of peat forms. Heating and compression of peat form lignite, bituminous coal, and finally anthracite, as pressure and temperature increases. Created by Karla Panchuk (2017, CC BY-NC-SA 4.0) Component photographs: Lignite: U. S. Geological Survey (n.d.) Public Domain view source Description (from the website):

Closeup of Pliocene lignite coal from a Balkan endemic nephropathy (BEN) village in Serbia. Lignite is low rank, or relatively unaltered (soft, or "brown") coal, and is characterized by a brownish color and appearance that often resembles wood. This lignite releases copious amounts of dissolved organic substances into groundwater.

Images by Roger Weller/Cochise College:

Photos are copyright free for non-commercial educational uses. Just credit photos to R.Weller/Cochise College.

Peat Anthracite Bituminous Coal

Main_depositional_environments v1

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basins

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Triassic-Sulphur

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Cross-bedded

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Formation-of-cross-beds

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imbrication-of-clasts

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JamesStJohn_graded_bedding cropped

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Mud-cracks

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Upper-Cretaceous-Nanaimo-Group-rocks

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Turbidite-layers

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fluvial-sandstone

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Comox-Formation

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Pemaquid Point

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Created by Karla Panchuk (2018, CC BY 4.0). Components: Photograph of along-shore view: Joyce McBeth (2009), CC BY 4.0. View source. Photograph looking toward the ocean: Joyce McBeth (2009), CC BY 4.0. View source. Locator globe: Flappiefh (2013), derivative of Reisio (2005), Public Domain. View source.

shale_gneiss

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Figure 10.2 Shale is the parent rock of gneiss (pronounced "nice"). These rocks look very different, but gneiss can form when the atoms contained within the shale are re-arranged into new mineral structures. Created by Karla Panchuk (2018, CC BY-NC-SA 4.0) Component photographs by R. Weller/Cochise College:

Photos are copyright free for non-commercial educational uses. Just credit photos to R.Weller/Cochise College.

Shale Gneiss

Al2SiO5

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Created by Karla Panchuk (2018, CC BY-SA 4.0). Photographs by Rob Lavinsky/ iRocks.com (pre-2010), CC BY-SA 3.0: Andalusite Kyanite Sillimanite

clay experiment

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Figure 10.6 Foliation that develops when minerals are squeezed and deform.

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image007

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bedding and foliation

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foliation and crystal habit

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Figure 10.9 A foliated metamorphic rock called phyllite (upper left). The satin sheen comes from the alignment of minerals. Lower left: A view of the same kind of rock under a microscope showing mica crystals (colourful under polarized light) aligned in bands. The region outlined in a red dashed line shows a lens of quartz crystals that do not display alignment. Upper right- stacks of platy mica crystals. Lower right- a blocky quartz crystal. Created by Karla Panchuk (2018, CC BY-SA 4.0) Component photographs: Phyllite: Przykuta (2011) CC BY-SA 3.0 view source Photomicrograph: Petr Brož (2007) CC BY-SA 3.0 view source Mica: Rob Lavinsky, iRocks.com (pre-2010) CC BY-SA 3.0 view source Quartz: Jan Helebrant (2018) CC BY-SA 2.0 view source

metaconglomerate

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Figure 10.10 Metaconglomerate consisting of quartz pebbles, displaying elongated clasts. Note that quartz pebbles have developed "wings" to varying degrees (e.g., white dashed ellipse). These are the result of quartz dissolving where stress is applied, and flowing away from the direction of maximum stress before recrystallizing (upper right sketch). Created by Karla Panchuk (2018, CC BY-NC-SA 4.0) Metaconglomerate photo by R. Weller/ Cochise College view source Terms of use (from the website):

Photo is copyright free for non-commercial educational uses. Just credit photo to R.Weller/Cochise College.

cleavage close up

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slaty cleavage

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mylonite with plastic defm of qtz

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Figure 10.13 Rocks from the Western Carpathians mountain range before and after. Left- An undeformed granitic rock containing the mica mineral biotite (Bt), plagioclase feldspar (Pl), potassium feldspar (Kfs), and quartz (Qtz). Right- a metamorphic rock (mylonite) resulting from extreme deformation of granitic rocks. Quartz crystals have been flattened and deformed. The other minerals have been crushed and deformed into a fine-grained matrix (Mtx). Source: Farkašovský, R., Bónová, K., & Košuth, M. (2016). Microstructural, modal and geochemical changes as a result of granodiorite mylonitisation – a case study from the Rolovská shear zone (Čierna hora Mts, Western Carpathians, Slovakia). Geologos 22(3), 171-190. doi: 10.1515/logos-2016-0019 View full text Granodiorite image (left image):

Fig. 2. Photomicrograph of the undeformed phaneritic granular biotite granodiorite composed mainly of plagioclase (Pl), quartz (Qtz), K-feldspar (Kfs) and biotite (Bt). Scale 1 mm. Crossed polarised light.

Mylonite image (right image):

Fig. 4. Photomicrograph of the biotite granodiorite mesomylonite (sample 5). The rock has a strong plane-parallel structure and mylonitic texture with abundant quartz porphyroclasts (Qtz) and aphaninic matrix (Mtx). Common microstructures are short shear bands (a), asymmetrical quartz porphyroclasts (b), ductile bending of the longitudinal quartz grains and matrix mineral aggregates (c), synthetic microfaults in quartz porphyroclasts (d), fragmentation of larger quartz grains (e). Sinistral shear sense. Scale 1 mm. Plane polarised light.

slate

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Created by Karla Panchuk (2018, CC BY-SA 4.0.) Components: Slate fragments photo: Vincent Anciaux (2005), CC BY-SA 3.0. View source. Slate outcrop photo: Gretarsson (2006), CC BY-SA 3.0. View source.

phyllite

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Created by Karla Panchuk (2018, CC BY-SA 4.0). Components: Hand sample photo: Chadmull (2006), Public Domain. View source. Outcrop photo: Laszlovszky András (2008), CC BY-SA 2.5. View source.

schist

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Figure 10.15 Schist, a medium- to high-grade foliated metamorphic rock. Top- Hand sample showing light reflecting off of mica crystals. Bottom- Close-up view of mica crystals and garnet. Created by Karla Panchuk (2018, CC BY-NC-SA 4.0) Photos by R. Weller/ Cochise College Hand sample Enlarged view Terms of use (from the website):

Photo is copyright free for non-commercial educational uses. Just credit photo to R.Weller/Cochise College.

gneiss

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Figure 10.17 Gneiss, a coarse-grained, high grade metamorphic rock, is characterized by colour bands. Top- Hand samples showing that colour bands can be continuous (left) or less so (right). Bottom- Gneiss in outcrop at Belteviga Bay, Norway. Notice the light and dark stripes on the rock. Created by Karla Panchuk (2018, CC BY-SA 4.0) Component photographs: Hand sample with wavy bands: Siim Sepp (2005) CC BY-SA 3.0 view source Hand sample with discontinuous bands: Anders Damberg, Geological Survey of Sweden (2011) CC BY 2.0 view source Belteviga Bay Outcrop: Erlend Bjørtvedt (2013) CC BY-SA 3.0 view source

amphibolite_pm20-28-300x225

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Figure 10.18 Amphibolite in thin section (2mm field of view), derived from metamorphism of a mafic igneous rock. Green crystals are the amphibole hornblende, and colourless crystals are plagioclase feldspar. Notice that the crystals are aligned horizontally. Source: D.J. Waters, University of Oxford view source/ view context Original figure caption:

Amphibolite This rock was originally a basic igneous rock (basalt or dolerite). When metamorphosed, the heating and compression changed the original minerals to hornblende (green) and feldspar (colourless), and gave the rock a banding of minerals. Field of view 2 mm.

Photographic images on this site are copyright University of Oxford (except where explicitly stated). They may be used without charge for bona fide educational purposes.

marble

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Figure 10.19 Marble is a non-foliated metamorphic rock with a limestone protolith. Left- Marble made of pure calcite is white. Upper right- Microscope view of calcite crystals within marble that are blocky and not aligned. Lower right- A quarry wall showing the "marbling" that results when limestone contains components other than calcite. Created by Karla Panchuk (2018, CC BY-NC-SA 4.0) Component photographs: Marble hand sample: U. S. Geological Survey (n.d.) Public Domain view source Marble outcrop: Denis Travin (2009) CC BY 2.0 view source Photomicrograph: D.J. Waters, University of Oxford view source/ view context Original figure caption:

Marble Metamorphosed limestones are called marble. The calcium carbonate re-forms itself into larger, interlocking crystals of calcite (e.g. the pearly-coloured crystals in the centre). The impurities are converted into new metamorphic minerals. In this case, the larger bold-coloured crystals are forsterite (magnesium silicate, a variety of olivine). Field of view 6 mm, polarising filters.

Photographic images on this site are copyright University of Oxford (except where explicitly stated). They may be used without charge for bona fide educational purposes.

quartzite

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hornfels

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Figure 10.21 Hornfels, a non-foliated metamorphic rock formed from a fine-grained protolith. Left: Hornfels from the Novosibirsk region of Russia from a sedimentary protolith. The dark and light bands preserve the bedding of the original sedimentary rock. The rock has been recrystallized during contact metamorphism and does not display foliation. (scale in cm). Right: Hornfels in thin section from a sedimentary protolith. Note that the brown mica crystals are not aligned. The dark band at the top reflects the layering within the sedimentary parent rock, similar to the way those layers are preserved in the sample on the left. Image sources: Hand sample photograph: Fedor (2006) Public Domain view source Photomicrograph: D.J. Waters, University of Oxford view source/ view context Original figure caption:

Hornfels Rocks close to a large igneous intrusion are heated to high temperatures but not deformed. Their minerals change, but they tend not to develop a new banding or cleavage. This makes a hard, fine-grained rock called a hornfels. This example was a fine-grained sedimentary rock, and the horizontal banding you can see is the original sedimentary layering. There are many small mica flakes, but they do not lie parallel to one another, as they would in a schist. Field of view 2.5 mm.

Photographic images on this site are copyright University of Oxford (except where explicitly stated). They may be used without charge for bona fide educational purposes.

Table-10-1-1024x462

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Table 10.1: A Rough Guide to the Effect of Metamorphism on Different Protoliths Mudrock Protolith Very Low Grade (150-300 C): slate Low Grade (300 - 450 C): phyllite Medium Grade (450 - 550 C): schist High Grade (above 550 C): gneiss Granite Protolith Very Low Grade through Medium Grade: no change High Grade: granite gneiss Basalt Protolith Very Low Grade through Low Grade: chlorite schist Medium Grade through High Grade: amphibolite (amphibole gneiss) Sandstone Protolith Very Low Grade: no change Low Grade: little change Medium Grade through High Grade: quartzite Limestone Protolith Very Low Grade: little change Low Grade through High Grade: marble

Migma_ss_2006

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SONY DSC

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Figure 10.22 Ptygmatic folding from Broken Hill, New South Wales, Australia. Ptygmatic folding happens when a stiff layer within a rock is surrounded by weaker layers. Folding causes the stiff layer to crinkle while the weaker layers deform around it. Source: Roberto Weinberg view source Terms of use (from the website):

Copyright 2004-2011 by Roberto Weinberg. All rights reserved. Unlimited permission to copy or use is hereby granted for non-profit driven enterprise subject to inclusion of this copyright notice and my World Wide Web URL: http://users.monash.edu.au/~weinberg. I would very much appreciate an email stating how this material will be used. Thanks, RW.

JStJohn_BlackMarinace

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regionalmeta

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seafloor metamorphism

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800px-Archean_Greenstone_Pillow_Lava_in_Michigan_USA_3-300x188

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image022

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Franciscan-Complex-

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image032-290x300

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volcanic_arc

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impact meta

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fault_breccia

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Figure 10.33 Fault breccia, created when shear stress along a fault breaks up rocks. Left- close-up view of fault breccia clearly showing dark angular fragments. Right- A fault zone containing fragments broken from the adjacent walls (dashed lines). Note that the deformation does not extend far past the margins of the fault zone. Created by Karla Panchuk (2018, CC BY 4.0). Component photographs: French Grand Antique Marble (fault breccia): James St. John (2014) CC BY 2.0 view source Lake Vermillion Formation fault: James St. John (2015) CC BY 2.0 view source

Mylonite

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Figure 10.34 Mylonite, a rock formed by dynamic metamorphism. Left: An outcrop showing the early stages of mylonite development, called protomylonite. Notice that the deformation does not extend to the rock at the bottom of the photograph. Middle: Mylonite showing ribbons formed of drawn-out crystals. Right: Microscope view of mylonite with mica (colourful crystals) and quartz (grey and black crystals). This is a case where the shape of quartz crystals is controlled more by stress than by crystal habit. Created by Karla Panchuk (2018, CC BY-SA 4.0) Components: Protomylonite photograph: Wilfried Bauer (1995) CC BY-SA 3.0 view source Mylonite photograph: Woudloper (2006) CC BY-SA 1.0 view source Photomicrograph: Strekeisen (2017) CC BY-SA 4.0 view source

facies_metatypes

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meta_locations

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image027

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Meguma_with_ranges

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Meguma_scenario

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metamorphism-and-alteration-around-a-pluton

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calcite-vein-2

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image037

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Mt_Garibaldi_with_location

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Figure 11.1 Mt. Garibaldi (in the background), near Squamish B.C., is one of Canada’s most recently active volcanoes, last erupting approximately 10,000 years ago. It is also one of the tallest, at 2,678 m in height. Created by Karla Panchuk (2017) CC BY-SA 4.0 Components: Mt. Garibaldi Photograph: Michael Scheltgen (2006) CC BY 2.0 view source World map: Ssolbergj (2006) GNU Free Documentation License Version 1.2 view source Canada map: Lokal_Profil (2007) CC BY-SA 2.5 view source

Eruption of Vesuvius by Moonlight

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Kamoamoa_Fissure_Eruption

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Figure 11.3 Kamoamoa fissure eruption on the flanks of the Hawai'ian volcano Kilauea in March of 2011. Created by Karla Panchuk (2017) CC BY-SA 4.0 Components: Kamoamoa Photograph: United States Geological Survey (2011) Public Domain view source World map: TUBS (2011) CC BY-SA 3.0 view source Hawai'i map: NordNordWest (2009) CC BY-SA-3.0-DE view source

stratovolcano

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Caldera_USGS

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Santorini_caldera_photo

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Figure 11.6 The Greek Island of Santorini. Left: Aerial view of the island forming a ring around a flooded caldera. Right: A view from the rim of the caldera. The other side of the rim is visible in the distance. Created by Karla Panchuk (2017) CC BY-SA 4.0 Components: Santorini satellite image: NASA/GSFC/MITI/ERSDAC/JAROS, and U.S./Japan ASTER Science Team (2000) Public Domain view source World map: TUBS (2011) CC BY-SA 3.0 view source Photograph across the caldera: Klearchos Kapoutsis (2010) CC BY 2.0 view source

fumarole_USGS

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StHelens_lava_dome_feuerborn

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Lava_comparison

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lava tubes

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ropy_lava

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aa

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Pillow_lavas

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DSC05018

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Svartifoss - Iceland

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Volcanic_ash

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lapilli

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Figure 11.17 Lapilli are pyroclasts ranging between 2 mm and 64 mm in size. Upper left: lapilli from the site of the ancient city of Pompeii. Lower left: Pele's tears, a type of lapilli that forms when droplets of lava fly through the air. Right: Pele's hair, which form when Pele's tears are drawn out into thin threads as they fly. Sources: Upper left: Pauline (2009) CC BY-NC-ND 2.0. view source Lower left, right: Karla Panchuk (2017) CC BY 4.0. Scales have been added to images on the lower left and right according to field-of-view dimensions included with image descriptions. Photograph for lower left (Pele's tears): James St. John (2014) CC BY 2.0 view source Photograph for right (Pele's hair): James St. John (2007) CC BY 2.0 view source

Kilauea Block

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bomb_St-John

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pumice_kp

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mafic_vesicular

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volcano_size_comparison

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shield_volcano_nolabels

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stratovolcano_Cotopaxi

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Figure 11.24 Composite volcano. Cotopaxi in Ecuador exhibits the upward-steepening cone characteristic of composite volcanoes. Diagram of a composite volcano showing alternating layers of lava and tephra. Top image assembled by Karla Panchuk (2017, CC BY 4.0) Cotopaxi Photo: Simon Matzinger (2014) CC BY 2.0 Blank locator map: Flappiefh (2013), derivative of Reisio (2005), Public Domain. view source Lower image: Karla Panchuk (2017) CC BY 4.0.

cinder_cone_Capulin

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Figure 11.25 Cinder cone. These small, straight-sided volcanoes are made of volcanic fragments ejected when gas-rich basaltic lava erupts. Created by Karla Panchuk (2017, CC BY 4.0) Components: Capulin photograph: R. D. Miller, U. S. Geological Survey (1980) Public Domain view source Blank locator map: Flappiefh (2013), derivative of Reisio (2005), Public Domain. view source

Hawaiian_eruption

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Strombolian_Etna

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Figure 11-27 Strombolian eruption of Mt. Etna. Sputtering lava forms a smaller cinder cone around a vent within the crater of Etna. Created by Karla Panchuk (2017, CC BY-SA 4.0) Components: Mt. Etna photograph: Robin Wylie (2012) CC BY 2.0 view source Georgia locator globe: Izzedine (2010) CC BY-SA 3.0 view source

Lacroix_1902_Pelee

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Figure 11-28 A series of photos taken by Alfred Lacroix during the eruption of Mt. Pelée on May 8, 1902 showing the development of the pyroclastic flow that destroyed the city of St. Pierre and its nearly 30,000 inhabitants. Created by Karla Panchuk (2017, CC BY 4.0) Components: Mt. Pelee pyroclastic flow sequence: A. Lacroix (1902) Public Domain view original Photographs published in Lacroix, A. (1904) Montagne Pelée et ses Éruptions, Paris: Masson et Cie Éditeurs, 1904 Blank locator map: Flappiefh (2013), derivative of Reisio (2005), Public Domain. view source

StPierre_stereoview

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Redoubt_1990

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Figure 11-30 Plinian eruption of Mt. Redoubt in Alaska on April 21, 1990. Created by Karla Panchuk (2017, CC BY 4.0) Components: Mt. Redoubt photograph: R. Clucas, U. S. Geological Survey (1990) Public Domain view source Blank north-pole locator map: Flappiefh (2013), derivative of Reisio (2005), Public Domain. view source

Eyjafjallajokull_hydrovolcanic

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Figure 11-31 Hydrovolcanic eruption of Eyjafjallajökull in April of 2010. Left- Eruptive column with volcanic lightning. Volcanic lightning is caused by the static electricity generated by volcanic ash particles rubbing together. Right- Another view of the ash cloud, with westward winds carrying ash toward Europe where it would disrupt air traffic. Created by Karla Panchuk (2017, CC BY 4.0) Components: Volcanic lightning photograph: Terje Sørgjerd (2010) CC BY-SA 3.0 view source Ash cloud photograph: Henrik Thorburn (2010) CC BY 3.0 view source Blank north pole locator map: Flappiefh (2013), derivative of Reisio (2005), Public Domain. view source

plate tectonic settings with volcanism JY3121

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Hawaiian_hotspot

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CRBG_LIP

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Ekati_mine

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Figure 11.37 The Ekati diamond mine in the Northwest Territories, part of the Lac de Gras kimberlite field. Created by Karla Panchuk (2017, CC BY-SA 4.0) Components: Ekati photograph: J. Pineau (2010) CC BY-SA 3.0 view source Blank north pole locator map: Flappiefh (2013), derivative of Reisio (2005), Public Domain. view source

UNOSAT_A3_Natural_Portrait_VO20210523COD_Geological_Information_MountNyiragongo_DRCongo_28May2021_v1.pdf

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Laki_fissure_(2)

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eruption-of-Mt.-Mayon

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USGS_Lahar_StHelens_img1107_900w_602h

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800px-MSH80_bulge_on_north_side_04-27-80

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SONY DSC

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SONY DSC

USGS_Gas-sampling

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Hualalai-volcano

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Canadian_volcanism

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Figure 11.45 Canada's volcanic regions are located in British Columbia and the Yukon Territory. Volcanism is associated with three tectonic settings: the subduction zone along the west coast (Garibaldi Volcanic Belt, Wrangell Volcanic Belt), a continental rift zone (Wells Gray-Clearwater Volcanic Field, Stikine Volcanic Belt), and a mantle plume (Anahim Volcanic Belt). Source: Volcanoes Canada, Canadian Hazards Information Service, Government of Canada (view source) This image is used under Natural Resources Canada's Terms and Conditions for non-commercial reproduction.

Non-Commercial Reproduction: Ownership and usage of content provided on this site Materials on this website were produced and/or compiled for the purpose of providing Canadians with access to information about the programs and services offered by the Government of Canada. You may use and reproduce the materials as follows: Non-commercial reproduction Unless otherwise specified you may reproduce the materials in whole or in part for non-commercial purposes, and in any format, without charge or further permission, provided you do the following: -exercise due diligence in ensuring the accuracy of the materials reproduced -indicate both the complete title of the materials reproduced, as well as the author (where available) -indicate that the reproduction is a copy of the version available at [URL where original document is available] Commercial reproduction Unless otherwise specified, you may not reproduce materials on this site, in whole or in part, for the purposes of commercial redistribution without prior written permission from the copyright administrator.

Garibaldi_Volcanic_Belt-en.svg

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The_Table1

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1024px-Anahim_Volcanic_Belt-en.svg

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Rainbow_Range

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953px-Wells_Gray-Clearwater_Volcanic_Field-en.svg

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Edziza_nass5518

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Christchurch_demo

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Figure 12.1 Demolition of a structure damaged when an earthquake of magnitude 6.3 struck Christchurch, New Zealand on February 22, 2011. Created by Karla Panchuk (2017, CC BY-SA 4.0) Components: Demolition Photograph: Terry Phillpot (2012) CC BY-NC 2.0 view source New Zealand locator map: Gringer (2010) Public Domain view source New Zealand map: L. J. Holden (2007) CC BY-SA 3.0 view source

Vancouver_Island_nomap

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Figure 12.2 Damage to an elementary school in Courtenay, British Columbia after a magnitude 7.3 earthquake on Sunday, June 23, 1946. Left: A hole left after the chimney collapsed through the roof. Right: Damage inside the school. In addition to damaging structures, the earthquake triggered numerous slope failures. School building with damaged roof: Earthquakes Canada, Natural Resources Canada (n.d.) view source Interior of school: Earthquakes Canada, Natural Resources Canada (n.d.) view source Note: School images and other images of damage from the 1946 earthquake can be accessed at Damage photographs from the M7.3 Vancouver Island Earthquake of 1946 Photographs are used under Natural Resources Canada's Terms and Conditions for non-commercial reproduction.

Non-Commercial Reproduction: Permission to reproduce Government of Canada works, in part or in whole, and by any means, for personal or public non-commercial purposes, or for cost-recovery purposes, is not required, unless otherwise specified in the material you wish to reproduce. A reproduction means making a copy of information in the manner that it is originally published – the reproduction must remain as is, and must not contain any alterations whatsoever. The terms personal and public non-commercial purposes mean a distribution of the reproduced information either for your own purposes only, or for a distribution at large whereby no fees whatsoever will be charged. The term cost-recovery means charging a fee for the purpose of recovering printing costs and other costs associated with the production of the reproduction. Users are required to: - Exercise due diligence in ensuring the accuracy of the materials reproduced; - Indicate both the complete title of the materials reproduced, as well as the author organization; and - Indicate that the reproduction is a copy of an official work that is published by the Government of Canada and that the reproduction has not been produced in affiliation with, or with the endorsement of the Government of Canada. Unless otherwise specified, this authorization is also applicable to all published information regardless of its format.

Elastic_rebound

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Rupture_focus_epicentre

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stress-changes-e1439324273866

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Haida_Gwaii_aftershocks

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Figure 12.6 Magnitude 7.8 Haida Gwaii earthquake and aftershocks. Mainshock (red circle marks the epicentre) occurred on October 28th, 2012. Aftershocks are for the period from October 28th to November 10th of 2012. Although the epicentre is near a transform boundary, the rupture was influenced more by compression related to the subduction zone. Created by: Karla Panchuk (2017, CC BY 4.0) Components: Base map with earthquake epicentres: U. S. Geological Survey, Latest Earthquakes (Public Domain) view interactive map World map: derivative of Ssolbergj (2006) CC BY 3.0 view source

Episodic-tremor

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body_waves

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seismic-surface

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Figure 12.9 Surface waves travel along Earth's surface and have a diminished impact with depth. Rayleigh waves (left) cause a rolling motion, and Love waves (right) cause the ground to shift from side to side. Created by Steven Earle (2015, CC BY 4.0) view source Components: Rayleigh wave: U. S. Geological Survey (n.d.) Public Domain view source view context Love wave: U. S. Government (n.d.) Public Domain view source Note: Author information is absent, and the source url provided (http://www.geo.mtu.edu/UPSeis/love_web.jpg) is broken.

seismograph

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Figure 12.10 How a seismograph records earthquakes. Left: Seismograph schematic Created by Karla Panchuk (2018) CC BY-NC-SA 4.0 Schematic image: Incorporated Research Institutions for Seismology (2012) How Does A Seismograph Work? view source IRIS Image Gallery Terms of Use Right: Created by Karla Panchuk (2018) CC BY-SA 4.0 Seismograph photo: Z22 (2014) CC BY-SA 3.0 view source

travel_time_IRISmod

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EQ_location_IRIS

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Figure 12.12 Locating earthquakes by drawing three circles with radii of lengths determined from P-wave and S-wave travel times. Station names (SOCO, TEIG, SSPA) correspond to seismograms in Figure 12.11. Source: Incorporated Research Institutions for Seismology (n.d.) How Are Earthquakes Located? view source IRIS Image Gallery Terms of Use:

I. LICENSE GRANT 1.1 Subject to the restrictions set forth in Article II, IRIS hereby grants to User a limited, non-exclusive, non-transferable, revocable, worldwide license to download any photos or other materials ("Images") made available to User via the IRIS Image Gallery solely for its ordinary personal and/or educational/academic/professional purposes.

Amplitude

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EQ_Frequency_IRIS

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Vancouver-Island

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Figure 12.15 Intensity map for the M7.3 Vancouver Island earthquake on June 23, 1946. Source: Earthquakes Canada, Natural Resources Canada (2016) view source This image is used under Natural Resources Canada's Terms and Conditions for non-commercial reproduction.

Non-Commercial Reproduction: Permission to reproduce Government of Canada works, in part or in whole, and by any means, for personal or public non-commercial purposes, or for cost-recovery purposes, is not required, unless otherwise specified in the material you wish to reproduce. A reproduction means making a copy of information in the manner that it is originally published – the reproduction must remain as is, and must not contain any alterations whatsoever. The terms personal and public non-commercial purposes mean a distribution of the reproduced information either for your own purposes only, or for a distribution at large whereby no fees whatsoever will be charged. The term cost-recovery means charging a fee for the purpose of recovering printing costs and other costs associated with the production of the reproduction. Users are required to: - Exercise due diligence in ensuring the accuracy of the materials reproduced; - Indicate both the complete title of the materials reproduced, as well as the author organization; and - Indicate that the reproduction is a copy of an official work that is published by the Government of Canada and that the reproduction has not been produced in affiliation with, or with the endorsement of the Government of Canada. Unless otherwise specified, this authorization is also applicable to all published information regardless of its format.

Earthquakes_and_plates

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Figure 12.8 Earthquakes greater than magnitude 5, from 2000 to 2008. Bands of earthquakes mark tectonic plates. Narrow bands with shallow earthquakes (marked in red) indicate transform boundaries or mid-ocean ridge divergent boundaries. Wider bands with earthquakes at a range of depths are subduction zones. Wide bands of scattered earthquakes mark continent-continent convergent margins (e.g., between the Indian and Eurasian plates), or continental rift zones (e.g., in eastern Africa). Created by Karla Panchuk (2017, CC BY-NC-SA 4.0) The base map for this image (everything exclusive of plate and ocean basin labels) is hosted by the National Science Foundation Multimedia Gallery (view source). The base map should be attributed to Lisa Christiansen, Caltech Tectonics Observatory. Base map description:

More about this Image: Scientists at Caltech's Tectonics Observatory (TO) have been investigating the Sumatra region to shed light on what happened during the recent earthquakes and to improve our understanding of the seismic and tsunami hazard associated with such plate boundaries. New research results by TO scientists show how this region's massive earthquakes (magnitude greater than 8.0) are connected to the continual deformation of Earth's surface, which occurs even during calm times in between the massive quakes (these calm times are called interseismic periods). TO scientists combine field measurements of coral growth patterns, data from GPS stations and sophisticated computer models. These research results provide a new way to estimate locations and magnitudes of future giant quakes. It is a step toward estimating when the next big earthquake will occur in Sumatra and similar regions previously thought to be at low risk for large earthquakes, such as in China, Java, Japan and Peru. (Date of Image: November 2008)

The terms of use for NSF Multimedia Gallery materials not credited to the NSF are the following:

General Restrictions: Images and other media in the National Science Foundation Multimedia Gallery are available for use in print and electronic material by NSF employees, members of the media, university staff, teachers and the general public. All media in the gallery are intended for personal, educational and nonprofit/non-commercial use only.

Div_Trans_Eq

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Baikal_Rift_Eqs

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Conv_Aleut_Eqs

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SZearthquakes

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Inda_Asia_EQ

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India-Asia-convergent-boundary

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Charlevoix_ISZ

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Figure 12.15 Charlevoix seismic zone, site of intraplate earthquakes. The location of the Charlevoix seismic zone is indicated by the star on the map of Canada. Dots indicate earthquake epicentres. The size of the dots indicates magnitude. White lines indicate fault segments. The dashed circle marks the edge of a crater formed by a meteorite impact 342 million years ago. Created by Karla Panchuk (2017), CC BY-SA 4.0 Components: Satellite image: Satellite map layer, Latest Earthquakes, USGS Earthquake Hazards Program (public domain) visit Latest Earthquakes Map of Canada: Paul Robinson (2007) CC BY-SA 2.5, derivative of Lokal_Profil (2007) CC BY-SA 2.5 view source Data: Epicentre locations: Earthquakes Canada, Natural Resources Canada (2016) Earthquake zones in Eastern Canada Visit website Fault and crater locations: Based on Figure 4 of Hongyu, Y., Liu, Y., Harrington, R. M., & Lamontagne, M. (2016). Seismicity along St. Lawrence Paleorift Faults Overprinted by a Meteorite Impact Structure in Charlevoix, Québec, Eastern Canada. Bulletin of the Seismological Society of America 106(6), 2663-2673. doi: https://doi.org/10.1785/0120160036

Cypress-Freeway

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Izmit_1999_USGS

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Niigata

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unconsolidated-sedimentary-layers

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Liquefaction

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The-Las-Colinas-debris

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SanFranburning

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tsunami

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Figure 12.31 Tsunami triggered along a subduction zone. Top: The overriding crust is deformed because it is locked to the subducting slab. Middle: When the locked zone ruptures, the crust rebounds, and waves are created. Bottom: Tsunami waves have small amplitudes in the deep ocean, but once in shallow water, they slow down, causing the waves to become taller and closer together. Created by: Karla Panchuk (2018) CC BY-NC-SA 4.0 Components: Top and middle modified after Steven Earle (2015) CC BY 4.0 view source / view context Bottom: Modified after COMET/UCAR. view source / view context Full attribution: The source of this material is the COMET® Website at http://meted.ucar.edu/ of the University Corporation for Atmospheric Research (UCAR), sponsored in part through cooperative agreement(s) with the National Oceanic and Atmospheric Administration (NOAA), U.S. Department of Commerce (DOC). ©1997-2017 University Corporation for Atmospheric Research. All Rights Reserved. COMET/UCAR terms of use (excerpt):

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tsunami

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Parkfield-segment

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SF_probability_2014

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Cariboo_folds

kqzheng — Thu, 19 Aug 2021 18:11:12 +0000

Created by Karla Panchuk (2017, CC BY-SA 4.0) Figure 13.1 Folded rocks in the Cariboo Mountains of BC. Created by: Karla Panchuk (2018, CC BY-NC-SA) Components: Photograph: Drew Brayshaw (2009) CC BY-NC 2.0 view source Canada map: Lokal_Profil (2007) CC BY-SA 2.5 view source

stress-1024x542

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diff_stress_1

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Pennisi_bench_red

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brittle

kqzheng — Thu, 19 Aug 2021 18:11:13 +0000

Potash_defm

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Boudinage_v2

kqzheng — Thu, 19 Aug 2021 18:11:14 +0000

BDTZ

kqzheng — Thu, 19 Aug 2021 18:11:14 +0000

Structure_types

kqzheng — Thu, 19 Aug 2021 18:11:15 +0000

fold_anatomy

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fold_asymmetrical

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overturned_fold

kqzheng — Thu, 19 Aug 2021 18:11:22 +0000

recumbent

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Triassic-Quatsino

kqzheng — Thu, 19 Aug 2021 18:11:23 +0000

Pennsylvania_fold_aerial

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plunging_fold

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fold_topography

kqzheng — Thu, 19 Aug 2021 18:11:24 +0000

1280px-Joints_1_Rygel

kqzheng — Thu, 19 Aug 2021 18:11:31 +0000

1280px-Half_dome_yosemite_nationalpark_t1

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depiction-of-joints

kqzheng — Thu, 19 Aug 2021 18:11:32 +0000

joints-developed-in-a-rock

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intrusive-rocks-on-Quadra-Island

kqzheng — Thu, 19 Aug 2021 18:11:33 +0000

Fault_hangingwall_footwall

kqzheng — Thu, 19 Aug 2021 18:11:33 +0000

dip_slip_faults

kqzheng — Thu, 19 Aug 2021 18:11:34 +0000

Figure 13.25 Dip slip faults. Normal faults are caused by tension, while reverse faults happen during compression. Source: Karla Panchuk (2018) CC BY-SA 4.0, modifed after Woudloper (2010) CC BY-SA 3.0 view source

strike_slip_faults

kqzheng — Thu, 19 Aug 2021 18:11:37 +0000

graben-and-horst-structures

kqzheng — Thu, 19 Aug 2021 18:11:38 +0000

thrust-fault

kqzheng — Thu, 19 Aug 2021 18:11:39 +0000

McConnell-Thrust

kqzheng — Thu, 19 Aug 2021 18:11:39 +0000

McConnell-Thrust-at-Mt.-Yamnuska-1024x411

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orogeny

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Figure 13.32 Orogeny in an ocean-continent collision zone. Mountains form from subduction zone volcanism, and from large sheets of rock that are thrust inland and folded. Materials accumulating on the leading edge of the continent in an accretionary wedge are eventually smashed onto the continent, adding to continental crust. Created by Karla Panchuk (2018, CC BY 4.0) Modified after Ron Blakey, NAU Geology (n.d.) view source Terms of use:

All maps and photographs on this page (Geology Illustrated) are copyright to me, Ron Blakey, and may be used, with citation to Ron Blakey, NAU Geology, for educational, non-commercial purposes.

orogeny_continent-continent

kqzheng — Thu, 19 Aug 2021 18:11:48 +0000

Figure 13.33 Orogeny by continent-continent collision. The formation of Pangea included the merging of Africa and North America. This closed an ocean basin and stopped subduction along the coast of North America. Volcanism ended with the closure of the ocean basin, but mountains continued to grow through folding and faulting. Created by Karla Panchuk (2018, CC BY 4.0) Modified after Ron Blakey, NAU Geology (n.d.) view source Terms of use:

All maps and photographs on this page (Geology Illustrated) are copyright to me, Ron Blakey, and may be used, with citation to Ron Blakey, NAU Geology, for educational, non-commercial purposes.

fault-block_mountains

kqzheng — Thu, 19 Aug 2021 18:11:51 +0000

Figure 13.34 Fault-block mountains formed in a rift zone. Magma can move up along normal faults, resulting in igneous intrusions, or volcanic eruptions. Over time, valleys between elevated blocks will fill with sediment as the blocks erode. Created by Karla Panchuk (2018, CC BY 4.0) Modified after Ron Blakey, NAU Geology (n.d.) view source Terms of use:

All maps and photographs on this page (Geology Illustrated) are copyright to me, Ron Blakey, and may be used, with citation to Ron Blakey, NAU Geology, for educational, non-commercial purposes.

strike-and-dip

kqzheng — Thu, 19 Aug 2021 18:11:52 +0000

compass

kqzheng — Thu, 19 Aug 2021 18:11:53 +0000

anticline-and-a-dyke-in-cross-section

kqzheng — Thu, 19 Aug 2021 18:11:53 +0000

Putting-strike-and-dip-on-a-map

kqzheng — Thu, 19 Aug 2021 18:11:53 +0000

Johnston-Creek

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water-cycle

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storage-reservoirs

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Cawston-Creek-e1438915203759

kqzheng — Thu, 19 Aug 2021 18:11:56 +0000

Cawston-Creek-profile

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Metro_Vancouver_Watershed_Boundaries

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dendritic

kqzheng — Thu, 19 Aug 2021 18:11:57 +0000

deranged-and-radial

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typical-graded-stream

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Similkameen-river

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Juan-de-Fuca-Trail

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Wandeling bij Lillooet

kqzheng — Thu, 19 Aug 2021 18:11:59 +0000

Figure 14.13 Terraces on the Fraser River north of Lillooet, BC (above the river on the left-hand side of the image). Source: Wikimedia user “China Crisis” (2007) CC BY-SA 3.0. view source

Davis-cycle-of-erosion

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relative-velocity-of-stream-flow

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grain-225x300

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transportation-of-sediments

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Cascade-Falls-area

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Kicking-Horse-River

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Coldwater-River

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Bonnell-Creek

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Nowitna-River

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delta-of-the-Fraser-River

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Stikine-River

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Qualicum-River

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Red-river-Floodway

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Alberta-floods

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Flooding-in-Calgary

kqzheng — Thu, 19 Aug 2021 18:12:07 +0000

1965-Hope-Slide-as-seen-in-2014

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slope force balance

kqzheng — Thu, 19 Aug 2021 18:12:09 +0000

Created by Karla Panchuk (2021, CC BY-NC-SA 4.0. Adapted from Steven Earle (2015), CC BY 4.0. View source. Background image adapted from the photograph "Alberta Rocky Mountains" by Dave Bloggs (2017), CC BY 2.0. View source.

Relative-stability-of-slopes-as-a-function-of-the-orientation-of-weaknesses-1024x356

kqzheng — Thu, 19 Aug 2021 18:12:10 +0000

Glacial-outwash-and-Glacial-till-1024x483

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sand sorter

kqzheng — Thu, 19 Aug 2021 18:12:10 +0000

Depiction-of-dry-moist-and-saturated-sand

kqzheng — Thu, 19 Aug 2021 18:12:11 +0000

Sand-and-Water-1024x768

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OsoWA

kqzheng — Thu, 19 Aug 2021 18:12:11 +0000

contribution-of-freeze-thaw-to-rock-fall

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talus-slope-and-The-results-of-a-rock-fall-1024x384

kqzheng — Thu, 19 Aug 2021 18:12:12 +0000

DownieSlideGoogleEarth

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RockBoltSeatoSky

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2010-Mt.-Meager-rock-avalanche-

kqzheng — Thu, 19 Aug 2021 18:12:14 +0000

depiction-of-the-contribution-of-freeze-thaw

kqzheng — Thu, 19 Aug 2021 18:12:14 +0000

Creep

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motion-of-unconsolidated-sediments-in-an-area-of-slumping-1024x456

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slump-along-the-banks-of-a-small-coulee-near-Lethbridge

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A-slump-and-an-associated-mudflow

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lower-part-of-debris-flow-within-a-steep-stream

kqzheng — Thu, 19 Aug 2021 18:12:17 +0000

road-constructed-by-cutting-into-a-steep-slope

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motion-monitoring-device-at-the-Checkerboard-Slide-near-Revelstoke

kqzheng — Thu, 19 Aug 2021 18:12:24 +0000

RainierSeattle

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Rainier_lahars_USGS

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snow-avalanche-shelter

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mitigating-debris-flows-on-the-Sea-to-Sky-Highway-1024x313

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The-Barrier

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Lemaire_Channel

kqzheng — Thu, 19 Aug 2021 18:12:27 +0000

Figure 16.1 Antarctic Peninsula. Antarctica was not always covered by ice. A change in the Earth system triggered the onset of Antarctic glaciation approximately 40 million years ago. Created by Karla Panchuk (2018, CC BY-SA 4.0) Components: Lemaire Channel photo: Liam Quinn (2011) CC BY-SA 2.0 view source Locator globe: Bosonic dressing (2009) CC BY-SA 3.0 view source

Earth System example

kqzheng — Thu, 19 Aug 2021 18:12:27 +0000

Figure 16.2 An example of Earth-system interactions. The opening of the Drake Passage changed how ocean currents moved heat around the planet, and cooled Antarctica until it froze over. Created by Karla Panchuk (2018, CC BY-NC-SA 4.0) Components: Earth cutaway: Karla Panchuk (2018) CC BY 4.0 view source Sun: NASA/SDO (AIA) (2010) Public Domain view source Maps: Woods Hole Oceanographic Institution (n.d.) view source Terms of use for images from Woods Hole Oceanographic Institution (from the website):

WHOI Copyright Statement SHARE THIS: Text, graphics and other material contained on this Web site is intended solely for scholarly use by the academic and scientific community. Information contained includes material proprietary to Woods Hole Oceanographic Institution. Use or reproduction of any material herein for any commercial purpose (including but not limited to, textbooks, exhibits, and products for sale) is prohibited without the prior written consent of Woods Hole Oceanographic Institution's Director of Digital Assets at images@whoi.edu.

Daily_Avg_Temp_YXE

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solar_evolution

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orbital_cycles

kqzheng — Thu, 19 Aug 2021 18:12:28 +0000

Figure 16.5 Cycles in Earth’s orbit. Left: The shape of Earth’s orbit (its eccentricity) changes over 100,000 year cycles from more circular to more elliptical. Middle: Over 41,000 year periods, Earth’s axis of rotation nods toward and away from the sun. Right: Over 21,000 year cycles, Earth wobbles on its axis of rotation. Modified by Karla Panchuk (2017, CC BY 4.0) after Steven Earle (2015) CC BY 4.0 view source Components: Eccentricity zero: NASA/Mysid (2008) Public Domain view source Obliquity: NASA/Mysid (2008) Public Domain view source Precession: NASA/Mysid (2008) Public Domain view source

effect-of-precession-on-insolation-in-the-northern-hemisphere-summers-1024x434

kqzheng — Thu, 19 Aug 2021 18:12:28 +0000

insolation_all

kqzheng — Thu, 19 Aug 2021 18:12:29 +0000

Figure 16.7 Comparison of orbital cycles, insolation, and climate data for a 1.6 million year period. Created by Karla Panchuk (2017, CC BY-SA 4.0) Components: Graphs: Incredio (2009) CC BY 3.0 view source Orbital cycle diagrams: Steven Earle (2015) CC BY 4.0 view source Antarctica map: Bosonic Dressing (2009) CC BY-SA 3.0 view source

sunspots

kqzheng — Thu, 19 Aug 2021 18:12:30 +0000

sunspot_data

kqzheng — Thu, 19 Aug 2021 18:12:30 +0000

vertical-movement-of-water-along-a-north-south-cross-section

kqzheng — Thu, 19 Aug 2021 18:12:30 +0000

thermohaline-circulation-system

kqzheng — Thu, 19 Aug 2021 18:12:31 +0000

AMOC_short-term

kqzheng — Thu, 19 Aug 2021 18:12:32 +0000

Thornalley et al

kqzheng — Thu, 19 Aug 2021 18:12:33 +0000

Carboniferous_Scotese

kqzheng — Thu, 19 Aug 2021 18:12:36 +0000

Figure 16.14 Glaciation on the supercontinent Gondwana. Paleogeographic reconstruction for 306 million years ago. Source: C. R. Scotese, PALEOMAP Project (www.scotese.com) Terms of use (from the website):

These maps may be used or modified in any manner for personal use, teaching, research or in scientific publications as long as appropriate credit is given to the author (see below). These maps may not be copied, resold, used or modified in any manner for commercial purposes, such as consulting reports, trade journals or the popular press, textbooks, videos, educational CD-ROMS, computer animations, museum exhibits, web sites on the Internet or for any other commercial use, without the express written consent of the author. Links may be made from any web site on the internet to the PALEOMAP website, www.scotese.com.

Variations-in-the-ENSO-index

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ENSO-mechanism

kqzheng — Thu, 19 Aug 2021 18:12:37 +0000

Nino_winterandsummer_620_from_climate.gov__0

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Nina_winterandsummer_620_from_climate.gov_

kqzheng — Thu, 19 Aug 2021 18:12:38 +0000

Typical-albedo-values-for-Earth-surfaces-1024x339

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1280px-Clearcutting-Oregon

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co2-vibrations-2

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CO2-temp-800k

kqzheng — Thu, 19 Aug 2021 18:12:39 +0000

Figure 16.22 Variations in atmospheric CO2 levels and temperature over the last 800,000 years. Top- CO2 concentration from ice core data in Lüthi et al (2008). The dashed line shows a recent measurement of atmospheric CO2 levels from the Mauna Loa Observatory. Bottom: Temperature record derived from oxygen isotope measurements of water in ice cores. Data from Jouzel et al (2008). Reference line (lower dashed line) is the average temperature for the past 1000 years. The upper dashed line represents the global surface temperature for 2016 from NASA's Goddard Institute for Space Studies. Terms of use for this National Research Council image (from the website):

Evidence, Impacts, and Choices Figure Gallery These images were featured in the National Research Council booklet, Climate Change: Evidence, Impacts, and Choices. They are made freely available here for redistribution, on the condition that the original authors are cited.

Data Sources: Lüthi, D., M. Le Floch, B. Bereiter, T. Blunier, J.-M. Barnola, U. Siegenthaler, D. Raynaud, J. Jouzel, H. Fischer, K. Kawamura, and T.F. Stocker. 2008. High-resolution carbon dioxide concentration record 650,000-800,000 years before present. Nature, Vol. 453, pp. 379-382, 15 May 2008. Get data from NOAA Climatic Data Center. Jouzel, J., V. Masson-Delmotte, O. Cattani, G. Dreyfus, S. Falourd, G. Hoffmann, B. Minster, J. Nouet, J.M. Barnola, J. Chappellaz, H. Fischer, J.C. Gallet, S. Johnsen, M. Leuenberger, L. Loulergue, D. Luethi, H. Oerter, F. Parrenin, G. Raisbeck, D. Raynaud, A. Schilt, J. Schwander, E. Selmo, R. Souchez, R. Spahni, B. Stauffer, J.P. Steffensen, B. Stenni, T.F. Stocker, J.L. Tison, M. Werner, and E.W. Wolff. 2007. Orbital and Millennial Antarctic Climate Variability over the Past 800,000 Years. Science, Vol. 317, No. 5839, pp.793-797, 10 August 2007. Get data from NOAA Climatic Data Center. Mauna Loa Observatory NASA/ Goddard Institute for Space Studies

CSIRO_Antarctic_Ice_sm

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800px-Ice_Core_Vitals_(5433412179)

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1024px-NICL_Freezer

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Climate_zones

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varves

kqzheng — Thu, 19 Aug 2021 18:12:41 +0000

Figure 16.27 Varves in a core from Canoe Brook, Drummerston, Vermont. The top of the core is toward the right side of the photograph. Image with labels added: Karla Panchuk (2017) CC BY-NC-SA 4.0 Varve photograph: Jack Ridge/ North American Glacial Varve Project (2008) view source Terms of use (from the website):

Referencing this Site If you use information on this web site for your own academic writing, web postings, or research you should reference this web site. The references listed in our text should be used for previously published material. For material from the web site that has not been published before, including all images unless otherwise indicated, please use the reference below with the date on which the web site was accessed. Images with specific credits to outside sources in their captions have been used with special permission. The use or reproduction of these images outside of the current web site requires permission from their respective owners. APA Style: "Ridge, J.C. (September 11, 2018) The North American Glacial Varve Project. Retrieved from http://eos.tufts.edu/varves. This work is sponsored by The National Science Foundation and The Department of Earth and Ocean Sciences, Tufts University, Medford, Massachusetts."

Lake Ontario grid

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Lake Ontario results

kqzheng — Thu, 19 Aug 2021 18:12:42 +0000

model-data comparison

kqzheng — Thu, 19 Aug 2021 18:12:42 +0000

World-population-growth-over-the-past-12000-years-

kqzheng — Thu, 19 Aug 2021 18:12:43 +0000

CO2_d13C

kqzheng — Thu, 19 Aug 2021 18:12:43 +0000

Carbon_cycle

kqzheng — Thu, 19 Aug 2021 18:12:43 +0000

anthro_emissions_by_sector

kqzheng — Thu, 19 Aug 2021 18:12:45 +0000

Figure 16.34 Flow diagram illustrating the pathways through which human activities produce greenhouse gases. The diagram connects the items in each column with flows that ultimately lead to the type of fuel used, and the greenhouse gasses produced. The width of each band is proportional to the quantity flowing from one column to the next. Note that F-Gas refers to anthropogenic fluorinated gases, which are extremely powerful greenhouse gases. Source: Fischedick et al. (2014), Figure 10.1, based on Bajželj et al. (2013). Fischedick M., Roy, J., Abdel-Aziz, A., Acquaye, A., Allwood, J. M., Ceron, J.-P., Y. Geng, Y., Kheshgi, H., Lanza, A., Perczyk, D., Price, L., Santalla, E., Sheinbaum, C., and Tanaka, K. (2014). Industry. In: Climate Change 2014: Mitigation of Climate Change. Contribution of Working Group III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Edenhofer, O., Pichs-Madruga, R., Sokona, Y., Farahani, E., Kadner, S., Seyboth, K., Adler, A., Baum, I., Brunner, S., Eickemeier, P., Kriemann, B., Savolainen, J., Schlömer, S., von Stechow, C., Zwickel, T., and Minx, J. C. (eds.). Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. Retrieved from https://www.ipcc.ch/pdf/assessment-report/ar5/wg3/ipcc_wg3_ar5_chapter10.pdf Bajželj, B., Allwood, J. M., and Cullen, J. M. (2013). Designing Climate Change Mitigation Plans That Add Up. Environmental Science & Technology 47, 8062-8069. doi: 10.1021/es400399h. Retrieved from http://pubs.acs.org/doi/pdf/10.1021/es400399h Terms of use (from the website):

IPCC Report Graphics Unless otherwise stated, the information available on this website, including text, logos, graphics, maps, images, audio clips and electronic downloads is the property of the IPCC and is protected by intellectual property laws. You may freely download and copy the material contained on this website for your personal, non-commercial use, without any right to resell, redistribute, compile or create derivative works therefrom, subject to more specific restrictions that may apply to specific materials. Reproduction of a limited number of figures or short excerpts of IPCC material is authorized free of charge and without formal written permission provided that the original source is properly acknowledged, with mention of the name of the report, the publisher and the numbering of the page(s) or the figure(s). Permission can only be granted to use the material exactly as it is in the report. Please be aware that figures cannot be altered in any way, including the full legend.

Hockey_stick_annotated 2

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projected sea level change

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Figure 16.36 Measured and projected change in global average sea level. Data come from proxy records as well as from direct measurements from tidal gauges and satellite data. Projected sea level rise could be as little as 33 cm over 1800 levels, or as much as 206 cm. Created by: Karla Panchuk (2018) Modified after Steven Earle (2015) CC BY 4.0 view source Original diagram from J. Willis, Jet Propulsion Laboratory (2013) Click here to view the source and access more information about this image, including access to data. Terms of use (from the website):

When citing this figure, please reference J. Willis, Jet Propulsion Laboratory. Free to use with credit to the original figure source.

erlaac2f0f1_hr

kqzheng — Thu, 19 Aug 2021 18:12:47 +0000

ice melt contribution

kqzheng — Thu, 19 Aug 2021 18:12:48 +0000

SPM_fig4_revTFS_stockholm_260913AN_rev1

kqzheng — Thu, 19 Aug 2021 18:12:49 +0000

Figure 16.39 Anthropogenic and natural contributions to radiative forcing in 2011 compared to 1750. Source: IPCC (2013). Summary for Policymakers. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker,T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. View source Terms of use (from the website)

You may freely download and copy the material contained on this website for your personal, non-commercial use, without any right to resell or redistribute it or to compile or create derivative works there from, subject to more specific restrictions that may apply to specific materials. Reproduction of limited number of figures or short excerpts of IPCC material is authorized free of charge and without formal written permission provided that the original source is properly acknowledged, with mention of the complete name of the report, the publisher and the numbering of the page(s) or the figure(s). Permission can only be granted to use the material exactly as it is in the report. Please be aware that figures cannot be altered in any way, including the full legend. For media use it is sufficient to cite the source while using the original graphic or figure. In line with established Internet usage, any external website may provide a hyperlink to the IPCC website or to any of its pages without requesting permission.

projected warming

kqzheng — Thu, 19 Aug 2021 18:12:49 +0000

Figure 16.40 Model projections to 2100 for surface temperatures (a) and the extent of sea ice in September in the Northern Hemisphere (b). Black line: measurements; Grey shading: model attempts to simulate conditions from 1950 to 2005; Blue: best-case scenario with peak CO2 emissions in 2020 and zero emissions by 2080; Red: worst-case scenario with no decline in emissions. Numbers indicate the number of models run for each scenario. Shading indicates range of uncertainty. Source: IPCC (2013). Summary for Policymakers. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker,T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. View source Terms of use (from the website)

You may freely download and copy the material contained on this website for your personal, non-commercial use, without any right to resell or redistribute it or to compile or create derivative works there from, subject to more specific restrictions that may apply to specific materials. Reproduction of limited number of figures or short excerpts of IPCC material is authorized free of charge and without formal written permission provided that the original source is properly acknowledged, with mention of the complete name of the report, the publisher and the numbering of the page(s) or the figure(s). Permission can only be granted to use the material exactly as it is in the report. Please be aware that figures cannot be altered in any way, including the full legend. For media use it is sufficient to cite the source while using the original graphic or figure. In line with established Internet usage, any external website may provide a hyperlink to the IPCC website or to any of its pages without requesting permission.

Numbers-of-various-types-of-disasters-between-1971-and-2010

kqzheng — Thu, 19 Aug 2021 18:12:49 +0000

Figure 16.41 Numbers of climate-system of disasters between 1971 and 2010. Source: WMO (2015) Atlas of Mortality and Economic Losses from Weather, Climate and Water Extremes: 1970-2012. View source (p. 9) Terms of use (from the website):

Unless otherwise stated, the information available on this website, including text, logos, graphics, maps, images, audio clips and electronic downloads, is the property of WMO and is protected by intellectual property law. You may freely download and copy the WMO material contained on this website for your personal, non-commercial use, without any right to resell, redistribute, compile or create derivative works therefrom, subject to more specific restrictions that may apply to specific materials. Reproduction of short excerpts of WMO materials, figures and photographs on this website is authorized free of charge and without formal written permission provided that the original source is acknowledged. Reproduction of videos files are authorized free of charge and without formal written permission provided that the original source is acknowledged and subject to the standard creative commons licensing conditions.

russia_anomaly_small

kqzheng — Thu, 19 Aug 2021 18:12:50 +0000

1280px-Hurricane_Sandy_damage_Long_Beach_Island

kqzheng — Thu, 19 Aug 2021 18:12:50 +0000

Relationship-between-Atlantic-tropical-storm-cumulative-annual-intensity-and-Atlantic-sea-surface-temperatures-1024x657

kqzheng — Thu, 19 Aug 2021 18:12:50 +0000

Global precipitation

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degrading-permafrost-site-1024x591

kqzheng — Thu, 19 Aug 2021 18:12:51 +0000

Mountain-pine-beetle-damage-1024x765

kqzheng — Thu, 19 Aug 2021 18:12:51 +0000

athabasca-2

kqzheng — Thu, 19 Aug 2021 18:12:52 +0000

now-you-see-it

kqzheng — Thu, 19 Aug 2021 18:12:52 +0000

Greenland

kqzheng — Thu, 19 Aug 2021 18:12:53 +0000

antarctic-greenland-2

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Antarctic-Ice-Sheet

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Schematic-ice-flow-diagram

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formation-of-glacial-ice

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equilibrium-line

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ice_stress_mod_JM

kqzheng — Thu, 19 Aug 2021 18:12:54 +0000

Spragg_Mt_Cook_red

kqzheng — Thu, 19 Aug 2021 18:12:55 +0000

flow-rates-2

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Zackowitz_Byron_Glacier

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flow-before-after-2

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Mt.-Robson

kqzheng — Thu, 19 Aug 2021 18:12:56 +0000

Receding_glacier_landscape_LMB

kqzheng — Thu, 19 Aug 2021 18:12:57 +0000

U-shaped-valley

kqzheng — Thu, 19 Aug 2021 18:12:57 +0000

Woodf1a

kqzheng — Thu, 19 Aug 2021 18:12:57 +0000

alpine-glaciation-erosion

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aletsch-2

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squamish-2

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Jeff_P_Trappers_Peak

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Jeff_Hollett_Peyto_Lake

kqzheng — Thu, 19 Aug 2021 18:12:59 +0000

bering_etm_2002272_lrg

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glacier

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till-2

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gf-2

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kettle-2

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McKenna_esker

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Drumlins_sm

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gl-gm-2

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major-past-glaciations

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Cenozoic_temp_trends

kqzheng — Thu, 19 Aug 2021 18:13:10 +0000

Antarctic-Circumpolar

kqzheng — Thu, 19 Aug 2021 18:13:10 +0000

Foram-oxygen-isotope-record

kqzheng — Thu, 19 Aug 2021 18:13:11 +0000

Glacials-and-Interglacials-

kqzheng — Thu, 19 Aug 2021 18:13:11 +0000

768px-Iceage_north-intergl_glac_hg

kqzheng — Thu, 19 Aug 2021 18:13:11 +0000

Highland-Valley-Copper-Mine-at-Logan-Lake-1024x572

kqzheng — Thu, 19 Aug 2021 18:13:12 +0000

main-components-of-a-tablet-computer

kqzheng — Thu, 19 Aug 2021 18:13:12 +0000

value-of-various-Canadian-mining-sectors-in-2013

kqzheng — Thu, 19 Aug 2021 18:13:13 +0000

nickel-smelter-at-Thompson-1024x547

kqzheng — Thu, 19 Aug 2021 18:13:13 +0000

black-smoker-on-the-Juan-de-Fuca-Ridge-1024x595

kqzheng — Thu, 19 Aug 2021 18:13:14 +0000

the-formation-of-a-porphyry-deposit

kqzheng — Thu, 19 Aug 2021 18:13:14 +0000

1024px-Banded_iron_formation_Dales_Gorge

kqzheng — Thu, 19 Aug 2021 18:13:14 +0000

20.8

kqzheng — Thu, 19 Aug 2021 18:13:14 +0000

Underground-at-the-Myra-Falls-Mine

kqzheng — Thu, 19 Aug 2021 18:13:15 +0000

Schematic-cross-section-of-a-typical-underground-mine

kqzheng — Thu, 19 Aug 2021 18:13:15 +0000

Entrance-to-an-exploratory-decline-1024x545

kqzheng — Thu, 19 Aug 2021 18:13:15 +0000

tailings-pond-at-the-Myra-Falls-Mine

kqzheng — Thu, 19 Aug 2021 18:13:16 +0000

The-tailings-pond-at-Myra-Falls-Mine-with-settling-ponds

kqzheng — Thu, 19 Aug 2021 18:13:16 +0000

The-Mt.-Polley-Mine-area-prior-to-the-dam-breach

kqzheng — Thu, 19 Aug 2021 18:13:16 +0000

Mt.-Polley-Mine-area-after-the-tailings-dam-breach

kqzheng — Thu, 19 Aug 2021 18:13:16 +0000

Sand-and-gravel-in-an-aggregate-pit-1024x529

kqzheng — Thu, 19 Aug 2021 18:13:17 +0000

Triassic-Quatsino-Formation-limestone-1024x535

kqzheng — Thu, 19 Aug 2021 18:13:18 +0000

Slate-used-as-a-facing-material-1024x753

kqzheng — Thu, 19 Aug 2021 18:13:18 +0000

Coal_Rank_USGS

kqzheng — Thu, 19 Aug 2021 18:13:18 +0000

Highvale-Mine-and-the-Sundance

kqzheng — Thu, 19 Aug 2021 18:13:19 +0000

depth-and-temperature-limits-for-biogenic-gas

kqzheng — Thu, 19 Aug 2021 18:13:19 +0000

Migration-of-oil-and-gas

kqzheng — Thu, 19 Aug 2021 18:13:19 +0000

Seismic-section-through-the-East-Breaks-Field

kqzheng — Thu, 19 Aug 2021 18:13:19 +0000

Interpreting-a-Seismic-Profile-

kqzheng — Thu, 19 Aug 2021 18:13:20 +0000

Schematic-cross-section-of-northern-Alberta-1024x470

kqzheng — Thu, 19 Aug 2021 18:13:20 +0000

Depiction-of-the-process-of-directional-drilling-and-fracking-1024x434

kqzheng — Thu, 19 Aug 2021 18:13:21 +0000

Diamond-mines-in-the-Lac-de-Gras-region-1024x509

kqzheng — Thu, 19 Aug 2021 18:13:21 +0000

Arizona-Grand-Canyon-SE

kqzheng — Thu, 19 Aug 2021 18:13:22 +0000

geological-map-of-England-and-Wales-e1439316487427

kqzheng — Thu, 19 Aug 2021 18:13:22 +0000

Eons

kqzheng — Thu, 19 Aug 2021 18:13:22 +0000

Phanerozoic_eon

kqzheng — Thu, 19 Aug 2021 18:13:23 +0000

Cenozoic_era

kqzheng — Thu, 19 Aug 2021 18:13:24 +0000

1280px-Delta_Formation.svg_-1024x572-1

kqzheng — Thu, 19 Aug 2021 18:13:26 +0000

Cretaceous-Nanaimo

kqzheng — Thu, 19 Aug 2021 18:13:26 +0000

principle_of_inclusions

kqzheng — Thu, 19 Aug 2021 18:13:27 +0000

Grand Canyon unconformity

kqzheng — Thu, 19 Aug 2021 18:13:27 +0000

unconformities

kqzheng — Thu, 19 Aug 2021 18:13:27 +0000

Earth history_cropped

kqzheng — Thu, 19 Aug 2021 18:13:36 +0000

age-of-a-rock

kqzheng — Thu, 19 Aug 2021 18:13:37 +0000

septum-of-an-ammonite

kqzheng — Thu, 19 Aug 2021 18:13:37 +0000

foraminifera

kqzheng — Thu, 19 Aug 2021 18:13:38 +0000

decay-of-40K

kqzheng — Thu, 19 Aug 2021 18:13:46 +0000

Crystals-of-potassium-feldspar

kqzheng — Thu, 19 Aug 2021 18:13:46 +0000

Table_19-2

kqzheng — Thu, 19 Aug 2021 18:13:47 +0000

Table 19.2. Commonly used isotope systems for dating geological materials Potassium-Argon: Half-life 1.3 Ga, useful range 10 ka - 4.57 Ga. Widely applicable because most rocks contain potassium Uranium-Lead: Half-life 4.5 Ga, useful range 1 Ma - 4.57 Ga. The rock must contain uranium-bearing minerals (felsic igneous rocks) Rubidium-Strontium: Half-life 47 Ma, useful range 10 Ma - 4.57 Ga. Less precision than other methods for old rocks Carbon-Nitrogen (radiocarbon dating): Half-life 5,730 a, useful range 100 a to 60 ka. Sample must contain wood, bone, or carbonate minerals; can be applied to young sediments

sedimentary-rocks

kqzheng — Thu, 19 Aug 2021 18:13:47 +0000

Radiocarbon

kqzheng — Thu, 19 Aug 2021 18:13:47 +0000

tree-ring-dating

kqzheng — Thu, 19 Aug 2021 18:13:55 +0000

Washington

kqzheng — Thu, 19 Aug 2021 18:13:55 +0000

magnetic-field-reversals

kqzheng — Thu, 19 Aug 2021 18:13:56 +0000

magnetized-oceanic-crust

kqzheng — Thu, 19 Aug 2021 18:13:56 +0000

pattern-of-magnetism

kqzheng — Thu, 19 Aug 2021 18:13:57 +0000

periodic-table-e1442860182846

kqzheng — Thu, 19 Aug 2021 18:14:06 +0000

Plate_tectonics_map

kqzheng — Thu, 19 Aug 2021 22:01:59 +0000

image045_2-1

kqzheng — Thu, 19 Aug 2021 22:02:01 +0000

divergent-boundary-processes-1

kqzheng — Thu, 19 Aug 2021 22:02:01 +0000

rift-formation-e1439319064564-1

kqzheng — Thu, 19 Aug 2021 22:02:01 +0000

image0551-1

kqzheng — Thu, 19 Aug 2021 22:02:01 +0000

ocean-continent-1

kqzheng — Thu, 19 Aug 2021 22:02:02 +0000

image0591-1

kqzheng — Thu, 19 Aug 2021 22:02:02 +0000

image061-1

kqzheng — Thu, 19 Aug 2021 22:02:03 +0000

image063-1

kqzheng — Thu, 19 Aug 2021 22:02:03 +0000

Pangea-breakup-507x1024-1-1

kqzheng — Thu, 19 Aug 2021 22:02:03 +0000

image067-1

kqzheng — Thu, 19 Aug 2021 22:02:03 +0000

Pangea-ultima-494x1024-1-1

kqzheng — Thu, 19 Aug 2021 22:02:04 +0000

Wilson-cycle-1

kqzheng — Thu, 19 Aug 2021 22:02:04 +0000

Plate_tectonics_map

kqzheng — Thu, 19 Aug 2021 22:10:02 +0000

image045_2

kqzheng — Thu, 19 Aug 2021 22:10:04 +0000

divergent-boundary-processes

kqzheng — Thu, 19 Aug 2021 22:10:04 +0000

rift-formation-e1439319064564

kqzheng — Thu, 19 Aug 2021 22:10:04 +0000

image0551

kqzheng — Thu, 19 Aug 2021 22:10:05 +0000

ocean-continent

kqzheng — Thu, 19 Aug 2021 22:10:05 +0000

image0591

kqzheng — Thu, 19 Aug 2021 22:10:05 +0000

image061

kqzheng — Thu, 19 Aug 2021 22:10:06 +0000

image063

kqzheng — Thu, 19 Aug 2021 22:10:06 +0000

Pangea-breakup-507x1024

kqzheng — Thu, 19 Aug 2021 22:10:06 +0000

These maps may be used or modified in any manner for personal use, teaching, research or in scientific publications as long as appropriate credit is given to the author (see below). These maps may not be copied, resold, used or modified in any manner for commercial purposes, such as consulting reports, trade journals or the popular press, textbooks, videos, educational CD-ROMS, computer animations, museum exhibits, web sites on the Internet or for any other commercial use, without the express written consent of the author. Links may be made from any web site on the internet to the PALEOMAP website, www.scotese.com.

Links to maps: Early Jurassic (195 Ma) Late Jurassic (152 Ma) Late Cretaceous (94 Ma) End Cretaceous (66 Ma)

image067

kqzheng — Thu, 19 Aug 2021 22:10:06 +0000

Pangea-ultima-494x1024

kqzheng — Thu, 19 Aug 2021 22:10:07 +0000

These maps may be used or modified in any manner for personal use, teaching, research or in scientific publications as long as appropriate credit is given to the author (see below). These maps may not be copied, resold, used or modified in any manner for commercial purposes, such as consulting reports, trade journals or the popular press, textbooks, videos, educational CD-ROMS, computer animations, museum exhibits, web sites on the Internet or for any other commercial use, without the express written consent of the author. Links may be made from any web site on the internet to the PALEOMAP website, www.scotese.com.

Links to maps: Modern World Future World (+50 Ma) Future World (+150 Ma) Future World (+250 Ma)

Wilson-cycle

kqzheng — Thu, 19 Aug 2021 22:10:08 +0000

USGS_Kīlauea_MultimediaFile-978

kqzheng — Thu, 19 Aug 2021 23:45:01 +0000

711px-Carbon_cycle

kqzheng — Fri, 20 Aug 2021 16:59:34 +0000

Roche_moutonnée_below_Myot_Hill_-_geograph.org.uk_-_164736

kqzheng — Fri, 20 Aug 2021 17:30:47 +0000

H5PLicense

clalonde — Fri, 20 Aug 2021 17:33:01 +0000

Interpreting-a-Seismic-Profile-

kqzheng — Fri, 20 Aug 2021 17:41:50 +0000

Delta_Formation.svg

kqzheng — Fri, 20 Aug 2021 17:45:14 +0000

Cover Image

hfriedman — Mon, 30 Jan 2023 23:54:36 +0000

Mount Burgess towers

kqzheng — Wed, 26 Apr 2023 21:47:34 +0000

Emerald Lake Lodge

kqzheng — Wed, 26 Apr 2023 21:48:33 +0000

luckysocks

kqzheng — Wed, 26 Apr 2023 21:59:22 +0000

Tyrannosaurus rex

kqzheng — Wed, 26 Apr 2023 22:07:24 +0000

STSCI-H-p0612a-2400x1525

kqzheng — Thu, 04 May 2023 22:38:10 +0000

spectra

kqzheng — Thu, 04 May 2023 22:43:35 +0000

file-60dc950ec9013

kqzheng — Thu, 04 May 2023 22:59:16 +0000

solar system4

kqzheng — Thu, 04 May 2023 23:09:34 +0000

nebula

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disk2

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gaps3

kqzheng — Thu, 04 May 2023 23:11:48 +0000

planet 4

kqzheng — Thu, 04 May 2023 23:13:25 +0000

planet chart

kqzheng — Thu, 04 May 2023 23:19:46 +0000

Foreshocks, mainshocks, & after shocks

jgray — Wed, 07 Jun 2023 16:31:49 +0000

118-1

kqzheng — Wed, 07 Jun 2023 16:49:45 +0000

solarsystem-1

acheveldave — Wed, 07 Jun 2023 16:50:52 +0000

Igneous rock layers

jgray — Wed, 07 Jun 2023 16:52:39 +0000

118-3

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118-4

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118-5

kqzheng — Wed, 07 Jun 2023 16:55:53 +0000

solarsystem-2

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solarsystem-3

acheveldave — Wed, 07 Jun 2023 16:58:54 +0000

solarsystem-4

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solarsystem-5

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105-1

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105-2

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105-3

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105-4

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105-5

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105-6

kqzheng — Wed, 07 Jun 2023 17:10:58 +0000

Enhanced-colour view of an outcrop from Horseshoe Bay

jgray — Wed, 07 Jun 2023 17:15:11 +0000

120-6

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120-5

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120-1

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120-2

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120-3

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120-4

kqzheng — Wed, 07 Jun 2023 17:21:04 +0000

Moraine

jgray — Wed, 07 Jun 2023 17:26:14 +0000

83-1

acheveldave — Wed, 07 Jun 2023 17:33:56 +0000

83-2

acheveldave — Wed, 07 Jun 2023 17:34:55 +0000

135-1

kqzheng — Wed, 07 Jun 2023 17:36:54 +0000

135-BA

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135-2

kqzheng — Wed, 07 Jun 2023 17:41:09 +0000

135-3

kqzheng — Wed, 07 Jun 2023 17:44:19 +0000

Basalt in Iceland

jgray — Wed, 07 Jun 2023 17:48:58 +0000

102-1

acheveldave — Wed, 07 Jun 2023 17:50:51 +0000

102-2

acheveldave — Wed, 07 Jun 2023 17:51:11 +0000

Thin soil in steep slope in Iceland

jgray — Wed, 07 Jun 2023 17:51:53 +0000

Grassy hillside slipping

jgray — Wed, 07 Jun 2023 17:53:21 +0000

Cleanup after storm

jgray — Wed, 07 Jun 2023 17:55:46 +0000

Slap of rock inching down

jgray — Wed, 07 Jun 2023 17:58:29 +0000

Crumbled mountainside

jgray — Wed, 07 Jun 2023 17:59:54 +0000

155-1

kqzheng — Wed, 07 Jun 2023 18:00:15 +0000

155-1C

kqzheng — Wed, 07 Jun 2023 18:00:58 +0000

Mud after heavy rains

jgray — Wed, 07 Jun 2023 18:02:56 +0000

155-2

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155-2C

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155-3

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Folds

jgray — Wed, 07 Jun 2023 18:06:40 +0000

157-1a

acheveldave — Wed, 07 Jun 2023 18:06:42 +0000

157-1b

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157-1c

acheveldave — Wed, 07 Jun 2023 18:06:45 +0000

V-shape

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157-2a

acheveldave — Wed, 07 Jun 2023 18:12:18 +0000

157-2b

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157-2c

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157-3a

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157-3b

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157-3c

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157-4a

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157-4b

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file-60f1f00c3b698

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file-60fd7a73f0f2a

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196-question2

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196-question1

hfriedman — Tue, 20 Jun 2023 16:22:38 +0000

file-60faf2f4e042b

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9-3b

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9-1a

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9-1b

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9-2

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9-3a

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file-60f9c1a3e813b

jgray — Tue, 20 Jun 2023 16:31:57 +0000

151

hfriedman — Tue, 20 Jun 2023 16:37:02 +0000

file-5f6e12106c635

jgray — Tue, 20 Jun 2023 16:39:06 +0000

Karla Panchuk. Photo: Tadd and Debbie Ottman (2007) CC BY. View source.

file-5f6e1509cbdf1

jgray — Tue, 20 Jun 2023 16:40:17 +0000

Source: Karla Panchuk. Photo: Daniel Case (2007) CC BY-SA. View source.

126-1

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126-2

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file-60f6eaabe7fb0

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file-60f7160190037

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file-60f5d72262bb9

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17-1

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file-5f4d48f060590

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file-60e8a2c0ab629

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8x10.ai

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27-1

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file-60ea1f4fcc561

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file-60ea1c2a95844

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file-60ea16fb6f95d

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file-60ea1b21b5ed1

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file-60ea194399749

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file-60ea13fd1131f

jgray — Tue, 20 Jun 2023 17:53:53 +0000

1.1 What Is Geology?

kqzheng — Sun, 27 Aug 2017 20:05:24 +0000

The Study of Rocks?

Imagine that someone has just asked you what geology is. If your answer is, “It’s the study of rocks,” then you’re in for a surprise! While rocks are important to geology, some geologists don’t study rocks at all. Geologists might study water, climate, organisms in extreme environments, or environments where rocks might form in the future. If your answer is “It’s the study of Earth,” you’d be getting closer. Geologists do study Earth—its interior and its exterior surface, the rocks and other materials around us, and the processes that formed those materials. They also study how Earth has changed over the vast time-span of its history, and how it might change in the future. Even so, you’d still be missing the work done by geologists who study other planets or solid non-planetary objects in space. Perhaps reading this chapter will be the start to your career as the head geologist on a Mars survey crew locating water resources for a new colony...

Earth's Geology Helps Us Understand Ancient Mars These photographs are of the same type of rock, but one is on Mars (left) and the other is on Earth (right). On Earth this type of rock is formed when rapidly moving water carries broken pieces of rock, smooths them off, then drops them on a stream bed. By knowing how this rock forms on Earth, we can tell that Mars also had water flowing rapidly over its surface at some time in its history. [caption id="attachment_22" align="aligncenter" width="650"] Figure 1.2 Left: Link outcrop on Mars photographed on 2 September 2012. Right: A similar outcrop in the Northwest Territories. Source: NASA/JPL-Caltech/MSSS and PSI (2012). Public Domain. Image source.[/caption]

Geology Is a Science

Geological questions are investigated by collecting evidence, hypothesizing about what that evidence could mean, and finding ways to test whether those ideas make sense or not. Geological science is done with the same rigour as any other science, and this means that a lot of creativity is sometimes required to figure out how to test hypotheses. For instance, how do you test a hypothesis about what Earth was like 3 billion years ago? Some geoscientists test such hypotheses by trying to find modern-day environments that are similar to ancient Earth environments. Geology is an interdisciplinary science. Geologists must understand and apply other sciences—physics, chemistry, biology, mathematics, astronomy, and more—in their work. Geoscientists often consult with or work with scientists in other disciplines. An aspect of geology that is unlike most of the other sciences is the role played by the vastness of time. When geologists study the evidence around them, they are often observing the results of events that took place thousands, millions, and even billions of years in the past, and which may still be ongoing. Geologists study not only processes that happened long ago, but processes that happen at incredibly slowly. Changes in the shape of Earth's surface might only happen at rates of mm to cm per year, but because of the amount of time available, tiny changes can result in expansive oceans forming, or entire mountain ranges being worn away.

Geological Time in the Canadian Rocky Mountains The photographs below show changes that happened in the blink of an eye, geologically speaking. At the same time, they also illustrate the vastness of geological time. The past 100 years. On the right of both photos is Rearguard Mountain, which is a few kilometres northeast of Mount Robson. Mount Robson is the tallest peak in the Canadian Rockies, at 3,954 m. The large glacier in the middle of the photo is the Robson Glacier. The river flowing from Robson Glacier drains into Berg Lake in the bottom right. The photo on the right was taken around 1908 by the Canadian geologist and artist Arthur Philemon Coleman, and gives an indication of how much the glacier has receded in the last hundred years due to rapid climate change. The past two million years. This area—like much of Canada—has been covered repeatedly by glaciers over the past 2 million years. Those glaciers scoured away rocks to form the valley on the left. 500 million years ago. The rocks making up the mountains formed in ocean water over 500 million years ago. A few hundred million years later, a great collision between Earth's tectonic plates pushed those rocks east for 10s to 100s of km, and 1000s of m upward. [caption id="attachment_23" align="aligncenter" width="700"] Figure 1.3 Rearguard Mountain and Robson Glacier in Mount Robson Provincial Park, BC. Left: Robson Glacier today, retreating up the valley. Right: Robson Glacier circa 1908. Sources: Left- Karla Panchuk (2017) CC BY-SA 4.0 with photo by Steven Earle (2015) CC BY 4.0. Image source. Right: A.P. Coleman (c. 1908) Public Domain. Click the image for more attributions.[/caption]

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How does the element of time make geology different from the other sciences, such as chemistry and physics? Geology requires that we consider vast amounts of time, and think about the effects that accumulate over thousands, millions, or even billions of years.

References

Victoria University Library (2009) A. P. Coleman exhibition. https://library.vicu.utoronto.ca/apcoleman/index.htm

1.2 Why Study Earth?

kqzheng — Sun, 27 Aug 2017 20:05:25 +0000

Earth is our home, and by studying geology we can learn how to live here safely, and how to use the resources we need in a sustainable way:

We can learn to minimize our risks from earthquakes, volcanoes, slope failures, and damaging storms.
We can study rocks and the fossils they contain to understand the evolution of our environment and the life within it.
We can learn how and why Earth’s climate changed in the past, and use that knowledge to understand both natural and human-caused climate change..
We can measure how human activities have altered the environment, and learn how to prevent and sometimes repair the damage.
We rely on Earth for resources such as soil, water, metals, industrial minerals, and energy, and we need to know how to find these resources and exploit them sustainably.

Arguably, studying Earth is more important than ever. Moving away from fossil fuels as an energy source means more extraction of mineral resources, not less, because those resources are needed to build clean-energy technologies, such as batteries. It's essential that we learn how to do so without repeating the mistakes of the past. Studying Earth is also important for helping to keep people safe as the climate changes. Even people who don't necessarily plan to be geologists—e.g., policy makers, community leaders, and the general public—can benefit from understanding how their environment works.

Spotting Everyday Hazards Look at the slides below and see if you can spot a common hazard related to how Earth's surface changes over time, but that humans can alleviate by planning ahead.

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Can you spot the potential hazard? Hint: Compare this image with the next one in the slide deck. [caption id="attachment_1445" align="aligncenter" width="600"] Mount Burgess towers above Emerald Lake and a cluster of cabins at Yoho National Park. Photo: Jack Borno[/caption] [caption id="attachment_1446" align="aligncenter" width="600"] Emerald Lake Lodge at Yoho National Park. Photo: Mehrdad Rezvanpour[/caption] The cabins at Emerald Lake weren't always this close to the lake. Over time, waves washed away at the slope, bringing the shore closer and closer. With no preventative measures, the waves will erode the material beneath the cabins, and they will collapse into the lake. The Emerald Lake Lodge is also very close to the water, but the slope is protected from wave erosion by a barrier of rocks.

1.3 What Do Geologists Do?

kqzheng — Sun, 27 Aug 2017 20:05:26 +0000

[caption id="attachment_28" align="aligncenter" width="650"] Figure 1.4 Geologists at work on the island of Spitsbergen, part of the Svalbard archipelago. The islands are located in the Arctic Ocean north of Norway. Source: Gus MacLeod (2007) CC BY-NC-ND 2.0. Image source.[/caption] Geologists do a lot of different things. Many of the jobs are the things you would expect:

Geologists work in the resource industry, including mineral exploration and mining, and exploring for and extracting sources of energy.
They do hazard assessment and mitigation (e.g., assessment of risks from slope failures, earthquakes, and volcanic eruptions).
They study the nature of the subsurface for construction projects such as highways, tunnels, and bridges.
They use information about the subsurface for water supply planning, development, and management, and to decide how best to contain contaminants from waste.

Geologists also do the research that makes practical applications of geology possible. Some geologists spend their summers trekking through the wilderness to make maps of the rocks in a particular location, and collect clues about the geological processes that occurred there. Some geologists work in laboratories analyzing the chemical and physical properties of rocks to understand how the rocks will behave when forces act on them, or when water flows through them. Some geologists specialize in inventing ways to use complex instruments to make these measurements. Geologists study fossils to understand ancient animals and environments, and go to extreme environments to understand how life might have originated on Earth. Some geologists help NASA understand the data they receive from objects in space. Geological work can be done indoors in offices and labs, but some people are attracted to geology because they like to be outdoors. Many geological opportunities involve fieldwork in places that are as amazing to see as they are interesting to study. Sometimes these are locations where few people have ever set foot, and where few ever will again.

Living with a Volcano [caption id="attachment_29" align="aligncenter" width="650"] Figure 1.5 Ash plume from the eruption of La Soufrière on the island of St. Vincent on 9 April, 2021. In the Red Zone near the volcano, risk of life-threatening volcanic hazards is highest. An evacuation was ordered on 8 April, 2021. Source: Karla Panchuk (2021) CC BY-SA. Photograph by Lauren Dauphin, NASA Earth Observatory.[/caption]

On 9 April 2021, the volcano La Soufrière on the island of St. Vincent erupted explosively. A day earlier, seismic activity at the volcano caused the Prime Minister of St. Vincent and the Grenadines to call for an evacuation of the Red Zone in the northern part of the island, closest to the volcano. By 12 April, 16,000 people had been evacuated to safety. Many people with a wide range of knowledge, skills, and experience are needed to manage this kind of evacuation. Which of the roles below include geoscientists? Select as many as apply.

Planning an evacuation strategy
Assessing the risk of an eruption
Planning and organizing humanitarian relief
Communicating with the public

To check your answers, navigate to the below link to view the interactive version of this activity.

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References

Hansen, K. (2021, April 9). Eruption at La Soufrière. NASA Earth Observatory. https://earthobservatory.nasa.gov/images/148176/eruption-at-la-soufriere

Jones, D. (2021, April 12). From bad to worse: La Soufrière volcano continues to erupt. NPR. https://www.npr.org/2021/04/12/986302206/from-bad-to-worse-la-soufriere-continues-to-erupt

Lovell, E., & Wilkinson, E. (Accessed 2021, April 30). Managing multi-hazard disaster risk in St Vincent and the Grenadines. Overseas Development Institute. https://odi.org/en/publications/multimedia/managing-multi-hazard-disaster-risk-in-st-vincent-and-the-grenadines/

Romo, V., & Newman, S. (2021, April 9). Volcano erupts on Caribbean Island Of St. Vincent as evacuation continues. NPR. https://www.npr.org/2021/04/09/985626157/threat-of-volcanic-eruption-forces-residents-to-flee-st-vincent

1.4 We Study Earth Using the Scientific Method

kqzheng — Sun, 27 Aug 2017 20:05:27 +0000

Science Is a Process

Some people consider science to be a set of facts about nature, but a better description of science is that it's a means of collecting those facts in as reliable a way as possible. Carl Sagan, an astronomer and author, put it this way:

Science is more than a body of knowledge. It is a way of thinking; a way of skeptically interrogating the universe with a fine understanding of human fallibility. (The Charlie Rose Show, 1996)

The understanding of human fallibility that Sagan refers to is the awareness that human brains take shortcuts in reasoning. Sometimes we're conscious of these shortcuts, but other times we're unaware. An example would be the tendency to have more confidence in a fact that is repeated by multiple sources than in a fact stated only once. If you do an Internet search and see the same fact repeated on many websites, it might seem that the fact has been independently confirmed by many people. However, it could simply be that everyone used the same Wikipedia article for their research. The scientific method is a way to reduce the likelihood that errors in reasoning will lead to flawed conclusions. In its most basic form, the scientific method involves formulating an idea about how the world works—a hypothesis—and then finding a way to test it to see if it’s actually true, and should be accepted. The term theory is often used in everyday language as a synonym for hypothesis, but that's not what a scientist means when they talk about a theory. What they're referring to is a hypothesis that has been tested over and over again, and passed every single test. Saying that an idea is a hypothesis is like suggesting, “Maybe the world works this way.” Saying that an idea is a theory is like concluding, “It’s extremely unlikely that the world works in a way other than this.” Another term commonly used to describe a scientific idea of great certainty is law. But don’t confuse a law with a theory. Whereas theories are explanations of phenomena, laws are descriptions that apply under specific circumstances. For example, the law of conservation of mass tells us that mass is never lost or gained in physical interactions. An object might break into two, but the total mass of the two parts is the same as the mass of the original. The law doesn’t explain why mass works that way, but it is a reliable rule to use when doing physics calculations.

Theory, Hypothesis, or Law?

Complete this summary of theory, hypothesis, and law by putting the words into the correct blank. “ , this should work” is how people sometimes express uncertainty about whether they will be successful or not, but this isn't the correct terminology. If they mean that they're trying out an untested idea, it would be more accurate to say, “ , this should work.” If they're worried about whether reality will match up with predictions that are based on mathematical descriptions of physical phenomena, what they really mean is, “ , this should work.” Fill-in-the-blank options:

Hypothetically
In theory
By law

To check your answers, navigate to the below link to view the interactive version of this activity.

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An Example of the Scientific Method at Work

Imagine a field trip to the stream shown in Figure 1.6. Notice that the rocks in and along the stream are rounded off rather than having sharp edges. We might hypothesize that the rocks were rounded because as the stream carried them, they crashed into each other and pieces broke off. [caption id="attachment_32" align="aligncenter" width="650"] Figure 1.6 Hypothesizing about the origin of round rocks in a stream. Source: Steven Earle (2015) CC BY 4.0 view source[/caption] If that hypothesis is correct, then the further we go downstream, the rounder and smaller the rocks should be. Going upstream we should find that the rocks are more angular and larger. If we were patient we could also test the hypothesis by marking specific rocks and then checking back to see if those rocks have become smaller and more rounded as they moved downstream. If the predictions turn out to be correct, we must still be careful about how much certainty to attach to our hypothesis. Although our hypothesis might seem to us to be the only reasonable explanation, someone could argue that we have the mechanism wrong, and the rocks weren't rounded by bumping into each other. If our experiment didn't specifically check for the mechanism (e.g., by looking to see if chips fall off the rocks and the rocks are made smoother) then we would have to acknowledge the possibility. We needn't abandon the hypothesis as a useful tool for making predictions, but it is necessary to be open to the possibility that other things might be going on. If someone demonstrates conclusively that our hypothesis is wrong, then we have to discard the hypothesis and come up with a better one. A good hypothesis is testable. Someone might argue that an extraterrestrial organization creates rounded rocks and places them in streams when nobody is looking. There's no practical way to test this hypothesis to confirm it, and there's no way to prove it false. Even if we never see aliens at work, we still can't say they haven't been, because according to the hypothesis they only work when people aren't looking. Compare this to our original hypothesis which allows us to make testable predictions such as rocks getting smaller and rounder downstream. Our original hypothesis gives us a way to see how realistic it is, whereas the alien hypothesis gives us no way to know if it makes sense or not.

Lucky Socks Science

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Can these hypotheses about lucky socks be tested?

Hypothesis 1: I won yesterday’s lottery because I was wearing my lucky socks.
Hypothesis 2: Wearing lucky socks will improve my chances of winning the lottery.

1.5 Three Big Ideas: Geological Time, Uniformitarianism, and Plate Tectonics

kqzheng — Sun, 27 Aug 2017 20:05:29 +0000

In geology there are three big ideas that are fundamental to the way we think about how Earth works. The ideas are like the sound track to a movie—sometimes we might not even notice them, but they nevertheless affect our perception of what's happening. In the rest of this book these ideas may be mentioned explicitly in some cases, but in other cases they won't be discussed by name, and it will be helpful for you to realize that they're relevant.

Geological Time (Deep Time)

Earth is approximately 4.6 billion years old (4,600,000,000 years), which is a long time for geological events to unfold and changes to happen. The changes themselves might be tiny—over a year, a chemical reaction might eat away a few layers of atoms at the surface of a rock. Over hundreds of millions of years, however, the chemical reaction could cause a mountain range to crumble into grains of sand, and be swept away by rivers. For geologists who study very, very slow processes, 10 million years might be a short time, and 1 million years might be trivial. For these geologists, intervals of 1 million years aren't even useful to consider, because the changes over that time are too small to see in the rocks that accumulated. As you read through this book, keep in mind that the well of geologic time is indeed deep, and "ancient" is defined in a whole new way.

Need Some Help Visualizing Geological Time? Watch the animation Four Ways to Understand the Earth’s Age to see four analogies for geological time. https://youtu.be/tkxWmh-tFGs

Expressing Geological Time in Numbers

Special notation is used for geological time because, as you might imagine, writing all those zeroes can become tiresome. Table 1.1 shows common abbreviations you will see throughout this book.

Table 1.1 Abbreviations Used to Describe Geological Time
Abbreviation	Meaning	Example
Ga	giga annum or billions of years	Earth is 4.6 Ga old.
Ma	mega annum or millions of years	Earth is 4,600 Ma old.
ka	kilo annum or thousands of years	The last glacial cycle ended 11,700 years ago, or 11.7 ka.

How Many Years Is That?

Can you write each of these in units of years? Use commas to break up the numbers (e.g., 4,000,000).

2.75 ka =
0.93 Ga =
14.2 Ma =

Hint: Multiply why what k,[footnote]This is the same as moving the decimal 3 places to the right.[/footnote] G,[footnote]This is the same as moving the decimal 9 places to the right.[/footnote] or M[footnote]This is the same as moving the decimal 6 places to the right.[/footnote] stands for. To check your answers, navigate to the below link to view the interactive version of this activity.

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Expressing Geological Time Using the Geological Time Scale

The geological time scale (Figure 1.7) is a way of breaking down geological time according to important events in Earth's history. Time is divided into eons, eras, periods, and epochs, and these intervals are referred to by names rather than by years. Giving intervals of geologic time names rather than using numbers makes sense because we won't always know the age in years (the absolute age) of a rock or fossil, but we can place it in context based on our knowledge of the geological record. We can describe its relative age by saying that it's older than or younger than another rock or fossil. [caption id="attachment_35" align="aligncenter" width="2200"] Figure 1.7 Geologic Society of America Geologic Time Scale, 2012. Source: Walker, J.D., Geissman, J.W., Bowring, S.A., and Babcock, L.E., compilers (2012) Geologic Time Scale v. 4.0: Geological Society of America, doi: 10.1130/2012.CTS004R3C. Download PDF[/caption]

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Tyrannosaurus rex lived from 66 to 68 million years ago. Does T. rex belong in a Jurassic park? No. The Jurassic Period was from 201 to 145 million years ago. This is much too early for a T. rex.

The tricky thing about the geologic time scale is that the boundaries are always changing. As our knowledge of the absolute age of an event improves with new discoveries, it might be necessary to nudge a boundary earlier or later. Sometimes the original reason for defining a boundary no longer holds, but we agree to use it anyway. For example, the Phanerozoic Eon (the last 542 million years) is named for the time during which visible (phaneros) life (zoi) is present in the geological record, and its start was meant to mark the first appearance of these organisms. We now have evidence that large organisms—those that leave fossils visible to the naked eye—have existed longer than that, first appearing by 600 Ma at the latest.

An Early Definition of the Proterozoic Notice that in Figure 1.7 the Proterozoic Eon precedes the Phanerozoic Eon. This was not always the case. Figure 1.8 shows an excerpt from a periodical published in 1879, in which the Proterozoic is defined as covering the Cambrian through Silurian. The author refers to "the most extreme adherents of the Murchisonian party in geology," a reference to the contentious assertion by Scottish geologist Roderick Murchison (1792-1871) that the Silurian Period should encompass the Cambrian and Ordovician periods as well. [caption id="attachment_36" align="aligncenter" width="700"] Figure 1.8 An excerpt from the periodical The Annals and Magazine of Natural History (1879) in which the name "Proterozoic" is assigned to the Cambrian, Ordovician, and Silurian periods instead of to the time preceding the Cambrian. Source: Karla Panchuk (2017) CC BY 4.0 Read the book[/caption]

A Way To Think About Geological Time

A useful mechanism for understanding geological time is to scale it down into one year. The origin of the solar system and Earth at 4.6 Ga would be represented by January 1, and the present year would be represented by the last tiny fraction of a second on New Year’s Eve. At this scale, each day of the year represents 12.5 million years; each hour represents about 500,000 years; each minute represents 8,694 years; and each second represents 145 years. Some significant events in Earth’s history, as expressed on this time scale, are summarized in Table 1.2.

Table 1.2 Some Important Dates Expressed As If All of Geological Time Were Condensed Into One Year *Source: Karla Panchuk (2021) CC BY 4.0, modified after Steven Earle (2015) CC BY 4.0 view original*
Event	Approximate Date	Calendar Equivalent
Formation of oceans and continents	4.5 - 4.4 Ga	first week of January
Evolution of the first primitive life forms	3.8 Ga	end of February
Formation of British Columbia’s oldest rocks	2.0 Ga	July
Evolution of the first multi-celled animals	600 Ma	beginning of November
Animals first crawled onto land	360 Ma	end of November
Vancouver Island reached North America and the Rocky Mountains were formed	90 Ma	December 16
Extinction of the non-avian dinosaurs	66 Ma	December 18
The last time atmospheric CO2 levels were above 400 ppm (Tripati et al., 2009)	16 Ma	6:20 p.m. December 30
Beginning of the Pleistocene ice age	2 Ma	10:10 p.m., December 31
Oldest radiocarbon date from people living in Canada (British Columbia)	13.8 ka	11:58 p.m., December 31
First indication of fossil fuel impacts on atmospheric CO2 levels (~280 ppm)	221 years ago	1.5 seconds before midnight, December 31
Atmospheric CO2 levels exceed 400 ppm	8 years ago	0.06 seconds before midnight, December 31

Uniformitarianism

Uniformitarianism is the notion that the geological processes occurring on Earth today are the same ones that occurred in the past. This is an important idea because it means that observations we make today about geological processes can be used to interpret and understand the rock record. While this idea might not seem remarkable today, it was ground breaking and even controversial for its time. Many people who heard about it for the first time thought about the age of the Earth in thousands of years, but uniformitarianism required them to think on timescales almost too vast to comprehend. For some, this implied questioning their most deeply held religious beliefs. The idea of uniformitarianism can be traced back to James Hutton, first in a paper he read to the Royal Society in 1784, and later in his book Theory of the Earth, first published in 1788. (Read this book at Project Gutenberg.) In 1788 he explained, "In examining things present, we have data from which to reason with regard to what has been; and, from what has actually been, we have data for concluding with regard to that which is to happen hereafter." Charles Lyell, also a Scottish geologist, expanded on Hutton's ideas and incorporated those of may other early geologists in his own book Principles of Geology. According to Lyell, "religious prejudice" was a major stumbling block to the acceptance of these ideas, but also Hutton and other scientists should have written less wordy books, and included more pictures. Charles Lyell's thinking about uniformitarianism is paraphrased as "the present is the key to the past." As he put it in his own less-wordy more-pictures book, a belief in the "permanency of the laws of nature" means that a geologist

"will deem it incumbent on him to examine with minute attention all the changes now in progress on the Earth, and will regard every fact collected respecting the causes in diurnal action, as affording him a key to the interpretation of some mystery in the archives of remote ages."

In other words, learning how things work today will tell you about how they worked in the past. Paraphrasing uniformitarianism as "the present is the key to the past" has led some to view it as an oversimplification, because not all geological processes occurring today occurred at all times in the geological past. Some important chemical reactions that happen on Earth's surface today require abundant oxygen in the atmosphere, and could not have occurred prior to Earth developing an oxygen-rich atmosphere. Furthermore, there was a time in Earth's history when continents as we know them hadn't yet developed. Some events, such as devastating impacts by objects from space, have never been witnessed on the same scale by humans. We must be cognizant of the fact that conditions were different at different times in Earth's history, and take that into account when interpreting the rock record. Despite the different past conditions on Earth as a whole, there still exist environments today where some of these conditions are present. These environments are like tiny samples of what Earth used to be like. This means we can still use present conditions to inform us about the past, but we have to think carefully about ways that such environments today differ from the ancient environments that no longer exist. Arguably, saying that "the present is the key to the past" is an oversimplification on Lyell's part isn't exactly fair to Lyell, given his phrasing about the "permanency of the laws of nature." (Author's note: I have been unable to find that exact "key to the past" quote so far, so at this point we have to allow for the possibility that Lyell didn't oversimplify at all, and he's getting in trouble for what someone else said. I will update this if I find out differently). If we take his meaning to be that the basic way that physics and chemistry work has always been the same, then we are still using that principle to interpret the Mars rocks in Figure 1.2. Imagine what Lyell would have thought about this idea being used to infer the presence of an ancient river on Mars!

Check Your Understanding

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True or False? Because "the present is the key to the past" uniformitarianism is only backward-looking, and thus only good for understanding Earth in the past. Answer False. Looking at how Earth-processes work today can help us understand past Earth because nature worked the same way then as now. This means we can also use information about past geologic events to understand events happening now, or what might happen in the future.

Plate Tectonics

The theory of plate tectonics—the idea that Earth's surface is broken into large moving fragments, called plates—profoundly changed our perspective on how the Earth works. Before plate tectonic theory, we couldn't answer questions like "How did that mountain range get there?" and "Why do earthquakes happen where they do?" This critical framework is also very recent—the papers that led to the widespread acceptance of plate tectonics were published at about the same time that we got cordless power tools, contact lenses, and satellite television. Earth has 15 large tectonic plates (Figure 1.9), and numerous smaller ones. (A more detailed map of Earth's tectonic plates can be found at Plate Tectonics Map.) Tectonic plates move in a variety of directions and at a variety of speeds, although on average they move a few millimetres per year. In Figure 1.9, the black arrows show the direction of motion. The size of arrows indicates how fast the plate is moving. Plates with arrows going in more than one direction are rotating. Plate tectonic traffic is complicated! The explanatory power of plate tectonic theory comes from what happens when plates interact along their margins. Plate boundary interactions (red arrows) include collisions (→ ←), separation (← →), or sliding along each other (↗ ↙). [caption id="attachment_37" align="aligncenter" width="650"] Figure 1.9 Earth's fifteen largest tectonic plates. Black arrows show the direction of plate motions. The length of the arrow indicates velocity. Red arrows show how plates move relative to each other. Source: Steven Earle (2015) CC BY 4.0. view source Modified after U. S. Geological Survey (1996) Public Domain view original[/caption] Before plate tectonic theory, we made observations but could only guess at mechanisms. It was like watching the hands on a clock and trying to guess what moves them. After plate tectonics it was like being able to open the clock and not only watch the gears turn, but realize for the first time that there are such things as gears. Plate tectonics not only explains why things have happened, but also allows us to predict what might happen in the future. Plate tectonics is covered in more detail later, however the key point is that Earth's outer layer consists of rigid plates that are constantly interacting with each other as they move around the Earth. The plates can move because they are floating on a layer of weak rock that deforms as the plates travel, much the same way the filling in a peanut butter and jelly sandwich allows you to slide the top layer of bread across the bottom layer. Whether the plates move away from each other, collide, or just slide past each other determines things like the locations of mountain belts and volcanoes, where earthquakes happen, and the shapes and sizes of oceans and continents.

Check Your Understanding

Are any of the other "big ideas" involved in plate tectonic theory? Choose any that apply.

Yes, deep time is required.
Yes, uniformitarianism applies.
No, plate tectonic theory works all by itself.

To check your answers, navigate to the below link to view the interactive version of this activity.

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Glossary

kqzheng — Sun, 27 Aug 2017 23:44:28 +0000

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

A

aa a lava flow that solidifies with a blocky high-relief surface ablation melting of ice in the context of glaciation ablation till till that is formed when englacial and supraglacial sediments are deposited because the ice that was supporting them melts abyssal plain the flat surface of the deep ocean, typically beyond the limits of the continental slopes abyssal pelagic zone the deeper parts of the ocean, between 4000 and 6000 m. accretion (plate tectonics) the process by which continental blocks (terranes) are added to existing continental areas accretion (planetary) the process by which solid celestial bodies are added to existing bodies during collisions acid rock drainage (acid mine drainage) the production of acid from oxidation of sulphide minerals (especially pyrite) in either naturally or anthropogenically exposed rock aeolian processes related to transportation and deposition of sediments by wind aerobic processes that take place in the presence of abundant oxygen aerosol an aggregate of fine solid particles or a small droplet of liquid suspended in the air aftershock an earthquake that can be shown to have been caused by another earthquake aggregate unconsolidated materials (typically sediments) that are used in the construction industry albedo the reflectivity of a surface of a planet (expressed as the percentage of light that reflects) albite sodium-rich plagioclase feldspar alpine glacier a glacier formed in a mountainous region and confined to a valley (same as valley glacier) amphibole a double-chain ferromagnesian silicate mineral (e.g., hornblende) amphibolite a foliated metamorphic rock in which the mineral amphibole as an important component amplification in the context of seismic shaking the process by which the amplitude of the seismic waves are enhanced amplitude for any type of wave, the difference in height between a crest and the adjacent trough anaerobic processes that take place without oxygen andesite a volcanic rock of intermediate composition anion a negatively charged ion angular unconformity a geological boundary at the base of a sedimentary layer where the sedimentary rock beneath has been tilted or folded and then eroded anorthite calcium-rich plagioclase feldspar Antarctic Bottom Water water at abyssal depths in the ocean that forms from the sinking of dense cold water adjacent to Antarctica anticline an upward fold where the beds are known not to be overturned anthracite a high grade of coal (92 to 98% carbon) that is formed from deep burial and weak metamorphism anthropogenic resulting from the influence of humans antiform an upward fold where it is not known if the beds have been overturned apparent polar wandering path a path of seeming varying magnetic pole positions defined by paleomagnetic data, which is in fact an artefact of the motion of contients aphanitic an igneous texture characterized by crystals that are too small to see with the naked eye aquifer a body of rock or sediment that has sufficient permeability to allow it to be used as a source of groundwater aquitard a body of rock or sediment that has insufficient permeability to allow it to be used as a source of groundwater arch a rock weathering remnant in the form of an arch (typically along a coast and resulting from wave erosion) arenite a sandstone with less than 15% silt and clay arête a sharp ridge that separates adjacent glacially carved valleys arkose a sandstone with more than 10% feldspar and more feldspar than lithic fragments arkosic arenite an arkose with less than 15% clay/silt matrix artesian well a well that is completed in a confined aquifer and in which the water level in the well rises above the top of the aquifer asteroid a rocky body orbiting the Sun asteroid belt the region between the orbits of Mars and Jupiter that is populated with many asteroids asthenosphere the part of the mantle, from about 100 to 200 km below surface, within which the mantle material is close to its melting point, and therefore relatively weak asymmetrical (folds) where the two sides of the fold make significantly different angles with respect to the axial plane atoll a ring-shaped carbonate (or coral) reef or series of islands atomic mass the total number of neutrons plus protons in an atom atomic number the total number of protons in an atom attitude the orientation of a sloping geological feature, such as a bedding plane or fracture aureole a zone of metamorphism around a source of heat such as a magma body axial plane a plane that can be traced through all of the hinge lines of a fold

B

back reef the zone of shallow water on the shore-side of a reef background (geochemistry) the typical level of an element in average rocks or sediments backwash the wash of wave water down the slope of a beach banded iron formation an iron-bearing sedimentary rock that is rich in minerals such as hematite and magnetite, and interbedded with chert stained red by hematite bank-full stage the water level of stream when it is in flood and just about to flow over its banks barrier reef a carbonate (or coral) reef that forms a barrier to waves along a coast basal sliding the motion of glacial ice along the base of a glacier that is warm enough to have liquid water basalt a volcanic rock of mafic composition base level (stream) the base level is the lowest level that a stream can erode down to, as defined by the ocean, a lake or another stream that it flows into batholith an irregular body of intrusive igneous rock that has an exposed surface of at least 100 km²bathypelagic zone the moderately deep parts of the ocean, between 1000 and 4000 m. baymouth bar a spit that extends across the mouth of a bay beach face the part of the beach that is relatively steep and lies between the high and low tide levels bed a sedimentary layer bed load the fraction of a stream’s sediment load that typically rests on the bottom and is moved by saltation and traction bedding repeated layering in a sedimentary rock bentonite a smectite clay that has strong swelling properties and is effective at absorbing dissolved ions berm a flat area of a beach in the backshore area (above the high tide level) big-bang theory the theory that the universe started by expanding suddenly from a single point approximately 13.77 billion years ago biochemical sedimentary rock a rock formed when biological processes cause ions to precipitate (e.g., when organisms make shells of calcite or silica) biotite a sheet-silicate mineral (mica) that includes iron and or magnesium, and is therefore a ferromagnesian silicate biozone a stratigraphic interval that can be defined on the basis of a specific fossil bituminous coal a medium-grade type of coal with 70 to 92% carbon blueschist 1. (metamorphic rock) a schist with blue colouring due to the presence of the mineral glaucophane. Formed in subduction zones. 2. (metamorphic facies) a facies characterized by relatively low temperatures and high pressures, such as can exist within a subduction zone body wave a seismic wave that travels through rock (e.g., a P-wave or an S-wave) boulder a sediment clast with a diameter of at least 256 mm Bowen's reaction series the scheme that defines the typical order of crystallization of minerals from magma as the magma cools braided a stream pattern which is characterized by abundant sediment and numerous intertwining channels around bars breakwater a structure built offshore in order to deflect the energy of waves breccia a sedimentary or volcanic rock texture characterized by angular clasts brunisol a relatively immature forest soil, lacking in well-defined horizons burial when a layer of sediment is covered by subsequent sediment accumulation

C

caldera a volcanic depression that forms when part of the volcano collapses into an empty magma chamber caliche a white calcium-carbonate rich layer within soils in arid regions calving the loss of ice from the front of a glacier by collapse into water Canadian Shield the exposed part of the continent Laurentia carbonate a mineral for which the anion is CO₃^-2carbonate compensation depth the depth in the ocean below which carbonate minerals are soluble cation a positively charged ion cementation the process by which minerals are precipitated between grains in sediments, locking the grains together Cenozoic the most recent of the eras, representing the past 65.5 Ma of geological time chemical sedimentary rock a sedimentary rock comprised of material that was transported as ions in solution, then precipitated by inorganic means (e.g., precipitation triggered by evaporation) chemical weathering chemical reactions at Earth's surface which break down rocks and minerals chernozem black soil typical of grasslands in cold climates such as the Canadian Prairies chert very fine-grained sedimentary rock formed almost entirely of silica chilled margin edges of a pluton which cool rapidly through contact with country rock, resulting in finer grain sizes than in the interior of the pluton chlorite ferromagnesian sheet silicate mineral, typically present as fine crystals and forming from the low-temperature metamorphism of mafic rock cinder cone steep-sided volcano comprised almost entirely of loose rock fragments, and typically formed during a single eruptive event cirque a steep-sided semi-circular basin eroded by an alpine glacier at the head of its valley clast a sedimentary fragment of mineral or rock clastic sedimentary rock a sedimentary rock comprised of material that was transported as clasts or fragments clay sediment particle that is less than 1/256 mm in diameter clay mineral a hydrous sheet-silicate mineral that typically exists as clay-sized grains claystone a sedimentary rock comprised mostly of clay-sized grains cleavage tendency for a mineral to break along smooth planes that are predetermined by its lattice structure climate feedback a case in which the effects of a climate forcing trigger other changes which either amplify or mute the effects of the initial forcing climate forcing a mechanism, such as a change in greenhouse gas levels, that causes the climate to change coal an organic sedimentary rock formed by the compression and heating of vegetative organic matter. Types of coal include lignite, bituminous coal, and anthracite. coal-bed methane methane that is trapped within the pores of coal within a coal seam coastal straightening the tendency for an irregular coast to be straightened over time by coastal erosion processes cobble sediment particle that is between 64 and 256 mm in diameter col the low point or pass along a ridge between two glacial valleys columnar jointing the fractures in volcanic rock forming columns that are typically 6-sided, resulting from cooling and contraction of the rock composite volcano (or stratovolcano) a volcano that is constructed of alternating layers of pyroclastic debris and lava flows concentrate (mining) a product of ore processing that includes a specific ore mineral, separated from the rest of the rock concordant parallel to pre-existing layering or foliation within a rock cone of depression the depression of the water table around a well that is heavily pumped confined aquifer an aquifer that lies below a confining layer confining layer an aquitard that overlies an aquifer and restricts the flow of water down from the surface confining pressure pressure resulting from the weight of overlying rocks conglomerate a sedimentary rock that is comprised predominantly of rounded grains that are larger than 2 mm contact metamorphism metamorphism that takes place adjacent to a source of heat, such as a body of magma continental drift the concept that tectonic plates can move across the surface of the Earth continental glacier a glacier that covers a significant part of a continent and has an area of at least 50,000 km²continental shelf the shallow (typically less than 200 m) and flat sub-marine extension of a continent continental slope the steeper part of a continental margin, that slopes down from a continental shelf towards the abyssal plain contractionism the now discredited theory that mountain ranges formed as a result of the contraction of the Earth convergent boundary a plate boundary at which the two plates are moving towards each other Cordilleran Ice Sheet the continental glacier that covered part of western North America, including almost all of British Columbia, part of the Yukon, and part of northern Washington, during the Pleistocene glaciations core the metallic interior part of the Earth, extending from a depth of 2900 km to Earth's centre core-mantle boundary (CMB) the boundary, at 2900 km depth, between the mantle and the core Coriolis effect the tendency for moving bodies (e.g., ocean currents) to rotate on the surface of the Earth, clockwise in the northern hemisphere and counter-clockwise in the southern hemisphere cosmic microwave background (CMB) a radiation "fog" left over from the an early stage in the development of the universe, when the universe was too dense to allow photons to travel far without being scattered country rock the original rock of a region, into which younger rock (typically igneous) rock has been intruded covalent bond a bond between two atoms in which electrons are shared crater a volcanic depression that is related to a specific volcanic vent craton a region of ancient (typically Precambrian) crystalline rock (equivalent to a shield) creep the very slow (mm to cm per year) flow of unconsolidated material on a gentle slope crest the highest point on a wave crevasse an open fissure on the surface of a glacier cross bedding small-scale inclined bedding within larger horizontal beds crust the uppermost layer of the Earth, ranging in thickness from about 5 km (in the oceans) to over 50 km (on the continents) cryptocrystalline refers to the texture of a rock or mineraloid in which crystals are so small that they are almost undetectable even with magnification cyanobacteria photosynthetic bacteria that evolved in the early Archean

D

D" layer (d-double-prime layer) a low seismic velocity zone within the basal 200 km of the mantle debris flow a gravity-driven flow of water and sediment that includes a significant proportion of coarse (cobble to boulder) material decline (mining) a sloped tunnel used to access lower parts of a mine with wheeled equipment decompression melting melting (or partial melting) of rock resulting from a reduction in pressure without a significant reduction in temperature Deep time (geologic time). The long timescales (millions to billions of years) over which geologic processes happen. dendritic a pattern of drainage channels that resembles the branches in a tree density weight per volume of a substance (e.g., g/cm³) deposition when sediments are dropped out of the medium carrying them, and begin to accumulate in layers deranged (drainage) a pattern of drainage channels that is chaotic detrital referring to fragments of rocks or minerals diatom photosynthetic algae that make their tests (shells) from silica differentiation the un-mixing of a molten planetary body, resulting in the formation of a metallic core and a silicate mantle dike a tabular intrusive igneous body that is discordant to any existing layering in the country rock diorite an intermediate intrusive igneous rock dip the angle below horizontal at which a sedimentary bed or other feature slopes directed pressure (also, differential stress, directional pressure) pressure which is greater in one direction than in others (e.g., compression, tension) discharge the volume of water flow in a stream expressed in terms of volume per unit time (e.g., m³/s) discharge area the part of an aquifer where groundwater discharge takes place disconformity a boundary between parallel sedimentary layers where some erosion of the lower layer has taken place discordant when a geological feature is not parallel to any existing layering in the country rock dissolution when water molecules take a substance apart by capturing its ions and keeping them separated (a type of chemical weathering) divalent an ion with a charge or +2 or -2 divergent a plate boundary at which the two plates are moving away from each other dodecahedron an object with twelve surfaces, such as a garnet crystal dolomite a calcium-magnesium carbonate mineral (Ca,Mg)CO₃. Also, a rock made out of that mineral (see also dolostone) dolomitization the addition of magnesium to limestone during which some or all of the calcium carbonate is converted to dolomite dolostone a carbonate rock made up primarily of the mineral dolomite drainage basin the catchment area of a stream, including the area where all surface water drains into the stream drop stone a fragment of rock within otherwise fine-grained sediment that has been dropped from floating ice on a body of water drumlin a streamlined glacial erosional feature comprised of sediments and/or bedrock dyke see dike

E

eccentricity (Milankovitch cycles) the degree to which the sun is offset from the geometric centre of the Earth’s orbit eclogite a garnet-pyroxene-glaucophane bearing rock that is the product of high-pressure metamorphism of oceanic crustal rock, typically within a subduction zone effusive a volcanic eruption dominated by the relatively gentle flow of lava El Niño a periodic climatic situation in which warm water extends all or most of the way to the eastern edge of the equatorial Pacific elastic deformation deformation from which a material can fully recover if the stress is removed electron sub-atomic particle with a single negative charge end moraine sediment deposit that accumulates at the front of a glacier englacial within a glacier, referring especially to sediment carried within the glacial ice epicentre the location on the surface vertically above the location (i.e., “hypocentre” or “focus”) where an earthquake takes place epipelagic zone the upper layer of water (0 to 200 m) in areas of the open ocean epithermal deposit a mineral deposit formed near to surface in an area of hydrothermal activity, typically associated with a body of magma equilibrium line (glacier) the line between the zone of accumulation and the zone of ablation (in late summer the equilibrium line is the boundary between snow-covered ice and bare ice) equipotential lines (groundwater) lines connecting locations with equal hydraulic head or water pressure erosion the process of transporting sediments away from their source esker a ridge of sediment deposited by a sub-glacial stream eustatic sea level change sea level change related to a change in the volume of the oceans, typically because of an increase or decrease in the amount of glacial ice on land evaporite a chemical sedimentary rock that forms when evaporation concentrates the ions in a solution to the point where they begin to precipitate out exfoliation (weathering) the fracturing of rock that results from a reduction in the pressure when overlying rock is eroded away exoplanet a planet that orbits a star other than the sun extrusive igneous rock that cooled at Earth's surface

F

fall (mass wasting) the vertical or near-vertical downward movement of rock fault boundary in rock or sediment along which displacement has taken place feedback when one process triggers others which either amplify or mute the original process feldspar a very common framework silicate mineral feldspathic arenite a sandstone consisting predominantly of sand-sized grains and cement (less than 15% fine-grained matrix), and with more than 10% feldspar grains felsic silica rich (>65% SiO₂) in the context of magma or igneous rock ferric the oxidized form of an ion of iron (Fe³⁺) ferromagnesian referring to a silicate mineral that contains iron and or magnesium ferrous the reduced (non-oxidized) form of an ion of iron (Fe²⁺) fetch the distance over which wind blows to form waves finger lake a lake that occupies a glacial valley firn the granular transitional state between snow and ice within a glacier flood plain the area that is occupied by water when a stream floods and overtops its banks flow a mass-wasting event where material moves which is saturated with water flow path the path that groundwater flows along between a recharge area and a discharge area flowing artesian well an artesian well in which the water level naturally rises above the surface of the ground flux melting melting of rock that is facilitated by the addition of a flux (typically water) which lowers the rock's melting point focus (earthquake) the actual point below surface at which an earthquake takes place (equivalent to hypocentre) foliation the alignment of mineralogical or structural features of a rock – especially a metamorphic rock footwall the lower surface of a non-vertical fault foraminifera single-celled protist with a shell that is typically made of CaCO₃fore-reef the zone on the ocean side of a reef formation a unit of sedimentary rock that is lithologically consistent and sufficiently thick and extensive to be shown on a geological map at the scale that is typically used in the area in question fracking fracturing rock by injecting water and chemicals down a well at very high pressure (equivalent to hydraulic fracturing) fractional crystallization the sequential crystallization of minerals from magma, and the physical separation of early-forming crystals from the magma in the area where they crystallized fracture a break within a body of rock in which the rock on either side is not displaced fringing reef a reef adjacent to a shoreline where there is either a very narrow back reef area or none at all (in which case the reef is effectively attached to the shore) frost line (also, snow line) in the context of newly forming planetary systems, the distance beyond a star at which volatile components (e.g., water, carbon dioxide, methane, ammonia etc.) are frozen frost wedging physical weathering caused when the expansion of freezing water pries rock apart

G

Ga (giga annum) billions of years before the present gabbro a mafic intrusive igneous rock Gaia hypothesis the hypothesis advanced by James Lovelock that the organisms have affected the atmosphere and oceans such that conditions on Earth have been kept habitable, in spite of significantly changing energy received from the Sun galaxy gravitationally-bound system of stars and interstellar matter gas giant a large planet composed mostly of hydrogen and helium (e.g. Jupiter) Geological time (deep time). The long timescales (millions to billions of years) over which geologic processes happen. Geology. The study of Earth, its materials, and the natural processes acting on and within it. geosyncline kilometres thick deposit of sediments that has accumulated along the edge of a continent and is sufficient mass to depress the crust beneath it geothermal gradient the rate of increase of temperature with depth in the Earth (typically around 30˚C/km within the crust) giant impact hypothesis the theory that the Moon formed when a Mars-sized planet (Theia) collided with the Earth at 4.5 Ga glacial period a period of Earth’s history during which glacial ice was present over a sufficient extent to have left recognizable evidence glacial groove a straight line created on a rock surface by erosion by a rock fragment embedded in the base of glacial ice (larger and deeper than a glacial striation) glacial striation a straight line created on a rock surface by erosion by a rock fragment embedded in the base of glacial ice (finer than a glacial groove – typically less than 1 cm wide) glacier a long lasting (centuries or more) body of ice on land that moves under its own weight glaciofluvial referring to sediments deposited from a stream that is derived from a glacier glaciolacustrine referring to sediments deposited within a lake in a glacial environment glaciomarine referring to sediments deposited within the ocean in a glacial environment glaucophane a blue sodium-magnesium-bearing amphibole mineral that forms during metamorphism at high pressures and relatively low pressures, typically within a subduction zone gneiss high-grade foliated metamorphic rock in which the mineral components are separated into bands of different composition graben a down-dropped fault block, bounded on either side by normal faults grade 1. (mineral deposit) the amount of a specific metal or mineral expressed as a proportion of the whole rock. 2. (coal) the extent to which carbon has been concentrated within the coal, and the possible energy output on combustion has increased graded bedding an individual sedimentary layer that shows a distinctive gradation in grain size (normal graded bedding is finer towards the top, reverse graded bedding is coarser towards the top) gradient the slope of a stream bed over a specific distance, typically expressed in m per km grain size the diameter of a fragment (clast) of sediment granite a felsic intrusive igneous rock granule a sedimentary particle ranging in size from 2 to 4 mm in diameter greenhouse gas a gaseous molecule with 3 or more atoms that is able to absorb infrared radiation greenhouse effect (climate) the ability of an atmosphere to absorb infrared radiation due to the presence of greenhouse gases greenschist 1. (metamorphic rock) a foliated metamorphosed rock (typically derived from basalt) in which the green colouration is derived from either chlorite, epidote, or green amphibole. 2. (metamorphic facies) low-grade metamorphic facies characteristic of regional metamorphism greenstone a non-foliated metamorphosed rock (typically derived from basalt) in which the green colouration is derived from either chlorite, epidote or green amphibole. Can be formed by hydrothermal metamorphism on the ocean floor. greywacke a sandstone with more than 15% silt and clay, and with a significant proportion of sand-sized rock fragments groundwater water that lies beneath the surface of the ground group a stratigraphically continuous series of related formations groyne a man-made structure extending from the shore built to deflect the energy of waves gyre a closed circular ocean current

H

habit a characteristic crystal form or combination of forms of a mineral habitable zone the region around a star that is considered to be suitable for a life-bearing planet Hadean the first eon of Earth history, extending from 4.57 to 3.80 Ga halide a mineral in which the anion is one of the halide elements (e.g., halite – NaCl or fluorite - CaF₂) halite NaCl, a halide mineral which consititutes table salt halogen an element in the second-last column of the periodic table that forms anions with a negative-1 charge hanging valley a glacial valley created by a tributary glacier which does not erode as deeply as the main-valley glacier that it joins hanging wall the upper surface of a non-vertical fault headland a point extending out to sea horn a peak that has been eroded on at least three sides by glaciers hornfels a fine-grained metamorphic rock that is not foliated. It can have a variety of parent rocks. horst an uplifted fault block, bounded on either side by normal faults hot spot the surface area of volcanism and high heat flow above a mantle plume hydrated mineral a mineral that includes either hydroxyl (OH) or water (H₂O) in its chemical formula (e.g., gypsum CaSO₄.2H₂O) hydraulic conductivity an expression of the rate at which a liquid will flow through a porous medium, as determined by the permeability of the medium and the viscosity of the liquid hydraulic fracturing fracturing rock by injecting water and chemicals down a well at very high pressure (equivalent to fracking) hydrolysis a reaction between a mineral and water in which H+ ions are added to the mineral and a chemically equivalent amount of cations are released into solution hydrothermal refers to hot water solutions and processes involving hot water solutions hydrothermal alteration chemical alteration of minerals by hot water solutions hydroxide the anion OH^- or an mineral that includes that anion hypocentre the actual point below surface at which an earthquake takes place (equivalent to focus) Hypothesis. An informed speculation about how the world works. Scientific hypotheses must be testable.

I

ice giant a planet that is comprised mainly of gases heavier than hydrogen and helium, including oxygen, carbon, nitrogen, and sulfur (e.g., Uranus and Neptune) igneous a rock formed from the cooling of magma illite a clay mineral with a composition similar to that of muscovite mica imbricate aligned and overlapping, like the tiles on a roof index fossil a fossil with a distinctive appearance and a wide geographic range but from a relatively restricted time range, thus making it useful for dating a correlating rocks from different regions (the most useful index fossils are from organisms that lived for less than a million years) index mineral (metamorphic rocks) a mineral with a stability range of pressures and temperatures sufficiently narrow so as to be useful in indicating the pressures and temperatures at which a metamorphic rock formed. inert in chemistry, an element that does not readily react with other elements (e.g., neon) infiltration the recharge of groundwater from the downward percolation of surface water insolation a measure of the intensity of solar energy at a specific location or time (expressed in W/m2) intensity in seismology, a qualitative measure of the amount of shaking at specific location, based on what was felt by observers, or the amount of damage done Intergovernmental Panel on Climate Change (IPCC) an international body established in 1988 by the UN’s World Meteorological Organization and the UN Environment Program to prepare periodic reports on the status of global climate change and its mitigation intrusive an igneous rock (pluton) that has cooled slowly beneath the surface ionic bond a bond in which electrons are transferred from one atom to another, thus forming ions ion an atom that has either gained or lost electrons and has thus become charged (or a group of atoms that also has a charge – e.g., HCO₃^-) isoclinal fold a tight fold in which the limbs are parallel to each other isostasy the equilibrium between a block of crust floating on the underlying plastic mantle isostatic sea-level change the effect on relative sea level of a vertical adjustment of the crust resulting from a change in the mass of the crust (e.g., from losing or gaining ice) isotherm a surface or line drawn to represent points at the same temperature. (iso = same) isotope a form of an element that differs from other forms because it has a different number of neutrons (e.g., ¹⁶O has 8 protons and 8 neutrons while ¹⁸O has 8 protons and 10 neutrons)

J

joint a fracture in rock where the rock on one side has not moved relative to the other side jointing the formation of joints Jovian planet a gas giant planet

K

ka (kilo annum) thousands of years before the present kaolinite a clay mineral that does not have cations other than Al and Si karst the solutional erosion of an area with soluble rock (typically limestone) to form depressions and caves kettle a depression formed at the front of a large glacier when a stranded ice block that was surrounded by sediment eventually melts kettle lake a lake that forms within a kettle kimberlite an ultramafic volcanic rock that originates at significant depth (> 200 m) in the mantle (some kimberlites include diamonds) Kuiper belt a region of the Solar System beyond the orbit of Neptune that is populated by small objects and dwarf planets (including Pluto)

L

laccolith concordant intrusion in which the central part has bulged upward lahar a mudflow or debris flow that is either caused by a volcanic eruption, or forms on the flank of a volcano as a result of flooding not related to an eruption landfill gas gases produced within a landfill during the microbial breakdown of landfill components (most are dominated by carbon dioxide and methane) large igneous province (LIP) a very large area of mafic volcanic rock produced by a massive eruption typically related to a mantle plume lateral moraine a deposit of rocky material that forms along the margin of a valley or alpine glacier, mostly from the freeze-thaw release of material from the steep slopes above lattice the regular and repeating three-dimensional structure of a mineral Laurentide Ice Sheet the continental glacier that extended across central eastern North America during the Pleistocene, covering most of Canada and a significant part of the United States lava molten rock on Earth's surface (cf. magma) lava levée a ridge that forms along the edge of a lava flow because the magma at the edge cools faster than that in the middle lava tube a tube that forms as mafic lava flows along a channel and lava leveés build up on either side, eventually forming a roof (once a lava tube forms it insulates the flowing magma, allowing it to stay hot a liquid for longer and therefore flow much further) Law. A description that always applies to a scientific phenomenon, given specific conditions. leachate in the context of landfills, the liquid (rainwater) that passes through the waste and becomes contaminated with soluble components from the waste levée on a stream, the ridge that naturally forms along the edge of the channel during flood events level in mining, a horizontal mine opening light year the distance that light can travel in one year (9.4607 x 10¹² km) lignite a low-grade type of coal with less than 70% carbon limbs the layers of rock on either side of a fold limestone a biochemical sedimentary rock that is comprised mostly of calcite liquefaction the tendency for unconsolidated and water saturated sediments to lose strength during seismic shaking lithic arenite an arenite in which there is more than 10% lithic clasts and in which there are more lithic clasts than feldspar clasts (see also arenite) lithic clasts fragments of another rock which are included in the sand-sized grains in sandstone, or in the larger grains in conglomerate lithification the conversion of unconsolidated sediments into rock by compaction and cementation lithosphere the rigid outer part of the Earth, including the crust and the mantle down to a depth of about 100 km lithostatic pressure pressure due to the weight of overlying rocks lodgement till sediment that accumulates at the base of a glacier and typically has a wide range of grain sizes (including clay) and is well compacted long axis in a crystal, clast, or grain, the direction in which the length would be the greatest longshore current the movement of water along a shoreline produced by the approach of waves at an angle to the shore longshore drift the movement of sediment along a shoreline resulting from a longshore current and also from the swash and backwash on a beach face Love wave a surface seismic wave, with horizontal motion, that develops in relatively weak (e.g., unconsolidated) materials at surface luvisol a cold climate forest soil formed in which clay has been removed from the A horizon and relocated into the B horizon

M

Ma (Mega annum) millions of years before the present mafic silica poor (<45% SiO2) in the context of magma or igneous rock, and containing ferromagnesian minerals such as olivine and pyroxene) magma molten rock within Earth's interior (cf. lava) magnetic chronology the study of the timing of reversals of the Earth’s magnetic field, and the application of that understanding to dating geological materials magnitude a measure of the amount of energy released by an earthquake mantle the middle layer of the Earth, dominated by iron and magnesium rich silicate minerals and extending for about 2900 km from the base of the crust to the top of the core mantle plume a plume of hot rock (not magma) that rises through the mantle (either from the base or from part way up) and reaches the surface where it spreads out and also leads to hot-spot volcanism marble a non-foliated metamorphic rock derived from a limestone or dolostone protolith, in which the calcite or dolomite has been recrystallized into larger crystals mass wasting the mass failure, by gravity, of rock or unconsolidated material on a slope matrix finer-grained material between larger clasts within a sedimentary rock maturity the degree to which a sediment or sedimentary rock exhibits characteristics of prolonged physical and chemical weathering and transpor,t meander cutoff the formation of a shorter stream channel across the narrow boundary between two meanders on a stream meandering the sinuous path taken by a stream within a wide flat flood plain mechanical weathering (also, physical weathering) weathering that occurs when physical processes cause a rock to break into smaller pieces without changing the chemical composition medial moraine a lateral moraine that has been shifted towards the centre of a valley glacier at a point where two glaciers meet member a subdivision of a formation mesopelagic zone the upper middle zone of the open ocean extending from 200 to 1000 m depth metallic lustre the lustre of a mineral into which light does not penetrate but reflects off of the surface without being scattered (i.e., shines reflects light like a shiny metal) metallic bond a type of bond in which abundant electrons are easily shared amongst cations metamorphic facies a group of metamorphic rocks formed under the same range of pressures and temperature conditions, but from different parent rocks metamorphic grade refers to the intensity of metamorphism, and increases as pressure and temperature increase metamorphism the transformation of a parent rock into a new rock as a result of heat and pressure that leads to the formation of new minerals, or recrystallization of existing minerals, without melting metasomatism metamorphism facilitated by ion transfer through water, and which results in a substantial change in the chemical composition (not just the mineral content) of a rock meteoroid a small fragment of stony or metallic debris in space methane hydrate a combination of water ice and methane in which the methane is trapped inside “cages” in the ice mica a sheet-silicate mineral (e.g., biotite, muscovite) migmatite rock that is part metamorphic and part igneous, formed at very high grades of metamorphism when a part of the parent rock starts to melt Milankovitch cycles millennial-scale variations in the orbital and rotational parameters of the Earth that have subtle effects on the Earth’s climate Mohorovičić discontinuity (Moho) the boundary between the crust and the mantle moment magnitude a way of estimating earthquake magnitude based on the area of the rupture surface and the amount of displacement monogenetic a volcano that forms in a single eruptive event moraine lake a finger lake that forms within a glacial valley and is dammed by an end moraine mud crack a dessication crack formed when mud shrinks as it dries mudflow a mass-wasting event involving the flow of mud (sand, silt and clay) within a channel mudrock an inclusive term for mudstone, shale and claystone mudstone a fine-grained clastic sedimentary rock with a mixture of silt-sized and clay-sized particles muscovite a potassium-bearing non-ferromagnesian mica

N

native element (also, native element mineral) a mineral that consists of only one element (e.g., native gold) nebula a large cloud of dust and gas in space, frequently hosting the formation of stars negative feedback a process that results in a decrease in that process (in the context of climate change it is a process that reduces the change in climate, such as the enhanced growth of vegetation in response to an increase in atmospheric carbon dioxide) neutron a sub-atomic particle with a mass of 1 and a charge of 0 nonconformity a geological boundary where non-sedimentary rock is overlain by sedimentary rock non-ferromagnesian mineral a silicate mineral that does not contain iron or magnesium (e.g., feldsspar) non-metallic lustre the lustre of a mineral into which light does penetrate, or which does not produce a bright reflection normal fault a non-vertical fault along which the hanging wall (upper surface) has moved down relative to the footwall normal force the component of the gravitational force that acts directly into the slope North Atlantic Deep Water deep Atlantic Ocean water that has descended in the far north of the basin in the area between Scandinavia and Greenland nunatuk a rocky peak that extends above the ice level of a continental glacier

O

obliquity (Milankovitch cycles) the angle of the tilt of the Earth’s rotational axis with respect to the plane of its orbit around the sun ocean plain the extremely flat surface of the deep ocean floor in areas unaffected by plate tectonic processes and volcanism oil window the depth range, which is approximately 2000 to 4000 m, within which the temperature is appropriate for the formation of oil from organic matter in sedimentary rock ooid a small (approximately 1 mm) sphere of calcite formed in areas of tropical shallow marine water with strong currents olivine a silicate mineral made up of isolated silica tetrahedra and with either iron or magnesium (or both) as the cations Oort cloud a spherical cloud of icy objects extending from between about 5,000 and 500,000 astronomical units (Sun-Earth distances) from the Sun (thought to be a source area of comets) open-pit mine a mine that is open to the surface organic sedimentary rock a sedimentary rock consisting of materials made of carbon-hydrogen bonds (e.g., animal and plant material) outcrop a surface exposure of rock that is part of the crust (bedrock) outwash plain an extensive region of sand and gravel deposited by streams flowing out of a glacier (same as sandur) overturned a geological feature that has been tilted to the point where it is upside down oxbow a part of a stream meander that has become isolated from the rest of the stream as the result of a meander cutoff oxidation the reaction between a mineral and oxygen oxide a mineral in which the anion is oxygen (e.g., hematite Fe₂O₃)

P

pahoehoe a lava flow with a ropy surface texture formed when the surface cools and hardens while the lava beneath is still flowing paleomagnetic characterized by past variations in the intensity and polarity of the Earth’s magnetic field Pangea che supercontinent that existed between approximately 300 and 180 Ma paraconformity an interruption representing a period of non-deposition, without tilting or erosion, in a sequence of sedimentary rocks parasitic fold a fold within a fold parent rock (also, parent material, protolith) the rock that was already in existence when a process of metamorphism started, or the rock from which sediments were derived partial melting the process during which a only specific mineral components of a rock melt parting a narrow gap between individual sedimentary layers passive margin a boundary between a continent and an ocean at which there is no tectonic activity (e.g., the eastern edge of North America) paternoster lake one of a series of rock basin lakes peat a product of the first stage of coal formation, where vegetative material undergoes limited decomposition in a low-oxygen, acidic environment pebble a sedimentary particle ranging in size from 2 to 64 mm (includes granule) pelagic the part of a lake or the ocean that is not close to shore permafrost ground that remains frozen for two or more years permanentism the now discredited theory that the features on the Earth have not changed significantly over geological time permeability an expression of the ease with which liquid will flow through a porous medium phaneritic a rock texture in which the individual crystals or grains are visible to the naked eye Phanerozoic the most resent eon of geological time, encompassing the Paleozoic, Mesozoic and Cenozoic eras phenocryst a relatively large crystal within an igneous rock phyllosilicate a silicate mineral in which the silica tetrahedra are made up of sheets phosphate a mineral in which the anion is PO₄^3-photic zone the upper 200 m of the ocean or a lake, where, depending on the turbidity of the water, light can penetrate phreatic eruption a steam-drive volcanic eruption that takes place when surface or near-surface water is heated by volcanic activity phyllite a metamorphic rock with slaty cleavage and a sheen on the surface produced by aligned micas physical weathering (also, mechanical weathering) weathering that occurs when physical processes cause a rock to break into smaller pieces without changing the chemical composition pillow a pillow-shaped mass of volcanic rock (typically basalt) formed when magma erupts beneath the surface pillow lava a volcanic rock (typically basalt) that is made up primarily of pillows pipe a cylindrical body of igneous rock. May feed a volcano or connect plutons Plate. A fragment of Earth’s surface consisting of lithosphere (crust and uppermost mantle). Plate tectonics. The concept that the Earth’s crust and upper-most mantle (lithosphere) is divided into a number of plates that move independently on the surface and interact with each other at their boundaries Plinian eruption a large volcanic eruption in which a column of hot tephra and gases rises many kilometres into the atmosphere pluton a body of igneous rock formed by cooling within the Earth (i.e., a body of intrusive igneous rock) podzol a soil with well-developed horizons formed in temperate forested regions podzolization the process of the formation of podsol polar wandering path see: apparent polar wandering path polymerize the formation of molecular chains within a fluid (e.g., a magma) that lead to an increase in the fluid’s viscosity polymorphs two or more minerals with the same chemical formula but different crystal structures porosity the percentage of open pore space within a body of rock or sediment porphyritic an igneous texture in which some of the crystals are distinctively larger than the rest porphyry deposit a mineral deposit (of copper or molybdenum especially) in which part of the host rock is a porphyritic stock positive feedback a process that results in an increase in that process (in the context of climate change it is a process that enhances the change in climate, such as the reduced reflectivity of the Earth’s surface when ice melts) potassium feldspar feldspar with the formula KAlSi₃O₈, and which is a common constituent of felsic igneous rocks potentiometric surface the imaginary surface defined by the levels to which water would rise in a series of wells drilled into a confined aquifer precession (Milankovitch cycles) the variation in the direction at which the Earth’s rotational axis is pointing pressure-release cracking cracking of a rock which occurs when overlying rocks are removed by erosion and the outer layer of the rock expands principle of cross-cutting relationships the principle that a body of rock that cuts across or through another body of rock is younger than that other body principle of faunal succession the principle that life on Earth has evolved in an orderly way, and that we can expect to always find fossils of a specific type in rocks of a specific age principle of inclusions the principle that inclusions within a body of rock must be older than the rock principle of original horizontality the principle that sedimentary beds are originally deposited in horizontal layers principle of superposition the principle that in a sequence of layered rocks that is not overturned or interrupted by faulting, the oldest will be at the bottom and the youngest at the top proglacial referring to the area in front of a glacier protolith (also, parent rock) the rock which was altered to produce a metamorphic rock proton a sub-atomic particle with a mass of 1 and a charge of 1 protoplanetary disk a rotating cloud of gas and dust surrounding a young star pumice a highly vesicular (filled with holes left by gas bubbles) felsic volcanic rock (typically composed mostly of glass) p-wave a seismic body wave that is characterized by deformation of the rock in the same direction that the wave is propagating (compressional vibration) pyroclastic volcanic material formed during an explosive eruption pyroclastic density current a body of hot pyroclastic rock and gases that is flowing rapidly down the flank of a volcano pyroxene a single chain silicate mineral

Q

quartz a silicate mineral with the formula SiO₂quartz sandstone (also, quartz arenite) a sandstone in which more than 90% of the grains are quartz quartzite a non-foliated metamorphic rock formed from the contact or regional metamorphism of sandstone

R

radial (drainage) a pattern of streams radiating out from a central point, typically an isolated mountain radioactivity the natural transformation of unstable isotopes into new elements radiolaria microscopic (0.1 to 0.2 mm) marine protozoa that produce silica shells Rayleigh wave a surface seismic wave, with vertical motion recharge the transfer of surface water into the ground to become groundwater recharge area an area of an aquifer where recharge is predominant over discharge recrystallization during metamorphism, mineral crystals dissolving and reforming as larger crystals rectangular drainage a pattern in which tributaries typically flow at right angles to each other and meet at right angles recumbent fold a fold that is overturned such that its limbs are close to horizontal redshift the increase in wavelength of light resulting from the fact that the source of the light is moving away from the observer reef a mound of carbonate formed in shallow tropical marine environments by corals, algae and a wide range of other organisms regional metamorphism metamorphism caused by burial of the parent rock to depths greater than 5 km (typically takes place beneath mountain ranges, and extends over areas of hundreds of km²) remnant magnetism magnetism of a body of rock that formed at the time the rock formed and is consistent with the magnetic field orientation that existed at that time and place (see also paleomagnetism) reservoir rock rock into which petroleum has migrated and is now trapped residual soil soil formed by weathering of the underlying rock or sediment retrograde metamorphism metamorphism that transforms a higher grade metamorphic rock into a lower grade metamorphic rock reverse fault a non-vertical fault along which the hanging wall (upper surface) has moved up relative to the footwall rhyolite a felsic volcanic rock ridge push the concept that at least part of the mechanism of plate motion is the push of oceanic lithosphere down from a ridge area rip current a strong flow of water outward from a beach ripple a series of small parallel ridges formed within sediment that has accumulated in moving water or wind rip-rap angular rock fragments, typically boulder sized, used to armour slopes and shorelines against erosion roche moutonée a product of glaciation in which a bedrock protrusion is eroded into a streamlined shape that has a broken or jagged leading (down-ice) edge rock avalanche a rapid turbulent flow of broken bedrock fragments down a steep slope rock basin lake a lake situated in a rock basin carved at the upper end of an alpine glacier rock cleavage the tendency of a rock to break along planes defined by foliation rock cycle the series of processes through which rocks are transformed from one type to another rock fall the near-vertical fall or bouncing of rock released from a steep slope rock slide the translational motion of an essentially intact body of rock down a slope (rock slides are typically slow, because once they start to move fast the rock body becomes fragmented and then flows as a rock avalanche) root wedging a physical weathering process in which roots grow into cracks in rocks and force them open rounding describes the extent to which clasts have had their edges and corners smoothed off runoff flow of water down a slope, either across the ground surface, or within a series of channels rupture breaking of rock subject to stress, typically resulting in an earthquake rupture surface the area over which rock rupture takes place during an earthquake

S

sackung an escarpment or trough at the top of a slow-moving rock slide (sackungen) saltation the bouncing of particles along a stream bottom or desert floor salt wedging a physical weathering process in which water with dissolved salt flows into a crack, and as the water evaporates, salt crystals grow and push the crack open sand a mineral or rock fragment ranging in size from 1/16th to 2 mm sandstone a rock that is primarily comprised of sand-sized particles sandur an extensive region of sand and gravel deposited by streams flowing out of a glacier (same as outwash plain) saturated zone the part of an aquifer, or any body of rock, that is saturated with water schist a foliated metamorphic rock with crystals large enough to be visible to the unaided eye Scientific method. A way to collect scientific facts in as reliable a way as possible. A hypothesis is formulated and tested. Whether the hypothesis passes or fails the test will determine whether it is kept or discarded. sea cave a shallow cave formed on a rocky shore by wave erosion sea cliff a coastal escarpment that is typically eroding inland as a result of wave action sea-floor spreading the formation of new oceanic crust by volcanism at a divergent plate boundary sector collapse the sudden collapse of a significant part of the flank of a volcano sedimentary rock rock that has formed by the lithification of sediments or by the precipitation of ions from water sediments unconsolidated (loose) particles of mineral or rock seismic pertaining to earthquakes seismic moment a measurement of an earthquake’s energy based on longwave vibrations, or on the product of the fault area and displacement seismic reflection sounding measurement of the properties of sediments based on detection of sounds generated at surface and reflected from layers beneath the surface septae calcareous partitions between the successive living chambers in a cephalopod septic system a system constructed to facilitate the dispersion and detoxification of sewage (typically includes a septic tank and a drainage field) shaft a vertical opening at a mine shale a silt- and clay-rich rock that has evidence of layering shatter cone conical nested fractures that result from extraterrestrial impacts. Cones point toward the impact. shear force the component of the gravitational force in the direction parallel to a slope shear strength the strength of a body of rock or sediment that counteracts the shear force shear stress the stress placed on a body of rock or sediment adjacent to a fault sheeted dikes a series of near-vertical dykes formed in the vicinity of a spreading ridge when magma from depth flows into fractures formed by extensional forces sheet silicate a silicate mineral in which the silica tetrahedra are combined within sheets sheetwash overland flow of water, typically related to a heavy precipitation event shield a region of ancient (typically Precambrian) crystalline rock (equivalent to a craton) shield volcano a low-profile volcano formed primarily from eruptions of low-viscosity mafic magma shocked quartz quartz crystals in which the structure has been deformed by sudden, intense pressure. Deformation is visible as parallel lines within the crystal. with damage along parallel plains Sial (sialic) an outdated term referring to rock or magma in which silica and aluminum are the predominant components (generally equivalent to felsic) silica a form of the mineral quartz (SiO₂) silica tetrahedron an ion which is a combination of 1 silicon atom and 4 oxygen atoms that form a tetrahedron shape (SiO₄^4-) silicate a mineral that includes silica tetrahedra silicon the 14th element silicone resin or caulking made from silicon-oxygen chains and various organic molecules sill a tabular igneous intrusion (pluton) that is parallel to existing layering in the country rock silt sedimentary particles ranging is size from 1/256th to 1/16th of a mm siltstone a clastic sedimentary rocks consisting predominately of silt-sized particles Sima (simatic) an outdated term referring to rock or magma in which silica, magnesium and iron are the predominant components (generally equivalent to mafic) skarn the contact metamorphism (and metasomatism) of limestone slab pull the concept that at least part of the mechanism of plate motion is the pull of oceanic lithosphere down into the mantle slate a fine-grained metamorphic rock that splits easily into sheets slaty cleavage the tendency for slate or phyllite to split into sheets (note that this is the only situation in this textbook where the term “cleavage” is applied to a rock as opposed to a mineral) slide the downward movement of rock or sediment on a slope as an intact mass slump a slide in which the nature of the motion is rotational (typically only develops in unconsolidated sediments) smectite a fine-grained sheet silicate mineral that can accept water molecules into interlayer spaces, resulting is swelling smelter a refinery at which minerals are processed to produce pure metals snow line (frost line) in the context of newly forming planetary systems, the distance beyond a star at which volatile components (e.g., water, carbon dioxide, methane, ammonia etc.) are frozen soil horizon a layer, within a well-developed soil, that is physically or chemically different from layers above or below solar system a star and the planets surrounding it. Sometimes used specifically for the sun and its planets, and planetary system used for other stars solar wind a stream of ionized (charged) particles away from the sun solid solution the substitution of one element for another in a mineral (e.g., in Bowen's reaction series there exists a continuum of plagioclase feldspar where calcium becomes progressively less common, and sodium more so) solifluction the flow of water saturated sediment or soil over a stronger and less permeable substrate sorting the extent to which the grain size within a sample of sediment is similar. Well-sorted sediments have very similar grain sizes, and poorly-sorted sediments have a variety of grain sizes. source rock the sedimentary rock from which petroleum originates prior to its migration into a reservoir rock speleothem a cave structure formed when calcium carbonate precipitates (see also stalactite, stalagmite) sphericity the extent to which a grain is the same diameter in all dimensions (e.g., more like a sphere, but without implying roundness or smoothness) spit a sand or coarser deposit extending from shore out into open water spring a flow of groundwater onto the surface stack a prominent rocky island that is a remnant of the erosion of a headland stage the level of water in a stream stalactite a cone-shaped speleothem that is suspended from the roof of a cave stalagmite a cone-shaped speleothem that forms on the floor of a cave step-pool a characteristic of stream flow in which water flows from one pool to another, typically on a stream with a steep gradient stock an irregular pluton with n exposed area less than 100 km²stoping the fracturing and incorporation of fragments of country rock as a magma body moves upward through the crust strain the deformation of rock that is subjected to stress streak the mark left on a porcelain plate when a mineral sample is ground to a powder by being rubbed across the plate (typically provides a more reliable depiction of the colour than the whole sample) stream any body of flowing water stress a force applied to a rock (specifically, the force per unit area) stress transfer the change in the pattern of stress on a region of rock as a result of an earthquake (typically stress is reduced in the area of a rupture zone, but is increased elsewhere in the vicinity) strike the compass direction of a horizontal line on a sloped surface (e.g., bedding plane, fracture etc.) strike-slip fault a fault that is characterized by motion that is close to horizontal and parallel to the strike direction of the fault subaerial eruption a volcanic eruption that takes place on land subaqueous eruption a volcanic eruption that takes place under water subducted when part of a plate is forced beneath another plate along a subduction zone subduction zone the sloping region along which a tectonic plate descends into the mantle beneath another plate subglacial beneath a glacier sulphate a mineral in which the anion is SO₄^2-sulphide a mineral in which the anion is S^2-supergroup a stratigraphically continuous series of related groups superterrane a number of terranes that are contiguous supraglacial on the surface of a glacier surf zone the near-shore zone where waves are breaking into surf suture the line on the surface of a cephalopod that marks the boundary between a septum and the outer shell swash the upward motion of a wave on a beach (typically takes place at the same angle that the waves are approaching the shore) s-wave a seismic body wave that is characterized by deformation of the rock transverse to the direction that the wave is propagating symmetrical a fold in which the limbs are at the same angle to the hinge syncline a downward fold where the beds are known not to be overturned synform a downward fold where it is not known if the beds are overturned

T

tabular referring to a structure that is sheet-like (or like a table top). See also dike, sill tailings the fine-grained waste rock from a plant used to concentrate ore minerals talus slope a sloped deposit of angular rock fragments at the base of a rocky escarpment tarn a lake within a rock basin tectonic plate a fragment of the lithosphere that moves across the surface of the Earth as a single unit tectonic sea level change relative sea level change related to the vertical motion of a crustal block caused by tectonic processes tephra fragments of volcanic rock (including volcanic ash) ejected during an explosive eruption terminal moraine and end moraine that marks the farthest forward advance of a glacier terrane a block of crust that has geological features which are distinctive from neighbouring regions, and is assumed to have been moved from elsewhere by tectonic processes terrestrial planet a planet with a rocky mantle and crust, and metallic core (e.g., Earth) terrigenous referring to sedimentary particles that originated on a continent test the shell-like hard parts (either silica or carbonate) of small organisms such as radiolarian and foraminifera Theory. A hypothesis that has passed repeated and rigorous testing. thrust fault a low angle reverse fault till unsorted sediment transported and deposited by glacial ice tiltmeter a sensitive instrument used to monitor subtle changes in the tilt of the land, particularly in studies of active volcanoes tombolo a sand or coarser deposit connecting an island or rocky prominence to a larger body of land traction a force that contributes to the movement of particles situated on a stream bed or desert floor transform fault a boundary between two plates that are moving horizontally with respect to each other transportation refers to moving sediments from one location to another transported soils soils which form on sediments that have been moved from their original location. The soils themselves have not been transported. travertine a deposit of calcium carbonate that forms at springs, hot springs or within limestone caves trellis a drainage pattern in which tributaries typically flow parallel to one other but meet at right angles trigger an event, such as an earthquake or a heavy rainfall, that starts a mass wasting event trough the lowest point of a wave truncated spur the steep end of a ridge or arête that has been eroded by a main-valley glacier tsunami a long-wavelength wave produced by the vertical motion of the floor of the ocean or a large lake, typically related either to an earthquake or a sub-marine mass wasting event tufa a form of travertine that is especially porous as it forms around existing vegetative material. tuya a flat-topped volcanic hill or mountain that formed when an eruption took place beneath a glacier and the melting led to the formation of a lake that then resulted in the wave-erosion of the top of the volcano

U

unconfined aquifer an aquifer that is not overlain by a confining layer unconformity a geological boundary at the base of a sedimentary layer unconformity-type uranium deposit a uranium deposit that has formed at a nonconformity between sandstone and older rock uncompressed density the density of planetary material that it would have it was not compressed by the planets gravitational force underground storage tank (UST) an underground tank for storing liquids, most commonly for liquid fuel Uniformitarianism. The idea that geological processes today can be used to understand geological processes in the past. unsaturated zone the rock or sediment above the water table U-shaped valley a relatively straight valley with a flat bottom and steep sides that has been carved by a valley glacier

V

valley glacier a glacier formed in a mountainous region and confined to a valley (same as alpine glacier) varve a recognizable layer within sediments that represents a single year of deposition vesicular an igneous texture characterized by holes left by gas bubbles volcanic glass lava that has cooled within minutes, not allowing time for the formation of crystals volcanic-hosted massive sulphide a mineral deposit hosted by volcanic rocks and including zones where most of the rock is made up of sulphide minerals (including ore minerals and pyrite)

W

wacke a sandstone with more than 15% clay and silt water table the upper surface of the saturated zone in an unconfined aquifer wave base the depth of water that is affected by the sub-surface orbital motion of wave action (approximately one-half of the wavelength) wave-cut platform a nearly-horizontal bench of rock eroded by waves within the surf zone (equivalent to wave-cut terrace) wavelength the distance between the crests of two waves weathering a range of processes taking place in the surface environment, through which solid rock is transformed into sediment and ions in solution wedging physical (mechanical) weathering processes which involve forcing open cracks in a rock (see also frost wedging, root wedging, salt wedging) Western Canada Sedimentary Basin a large basin in the western interior of Canada, east of the Rocky Mountains, extending from the northern United States to the Northwest Territories Wisconsin Glaciation the most recent advance of the Pleistocene glaciations, extending from 85 to 11 ka

X

xenolith (zee-know-lith) A fragment of country rock incorporated into igneous rock, commonly as a result of stoping

Y

youthful stream a stream that is actively down-cutting its valley in an area that has recently been uplifted

Z

zone of ablation the part of a glacier, below the equilibrium line, where there is net loss of ice mass due to melting and calving zone of accumulation the part of a glacier, above the equilibrium line, where there is net gain of ice mass because not all of the snow that falls each winter is able to melt during the following summer

Appendix A. List of Geologically Important Elements and the Periodic Table

kqzheng — Sun, 27 Aug 2017 23:44:22 +0000

The following table includes 36 of the geologically important elements, listed alphabetically by their element name, along with their atomic number and the atomic mass of their most stable isotope. The geologically most important elements are bolded, and the eight main elements of silicate minerals are identified with an asterisk (*).

Symbol	Name	Atomic No.	Atomic Mass
Al*	Aluminum	13	27
As	Arsenic	33	75
Ba	Barium	56	137
Be	Beryllium	4	9
B	Boron	5	11
Cd	Cadmium	48	112
Ca*	Calcium	20	40
C	Carbon	6	12
Cl	Chlorine	17	35
Cr	Chromium	24	52
Co	Cobalt	27	59
Cu	Copper	29	64
F	Flourine	9	19
Au	Gold	79	197
He	Helium	2	4
H	Hydrogen	1	1
Fe*	Iron	26	56
Pb	Lead	82	207
Mg*	Magnesium	12	24
Mn	Manganese	25	55
Mo	Molybdenum	42	96
Ne	Neon	10	20
Ni	Nickel	28	59
N	Nitrogren	7	14
O*	Oxygen	8	16
P	Phosphorus	15	31
Pt	Platinum	78	195
K*	Potassium	19	39
Si*	Silicon	14	28
Ag	Silver	47	108
Na*	Sodium	11	23
Sr	Strontium	38	88
S	Sulfur	16	32
Ti	Titanium	22	48
U	Uranium	92	238
Zn	Zinc	30	65

The periodic table is a list of all of the elements arranged in groups according to their atomic configuration. In this table the elements are colour-coded according to their chemical and physical properties. For an accessible version of the periodic table please see Syngenta Period Table of Elements.

Versioning History

kqzheng — Fri, 20 Aug 2021 20:10:34 +0000

This page provides a record of edits and changes made to this book since its initial publication. Whenever edits or updates are made in the text, we provide a record and description of those changes here. If the change is minor, the version number increases by 0.01. If the edits involve substantial updates, the version number increases to the next full number. The files posted by this book always reflect the most recent version. If you find an error in this book, please fill out the Report an Error form.

Version	Date	Change	Details
1.01	September 1, 2015	Original book published.
2.01	December, 2019	First University of Saskatchewan Edition published.	Book was adapted by Karla Panchuk. Chapters reordered; Expanded content on topics like Earth-system change, glaciation, and mass wasting; Updated coverage of recent events and research; and Inserted additional images to support written content. This book includes approximately 580 figures, 40% of which were modified, added, or created as original works for this edition.
3.01	August 20, 2021	Book republished by BCcampus as Physical Geology – H5P Edition	Over 200 interactive H5P activities were added.
3.02	July 4, 2023	Improved offline access to interactive content.	Static versions of many H5P activities were added to the PDF and EPUB versions of the book for offline access.
3.03	February 6, 2026	Replaced incorrect image.	In section 5.3 Mineral Groups, inserted the correct image under the heading "Oxide Minerals: O²⁻ Anion."

Acknowledgements

jgray — Tue, 28 Mar 2023 18:36:50 +0000

Physical Geology, H5P Edition is not just one book, not just one project, and not just one person. This edition was supported by a BCcampus H5P OER Development Grant. It benefited immensely from technical support, insights, and encouragement by Alan Levine and Clint Lalonde of BCcampus. The blame for getting me started with H5P and making me aware of its potential falls squarely on the shoulders of JR Dingwall, instructional designer and edtech guru at the University of Saskatchewan. Modifications to the textbook were also supported by UBC Okanagan's Department of Earth, Environmental, and Geographic Sciences as part of an on-going project to further adapt the book. Tireless and extensive logistical and administrative support were provided by Pearl Glute and Jeff Myhre. H5P Edition is an adaptation of the First University of Saskatchewan Edition of this textbook. My work on that edition was supported by a grant from the University of Saskatchewan Open Educational Resources Fund to myself, Joyce McBeth, and Tim Prokopiuk of the Department of Geological Sciences at the University of Saskatchewan. Joyce adapted Chapters 14, 15, and 17, and contributed invaluable edits and feedback. Tim contributed edits and selected rock samples for me to photograph from the department’s collection. Lyndsay Hauber assisted with updates to image attributions for the chapter on plate tectonics. Donna Beneteau and Doug Milne of the College of Engineering, and Zoli Hajnal of Geological Sciences gave me a tour of the Geological Engineering Rock Mechanics Facility, and helped me to photograph their experiments. Heather Ross and Nancy Turner at the Gwenna Moss Centre for Teaching and Learning were instrumental through their support and encouragement on this project and through discussions with them about open textbooks and OER. The First University of Saskatchewan Edition owes its existence to the original version of Physical Geology by Steven Earle, written for the BCcampus Open Textbook project. It has been a comprehensive and solid foundation upon which to build adapted works.

Image Sources

This project would not be possible without the generosity of many individuals and organizations who shared their work with a Creative Commons license or under other open licensing terms. The following is a list of valuable image resources, as much as it is an acknowledgement of contributions: Roger Weller has made available thousands of his high-quality rock and mineral photographs through his website hosted by Cochise College, and granted permission for their non-commercial educational use. His photos have been used extensively throughout this project. Roger's usage stipulation has led to thoughtful discussions about what the appropriate way is to license derivative materials that make use of non Creative-Commons content. I have concluded that the best way to ensure that his wishes are respected is to license materials I make with his photographs as CC BY-NC-SA. This permits free sharing and remixing, but stipulates no commercial use, and that all derivative works must be shared with a non-commercial license. James St. John is a geologist and paleontologist who has contributed (as of August 2021) 80,190 high-quality geology-related photographs to the photo-sharing website Flickr. His photographs cover a wide range of rocks and minerals, and rarely has there been an image that I needed but couldn't find in his work. His Flickr account is remarkable for the abundance and quality of photographs, but also because he includes detailed descriptions of his images, making it possible for me to verify that an image is what I think it is, and gather useful background information. He has shared his images with a CC BY license, which I appreciate greatly because it allows me to combine them with content having more restrictive licenses. You can find his teaching website here, and video footage from his Yellowstone geyser project here. The U. S. Geological Survey has contributed innumerable images to the public domain. The Hawaiian Volcano Observatory in particular is my go-to source for both the latest in volcano photos, and for fascinating historical images. Data and images from the USGS Earthquake Hazards Program Latest Earthquakes map have been invaluable. I have used NASA images for views of Earth as much as I have for views of space and other planets. It is truly remarkable that in spite of the vast resources and expertise needed to acquire these photographs, they are free to view, use, and learn from. Among the many teaching resources offered by IRIS (Incorporated Research Institutions for Seismology) are beautifully designed images for explaining earthquakes and seismology. When all other sources failed, the odds were good that Robert Lavinsky (www.iRocks.com), Mike Norton, or Michael Rygel had contributed exactly the right photograph to Wikimedia Commons.

Chapter 1 H5P Offline Copies

kqzheng — Wed, 26 Apr 2023 21:50:03 +0000

1.2 Why Study Earth?

Spotting Everyday Hazards

1.3 What Do Geologists Do?

Living with a Volcano

Planning and organizing humanitarian relief
Communicating with the public
Assessing the risk of an eruption
Planning an evacuation strategy

1.4 We Study Earth Using the Scientific Method

Theory, Hypothesis, or Law?

Complete this summary of theory, hypothesis, and law by putting the words (Hypothetically, In theory, By law) into the correct blank.

“ , this should work” is how people sometimes express uncertainty about whether they will be successful or not, but this isn't the correct terminology.
If they mean that they're trying out an untested idea, it would be more accurate to say, “ , this should work.”
If they're worried about whether reality will match up with predictions that are based on mathematical descriptions of physical phenomena, what they really mean is, “ , this should work.”

1.5 Three Big Ideas: Geological Time, Uniformitarianism, and Plate Tectonics

How Many Years Is That?

Can you write each of these in units of years? Use commas to break up the numbers (e.g., 4,000,000).

2.75 ka = years. (Hint: Multiply by what "k" stands for. This is the same as moving decimal 3 places to the right.)
0.93 Ga = years. (Hint: Multiply by what "G" stands for. This is the same as moving the decimal 9 places to the right.)
14.2 Ma = years. (Hint: Multiply by what "M" stands for. This is the same as moving the decimal 6 places to the right.)

Uniformitarianism: Check Your Understanding

Because "the present is the key to the past" uniformitarianism is only backward-looking, and thus only good for understanding Earth in the past.

True
False

Plate Tectonics: Check Your Understanding

Are any of the other "big ideas" involved in plate tectonic theory? Choose any that apply.

Yes, deep time is required.
Yes, uniformitarianism applies.
No, plate tectonic theory works all by itself.

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Chapter 1 Summary & Key Term Check

kqzheng — Sun, 27 Aug 2017 20:05:30 +0000

Chapter 1 Main Ideas

1.1 What is Geology?

Geology is the study of Earth. It is an integrated science that involves the application of many of the other sciences. Geologists must take into account the fact that the geological features we see today may have formed thousands, millions, or even billions of years ago, and over very long time spans.

1.2 Why Study Earth?

Geologists study Earth out of curiosity and for other, more practical reasons, including understanding the evolution of life on Earth; searching for resources; understanding risks from geological events such as earthquakes, volcanoes, and slope failures; and documenting past environmental and climate changes so that we can understand how human activities are affecting Earth.

1.3 What Do Geologists Do?

Geologists work in the resource industry, and in efforts to protect the environment. Geologists work to minimize the risks from geological hazards (e.g., earthquakes), and to help the public understand those risks. Geologists investigate Earth materials in the field, in and in the lab.

1.4 We Study Earth Using the Scientific Method

Scientific inquiry requires a careful process of making a hypothesis and then testing it. If a hypothesis doesn't pass the test, it's time for a new one. A theory is a hypothesis that has been tested repeatedly and never failed a test. A law is a description of a natural process.

1.5 Three Big Ideas: Geological Time, Uniformitarianism, and Plate Tectonics

Geological time: Earth is approximately 4,600,000,000 years old; that is, 4.6 billion years or 4.6 Ga or 4,600 Ma. It’s such a huge amount of time that even extremely slow geological processes can have an enormous impact. Uniformitarianism: Processes that occur today also occurred in the geologic past. We can use our observations of the present to understand the processes that shaped Earth throughout its history. Plate tectonics: Earth's surface is broken into plates that move and interact with each other. The interactions between these plates are key for understanding the mechanisms behind geologic processes.

Key Term Check

What key term from Chapter 1 is each card describing? Turn the card to check your answer. [h5p id="22"]

6.1 What Is A Rock?

kqzheng — Sun, 01 Oct 2017 21:18:37 +0000

A rock is a solid mass of geological materials. Geological materials include individual mineral crystals, inorganic non-mineral solids like glass, pieces broken from other rocks, and even fossils. The geological materials in rocks may be inorganic, but they can also include organic materials such as the partially decomposed plant matter preserved in coal. A rock can be composed of only one type of geological material or mineral, but many are composed of several types (Figure 6.2). [caption id="attachment_210" align="aligncenter" width="650"] Figure 6.2 Rocks versus minerals. Rocks in the image are made up of crystals of one or more minerals. Source: Karla Panchuk (2021), CC BY-NC-SA. Photographs by R. Weller/ Cochise College and James St. John. Click for attributions.[/caption]

Concept Check: Rock or Mineral?

Whether we refer to something as a rock or mineral may depend on context. Here is a story to illustrate. Fill in each blank space with either rock or mineral. Emily went for a walk and found two interesting geological specimens. One was white and the other was multi-coloured. After investigating, she learned that the white was quartzite, and made entirely of crystals of the quartz. Even though it was quartzite, she added it to her collection as an example of quartz because it had the same physical properties. Emily learned that the multi-coloured was granite. Granite contains quartz crystals, but it also contains crystals of the potassium feldspar and the biotite. One day Emily’s friend Liz came for a visit because she heard that Emily could show her what quartz looked like. Unfortunately, Emily had put her specimen collection with quartz samples in a safe place, so try as she might, she couldn't find it. Instead, she showed Liz the granite and pointed out which crystals within the granite were the quartz. To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="65"]

Three Main Types of Rock

Rocks are grouped into three main categories based on how they form. Igneous rocks form when melted rock cools and solidifies. Sedimentary rocks form when fragments of other rocks are buried, compressed, and cemented together; or when minerals precipitate from solution, either directly or with the help of an organism. Metamorphic rocks form when heat and pressure alter a pre-existing rock. Although temperatures can be very high, metamorphism does not involve melting of the rock.

Do You Know Your Rock Types? [h5p id="66"]

6.2 The Rock Cycle

kqzheng — Sun, 01 Oct 2017 21:22:51 +0000

The rock components of the crust are slowly but constantly being changed from one form to another. The processes involved are summarized in the rock cycle (Figure 6.3). The rock cycle is driven by two forces:

Earth’s internal heat, which causes material to move around in the core and mantle, driving plate tectonics.
The hydrological cycle-movement of water, ice, and air at the surface. The hydrological cycle is powered by the sun.

[caption id="attachment_213" align="aligncenter" width="864"] Figure 6.3 The rock cycle describes processes that form the three types of rock: igneous, sedimentary, and metamorphic. These same processes can turn one type of rock into another. Source: Karla Panchuk (2017) CC BY-SA 4.0. Click for more attributions.[/caption] The rock cycle is still active on Earth because our core is hot enough to keep the mantle moving, the atmosphere is relatively thick, and there is liquid water. On some other planets or their satellites (e.g., Mercury), the rock cycle is virtually dead because the core is no longer hot enough to drive mantle convection, and there is no atmosphere or liquid water.

Common Misconception Warning! The rock cycle is not like the life cycle of an organism where a rock must pass through all of the processes or stages, and in a particular order. The rock cycle is more like a choose-your-own-adventure. A rock's history can branch off along any pathway, or just stop altogether at a particular point.

We can start anywhere we like to describe the rock cycle, but it’s convenient to start with magma. Magma is melted rock located within the Earth. Rock can melt at between about 800 °C and 1300 °C, depending on the minerals in the rock, and the pressure the rock is under. If it cools slowly within the Earth (over centuries to millions of years), magma forms intrusive igneous rocks. If magma erupts onto the surface, we refer to it as lava. Lava cools rapidly on Earth's surface (within seconds to years) and forms extrusive igneous rocks (Figure 6.4).[footnote]Remember the difference between intrusive and extrusive igneous rocks by recalling that INtrusive rocks form withIN the Earth, and EXtrusive rocks form when lava EXits the Earth's crust.[/footnote] [caption id="attachment_214" align="aligncenter" width="500"] Figure 6.4 Lava flowing from Kīlauea Volcano, Hawai`i. Source: J. D. Griggs, U. S. Geological Survey (1985), Public Domain. Image source.[/caption] Mountain building lifts rocks upward where they are acted upon by weathering. Weathering includes chemical processes that break rocks apart, as well as physical processes. Figure 6.5 shows the result of rocks in mountains being broken apart when water gets into cracks, freezes, and forces the cracks wider. Uplift through mountain building is how rocks once buried deep within Earth can be exposed at Earth's surface. [caption id="attachment_215" align="aligncenter" width="500"] Figure 6.5 Mountains being broken apart by the wedging action of ice near La Madaleta Glacier, Spain. Source: Luis Paquito (2006), CC BY-SA 2.0. Image source.[/caption] The weathering products—mostly small rock and mineral fragments—are eroded, transported, and then deposited as sediments. Transportation and deposition occur through the action of glaciers, streams, waves, wind, and other agents. Figure 6.6 shows transportation of fine-grained sediment particles by wind during the Great Depression in the 1930s. [caption id="attachment_216" align="aligncenter" width="500"] Figure 6.6 Wind transports sediment in a dust storm near Okotoks, Alberta, Canada in July of 1933. Source: Glenbow Museum Archives, File Number NA-2199-1 (1933), Public Domain. Image source.[/caption] Sediments are deposited in stream channels, lakes, deserts, and the ocean. Some depositional settings result in characteristic sedimentary structures, such as the ripples that formed when flowing water moved sand along the bottom of the South Saskatchewan River (Figure 6.7). [caption id="attachment_217" align="aligncenter" width="500"] Figure 6.7 Sand ripples along the South Saskatchewan River, near Saskatoon SK. Ruby for scale. Source: Karla Panchuk (2008), CC BY-SA 4.0. Image source.[/caption] Unless sediments are re-eroded and moved along, they'll eventually be buried by more sediments. At depths of hundreds of metres or more, sediments become compressed, forcing particles closer together. Mineral crystals grow around and between the particles, binding them together (cementing them). The hardened cemented sediments are sedimentary rock. Figure 6.8 shows an example of an ancient sedimentary rock in which ripple structures are preserved, and visible in cross-section as wavy lines. [caption id="attachment_218" align="aligncenter" width="500"] Figure 6.8 Ripples preserved in 1.2 Ga old sandstone. Notice the wavy lines above the coin. This is a side view of the ripples. Source: Anne Burgess (2008), CC BY-SA 2.0. Image source.[/caption] Rocks that are buried very deeply within the crust can reach pressures and temperatures much higher than those at which sedimentary rocks form. Existing rocks that are heated up and squeezed under those extreme conditions are transformed into metamorphic rocks (Figure 6.9). The transformation to a metamorphic rock can happen through physical changes, such as when the minerals making up an existing rock re-form into larger crystals of the same mineral. It can also happen through chemical changes, when minerals within the rock react to form new minerals. [caption id="attachment_219" align="aligncenter" width="500"] Figure 6.9 Limestone, a sedimentary rock formed in marine waters, has been altered by metamorphism into this marble visible on Quadra Island, BC. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Practice with the Rock Cycle [h5p id="67"]

Chapter 6 Summary & Key Term Check

kqzheng — Sun, 01 Oct 2017 21:33:36 +0000

Chapter 6 Main Ideas

6.1 What Is a Rock?

A rock is a solid mass of geological materials. Geological materials include individual mineral crystals, inorganic non-mineral solids like glass, pieces broken from other rocks, and even fossils.

6.2 The Rock Cycle

There are three main types of rock. Igneous rocks form when melted rock cools and solidifies. Sedimentary rock forms from fragments of other rocks, or when crystals precipitate from solution. Metamorphic rocks form when existing rocks are altered by heat, pressure, and/or chemical reactions. The rock cycle summarizes the processes that contribute to transformation of rock from one type to another. The rock cycle is driven by Earth’s internal heat, and by processes happening at the surface that are driven by solar energy.

Key Term Check

What key term from Chapter 6 is each card describing? Turn the card to check your answer. [h5p id="68"]

11.1 What Is A Volcano?

kqzheng — Wed, 20 Sep 2017 00:56:31 +0000

Volcanoes Are Where Magma Emerges

A volcano is a location where molten rock flows out, or erupts, onto Earth’s surface as lava. Volcanic eruptions can happen on land or underwater. Some volcanic eruptions flow from mountains (such as Mount Garibaldi in Figure 11.1), but others do not. Fissure eruptions (Figure 11.3) are volcanic eruptions flowing from long cracks in the Earth. [caption id="attachment_404" align="aligncenter" width="720"] Figure 11.3 Kamoamoa fissure eruption on the flanks of the Hawai'ian Kīlauea Volcano in March of 2011. Source: Karla Panchuk (2017) CC BY-SA 4.0. Photograph: U. S. Geological Survey (2011) Public Domain view source. Click for more attributions.[/caption]

Volcano Anatomy

The main parts of a volcano are shown in Figure 11.4. When volcanoes erupt, magma moves upward from a magma chamber and into a vent or conduit. It flows out from a crater at the top, or sometimes emerges at a secondary site on the side of the volcano resulting in a flank eruption. Erupted materials accumulate around the vent forming a volcanic mountain. The accumulated material might consist of layers of solidified lava, called lava flows, but it might also include fragments of various sizes that have been thrown from the volcano. [caption id="attachment_405" align="aligncenter" width="1527"] Figure 11.4 The parts of a volcano (not to scale). Source: Karla Panchuk (2017), CC BY 4.0.[/caption]

Crater or Caldera?

A crater is the basin above a volcano's vent. Craters have diameters on the scale of 10s to 100s of metres. A caldera is a bowl-shaped structure that resembles a crater, but it's much larger (km in scale) and forms when a volcano collapses in on itself. The process is illustrated in Figure 11.5, going from left to right. It begins when an eruption occurs, and the magma chamber beneath the volcano is drained. If a significant part of a volcano's mass is supported by magma within the chamber, then depleting the magma also reduces the support for the volcano. [caption id="attachment_406" align="aligncenter" width="1770"] Figure 11.5 Formation of a caldera. Calderas are the result of a volcano collapsing into a drained magma chamber. Source: Karla Panchuk (2017), CC BY 4.0. Modified after U. S. Geological Survey (2002), Public Domain. Image source.[/caption] The loss of support causes part of the volcano to collapse into the void in the magma chamber, leaving behind a broad basin rimmed by the remnants of the volcano. Over time, the basin can fill with water. If there is still activity within the magma chamber, magma may force its way upward again, causing the floor of the caldera to be lifted, or erupting to form a new volcano within the caldera. The island of Santorini (Figure 11.6) is an example of a caldera. The island itself is the rim of the caldera, and the bay is the flooded basin. The two small islands in the middle of the bay formed from magma refilling the chamber that feeds the volcano, as in the far right of Figure 11.5. The caldera formed after an enormous eruption between 1627 and 1600 BCE[footnote]Friedrich, W. L., Kromer, B., Friedrich, M., Heinemeier, J., Pfeiffer, T., & Talamo, S. (2006). Santorini Eruption Radiocarbon Dated 1627-1600 B.C. Science (312)5773, 548. doi: 10.1126/science.1125087[/footnote]. The eruption is thought to have contributed to the downfall of the Minoan civilization, and some speculate that it might also be the source of the myth of Atlantis, a story about a lost continent that sank beneath the sea after a natural disaster. [caption id="attachment_407" align="aligncenter" width="1080"] Figure 11.6 The Greek Island of Santorini. Left: Aerial view of the island forming a ring around a flooded caldera. Right: A view from the rim of the caldera. The other side of the rim is visible in the distance. Source: Karla Panchuk (2017), CC BY-SA 4.0. Click for more attributions.[/caption]

Practice with Volcano Terminology [h5p id="128"]

11.2 Materials Produced by Volcanic Eruptions

kqzheng — Wed, 20 Sep 2017 00:56:33 +0000

Volcanic eruptions produce three types of materials: gas, lava, and fragmented debris called tephra.

Volcanic Gas

Magma contains gas. Most of the gas emitted by volcanoes is water, but gas also contains carbon dioxide (CO₂), sulphur dioxide (SO₂), and hydrogen sulphide (H₂S), in order of decreasing abundance. .At high pressures, the gases are dissolved in the magma, but if the pressure decreases, the gas comes out of solution, forming bubbles, similar to what happens when you open a bottle of pop. Pop is bottled under pressure, forcing carbon dioxide gas to dissolve into the fluid. As long as the bottle is closed and the pressure remains high, you'll see few to no bubbles in the pop. But if you open the bottle, air rushes out, decreasing the pressure on the pop. The pop will begin to fizz as carbon dioxide gas comes out of solution and forms bubbles. Volcanoes release gases when erupt, and through openings called fumaroles (Figure 11.7). They can also release gas into soil and groundwater. [caption id="attachment_410" align="aligncenter" width="500"] Figure 11.7 A fumarole at Puʻu ʻŌʻō Crater, Hawaii. The yellow crust along the margin of the fumarole is made of sulphur crystals. The crystals form when sulphur vapour cools as it is released from the fumarole. Source: U. S. Geological Survey (2016), Public Domain. Image source.
[/caption]

Lava

The ease with which lava flows, and the structures it forms depend on how much silica and gas are in the lava. The more silica, the more polymerization (formation of long molecules) happens, stiffening the lava. The stiffness of lava is described in terms of viscosity: lava that flows easily has low viscosity, and lava that is sticky and stiff has high viscosity. In general, higher-silica (felsic) lava contains more gas than low-silica lava. When the gas forms into bubbles, viscosity increases further. Consider the pop analogy again: If you shook the bottle vigorously, then opened it, the pop would gush out in a thick, frothy flow. In contrast, if you took care to not shake the bottle before opening it, you could pour out a thin stream of fluid. The presence of gas not only makes high-silica lava more viscous, but can affect mafic lavas in that way as well.

Concept Check: Viscosity

Write the words into the correct boxes. If a liquid flows easily, it has viscosity. If it's sticky and sluggish, it has a viscosity. For example, consider molasses, water, and cooling oil. If you arranged them from lowest viscosity to highest viscosity, the order would be , then , then . Fill-in-the-blank options:

low
molasses
high
cooking oil
water

To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="201"]

Chemical Composition Affects the Thickness and Shape of Lava Flows

A Quick Review Before You Continue

Write the words into the correct boxes.The terms "mafic," "intermediate," and "felsic" refer to the composition of igneous materials. There is less silica and more magnesium and iron in materials. There is more silica, sodium, and aluminum in materials. Intermediate materials fall in the range between mafic and felsic materials. When talking about lavas, terms related to composition are used interchangeably with the terms related to volcanic rock type. For example, a mafic lava can also be called a lava. Intermediate lavas are sometimes called , and felsic lavas are called . Fill-in-the-blank options:

mafic
felsic
basaltic
rhyolitic
andesitic

To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="129"]

The thickness and shape of a lava flow depends on its viscosity. The greater the viscosity, the thicker the flow, and the shorter the distance it can go before solidifying. Highly viscous lava might not flow very far at all, and simply accumulate as a bulge, called a lava dome, in a volcano’s crater. Figure 11.8 shows a dome formed from rhyolitic lava in the crater of Mt. St. Helens. [caption id="attachment_411" align="aligncenter" width="500"] Figure 11.8 Lava dome in the crater of Mt. St. Helens. Source: Terry Feuerborn (2011), CC BY-NC 2.0. Image source.[/caption] Less viscous andesitic lava can travel further, as with the thick flow in Figure 11.9 (right). The left of Figure 11.9 shows thin streams of freely-flowing, low-silica, low-viscosity basaltic lava. [caption id="attachment_412" align="aligncenter" width="650"] Figure 11.9 Lava flows. Left: A geologist collects a sample from a basaltic lava flow in Hawaii. Right: an andesitic lava flow from Kanaga Volcano in the Aleutian Islands. Source: Left- U. S. Geological Survey (2014), Public Domain. Image source.; Right- Michelle Combs, U. S. Geological Survey (2015), Public Domain. Image source.[/caption] Low-viscosity basaltic lava flows may travel extended distances if they move through conduits called lava tubes. These are tunnels within older solidified lava flows. Figure 11.10 (top) shows a view into a lava tube through a hole in the overlying rock, called a skylight. Figure 11.10 (bottom) shows the interior of a lava tube, with a person for scale. Lava tubes form naturally and readily because flowing mafic lava preferentially cools near its margins, forming solid lava levées that eventually close over the top of the flow. Lava within tubes can flow for 10s of km because the tubes insulate the lava from the atmosphere and slow the rate at which the lava cools. The Hawai'ian volcanoes are riddled with thousands of old, drained lava tubes, some as long as 50 km. [caption id="attachment_413" align="aligncenter" width="500"] Figure 11.10 Lava tubes. Top: An opening in the roof of a lava tube (called a skylight) permitting a view of lava flowing through the tube (Puʻu ʻŌʻō crater, Kīlauea). The opening is approximately 6 m across. Bottom: Inside a lava tube that channelled lava away from Mt. St. Helens in an eruption 1,895 years ago. Sources: Top: U. S. Geological Survey (2016), Public Domain. Image source. Bottom: Thomas Shahan (2013), CC BY-NC 2.0. Image source.
[/caption]

Lava Structures

Pahoehoe

Lava flowing on the surface can take on different shapes as it cools. Basaltic lava with an unfragmented surface (e.g., Figure 11.9, right), is called pahoehoe. (pronounced pa-hoy-hoy). Pahoehoe can be smooth and billowy. It can also develop a wrinkled texture, called ropy lava (Figure 11.11). Ropy lava forms when the outermost layer of the lava cools and develops a skin (visible as a dark layer in Figure 11.11, left), but the skin is still hot and thin enough to be flexible. The skin is stiffer than the lava beneath it, and is dragged by flowing lava and folded up into wrinkles. Figure 11.11 (right) is a close-up view after a cut has been made to show the internal structure of a wrinkled lava flow. Notice the many holes, or vesicles, within the lava, formed when the lava solidified around gas bubbles. [caption id="attachment_414" align="aligncenter" width="650"] Figure 11.11 Ropy lava from Hawaii. Left: Ropy texture forming as a thin surface layer of lava cools and is wrinkled by the motion of lava flowing beneath it. Right: Cross-section view of ropy lava. Sources: Left: Z. T. Jackson (2005), CC BY NC-ND 2.0. Image source.; Right: Fiddledydee (2011), CC BY-NC 2.0. Image source..[/caption]

A'a and Blocky Lava

When the outer layer of the lava flow can't accommodate the motion of lava beneath by deforming smoothly, the outer layer will break into fragments as lava moves beneath it. This could happen if the lava flow develops a thicker, more brittle outer layer, or if it moves faster. The result is a sharp and splintery rubble-like lava flow called a’a (pronounced like "lava" but without the l and v). Figure 11.12 (left) shows a close-up view of the advancing front of an a’a lava flow (the flow is moving toward the viewer). Figure 11.12 (right) shows an a’a lava flow viewed from the side. Compare the texture of the a’a flow with the texture of the lighter-grey pahoehoe lava in the foreground of the picture. [caption id="attachment_415" align="aligncenter" width="650"] Figure 11.12 Aa lava flows. Left: Close-up view of a'a forming during an eruption of Pacaya Volcano in Guatemala. Field of view approximately 1 m across. Right: Rubbly reddish-brown a'a lava flow viewed from Chain of Craters Road, Hawai’i Volcanoes National Park. Pahoehoe is visible in lighter grey in the foreground. Sources: Photo of Hawaiian aa and pahoehoe: Roy Luck (2009), CC BY 2.0. Image source. Pacaya a'a: Greg Willis (2008), CC BY-SA 2.0 (labels added). Image source..[/caption] Higher viscosity andesitic lava flows also develop a fragmented surface, called blocky lava. This is visible in the toe of the andesitic lava flow from Figure 11.9 (right). The difference between a’a and the andesitic blocky lava is that the blocky lava has fragments with smoother surfaces and fewer vesicles.

Lava Pillows

When lava flows into water, the outside of the lava cools quickly, making a tube (Figure 11.13 (top left)). Blobs of lava develop at the end of the tube (Figure 11.13 (top right)), forming pillows. The bottom left of Figure 10.13 shows pillows covering the sea floor, and the bottom right shows the distinctive rounded shape of pillows in outcrop. Because pillows always form underwater, finding them in the rock record gives us information that the environment was underwater. [caption id="attachment_416" align="aligncenter" width="650"] Figure 11.13 Pillow lavas. Top left: A tube of lava extruding underwater. Hot lava can be seen through cracks in the wall of the tube. The image is approximately 1 m across. (Pacific Ocean, near Fiji). Top right: The rounded end of a tube with cracks showing the lava within. (Pacific Ocean, near Fiji). Bottom left: sea floor near the Galápagos Islands covered with pillow lavas. Bottom right: A boulder made of 2.7 billion year old pillows derived from the Ely Greenstone in north-eastern Minnesota. Sources: Top left- NSF and NOAA (2010), CC BY 2.0. Image source.; Top right- NSF and NOAA (2010), CC BY 2.0. Image source.; Bottom left- NOAA Okeanos Explorer Program, Galápagos Rift Expedition 2011 (2011), CC BY 2.0. Image source.; Bottom right- James St. John (2015), CC BY 2.0. Image source..[/caption]

Columnar Joints

When lava flows cool and solidify, they shrink. Long vertical cracks, or joints, form within the brittle rock to allow for the shrinkage. Viewed from above, the joints form polygons with 5, 6, or 7- sides, and angles of approximately 120º between sides (Figure 11.14). [caption id="attachment_417" align="aligncenter" width="500"] Figure 11.14 Columnar joints viewed from above, Giant's Causeway, Northern Ireland. Source: Meg Stewart (2012), CC BY-SA 2.0. Image source.[/caption] Figure 11.15 shows a side view of columnar joints in a basaltic lava flow in Iceland. [caption id="attachment_418" align="aligncenter" width="500"] Figure 11.15 Columnar joints in a basaltic lava flow, Svartifoss (Black Fall) Vatnajökull National Park, Iceland. Source: Ron Kroetz (2015), CC BY-ND 2.0. Image source.[/caption]

Practice with Structures Made by Lava [h5p id="130"]

Pyroclastic Materials

The pop bottle analogy illustrates another key point about gas bubbles in fluid, which is that the bubbles can propel fluid. In the same way that shaking a pop bottle to make more bubbles will cause pop to gush out when the bottle is opened, gas bubbles can violently propel lava and other materials from a volcano, creating an explosive eruption. Collectively, loose material thrown from a volcano is referred to as tephra. Individual fragments are referred to in general terms as pyroclasts, so sometimes tephra is also referred to as pyroclastic debris. Pyroclasts are classified according to size.

Volcanic Ash

Particles less than 2 mm in diameter are called volcanic ash. Volcanic ash consists of small mineral grains and glass. Figure 11.16 shows volcanic ash on three scales: in the upper left is ash from the 2010 eruption of Eyjafjallajökull in Iceland. The image was taken with a scanning electron microscope at approximately 1000 times magnification. In the upper right is ash from the 1980 eruption of Mt. St. Helens, collected in Yakima, Washington, about 137 km northeast of Mt. St. Helens. Individual particles are under 1 mm in size. Figure 11.16 (bottom) shows a village near Mt. Merapi in Indonesia dusted in ash after an eruption 2010. [caption id="attachment_419" align="aligncenter" width="650"] Figure 11.16 Volcanic ash. Upper left: Ash from 2010 eruption of Eyjafjallajökull in Iceland, magnified approximately 1000x. Upper right- Ash from the 1980 eruption of Mt. St. Helens, collected at Yakima, Washington. Bottom: Indonesian village after the eruption of Mt. Merapi in 2010. Sources: Upper left: Birgit Hartinger, AEC (2010), CC BY-NC-ND 2.0. Image source. Upper right: James St. John (2014), CC BY 2.0 (scale added). Image source. Bottom: AusAID/Jeong Park (2010), CC BY 2.0. Image source.[/caption]

Lapilli

Fragments with dimensions between 2 mm and 64 mm are classified as lapilli. Figure 11.17 (upper left) shows lapilli at the ancient city of Pompeii, which was buried when Mt. Vesuvius erupted in 79 C.E. Figure 11.17 (lower left) is a form of lapilli called Pele's tears, named after the Hawai'ian diety Pele. Pele's tears form when droplets of lava cool quickly as they are flung through the air. Rapidly moving through the air may draw the Pele's tears out into long threads called Pele's hair (Figure 11.17, right). The dark masses in Figure 11.17 (right) within the Pele's hair are Pele's tears. [caption id="attachment_420" align="aligncenter" width="650"] Figure 11.17 Lapilli are pyroclasts ranging between 2 mm and 64 mm in size. Upper left: lapilli from the site of the ancient city of Pompeii. Lower left: Pele's tears, a type of lapilli that forms when droplets of lava fly through the air. Right: Pele's hair, which form when Pele's tears are drawn out into thin threads as they fly. Sources: Upper left: Pauline (2009), CC BY-NC-ND 2.0. Image source.; Lower left: James St. John (2014), CC BY 2.0 (scale added) Image source.; Right: James St. John (2009), CC BY 2.0 (scale added) Image source.[/caption]

Blocks and Bombs

Fragments larger than 64 mm are classified as blocks or bombs, depending on their origin. Blocks are solid fragments of the volcano that form when an explosive eruption shatters the pre-existing rocks. Figure 11.18 shows one of many blocks from an explosive eruption at the Halema‘uma‘u crater at Kīlauea Volcano in May of 1924. The block has a mass of approximately 7 tonnes and landed 1 km from the crater. [caption id="attachment_421" align="aligncenter" width="500"] Figure 11.18 Volcanic block weighing approximately 7 tonnes thrown 1 km from the Halema‘uma‘u crater at Kīlauea Volcano on May 18, 1924. Source: U. S. Geological Survey (1924), Public Domain. Image source.[/caption] Bombs form when lava is thrown from the volcano and cools as it travels through the air. Traveling through the air may cause the lava to take on a streamlined shape, as with the example in Figure 11.19. [caption id="attachment_422" align="aligncenter" width="500"] Figure 11.19 Volcanic bomb with a streamlined shape. Source: James St. John (2016), CC BY 2.0 (scale added). Image source.[/caption]

Effects of Gas on Lapilli and Bombs

The presence of gas in erupting lava can cause lapilli and bombs to take on distinctive forms as the lava freezes around the gas bubbles, giving the rocks a vesicular (hole-filled) texture. Pumice (Figure 11.20) forms from gas-filled felsic lava. Figure 11.20 (right), shows a magnified view of the sample on the left. The dark patches in the photograph are mineral crystals that formed in the magma chamber before the lava erupted. Pumice floats on water because some of the holes are completely enclosed, and air-filled. [caption id="attachment_423" align="aligncenter" width="650"] Figure 11.20 Lapilli-sized pumice collected from the shores of Lake Atitlán in Guatemala by H. Herrmann. The lake is a flooded caldera, and is surrounded by active volcanoes. Right: Magnified view showing vesicular structure and amphibole crystals (dark patches). Source: Karla Panchuk (2017), CC BY 4.0.[/caption] The mafic counterpart to pumice is scoria (Figure 11.21, left). Mafic lava can also form reticulite (Figure 11.21, right), a rare and fragile rock in which the walls surrounding the bubbles have all burst, leaving behind a delicate network of glass. [caption id="attachment_424" align="aligncenter" width="650"] Figure 11.21 Mafic lapilli with vesicular textures. Left: Scoria from Mount Fuji, Japan. Scoria is the denser mafic counterpart to pumice. Right: Reticulite from Kīlauea Volcano. Reticulite is a delicate network of volcanic glass that forms when the walls separating gas bubbles pop. Sources: Left- James St. John (2014), CC BY 2.0 (scale added). Image source. Right- James St. John (2014), CC BY 4.0 (scale added). Image source..[/caption]

Practice with Types of Pyroclastic Material [h5p id="131"]

References

U. S. Geological Survey (2013) Mt. St. Helens national volcanic monument. https://volcanoes.usgs.gov/volcanoes/st_helens/st_helens_geo_hist_106.html

11.3 Types of Volcanoes

kqzheng — Wed, 20 Sep 2017 00:56:35 +0000

The products of volcanism that build volcanoes and leave lasting marks on the landscape include lava flows that vary in viscosity and gas content. They also include tephra ranging in size from less than a mm to blocks with masses of many tonnes. Individual volcanoes vary in the volcanic materials they produce, and this affects the size, shape, and structure of the volcano. There are three types of volcanoes: cinder cones (also called spatter cones), composite volcanoes (also called stratovolcanoes), and shield volcanoes. Figure 11.22 illustrates the size and shape differences amongst these volcanoes. Shield volcanoes, which get their name from their broad rounded shape, are the largest. Figure 11.22 shows the largest of all shield volcanoes—in fact, the largest of all volcanoes on Earth—Mauna Loa, which makes up a substantial part of the Island of Hawai‘i and has a diameter of nearly 200 km. The summit of Mauna Loa is presently 4,169 m above sea level, but this represents only a small part of the volcano. It rises up from the ocean floor at a depth of approximately 5,000 m. Furthermore, the great mass of the volcano has caused it to sag downward into the mantle by an additional 8,000 m. In total, Mauna Loa is a 17,170 m thick accumulation of rock. [caption id="attachment_427" align="aligncenter" width="1917"] Figure 11.22 Comparison of volcano sizes and shapes. Broad, rounded shield volcanoes are the largest, followed by cone-shaped composite volcanoes. Straight-sided cinder cones are the smallest, and barely visible in the scale of the diagram. Source: Karla Panchuk (2017), CC BY 4.0. Modified after Steven Earle (2015), CC BY 4.0. View original.[/caption] Kīlauea Volcano is also a shield volcano, albeit a much flatter one. Kīlauea Volcano rises only 18 m about the surrounding terrain, and is almost not visible in the scale of the diagram, however it still stretches over a distance of 125 km along the eastern side of the Island of Hawai‘i. Composite volcanoes are the next largest. Mt. St. Helens is shown on the left of Figure 11.22. It rises 1,356 m above the surrounding terrain in the Cascade Range of the western United States, and has a diameter of approximately 6 km. Composite volcanoes tend to be no more than 10 km in diameter. Unlike shield volcanoes, composite volcanoes have a distinctly conical shape, with sides that steepen toward the summit. Cinder cones are the smallest, and almost too small to see next to a volcano like Mauna Loa. Eve Cone is a cinder cone on the flanks of Mt. Edziza in northwestern British Columbia. It rises 172 m above the landscape, and has a diameter of under 500 m. Cinder cones have straight sides, unlike upward-steepening composite volcanoes, or rounded shield volcanoes.

Volcano Structure

Shield Volcanoes

Shield volcanoes, like the Sierra Negra volcano in the Galápagos Islands (Figure 11.23, top), get their gentle hill-like shape because they are built of successive flows of low-viscosity basaltic lava (Figure 11.23, bottom). The low viscosity of the lava means that it can flow for long distances, resulting in the greater size of shield volcanoes compared to composite volcanoes or cinder cones. [caption id="attachment_428" align="aligncenter" width="650"] Figure 11.23 Shield volcano. Top: The Sierra Negra volcano in the Galápagos Islands exhibits the low, rounded shape characteristic of shield volcanoes. Bottom: Diagram of a shield volcano island, showing the build up of basaltic lava flows (not to scale). Sources: Top- BRJ INC. (2012), CC BY-NC-ND 2.0. Bottom- Karla Panchuk (2017), CC BY 4.0.[/caption]

Composite Volcanoes (Stratovolcanoes)

Composite volcanoes, like Cotopaxi in Figure 11.24 (top), consist of layers of lava alternating with layers of tephra (blocks, bombs, lapilli, and ash; Figure 11.24, bottom). The layers (strata) is where the alternative name, stratovolcano comes from. Cotopaxi displays the characteristic shape of composite volcanoes, which have slopes that get steeper near the top of the volcano. The change in the slope reflects the accumulation of tephra fragments near the volcano's vent. Composite volcanoes typically erupt higher viscosity andesitic and rhyolitic lavas, which do not travel as far from the vent as basaltic lavas do. This results in volcanoes of smaller diameter than shield volcanoes. A notable exception is Mt. Fuji in Japan, which erupts basaltic lava. [caption id="attachment_429" align="aligncenter" width="1008"] Figure 11.24 Composite volcano. Top: Cotopaxi in Ecuador exhibits the upward-steepening cone characteristic of composite volcanoes. Bottom: Diagram of a composite volcano showing alternating layers of lava and tephra (not to scale). Sources: Karla Panchuk (2017), CC BY 4.0; Top photo by Simon Matzinger (2014), CC BY 2.0. Click for more attributions.[/caption] From a geological perspective, composite volcanoes tend to form relatively quickly and do not last very long. If volcanic activity ceases, it might erode away within a few tens of thousands of years. This is largely because of the presence of pyroclastic eruptive material, which is not strong.

Cinder Cones (Spatter Cones)

Cinder cones, like Mt. Capulin in Figure 11.25, have straight sides and are typically less than 200 m high. Most are made up of fragments of scoria (vesicular rock from basaltic lava) that were expelled from the volcano as gas-rich magma erupted. Because cinder cones are made up almost exclusively of loose fragments, they have very little strength. They can be eroded away easily, and relatively quickly. [caption id="attachment_430" align="aligncenter" width="650"] Figure 11.25 Cinder cone. These small, straight-sided volcanoes are made of volcanic fragments ejected when gas-rich basaltic lava erupts. Sources: Karla Panchuk (2017) CC BY 4.0, with photograph by R. D. Miller, U. S. Geological Survey (1980), Public Domain. Image source. Click for more attributions.[/caption]

Practice with Types of Volcanoes

Fill in the missing words in these descriptions of volcanoes. are large, broad, and gently sloped, and form from (Basaltic or rhyolitic) lava. are small, straight-sided volcanoes that form from (Basaltic or rhyolitic) lava that sputters out and becomes tephra. are intermediate in size, and have sides that become steeper near the top. They are built of layers of lava and (Another way of saying pyroclastic debris), and most often are (Basaltic or andesitic) to rhyolitic in composition. To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="132"]

Now that you're warmed up, try this: [h5p id="133"]

References

Rubin, K. (n.d.) Mauna Loa Volcano. https://www.soest.hawaii.edu/GG/HCV/maunaloa.html

11.4 Types of Volcanic Eruptions

kqzheng — Wed, 20 Sep 2017 00:56:37 +0000

Volcanoes produce a variety of materials when they erupt. The characteristics of the eruptions themselves also vary from one volcano to the next, and sometimes from one eruption to the next for the same volcano. Eruptions are classified according how explosive they are, and the height of their eruption column—how high they blast material into the air. Both the explosiveness of an eruption and the height of the eruption column are related in part to the composition of magma and the amount of gas it contains. In particular, magmas with more silica erupt more explosively. The higher the silica content, the greater the viscosity of the magma. This means more pressure can build up before the magma is forced out of the volcano. Magma with more silica also tends to contain more dissolved gas. The gas helps to propel magma out of the volcano, in the same way that the bubbles in a shaken bottle of pop cause the pop to foam out when the lid is removed. There are four types of eruptions with properties determined mostly by the silica content of magma, and the amount of gas it contains. In order of increasing explosiveness, these are Hawai'ian, Strombolian, Vulcanian, and Plinian eruptions. Any composition of magma can have an explosive eruption if the magma suddenly encounters water. Hot magma contacting groundwater or seawater causes the water to flash to steam. Explosive eruptions driven by water are called hydrovolcanic (or phreatic) eruptions.

Hawai‘ian Eruptions

Hawai‘ian eruptions are named after the characteristic eruptions of volcanoes of the Hawai‘ian islands. Hawai‘ian eruptions are effusive (flowing) rather than explosive because they erupt low-viscosity basaltic lava. Hawai‘ian eruptions form shield volcanoes and can also take the form of fissure eruptions. Fissure eruptions occur when lava erupts from long cracks in the ground rather than from a central vent. Figure 11.26 shows examples from two eruptions on of Hawai‘i. In the upper left and right are images from the November 1959 eruption of Kīlauea Iki Crater. The upper left shows a fissure eruption and effusive flow of lava. Burning trees appear as bright spots toward the bottom of the photo. Figure 11.26 (right) shows a lava fountain reaching 425 m above Kīlauea Iki Crater. U. S. Geological Survey scientists reported that volcanic bombs up to 60 cm across smashed the guard rail and dented the asphalt on the road. Figure 11.26 (lower left) shows Hawaiian Volcano Observatory (HVO) scientists making a quick getaway, with lava fountains from Mauna Loa Volcano in the background. [caption id="attachment_433" align="aligncenter" width="650"] Figure 11.26 Hawai‘ian eruptions. Top left: Fissure eruption at Kīlauea Iki Crater in November of 1959. Bottom left: Lava fountains from an eruption of Mauna Loa Volcano in 1984. Right: Lava fountain from Kīlauea Iki Crater eruption in November of 1959. Sources: Top left- U. S. Geological Survey (1959), Public Domain. Image source. Bottom left: R. B. Moore, U. S. Geological Survey (1984), Public Domain. Image source. Right- U. S. Geological Survey (1959), Public Domain. Image source.[/caption] The photographs of the Kīlauea Iki Crater and Mauna Loa Volcano eruptions make the point that while Hawai‘ian eruptions are considered "gentle" eruptions, this is a relative term. "Gentle" eruptions range from lava flows that can be safely sampled by trained personnel, as in Figure 11.5, to lava fountains that soar hundreds of metres above the tree tops and rain large and dangerous rocks upon the surroundings.

Strombolian Eruptions

Strombolian eruptions, named for Mt. Stromboli in Italy, occur when basaltic lava has higher viscosity and higher gas content. The sticky lava is ejected in loud, violent, but short-lived spattery eruptions. Clumps of gas-rich lava thrown 10s to 100s of metres in the air accumulate as scoria in a pile around the vent, forming cinder cones. Figure 11.27 shows a Strombolian eruption in the crater of Mt. Etna. A smaller cinder cone is forming around the vent as lava sputters out of it. [caption id="attachment_434" align="aligncenter" width="550"] Figure 11.27 Strombolian eruption of Mt. Etna. Sputtering lava forms a smaller cinder cone around a vent within the crater of Etna. Source: Karla Panchuk (2017), CC BY-SA 4.0. Photograph- Robin Wylie (2012), CC BY 2.0. Image source. Click for more attributions.[/caption]

Vulcanian Eruptions

Vulcanian eruptions get their name from the volcanic Italian island of Vulcano, which itself takes the name of the Roman god of fire, Vulcan. In Roman mythology, Vulcan was the maker of armour and weaponry for the gods, and volcanic eruptions were attributed to him working in his forge. Vulcanian eruptions are far more explosive than Strombolian eruptions, and can blast tephra and gas to a height of 5 to 10 km. The explosiveness is related to a build-up of pressure as the higher viscosity of intermediate silica content lava restricts the escape of gas. Vulcanian eruptions produce large quantities of ash in addition to blocks and bombs. The Vulcanian eruption of Mt. Pelée on the island of Martinique in 1902 resulted in the first detailed documentation by geologists of a devastating phenomenon that is now referred to as a pyroclastic flow (Figure 11.28). Volcanic debris from the collapse of a lava dome on Mt. Pelée combined with hot gas to form a searing avalanche that raced down the mountain, over the city of St. Pierre, and into the harbour. [caption id="attachment_435" align="aligncenter" width="650"] Figure 11.28 A series of photos taken by Alfred Lacroix during the eruption of Mt. Pelée on May 8, 1902 showing the development of the pyroclastic flow that destroyed the city of St. Pierre and nearly 30,000 inhabitants. Source: Karla Panchuk (2017), CC BY 4.0. Photograph: Alfred Lacroix (1902), Public Domain. Click for more attributions.[/caption] The French geologist Alfred Lacroix described what he saw as a "nuée ardente," or thick fiery cloud. The following first-hand account was published in Cosmopolitan Magazine in July of 1902, attributed to Ellery S. Scott, a sailor on the steamship Roraima:

"In idle interest, I turned my glass toward Mont Pelee. It was at that very moment that the whole top of the mountain seemed blown into the air. The sound that followed was deafening. A great mass of flames, seemingly a mile in diameter, with twisting giant wreaths of smoke, rolled thousands of feet into the air, and then overbalanced and came rolling down the seamed and cracked sides of the mountain. Foot hills were overflowed by the onrushing mass. It was not mere flame and smoke. It was molten lava, giant blocks of stone and a hail of smaller stones, with a mass of scalding mud intermingled.

For one brief moment I saw the city of St. Pierre before me. Then it was blotted out by the overwhelming flood. There was no time for the people to flee. They had not even time to pray.... I had called to Carpenter Benson to start the windlass, but before he could move, the "Roraima" rolled almost on her port beam-ends, and then as suddenly went to starboard. The funnel, masts and boats went by the board in an instant. The decks were swept clean. The hatches were staved in. The next instant a hail of fire and red-hot stones was upon the ship. Then came the scalding mud. The saloon was ablaze. The ship seemed doomed. Men were struck down all around me by flaming masses of lava. From bright sunlight the air became dense as midnight. The smoke that rolled down from the crater's mouth had blotted the sun from our vision."

Scott's account vividly describes of the speed of the pyroclastic flow. In some cases, pyroclastic flows travel at speeds greater than 700 km/h. They are able to travel rapidly because they behave like a fluid, and can also ride on a cushion of hot gas. Scott says the city was "blotted out by a flood," yet the lower parts of buildings remained (Figure 11.29), and human remains were found in streets and homes where they had fallen. The ruins of St. Pierre look as though the top of the city were shaved off, and that's effectively what happened as the pyroclastic flow rushed across it, buoyed by gas.

[caption id="attachment_436" align="aligncenter" width="650"] Figure 11.29 Two stereographs of the ruins of St. Pierre, published in 1902. Stereographs are viewed with a stereoscope to make an image appear three dimensional. Top- "St. Pierre, 'the city of dead,' Mt. Pelee smoking, Martinique"; Bottom- "Overlooking the mud-filled Roxelane River bed, and ash-covered ruins, to Mont Pelée, St. Pierre, Martinique." Source: Top- Boston Public Library (2013), CC BY 2.0. Image source.; Bottom- Boston Public Library (2013), CC BY 2.0. Image source.[/caption]

The vast majority of fatalities from the eruption were caused by the heat of pyroclastic flow. Examination of the ruins of St. Pierre revealed that glass had melted, but copper had not, putting the temperature at between 700 ºC and 1000 ºC (1292 ºF to 1832 ºF).

Plinian Eruptions

Plinian eruptions are explosive eruptions of intermediate to felsic lava, and can form eruptive columns up to 45 km high. The origin of the name is the eruption of Vesuvius in 79 CE, which buried the towns of Pompeii and Herculaneum. The Roman admiral Gaius Plinius Secundus, also known as Pliny the Elder, attempted a rescue mission when he saw the column of ash and debris above Vesuvius, but died suddenly of unknown causes without being able to reach Herculaneum. A more recent Plinian eruption was that of Mt. Redoubt on April 21, 1990, shown in Figure 11.30. Pyroclastic flows resulted, as did lahars, landslides that formed when glaciers melted and turned volcanic ash into mud. The shape of the eruptive column, with parts of the column appearing to spread out in flat layers at different levels, reflects differences in atmospheric characteristics. [caption id="attachment_437" align="aligncenter" width="650"] Figure 11.30 Plinian eruption of Mt. Redoubt in Alaska on April 21, 1990. Source: Karla Panchuk (2017), CC BY 4.0. Photograph: R. Clucas, U. S. Geological Survey (1990), Public Domain. Image source. Click for more attributions.[/caption]

Hydrovolcanic (Phreatic) Eruptions

Hydrovolcanic eruptions can be far more explosive than Plinian eruptions. They occur when water in the form of groundwater, seawater, or even melting glacial ice or snow comes into contact with magma. Heat from the magma changes water suddenly to steam, which can expand to more than a thousand times the original volume of water. The sudden expansion results in an explosive force that can blast a volcano to pieces and create large amounts of volcanic ash. In April of 2010, activity by the Icelandic volcano Eyjafjallajökull (Figure 11.31) melted the glacier above it, releasing large quantities of water and triggering a hydrovolcanic eruption. Ash rose in a plume 10 km high, and was blown westward and into the skies over Europe. Volcanic ash can damage or destroy aircraft engines, so the precaution was taken to prohibit air travel for a 5-day period. The enormous economic impact of stopping flights has led to numerous studies about the best way to deal with similar events with volcanic ash in the future. [caption id="attachment_438" align="aligncenter" width="720"] Figure 11.31 Hydrovolcanic eruption of Eyjafjallajökull in April of 2010. Left- Eruptive column with volcanic lightning. Volcanic lightning is caused by the static electricity generated by volcanic ash particles rubbing together. Right- Another view of the ash cloud, with westward winds carrying ash toward Europe where it would disrupt air traffic. Source: Karla Panchuk (2017), CC BY-SA 4.0. Left photograph: Terje Sørgjerd (2010), CC BY-SA 3.0. Image source. Right photograph: Henrik Thorburn (2010), CC BY 3.0. Image source. Click for more attributions.[/caption]

Practice with Types of Volcanic Eruptions When you've completed this exercise, you'll have a handy summary table that you can refer to. You can keep a copy by doing a screen capture. [h5p id="134"]

References

Bressan, D (2012). Geology scene investigation: Death by volcanic fire. https://blogs.scientificamerican.com/history-of-geology/geology-scene-investigation-death-by-volcanic-fire

British Geological Survey (n.d.). Eyjafjallajökull eruption, Iceland | April/May 2010. http://www.bgs.ac.uk/research/volcanoes/icelandic_ash.html

Scott, E. S. (2011). Eyewitness account to eruption of Mont Pelee Matinique St Pierre Fort de France. Digital History Project. https://web.archive.org/web/20121012160805/http://www.digitalhistoryproject.com/2011/09/eyewitness-account-to-eruption-of-mont.html

Rosen, J. (2015). Benchmarks: May 8, 1902: The deadly eruption of Mount Pelée. https://www.earthmagazine.org/article/benchmarks-may-8-1902-deadly-eruption-mount-pelee

U. S. Geological Survey (1997). Pyroclastic flows. https://pubs.usgs.gov/gip/msh//pyroclastic.html

11.5 Plate Tectonics and Volcanism

kqzheng — Wed, 20 Sep 2017 00:56:40 +0000

So far we've discussed volcanoes in terms of the kinds of volcanic mountains they form, the materials they produce, and the style of eruption they have. All of these characteristics can be tied together into a big picture by considering the plate tectonic settings in which magma forms (Figure 11.32). The vast majority of volcanoes are present along plate tectonic boundaries. [caption id="attachment_441" align="aligncenter" width="2176"] Figure 11.32 Plate tectonic settings of volcanism. Volcanoes along subduction zones are the result of flux melting (lowering the melting point by adding water). Decompression melting produces volcanoes along divergent margins (ocean spreading centres and continental rift zones), as well as above mantle plumes. Contact between hot mafic partial melts and felsic rocks can trigger partial melting of the felsic rocks (melting from conduction). Source: Karla Panchuk (2021), CC BY 4.0. Modified after U. S. Geological Survey (1999), Public Domain. View original.[/caption] There are four main scenarios to consider:

Divergent boundaries where melting is triggered by decompression
Subduction zones (ocean-ocean and ocean-continent convergent boundaries) where flux melting occurs as water is released from subducting ocean crust
Hot spots where plumes of hot mantle material rise up, then melt as a result of decompression.
Melting by conduction when magma transfers heat to rocks having a lower melting temperature.

Decompression Causes Volcanism Along Spreading Centres and Rift Zones

At an ocean spreading ridge (centre of Figure 11.32), convection moves hot mantle rock slowly upward at rates of cm per year. At roughly 60 km below the surface, the mantle rocks have decompressed is enough to permit partial melting of approximately 10% of the ultramafic rock. Mafic magma is produced, and it moves up toward the surface. Magma fills vertical fractures produced by the spreading and spills out onto the sea floor making pillow lavas and lava flows. Spreading-ridge volcanism is taking place approximately 200 km offshore from the west coast of Vancouver Island. In continental rift zones where continental crust is thinning (far right in Figure 11.32), a similar decompression process occurs, triggering partial melting of ultramafic mantle rocks. However, if the continental crust above the region where melting occurs has a lower melting temperature than the mafic melt that is produced, the continental crust will also melt. Continental rift zones can have a range of volcano types. If mafic magma erupts, shield volcanoes, broad lava flows, and cinder cones result. However, if rocks of other compositions are melted and added in, or the mafic magma undergoes fractional crystallization before erupting, then composite volcanoes will also form.

Water Causes Partial Melting Along Subduction Zones

At an ocean-continent convergent boundary (Figure 11.32, right) or ocean-ocean convergent boundary (Figure 11.32, left), oceanic crust is pushed down into the mantle. Although temperatures are high, the slab is kept from melting by high pressures. However, under these conditions minerals in the slab release water from within their crystal structures. The water lowers the melting point of rock above the slab, and partial melting is triggered within the mantle. Mafic magma rises through the mantle to the base of the crust. There it contributes to partial melting of crustal rock, and more felsic material is added to the magma. The magma, now intermediate in composition, continues to rise and assimilate crustal material. In the upper part of the crust, it accumulates into plutons. Over time, fractional crystallization of magma within the pluton can make it even more silica-rich. From time to time, the magma from the plutons rises toward surface, leading to volcanic eruptions. Composite volcanoes with Vulcanian or Plinian eruption styles are characteristic of the volcanic arcs that form in subduction zones, although in the Trans-Mexico Volcanic Belt, Strombolian eruptions produce short-lived cinder cones. Where two margins of oceanic crust collide, the volcanic arc will be a chain of volcanic islands. Where continental and oceanic crust collide, there will be a volcanic arc on the continental crust.

Mt. St. Helens: A Composite Volcano in the Cascades Range Continental Volcanic Arc

[caption id="attachment_1719" align="aligncenter" width="400"] On May 18, 1980 at 8:32 a.m., a M5.1 earthquake shook Mt. St. Helens.[/caption] [caption id="attachment_1720" align="aligncenter" width="550"] Before and after views of Mt. St. Helens.[/caption] It marked the start of a 9-hour (hint: Hawai'ian or Plinian?) eruption with a 24 km high eruption column and multiple (hint: a type of hazard associated with explosive eruptions) flows. By the time the eruption was over, a large part of the volcano had been blasted away. The explosive eruption was driven by gas-rich felsic magma. However not all of Mt. St. Helens’ eruptions have been of (hint: basaltic or rhyolitic?) material. The (hint: two words meaning an underground conduit for lava) shown here is from a time when Mt. St. Helens erupted (hint: basaltic or rhyolitic?) lava. The iMUSH (Imaging Magma Under St. Helens) project has investigated beneath Mt. St. Helens to understand where the magma came from. [caption id="attachment_1722" align="aligncenter" width="400"] Based on: Kiser, E., Palomeras, I., Levander, A., Zelt, C., Harder, S., Schmandt, B., Hansen, S., Creager, K., & Ulberg, C. (2016). Magma reservoirs from the upper crust to the Moho inferred from high-resolution Vp and Vs models beneath Mount St. Helens, Washington State, USA. Geology (44)6, 411-414. DOI: 10.1130/G37591.1[/caption] There is not one, but three (hint: spaces in which magma is stored) in the area. Earthquakes in the 24 hours after the 1980 eruption suggested magma movement at between 5 and 14 km depth. Earthquakes from 1980 to 2005 indicate movement of magma even deeper in a chamber that extends all the way down to the (hint: layer beneath the crust) . The complex history of Mt. St. Helens could reflect compositional changes in the small chamber over time. As (hint: the process where more mafic minerals form first, and then settle out of magma) proceeds, magma becomes more (hint: felsic or mafic?). But movement of more mafic magma from the larger chamber could change the chemical composition of eruptions. The larger chamber may be connected to a chamber feeding a nearby volcanic field, with (hint: large volcanoes with gently curved slopes) and (hint: small straight-sided volcanoes), where basalt makes up 80% of erupted materials. To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="135"]

Mantle Plumes Can Cause Volcanism Away from Plate Boundaries

Mantle plumes are rising columns of hot solid rock. The column may be kilometres to 10s of kilometres across, but near the surface it spreads out to create a mushroom-like head that is 10s to over 100 kilometres across. Mantle plumes are different from the convection that normally occurs beneath ocean spreading centres: plumes rise approximately 10 times faster than mantle convection normally occurs, and may originate deep in the mantle, possibly just above the core-mantle boundary. When a mantle plume rises to the base of the lithosphere, the pressure is low enough to permit partial melting of the plume material, producing mafic magma. Heat carried by the mantle plume may also melt rock adjacent to the plume. The magma rises and feeds hotspot volcanoes. The lithospheric plate above the mantle plume is moving across the plume, so a chain of hotspot volcanoes can result as existing hotspot volcanoes are slowly moved away from the mantle plume, and new volcanoes form in the lithosphere. Many shield volcanoes are associated with mantle plumes, including those that make up the Hawai'ian islands. All of the Hawai'ian volcanoes are related to the mantle plume that currently lies beneath Mauna Loa, Kilauea, and Lōʻihi (Figure 11.33, top). There is evidence of crustal magma chambers beneath all three active Hawai'ian volcanoes. At Kīlauea, the magma chamber appears to be several kilometres in diameter, and is situated between 8 km and 11 km below surface (Lin et al., 2014). In this area, the Pacific Plate is moving northwest at a rate of about 7 cm/year. This means that the earlier formed—and now extinct—volcanoes have now moved well away from the mantle plume. The hotspot may have been present for at least 85 million years (Regelous et al., 2003), if this process is responsible for the long chain of eroded and submerged mountains stretching to the Aleutian Trench (Figure 11.33, bottom). [caption id="attachment_442" align="aligncenter" width="650"] Figure 11.33 Hawai'ian hotspot volcanoes and volcanic chain. Top- A mantle plume beneath Hawai'i supplies magma to Mauna Loa Volcano, Kīlauea Volcano, and Lōʻihi Seamount. Volcanoes to the northwest are no longer active because they have moved away from the plume. Bottom- Bathymetric (depth) map showing the chain of islands stretching toward the Aleutian Trench, and marking the progress of the Pacific Plate over the mantle plume. Source: Top- J. E. Robinson, U. S. Geological Survey (2006), Public Domain. Image source. Bottom- National Geophysical Data Center/ U. S. Geological Survey (2006), Public Domain (labels added). Image source.[/caption] Kīlauea Volcano is approximately 300 ka old, while neighbouring Mauna Loa Volcano is over 700 ka and Mauna Kea Volcano is over 1 Ma. If volcanism continues above the Hawaii mantle plume in the same manner that it has for the past 85 Ma, it is likely that Kīlauea Volcano will continue to erupt for at least another 500,000 years. By that time, its neighbour, Lōʻihi Seamount, will have emerged from the sea floor, and its other neighbours, Mauna Loa and Mauna Kea, will have become significantly eroded, like their cousins, the islands to the northwest.

Large Igneous Provinces (LIPs)

While the Hawaii mantle plume has produced a relatively low volume of magma for approximately 85 Ma, other mantle plumes are less consistent, and some generate massive volumes of magma over relatively short time periods. Although their origin is still controversial, it's thought that the volcanism leading to large igneous provinces (LIPs) is related to very high volume but relatively short duration bursts of magma from mantle plumes. An example of an LIP is the Columbia River Basalt Group, which extends across Washington, Oregon, and Idaho in the United States (Figure 11.34). This volcanism, which covered an area of about 160,000 km² with basaltic rock up to several hundred metres thick, took place between 17 and 14 Ma. [caption id="attachment_443" align="aligncenter" width="550"] Figure 11.34 Part of the Columbia River Basalt Group at Frenchman Coulee, eastern Washington, United States. All of the flows visible here have formed large (up to two metres in diameter) columnar basalts, a result of relatively slow cooling of flows that are tens of m thick. The inset map shows the approximate extent of the 17 to 14 Ma Columbia River Basalts, with the location of the photo shown as a star. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] The mantle plume that is assumed to be responsible for the Columbia River LIP is now situated beneath the Yellowstone area, where it leads to felsic volcanism. Over the past 2 Ma, three very large explosive eruptions at Yellowstone have yielded approximately 900 km³ of felsic magma. This is approximately 900 times the volume of the 1980 eruption of Mt. St. Helens, but only 5% of the volume of mafic magma in the Columbia River LIP. Most other LIP eruptions are much bigger. The Siberian Traps (also basalt), which erupted at the end of the Permian period at 251 Ma, are estimated to have produced approximately 40 times as much lava as the Columbia River LIP. The largest known LIP is the Ontong Java Plateau, located in the southwest Pacific Ocean. It formed around 122 Ma, presently covers 1,500,000 km2, and has a volume of 5,000,000 km³. But this is only a small fraction of its original size, because the majority of it has been subducted, and it may have been split into pieces that have been classified as separate LIPs.

Kimberlites

Kimberlite pipes are carrot-shaped cones of ultramafic rock. They form from the explosive eruption of mantle plumes originating at depths of 150 to 450 km in the mantle. The plume makes its way to the surface quickly (over hours to days), having little interaction with the surrounding rocks, and thus preserving a sample of the ultramafic mantle. As the plume nears the surface, a build-up of gas causes it to pick up speed, and by the time it reaches the surface it may be travelling faster than the speed of sound. The explosiveness of kimberlite eruptions means that they don't form volcanic mountains on the surface, but leave circular holes in the ground. Kimberlite eruptions that originate at depths greater than 200 km beneath old, thick, continental crust travel through the region of the mantle where diamond is stable. In some cases, such as in Saskatchewan and the Northwest Territories, kimberlites bring diamond-bearing material to the surface. All of Earth's diamond deposits are thought to have originated in this way. Diamond mines in kimberlites, such as the Ekati Mine in the Northwest Territories, are easy to spot by the characteristic circular hole that develops as miners excavate the cone-shaped structure (Figure 11.35). The kimberlites at Ekati erupted between 45 and 60 Ma. Many kimberlites are older, and some much older. There have been no kimberlite eruptions in recent times, but the youngest known kimberlites are in the Igwisi Hills in Tanzania and are only about 10,000 years old. The next youngest date to approximately 30 Ma. [caption id="attachment_444" align="aligncenter" width="550"] Figure 11.35 The Ekati diamond mine in the Northwest Territories, part of the Lac de Gras kimberlite field. Source: Karla Panchuk (2017) CC BY-SA 4.0; Photograph by J. Pineau (2010), CC BY-SA 3.0. Image source. Click for more attributions.[/caption]

References

Kiser, E., Palomeras, I., Levander, A., Zelt, C., Harder, S., Schmandt, B., Hansen, S., Creager, K., & Ulberg, C. (2016). Magma reservoirs from the upper crust to the Moho inferred from high-resolution Vp and Vs models beneath Mount St. Helens, Washington State, USA. Geology, (44)6, 411-414. DOI: 10.1130/G37591.1

Lin, G, Amelung, F, Lavallee, Y, and Okubo, P. (2014). Seismic evidence for a crustal magma reservoir beneath the upper east rift zone of Kilauea volcano, Hawaii. Geology, 42(3), 187-190. DOI: 10.1130/G35001.1

Regelous, M., Hofmann, A. W., Abouchami, W., & Galer, S. J. G. (2003) Geochemistry of lavas from the Emperor Seamounts, and the geochemical evolution of Hawaiian magmatism from 85 to 42 Ma. Journal of Petrology 44(1), 113-140. DOI: 10.1093/petrology/44.1.113.

U. S. Geological Survey, Volcano Hazards Program (n.d.). Indian Heaven volcanic field. https://volcanoes.usgs.gov/volcanoes/indian_heaven/

U. S. Geological Survey, Volcano Hazards Program (n.d.). Mount St. Helens: 1980 cataclysmic eruption. https://volcanoes.usgs.gov/volcanoes/st_helens/st_helens_geo_hist_99.html

11.6 Volcanic Hazards

kqzheng — Wed, 20 Sep 2017 00:56:41 +0000

Volcanic Hazards Are More Complex than Lava Flows

The basaltic lava flows produced by volcanoes on the island of Hawai'i are responsible for extensive damage to homes, infrastructure, and habitats. Figure 11.36 shows lava flows (in black) from the Puʻu ʻŌʻō crater of Kīlauea Volcano. The lava flow destroyed the house, and is encroaching on the transfer station. Smoke in the background marks locations where additional flows have broken out and are burning vegetation. In spite of the damage that lava flows can cause, they are not the volcanic hazard with the greatest direct impact on lives and infrastructure over all. Even the relatively free-flowing Hawai'ian basaltic lava moves slowly enough that it can be escaped on foot. [caption id="attachment_1058" align="aligncenter" width="1024"] Figure 11.36 Lava flow from Kīlauea's Puʻu ʻŌʻō crater. Lava (in black) has destroyed a house and threatens a transfer station. Source: U. S. Geological Survey (2014) Public Domain. view source[/caption] It would be a mistake, however, to assume that being safe from lava flows is just a matter of having everyone walk away in a timely fashion. In May of 2021, a Hawai'ian style eruption of Mt. Nyiragongo in the Democratic Republic of the Congo resulted in more than 30 fatalities, including:

Two people who didn't have the assistance they needed to leave their homes to escape the lava.
Fatalities from collisions as people tried to flee in vehicles.
Fatalities from asphyxiation when people tried to walk to safety over cooled lava flows, but were suffocated by gases still rising from the lava.

There were additional complications with adverse outcomes that won't likely be fully assessed for some time:

Approximately 230,000 people were displaced without adequate supplies—including food or clean drinking water—resulting in outbreaks of cholera.
Water supply was cut off for most of the city of Goma (pop. 670,000) because of lava flows.
1400 children were separated from their parents.

The map in Figure 11.37 shows a preliminary assessment of the situation as of 28 May 2021. Notice that all of this disruption happened with lava flows that only barely crossed into the official city boundary of Goma. A further potential complication is that Goma is situated on the north shore of Lake Kivu, which has the potential to undergo limnic eruptions of methane if the lake temperature or conditions on the lake floor are altered too substantially (e.g., if a rift opened within the lake). [caption id="attachment_447" align="aligncenter" width="724"] Figure 11.37 Map of lava flow and tremors from the Mount Nyiragongo eruption in May of 2021. Source: UNITAR (2021), Public Domain. Image source. Click to access an interactive version of this map.[/caption] This summary of the broader consequences of a "gentle" eruption of Mt. Nyiragongo highlights the fact that the largest impact and the greatest suffering are typically not caused by the direct effects of volcanic eruptions. They aren't even caused by the most immediate effects, such as the shorter-term consequences of people fleeing without adequate humanitarian support. Far more devastating are large-scale (including global) changes to climate and environmental conditions leading to widespread respiratory distress, toxicity, famine, and habitat destruction, accounting for approximately 8 million deaths during historical times. In contrast, direct effects have accounted for fewer than 200,000, or 2.5% of the total, in that time.

Volcanic Gas and Tephra Emissions

Volcanic gases and volcanic debris are far more dangerous hazards than lava flows alone. Large volumes of rock and gases are emitted during major Plinian eruptions at composite volcanoes, and a large volume of gas is released during some very high-volume effusive eruptions. Gases and fine particles of volcanic ash can cause respiratory distress and poisoning, and ash poses a risk for aircraft. Most of the tephra from large explosive eruptions ascends high into the atmosphere, and some of it is distributed around Earth by high-altitude winds. The larger components (larger than 0.1 mm) fall closer to the volcano, and the accumulation of tephra from large eruptions can cause serious damage and casualties. When the large eruption of Mt. Pinatubo in the Philippines occurred in 1991, tens of centimetres of ash accumulated in fields and on rooftops in the surrounding populated region. Heavy typhoon rains hit the island at the same time and added to the weight of the tephra. The weight was too much for roofs to bear, and thousands of structures collapsed, causing at least 300 of the 700 deaths attributed to the eruption. One of the long-term effects of adding volcanic particles and gases to the atmosphere is cooling. Over an eight-month period in 1783 and 1784, a massive effusive eruption took place at the Laki volcano in Iceland (Figure 11.38). Although there was relatively little volcanic ash involved, a massive amount of sulphur dioxide was released into the atmosphere, along with a significant volume of hydrofluoric acid (HF). The sulphur dioxide combined with water to make sulphate aerosols, which block incoming solar energy. [caption id="attachment_448" align="aligncenter" width="550"] Figure 11.38 A view of the Laki volcano fissure in Iceland. Source: Petr Brož (2009), CC BY-SA 3.0. Image source.[/caption] The accumulation of sulphate aerosols over that 8 months led to dramatic cooling in the northern hemisphere. There were serious crop failures in Europe and North America, and a total of 6 million people are estimated to have died from famine and respiratory complications. In Iceland, poisoning from the HF resulted in the death of 80% of sheep, and 50% of cattle. The ensuing famine, along with HF poisoning, resulted in more than 10,000 human deaths, about 25% of the population.

Pyroclastic Flows

In a typical explosive eruption at a composite volcano, the tephra and gases are ejected with explosive force and sent high up into the atmosphere. As the eruption proceeds, and the amount of gas in the rising magma starts to decrease, and less gas is supplied to the eruption column. Parts of the column will become denser than air, leading the column to collapse and flow downward along the flanks of the volcano (Figure 11.39), picking up speed as it cools. [caption id="attachment_449" align="aligncenter" width="550"] Figure 11.39 The Plinian eruption of Mt. Mayon, Philippines in 1984. Although most of the eruption column is ascending into the atmosphere, pyroclastic flows are traveling down the sides of the volcano in several places. Warnings were issued in time to evacuate 73,000 people. Source: C. G. Newhall, U. S. Geological Survey (1984), Public Domain. Image source.[/caption] Pyroclastic flows can travel over water, in some cases for many kilometres. In 1902 the pyroclastic flow from the eruption of Mt. Pelée traveled out into the harbour and destroyed several wooden ships anchored there. The pyroclastic flow from the 1883 eruption of Krakatau traveled 80 km across the Sunda Straits and claimed victims on the southwest coast of Sumatra. It also triggered a tsunami. One of the most famous pyroclastic flows occurred when Mt. Vesuvius erupted in 79 CE. It buried the cities of Pompeii and Herculaneum, killing an estimated 18,000 people.

Lahar

A lahar is any flow of mud or debris that is related to a volcano (Figure 11.40, from the eruption of Mt. St. Helens in 1980). Most are caused by melting snow and ice during an eruption, as was the case with the lahar that destroyed the Colombian town of Armero in 1985 when the volcano Nevado del Ruiz caused the ice dam on a glacial lake to fail. The resulting lahar killed 23,000 people in Armero, about 50 km from the volcano. [caption id="attachment_450" align="aligncenter" width="550"] Figure 11.40 Mud left behind from the lahar after the May 18, 1980 eruption of Mt. St. Helens. The lahar carried the boulder to its present location. Source: L. Topinka, U. S. Geological Survey (1980), Public Domain. Image source.[/caption] Lahars can also happen when there is no volcanic eruption, because composite volcanoes tend to be weak and easily eroded. In October 1998, category 5 hurricane Mitch slammed into the coast of Central America. Damage was extensive and 19,000 people died. Fatalities were largely because of mudflows and debris flows triggered by intense rainfall , with some regions receiving almost 2 m of rain over a few days. At Casita Volcano in Nicaragua, the heavy rains weakened rock and volcanic debris on the upper slopes, resulting in a debris flow that rapidly built in volume as it raced down the steep slope. It struck the towns of El Porvenir and Rolando Rodriguez killing more than 2,000 people. Both towns were newly buit, but without planning approval in an area that was known to be at risk of lahars.

Sector Collapse and Debris Avalanche

In the context of volcanoes, sector collapse or flank collapse is the catastrophic failure of a significant part of an existing volcano, creating a large debris avalanche. This hazard was first recognized with the failure of the north side of Mt. St. Helens immediately prior to the large eruption on May 18, 1980. In the weeks before the eruption, a large bulge had formed on the side of the volcano (Figure 11.41) as magma moved from depth into a magma chamber within the mountain itself. Early on the morning of May 18, a moderate earthquake struck and destabilized the bulge, leading to Earth’s largest observed landslide in historical times. The failure of this part of the volcano exposed the underlying magma chamber, causing it to explode sideways. This in turn exposed the conduit leading to the magma chamber below, resulting in a Plinian eruption lasting nine hours. [caption id="attachment_451" align="aligncenter" width="550"] Figure 11.41 Bulge forming on the north side of Mt. St. Helens, April 27 1980. Source: P. Lipman, U. S. Geological Survey (1980), Public Domain. Image source.[/caption]

Practice with Types of Volcanic Hazards

Lava flows are not the volcanic hazard with the greatest impact on lives and infrastructure. Fill in the words to complete these descriptions of other volcanic hazards. Volcanic can be toxic and cause respiratory distress. It can also react with (hint: carbon dioxide or water?) in the atmosphere to form aerosols that cause (hint: warming or cooling?) of the climate. Fine-grained tephra called can accumulate on structures and cause the structures to collapse. If an eruption column collapses, a can occur, where tephra travels rapidly down the mountain on a cushion of hot gas. If volcanic debris is mixed with water, either during an eruption or afterward, a flow of mud and debris called a can occur. Pressure and deformation of a volcano can cause the mountain to come crashing down in a catastrophic failure called a . To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="136"]

11.7 Monitoring Volcanoes and Predicting Eruptions

kqzheng — Wed, 20 Sep 2017 00:56:43 +0000

Six Important Signs of a Pending Eruption

In 2005, U. S. Geological Survey geologist Chris Newhall made a list of the six most important signs of an imminent volcanic eruption. They are:

Gas leaks — the release of gases (mostly H₂O, CO₂, and SO₂) from the magma into the atmosphere through cracks in the overlying rock
Bulging — the deformation of part of the volcano, indicating that a magma chamber at depth is swelling or becoming more pressurized
Seismicity — many (hundreds to thousands) of small earthquakes, indicating that magma is on the move. The quakes may be the result of the magma forcing the surrounding rocks to crack, or a harmonic vibration that is evidence of magmatic fluids moving underground.
Seismicity ceases — a sudden decrease in the rate of earthquake activity. This may indicate that magma has stalled, and that something is about to give way
Big bump — a pronounced bulge on the side of the volcano (like the one at Mt. St. Helens in 1980), which may indicate that magma has moved close to surface
Steam — steam eruptions (phreatic eruptions) that happen when magma near the surface heats groundwater to the boiling point. The water eventually explodes into steam, sending fragments of the overlying rock far into the air.

With these signs in mind, we can make a list of the equipment we should have and the actions we can take to monitor a volcano and predict when it might erupt.

Methods for Monitoring and Predicting Volcanic Eruptions

Assessing Seismicity

The simplest and cheapest way to monitor a volcano is with seismometers, instruments that detect vibration. In an area with several volcanoes that have the potential to erupt (e.g., the Squamish-Pemberton area), a few well-placed seismometers can provide an early warning that something is changing beneath one of the volcanoes. There are currently enough seismometers in the Lower Mainland and on Vancouver Island to provide this information. You can view a map of Canadian National Seismograph Network here: CN Station Book Index. If there is seismic evidence that a volcano is coming to life, more seismometers should be placed in locations within a few tens of kilometres of the source of the activity (Figure 11.42). This will allow geologists to determine the exact location and depth of the seismic activity so that they can see where the magma is moving. [caption id="attachment_454" align="aligncenter" width="550"] Figure 11.42 A seismometer installed in 2007 in the vicinity of the Nazco Cone, BC. Source: Cathie Hickson (n.d.), used with permission.[/caption]

Using Earthquakes to Monitor Volcanoes: Interactive Video Watch the video and answer the questions that pop up. [h5p id="137"]

Detecting Gases

Water vapour quickly turns into clouds of liquid water droplets and is relatively easy to detect just by looking, but CO₂ and SO₂ are not as obvious. It’s important to be able to monitor changes in the composition of volcanic gases, and we need instruments to do that. Some can be monitored from a distance (from the ground or even from the air) using infrared devices, but to obtain more accurate data, we need to sample the air and do chemical analysis. This can be achieved with instruments placed on the ground close to the source of the gases, or by collecting samples (Figure 11.43) and analyzing them in a lab. [caption id="attachment_455" align="aligncenter" width="535"] Figure 11.43 A geologist collects a gas sample from Sherman Crater, Mt. Baker, Washington. Gas is drawn through a titanium tube inserted in a fumarole, and collected in a glass vacuum flask. Source: D. Tucker, U. S. Geological Survey (2006), Public Domain. Image source.[/caption]

Measuring Deformation

There are two main ways to measure ground deformation at a volcano. One is known as a tiltmeter, which is a sensitive three-directional level that can detect small changes in the tilt of the ground at a specific location. Another is through the use of GPS (global positioning system) technology (Figure 11.44). GPS is more effective than a tiltmeter because it provides information on how far the ground has actually moved — east-west, north-south, and up-down. [caption id="attachment_456" align="aligncenter" width="569"] Figure 11.44 A GPS unit installed at Hualālai Volcano, Hawaii. The dish-shaped antenna on the right is the GPS receiver. The antenna on the left is for communication with a base station. Source: U. S. Geological Survey (n.d.) Public Domain. view source[/caption]

Putting It All Together

By combining information from these types of sources, along with careful observations made on the ground and from the air, and a thorough knowledge of how volcanoes work, geologists can get a good idea of the potential for a volcano to erupt in the near future (months to weeks, but not days). They can then make recommendations to authorities about the need for evacuations and restricting transportation corridors. Our ability to predict volcanic eruptions has increased dramatically in recent decades because of advances in our understanding of how volcanoes behave and in monitoring technology. Providing that careful work is done, there is no longer a large risk of surprise eruptions, and providing that public warnings are issued and heeded, it is less and less likely that thousands will die from sector collapse, pyroclastic flows, ash falls, or lahars. Indirect hazards are still very real, however, and we can expect the next eruption like the one at Laki in 1783 to take an even greater toll than it did then, especially since there are now roughly eight times as many people on Earth.

Practice: Do You Know the Signs?

Match the words into the correct boxes to complete this description of ways to know a volcano might be about to erupt.If an eruption is about to happen, the amount and composition of released by the volcano might change. Bulging of the volcano is a sign that the is swelling or becoming more pressurized. It could also mean that magma is getting close to the . Earthquakes can mean that magma is moving or causing surrounding rocks to crack. We can tell the difference because movement of magma creates whereas breaking rocks shows up on a seismograph as . Phreatic eruptions occur when magma near the surface heats to the boiling point. Fill-in-the-blank options:

groundwater
volcanic tremors
gas
surface
sudden shocks
magma chamber

To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="138"]

11.8 Volcanoes in Canada

kqzheng — Mon, 25 Sep 2017 01:29:43 +0000

Canada's volcanically active regions are located in British Columbia and the Yukon Territory (Figure 11.45). At least 49 eruptions have occurred within these regions in the last 10,000 years. There are five volcanic regions associated with three types of plate tectonic settings: a subduction zone, a mantle plume, and a continental rift zone. [caption id="attachment_459" align="aligncenter" width="494"] Figure 11.45 Canada's volcanic regions are located in British Columbia and the Yukon Territory. Volcanism is associated with three tectonic settings: the subduction zone along the west coast (Garibaldi Volcanic Belt, Wrangell Volcanic Belt), a continental rift zone (Wells Gray-Clearwater Volcanic Field, Stikine Volcanic Belt), and a mantle plume (Anahim Volcanic Belt). Source: Volcanoes Canada, Canadian Hazards Information Service, Natural Resources Canada (n.d.), Image source. Click for copyright information.[/caption]

Subduction Zone Volcanism: Wrangell and Garibaldi Volcanic Belts

The Wrangell Volcanic Belt is the result of subduction beneath the North American Plate. Volcanoes in the Canadian part of the Wrangell Volcanic Belt erupted between 17.8 and 10.4 million years ago. They were fed by lava that seeped up along a leaky transform fault. Southwestern British Columbia is at the northern end of the Juan de Fuca subduction zone, and part of the Cascade Volcanic Arc that extends south through Washington and Oregon. The Canadian part of the Cascade Arc has had a lower rate and volume of volcanism than U. S. portions. One reason is that the northern part of the Juan de Fuca Plate is subducting more slowly than the rest of the plate, or else has stalled. The Garibaldi Volcanic Belt has several volcanic centres , or regions where volcanism has caused multiple volcanoes to develop (Figure 11.46). [caption id="attachment_460" align="aligncenter" width="576"] Figure 11.46 Volcanic centres within the Garibaldi Volcanic Belt. The most recent eruption was 2,350 years ago at Mt. Meager. Source: Sémhur (2007), CC BY-SA 4.0. Image source.
[/caption] The most recent volcanic activity in this area was 2,350 years ago at Mt. Meager. An explosive eruption similar in magnitude to that of Mt. St. Helens in 1980 spread ash as far east as Alberta. There was also significant volcanic activity at Mt. Price and Mt. Garibaldi approximately 10,000 years ago as glacial ice receded. In both cases, lava and tephra built up against glacial ice. The western side of Mt. Garibaldi failed by sector collapse when the ice melted, leaving rocks unsupported. Eruption beneath glacial ice resulted in the formation of a tuya—a steep-sided, flat-topped volcano—called The Table near Mt. Garibaldi (Figure 11.47). [caption id="attachment_461" align="aligncenter" width="550"] Figure 11.47 The Table, a tuya near Mt. Garibaldi. Tuyas form when volcanoes erupt beneath ice, and their shape is determined by rapid cooling beneath the ice sheet. Source: Andre Charland (2004), CC BY 2.0. Image source.[/caption]

Mantle Plume Volcanism: Anahim Volcanic Belt

The chain of volcanic complexes and cones extending from Milbanke Sound to Nazko Cone is interpreted as being related to a mantle plume currently situated close to the Nazko Cone, just west of Quesnel (Figure 11.48). The North American Plate is moving in a westerly direction at about 2 cm per year with respect to this plume, and the series of now partly eroded shield volcanoes between Nazco and the coast is interpreted to have been formed by the plume as the continent moved over it. [caption id="attachment_462" align="aligncenter" width="1024"] Figure 11.48 Anahim Volcanic Belt, the result of a mantle plume beneath the North American Plate. Source: Sémhur (2007), CC BY-SA 4.0. Image source.
[/caption] The Rainbow Range, which formed at approximately 8 Ma, is the largest of these older volcanoes. It has a diameter of about 30 km and an elevation of 2,495 m (Figure 11.49). The name “Rainbow” refers to the bright colours displayed by some of the volcanic rocks as they weather. [caption id="attachment_463" align="aligncenter" width="550"] Figure 11.49 Tsitsutl, the "painted mountain" within the Rainbow Range of the Anahim Volcanic Belt. The vibrant colours of the Rainbow Range are the result of chemical weathering. Source: Drew Brayshaw (2015), CC BY-NC 2.0. Image source.[/caption]

Rift-Related Volcanism: Wells Gray-Clearwater Volcanic Field and Stikine Volcanic Belt

While British Columbia is not about to split into pieces, two areas of volcanism are related to rifting, or at least to stretching-related fractures that might extend through the crust. These are the Wells Gray-Clearwater volcanic field southeast of Quesnel (Figure 11.50), and the Stikine Volcanic Belt (also called the Northern Cordillera Volcanic Province), which ranges across the northwestern corner of the province. [caption id="attachment_464" align="aligncenter" width="550"] Figure 11.50 Wells Gray-Clearwater Volcanic Field is the result of extension in the crust. Source: Sémhur (2007) CC BY-SA 4.0. Image source.
[/caption] The Stikine Volcanic Belt includes Canada's most recent volcanic eruption, a cinder cone and mafic lava flow that formed around 250 years ago at the Tseax River Cone in the Nass River area north of Terrace. According to Nisga’a oral history, lava overran a village on the Nass River, and 2000 lives were lost. The region is now part of the Anhluut’ukwsim Laxmihl Angwinga’asanskwhl Nisga’a (Nisga’a Memorial Lava Bed Park). The Mount Edziza Volcanic Field near the Stikine River is a large area of lava flows, sulphurous ridges, and cinder cones. The most recent eruption in this area was about 1,000 years ago. While most of the other volcanism in the Edziza region is mafic and involves lava flows and cinder cones, Mt. Edziza itself (Figure 11.51) is a composite volcano with rock compositions ranging from rhyolite to basalt. A possible explanation for the presence of composite volcanism in an area dominated by mafic flows and cinder cones is that there is a magma chamber beneath this area, within which magma differentiation is taking place. [caption id="attachment_465" align="aligncenter" width="550"] Figure 11.51 Mount Edziza, in the Stikine Volcanic Belt, BC, with Eve Cone in the foreground. Source: NASS5518 (2008), CC BY 2.0. Image source.[/caption]

Practice: Why Does Canada Have Volcanoes? [h5p id="139"]

References

Geological Survey of Canada (n.d.) Catalog of Canadian volcanoes: Anahim volcanic belt. https://web.archive.org/web/20080513070258/http://gsc.nrcan.gc.ca/volcanoes/cat/feature_garibaldi_e.php

Geological Survey of Canada (n.d.) Catalog of Canadian Volcanoes: Garibaldi volcanic belt: Garibaldi Lake volcanic field. https://web.archive.org/web/20080513070258/http://gsc.nrcan.gc.ca/volcanoes/cat/feature_garibaldi_e.php

Skulski, T., Francis, D., & Ludden, J. (1991) Arc-transform magmatism in the Wrangell volcanic belt. Geology, (19)1, 11-14. doi:10.1130/0091-7613(1991)019<0011:ATMITW>2.3.CO;2

Trop, J. M., Hart, W. K., Snyder, D., & Idleman, B. (2012). Miocene basin development and volcanism along a strike-slip to flat-slab subduction transition: Stratigraphy, geochemistry, and geochronology of the central Wrangell volcanic belt, Yakutat-North America collision zone. Geosphere, (8)4, 805-834. doi:10.1130/GES00762.1

Volcanoes Canada, Canadian Hazards Information System, Natural Resources Canada (n.d.). Where are Canada's volcanoes? http://chis.nrcan.gc.ca/volcano-volcan/can-vol-en.php

Chapter 11 Summary & Key Term Check

kqzheng — Wed, 27 Sep 2017 04:03:20 +0000

Chapter 11 Main Ideas

11.1 What Is A Volcano?

Volcanoes are places where molten rock escapes to Earth's surface. Some volcanoes are cone-shaped or hill-shaped mountains, and some eruptions happen along fissures. Eruptions are fed by a magma chamber beneath the volcano. Sometimes a volcano collapses into empty space in the magma chamber beneath, forming a caldera.

Practice Again

Basic volcano terminology

Extra Review

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What's the difference between a caldera and a crater? A crater an opening at the top of a volcano where magma escapes. Craters are tens to hundreds of metres in scale. A caldera is a structure that forms when a magma chamber beneath the volcano is emptied, and the unsupported part of the volcano falls in.

11.2 Materials Produced by Volcanic Eruptions

Volcanoes produce gas, lava flows, and debris called tephra. The characteristics of a lava flows depend on whether the lava is thin and runny (mafic with low gas content) or thick and sticky (felsic with high gas content). Tephra is classified according to size. Ash is less than 2 mm in diameter, lapilli is between 2 mm and 64 mm, and blocks and bombs are larger than 64 mm.

Practice Again

11.3 Types of Volcanoes

Cinder cones are relatively small straight-sided volcanoes that are composed mostly of mafic rock fragments. Composite volcanoes consist of alternating layers of lava flows and tephra. They tend to be intermediate to felsic in composition, and get steeper toward the top. Shield volcanoes are broad, low, hill-like volcanoes that form from layers of low-viscosity mafic lava.

Practice Again

Types of volcanoes

Extra Review

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The three types of volcanoes—composite volcanoes, cinder cones, and shield volcanoes—are different because of the volcanic products that make them up. What volcanic products make up each of these? Shield volcanoes are built of thin, runny lava flows. Composite volcanoes are built of lava flows interspersed with volcanic debris (tephra), ranging from ash to blocks and bombs. Cinder cones are almost entirely tephra.

11.4 Types of Volcanic Eruptions

Volcanic eruptions can be classified according to how explosive they are, and how high into the atmosphere they blast material. Hawai'ian eruptions are relatively gentle effusive eruptions of low-viscosity mafic lava, and form shield volcanoes. Strombolian eruptions are more vigorous eruptions of mafic tephra. They blast material hundreds of metres into the air. The tephra falls out of the atmosphere to form a cinder cone. Vulcanian eruptions are explosive eruptions of intermediate to felsic composition lava, producing pyroclastic flows and eruptive columns from 5 to 10 km high. Plinian eruptions are highly explosive eruptions of felsic lava, and can produce eruption columns up to 45 km high. Both Vulcanian and Plinian eruptions are associated with composite volcanoes. Hydrovolcanic eruptions are the explosive result of magma or lava interacting with water, and rapidly changing the water to steam.

Practice Again

Different types of volcanic eruptions

Extra Review

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What determines whether a volcano will erupt effusively (as in a Hawai’ian eruption) or explosively (as in a Vulcanian or Plinian eruption)? Does the same rule apply to hydrovolcanic eruptions? The composition of magma will determine how explosive a volcano will be. Effusive eruptions happen when magma is mafic in composition, because basaltic lava flows easily and has less gas to build pressure. Explosive Vulcanian and Plinian eruptions are the result of intermediate to felsic magmas with a high gas content. Pressure from gas combined with the stickiness of the lava causes explosive eruptions. Hydrovolcanic eruptions are different because the explosiveness comes from interacting with water. Lava of any composition can cause an explosion when encountering water because it can turn water to steam almost instantly.

11.5 Plate Tectonics and Volcanism

Volcanism is closely related to plate tectonics. Most volcanoes are associated with convergent plate boundaries (at subduction zones), where magma is formed when water from a subducting plate acts as a flux to lower the melting temperature of the adjacent mantle rock. Volcanic activity also occurs at divergent boundaries and areas of continental rifting. At divergent boundaries magma forms because of decompression melting. Decompression melting also takes place within a mantle plume. In ocean-continent collision zones, and in continental rift zones, magma compositions—and thus the nature of volcanism—can be impacted by conduction melting of surrounding non-mantle rocks.

Practice Again

Types of volcanism in different plate tectonic settings

Extra Review

Plate tectonic setting can influence the composition of lava erupting from a volcano, and thus the type of volcano and type of volcanic eruption. Explain how tectonic setting would influence the type of volcano and eruption for the following:

A composite volcano undergoes a Plinian eruption along a subduction zone where oceanic crust is colliding with continental crust.
A hotspot shield volcano in the middle of a plate of oceanic lithosphere undergoes a Hawai’ian eruption.

To check your answers, navigate to the below link to view the interactive version of this activity.

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11.6 Volcanic Hazards

Most direct volcanic hazards are related to volcanoes that erupt explosively, especially composite volcanoes. Pyroclastic flows, some as hot as 1000 ˚C, can move at hundreds of km/h and will kill anything in the way. Lahars—volcano-related mudflows—can be large enough to destroy entire towns. Lava flows are also destructive, but tend to move slowly enough to permit people to get to safety. Indirect hazards claim far more lives than direct hazards, and include famine related to volcanically-induced climate cooling.

11.7 Monitoring Volcanoes and Predicting Eruptions

Clues that a volcanic eruption might soon occur include earthquakes, a change in the type and amount of gases released, and changes in the shape of the volcano as magma moves within it. Volcanoes are monitored using seismometers to detect earthquakes, volcanic gases are sampled and analyzed, and instruments are used to detect deformation of the volcano. These tools make it possible to assess the hazard posed by a given volcano, and the risk of eruption.

11.8 Volcanoes in British Columbia

British Columbia and the Yukon Territory include examples of volcanoes that form as a result of fluid-induced melting along a subduction zone (the Wrangell and Garibaldi volcanic belts), as a result of decompression where the crust is thinning and stretching (Stikine Volcanic Belt and Wells Gray-Clearwater Volcanic Field), and because of mantle plume activity (Anahim Volcanic Belt).

Key Term Check

What key term from Chapter 11 is each card describing? Turn the card to check your answer. [h5p id="144"]

12.1 What is an Earthquake?

kqzheng — Thu, 09 Nov 2017 21:54:20 +0000

Earthquake Shaking Comes from Elastic Deformation

Earthquakes occur when rock ruptures (breaks), causing rocks on one side of a fault to move relative to the rocks on the other side. Although motion along a fault is part of what happens when an earthquake occurs, rocks grinding past each other isn't what creates the shaking. In fact, it could be said that the earthquake happens after rocks have undergone most of the displacement. Consider this: if rocks slide a few centimetres or even metres along a fault, would that motion alone explain the incredible damage caused by some earthquakes? If you were in a car that suddenly accelerated then stopped, you'd feel a jolt. But earthquakes aren't a single jolt. Buildings can swing back and forth until they shake themselves to pieces, train tracks can buckle and twist into s-shapes, and roads can roll up and down like waves on the ocean. During an earthquake, rock is not only slipping—it's also vibrating like a plucked guitar string. Rocks might seem rigid, but when stress is applied, they may stretch. If there hasn't been too much stretching, a rock will snap back to its original shape once the stress is removed. Deformation that is reversible is called elastic deformation. Rocks that are stressed beyond their ability to stretch can rupture, allowing the rest of the rock to snap back to its original shape. The snapping back—called elastic rebound—causes the rock to vibrate, and this is what causes the shaking during an earthquake. Figure 12.3 (top) shows this sequence of events. Stress is applied to a rock and deforms it. The deformed rock ruptures, forming a fault. After rupturing, the rock above and below the fault snaps back to the shape it had before deformation. [caption id="attachment_473" align="aligncenter" width="650"] Figure 12.3 Elastic deformation, rupture, and elastic rebound. Top: Stress applied to a rock causes it to deform by stretching. If the stress becomes too much for the rock, it ruptures, forming a fault. The rock snaps back to its original shape in a process called elastic rebound. Bottom: On an existing fault, asperities keep rocks on either side of the fault from sliding. Stress deforms the rock until the asperities break, releasing the stress, and causing the rocks to spring back to their original shape. Source: Karla Panchuk (2017), CC BY 4.0. Modified after Steven Earle (2015), CC BY 4.0. View original.[/caption] Ruptures can also occur along pre-existing faults (Figure 12.3, bottom). The rocks on either side of the fault are locked together because bumps along the fault, called asperities, prevent the rocks from moving relative to each other. When the stress is great enough to break the asperities, the rocks on either side of the fault can slide again. While the rocks are locked together, stress can cause elastic deformation. When asperities break and release the stress, the rocks undergo elastic rebound and return to their original shape.

Rupture Surfaces Are Where the Action Happens

Images like 12.3 are useful for illustrating elastic deformation and rupture, but they can be misleading. The rupture that happens doesn't necessarily break the rock through and through. Rupture and displacement only happen along a subsection of a fault, called the rupture surface, although the rupture surface could extend 10s to 100s of kilometres. In Figure 12.4, the rupture surface is the dark pink patch. It takes up only a part of the fault plane (lighter pink). The fault plane represents the surface where the fault exists, and where ruptures have happened in the past. Although the fault plane is drawn as being flat in Figure 12.4, faults are not actually perfectly flat. The location on the fault plane where the rupture happens is called the hypocentre or focus of the earthquake (Figure 12.4, right). The location on Earth's surface immediately above the hypocentre is the epicentre of the earthquake. [caption id="attachment_474" align="aligncenter" width="1024"] Figure 12.4 Rupture surface (dark pink), on a fault plane (light pink). Left: In this example, the near side of the fault is moving to the left, and the lengths of the arrows within the rupture surface represent relative amounts of displacement. Coloured arrows represent propagation of failure on a rupture surface, starting at the dark blue heavy arrow and propagating left (green arrows) then right (yellow arrows). Right: An earthquake's hypocentre (or focus) is the location on the fault plane where the rupture happens. Its epicentre (red star) is the location on the surface above the hypocentre. Source: Left: Steven Earle (2015), CC BY 4.0. Image source. Right: Karla Panchuk (2017) CC BY 4.0.
[/caption] Within the rupture surface, the amount of displacement varies. In Figure 12.4, the larger arrows indicate where there has been more displacement, and the smaller arrows where there has been less. Beyond the edge of the rupture surface there is no displacement at all. Notice that this particular rupture surface doesn't extend to the land surface of the diagram. The size of a rupture surface and the amount of displacement along it will depend on a number of factors, including the type and strength of the rock, and the degree to which the rock was stressed beforehand. The magnitude of an earthquake will depend on the size of the rupture surface and the amount of displacement. A rupture doesn’t occur all at once along a rupture surface. It starts at a single point and spreads rapidly from there. Figure 12.4 illustrates a case where rupturing starts at the heavy blue arrow in the middle, then continues through the lighter blue arrows. The rupture spreads to the left side (green arrows), then the right (yellow arrows). Depending on the extent of the rupture surface, the propagation of failures (incremental ruptures contributing to making the final rupture surface) from the point of initiation is typically completed within seconds to several tens of seconds. The initiation point isn’t necessarily in the centre of the rupture surface; it may be close to one end, near the top, or near the bottom.

Concept Check: What Is an Earthquake?

Write the words into the correct blank. When rocks are stretched, then snap back to their original shape in a process called . They vibrate like a plucked guitar string, and this is what we feel as an . Rocks break on a surface that takes up only part of the plane. (Earthquakes can happen in many locations on this plane over time.) The location where slip happens on the fault plane within the Earth is the . The is the location on the surface above actual slip. Fill-in-the-blank options:

elastic rebound
fault
earthquake
epicentre
rupture
hypocentre or focus

To check your answers, navigate to the below link to view the interactive version of this activity.

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Shifting Stress Causes Foreshocks and Aftershocks

Earthquakes don't usually occur in isolation. There's often a sequence of smaller earthquakes before a larger one, and then progressively smaller earthquakes after. The largest earthquake in the series is the mainshock. The smaller ones that come before are foreshocks, and the smaller ones that come after are aftershocks. These descriptions are relative: for example, the strongest earthquake in a series is classified as the mainshock, but if another even bigger one comes after it, the bigger one is called the mainshock, and the earlier one is reclassified as an foreshock. Aftershocks and foreshocks are a larger-scale version of what happens when the failure on a rupture surface propagates from one part of that surface to another. The rupture illustrated in Figure 12.4 reduced stress in one area, but in doing so, transferred stress to others (Figure 12.5). Imagine your favourite action hero suspended from a frayed rope that's breaking strand by strand. When a strand breaks, the tension on that strand is released, but the remaining strands must still hold up the hero. If another strand breaks under the increased burden, the remaining strands have an even greater burden than before. (Ideally the hero will escape before the entire rope gives out...you can imagine that now, so our story has a pleasant outcome.) In the same way that stress causes one strand after another to fail, a rupture can trigger subsequent ruptures nearby. [caption id="attachment_475" align="aligncenter" width="550"] Figure 12.5 Stress changes related to an earthquake. Stress decreases in the area of the rupture surface, but increases on adjacent parts of the fault. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] Numerous aftershocks were associated with the magnitude 7.8 earthquake that struck Haida Gwaii in October of 2012 (Figure 12.6; mainshock in red, aftershocks in grey). Some of the stress released by the mainshock was transferred to other nearby parts of the fault, and contributed to a cascade of smaller ruptures, including along other nearby faults. Notice how the aftershocks from the Haida Gwaii earthquake are scattered rather than located only on the main faults shown in the image. [caption id="attachment_476" align="aligncenter" width="650"] Figure 12.6 Magnitude 7.8 Haida Gwaii earthquake and aftershocks. Mainshock (red circle marks the epicentre) occurred on October 28th, 2012. Aftershocks are for the period from October 28th to November 10th of 2012. Although the epicentre is near a transform boundary, the rupture was influenced more by compression related to the subduction zone. Source: Karla Panchuk (2017) CC BY 4.0. Base map with epicentres from the U. S. Geological Survey Latest Earthquakes tool view interactive map. Subduction zone after Wang et al. (2015). Click for more attributions.[/caption] The effects of stress transfer may not show immediately. Aftershocks can be delayed for hours, days, weeks, or even years. Because stress transfer affects a region, not just a single fault, and because there can be delays between the event that transferred stress and the one that was triggered by the transfer, it can sometimes be hard to be know whether one earthquake is actually associated with another, and whether a foreshock or aftershock should be assigned to a particular mainshock.

Concept Check: Foreshocks, Mainshocks, & Aftershocks

[caption id="attachment_1669" align="aligncenter" width="659"] Circle sizes indicate the magnitude of earthquakes. Bigger circles are bigger earthquakes.[/caption] Fill in the correct letters to indicate which earthquake is a foreshock, mainshock, and aftershock. Earthakes and are foreshocks. Earthquake is the mainshock. Earthquake is an aftershock. To check your answers, navigate to the below link to view the interactive version of this activity.

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Episodic Tremor and Slip

Episodic tremor and slip (ETS) is periodic slow sliding along part of a subduction boundary. It doesn't produce recognizable earthquakes, but does produce seismic tremor (observed as rapid seismic vibrations on instruments). It was first discovered on the Vancouver Island part of the Cascadia subduction zone by Geological Survey of Canada geologists Herb Dragert and Gary Rogers.[footnote]Rogers, G. and Dragert, H. (2003). Episodic tremor and slip on the Cascadia subduction zone: The chatter of silent slip. Science, 300, 1942-1943.[/footnote] The boundary between the subducting Juan de Fuca plate and the North America plate can be divided into three segments (Figure 12.7). The cold upper part of the boundary is the locked zone. There the plates are stuck together for long periods of time. When slip does occur, it generates very large earthquakes. The last time the locked zone along Canada's west coast slipped was January 26, 1700. It caused an earthquake of magnitude 9. [caption id="attachment_477" align="aligncenter" width="650"] Figure 12.7 Episodic tremor and slip along the Cascadia subduction zone. The Juan de Fuca plate is locked to the North American plate at the top of the subduction zone, but lower down it is slipping continuously. In the intermediate (ETS) zone, the plate alternately sticks and slips on a regular schedule. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] The warm lower part of the boundary, called the continuous slip zone, is sliding continuously because the warm rock is weaker. The central part of the boundary, the ETS zone, isn’t cold enough to be stuck, but isn’t warm enough to slide continuously. Instead it slips episodically approximately every 14 months for about 2 weeks, moving a few centimetres each time. It might seem that periodic slip along this part of the plate helps to reduce tension, and thus reduce the risk of a large earthquake. In fact, the opposite is likely the case. The movement along the ETS part of the plate boundary transfers stress to the adjacent locked part of the plate. During the two-week ETS period, the transfer of stress means an increased chance of a large earthquake. Since 2003, ETS processes have also been observed in subduction zones in Mexico and Japan.

Additional Resources

IRIS Teachable Moment slides for the October 2012 Haida Gwaii earthquake

References

Wang, K., Jiangheng, H., Schulzeck, F., Hyndman, R. D., and Riedel, M. (2015). Thermal condition of the 27 October 2012 Mw 7.8 Haida Gwaii subduction earthquake at the obliquely convergent Queen Charlotte Margin. Bulletin of the Seismological Society of America, 105(2B), 1290–1300. doi: 10.1785/0120140183

14.1 The Hydrological Cycle

kqzheng — Mon, 20 Nov 2017 19:54:03 +0000

Water is constantly on the move. It is evaporated from the oceans, lakes, streams, the surface of the land, and from plants (transpiration) by solar energy (Figure 14.2). It's transported in its gaseous form through the atmosphere by the wind and condenses to form clouds of water droplets or ice crystals. It falls to the Earth’s surface as rain or snow and flows through streams, into lakes, and eventually back to the oceans. Water on the surface and in streams and lakes infiltrates the ground to become groundwater. Groundwater slowly moves through soils, surficial materials, and pores and cracks in the rock. The groundwater flow paths can intersect with the surface and the water can then move back into streams, lakes and oceans.

[caption id="attachment_568" align="aligncenter" width="1024"] Figure 14.2 The various components of the water cycle. Black or white text indicates the movement or transfer of water from one reservoir to another. Yellow text indicates the storage of water. Source: Steven Earle (2015), CC BY-SA 3.0. Image source. Modified after Ingwik (2010), CC-BY-SA 3.0. Image source.[/caption]

Water is stored in various reservoirs as it moves across and through the Earth. A reservoir is a space that stores water. It can be a space we can easily visualize (such as a lake) or a space that's more difficult to visualize, such as the atmosphere or the groundwater in a region. The largest reservoir is the ocean, accounting for 97% of the total volume of water on Earth (Figure 14.3). Ocean water is salty, but the remaining 3% of water on Earth is fresh water. Two-thirds of our fresh water is stored in the ground and one-third is stored in ice. The remaining fresh water (about 0.03% of the total) is stored in lakes, streams, vegetation, and the atmosphere.

[caption id="attachment_569" align="aligncenter" width="254"] Figure 14.3 Earth's water reservoirs. Glacial ice is represented by the white band, groundwater the red band, and surface water the very thin blue band at the top. The 0.001% stored in the atmosphere is not shown. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]To put these percentages in perspective, we can compare a 1 litre container of water to the entirety of the Earth’s water supply (Figure 14.4). We start by almost filling the container with 970 ml of water and 34 g of salt, to simulate all the sea water on Earth. Then we add one regular-sized (~20 ml) ice cube to represent glacial ice and two teaspoons (~10 ml) of groundwater. All of the water that we see around us in lakes and streams and in the atmosphere can be represented by adding three more drops of water from an eyedropper. [caption id="attachment_796" align="aligncenter" width="186"] Figure 14.4 All of Earth’s water in a 1 litre container: three drops represent all fresh water in lakes, streams, and wetlands, plus all atmospheric water in the atmosphere. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Although the water in the atmosphere is only a small proportion of the total water on Earth, the volume is still very large. At any given time, there is the equivalent of approximately 13,000 km³ of water in the air in the form of water vapour and water droplets in clouds. Water is evaporated from the oceans, vegetation, and lakes at a rate of 1,580 km³ per day, and each day nearly the same volume falls back as rain and snow over the oceans and land. Most of the precipitation that falls onto land returns to the ocean in the form of stream flow (117 km³/day) and groundwater flow (6 km³/day). Most of the rest of this chapter is about this 117 km³/day of streamflow.

How Long Does Water Stay in the Atmosphere and Ocean?

Grab your calculator and math this out!Don't worry: this just involves your ÷ button, and if you're not sure what to do, click on the tips. The residence time of a water molecule in the atmosphere (or any of the other reservoirs) can be estimated by dividing the total amount of water in the reservoir by the rate at which it is removed. For the atmosphere, we know that the reservoir size is 13,000 km³, and the rate is 1,580 km³/day. This means that on average, a water molecule stays in the atmosphere for only about (hint: enter 13000 ÷ 1580 into your calculator to get a time in days) days! (Round to the nearest day.) “Average” needs to be emphasized here because some molecules remain in the air for only a few hours, while others may remain in the air for weeks. For the ocean, the volume of the oceans 1,338,000,000 km³ and the rate of removal of water from the oceans is approximately the same as the atmosphere (1,580 km³/day). The average residence time of a water molecule in the ocean is (hint: enter 1338000000 ÷ 365 into your calculator to get a time in days and convert it to years)years. (Round to the nearest year.) To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="159"]

14.2 Drainage Basins

kqzheng — Mon, 20 Nov 2017 19:54:06 +0000

A stream is a body of flowing surface water of any size, ranging from a tiny trickle to a mighty river. The area from which the water flows to form a stream is known as its drainage basin or watershed. All of the precipitation (rain or snow) that falls within a drainage basin eventually flows into its stream, unless some of this water is able to cross into an adjacent drainage basin via groundwater flow. An example of a drainage basin is shown in Figure 14.5. [caption id="attachment_572" align="aligncenter" width="400"] Figure 14.5 A schematic diagram of the drainage basin of Cawston Creek near Keremeos, BC. The blue line shows the extent of the drainage basin. The dashed red line is the drainage basin of one of its tributaries. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

An important characteristic of streams is their gradient: the rate of change in elevation with distance along the stream. A steep gradient has a rapid change in elevation with horizontal distance, and a shallow gradient has a slow change in elevation with horizontal distance. Cawston Creek drainage basin in BC is approximately 25 km² and is a typical small drainage basin within a very steep glaciated valley. As shown in Figure 14.6, the upper and middle parts of the creek have steep gradients averaging about 200 m/km but ranging from 100 to 350 m/km, and the lower part, within the valley of the Similkameen River, is relatively flat at <5 m/km.

The shape of the valley has changed through time to result in the shape we see now. First, there was tectonic uplift (related to tectonic plate convergence). Then the landscape changed due to stream erosion and mass wasting (landslides). This was followed by several episodes of glacial erosion. Finally, there was post-glacial stream erosion up to the present time. The lowest elevation of Cawston Creek (275 m, where the creek flows into the Similkameen River) is its base level. Cawston Creek cannot erode below this level unless the Similkameen River erodes deeper into its flood plain (the area that is inundated during a flood). Base level is the elevation where a stream will no longer erode deeper into the bedrock or sediments it flows through, because the elevation of the stream does not drop below this level, and further erosion can only occur if there is an elevation drop to propel the water deeper due to the force of gravity.

The ocean is the ultimate base level, but lakes and other rivers act as base levels for many smaller streams. [caption id="attachment_573" align="aligncenter" width="1024"] Figure 14.6 Profile of the main portion of Cawston Creek near Keremeos, BC. The maximum elevation of the drainage basin is about 1,840 m, near Mount Kobau. The base level is 275 m, at the Similkameen River. As shown, the gradient of the stream can be determined by dividing the change in elevation between any two points (rise) by the distance between those two points (run). Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]The water supply for the Vancouver, BC metropolitan area comes from three large drainage basins on the north shore of Burrard Inlet, as shown in Figure 14.7. This map illustrates the concept of a drainage basin divide. The boundary between two drainage basins is the ridge of land between them. A drop of rain falling on the boundary between the Capilano and Seymour drainage basins, for example, could flow into either basin. Rain falling on the Capilano basin side cannot flow into the Seymour drainage basin, because of the drainage basin divide. [caption id="attachment_574" align="aligncenter" width="550"] Figure 14.7 The three drainage basins that supply water to the metropolitan Vancouver, BC area. Source: Alaidlaw (2016), CC BY-SA 2.0. Image source.[/caption]

The pattern of tributaries within a drainage basin depends largely upon the type of underlying rock, and on structures within that rock such as folds, fractures, and faults. Three types of drainage patterns are illustrated in Figure 14.8. Dendritic patterns, which are by far the most common, develop in areas where the rock (or unconsolidated material) beneath the stream does not have structures that control the stream flow patterns such as folds and joints; the materials can be eroded by the stream equally easily in all directions. Most areas of British Columbia have dendritic patterns, as do most areas of the prairies and the Canadian Shield.

Trellis drainage patterns typically develop where sedimentary rocks have been folded or tilted, and then eroded to varying degrees depending on their resistance to erosion. The Rocky Mountains of BC and AB have some fine examples of trellis drainage.

Rectangular patterns develop in areas that have very little topography and a system of bedding planes, fractures, or faults that form a rectangular network. Rectangular drainage patterns are rare in Canada. [caption id="attachment_575" align="aligncenter" width="550"] Figure 14.8 Typical dendritic, trellis, and rectangular stream drainage patterns. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]In many parts of Canada, especially relatively flat areas with thick glacial sediments, and throughout much of Canadian Shield in eastern and central Canada, drainage patterns are chaotic , also known as deranged (Figure 14.9, left). Lakes and wetlands are common in this type of environment. Radial drainage (Figure 14.9, right) is a pattern that forms around isolated mountains (such as volcanoes) or hills. The individual streams that radiate out from the hill typically have dendritic drainage patterns. [caption id="attachment_576" align="aligncenter" width="400"] Figure 14.9 Left: a typical deranged pattern; right: a typical radial drainage pattern developed around a mountain or hill. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Practice with Drainage Networks [h5p id="160"]

The process of a stream eroding downward into bedrock is called downcutting. Over geological time, and during tectonic quiescence, a stream will erode its drainage basin into a smooth profile similar to that shown in Figure 14.10. This is called a graded stream. Graded streams are steepest in their headwaters and their gradient gradually decreases toward their mouths. Ungraded streams are still in the process of rapidly eroding and downcutting their drainage basin, they have steep sections at various points, and typically have rapids and waterfalls at numerous locations along their lengths (e.g., Cawston Creek, Figure 14.6). [caption id="attachment_577" align="aligncenter" width="1024"] Figure 14.10 The topographic profile of a typical graded stream. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

A graded stream can become ungraded if there is renewed tectonic uplift, or if there is a change in the base level. Base level changes can occur due to tectonic uplift or some other reason such as construction of a dam downstream. As stated earlier, the base level of Cawston Creek is defined by the level of the Similkameen River, but this can change, and has done so in the past. Figure 14.11 shows the valley of the Similkameen River in the Keremeos area. The river channel is just beyond the row of trees. The green field in the distance is underlain by material eroded from the hills behind and deposited by a small creek (not Cawston Creek) adjacent to the Similkameen River when its level was higher than it is now. Some time in the past several centuries, the Similkameen River eroded down through these deposits (forming the steep bank on the other side of the river), and the base level of the small creek was lowered by about 10 m. Over the next few centuries, this creek will erode through the sediments and eventually become graded again.

[caption id="attachment_578" align="aligncenter" width="550"] Figure 14.11 An example of a change in the base level of a small stream that flows into the Similkameen River near Keremeos. The previous base level was near the top of the sandy bank. The current base level is the Similkameen River, located on the far side of the line of trees. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Another example of a change in base level can be seen along the Juan de Fuca Trail on southwestern Vancouver Island. As shown in Figure 14.12, many of the small streams along this part of the coast flow into the ocean as waterfalls. It is evident that the land in this area has risen by about 5 m in the past few thousand years, probably in response to deglaciation. The streams that used to flow directly into the ocean now have a lot of downcutting to do before they will be a graded stream again.

[caption id="attachment_579" align="aligncenter" width="650"] Figure 14.12 Two streams with a lowered base level on the Juan de Fuca Trail, southwestern Vancouver Island. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] Sediments accumulate within the channel and flood plain of a stream, and then, if the base level changes, or if there is less sediment supplied to the stream to deposit, the stream may cut down through these existing sediments to form terraces. A terrace on the Similkameen River is shown in Figure 14.11 and terraces on the Fraser River are shown in Figure 14 .13. [caption id="attachment_651" align="aligncenter" width="550"] Figure 14.13 Terraces on the Fraser River north of Lillooet, BC (above the river on the left-hand side of the image). Source: China Crisis (2007), CC BY-SA 3.0. Image source.[/caption] In the late 19th century, American geologist William Davis proposed that streams and the surrounding terrain develop in a cycle of erosion (Figure 14.14). Following tectonic uplift , the stream patterns are immature. Streams erode quickly, developing deep V-shaped valleys that tend to follow relatively straight paths. Gradients are high, and profiles are ungraded. Rapids and waterfalls are common. As the landscape matures, the streams erode wider valleys and deposited thick sediment layers. Even after maturity, gradients are slowly reduced and grading increases. In old age, streams are surrounded by rolling hills, and they occupy wide sediment-filled valleys. Meandering patterns are common, and erosion now is focused towards the channel walls, with little downcutting. [caption id="attachment_581" align="aligncenter" width="600"] Figure 14.14 A depiction of the Davis cycle of erosion: a: initial stage, b: youthful stage, c: mature stage, and d: old age . Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]Davis’s work was done long before the idea of plate tectonics, and he was also not familiar with the impacts of glacial erosion on streams and their environments. While some parts of his idea are out of date, it is still a useful way to understand streams and their evolution. Plate tectonic activity and other processes such as isostatic rebound after glaciation results in uplift that alters stream gradients, so streams are constantly adjusting due to these changing conditions. It would be relatively rare to find a stream that is able to mature through all of these stages without interruption.

14.3 Stream Erosion and Deposition

kqzheng — Mon, 20 Nov 2017 19:54:07 +0000

Stream Velocity Depends on the Shape and Size of the Channel

Flowing water is a very important mechanism for both erosion and deposition. Water flow in a stream is primarily related to the stream’s gradient, but it's also controlled by the geometry of the stream channel (FIgure 14.15). Water flow velocity decreases due to friction along the stream bed. The stream is thus slowest at the bottom and edges and fastest near the surface and in the middle of the stream (where there is the least amount of friction). The velocity just below the surface of the water is typically a little higher than right at the surface because of friction between the water and the air. On a curved section of a stream, flow is fastest on the outside of the curve and slowest on the inside of the curve.

[caption id="attachment_584" align="aligncenter" width="650"] Figure 14.15 The relative velocity of stream flow depending on whether the stream channel is straight or curved (left). (Right) it is also dependent on the water depth. The length of each of the arrows indicates the relative velocity of the stream at that position in the channel. Shorter arrows mean slower flow. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Another important factor influencing stream-water velocity is the discharge, or volume of water passing a point in a unit of time (e.g., m³/second). Water levels rise during a flood and due to the higher discharge of water the stream flow velocity increases. The higher discharge also increases the cross-sectional area of the stream, so it fills up the channel. In a flood it may overflow the banks. Another factor that affects stream-water velocity is the size of sediments on the stream bed. Large particles tend to slow the flow more than small ones.

Sediment Transport Depends on Stream Velocity and Turbulence

[caption id="attachment_585" align="alignright" width="152"]
Figure 14.16 How quickly a grain settles to the bottom of a stream depends on its mass and the friction between the grain and the water. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

If you drop a piece of gravel into a glass of water, it will sink quickly to the bottom. If you drop a grain of sand into the same glass, it will sink more slowly. A grain of silt will take longer yet to get to the bottom, and a particle of fine clay will take a long time settle out. The rate of settling is determined by the balance between gravity and friction (Figure 14.16). Friction between the grain and water will slow the grain's fall.

One of the key principles of sedimentary geology is that the ability of a moving medium (air or water) to move sedimentary particles and keep them moving is dependent on the velocity of flow. The faster the medium flows, the larger the particles it can move. As you probably know from intuition and from experience, streams that flow rapidly tend to be turbulent (flow paths are chaotic and the water surface appears rough) and the water may be muddy. In contrast, streams that flow more slowly tend to have laminar flow (straight-line flow and a smooth water surface) and clearer water. Turbulent flow is more effective than laminar flow at keeping sediments suspended within the water.

Particles within a stream are transported in different ways depending on their size (Figure 14.17). Large particles which rest on the stream bed are known as the bedload. The bedload may only be transported when the flow rate is rapid and under flood conditions. They are transported by saltation (bouncing along, and colliding with other particles) and by traction (being pushed along by the force of the flow).

[caption id="attachment_586" align="aligncenter" width="550"] Figure 14.17 Modes of transportation of sediments and dissolved ions (represented by red dots with + and – signs) in a stream. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Smaller particles may rest on the bottom occasionally, where they can be transported by saltation and traction, but they can also be held in suspension in the flowing water (the suspended load), especially at higher flow velocities.

Stream water also has a dissolved load, which represents (on average) about 15% of the mass of material transported, and includes ions such as calcium (Ca⁺²) and chloride (Cl^-) in solution. The solubility of these ions is not affected by flow velocity.

If you look at a typical stream, there are always some sediments being deposited, some staying where they are, and some being eroded and transported. This changes over time as the discharge of the river changes in response to changing weather conditions.

The Hjulström-Sundborg Diagram Summarizes What Happens to Grains of Different Sizes at Different Stream Velocities

The faster water is flowing, the larger the particles that can be kept in suspension and transported within the flowing water. However, as Swedish geographer Filip Hjulström discovered in the 1940s, the relationship between grain size and the likelihood of a grain being eroded, transported, or deposited is not as simple as one might imagine (Figure 14.18). Consider, for example, a 1 mm grain of sand. If it's resting on the bottom of the stream, it will stay there until the flow velocity is high enough to erode it (~20 cm/s). But once it's in suspension, that same 1 mm particle will remain suspended as long as the velocity doesn’t drop below 10 cm/s. For a 10 mm gravel grain, the velocity is 105 cm/s to be eroded from the bed but only 80 cm/s to remain in suspension. [caption id="attachment_810" align="aligncenter" width="550"] Figure 14.18 The Hjulström-Sundborg diagram showing the relationships between particle size and the tendency to be eroded, transported, or deposited, at different current velocities. Source: Steven Earle (2014), CC BY 4.0. Image source.[/caption] On the other hand, a 0.01 mm silt particle only needs a velocity of 0.1 cm/s to remain in suspension, but requires 60 cm/s to be eroded. In other words, a tiny silt grain requires a greater velocity to be eroded than a grain of sand that is 100 times larger! For clay-sized particles, the discrepancy is even greater. In a stream, the most easily eroded particles are small sand grains between 0.2 mm and 0.5 mm. Anything smaller or larger requires a higher water velocity to be eroded and entrained in the flow. The reason for this is that small particles, especially tiny grains of clay, possess a net surface charge, hence experience a strong tendency to stick together, and so are difficult to erode from the stream bed.

It's important to be aware that a stream can both erode and deposit sediments at the same time. At 100 cm/s, for example, silt, sand, and medium gravel will be eroded from the stream bed and transported in suspension, coarse gravel will be transported by saltation and traction, pebbles will be transported by both saltation and traction, and will also be deposited. Cobbles and boulders will remain stationary on the stream bed.

Practice with the Hjulström-Sundborg Diagram [h5p id="161"]

Natural Levees Form Because of Changes in Stream Velocity

A stream typically reaches its greatest velocity when it is close to flooding over its banks. This is known as the bank-full stage, as shown in Figure 14.19. When the flooding stream overtops its banks and occupies the wide area of its flood plain, the water has a much larger area to flow through and the velocity drops dramatically. As water flows from the channel out across the flood plain, it slows down and starts to deposit its sediment load. This forms an elevated bank known as a levee along the edges of the channel. The coarsest and thickest sediments are deposited near the channel banks, with particle size and thickness decreasing as you move further into the flood plain. People also build levees as flood control measures; the idea for this engineered solution to floods came from the naturally-build levees that form during floods. [caption id="attachment_811" align="aligncenter" width="500"] Figure 14.19 The development of natural levees during flooding of a stream. The sediments of the levee become increasingly fine away from the stream channel, and even finer sediments — clay, silt, and very fine sand — are deposited across most of the flood plain. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

14.4 Stream Types

kqzheng — Mon, 20 Nov 2017 19:54:09 +0000

Stream channels can be straight or curved, deep or shallow, cleared or filled with coarse sediments. The cycle of erosion has some influence on the nature of a stream, but there are several other factors that are important.

Youthful streams that are actively downcutting their channels tend to be relatively straight and are typically ungraded (meaning that rapids and waterfalls are common). As shown in Figures 14.1 and 14.20, youthful streams commonly have a step-pool morphology, meaning that the stream consists of a series of pools connected by rapids and waterfalls. They also have steep gradients, and steep and narrow V-shaped valleys. In some cases these valley walls are steep enough to be called canyons.

[caption id="attachment_589" align="aligncenter" width="400"] Figure 14.20 The Cascade Falls area of the Kettle River, near Christina Lake, BC. This stream has a step-pool morphology and a deep bedrock channel. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

In mountainous terrain, such as that in western AB and BC, steep youthful streams typically flow into wide and relatively low-gradient U-shaped glaciated valleys. The youthful streams have high sediment loads, and when they flow into the lower-gradient glacial valleys where the velocity is no longer high enough to carry all of the sediment, braided stream patterns develop, characterized by a series of narrow channels separated by gravel bars (Figure 14.21).

[caption id="attachment_590" align="aligncenter" width="550"] Figure 14.21 The braided channel of the Kicking Horse River at Field, BC. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Braided streams can develop anywhere where there is more sediment than a stream is able to transport. One such environment is in volcanic regions, where explosive eruptions produce large amounts of unconsolidated material that gets washed into streams. The Coldwater River next to Mt. St. Helens in Washington State is a good example of such a braided stream (Figure 14.22).

[caption id="attachment_591" align="aligncenter" width="550"] Figure 14.22 The braided Coldwater River, Mt. St. Helens, Washington. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]A stream that occupies a wide, flat flood plain with a low gradient typically carries only sand-sized and finer sediments and develops a sinuous flow pattern. As you saw in Figure 14.15, when a stream flows around a bend, the main current of the stream flows near the outside portion of the bend. This leads to erosion of the banks on the outside of the bend, and deposition of a point bar on the inside of the bend (Figure 14.23). Over time, the sinuosity of the stream becomes increasingly exaggerated, and the channel migrates throughout its flood plain, forming a meandering pattern. [caption id="attachment_592" align="aligncenter" width="550"] Figure 14.23 The meandering channel of the Bonnell Creek, Nanoose, BC. The stream is flowing toward the viewer. The sand and gravel point bar must have formed when the creek was higher and the flow faster than it was when the photo was taken, as the current stream velocity is too low to carry such coarse sediments. Most erosion and deposition take place during flooding events. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]A well-developed meandering river is shown in Figure 14.24. The meander in the middle of the photo has reached the point where the thin neck of land between two parts of the channel is about to be eroded through. When this happens, an oxbow lake will form. These are small cut off bends from earlier curves in the river; several are visible outside the path of the main stream in Figure 14.24. [caption id="attachment_593" align="aligncenter" width="550"] Figure 14.24 The meandering channel of the Nowitna River, Alaska. Numerous oxbow lakes are present, and another meander cutoff will soon take place. Source: Oliver Kumis (2002), CC-BY-SA 2.0. Image source.[/caption]At the point where a stream enters a body of water such as a lake or the ocean, the flow rates drops dramatically, and sediment is deposited. Over time, as more and more sediments are deposited, the sediments form a distinctive triangular shape (with the bottom broad part of the triangle facing the ocean or lake and the point of the triangle facing upstream). This is called a delta; these are named after the Greek letter delta which is in the shape of a triangle. The Fraser River has created a large delta in BC where the river meets the Strait of Georgia (Figure 14.25). Much of the Fraser delta is very young in geological terms. Shortly after the end of the last glaciation (10,000 years ago), the delta did not extend past New Westminster. Since that time, all of the land that makes up Richmond, Delta, and parts of New Westminster and south Surrey has formed from sediment depositing from the Fraser River. You can see a more detailed description of the Fraser delta on the Geoscape Vancouver website: Vancouver-FraserDelta - CGEN Archive. [caption id="attachment_594" align="aligncenter" width="550"] Figure 14.25 The delta of the Fraser River and the plume of sediment that extends across the Strait of Georgia. The land outlined in red has formed over the past 10,000 years. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Putting It Together [h5p id="162"] [h5p id="163"]

14.5 Flooding

kqzheng — Mon, 20 Nov 2017 19:54:10 +0000

The discharge levels of streams are highly variable depending on the time of year and on variations in the weather from one year to the next. In Canada, most streams show discharge variability similar to that of the Stikine River in northwestern BC, illustrated in Figure 14.26. The Stikine River has its lowest discharge levels in the depths of winter when freezing conditions persist throughout most of its drainage basin. Discharge starts to rise slowly in May, and then rises dramatically through the late spring and early summer as the winter snow melts. For the year shown, the minimum discharge of the Stikine River was 56 m³/s in March, and the maximum was 37 times higher at 2,470 m³/s in May.

[caption id="attachment_597" align="aligncenter" width="550"] Figure 14.26 Variations in discharge of the Stikine River during 2013. Source: Steven Earle (2015), CC BY 4.0. Image source. Data from Water Survey of Canada, Government of Canada.[/caption]

Streams in coastal areas of southern British Columbia show a very different pattern from those in most of the rest of the country. In this region, the drainage basins receive a lot of rain (rather than snow) during the winter and also do not remain entirely frozen throughout the winter. The Qualicum River on Vancouver Island typically has its highest discharge levels in January or February and its lowest levels in late summer (Figure 14.27). In 2013, the minimum discharge of the Qualicum River was 1.6 m³/s in August, and the maximum was 34 times higher at 53 m³/s in March.

[caption id="attachment_598" align="aligncenter" width="550"] Figure 14.27 Variations in discharge of the Qualicum River during 2013. Source: Steven Earle (2015), CC BY 4.0. Image source. Data from Water Survey of Canada, Government of Canada.[/caption]

When a stream’s discharge increases, both the water level (stage) and the velocity increase as well. Rapidly flowing streams become muddy, and large volumes of sediment are transported both in suspension and along the stream bed. In extreme situations, the water level reaches the top of the stream’s banks (the bank-full stage, see Figure 14.19), and if it rises further, it will overflow the banks and floods the surrounding terrain. In the case of mature or old-age streams, this could include a vast area of relatively flat ground known as a flood plain, which is the area that is typically covered with water during a major flood. Since fine river sediments are deposited on flood plains, they are ideally suited for agriculture, and thus are typically occupied by farms and residences, and in many cases, by towns or cities. Such infrastructure is highly vulnerable to damage from flooding, and the people that live and work there are at risk.

Most streams in Canada have the greatest risk of flooding in the late spring and early summer when stream discharges rise in response to melting snow. In some cases, this is exacerbated by spring storms. In years when melting is especially fast and/or spring storms are particularly intense, flooding can be very severe.

One of the worst floods in Canadian history took place in the Fraser Valley of BC in late May and early June of 1948. The early spring of that year had been cold, and a large snow pack in the interior was slow to melt. In mid-May, temperatures rose quickly and melting was accelerated by rainfall. Fraser River discharge levels rose rapidly over several days during late May, and the dykes built to protect the valley were breached in a dozen places. Approximately one-third of the flood plain was inundated, and many homes and other buildings were destroyed, but there were no deaths.

The Fraser River flood of 1948, which was the worst flood in the Fraser Valley in the past century, was followed by very high river levels in 1950 and 1972, and by relatively high levels several times since then, the most recent being 2007 (Table 13.1). In the years following 1948, millions of dollars were spent repairing and raising the existing dykes and building new ones. Since then damage from flooding in the Fraser Valley has been relatively limited.

Table 14.1 Ranking of the maximum stage and discharge values for the Fraser River at Hope between 1948 and 2008. Typical discharge levels are ~1,000 m³/s. Source: Data from Mannerstrom (2008) Comprehensive Review of Fraser River at Hope Flood Hydrology and Flows Scoping Study, Report prepared for the B.C. Ministry of the Environment.
Rank	Year	Month	Date	Stage (m)	Discharge (m³/s)
1	1948	May	31	11.0	15,200
2	1972	Jun	16	10.1	12,900
3	1950	Jun	20	9.9	12,500
4	1964	Jun	21	9.6	11,600
5	1997	Jun	5	9.5	11,300
6	1955	Jun	29	9.4	11,300
7	1999	Jun	22	9.4	11,000
8	2007	Jun	10	9.3	10,850
9	1974	Jun	22	9.3	10,800
10	2002	Jun	21	9.2	10,600

Serious flooding occurred in July 1996 in the Saguenay-Lac St. Jean region of Quebec. In this case, the floods were caused by two weeks of heavy rainfall followed by one day of exceptional rainfall. On July 19, 1996 there was 270 mm of rain, equivalent to the region’s normal rainfall for the entire month of July. Ten deaths were attributed to the Saguenay floods, and the economic toll was estimated at $1.5 billion.

Just a year after the Saguenay floods, the Red River in Minnesota, North Dakota, and Manitoba reached its highest level since 1826. As is typical for the Red River, the 1997 flooding was due to rapid snowmelt. Due to the south to north flow of the river, the flooding starts in Minnesota and North Dakota, where melting begins earlier, then extends northwards. The residents of Manitoba had plenty of warning that the 1997 flood was coming because there was severe flooding at several locations on the U.S. side of the border.

After the 1950 Red River flood, the Manitoba government built a channel around the city of Winnipeg to reduce the potential of flooding in the city (Figure 14.28). Known as the Red River Floodway, the channel was completed in 1964 at a cost of $63 million. Since then it has been used many times to alleviate flooding in Winnipeg, and is estimated to have saved many billions of dollars in flood damage. The massive 1997 flood (Figure 14.28, right side) was almost too much for the floodway; in fact the amount of water diverted was greater than the designed capacity. The floodway has recently been expanded so that it can be used to divert even more of the Red River’s flow away from Winnipeg.

[caption id="attachment_599" align="aligncenter" width="650"] Figure 14.28 (left) map of the Red River Floodway around Winnipeg, Manitoba; (right) aerial view of the southern (inlet) end of the floodway during the 1997 Red River flood. Sources: (left) Kmusser (2007), CC BY 2.5.Image source. (right) Natural Resources Canada 2012, courtesy of the Geological Survey of Canada (Photo 2000-118 by G.R. Brooks ).[/caption]

Canada’s most costly flood ever was the June 2013 flood in southern Alberta. The flooding was initiated by snowmelt and worsened by heavy rains in the Rockies due to an anomalous flow of moist air from the Pacific and the Caribbean. At Canmore, AB rainfall amounts exceeded 200 mm in 36 hours, and at High River, AB 325 mm of rain fell in 48 hours.

[caption id="attachment_600" align="aligncenter" width="550"] Figure 14.29 Map of the communities most affected by the 2013 Alberta floods (in orange) Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

In late June and early July, the discharges of several rivers in the area, including the Bow River in Banff, Canmore, and Exshaw, the Bow and Elbow Rivers in Calgary, the Sheep River in Okotoks, and the Highwood River in High River, reached levels that were 5 to 10 times higher than normal for that time of year (see Exercise 14.5). Large areas of Calgary, Okotoks, and High River were flooded, and five people died (see Figures 14.29 and 14.30). The cost of the 2013 flood is estimated to be approximately $5 billion.

[caption id="attachment_601" align="aligncenter" width="814"] Figure 14.30 Flooding in Calgary (June 21, left) and Okotoks (June 20, right) during the 2013 southern Alberta flood. Sources: (left) Ryan L.C. Quan (2013), CC BY-SA 3.0. Image source. (right) Stephanie N. Jones (2013), CC BY-SA 3.0. Image source. [/caption]One of the things that the 2013 flood of the Bow River teaches us is that we cannot predict when a flood will occur nor how big it will be, so in order to minimize damage and casualties we need to be prepared. Some ways of preparing include:

Mapping flood plains and not building within them
Building dykes or dams where necessary
Monitoring the winter snowpack, the weather, and stream discharges
Creating emergency plans
Educating the public on how to prepare for and respond to the threat of flooding

Flood Frequency

Have you ever heard of a major flood referred to as a 100-year flood, or maybe even a 500-year flood? What does that actually mean? When a flood is referred to as a 500-year flood, this means that in the known history of the stream, the discharge that caused the flood occurred once every 500 years, on average. The recurrence interval of a flood of that magnitude is 500 years. A recurrence interval of 500 years is the same as saying that every year, there is a 1 in 500 chance (0.2%) that a flood of that size will happen. That will be true regardless of whether a 500-year flood occurred the year before. In other words, if a 500-year flood occurred one year, a flood of the same magnitude could still happen the following year. Recurrence intervals are determined by noting how often a stream has a particular discharge rate. Having many years of data for a stream is important to establish the behaviour of the stream. However, basing predictions on the past history of the stream means assuming that the conditions that determine the discharge of the stream have not changed. For example, in the Canadian prairies it was once common practice to drain wetlands so the land could be used for farming. Wetlands can store additional water during snowmelt or heavy rains. Over time as wetlands became fewer, it meant that under the same conditions, more water entered streams than in the past, resulting in higher discharge more often than in the past. In that case, historical data would underestimate the frequency of higher discharge events. Urbanization also changes the conditions that affect discharge. The concrete and asphalt surfaces common in most towns and cities do not permit water to flow through them and into the ground. Storm drain systems rapidly route storm water to streams. This means water gets to the stream much faster than if it had to move as groundwater. As a result, where in the past discharge would have increased slowly some time after the precipitation event, and be spread out over time, runoff now hits streams rapidly and focused over a shorter period of time.

Putting It Together [h5p id="164"]

Chapter 14 Summary & Key Term Check

kqzheng — Mon, 20 Nov 2017 19:54:12 +0000

Chapter 14 Main Ideas

14.1 The Hydrological Cycle

Water is stored in the oceans, glacial ice, the ground, lakes, rivers, and the atmosphere. Its movement is powered by solar energy and gravity.

14.2 Drainage Basins

All of the precipitation that falls within a drainage basin flows into the stream that drains that area. Stream drainage patterns are determined by the type of rock within the basin. Over geological time, streams change the landscape that they flow within, and eventually they become graded, meaning their profile becomes a smooth curve. A stream can lose that gradation if there is renewed uplift or if their base level changes for some other reason such as construction of a dam downstream.

Practice Again

Types of drainage basins

14.3 Stream Erosion and Deposition

The processes of erosion and deposition of particles within streams are primarily driven by the velocity of the stream water. Erosion and deposition of different-sized particles can happen simultaneously in a stream. Some particles are moved along the bottom of a river while others are carried in suspension. It takes a greater velocity of water to erode a particle from a stream bed than it does to keep it in suspension. Ions are also transported in solution. When a stream rises and then occupies its flood plain, the velocity of water over the flood plains slows and natural levees form along the edges of the stream channel.

14.4 Stream Types

Youthful streams in steep areas erode most rapidly downwards, and they tend to have steep, rocky, and relatively straight channels. Where sediment-rich streams empty into areas with lower gradients, braided streams can form. Meandering streams are common in areas with even lower gradients where silt and sand are the dominant sediments. Meandering streams erode the walls of their channels more rapidly than the channel base. Deltas form where streams flow into standing water.

Practice Again

How stream characteristics change along a stream profile

14.5 Flooding

Most streams in Canada have their highest discharge rates in spring and early summer, although the highest discharge in many of BC’s coastal streams is in the winter. Floods happen when a stream rises high enough to spill over its banks and spread across its flood plain. Some of the more significant floods in Canada include the Fraser River flood of 1948, the Saguenay River flood of 1996, the Red River flood of 1997, and the Alberta floods of 2013. We can estimate the probability of a specific flood level based on the record of past floods, and we can take steps to minimize the impacts of flooding such as building floodways to divert excess water and not building in flood-prone areas.

Key Term Check

What key term from Chapter 14 is each card describing? Turn the card to check your answer. [h5p id="165"]

2.1 Starting with a Big Bang

kqzheng — Wed, 03 Jan 2018 00:43:00 +0000

[caption id="attachment_1329" align="aligncenter" width="1296"] Figure 2.2 The big bang. The universe began 13.8 billion years ago as a rapid expansion of space, energy, and matter. It continues to expand. Left: Timeline of the universe, moving to the right. The point of the universe "vessel" represents the moment of the big bang. The vessel gets wider as time progresses, representing the expansion of the universe. Right (top): Red patches are light from galaxies formed within a few 100 million years of big bang. Right (bottom): Mollwiede projection of the cosmic microwave background, a "fog" from when the universe was still very dense. Temperature variations indicate clumping of matter in the early universe. Source: Karla Panchuk (2021) CC BY 4.0.[/caption] Planets don't form in isolation. Planet formation happens as part of the larger process of solar system formation, and solar systems form where enough of the right chemical elements are available. The relative abundance of chemical elements in the universe depends on the processes that make atoms, and that relative abundance is what controls the properties of planets and the stars they orbit. In other words, Earth’s story begins at the very beginning with the construction of the elements that comprise it.

The Big Bang Was Not a Big Bang

According to the big bang theory, the universe began almost 14 billion years ago (Glaser, 2021 & Choi, 2020) with a rapid expansion of matter, energy, and space. The phrase “big bang” suggests an explosion. If you imagine an explosion within a room, you might visualize a blast of light with debris flying away from a central location and hitting the walls of the room. That's the wrong mental image for thinking about the big bang. Instead, you have to imagine that no room existed at the start. The room simply blinks into existence without a lot of drama or flaming debris, and proceeds to get bigger. Another difference between an explosion and the big bang is that the explosion ends. If you returned to the explosion room a few hours later you might see a small crater in the floor, and some charred rubble scattered around. In contrast, the big bang room would be much larger than when you last checked, and it would continue to expand before your eyes. In Figure 2.2 the pointed end of the universe "vessel" represents the big bang. As time advances (moving to the right in the diagram), the vessel gets wider. You might wonder how a universe can be created out of nothing. Creating a universe out of nothing is mostly beyond the scope of this chapter, but there is a way to think about it. The particles that make up the universe have opposites that cancel each other out, similar to the way that we can add the numbers 1 and -1 to get zero (also known as “nothing”). As far as math goes, having zero is exactly the same as having a 1 and a -1. It is also exactly the same as having a 2 and a -2, a 3 and a -3, two -1s and a 2, and so on. In other words, nothing is really the potential for something if you divide it into its opposite parts.

Big Bang Theory

You might have noticed that we refer to the big bang theory, not the big bang hypothesis. It might be to hard to imagine that we could have any certainty at all about something as fundamental as how the universe started, but there is ample evidence to support the big bang theory. Generally speaking, the universe seems to behave the way predictions based on the big bang theory say it should.

A Baby Picture of the Universe

The notion of seeing the past is often used metaphorically when we talk about ancient events, but in this case it's meant literally. In our everyday experience, when we watch an event take place, we perceive that we're seeing it as it unfolds in real time. In fact, this isn’t true. To see the event, light from that event must travel to our eyes. Light travels very rapidly, but not instantly. If we were watching a digital clock 1 m away from us change from 11:59 a.m. to 12:00 p.m., we would actually see it turn to 12:00 p.m. three billionths of a second after it happened. This isn’t enough of a delay to cause us to be late for lunch, but the universe is a very big place, and the “digital clock” in question is often much, much farther away. In fact, the universe is so big that it's convenient to describe distances in terms of light years, or the distance light travels in one year. What this means is that light from distant objects takes so long to get to us that we see those objects as they were at some considerable time in the past. For example, the star Proxima Centauri is 4.24 light years from the sun. If you viewed Proxima Centauri from Earth on January 1, 2018, you would actually see it as it appeared in early October 2013. We now have tools that are powerful enough to look deep into space and see the arrival of light from early in the universe’s history. Astronomers can detect light from approximately 380,000 years after the big bang is thought to have occurred. Physicists tell us that if the big bang happened, then only 380,000 years later, particles within the universe would still be very close together—so close that light wouldn’t be able to travel far without bumping into another particle and getting scattered in another direction. The effect would be to fill the sky with glowing fog, the “afterglow” from the formation of the universe. In fact, this is exactly what we see when we look at light from 380,000 years after the big bang. (European Space Agency, 2015) The fog is referred to as the cosmic microwave background (or CMB), and it has been carefully mapped throughout the sky. In Figure 2.2, the colourful patch at the narrow end of the universe vessel represents the fog that is measured today as the CMB. The inset CMB map of the universe is a Mollweide projection. This projection is usually used to show Earth's geography on a flat surface, but in this case it represents spherical space looking toward the sky rather than what's beneath our feet. Colour variations in the CMB map represent temperature variations. These variations translate to differences in the density at which matter was distributed in the early universe. The red patches are the highest density regions and the blue patches are the lowest density. Higher density regions represent the eventual beginnings of stars and planets. The CMB map in Figure 2.2 has been likened to a baby picture of the universe.

[caption id="attachment_1842" align="aligncenter" width="700"] This image, called the Hubble Ultra Deep Field, shows 28 of the more than 500 young galaxies that existed when the universe was less than 1 billion years old[/caption] Is this image really showing actual galaxies from when the universe was still young? Choose one:

Yes, this photo shows objects as they existed billions of years before the photo was taken.
No, impossible! We can't literally see into the past. An artist must have created this image.

To check your answer, navigate to the below link to view the interactive version of this activity.

[h5p id="23"]

The Universe is Still Expanding

The expansion that started with the big bang never stopped. It continues today, and we can see it happen by observing that large clusters of billions of stars, called galaxies, are moving away from us. (An exception is the Andromeda galaxy with which we are on a collision course.) The astronomer Edwin Hubble came to this conclusion when he observed that the light from other galaxies was red-shifted. The red shift is a consequence of the Doppler effect. This refers to how we see waves when the object that's creating the waves is moving toward us or away from us. Before looking at the Doppler effect as it pertains to light, it can be useful to see how it works on something more tangible. The duckling swimming in Figure 2.3 is generating waves as it moves through the water. It's generating waves in front and behind, but notice that the ripples ahead of the duckling are closer to each other than the ripples behind the duckling. The distance from one ripple to the next is called the wavelength. The wavelength is shorter in the direction that the duckling is moving, and longer as the duckling moves away. [caption id="attachment_45" align="aligncenter" width="450"] Figure 2.3 A duckling illustrates the Doppler effect in water. The ripples made in the direction the duckling is moving (blue lines) are closer together than the ripples behind the duckling (red lines). Source: Karla Panchuk (2021) CC BY 4.0. Click for more attributions.[/caption] When waves are in air as sound waves rather than in water as ripples, the different wavelengths manifest as sounds with different pitches: the short wavelengths have a higher pitch, and the long wavelengths have a lower pitch. This is why an observer will hear a change in the pitch of a car’s engine or train whistle as the car or train races past. For light waves, wavelength translates to colour. In the spectrum of light that we can see, shorter wavelengths are on the blue end of the spectrum, and longer wavelengths are on the red end of the spectrum. In Figure 2.4, the longer or shorter wavelengths of the water ripples at the top of the diagram are analogous to the longer or shorter wavelengths of light in the visible spectra below. [caption id="attachment_46" align="aligncenter" width="650"] Figure 2.4 Red shift in light from the supercluster BAS11 compared to the sun’s light. Black lines represent wavelengths absorbed by atoms (mostly hydrogen and helium). For BAS11 the black lines are shifted toward the red end of the spectrum compared to the sun. Source: Karla Panchuk (2021) CC BY-SA 4.0. Click for more attributions.[/caption]

Does the red shift mean that galaxies look red because they are moving away from us? Sometimes. The ancient galaxies in the Hubble Ultra Deep Field look red because they are moving away from us so rapidly. For other galaxies, the colour we see is shifted toward the red end of the spectrum and longer wavelengths, but not necessarily actually red.

Notice that the sun’s spectrum in the upper part of Figure 2.4 has black lines in it. The black lines are there because some colours are missing from the sun's light that reaches Earth. Different elements absorb light of specific wavelengths, and many of the black lines in Figure 2.4 represent colours that are absorbed by hydrogen and helium within the sun. This means the black lines are like a bar code that can tell us what a star is made of. The lower spectrum in Figure 2.4 is the light coming from BAS11, an enormous cluster of approximately 10,000 galaxies located 1 billion light years away. The black lines represent the same elements as in the sun’s spectrum, but they are shifted to the right toward the red end of the spectrum, because BAS11 is moving away from us as the universe continues to expand. To summarize, because almost all of the galaxies we can see have light that is red-shifted, it means they are all moving away from us. In fact, the farther away they are, the faster they are going. This is evidence that the universe is still expanding.

Composition of the Universe

In Figure 2.2, the "contents" of the vessel change as time progresses. A few minutes after the big bang, the universe was still too hot and dense to be anything but a sizzle of particles smaller than atoms. But as it expanded, it also cooled. Eventually particles that collided were able to stick together to form atoms, rather than being smashed apart again when other particles crashed into them. Those collisions produced hydrogen and helium, the most common elements in the universe. For a long time after the big bang, clouds of hydrogen and helium atoms drifted about a dark universe. The Dark Ages (bottom of Figure 2.2) were a time when the ingredients for stars existed, but—at first—the stars themselves did not yet exist. The first stars likely appeared within 100 million years of the big bang,⁴ but the universe remained in the Dark Ages for several 100 million years more because there was too much other material around the stars for their light to be visible from a distance.

How do we know that stars existed during the Dark Ages if we can't see them? The timing for the earliest stars comes from computer models that calculate the stage of the universe's development during which stars could be expected to form (Williams, 2020). In addition, even though astronomers today can't see visible light from these stars, they can look for other wavelengths of light that the stars emitted.

Do you understand what the red shift means? [h5p id="24"]

References

Choi, S. K. (2020). The Atacama Cosmology Telescope: a measurement of the Cosmic Microwave Background power spectra at 98 and 150 GHz. Journal of Cosmology and Astroparticle Physics, 2020(045). https://doi.org/10.1088/1475-7516/2020/12/045

Glaser, L. B. (2021, January 4). Astronomers agree: Universe is nearly 14 billion years old. Phys.org. https://phys.org/news/2021-01-astronomers-universe-billion-years.html

European Space Agency. (2015, February 5). Planck reveals first stars were born late. http://www.esa.int/Our_Activities/Space_Science/Planck/Planck_reveals_first_stars_were_born_late

Williams, M. (2020, November 3). An extreme simulation of the universe's first stars. Universe Today. https://www.universetoday.com/148666/an-extreme-simulation-of-the-universes-first-stars/

2.2 Planet-Forming Materials Come from the Remnants of Exploded Stars

kqzheng — Fri, 05 Jan 2018 03:21:21 +0000

Only four elements account for 95% of Earth's mass: oxygen (O), magnesium (Mg), silicon (Si), and iron (Fe). Most of the remaining 5% comes from aluminum (Al), calcium (Ca), nickel (Ni), hydrogen (H), and sulphur (S). We know that the big bang made hydrogen and helium, but where did the rest of the elements come from? The answer is that almost all of the other elements were made by stars. Sometimes stars are said to “burn” their fuel, but burning is not what's going on within stars. The burning that happens when wood in a campfire is turned to ash and smoke is a chemical reaction: heat causes the atoms that were in the wood and in the surrounding atmosphere to exchange partners. Atoms group in different ways, but the atoms themselves don't change. What stars do is change the atoms.

Stars Make Small Atoms into Bigger Ones

Heat and pressure within stars cause smaller atoms to smash together and merge into new, larger atoms. For example, when hydrogen atoms smash together and fuse, helium is formed. This process is called nuclear fusion. Large amounts of energy are released when some elements fuse within stars, and that's what causes stars to shine. Stars can form large quantities of elements as heavy as iron during their normal burning process. Side reactions can form heavier elements in small amounts. It takes larger stars to make elements as heavy as iron in large quantities. Our sun is an average star. After it uses up its hydrogen fuel to make helium, and some of that helium is fused to make small amounts of other elements, it will be at the end of its life. It will stop making new elements and will cool down and bloat until its middle reaches the orbit of Mars. In contrast, large stars end their lives in spectacular fashion. They explode as supernovae, casting off newly formed atoms into space, and triggering side reactions to make even more heavy atoms. It took many generations of stars creating heavier elements and casting them into space before heavier elements were abundant enough for planets like Earth to form.

Our Planet's Composition is No Accident.

To see the importance of element-forming processes for our planet, notice the large circles in the simplified periodic table in Figure 2.5. The big circles mark the ten most abundant elements in our Milky Way galaxy. Aside from hydrogen and some of the helium, the abundance of those elements is entirely controlled by star processes. Now find the Earth circles. These are the nine most abundant elements we listed earlier. Notice how many of the Earth circles are also big circles. Furthermore, notice how many of the Earth circles fall in the set of elements that require very massive stars to exist and explode. [caption id="attachment_49" align="aligncenter" width="1687"] Figure 2.5 Simplified periodic table of the elements showing the ten most abundant elements in the MIlky Way galaxy (H, He, O, C, Ne, Fe, N, Si, Mg, S) in big circles. The nine most abundant elements by mass on Earth (O, Mg, Si, Fe, Al, Ca, Ni, H) are shown as Earth circles. Source: Karla Panchuk (2021) CC BY 4.0. Data for the periodic table from Jennifer Johnson's Origin of the Elements in the Solar System.[/caption]

Do you know your chemical symbols? [h5p id="25"]

References

Johnson, J. (2017, January 9). Origin of the elements in the solar system. Science Blog from the Sloan Digital Sky Surveys. https://blog.sdss.org/2017/01/09/origin-of-the-elements-in-the-solar-system/

2.3 How to Build a Solar System

kqzheng — Fri, 05 Jan 2018 04:48:09 +0000

A solar system consists of a collection of objects orbiting one or more central stars. All solar systems start out the same way. They begin in a cloud of gas and dust called a nebula. Nebulae are some of the most beautiful objects that have been photographed in space. They have vibrant colours from the gases and dust they contain, and brilliant twinkling from the many stars that have formed within them (Figure 2.6). The gas consists largely of hydrogen and helium, and the dust consists of tiny mineral grains, ice crystals, and organic particles. [caption id="attachment_52" align="aligncenter" width="600"] Figure 2.6 The Pillars of Creation within the Eagle Nebula viewed in visible light (left) and near infrared light (right). Near infrared light captures heat from stars and allows us to view stars that would otherwise be hidden by dust. This is why the picture on the right appears to have more stars than the picture on the left. Source: NASA, ESA, and the Hubble Heritage Team (STScI/AURA) (2015) Public Domain.[/caption]

Step 1: Collapse a Nebula

A solar system begins to form when a small patch within a nebula (small by the standards of the universe, that is) begins to collapse upon itself. Exactly how this starts isn’t clear, although it might be triggered by the violent behaviour of nearby stars as they progress through their life cycles. Energy and matter released by these stars might compress the gas and dust in nearby neighbourhoods within the nebula. Once triggered, the collapse of gas and dust within that patch continues for two reasons. One reason is that gravitational force pulls gas molecules and dust particles together. But early in the process, those particles are very small, so the gravitational force between them isn’t strong. So how do they come together? The answer is that dust first accumulates in loose clumps for the same reason dust bunnies form under a bed: static electricity. Given the role of dust bunnies in the early history of the solar system, one might speculate that an accumulation of dust bunnies poses a substantial risk to one’s home (Figure 2.7). In practice, however, this is rarely the case. [caption id="attachment_53" align="aligncenter" width="650"] Figure 2.7 Dust bunnies have mixed reviews as pets, owing largely to people not knowing how to manage them once they start forming planets and stars. Source: Karla Panchuk (2021) CC BY 4.0. Click for more attributions.[/caption]

Step 2: Make a Disk with a Star at Its Centre

As the small patch within a nebula condenses, a star begins to form from material drawn into the centre of the patch. The remaining dust and gas settle into a protoplanetary disk that rotates around the star. The disk is where planets will eventually form. Figure 2.8 (top left) is an artist’s concept of a protoplanetary disk, and Figure 2.8 (top right) is an actual protoplanetary disk surrounding the star HL Tauri. Notice the dark rings within the HL Tauri protoplanetary disk. These are gaps formed by the collection of dust and debris by incipient planets, called protoplanets, as they orbit the star. There is an analogy for this in our own solar system, because the dark rings are akin to the gaps in the rings of Saturn (Figure 2.8, bottom), where moons can be found. [caption id="attachment_54" align="aligncenter" width="650"] Figure 2.8 Protoplanetary disks and Saturn’s rings. Top left: Artist's concept of a protoplanetary disk containing gas and dust, surrounding a new star. Top right: A photograph of the protoplanetary disk surrounding HL Tauri. The dark rings within the disk are thought to be gaps where newly forming planets are sweeping up dust and gas. Bottom left: A photograph of Saturn showing similar gaps within its rings. The bright spot at the bottom is an aurora, similar to the northern lights on Earth. Bottom right: a close-up view of a gap in Saturn’s rings showing a moon as a white dot. Source: Karla Panchuk (2021) CC BY 4.0. Click for more attributions.[/caption]

Step 3: Build Some Planets

Types of Planets

Broadly speaking, planets can be classified into three categories based on their composition (Figure 2.9). Terrestrial planets are planets like Earth, Mercury, Venus, and Mars, which have a metal core surrounded by rock. Jovian planets (also called gas giants) are planets like Jupiter and Saturn that are mostly hydrogen and helium. Ice giants are planets like Uranus and Neptune with rocky cores enveloped by water ice, methane (CH₄) ice, and ammonia (NH₃) ice. [caption id="attachment_55" align="aligncenter" width="650"] Figure 2.9 Three types of planets. Jovian (or gas giant) planets such as Jupiter consist mostly of hydrogen and helium. They are the largest of the three types. Ice giant planets such as Uranus are the next largest. They contain water, ammonia, and methane ice, and have rocky cores. Terrestrial planets such as Earth are the smallest, and they have metal cores covered by rocky mantles. Source: Karla Panchuk (2021) CC BY-NC-SA 4.0. Click for more attributions.[/caption]

What were the earliest planets like?

[h5p id="26"]

Astronomers looking for some of the earliest stars in the universe were surprised to find a planetary system called HIP 11952, which existed 12.8 billion years ago. This was very early in the universe’s history, when stars still consisted largely of hydrogen and helium. Did HIP 11952 have terrestrial planets? Answer: No. The planetary system had two Jupiter-sized gas giant planets. Gas giant planets contain large amounts of hydrogen, which was plentiful in the early universe. In contrast, terrestrial planets have heavier elements, especially silica, iron, magnesium, and nickel, that had yet to be manufactured by stars in sufficient abundance to form terrestrial planets.

Arrangement of Solar System Objects

The three types of planets are not mixed together randomly within our solar system. Instead they occur in a systematic way, with terrestrial planets closest to the sun, followed by the Jovian planets and then the ice giants (Figure 2.10). Smaller solar system objects follow this arrangement as well. The asteroid belt contains bodies of rock and metal. Bodies ranging from metres to hundreds of metres in diameter are classified as asteroids, and smaller bodies are referred to as meteoroids. In contrast, the Kuiper belt (Kuiper rhymes with piper), and the Oort cloud (Oort rhymes with sort), which are at the outer edge of the solar system, contain bodies composed of large amounts of ice in addition to rocky fragments and dust. [caption id="attachment_56" align="aligncenter" width="864"] Figure 2.10 Our solar system. Top: Solar system objects with distances to scale. Distances are in astronomical units (AU), where 1 AU is the average distance from Earth to the sun. The edge of the Kuiper belt extends to 50 AU (7.5 billion km), but this distance is minuscule compared to the size of the solar system as a whole, which extends to the edge of the Oort cloud, thought to be 15 trillion km away. Bottom: Solar system with the sun and planets to scale. The gas giants are the largest planets, followed by the ice giants, and then the terrestrial planets. Note that the planets in this diagram likely do not reflect the entire population of planets in our solar system because evidence suggests that large planets are present beyond the Kuiper belt. Source: Karla Panchuk (2018) CC BY-NC-SA 4.0. Click for more attributions.[/caption] This arrangement is related to the temperatures surrounding the young sun. The frost line (or snow line) marks the division between the inner part of the protoplanetary disk closer to the sun, where it was too hot to permit anything but silicate minerals and metal to crystalize, and the outer part of the disk farther from the sun, where it was cool enough to allow ice to form. As a result, the objects that formed in the inner part of the protoplanetary disk consist largely of rock and metal, while the objects that formed in the outer part consist largely of gas and ice.

Concept check: Frost line

Tau Ceti is a solar analog, meaning that it's very similar to our sun in terms of size, temperature, and colour. It's not exactly like the sun, however—it's slightly smaller, cooler, and older. How does Tau Ceti's frost line compare to that of the sun? Choose one:

Tau Ceti's frost line is farther from Tau Ceti than the sun's frost line is from the sun.
Tau Ceti's frost line is closer to Tau Ceti than the sun's frost line is to the sun.

To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="27"]

Stars don't start out doing nuclear fusion, but work up to that point by drawing material toward themselves until they attain sufficient mass and density for fusion to begin. During the last stage before fusion begins, young stars blast the protoplanetary disk around them with solar winds (winds made up of energetic particles), which drives lighter molecules toward the outer part of the disk, clearing away many of the materials that can be used to form planets. This puts some time constraints on when planet formation can happen.

Do you know your solar system objects?

Summarize the trends in size and composition of objects in the solar system by dragging the words into the correct boxes. Closest to the sun we find the small, rocky, with cores. Further out are the , which are the largest in the solar system. They consist mostly of , and have cores of rock and ice. Beyond these are the , which are next largest. They have a mantle of ice (not just water ice but ammonia and methane ice), and a rocky core. Smaller objects in the solar system include rocky bodies within the between Mars and Jupiter, and bodies of ice and dust in the and in the beyond Neptune. Fill-in-the-blank options:

gas giant planets
Oort cloud
metal
ice giant planets
terrestrial planets
hydrogen
asteroid belt
Kuiper belt

To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="28"]

Rules of the Accretion Game

The objects in our solar system formed by accretion. Early in this process, particles collected in fluffy clumps because of static electricity. As the clumps grew larger, gravity became more important and collected clumps into solid masses, and solid masses into larger and larger bodies. If you were one of these bodies in the early solar system, and participating in the "accretion game" with the goal of becoming a planet, you would have to follow some key rules:

1. Keep your velocity just right. If you move too fast and collide with another body, you both smash up and have to start again. If you move slowly enough, gravity will keep you from bouncing off each other and you can grow larger.

2. Your distance from the Sun determines how big you can get. If you're closer, there's less material for you to collect than if you're farther away.

3. At the beginning you can only collect mineral and rock particles. You have to grow above a certain mass before your gravity is strong enough to hang onto gas molecules, because gas molecules are very light.

4. As your mass increases, your gravity becomes stronger and you can grab material from farther away. The bigger you are, the faster you grow.

You would also have to watch out for some dangers:

In the early stages of the game, the protoplanetary disk is turbulent, and you and other objects can get thrown into different orbits or at each other. This might be a good thing, or it might not, depending on how the rules above apply to you.
If the game progresses to the point where there is no more material within your reach and you are not yet a planet, then it's game over.
If you slow down too much (e.g., from bumping into other objects), you could spiral into the sun (game over).
If another planet gets big enough, it can:
- Rip you apart and then swing the pieces around so fast that for the rest of the game you collide too hard with other pieces to grow any bigger (game over)
- Fling you out of the solar system (game over)
- Grab you for itself (game over)
- Trap you in an orbit around it, turning you into a moon (game over, and incredibly humiliating)

Winners and Losers

The outcome of the game is evident in Figure 2.10. Today eight official winners are recognized, with Jupiter taking the grand prize, followed closely by Saturn. Both planets have trophy cases with more than 60 moons each, and each has a moon that's larger than Mercury. Prior to 2006, Pluto was also counted a winner, but in 2006 a controversial decision revoked Pluto’s planet status. The reason was a newly formalized definition of a planet. Pluto fit the first two requirements: orbiting the sun, and being massive enough to be round. But it didn't fit the third requirement, that an object can only be considered a planet if it has also swept its orbit clean of other bodies. (This is another criterion related to the mass of the object, as per the rules of the accretion game.) Pluto is situated within the icy clutter of the Kuiper belt, so it doesn't fit this definition. Some of Pluto’s supporters argued that Pluto should have been grandfathered in, given that the definition came after Pluto was declared a planet, but to no avail. Pluto has not given up, and on July 13, 2015, it launched an emotional plea with the help of the NASA’s New Horizons probe. New Horizons sent back images of Pluto’s heart (Figure 2.11). On closer inspection, Pluto’s heart was discovered to be broken. [caption id="attachment_57" align="aligncenter" width="600"] Figure 2.11 Photographs of Pluto. Left: The heart-shaped region called Tombaugh Regio is outlined. This region is named after Pluto’s discoverer Clyde Tombaugh. Right: False-colour images show compositional variations in Tombaugh Regio. Source: Karla Panchuk (2015) CC BY 4.0. Left photo- NASA/APL/SwRI (2015) Public Domain Image source, Right photo- NASA/APL/SwRI (2015) Public Domain. Image source.[/caption]

Update: Still Hard Feelings about the Unplaneting of Pluto Objections to "unplaneting" Pluto are all emotional on some level (see the I Heart Pluto Festival 2021), but scientists can have emotions about their work and still do good science. They keynote speaker at the I Heart Pluto Festival, Dr. Alan Stern gave a talk called "Why Pluto Is a Planet, The Embarrassment of The IAU, and Why They Had It Coming." Dr. Stern currently leads the New Horizons Mission. In his talk, Dr. Stern argued first of all that a vote by the IAU was a terrible way to go about making a scientific decision, because whether something is true from a scientific perspective or not has nothing to do with how many people think it should be accepted as truth. As he put it, "The image of the IAU taking a vote was the single most damaging pedagogical event in science in probably a century, because to many people it was easy to reach the conclusion that science is arbitrary or political, which it is not.” Second, he argued that the definition was changed for the wrong reason, namely that more and more Pluto-sized objects were being discovered past Neptune. (Stern expects there to be hundreds found.) "Astronomers became afraid of astronomically large numbers,” he quipped, and didn't like the idea of calling all those objects planets. So what does Dr. Stern think the definition of a planet should be? People who specialize in studying planets define a planet as being massive enough to be round, but not so big that nuclear fusion happens. In other words, round, but not a star. The let the lower limit be defined by how a planet's mass influences its own shape, rather than by conditions around it that might be unrelated to the object itself (such as a bigger planet flinging debris at it). In the end, this is an argument about what a good reason is to classify an object one way or another. Dr. Stern is frustrated because he sees the IAU definition as being based on an irrelevant issue, that acknowledging more planets means naming them all. Granted, naming them all means an extraordinary amount of work for the IAU, because you can't just slap a label on a planet and call it official. There are rules about these things. (The IAU's webpage Naming of Astronomical Objects begins, "Celestial nomenclature has long been a controversial topic.") But as Dr. Stern says, “There are countless stars and there are countless planets—and who cares? It’s just the data.” For more on Dr. Stern's talk, see Yes, Pluto Is A Planet Says NASA Scientist At The Site Of Its Discovery 91 Years Ago This Week by Jamie Carter (2021, February 15) in Forbes.

The Accretion Game and the Solar System Today

The rules and dangers of the planet-forming game help to explain many features of our solar system today.

Proximity to the sun explains why the terrestrial planets are so much smaller than the gas giant and ice giant planets.
Mars is smaller than it should be, given the rule that distance from the sun determines how much material a body can accumulate, and this can be explained by its proximity to Jupiter. Jupiter’s immense gravity interfered with Mars’ ability to accrete. Further evidence of Jupiter’s interference is the debris field that forms the asteroid belt. From time to time, Jupiter still flings objects from the asteroid belt out into other parts of the solar system, some of which have collided with Earth to catastrophic effect.
The Kuiper belt is an icy version of the asteroid belt, consisting of fragments left over from the early solar system. The material in the Kuiper belt is scattered because of Neptune’s gravity. From time to time, Jupiter interferes here as well, flinging Kuiper belt objects toward the sun and into orbit. As these objects approach the sun, the sun causes dust and gas to be blasted from their surface, forming tails. We know these objects as comets.
Comets may also come from the Oort cloud where gravitational forces from outside of the solar system can hurl objects from the Oort cloud toward the sun.

Can you build a solar system? [h5p id="29"]

2.4 Earth’s First 2 Billion Years

kqzheng — Fri, 05 Jan 2018 23:01:41 +0000

Setting the Stage

[caption id="attachment_60" align="aligncenter" width="1024"] Figure 2.12 An artists concept of Earth's earliest days. This interval of geologic time is referred to as the Hadean Eon. The object looming ominously in the background eventually made things very unpleasant. Source: Tim Bertelink (2016) CC BY-SA. Image source.[/caption] If you were to get into a time machine and visit Earth shortly after it formed (around 4.6 billion years ago), you would probably regret it. Large patches of Earth’s surface would still be molten, which would make landing your time machine very dangerous indeed. If you happened to have one of the newer time-machine models with hovering capabilities and heat shields, you would still face the inconvenience of having nothing to breathe but a tenuous wisp of hydrogen and helium gas, and depending on how much volcanic activity was going on, volcanic gases such as water vapour and carbon dioxide. Some ammonia and methane might be thrown in just to make it interesting, but there would be no oxygen. Assuming you had the foresight to purchase the artificial atmosphere upgrade for your time machine, it would all be for naught if you materialized just in time to see an asteroid—or worse yet, another planet—bearing down on your position. The moral of the story is that early Earth was a nasty place, and a time machine purchase is not something to take lightly.

How Earth Developed Interior Layers

How Do We Know the Composition of Earth's Layers?

In the previous section, you read about how terrestrial planets such as Earth have a rocky mantle and a metallic core. Earth's core is mostly a mixture of iron (Fe) and nickel (Ni). The mantle consists mostly of silicate minerals high in iron and magnesium (Mg). Silicate minerals are those having silica (Si) and oxygen (O) defining their basic structure and properties. If you think back to how elements are made, you will realize these elements are old friends we've already met. We can't observe the core directly, nor much of the mantle. Nevertheless, we can be confident in this assessment of Earth's composition because we know Earth's density.

Need a refresher on density? Density refers to how heavy an object is for its size, and it's helpful because it tells us about the materials that make up an object. You already have an intuitive sense of what density means. Imagine you received a package in the mail. You ordered a fluffy sweater and some books, so either could be in the package. You would know immediately which was in the package just by lifting it. If it felt heavy you’d know it contained books. Books have a greater density than a fluffy sweater. Similarly, density tells us something about the materials that make up a planet. Earth’s density reflects the fact that it has a metallic core and a rocky mantle. The iron making up Earth’s core has density of 8.0 g/cm³. A density of 3.25 g/cm³ is a good estimate for mantle rocks. Together these components give Earth a density of 5.5 g/cm³. Density is used to approximate the composition of other planets in our solar system, as well as the compositions of newly discovered planets in other solar systems.

We also know about Earth's composition because we have samples of broken up terrestrial planets and asteroids to look at. These are in the form of meteorites—mm-sized to meter-sized fragments originating in the asteroid belt. Meteorites consisting entirely of iron (Figure 2.13) are thought to represent core fragments from shattered asteroids and terrestrial planets. Stony meteorites represent the rocky exteriors. [caption id="attachment_61" align="aligncenter" width="300"] Figure 2.13 Iron meteorite. The interlaced pattern on the surface is made by mineral crystals. This meteorite is a fragment of the Seymchan pallasite meteorite found in Russia in June of 1967. Source: Opsoelder (2007), CC BY-SA 2.0 DE. Image source.[/caption]

Differentiation: Earth Unmixes Itself

Today Earth has a layered interior, but early in its history it was a disorganized mixture of whatever metallic and silicate minerals it grabbed up in its orbit around the young sun. The decay of radioactive elements, plus objects smashing into Earth heated it up enough to melt the iron within. Dense melted iron sank to the centre of out planet, forming its core, and the remaining silicate material formed the mantle. The process of separating melted iron and silicate minerals into layers within Earth is called differentiation. This makes it sound like a foregone conclusion that the melted iron would sink to Earth's centre, but think about a chocolate chip cookie for a minute, and imagine what would happen if you put it in the microwave long enough to melt the chips. Would you have a layer of chocolate at the base of the cookie, and a layer of chocolate-free cookie above? No, because some of the chocolate would remain trapped in pockets of cookie dough. Early Earth was an analogous scenario, except the melted iron was trapped in pockets of silicate mantle rocks. For a long time, scientists thought that the solution to this problem was to melt the entire Earth—silicate minerals and metal—so the metal could escape. More recently, a study demonstrated that when silicate rocks with pockets of iron melt were deformed in the lab (as we would expect early Earth to be deformed periodically by space objects slamming into it), it opened channels between blobs of melted iron, permitting the iron to flow (Figure 2.14). This would have allowed differentiation to proceed with far less heat involved. [caption id="attachment_62" align="aligncenter" width="1408"] Figure 2.14 Experimental results showing what happens when a mix of olivine and iron minerals (representing mantle material and core material, respectively) is melted in a lab. Top: Without additional deformation, the iron stays in blobs trapped between olivine crystals. Bottom: If the sample is deformed by twisting when the iron is melted, it is able to flow between the olivine crystals. Source: Karla Panchuk (2021) CC BY-NC-SA 4.0. Modified after Berg et al., (2017). Read the paper [PDF][/caption]

Capturing a Moment in Time

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Meteorites are part of how we know about the interiors of terrestrial planets. In the meteorite shown here, the silver crystals are an Fe-Ni mineral, and the colourful crystals between them are the mineral olivine. Olivine is a silicate mineral rich in iron and magnesium. Olivine and minerals with a similar composition make up the mantle.Based on what you know about the development of terrestrial planets, can you suggest a process that would explain the composition of this meteorite? Solution The meteorite formed when an asteroid was undergoing differentiation. The crystals reflect blobs of melted iron and silicate minerals in the process of separating into a metallic core and a silicate mantle. When the asteroid was shattered by a collision with another object, the blobs of melt crystallized in place, preserving evidence of differentiation. It is akin to freezing oil and water in the process of separating.

A Catastrophic Disruption

Although Earth had swept up a substantial amount of the material in its orbit as it was accreting, unrest within the solar system caused by changes in the orbits of Saturn and Jupiter was still sending many large objects on cataclysmic collision courses with Earth. The energy from these collisions repeatedly melted and even vaporized minerals in the crust, and blasted gases out of Earth’s atmosphere. Very old scars from these collisions are still detectable, although we have to look carefully to see them. For example, the oldest impact site discovered is the 3 billion year old Maniitsoq “crater” in west Greenland, although there is no crater to see. What is visible are rocks that were 20 km to 25 km below Earth’s surface at the time of the impact, but which nevertheless display evidence of deformation that could only be produced by intense, sudden shock. The evidence of the very worst collision that Earth experienced is not subtle at all. In fact, you've probably looked directly at it thousands of times already, perhaps without realizing what it is. That collision happened not long after Earth formed, with a planet named Theia, which was approximately the size of Mars (Figure 2.15). [caption id="attachment_63" align="aligncenter" width="410"] Figure 2.11 Artist’s impression of a collision between planets. A similar collision between Earth and the planet Theia might have given us our moon. Source: NASA/ JPL-Caltech (2009) Public Domain. view source.[/caption] When Theia slammed into Earth, Theia’s metal core merged with Earth’s core. Debris from the outer silicate layers was cast into space, forming a ring of rubble around Earth. The material within the ring coalesced into a new body in orbit around Earth, giving us our moon. Remarkably, the debris may have coalesced in 10 years or less! This scenario for the formation of the moon is called the giant impact hypothesis. Evidence for the giant impact hypothesis comes from chemical fingerprints in rocks on Earth and rocks on the moon. The moon has less iron that we would expect (thanks to Theia donating its core to us), and more elements that condense under high temperatures (thanks to having condensed from material vapourized into space). Prior to the chemical evidence for the giant impact hypotheses, other moon-forming hypotheses under consideration included Earth grabbing an already-formed moon as it tried to whiz by us, and a very gloopy melted Earth spinning fast enough to throw off a glob of itself into space.

Today's Atmosphere Took a Long Time to Develop

Earth’s first experiment with having an atmosphere did not succeed. It started out with a thin veil of hydrogen and helium gases that came with the material it accreted. But hydrogen and helium are very light gases, and they bled off into space. Earth’s second experiment with having an atmosphere went much better. Volcanic eruptions built up the atmosphere by releasing gases. The most common volcanic gases are water vapour and carbon dioxide (CO₂), but volcanoes release a wide variety of gases. Other important contributions include sulphur dioxide (SO₂), carbon monoxide (CO), hydrogen sulphide (H₂S), hydrogen gas, and methane (CH₄). Meteorites and comets also brought substantial amounts of water and nitrogen to Earth. It's not clear what the exact composition of the atmosphere was after Earth’s second experiment, but carbon dioxide, water vapour, and nitrogen were likely the three most abundant components. One thing we can say for sure about Earth’s second experiment is that there was effectively no free oxygen (O₂, the form of oxygen that we breathe) in the atmosphere. We know this in part because prior to 2 billion years ago, there were no rocks stained red from oxidized iron minerals. Iron minerals were present, but not in oxidized form. At that time, O₂ was produced in the atmosphere when the sun’s ultraviolet rays split water molecules apart, but chemical reactions removed the oxygen as quickly as it was produced. It wasn’t until well into Earth’s third experiment—life—that the atmosphere became oxygenated. Photosynthetic organisms used the abundant CO₂ in the atmosphere to manufacture their food, and released O₂ as a by-product. At first all of the oxygen was consumed by chemical reactions as before, but eventually the organisms released so much O₂ that it overwhelmed the chemical reactions. Oxygen began to accumulate in the atmosphere, although present levels of 21% oxygen didn’t occur until about 350 million years ago. Today the part of our atmosphere that isn’t oxygen consists largely of nitrogen (78%). The oxygen-rich atmosphere on our planet is life’s signature. If geologic processes were the only ones controlling our atmosphere, it would consist mostly of carbon dioxide, like the atmosphere of Venus. It's an interesting notion (or a disconcerting one, depending on your point of view) that for the last 2 billion years the light reflected from our planet has been beaming a bar code out to the universe, similar to the ones in Figure 2.4, except ours says “oxygen.” For 2 billion years, our planet has been sending out a signal that could cause an observer from another world to say, “That’s odd… I wonder what’s going on over there.”

Earth: No Longer a Smashing Time

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Earth’s surface was heavily bombarded by objects from space early in its history. Today heavy bombardment isn't a concern. In other words, although objects exist in space that could hit Earth, we don’t (usually) spend our time running for cover from flying space rocks.Given what you know about solar system formation and about Earth’s evolution over its first 2 billion years, can you suggest a reason or reasons why we no longer have this problem? Solution There are two reasons bombardment is less of a problem. First, early in the solar system’s history more debris was scattered throughout the solar system and thrown about as the solar system evolved. Over time, fragments have been thrown out of the solar system and accreted by planets. Second, we now have a thicker atmosphere than early Earth did. Smaller objects on a collision course with Earth will burn up in the atmosphere and not make it to Earth’s surface.

References

Berg, M. T. L., et al., (2017). Deformation-aided segregation of Fe-S liquid from olivine under deep Earth conditions: Implications for core formation in the early solar system. Physics of Earth and Planetary Interiors, 263, 38-54. http://dx.doi.org/10.1016/j.pepi.2017.01.004.

2.5 Are There Other Earths?

kqzheng — Fri, 05 Jan 2018 23:26:28 +0000

As of July 2021, 4,777 exoplanets—extrasolar planets, or planets outside of our solar system—have been confirmed. An additional 4,640 potential exoplanets have been detected and await confirmation. The majority of exoplanets have been detected using the transit technique, which involves looking for periodic fluctuations in light from a star from planets transiting in front of the star, causing it to appear dimmer (Figure 2.15). [caption id="attachment_67" align="aligncenter" width="650"] Figure 2.15 The transit method identifies stars with possible planets in orbit by looking for brief decreases in the brightness of the star. If this happens regularly, it could mean that a planet is periodically blocking some of the star's light. Source: Karla Panchuk (2021) CC BY-NC-SA. Modified after NASA/HESARC/TESS (2021), Public Domain. Image source. Click for more attributions.[/caption] If "other Earths" are defined as planets where we could walk out of a spaceship with no equipment other than a picnic basket, and enjoy a pleasant afternoon on a grassy slope near a stream, then it remains to be seen whether any of these planets fit the description (although NASA's Exoplanet Travel Bureau would like to make some suggestions for you to explore and find out for yourself; Figure 2.16.) [caption id="attachment_68" align="aligncenter" width="1623"] Figure 2.16 NASA Exoplanet Travel Bureau travel posters for two Earth-like planets, Trappist-1e, and Kepler-186f. I want to visit Trappist-1e. I want to visit Kepler-186f. Source: NASA-JPL/Caltech (2021), Public Domain. Go to the Travel Bureau.[/caption] On the other hand, if "other Earths" refers to rocky worlds approximately Earth’s size, and orbiting within their star’s habitable zone (the zone in which liquid water, and potentially life, can exist), then as of the October 2020 update of the Habitable Exoplanets Catalog, it's possible that we have found 60 such worlds. Part of the uncertainty about the 60 possible Earth-like worlds is related to their composition. Only five have been confirmed to be rocky, but it's tempting to conclude that the others are because they're similar in size to Earth. Remember the rules of the accretion game: you can only begin to collect gas once you're a certain size, and how much matter you collect depends on how far away from the sun you are. Given how large our gas giant and ice giant planets are compared to Earth, and how far away they are from the sun, we would expect that a planet similar in size to Earth, and a similar distance from its star, should be rocky. But it isn’t quite as simple as that. We're finding that the rules to the accretion game can result in planetary systems very different from our own. In the planetary systems we've observed, it's common to have planets larger than Earth orbiting closer to their star than Mercury does to the sun. Planets as large as Jupiter are rare, and where large planets do exist, they're much closer to their star than Jupiter is to the sun. To summarize, we need to be cautious about drawing conclusions from our own solar system, just in case we're basing those conclusions on something truly unusual. On the other hand, the seemingly unique features of our solar system would make planetary systems like ours difficult to spot. Using the transit method, small planets are harder to detect because they block less of a star’s light than larger planets. Larger planets farther from a star—like our gas giant planets—are difficult to spot because they don’t go past the star as frequently. If someone were observing our solar system, they might have to watch for up to 12 years to see Jupiter pass in front of the sun. For Saturn, they might have to watch for 30 years.

Key Ideas About the Hunt for Other Planets

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Questions:

The exoplanet Kepler-452b is within the habitable zone of its star. In our solar system, planets a similar distance from the sun are terrestrial planets. Does Kepler-452b’s distance from its star means it's a terrestrial planet?
None of the planetary systems discovered so far are like our solar system. Does this mean our solar system is unique in the universe?

Answers:

We can't say from Kepler-452b’s position alone that it's a terrestrial planet. The fact that we have terrestrial planets close to the sun makes sense in terms of the frost line, but it doesn't seem to be a hard-and-fast rule in other planetary systems.
The rules of the accretion game mean there are many complex interactions, so we shouldn’t expect to find a planetary system exactly like ours. At the same time, just because we haven’t found a similar planetary system doesn't mean there isn't one. Our methods work best for seeing large planets that orbit close to their stars, whereas our solar system has small planets close to the sun and larger ones farther away. That doesn’t mean our methods won’t eventually turn up a system like ours, just that they're more likely to turn up systems that are different.

If Habitable Zone Planets Are Terrestrial, Could We Live There?

The operational definition of “other Earths” involving a terrestrial composition, a size constraint of one to two times that of Earth, and location within a star’s habitable zone, does not preclude worlds incapable of supporting life as we know it. By those criteria, Venus is an “other Earth,” albeit right on the edge of the habitable zone for our sun. Venus is much too hot for us, with a constant surface temperature of 465°C (lead melts at 327°C). Its atmosphere is almost entirely carbon dioxide, and the atmospheric pressure at its surface is 92 times higher than on Earth. Any liquid water on its surface boiled off long ago. Yet the characteristics that make Venus a terrible picnic destination aren’t entirely things we could predict from its distance from the sun. They depend in part on the geochemical evolution of Venus, and at one time Venus might have been a lot more like a youthful Earth. These are the kinds of things we won’t know about until we can look carefully at the atmospheres and compositions of habitable-zone exoplanets. Using Our Solar System to Understand the Kepler-102 Exoplanet System

The star Kepler-102 is in the constellation Lyra. Five confirmed planets orbit Kepler-102. This table shows how those planets compare to Mercury, Earth, and Jupiter in terms of size, mass, and density. All of the Kepler-102 planets are closer to their star than Mercury is to the sun. Kepler-102d and Kepler-102f are the closest in size to Earth. Which is most likely to also have a rocky mantle and metallic core like Earth does?

Kepler-102d (Density of 8.9 g/cm³)
Kepler-102f (Density of 5.0 g/cm³)

Kepler-102b is closer to Jupiter's density than Earth's. Could Kepler-102b have a composition similar to Jupiter's (i.e., compressed gas with a rocky core), or maybe like an ice giant planet such as Uranus?

No, it's definitely not like those planets.
Yes, it could have a composition like Jupiter or Uranus.

Kepler-102c and Kepler-102d have densities similar to iron meteorites (thought to be from the cores of shattered planets). What example from our own solar system might explain the nature of those planets, assuming they are indeed mostly metallic? Kepler-102e is twice Earth's size. Its density is lower, but still consistent with a rocky mantle and metallic core. If it is indeed terrestrial, how might the size of its core relative to its mantle compare to that of Earth?

It has more core relative to mantle.
It has less core relative to mantle.

To check your answers, navigate to the below link to view the interactive version of this activity.

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Chapter 2 Summary & Key Term Check

kqzheng — Sat, 06 Jan 2018 03:09:09 +0000

Chapter 2 Main Ideas

2.1 Starting With a Big Bang

The universe began 13.8 billion years ago when energy, matter, and space expanded from a single point. Evidence for the big bang includes the cosmic “afterglow” from when the universe was still very dense. Also, red-shifted light from distant galaxies tells us the universe is still expanding.

2.2 Forming Planets from the Remnants of Exploding Stars

The big bang produced hydrogen and some helium, but heavier elements come from nuclear fusion reactions in stars. Large stars make elements such as silicon, iron, and magnesium, which are important in forming terrestrial planets. Large stars explode as supernovae and scatter the elements into space. Earth's composition is closely related to the abundance of elements made by stars.

2.3 How to Build a Solar System

Solar systems begin with the collapse of a cloud of gas and dust. Material drawn to the centre forms a star, and the remainder forms a disk around the star. Material within the disk clumps together to form planets. In our solar system, rocky planets are closer to the sun, and ice and gas giants are farther away. This is because temperatures near the sun were too high for ice to form, but silicate minerals and metals could solidify.

2.4 Earth's First 2 Billion Years

Early Earth was heated by radioactive decay, collisions with bodies from space, and gravitational compression. Heating caused molten metal to sink to Earth’s centre and form a core, and silicate minerals to form the mantle and crust. A collision with a planet the size of Mars knocked debris into orbit around Earth, and the debris coalesced into the moon. Earth’s atmosphere is the result of volcanic degassing, contributions by comets and meteorites, and photosynthesis.

2.5 Are There Other Earths?

The search for exoplanets has identified 60 planets that are similar in size to Earth and within the habitable zone of their stars. These are thought to be rocky worlds like Earth, but the compositions of these planets are not known for certain.

Key Term Check

What key term from Chapter 2 is each card describing? Turn the card to check your answer. [h5p id="34"]

Debunk It!

[h5p id="35"]

The meme shown below contains a quote that was attributed to a primary candidate in the 2016 U. S. presidential election. How many errors can you find?

Image description: Picture of an explosion. Over top, it says, "The big bang is merely a theory. If by chance it was true, how would you explain that the Earth didn't blow up in the explosion?"

Oh my, where to start… First, saying the big bang is “merely a theory” implies that theories have little to support them. In fact, theories are hypotheses that have been confirmed over and over again. The big bang theory in particular has a wide range of supporting evidence, including the cosmic microwave background, and the red shift. Second, the notion of an "explosion" is incorrect. What's being described is a release of energy that broke up matter. The big bang was the creation of energy, matter, and space, so it isn't an explosion in the "kaboom" sense. Finally, the Earth would not have blown up during the explosion (that wasn't actually an explosion) because Earth didn’t exist at the time of the big bang. The universe was almost entirely hydrogen and helium. It would take billions of years and many lifetimes of stars to make the heavier elements such as silicon, oxygen, iron, and magnesium, that are fundamental to making a terrestrial planet like Earth.

12.2 Seismic Waves and Measuring Earthquakes

kqzheng — Wed, 10 Jan 2018 01:48:45 +0000

The shaking from an earthquake travels away from the rupture in the form of seismic waves. Seismic waves are measured to determine the location of the earthquake, and to estimate the amount of energy released by the earthquake (its magnitude).

Types of Seismic Waves

Seismic waves are classified according to where they travel, and how they move particles.

Body Waves

Seismic waves that travel through Earth's interior are called body waves. P-waves are body waves that move by alternately compressing and stretching materials in the direction the wave moves. For this reason, P-waves are also called compression waves. The "P" in P-wave stands for primary, because P-waves are the fastest of the seismic waves. They are the first to be detected when an earthquake happens. A P-wave can be simulated by fixing one end of a spring to a solid surface, then giving the other end a sharp push toward the surface (Figure 12.8, top). The compression will propagate (travel) along the length of the spring. Some parts of the spring will be stretched, and others compressed. Any one point on the spring will jiggle forward and backward as the compression travels along the spring. [caption id="attachment_480" align="aligncenter" width="650"] Figure 12.8 Seismic waves simulated using a spring and rope attached to a fixed surface. Top: P-waves travel as pulses of compression. Bottom: S-waves move particles at right angles to the direction of motion. Source: Karla Panchuk (2018), CC BY 4.0. Adapted from Steven Earle (2015), CC BY 4.0. View original.[/caption] S-waves are body waves that move with a shearing motion, shaking particles from side to side. S-waves can be simulated by fixing one end of a rope to a solid surface, then giving the other end a flick (Figure 12.8, bottom). Any one point on the rope will move from side to side at a right angle to the direction in which the snaking motion is traveling. The "S" in S-wave stands for secondary, because S-waves are slower than P-waves, and are detected after the P-waves are measured. S-waves cannot travel through liquids. P-waves and S-waves can travel rapidly through geological materials, at speeds many times the speed of sound in air.

Surface Waves

When body waves reach Earth’s surface, some of their energy is transformed into surface waves, which travel along Earth's surface. Two types of surface waves are Rayleigh waves and Love waves (Figure 12.9). Rayleigh waves (R-waves) are characterized by vertical motion of the ground surface, like waves rolling on water. Love waves (L-waves) are characterized by side-to-side motion. Notice that the effects of both kinds of surface waves diminish with depth in Figure 12.9. Surface waves are slower than body waves, and are detected after the body waves. Surface waves typically cause more ground motion than body waves, and therefore do more damage than body waves. [caption id="attachment_481" align="aligncenter" width="700"] Figure 12.9 Surface waves travel along Earth's surface and have a diminished impact with depth. Rayleigh waves (left) cause a rolling motion, and Love waves (right) cause the ground to shift from side to side. Source: Steven Earle (2015), CC BY 4.0. Image source. Click for more attributions.[/caption]

Concept Check: Seismic Wave Types

Fill in the blanks to complete this summary of types of seismic waves. -waves (hint: P, S, R, or L?) are (hint: fast or slow?) body waves that move by compressing and stretching rock. -waves (hint: P, S, R, or L?) are (hint: fast or slow?) body waves that move by shearing rock. -waves (hint: P, S, R, or L?) are surface waves that produce a rolling motion. -waves (hint: P, S, R, or L?) are surface waves that produce a side-to-side motion. To check your answers, navigate to the below link to view the interactive version of this activity.

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Recording Seismic Waves Using a Seismograph

A seismometer is an instrument that detects seismic waves. An instrument that combines a seismometer with a device for recording the waves is called a seismograph. The graphical output from a seismograph is called a seismogram. Figure 12.10 (right) shows how a seismograph works. The instrument consists of a frame or housing that is firmly anchored to the ground. A mass is suspended from the housing, and can move freely on a spring. When the ground shakes, the housing shakes with it, but the mass remains fixed. A pen attached to the mass moves up and down on a rotating drum of paper, drawing a wavy line, the seismogram. The seismograph in Figure 12.10 (right) is oriented to measure vertical ground motion. The photo on the left shows a seismograph oriented to record horizontal ground motion. [caption id="attachment_482" align="aligncenter" width="650"] Figure 12.10 How a seismograph records earthquakes. Source: Left- Karla Panchuk (2018), CC BY-NC-SA 4.0. Modified after IRIS (2012) "How Does a Seismometer Work?" Image source; Right: Karla Panchuk (2018), CC BY-SA 4.0. Photo by Z22 (2014), CC BY-SA 3.0. Image source. Click for more attributions.[/caption] The pen and drum of a mechanical seismograph record the motion of the ground relative to the mass. However, unless an earthquake causes a large amount of ground motion directly beneath the seismograph, the height of the wave recorded on paper might be very small, making the seismogram difficult to analyze. The seismograph on the right has a device to amplify the ground motion, drawing larger waves that are easier to study. Modern seismographs record shaking as electrical signals, and are able to transmit those signals. This means seismologists need not return to the instrument to collect recordings before the records can be examined.

Finding The Location of an Earthquake

P-waves travel faster than S-waves. As the waves travel away from the location of an earthquake, the P-wave gets farther and farther ahead of the S-wave. Therefore, the farther a seismograph is from the location of an earthquake, the longer the delay between when the P-wave arrival is recorded, and the S-wave arrival is recorded. The delay between the P-wave and S-wave arrival appears as a widening gap in a diagram of P-wave and S-wave travel times (Figure 12.11, grey lines). P-wave and S-wave arrival times can be identified on seismograms. In the three seismograms in Figure 12.11, the arrivals of the P-waves and S-waves are marked with arrows, and the interval in minutes between the P-wave and S-wave arrivals are noted. The seismograms were recorded at three different seismic stations (earthquake monitoring locations equipped with seismographs). The distance of each station from the earthquake is determined by finding the distance along the graph where the gap between the P-wave and S-wave travel-time curves matches the delay between P-wave and S-wave arrivals on the seismogram. [caption id="attachment_483" align="aligncenter" width="720"] Figure 12.11 Using P-wave and S-wave travel times to determine how far seismic waves have travelled. Grey curves show the distance travelled by P-waves and S-waves after an earthquake occurs. P-waves are faster than S-waves, and the gap between them increases with time and distance. The delays between P-wave and S-wave arrivals on seismograms are matched to the curve to find the distances of seismic stations from the source of the seismic waves. Source: Karla Panchuk (2018), CC BY-NC-SA 4.0. Modified after IRIS (n.d.) "How Are Earthquakes Located?" Image source.[/caption] The delay between the P-wave and S-wave arrival at a seismic station can indicate how far the station is from the source of the earthquake, but not the direction the seismic waves came from. The possible locations of the earthquake can be represented on a map by drawing a circle around the seismic station, with the radius of the circle being the distance determined from the P-wave and S-wave travel times (Figure 12.12). If this is done for at least three seismic stations, the circles will intersect at the origin of the earthquake. This method is called triangulation. [caption id="attachment_484" align="aligncenter" width="720"] Figure 12.12 Locating earthquakes by drawing three circles with radii of lengths determined from P-wave and S-wave travel times. Station names (SOCO, TEIG, SSPA) correspond to seismograms in Figure 12.11. Source: IRIS (n.d.) "How Are Earthquakes Located?" Image source. Click for terms of use.[/caption]

Concept Check: Finding the Location of an Earthquake

Fill in the blanks to complete this description of how earthquakes are located by triangulation. The -waves (hint: P or S) generated by an earthquake are faster than -waves (hint: P or S). The farther seismic waves have to travel before reaching a seismic (hint: a monitoring location with a seisometer), the more the fast waves will gain on the slow ones, and the longer we have to wait after the fast waves arrive to see the slow ones. We can use that lag to figure out how far away the earthquake was. This information gives us the (hint: direction or distance?) to the earthquake, but not the (hint: direction or distance?). To find the actual location, we need the lag information measured from at least (hint: how many?) locations. Then we can draw a circle from each location with a radius that matches the (hint: direction or distance?) we figured out. The location of the earthquake is where all of the circles (hint: geometry term for crossing). To check your answers, navigate to the below link to view the interactive version of this activity.

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How Big Was It?

Earthquakes can be described in terms of their magnitude, which reflects the amount of energy released by the shaking. They can also be described in terms of intensity, which characterizes the impact of the shaking on people and their surroundings.

Earthquake Magnitude

Earthquake magnitudes are determined by measuring the amplitudes of seismic waves. The amplitude is the height of the wave relative to the baseline (Figure 12.13). Wave amplitude depends on the amount of energy carried by the wave. The amplitudes of seismic waves reflect the amount of energy released by earthquakes. [caption id="attachment_485" align="aligncenter" width="648"] Figure 12.13 Seismogram for a small earthquake that occurred near Vancouver Island in 1997. The maximum amplitude of the S-wave is indicated. Source: Karla Panchuk (2018) CC BY 4.0. Modified after Steven Earle (2015), CC BY 4.0. Image source.[/caption] The Richter magnitude scale uses the amplitudes of S-waves, and corrects for the decrease in amplitude that happens as the waves travel away from their source. The correction depends on how seismic waves interact with the specific rock types through which they travel, and therefore on local conditions, so the Richter magnitude is also referred to as the local magnitude. While news reports about earthquakes might still refer to the "Richter scale" when describing magnitudes, the number they report is most likely the moment magnitude. The moment magnitude is calculated from the seismic moment of an earthquake. The seismic moment takes into account the surface area of the region that ruptured, how much displacement occurred, and the stiffness of the rocks. Moment magnitude can capture the difference between short earthquakes and longer ones resulting from larger ruptures, even of both types of earthquakes generate the same amplitude of waves. The moment magnitude scale is also better for earthquakes that are far from the seismic station. Seismic wave measurements are still used to determine the moment magnitude, however different waves are used than for the local magnitude scale. The magnitude scale is a logarithmic one rather than a linear one- an increase of one unit of magnitude corresponds to a 32 times increase in energy release (Figure 12.14). There are far more low-magnitude earthquakes than high-magnitude earthquakes. In 2017 there were 7 earthquakes of M7 (magnitude 7) or greater, but millions of tiny earthquakes. [caption id="attachment_486" align="aligncenter" width="1024"] Figure 12.14 Earthquake magnitude and corresponding energy release. Energy release increases by approximately 32 times for each unit change in magnitude. Source: IRIS (n.d.) "How Often Do Earthquakes Occur?" Image source.[/caption]

Earthquake Intensity

Intensity scales were first used in the late 19th century, and then adapted in the early 20th century by Giuseppe Mercalli and modified later by others to form what we now call the Modified Mercalli Intensity Scale (Table 12.1). To determine the intensity of an earthquake, reports are collected about what people felt and how much damage was done. The reports are then used to assign intensity ratings to regions where the earthquake was felt.

Table 12.1 Modified Mercalli Intensity Scale [footnote]Source: U. S. Geological Survey (1989). The severity of an earthquake. *USGS General Interest Publication,* 1989-288-913[/footnote]
Severity	Details
I Not felt	Not felt except by a very few under especially favourable conditions
II Weak	Felt only by a few persons at rest, especially on upper floors of buildings
III Weak	Felt quite noticeably by persons indoors, especially on upper floors of buildings; many people do not recognize it as an earthquake; standing motor cars may rock slightly; vibrations similar to the passing of a truck; duration estimated
IV Light	Felt indoors by many, outdoors by few during the day; at night, some awakened; dishes, windows, doors disturbed; walls make cracking sound; sensation like heavy truck striking building; standing motor cars rocked noticeably
V Moderate	Felt by nearly everyone; many awakened; some dishes, windows broken; unstable objects overturned; pendulum clocks may stop
VI Strong	Felt by all, many frightened; some heavy furniture moved; a few instances of fallen plaster; damage slight
VII Very Strong	Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken
VIII Severe	Damage slight in specially designed structures; considerable damage in ordinary substantial buildings with partial collapse; damage great in poorly built structures; fall of chimneys, factory stacks, columns, monuments, walls; heavy furniture overturned
IX Violent	Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb; damage great in substantial buildings, with partial collapse; buildings shifted off foundations
X Extreme	Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations; rails bent
XI Extreme	Few, if any (masonry), structures remain standing; bridges destroyed; broad fissures in ground; underground pipelines completely out of service; earth slumps and land slips in soft ground; rails bent greatly
XII Extreme	Damage total; waves seen on ground surfaces; lines of sight and level distorted; objects thrown upward into the air

Intensity values are assigned to locations, rather than to the earthquake itself. This means that intensity can vary for a given earthquake, depending on the proximity to the epicentre and local conditions. For the 1946 M7.3 Vancouver Island earthquake, intensity was greatest in the central island region (Figure 12.15). In some communities within this region, chimneys were damaged on more than 75% of buildings. Some roads were made impassable, and a major rock slide occurred. The earthquake was felt as far north as Prince Rupert, as far south as Portland Oregon, and as far east as the Rockies, but with less intensity. [caption id="attachment_487" align="aligncenter" width="550"] Figure 12.15 Intensity map for the M7.3 Vancouver Island earthquake on June 23, 1946. Source: Earthquakes Canada, Natural Resources Canada (2016), Image source. Click for terms of use.[/caption] Intensity estimates are important as a way to identify regions that are especially prone to strong shaking. A key factor is the nature of the underlying geological materials. The weaker the underlying materials, the more likely it is that there will be strong shaking. Areas underlain by strong solid bedrock tend to experience far less shaking than those underlain by unconsolidated river or lake sediments. An example of this effect is the 1985 M8 earthquake that struck the Michoacán region of western Mexico, southwest of Mexico City. There was relatively little damage near the epicentre, but 350 km away in heavily populated Mexico City there was tremendous damage and approximately 5,000 deaths. The reason is that Mexico City was built largely on the unconsolidated and water-saturated sediment of former Lake Texcoco. These sediments resonate at a frequency of about two seconds, which was similar to the frequency of the body waves that reached the city. Consequently, the shaking was amplified. Survivors of the disaster recounted that the ground in some areas moved up and down by approximately 20 cm every two seconds for over two minutes. Damage was greatest to buildings between 5 and 15 storeys tall, because they also resonated at around two seconds, which amplified the shaking. Try It: Modified Mercalli Intensity Scale [h5p id="148"]

References

Ammon, C. J. (2001). Earthquake size. http://eqseis.geosc.psu.edu/~cammon/HTML/Classes/IntroQuakes/Notes/earthquake_size.html

12.3 Earthquakes and Plate Tectonics

kqzheng — Fri, 12 Jan 2018 08:13:06 +0000

Bands of earthquakes trace out plate boundaries (coloured dots, Figure 12.16). The depths of earthquakes and the width of the band, depend on the type of plate boundary. Mid-ocean ridges and transform margins have shallow earthquakes (usually less than 30 km deep), in narrow bands close to plate margins. Subduction zones have earthquakes at a range of depths, including some more than 700 km deep. Bands of earthquakes are wider along subduction zones because they take place throughout the subducting slab that extends beneath the opposing plate. Wide swaths of scattered earthquakes may correspond to continent-continent collision zones, such as between the Eurasian plate and the African, Arabian, and Indian plates to the south. Wide swaths of scattered earthquakes may also correspond to continental rift zones, such as in eastern Africa. [caption id="attachment_490" align="aligncenter" width="650"] Figure 12.16 Earthquakes greater than magnitude 5, from 2000 to 2008. Bands of earthquakes mark tectonic plates. Narrow bands with shallow earthquakes (marked in red) indicate transform boundaries or mid-ocean ridge divergent boundaries. Wider bands with earthquakes at a range of depths are subduction zones. Wide bands of scattered earthquakes mark continent-continent convergent margins (e.g., between the Indian and Eurasian plates), or continental rift zones (e.g., in eastern Africa). Source: Lisa Christiansen, Caltech Tectonics Observatory (2008). Image source. Plate and ocean basin labels added. Click for terms of use.[/caption] Earthquakes are also relatively common at a few locations away from plate boundaries. Some are related to the buildup of stress due to continental rifting or the transfer of stress from other regions, and some are not well understood. Locations include the Great Rift Valley area of Africa, the Lake Baikal area of Russia, and Tibet.

Earthquakes at Divergent and Transform Plate Boundaries

Earthquakes along divergent and transform plate margins are shallow (usually less than 30 km deep) because below those depths, rock is too hot and weak to avoid being permanently deformed by the stresses in those settings. If deformation is permanent, then removing the stress does not result in the rocks snapping back to their original shape. No snapping back means no shaking. Mid-ocean ridge divergent plate margins are offset by numerous transform faults (Figure 12.17). The locations of earthquakes along mid-ocean ridges, and the mechanisms for causing them, depend on how rapidly the mid-ocean ridges are spreading. The Pacific-Antarctic Ridge (left) is spreading relatively rapidly at 42 to 94 mm/year, depending on the location along the ridge. Rapid spreading causes rocks near the axis of the spreading centre to be hot and weak. As a result, most of the earthquakes (white dots) are located along transform faults, where rocks are cooler and stronger. Along rapidly spreading ridges, new ocean crust is bent upward into wide, high ridges. As spreading proceeds and crust moves away from the ridge, the bend is relaxed, and the crust stretches and breaks. This triggers earthquakes many kilometres away from the ridge. [caption id="attachment_491" align="aligncenter" width="1080"] Figure 12.17 Locations of earthquakes of magnitude 4 and greater from 1990 to 2010 along two mid-ocean ridges. Plate boundaries are marked in red. Arrows show the direction of plate motion. Left: Rapidly spreading Pacific-Antarctic ridge with earthquakes concentrated along transform faults. Right: Slowly spreading Southwest Indian Ridge, with earthquakes along both spreading segments and transform faults. Source: Karla Panchuk (2017) CC BY 4.0. Base maps with epicentres generated using the U. S. Geological Survey Latest Earthquakes website. Visit Latest Earthquakes[/caption] The Southwest Indian Ridge (right) spreads very slowly, at approximately 14 mm/year. Rocks are cooler and stronger along the slowly spreading ridge than along the rapidly spreading one. In the slow-spreading environment, earthquakes are generated when rocks along the ridge axis stretch and break. Earthquakes are more evenly distributed between divergent and transform segments of the boundary than they are along fast-spreading ridges. Earthquakes in continental rift zones are also shallow, but scattered more broadly than those along mid-ocean ridges. Lake Baikal (Figure 12.28), the world's oldest, deepest, and largest freshwater lake, formed 25 million years ago because of continental rifting. Note the scale in Figure 12.18, and compare how widely the earthquakes (blue dots) are spread in the Lake Baikal region, versus along the mid-ocean ridges in Figure 12.17. [caption id="attachment_492" align="aligncenter" width="650"] Figure 12.18 Blue circles mark the locations of earthquakes of M4 and greater from 1990 to 2010 along the Lake Baikal rift zone. White lines show some of the faults in the region. White lines with tick marks are normal faults related to spreading. Arrows show the direction of spreading. White lines without tick marks are transform faults. The Siberian Craton (shaded region) is strong 2 billion year old crust. Source: Karla Panchuk (2017) CC BY 4.0. Faults after U. S. Geological Survey (see references). Base maps (inset and rift views) with epicentres generated using the U. S. Geological Survey Latest Earthquakes website. Visit Latest Earthquakes[/caption] One reason for the difference in earthquake distribution in continental rift zones is that the rifts are only beginning to form. Faulting is "disorganized" within the continental crust. There is no well-established spreading centre, unlike mid-ocean ridges. Another reason is that the locations of faults, and thus earthquakes, in continental rift zones are affected by pre-existing geological structures within continental crust. In the case of the Lake Baikal rift, the strong, ancient crust of the Siberian Craton influences the orientation of the faults forming the rift. Faults run parallel to the craton near Lake Baikal. As rifting extends to the east, the part of the craton in the upper right of Figure 12.18 may deflect rifting southward.

Earthquakes at Convergent Boundaries

Subduction Zones

Along convergent plate margins with subduction zones, earthquakes range from shallow to depths of up to 700 km. Earthquakes occur where the two plates are in contact, as well as in zones of deformation on the overriding plate, and along the subducting slab deeper within the mantle. The result is that epicentres of earthquakes farther to the interior of the overriding plate will correspond to increasingly deep earthquakes. Where the Pacific plate subducts beneath the North American plate, forming the Aleutian volcanic arc (Figure 12.19), earthquakes increase in depth moving northward, following the Pacific plate into the mantle. Earthquakes between 0 and 33 km deep (red circles) occur closest to the subduction zone (red line; teeth point in the direction of the subducting slab). While there is some overlap, earthquakes between 33 and 70 km deep (white circles) occur in a band that reaches farther the north. Farthest north are the epicentres for earthquakes between 70 and 300 km deep (green dots). The deepest earthquake during the seven year interval shown in Figure 12.19 is represented by the large green dot farthest to the north. It occurred at a depth of 265 km. [caption id="attachment_493" align="aligncenter" width="650"] Figure 12.19 Earthquakes of M4.5 and greater from 2010 to 2017 along the Aleutian Trench subduction zone (red line; teeth point in the direction of the subducting slab). White arrows show the directions of plate movement. Circle colours indicate the depths of earthquakes (see legend, lower left). Earthquakes become deeper moving north from the subduction zone. Source: Karla Panchuk (2017) CC BY 4.0. Base maps with epicentres generated using the U. S. Geological Survey Latest Earthquakes website. Visit Latest Earthquakes[/caption] Earthquakes occur in subduction zones for a variety of reasons. Stresses associated with the collision of two plates cause deformation in the overriding plate, and thus shallow earthquakes. Shallow earthquakes also happen on the subducting slab when a locked zone (orange line, Figure 12.20) ruptures. The locked zone is where the largest earthquakes on Earth, called megathrust earthquakes, occur. There is the potential for a wider rupture zone on a gently dipping subduction zone boundary compared to other boundaries. [caption id="attachment_494" align="aligncenter" width="1024"] Figure 12.20 Factors contributing to earthquakes in subduction zones. Not all factors shown here are present in all subduction zones. Stress from bending, flexing, and stretching may cause ruptures. Changes in the mechanical properties of the mantle may affect how subucting slabs move, contributing additional stresses. The histogram at right shows the global average number of earthquakes at depth, for earthquakes greater than M5. The increase in earthquakes beneath 480 km may be caused in part by weakening as olivine transforms into high pressure/temperature polymorphs. Source: Karla Panchuk (2017) CC BY 4.0. Modified after Green et al. (2010).[/caption] If subduction is rapid, the subducting plate will bend more as it enters the mantle (slab A in Figure 12.20), causing the upper edge of the plate to stretch, and the interior and lower edge to be compressed. Stress from bending can cause shallow to intermediate earthquakes on these plates. Even without bending, the subducting slab can become stretched by its own weight as it falls into the mantle. The 410 km and 660 km discontinuities in Figure 12.20 mark boundaries where minerals transform into other, denser minerals that are stable at higher pressures and temperatures. When the subducting slab reaches the 660 km discontinuity (the top of the lower mantle), the increase in density in the surrounding mantle may slow down the leading edge of the sinking slab. Earthquakes can be generated when the slab is compressed by the lower mantle resisting its motion at the same time that the upper part of the slab continues to fall. Slower rates of subduction mean that the subducting slab will enter the mantle at a lower angle (slab B in Figure 12.20). These slabs might not have earthquakes from being bent downward into the mantle, as with slab A, but earthquakes may be triggered by changes in stress if the plate relaxes and unbends. The bar chart on the right of Figure 12.20 shows global average number of earthquakes that occur at different depths. Earthquakes are most abundant at the surface, and then fall to a minimum at 300 km. The number of earthquakes remains low until almost 500 km depth, and reaches a second peak around 600 km depth. The second peak might be explained by interactions between the subducting plate and the dense mantle beneath the 660 km discontinuity, but another hypothesis is that it is related to delayed mineral transformations. The subducting slab warms as it goes deeper into the mantle, but the warming is not uniform. The outer edges of the slab will warm before the interior does. The 410 km discontinuity is where olivine is transformed into the minerals wadsleyite and ringwoodite under normal mantle pressure and temperature conditions. However, if the interior of the subducting slab is still too cool at that depth, olivine will be retained to depths below 410 km. Olivine weakens prior to transforming into the high pressure minerals, and the weakening may make it easier for the slab to rupture.

Continent-Continent Convergence Zones

Where continents collide, earthquakes are scattered over a much wider area compared to earthquakes along mid-ocean ridges, transform margins, or subduction zones. An example is where the Indian plate collides with the Eurasian plate (Figure 12.21). At one time, India was a separate continent, and ocean crust separated India from the Eurasian plate. For a time, a subduction zone existed where ocean lithosphere from the Indian plate subducted beneath the Eurasian plate. But when the two land masses finally met, they became locked together and the subduction zone was closed off. Today the Indian plate is still pushing against the Eurasian plate in the regions indicated by the red arrows in Figure 12.21. The collision is accommodated by transform boundaries along the Indian plate. Regions of overall transform motion are indicated in Figure 12.21 with blue arrows. [caption id="attachment_495" align="aligncenter" width="648"] Figure 12.21 Earthquakes of M4.5 and greater from 1990 to 2017 along the collision zone between the Indian and Eurasian plates. Red lines- plate boundaries; red arrows- collision zones; blue arrows- transform zones. Source: Karla Panchuk (2017) CC BY 4.0. Base maps with epicentres generated using the U. S. Geological Survey Latest Earthquakes website. Visit Latest Earthquakes[/caption] The majority of earthquakes in Figure 12.21 occur at depths less than 70 km, however they are still abundant down to 150 km, and extend to more than 300 km depth at some locations. Deeper earthquakes may be caused by continued northwestward subduction of part of the Indian plate beneath the Eurasian plate in this area. Even though the area is no longer a subduction zone, the subducted slab still remains, and is subject to stresses that can trigger earthquakes. Some of the earthquakes in Figure 12.21 are related to the transform faults on either side of the Indian plate, and most of the others are related to the squeezing caused by the continued convergence of the Indian and Eurasian plates. That squeezing has caused the Eurasian plate to be thrust over the Indian plate, building the Himalayas and the Tibet Plateau to enormous heights. Most of the earthquakes of Figure 12.21 are related to the thrust faults shown in Figure 12.22 (and to hundreds of other similar ones that cannot be shown at this scale). The southernmost thrust fault in Figure 12.22 (the Main Boundary Fault) is equivalent to the convergent boundary in Figure 12.21. [caption id="attachment_496" align="aligncenter" width="650"] Figure 12.22 Schematic diagram of the India-Asia convergent boundary, showing examples of the types of faults along which earthquakes are focused. Source: Steven Earle (2015), CC BY 4.0. Image source. After D. Vuichard (Figure 2.3) in Ives and Messerli (1989).[/caption]

Intraplate Earthquakes

Intraplate earthquakes (within-plate earthquakes) are those that occur away from plate boundaries. Some intraplate earthquakes are related to human activities. When humans trigger earthquakes it is referred to as induced seismicity. In Saskatchewan there have been 20 earthquakes since 1985 (all less than magnitude 4), and the majority occurred near potash mines. Excavation changes the stress in surrounding rocks, so earthquakes may occur in the rocks above excavated parts of the mine. In Alberta, induced seismicity is triggered by hydraulic fracturing operations when water pressure increases along existing faults, causing them to slip. Intraplate earthquakes not related to human activities often occur along ancient rift zones. In eastern Canada, the Charlevoix seismic zone (approximately 100 km northeast of Québec City; Figure 12.23), is associated with a rift-zone faults that developed when an ancient ocean basin began to form more than 500 million years ago. Coincidentally, the rocks of the Charlevoix Seismic Zone are also fragmented because of a meteorite impact (the crater margin is indicated by the blue circle in Figure 12.23), weakening them further. While the Charlevoix zone is far from any boundary of the North American plate, tectonic forces acting on plate boundaries are still transmitted to the interior of the continent, contributing to the stress that causes the faults along the rift zone to rupture. [caption id="attachment_497" align="aligncenter" width="576"] Figure 12.23 Charlevoix seismic zone, site of intraplate earthquakes. The location of the Charlevoix seismic zone is indicated by the star on the map of Canada. Dots indicate earthquake epicentres. The size of the dots indicates magnitude. White lines indicate fault segments. The dashed circle marks the edge of a crater formed by a meteorite impact 342 million years ago. Source: Karla Panchuk (2017), CC BY-SA 4.0. Epicentre data from Earthquakes Canada. Click for more attributions and data sources.[/caption] Intraplate earthquakes can be large earthquakes. The Charlevoix seismic zone has had five earthquakes of magnitudes between 6 and 7 since 1663. The New Madrid seismic zone in the Mississippi River Valley had a series of four earthquakes with magnitudes between 7 and 8 in the winter of 1811-1812. The population of the region was sparse at the time, but today there are major cities in the New Madrid seismic zone, including Memphis, Tennessee, and St. Louis, Missouri.

Interactive Video Overview [h5p id="149"] Putting It Together: Earthquakes and Tectonic Plates [h5p id="150"]

References

Bao, X., and Eaton, D. W. (2016). Fault activation by hydraulic fracturing in western Canada. Science 354(6318), 1406-1409. doi: 10.1126/science.aag2583

Buck, W. R., Lavier, L. L., & Poliakov, A. N. B. (2005). Modes of faulting at mid-ocean ridges. Nature 434(7034), 719-23. doi: 10.1038/nature03358

Coastal and Marine Geology Program, U. S. Geological Survey (n.d.). Lake Baikal - A Touchstone for Global Change and Rift Studies. https://pubs.usgs.gov/fs/baikal/

DeMets, C., Gordon, R. G., & Argus, D. F. (2010). Geologically current plate motions. Geophysical Journal International 181, 1-80. doi: 10.1111/j.1365-246X.2009.04491.x

Earthquakes Canada, Natural Resources Canada (2016) Earthquake zones in Eastern Canada. http://www.seismescanada.rncan.gc.ca/zones/eastcan-en.php

Gendzwill, D. J., Horner, R. B., & Hasegawa, H. S. (1982). Induced earthquakes at a potash mine near Saskatoon, Canada. Canadian Journal of Earth Science 19, 466-475. doi: 10.1139/e82-038

Green, H. W. II, Chen, W.-P., & Brudzinski, M. R. (2010). Seismic evidence of negligible water carried below 400-km depth in subducting lithosphere. Nature 467, 828-830. doi:10.1038/nature09401

Hongyu, Y., Liu, Y., Harrington, R. M., & Lamontagne, M. (2016). Seismicity along St. Lawrence Paleorift Faults Overprinted by a Meteorite Impact Structure in Charlevoix, Québec, Eastern Canada. Bulletin of the Seismological Society of America 106(6), 2663-2673. doi: https://doi.org/10.1785/0120160036

Ives, J. D., Messerli, B. (1989). The Himalayan dilemma: Reconciling development and conservation. Routledge. https://www.nzdl.org/cgi-bin/library?e=d-00000-00---off-0hdl--00-0----0-10-0---0---0direct-10---4-------0-1l--11-en-50---20-about---00-0-1-00-0--4----0-0-11-10-0utfZz-8-10&cl=CL1.8&d=HASH0113acdd201c910d3d4d9d9d.5.2>=1

Myhill, R., & Warren, L. M. (2012). Fault plane orientations of deep earthquakes in the Izu-Bonin-Marianas subduction zone. Journal of Geophysical Research, 117, B06307. doi:10.1029/2011JB009047

Nishikawa, T., & Ide, S. (2015). Background seismicity rate at subduction zones linked to slab-bending-related hydration. Geophysical Research Letters, 42, 7081-7089. doi: 10.1002/2015GL064578

Sloan, R. A., Jackson, J. A., McKenzie, D., & Priestley, K. (2011). Earthquake depth distributions in central Asia, and their relations with lithosphere thickness, shortening and extension. Geophysics Journal International, 185, 1-29. doi: 10.1111/j.1365-246X.2010.04882.x

12.4 The Impacts of Earthquakes

kqzheng — Fri, 12 Jan 2018 08:29:25 +0000

Earthquakes can have direct impacts, such as structural damage to buildings from shaking, and secondary impacts, such as triggering landslides, fires, and tsunami. The types and extent of impacts will depend on local conditions where the earthquake strikes. The geological materials in the area matter, as does the type of terrain, and whether the region is near the coast or not. The extent of impact and type of damage will depend on whether the area is predominantly urban or rural, densely or sparsely populated, highly developed or underdeveloped. It will depend on whether the infrastructure has been designed to withstand shaking.

Damage to Structures from Shaking

As with the example of the 1985 Mexico earthquake, the geological foundations on which structures are built will affect the amount of shaking that occurs. Earthquakes produce seismic waves that vibrate at different rates, or frequencies. Waves with rapid vibrations have a high frequency, and waves with slower vibrations have lower frequencies. The energy of the higher frequency waves tends to be absorbed by solid rock. Lower frequency waves pass through solid rock without being absorbed, but are absorbed and amplified by soft sediments. It is therefore very common to see much worse earthquake damage in areas underlain by soft sediments than in areas of solid rock. During the 1989 Loma Prieta earthquake, parts of a two-layer highway in the Oakland area near San Francisco collapsed where they were built on soft sediments (Figure 12.24). [caption id="attachment_500" align="aligncenter" width="600"] Figure 12.24 A collapsed section of the Cypress Freeway in Oakland California. Source: U. S. Geological Survey (1989), Public Domain. Image source.[/caption] Building damage is also greatest in areas of soft sediments, and multi-storey buildings tend to be more seriously damaged than smaller ones. Buildings can be designed to withstand most earthquakes, and this practice is increasingly applied in earthquake-prone regions. Turkey is one such region, but even though Turkey had a relatively strong building code in the 1990s, adherence to the code was poor. Builders did whatever they could to save costs, including using inappropriate materials in concrete, and reducing the amount of steel reinforcing. The result was more than 17,000 deaths in the 1999 M7.6 Izmit earthquake (Figure 12.25). After two devastating earthquakes that year, Turkish authorities strengthened the building code further, but the new code has been applied only in a few regions, and enforcement of the code is still weak, as revealed by the amount of damage from a M7.1 earthquake in eastern Turkey in 2011. [caption id="attachment_501" align="aligncenter" width="650"] Figure 12.25 Damage from the 1999 M7.6 Izmit, Turkey earthquake. Source: Left; USGS (1999), Public Domain. Image source. Right: USGS (1999), Public Domain. Image source.[/caption]

Challenge: Which Seismic Wave Type Did This?

On March 27, 1964, an earthquake of magnitude 9.2 hit Alaska. The figure below shows damage to train tracks resulting from this earthquake. Which type of seismic waves—P-waves, S-waves, R-waves, or L-waves—caused this damage?This was caused by -waves, which are surface waves that move side to side in a snake-like fashion. To check your answers, navigate to the below link to view the interactive version of this activity.

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Structures underlain by sediments may be at risk of another hazard, called liquefaction, in which sediment is transformed into a fluid. When water-saturated sediments are shaken, the grains may lose contact with each other, and no longer support one another. Water between the grains holds them apart, causing the sediment to turn to mud and flow. The loss of support can lead to the collapse of buildings or other structures that might otherwise have sustained little damage. During the 1964 M7.6 earthquake in Niigata, Japan, liquefaction caused buildings to sink into the sediments (Figure 12.26). [caption id="attachment_502" align="aligncenter" width="650"] Figure 12.26 Collapsed apartment buildings in the Niigata area of Japan. The material beneath the buildings was liquefied by the 1964 earthquake. Source: DOC/NOAA/NESDIS/NCEI (1964), Public Domain. Image source.[/caption] Parts of the Fraser River delta are also prone to liquefaction-related damage. The region is characterized by a 2 m to 3 m thick layer of fluvial silt and clay above a layer of water-saturated fluvial sand that is at least 10 m thick (Figure 12.27). Under these conditions, seismic shaking can be amplified, and the sandy sediments will liquefy. This could lead to subsidence and tilting of buildings. Liquefaction can also contribute to slope failures and to fountains of sandy mud (sand volcanoes) in areas where there is loose saturated sand beneath a layer of more cohesive clay. Current building-code regulations in the Fraser delta area require that measures be taken to strengthen the ground beneath multi-story buildings prior to construction. [caption id="attachment_503" align="aligncenter" width="650"] Figure 12.27 Recent unconsolidated sedimentary layers in the Fraser River delta area (top) and the potential consequences in the event of a damaging earthquake. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Experiment with Liquefaction To see liquefaction for yourself, go to a sandy beach and find a place near the water’s edge where the sand is wet. While standing in one place on a wet part of the beach, start moving your feet up and down at a frequency of about once per second. Within a few seconds the previously firm sand will start to lose strength, and you’ll gradually sink in up to your ankles. If you can’t get to a beach, put some sand into a small container, saturate it with water, and then pour the excess water off. Shake the container gently to get the water to separate and then pour the excess water away. You may have to do this more than once. Place a small rock on the surface of the sand. It should sit there for hours without sinking in. Now, holding the container in one hand gently thump the side or the bottom with your other hand, about twice a second. The rock should gradually sink in as the sand around it becomes liquefied. [caption id="attachment_504" align="aligncenter" width="550"] Figure 12.28 Fine, damp sand before shaking (left) and after (right). Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] As you were moving your feet up and down or thumping the container, it’s likely that you soon discovered the most effective rate for getting the sand to liquefy. Stepping up and down as fast as you can (several times per second) on the wet beach would not have been effective, nor would you have achieved much by stepping once every several seconds. The body of sand vibrates most readily in response to shaking that is close to its natural harmonic frequency, and liquefaction is also most likely to occur at that frequency.

Slope Failure

Ground shaking during an earthquake can be enough to weaken rock and loose materials to the point of failure. Earthquakes can also trigger failures on slopes that are already weak. In January of 2011 a M7.6 earthquake offshore of El Salvador triggered slope failures that killed nearly 600 people (Figure 12.29). [caption id="attachment_505" align="aligncenter" width="450"] Figure 12.29 A slope gives way in a suburb of San Salvador after the January 2001 earthquake offshore of El Salvador. This is one of hundreds of slope failures resulting from the earthquake. Source: U. S. Geological Survey (2001), Public Domain. Image source.[/caption]

Fires

Fires are commonly associated with earthquakes because gas lines rupture and electrical lines are damaged when the ground shakes. Most of the damage in the great 1906 San Francisco earthquake was caused by massive fires in the downtown area of the city (Figure 12.30). Some 25,000 buildings were destroyed by those fires, which were fueled by gas leaking from broken pipes. Fighting the fires was difficult because water mains had also ruptured. Today the risk of fires can be reduced through P-wave early warning systems if utility operators can decrease pipeline pressure and break electrical circuits. [caption id="attachment_506" align="aligncenter" width="1024"] Figure 12.30 Fires in San Francisco following the 1906 earthquake. Source: Pillsbury Picture Co. (1906), Public Domain. Courtesy of the Library of Congress Prints and Photographs Division. Image source.[/caption]

Tsunami

Large earthquakes that take place beneath the ocean have the potential to displace large volumes of water. In a subduction zone, for example, the overriding plate becomes distorted by elastic deformation. It is squeezed laterally and pushed up (Figure 12.31 top). When an earthquake happens, the plate rebounds over an area of thousands of square kilometres, generating waves- a tsunami (Figure 12.31, middle). The waves spread across the ocean at velocities of several hundred kilometres per hour. Tsunami can make it to the far side of an ocean in about the same time as a passenger jet. [caption id="attachment_507" align="aligncenter" width="525"] Figure 12.31 Tsunami triggered along a subduction zone. Top: The overriding crust is deformed because it is locked to the subducting slab. Middle: When the locked zone ruptures, the crust rebounds, and waves are created. Bottom: Tsunami waves have small amplitudes in the deep ocean, but once in shallow water, they slow down, causing the waves to become taller and closer together. Source: Karla Panchuk (2018), CC BY-NC-SA 4.0. Top and middle modified after Steven Earle (2015), CC BY 4.0. Image source. Bottom modified after COMET/UCAR (1997-2017) view source. Click for COMET/UCAR attribution and terms of use.[/caption] Tsunami waves gain their height as they travel through shallower waters. In the deep ocean, the waves may be so small as to go undetected by ships, but when they are slowed down by interacting with the ocean floor, they can become much taller (Figure 12.31, bottom). In the tsunami following the 2004 Sumatra earthquake, the tallest waves were more than 30 m high. Subduction earthquakes must be large to generate significant tsunami. Earthquakes with magnitude less than 7 do not typically generate significant tsunami because the amount of vertical displacement of the sea floor is minimal. Sea-floor transform earthquakes, even large ones (M7 to M8), don’t typically generate tsunami either, because the motion is mostly side to side, not vertical. Tsunami can have an impact across an entire ocean basin. They spread across the ocean at velocities of several hundred kilometres per hour, and can make it to the far side of an ocean in about the same time as a passenger jet. In 1700 a rupture along the Cascadia thrust fault running from Vancouver Island to northern California resulted in a M9 earthquake. It generated a tsunami that travelled across the Pacific Ocean, and was recorded in Japan nine hours later. A computer simulation of the tsunami (Figure 12.32) shows how long it took tsunami waves from the Cascadia earthquake to travel across the Pacific Ocean, and how high the waves were. Notice that over all, the waves decrease in height moving away from the rupture, but they increase in height again as they reach the opposite side of the Pacific Ocean. [caption id="attachment_508" align="aligncenter" width="550"] Figure 12.32 Computer simulation of the tsunami from the 1700 M9 Cascadia earthquake. Colours show open-ocean wave heights. Contours show travel time in hours. Tsunami wave heights increase as the tsunami reached the western margin of the Pacific ocean. Source: NOAA/PMEL/Center for Tsunami Research (2011), Public Domain. Image source. / View context.[/caption]

Concept Check: Tsunami Hazard

The Australian Government publishes a brochure called Tsunami Information for Recreational Boaters. In it they say,“If your boat is at sea or in the deep ocean and a tsunami warning is issued, maintain your position and do not return to port until further advised.”Why is this good advice? To check your answers, navigate to the below link to view the interactive version of this activity

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References

Earthquakes Canada, Natural Resources Canada (2016). The M9 Cascadia Megathrust earthquake of January 26, 1700. http://www.earthquakescanada.nrcan.gc.ca/historic-historique/events/17000126-en.php

University Corporation for Atmospheric Research (2010). Propagation. Tsunamis. http://www.torbenespersen.dk/Publish/tsunami/print_4.htm#page_1.1.2

U. S. Geological Survey (2014). Indian Ocean tsunami remembered — Scientists reflect on the 2004 Indian Ocean that killed thousands. https://www2.usgs.gov/blogs/features/usgs_top_story/indian-ocean-tsunami-remembered-scientists-reflect-on-the-2004-indian-ocean-that-killed-thousands/

12.5 Forecasting Earthquakes and Minimizing Impacts

kqzheng — Fri, 26 Jan 2018 00:43:53 +0000

It has long been a dream of seismologists, geologists, and public safety officials to be able to accurately predict the location, magnitude, and timing of earthquakes on time scales that would be useful for minimizing danger to the public and damage to infrastructure (e.g., weeks, days, hours). Many methods of prediction that have been explored. These include using observations of warning foreshocks, changes in magnetic fields, episodic tremor and slip, changing groundwater levels, strange animal behaviour, patterns in the timing between earthquakes, and how stress is transferred after a rupture. So far, none of these has provided a reliable method. Although there are some reports of successful earthquake predictions, they are rare, and many are surrounded by doubtful circumstances.

The Parkfield Prediction Experiment

There was great hope for earthquake predictions late in the 1980s when attention was focused on part of the San Andreas Fault at Parkfield, approximately 200 km south of San Francisco, California. Between 1881 and 1965 there were five earthquakes at Parkfield. They were spaced at approximately 20-year intervals, all confined to the same 20 km-long segment of the fault, and all very close to M6 (Figure 12.33). Both the 1934 and 1966 earthquakes were preceded by small foreshocks exactly 17 minutes before the main quake. [caption id="attachment_511" align="aligncenter" width="650"] Figure 12.33 Earthquakes on the Parkfield segment of the San Andreas Fault between 1881 and 2004. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] The U. S. Geological Survey recognized this as an excellent opportunity to understand earthquakes and earthquake prediction, so they armed the Parkfield area with a huge array of geophysical instruments and waited. The next earthquake was expected to happen around 1987, but nothing happened! The “1987 Parkfield earthquake” finally struck in September 2004. Fortunately, all of the equipment was still there to record the earthquake, but it was no help from the perspective of earthquake prediction. There were no significant precursors to the 2004 Parkfield earthquake in any of the parameters measured, including tremors, changes in rock deformation, the magnetic field, the electrical conductivity of the rock, and creep (motion along the fault that is not accompanied by earthquakes). There was no foreshock. In other words, even though every available technique was used to monitor it, the 2004 earthquake came with no warning whatsoever.

Earthquake Probabilities

To be useful to the public and governments, earthquake predictions must be accurate most of the time, not just some of the time. If a prediction method is only accurate 10% of the time (and even that isn’t possible with the current state of knowledge), the public will lose faith in the process very quickly, and then will ignore all of the predictions. The hope for earthquake prediction is not dead, but it was hit hard by the Parkfield experiment. Today the focus of efforts in earthquake-prone regions is to provide forecasts of earthquake probability. Earthquake probabilities express the likelihood that an earthquake of a given magnitude will occur at a location within a given period of time. An example of this approach for the San Francisco Bay region of California is shown in Figure 12.34. Based on a wide range of information, including past earthquake history, accumulated stress from plate movement, and known stress transfer, seismologists and geologists have predicted the likelihood of a M6.7 or greater earthquake on each of eight major faults that cut through the region. The greatest probabilities are on the San Andreas, Rogers Creek/Hayward faults, and Calaveras/Paicines faults. There is a 72% chance that a major and damaging earthquake will take place somewhere in the region prior to 2043. [caption id="attachment_512" align="aligncenter" width="648"] Figure 12.34 Earthquake outlook for 2014-2047. Probabilities for individual faults are marked on the faults. Source: U. S. Geological Survey (2014), Public Domain. View report.[/caption]

Using Earthquake Probability Information

Decision makers can use forecasts of earthquake probability to assist with educating the public about earthquake risks, and to determine what action is necessary to make infrastructure earthquake-safe. Building safe infrastructure requires strong building codes, and enforcement of those codes. Building code compliance is robust in most developed countries, but is inadequate in many developing countries. New buildings are not the only ones requiring attention. Existing buildings — especially schools and hospitals — and other structures such as bridges and dams, must also be made safe. British Columbia began a multi-billion-dollar program in 2004 to make public schools safer for students. The program is focused on older public schools, because, according to the government, those built since 1992 already comply with modern seismic codes. Some schools would require too much work to make upgrading economically feasible and they are replaced. Where upgrading is feasible, the school is assessed carefully before any upgrade work is initiated. The seismic mitigation program identified 346 schools as being at high risk and in need of upgrades. As of December 2017, upgrades were completed at 168 schools, 28 schools were under construction or had approval to proceed with construction, and 150 did not yet have plans in place for upgrades. The program in British Columbia illustrates a challenge with seismic upgrades of public buildings, which is that governments must make adequate funds available for the upgrades to be done in a timely manner. The priority allocated to funding those projects will depend on how urgent the need for upgrades is considered to be, given other demands on public funds.

Earthquake Preparedness

Earthquake preparedness involves the formulation of public emergency plans, including escape routes, medical facilities, shelters, and food and water supplies. It also includes personal planning, such as emergency supplies (food, water, shelter, and warmth), escape routes from houses and offices, and communication strategies (with a focus on ones that don’t involve the cellular network).

References

Finn, W. D. L., & Dexter, A. (2012). Risk management plan for school seismic mitigation program. https://www2.gov.bc.ca/assets/gov/education/administration/resource-management/capital-planning/risk_management_plan_school_seismic_mitigation_program_2012.pdf

Province of British Columbia (n.d.). Seismic mitigation program. https://www2.gov.bc.ca/gov/content/education-training/administration/resource-management/capital-planning/seismic-mitigation-program

Province of British Columbia (2017). Seismic mitigation program progress report. https://www2.gov.bc.ca/assets/gov/education/administration/resource-management/capital-planning/seismic-mitigation/progress_report.pdf

Chapter 12 Summary & Key Term Check

kqzheng — Fri, 26 Jan 2018 02:19:06 +0000

Chapter 12 Main Ideas

12.1 What Is an Earthquake?

An earthquake is the shaking that results when a deformed body of rock snaps back to its original shape. The rupture is initiated at a point but quickly spreads across the area of a fault, with aftershocks initiated by stress transfer. Episodic tremor and slip is a periodic slow movement, accompanied by harmonic tremors, along the middle part of a subduction zone boundary.

Practice Again

Extra!

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Can plastic deformation of Earth’s crust (deformation that is not reversible) cause an earthquake? No. The vibration from earthquakes is a result of elastic rebound. This means that deformed rock has snapped back to its original shape. If deformation is not reversible, then elastic energy is not being stored, and rebound is not possible. Rocks may still be offset along a fault due to plastic deformation, but the offset happens as slow, creeping movement called aseismic slip (i.e., slip without seismicity/shaking).

12.2 Measuring Earthquakes

Earthquakes produce seismic waves that can be measured by a seismograph. The amplitudes of seismic waves are used to determine the amount of energy released by an earthquake- its magnitude. For the moment magnitude scale used today, the amount of energy released by an earthquake is proportional to the size of the rupture surface, the amount of displacement, and the strength of the rock. Intensity is a measure of the amount of shaking that occurs, and damage done at locations that experience an earthquake. Intensity will vary depending on the distance to the epicentre, the depth of the earthquake, and the type of geological materials present.

Practice Again

12.3 Earthquakes and Plate Tectonics

Most earthquakes happen at or near plate boundaries. Along divergent and transform boundaries earthquakes are shallow (less than 30 km depth), but at convergent boundaries they can be hundreds of kilometers beneath the surface. The largest earthquakes happen when a broad segment of the locked zone of a subduction zone ruptures. Intraplate earthquakes happen away from plate boundaries. They can be caused by human activities, or renewed motion on ancient faults.

Practice Again

Watch the interactive video

12.4 The Impacts of Earthquakes

Damage to buildings is the most serious consequence of most large earthquakes. The amount of damage is related to the type and size of buildings, how they're constructed, and the nature of the material on which they're built. Other important consequences are fires, damage to bridges and highways, slope failures, liquefaction, and tsunami.

12.5 Forecasting Earthquakes and Minimizing Impacts

There is no reliable technology for predicting earthquakes, but the probability of one happening within a certain time period can be forecast. We can minimize earthquake impacts by ensuring that the public is aware of the risk, that building codes are enforced, that existing buildings like schools and hospitals are seismically sound, and that both public and personal emergency plans are in place.

Key Term Check

What key term from Chapter 12 is each card describing? Turn the card to check your answer. [h5p id="154"]

13.1 Stress and Strain

kqzheng — Thu, 01 Feb 2018 01:32:33 +0000

Plate collisions and the accumulated weight of overlying rocks exert forces on rocks at depth. While the size of the force is important, it also matters whether the force is distributed over a wide region, or tightly focused on a small area. The same force will have a greater effect when acting over a small area than when acting over a larger area. If you have ever used snowshoes to walk across a snow bank without sinking in, you've taken advantage of the effects of distributing force (your mass acted upon by gravity) over a wider area (the area of your snowshoes rather than the soles of your boots). Stress is force adjusted for the area over which it is distributed. Strain is the change in shape that happens when rocks are deformed by stress.

Types of Stress

Stresses fall into two categories: normal stress acts at right angles to a surface, and shear stress acts parallel to a surface (Figure 13.2). Normal stress is subdivided into compression, when the stresses are squeezing a rock, and tension, when stress is pulling it apart. Rocks undergo compression in regions where plates are colliding, or where they're being buried beneath other rocks. Rocks experience tension where divergence is happening, such as when a continent is beginning the rifting process. Shear stress is characteristic of transform plate boundaries, where plates are moving side by side. [caption id="attachment_519" align="aligncenter" width="620"] Figure 13.2 Rocks can be affected by normal stress (compression and tension) or shear stress. Source: Karla Panchuk (2016), CC BY 4.0.[/caption] Although Figure 13.2 shows only one set of stress arrows for each scenario, rocks within the Earth are subject to stress from all directions. The relative size of the stresses in different directions will determine the response of the rock. Consider a deeply buried rock being stretched as a continent breaks apart (Figure 13.3). It is also being compressed by the weight of overlying sediments and rocks, but the stress from compression is relatively small compared to the tension from rifting. The net effect of stress acting on the rock will be determined more by the tension from rifting than by the compression from overlying rocks. [caption id="attachment_520" align="aligncenter" width="504"] Figure 13.3 A rock layer (dark brown) is compressed by the weight of rocks above, and stretched by rifting. The sizes of the arrows indicate the relative sizes of the stresses. Source: Karla Panchuk (2018), CC BY 4.0.[/caption] Rocks experience stress from all directions, but it is possible to break down stresses into three directions, just like a graph with x, y, and z axes. In diagrams showing these three directions, the sizes of arrows representing each direction will indicate the relative size of stresses, as they do in Figure 13.3. Analyzing stress in this way makes it much easier to describe the stresses operating on a rock, and to understand what their net effect will be.

Types of Strain

How a rock responds to stress depends on many factors. The "how" is not simply a matter of how much strain a rock will undergo, but what type of strain will occur. Is the deformation permanent or temporary? Does the rock break or does it deform without breaking?

Elastic Strain

Elastic strain is reversible strain. You can think of elastic strain as what happens to the elastic waistband of your favourite sweatpants when you put them on. The elastic stretches to let you into your pants, and once you're in them, it shrinks to keep them from falling down. When you take the pants off again, the elastic goes back to its original shape (ideally). Similarly, rocks undergoing elastic strain will snap back to their original shape once the stress is removed. Rocks snapping back to their original shape undergo elastic rebound. Elastic rebound of rocks on a large scale can have profound consequences, because the energy released causes the Earth to vibrate. We experience those vibrations as earthquakes.

Plastic Strain

If enough stress is applied, the changes that a material undergoes to accommodate the stress will leave it permanently deformed. When the stress is removed and the material does not go back to its original shape, this is called plastic strain.

Ductile or Brittle?

Ductile deformation refers to deformation that happens by flowing or stretching. The marble monument in Figure 13.4 is undergoing ductile deformation as it sags beneath its own weight. [caption id="attachment_521" align="aligncenter" width="550"] Figure 13.4 A marble monument at the Westminster Hall and Burying Ground in Baltimore, Maryland. The horizontal surface is undergoing slow ductile deformation as it sags beneath its own weight. The author Edgar Allan Poe is interred nearby. Source: Ray Pennisi (2007), CC BY-NC 2.0. Image source.[/caption] When a material breaks, it has undergone brittle deformation (Figure 13.5). The stone cylinders in Figure 13.5 are part of an experiment to test the strength of the rock. The cylinder on the right looked like the cylinder on the left before it was compressed, with force applied to the top and bottom. Strain gauges have been glued on to measure the amount of deformation lengthwise and across the cylinders. [caption id="attachment_522" align="aligncenter" width="450"] Figure 13.5 Cylinders of rock used to test the strength of rock under compression. The cylinder on the left has been equipped with strain gauges to measure the amount of deformation. The cylinder on the right has undergone brittle deformation after being compressed. Source: Karla Panchuk (2016), CC BY 4.0.[/caption] A material can undergo more than one kind of deformation when stress is applied. The barrel-shaped cylinder of potash in Figure 13.6 (right) originally looked like the cylinder on the left. The cylinder was compressed, with stress applied from the top and bottom. Initially, it underwent ductile deformation and thickened in the middle, creating the barrel shape. But as more stress was applied, the cylinder eventually underwent brittle deformation, resulting in the crack across the middle. [caption id="attachment_523" align="aligncenter" width="500"] Figure 13.6 Cylinders of potash before and after deformation. The potash underwent ductile deformation before it finally broke. Source: Karla Panchuk (2018), CC BY 4.0.[/caption]

Factors That Determine How A Rock Will Deform

A rock is not limited to exclusively brittle deformation, or exclusively ductile deformation. Even the deformed rock in Figure 13.5, which has clearly undergone brittle deformation, shows a slight curvature on the right side, near the top. This indicates that a small amount of ductile deformation occurred before brittle failure. For a given rock, deformation will be different depending on the amount of stress applied. Up to a point, rocks undergo elastic deformation, and will spring back to their original shape after the stress is removed. If more stress is applied, the rock may deform in a ductile manner. If stress increases further, the rock may fracture. The amount of stress required in each case will depend on the type of rock, as well as conditions such as pressure and temperature.

Composition

In general, sedimentary rocks will be more likely to undergo ductile deformation than igneous or metamorphic rocks under the same conditions. Rocks within each group will also deform differently. Boudinage structures (Figure 13.7) highlight the effect of composition on how rocks deform. These structures occur when a stronger rock more prone to brittle deformation is surrounded by weaker rocks prone to ductile deformation. The stronger rock will fracture into segments, called boudins, and the weaker rock will flow into the spaces between. In Figure 13.7 (top), the white layer reached the stage of pinching off, just before separating into segments. The surrounding black layer flowed in to fill the gap where the pinch was happening. Remarkably, the white layer itself contains a dark layer that has fragmented into boudins. Not all boudins break into blocky segments. Some display more ductile deformation (Figure 13.7, bottom). [caption id="attachment_524" align="aligncenter" width="504"] Figure 13.7 Two examples of boudinage structure. Top- The white layer has pinched off into segments, and the surrounding black layers have flowed into the gap forming between segments. Within the white layer is a thinner black layer that has also broken into segments. Bottom- Boudins displaying ductile deformation. Source: Top- Marek Cichanski (2012), CC BY-NC 2.0. Image source.. Bottom- Joyce McBeth (n.d.). CC BY 4.0.[/caption]

Temperature and Pressure

At higher temperatures, and under higher confining pressures, rocks are more likely to undergo ductile deformation. Confining pressure is the stress that a material experiences uniformly from all sides as a result of the weight of material above and around it. The pressure that a diver feels deep in the ocean is confining pressure due to the weight of water above and around the diver. This kind of confining pressure is called hydrostatic pressure. Within Earth, the confining pressure is due to the weight of overlying rocks. Confining pressure due to the weight of rocks is called lithostatic pressure. The rocks in Figures 13.5 and 13.6 experienced confining pressure from the atmosphere, and temperatures comfortable for the humans working in the lab. Under those conditions the rocks ultimately underwent brittle failure when they were compressed in the lab. Deep within the crust, the temperatures and confining pressures are far greater. Deep enough within the crust, both samples would undergo only ductile deformation if the same amount of stress were applied as in the experiment. The depth at which temperatures and confining pressures are high enough for rocks to go from brittle deformation to ductile deformation is called the brittle-ductile transition zone. The brittle-ductile transition zone occurs between approximately 10 km and 30 km depth, corresponding to temperatures around 300 ºC and greater. The depth at which temperatures reach 300 ºC at any particular location will depend on heat flow at that location. In continental crust, rocks at 300 ºC are deeper than in ocean crust. The change in pressure with depth also varies, depending on the mass and density of rocks. If depths are measured relative to sea level, the pressure at 10 km measured beneath a tall mountain belt will be greater than the pressure at 10 km measured within ocean crust. Experiments like those shown in Figures 13.5 and 13.6 can be used to determine where the brittle-ductile transition zone will be for a particular rock type. Experimenters apply stress to sample of a rock for a range of temperatures and confining pressures. They note the conditions under which the rock breaks or deforms in a ductile manner, and plot those on a graph (Figure 13.8). The results in Figure 13.8 are from experiments on limestone. The vertical axis is pressure. The more pressure, the deeper the rock would have to be within the Earth to experience that pressure. The white line represents the brittle-ductile transition zone. Above the white line are pressures and temperatures under which the limestone would fracture. Below the white line in the tan area are pressures and temperatures where the limestone would deform by flowing. Notice that the higher the temperatures, the less confining pressure is required for ductile deformation. [caption id="attachment_525" align="aligncenter" width="650"] Figure 13.8 Experimental results on limestone with tension applied (left) and compression applied (right). Source: Karla Panchuk (2018), CC BY 4.0. Modified after Heard (1960).[/caption]

How Stress Is Applied

The limestone experiments were performed by applying stress as tension (Figure 13.8 left) and again by applying stress as compression (right). When tension was applied, temperature and confining pressure had to be much higher before ductile deformation occurred. Under compression, ductile deformation was possible with far less confining pressure, and at lower temperatures. Strain rate, the rate at which deformation occurs, also makes a difference. If stress is applied at a rate that causes rapid deformation, the rock will be more likely to fracture than if deformation happens slowly. The marble slab in Figure 13.4 is a good example of this. It has sagged rather than broken because the rate of deformation has been very slow, at millimetres per decade.

Fluids

When rocks are under pressure, fluids trapped within the pore spaces of rocks- the gaps between grains- are also under pressure. Higher confining pressure is required for deformation to be ductile rather than brittle, but pressure from fluids, called pore pressure, resists the confining pressure. The result is that the effective confining pressure is lower than it would be without the fluids. Depending on the amount of pore pressure, and how close the rock is to the brittle-ductile transition zone, pore pressure could cause brittle failure in a rock that would otherwise undergo ductile deformation.

Stress and Geological Structures

Many different geologic structures can form when stress is applied to rocks. Structures form as a result of fracturing, tilting, folding, stretching, and squeezing (Figure 13.9). Some structures, like the fractures that make basalt columns (Figure 13.9, upper left), happen when rocks shrink due to cooling, but others are a consequence of plate tectonic forces. The types of structures that form depend on the plate tectonic setting and other geological conditions, making them valuable tools for understanding what happened to the rocks. The following sections address the different kinds of structures that form, and what information we can gather from these structures to learn more about the tectonic environment and regional geology. [caption id="attachment_526" align="aligncenter" width="550"] Figure 13.9 Structures resulting from deformation. Upper left- Fracturing in basalt near Whistler, British Columbia. Upper right- Tilting of sedimentary rock near Exshaw, Alberta. Lower left- Stretched limestone (light grey) and chert (dark grey) from Quadra Island, British Columbia. Lower right- Faulted shale near Cache Creek, British Columbia. Rocks above the fault moved up relative to those below. Source: Karla Panchuk (2018), CC BY 4.0. Photographs by Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Practice with Types of Deformation [h5p id="155"]

Many thin light and dark layers are bent into a flattened S-shape. What happened to shape the rock this way? Select all that apply. Need a closer look?
1. Plastic strain
2. Ductile deformation
3. Elastic strain
4. Brittle deformation
What happened to this rock? Select all that apply. Not sure what to focus on?
1. Elastic strain
2. Ductile deformation
3. Plastic strain
4. Brittle deformation
What's going on here?
- This is a structure called (hint: the broken blocks are called boudins) that forms when a (hint: stronger or weaker?) layer undergoes (hint: brittle or ductile?) deformation and breaks.
- It's surrounded by a (hint: stronger or weaker?) layer that undergoes (hint: brittle or ductile?) deformation and flows around the broken pieces.

References

Heard, H. C. (1960). Transition from brittle fracture to ductile flow in Solenhofen Limestone as a function of temperature, confining pressure, and interstitial fluid pressure. In D. Griggs & D. Handin (Eds.), Rock Deformation (A Symposium): Geological Society of America Memoir 79 (pp. 193-226). https://doi.org/10.1130/MEM79

15.1 Factors That Control Slope Stability

kqzheng — Mon, 20 Nov 2017 19:54:14 +0000

The potential for mass movement is important to consider when managing natural hazard risks to people and property. To understand the risks posed by mass movement, we first need to understand what makes a slope stable or unstable, and what can trigger materials to move.

Slope Angle and the Forces Acting On A Slope

A block of rock situated on a rock slope is pulled by gravity toward Earth’s centre (vertically down, Figure 15.2). We can split the vertical gravitational force into two components (vectors) relative to the slope: one pulling the block down parallel to the slope (the shear force), and the other pulling the block directly into (i.e., perpendicular) to the slope (the normal force). The shear force pulls the block down the slope, but the block doesn't move unless the shear force overcomes the strength of the bond between the block and the slope. The block might be only weakly connected, with friction being most important for keeping it there, or the block might still be part of the rock with more force being required to break it away. The strength of the relationship between the block and the slope is called the shear strength. For the gentle slope in Figure 15.2, the shear strength is greater than the shear force, so the block doesn't move. For the steeper slope, the shear force is greater (even though the rock weighs the same), and the block slides downhill. [caption id="attachment_608" align="aligncenter" width="650"] Figure 15.2 Differences in the shear and normal components of the gravitational force on slopes with differing steepness. The total gravitational force is the same in both cases, and the shear strength is the same. On the gentle slope, the shear force is less than the shear strength, so the block is stable. On the steep slope, the shear force is greater than the shear strength, so the block will slide. Source: Karla Panchuk (2021), CC BY-NC-SA 4.0. Click for more attributions.
[/caption] Slopes are created by uplift in the Earth’s crust, and modified by erosion. In areas with relatively recent uplift (such as most of British Columbia and the western part of Alberta in Canada), slopes tend to be quite steep. This is especially true where glaciation has taken place because glaciers in mountainous terrain create steep-sided U-shaped valleys. In areas without recent uplift (such as central Canada), slopes are less steep because erosion—including mass wasting—has been acting on the slopes for a long time. Note that mass wasting can happen even on relatively gentle slopes if the shear stress acting on the materials is greater than the materials’ shear strength.

Slope Strength in Rocks

Solid rocks tend to be strong, but rock strength varies widely. If we ignore issues such as fracturing and layering, then most crystalline rocks (e.g., granite, basalt, or gneiss) are very strong, while some metamorphic rocks (e.g., schist) are only moderately strong. Sedimentary rocks have variable strength. Dolostone and some limestone are strong, most sandstone and conglomerate are moderately strong, but there are some sandstones that are weak, and all mudstones are weak.

Fractures, metamorphic foliation (excluding banding in gneiss), or bedding can significantly reduce the strength of rock. In the context of mass wasting, these structures—which constitute planes of weakness—are parallel to the slope. In Figure 15.3, the bedding is almost perpendicular to the slopes at locations A and B. At location C, the bedding is nearly horizontal, and at location D it's actually parallel to the slope (a worrisome scenario). If these rocks gave out along their bedding planes, the most likely outcome is that the hill between B and C would slide along a bedding plane into the valley between C and D, as would one or more of beds making up the slope at D.

[caption id="attachment_609" align="aligncenter" width="550"] Figure 15.3 Relative stability of slopes. The stability is as a function of the orientation of planes of weakness (in this case bedding planes) relative to the slope orientations. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Internal variations in the composition and structure of rocks can significantly affect their strength. Schist, for example, may have layers that are rich in sheet silicates (micas) and these will tend to form weak layers.

Some minerals are more susceptible to weathering than others, and the weathered products are commonly quite weak (e.g., clay formed from feldspar). The side of Johnson Peak that failed in 1965 (Hope Slide) is made up of chlorite schist (metamorphosed sea-floor basalt) that has feldspar-bearing sills within it. The foliation and the sills are parallel to the steep slope. The schist is relatively weak to begin with, and the feldspar in the sills, which has been altered to clay, makes it even weaker.

Slope Strength in Unconsolidated Sediments

Unconsolidated (loose) sediments are generally weaker than sedimentary rocks because they're not cemented and, in most cases, have not been significantly compressed by overlying materials. Unconsolidated sediments can still bind together, and the strength of that binding is called cohesion. A cohesive sediment binds together strongly, and if you picked it up with a shovel it would stick together in a lump (e.g., sand mixed with clay, clay). A sediment that's not very cohesive is weakly bound and would probably fall apart if you picked it up with a shovel (e.g., sand, silt). The deposits that make up the cliffs at Point Grey, Vancouver, B.C. include sand, silt, and clay, overlain by sand. The finer deposits at Point Grey are relatively cohesive (they maintain a steep slope, Figure 15.4 left). The overlying sand is not very cohesive (relatively weak) and has a shallower slope because there are many slope failures in the sand deposit. [caption id="attachment_610" align="aligncenter" width="650"] Figure 15.4 Left: Glacial outwash deposits at Point Grey, Vancouver, B.C. The dark lower layer is made up of sand, silt, and clay. The light upper layer is well-sorted sand, which has experienced slope failure and formed a cone of talus. Right: Glacial till on Quadra Island, B.C. The till is strong enough to have formed a near-vertical slope. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

In contrast to poorly cohesive sediment deposits, glacial till can be as strong as some sedimentary rock. Glacial till is typically a mixture of clay, silt, sand, gravel, and larger clasts and forms and is compressed beneath tens to thousands of metres of glacial ice (Figure 15.4, right).

Angle of Repose

The steepness of a hill of unconsolidated material will depend on how well the material can withstand the force of gravity. The steepest possible angle that a slope can have and still be stable is called the angle of repose. The angle of repose will depend on many factors that govern how well sediment particles can stay together in a pile, but generally speaking, the smaller the grain size, and the more irregularly shaped the particles are, the higher the angle of repose. In Figure 15.5, the sand pile with a 39º angle of repose has smaller grains than the piles with 37º or 31º angles of repose. [caption id="attachment_611" align="aligncenter" width="500"] Figure 15.5 Angle of repose for sand piles with different grain sizes. Source: Karla Panchuk (2018), adapted from Andrew Dunn (2005), CC BY-SA 2.0. Image source.[/caption]

Importance of Water in Slope Stability

Apart from the type of material on a slope, the amount of water that the material contains is the most important factor controlling its strength. This is especially true for unconsolidated materials, but it also applies to bodies of rock. Granular sediments, like the sand at Point Grey, have lots of pore spaces between the grains. These spaces may be completely dry (filled only with air), moist (some spaces are water filled), or completely saturated (Figure 15.5).

[caption id="attachment_612" align="aligncenter" width="400"] Figure 15.5 | Depiction of dry, moist, and saturated sand. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]Water can increase the mass of the material on a slope, because the mass of the water is a component of the overall mass of the slope material. This increases the gravitational force pulling the slope materials down. A water saturated body of sediment with 25% porosity weighs approximately 13% more than it does when it is completely dry, so the gravitational shear force is also 13% higher. In the situation where the shear force and shear strength are closely matched, even a small change in shear force is enough to overcome the shear strength, and the block would move down the slope.

Sometimes Water Helps Stability

Unconsolidated sediments tend to be strongest when moist, because the small amounts of water at grain boundaries holds the grains together due to surface tension. Surface tension is the reason a droplet of water is dome-shaped rather than completely flat, and the reason some insects can walk on top of water. Dry sediments are held together only by the friction between grains. If the grains are well sorted, well rounded, or both, this cohesion is weak, because of minimal grain contact (i.e., the grains only touch in a few places). Saturated sediments tend to be the weakest of all because the water pushes the grains apart, decreasing friction between grains. Saturating a solid rock with water can also decrease its strength, especially if the rock has porosity, fractures, bedding planes, and/or clay-bearing zones. A sand castle (Figure 15.6) is an excellent example of the role of water in slope stability. Without water, the sand would be held up by friction between the grains alone, and would have the same shape as the sand piles in Figure 15.5. With too much water, the sand grains will be separated from each other by a layer of water, reducing the friction between the grains. That sand would slide into a sloppy puddle. [caption id="attachment_613" align="aligncenter" width="400"] Figure 15.6 A sand castle illustrates the importance of just the right amount of water for slope stability. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Water and Clay Minerals

Water also has an interesting effect on clay-bearing materials. All clay minerals will absorb a small quantity of water, which reduces the strength of the clay. The smectite clays (such as the bentonite used in cat litter) can absorb a lot of water, and this water pushes the clay sheets apart at a molecular level, which makes the clay swell. Smectite that has expanded in this way has almost no strength; it's extremely slippery. Thus, slopes containing smectite clay are more likely to experience slope failure when they are saturated.

Water Pressure

Water pressure is an important factor in slope failure. As you move deeper in saturated sediment, the pressure of the water goes up due to gravity acting on the column of water above it. This pressure is called hydrostatic pressure. The greater the depth below the surface of the water table (the point where the rock or sediments are saturated), the greater the water pressure acting on the materials. Holes are often drilled into rocks in road cuts to allow water to drain and relieve this water pressure. One of the hypotheses advanced to explain the 1965 Hope Slide is that cold conditions that winter caused small springs in the lower part of the slope to freeze, preventing water from flowing out. It's possible that water pressure gradually built up within the slope, weakening the rock mass to the extent that the shear strength was no longer greater than the shear force.

Triggers of Mass Wasting

Shear force is primarily related to slope angle, and once a slope angle is set, the shear force is constant. But shear strength can change quickly for a variety of reasons. Events that lead to a rapid decrease in shear strength are triggers for mass wasting.

An increase in water content is the most common trigger of mass wasting. This can result from rapid melting of snow or ice, heavy rain, or other events that change the pattern of water flow on and through the material. Rapid melting can be caused by a sudden increase in temperature (e.g., in spring or early summer), or by a volcanic eruption that melts snow and ice on a mountain top. Heavy rains are typically related to storms. The Oso landslide that occurred in Washington State, USA in 2014 (Figure 15.7) is an example of slope failure triggered by heavy rain. The flow buried the community of Steelhead Haven, killing 43 people. River erosion at the base of the slope made it even more susceptible to failure.

[caption id="attachment_614" align="aligncenter" width="550"] Figure 15.7 The Oso landslide, a flow that occurred in Washington State, USA 22 March 2014. Source: Matthew Sissel (2014) Public Domain. Image source[/caption]

Changes in water flow patterns can be caused by earthquakes, dammed streams from previous slope, or human structures that interfere with runoff (e.g., buildings, roads, or parking lots). An example of this is a deadly 2005 debris flow in North Vancouver, B.C. There had been previous slope failures in the area, and a report written in 1980 recommended that the municipal authorities and residents take steps to address surface and slope drainage issues. Unfortunately, little was done to address the issues. The failure happened during a rainy period but was likely triggered by excess runoff related to the roads at the top of this slope, and by landscape features including addition of fill to backyards in the area above the failure.

In some cases, a decrease in water content can lead to failure. This is most common with clean sand deposits (e.g., the upper layer in Figure 15.4 (left)). As water content drops, the surface tension holding grains together decreases.

Freezing and thawing can also trigger some forms of mass wasting. A block of rock might fall because of ice wedging, or thawing might release a block of rock that was frozen to a slope by a film of ice.

One other process that can weaken a body of rock or sediment is shaking. The most obvious source of shaking is an earthquake, but shaking from highway traffic, construction, or mining can also weaken rock. Several deadly mass wasting events (including snow avalanches) were triggered by the M7.8 earthquake in Nepal in April 2015.

What Controls Slope Stability?

Write the words into the correct blank. For a block of rock to remain on a slope, the forces working to hold it in place must be greater than the downslope component of , called the . By increasing the steepness of the slope, you can overcome the of the slope materials, and the block will slide down the hill. For solid rock, crystalline rock is than mudstone. Features like bedding planes and fractures will make any type of rock . Water content is an especially important consideration for unconsolidated materials. Dry sediments are held together weakly by , but just the right amount of moisture will keep sediments together by . With too much water, both of these stabilizing mechanisms are overcome. Just the right amount of water will allow you maintain a steep enough to build sand castles. Slope failure can be triggered by events such as adding too much water, disrupting grain contacts through shaking, and wedging open cracks. Fill-in-the-blank options:

weaker
shear strength
ice
earthquakes
stronger
angle of repose
gravity
heavy rainfall
friction
shear force
surface tension

To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="166"]

15.2 Classification of Mass Wasting

kqzheng — Mon, 20 Nov 2017 19:54:16 +0000

There are three criteria used to classify slope failures:

The type of material that failed (e.g., bedrock or unconsolidated sediment)
The mechanism of the failure (how the material moved as it failed
The rate of movement (how quickly the material moved)

The mechanism of the failure is the most important characteristic of a slope failure. The main mechanisms of are:

In a fall, material drops through the air, either vertically or nearly vertically.
In a slide, material moves as a coherent mass along a sloping surface, with little to no internal motion within the mass.
In a flow, failing material has internal motion (like a fluid) as it moves.

While we don't classify slope failures by the shape of the rupture surface (the rupture surface is the boundary between the slope and the sliding material), it's still an important feature used to describe mass wasting processes. Slope failures with curved rupture surfaces are called rotational slope failures, and slope failures with planar rupture surfaces are called translational slope failures.

Unfortunately, classifying slope failure is not as simple as the classification scheme above suggests. Many slope failures involve more than one type of motion, and it may not be possible to know how the material moved. The types of slope failure that are covered in this chapter are summarized in Table 15.1.

Table 15.1 Classification of slope failures based on type of material and motion.
Failure Type	Type of Material	Type of Motion	Rate of Motion
Rock fall	Rock fragments	Vertical or near-vertical fall (plus bouncing in many cases)	Very fast (>10s m/s)
Rock slide	A large rock body	Motion as a unit along a planar surface (translational sliding)	Typically very slow (mm/y to cm/y), but some can be faster
Rock avalanche	A large rock body that slides and then breaks into small fragments	Flow (at high speeds, the mass of rock fragments is suspended on a cushion of air)	Very fast (>10s m/s)
Creep or solifluction	Soil or other overburden; in some cases, mixed with ice	Flow (although sliding motion may also occur)	Very slow (mm/y to cm/y)
Slump	Thick deposits (m to 10s of m) of unconsolidated sediment	Motion as a unit along a curved surface (rotational sliding)	Slow (cm/y to m/y)
Mudflow	Loose sediment with a significant component of silt and clay	Flow (a mixture of sediment and water moves down a channel)	Moderate to fast (cm/s to m/s)
Debris flow	Sand, gravel, and larger fragments	Flow (similar to a mudflow, but typically faster)	Fast (m/s)

Rock Fall

Rock fragments can break off relatively easily from steep bedrock slopes, most commonly due to frost wedging. If you ever hike along a steep mountain trail on a cool morning, you'll probably hear the occasional fall of rock fragments onto a talus slope. Water in the cracks in the rock freezes and expands overnight, and this can break the rock apart. When the ice thaws in the morning sun, some of these broken fragments will fall to the slope below (Figure 15.8).

[caption id="attachment_617" align="aligncenter" width="500"] Figure 15.8 The contribution of freeze-thaw to a rock fall. Source: Steven Earle (2015), CC BY 4.0. Image source[/caption]

Talus slopes (also known as scree slopes) form from this fallen rock as it accumulates at the angle of repose below high rock walls. A typical talus slope near Keremeos in southern BC is shown in Figure 15.9 (left). In December 2014, a large block of rock split away from a cliff in this same area. It broke into smaller pieces that tumbled down the slope and crashed into the road, smashing the concrete barriers and gouging out large parts of the pavement (Figure 15.9, right). Luckily no one was hurt.

[caption id="attachment_618" align="aligncenter" width="650"] Figure 15.9 Left: A talus slope near Keremeos, B.C., formed by rock falls from the cliffs above. Right: The results of a rock fall onto a highway west of Keremeos in December 2014. Source: Steven Earle (2015), CC BY 4.0. Image source[/caption]

Rock Slide

A rock slide is a large body of rock that slips along a sloping surface. In most cases, the movement is parallel to a fracture, bedding plane, or metamorphic foliation plane; thus, most rock slides are translational slope failures. The speed of the movement can range from very slow to moderately fast. The word sackung describes the very slow motion of a block of rock (mm/y to cm/y) on a slope. A good example is the Downie Slide, a translational slide north of Revelstoke, BC (Figure 15.10). Foliation is present in the rock, and approximately parallel to the slope. It's acting as a plane of weakness, permitting a massive body of rock to inch its way down. The Downie Slide was recognized prior to the construction of the Revelstoke Dam, and was moving very slowly (a few cm/year) at the time of dam construction. Geological engineers were concerned that the presence of water in the reservoir (visible in Figure 15.10) could further weaken the plane of failure and accelerate the slide. The result would have been a catastrophic failure into the reservoir that would have sent a wall of water over the dam and into the community of Revelstoke. [caption id="attachment_619" align="aligncenter" width="650"] Figure 15.10 The Downie Slide, a sackung, on the shore of the Revelstoke Reservoir (above the Revelstoke Dam). The head scarp is visible at the top and a side-scarp along the left side. Source: Joyce McBeth (2018) CC BY 4.0, image © 2018 Google Earth, Data Google CNES / Airbus Data LDEO-Columbia, NSF, NOAA Data SIO, NOAA, U.S. Navy, NGA, GEBCO DigitalGlobe Landsat / Copernicus Province of BC.[/caption]

During the construction of the dam, the engineers tunneled into the rock at the base of the slide and drilled hundreds of drainage holes upward into the plane of failure. This allowed water to drain out more efficiently, reducing the hydrostatic pressure, and decreasing the rate of the slide. BC Hydro monitors this site continuously. The slide block is currently moving more slowly than it was prior to the construction of the dam.

In the summer of 2008, a large block of rock slid rapidly from a steep slope above Highway 99 near Porteau Cove, BC (between Horseshoe Bay and Squamish). The block crashed into the highway and adjacent railway and broke into many pieces, and the highway was closed for several days. Engineers and geoscientists took steps to stabilize the slope to decrease the risk of future rock falls. Rock bolts (long metal rods) were installed to anchor the blocks of rocks and prevent them falling Drainage holes were installed to drain water from the slope and decrease the water pressure. The rock along the slope is fractured parallel to the slope (Figure 15.11), and this almost certainly contributed to the failure, but it isn't known what actually triggered the event. The weather was dry and warm during the preceding weeks, and there was no significant earthquake in the region.

[caption id="attachment_620" align="aligncenter" width="419"] Figure 15.11 Site of the 2008 rock slide at Porteau Cove. Notice the prominent fracture set parallel to the surface of the slope. The slope has been stabilized with rock bolts (top arrow) and holes have been drilled into the rock to improve drainage (tube from drainage hole indicated with bottom arrow). Risk to passing vehicles from rock fall has been reduced by hanging mesh curtains (background), which secures loose material to the slope. Source: Joyce McBeth (2018) CC BY 4.0 after Steven Earle (2015) CC BY 4.0. Image source[/caption]

Rock Avalanche

If a rock slides and then starts moving quickly (at a rate of m/s), the rock is likely to break into many pieces. It will become a rock avalanche, in which the fragments of rock travel like a fluid, and are supported by a cushion of air within and beneath the moving mass. The 1965 Hope Slide was a rock avalanche, as was the famous 1903 Frank Slide in southwestern Alberta. The 2010 slide at Mt. Meager (west of Lillooet) was also a rock avalanche and rivals the Hope Slide as the largest slope failure in Canada during historical times (Figure 15.12).

[caption id="attachment_621" align="aligncenter" width="550"] Figure 15.12 The 2010 Mt. Meager landslide, showing where the slide originated (arrow, 4 km upstream). It then raced down a steep narrow valley and out into the wider valley in the foreground. Source: Mika McKinnon (2011), CC BY-SA-NC.[/caption]

Creep and Solifluction

The very slow—mm/y to cm/y—movement of soil or other unconsolidated material down slope is known as creep. Creep normally only affects the upper several centimetres of loose material, but in some cases sliding can also occur.

Creep can be facilitated by freezing and thawing because particles are lifted perpendicular to the surface by the growth of ice crystals within the soil, and then move downwards vertically due to gravity when the ice melts (Figure 15.13). The same effect can be produced by frequent wetting and drying of the soil. In cold environments, solifluction is a more intense form of freeze-thaw-triggered creep.

[caption id="attachment_622" align="aligncenter" width="550"] Figure 15.13 The contribution of freeze-thaw to creep. The blue arrows represent uplift caused by freezing in the wet soil underneath, while the red arrows represent depression by gravity during thawing. The uplift is perpendicular to the slope, while the drop is vertical. Source: Steven Earle (2015), CC BY 4.0. Image source[/caption]

Creep is most noticeable on moderate to steep slopes where trees or fence posts are consistently leaning in a downhill direction. Trees will try to correct their lean by growing upright, leading to a curved lower trunk known as a “pistol butt.” Creep can also generate terracettes, horizontal and repeating ridges on slopes (Figure 15.14). Historically, people thought terracettes formed where livestock or wild animals regularly travelled along slopes. While animals can accentuate terracettes, the primary reason terracettes form is creep.

[caption id="attachment_697" align="aligncenter" width="550"] Figure 15.14 Evidence of creep-generated terracettes on the Peace River hills in northeastern B.C. Source: Joyce McBeth (2018), CC BY 4.0.[/caption]

Slump

A slide is a mass movement where the material moves as a coherent mass. A slump is a type of slide that takes place within thick unconsolidated deposits. Slumps involve movement along one or more curved failure surfaces, and are thus rotational slope failures. Slumps have downward motion near the top and outward motion toward the bottom (Figure 15.15). They are typically caused by high water pressure within these materials on a steep slope.

[caption id="attachment_624" align="aligncenter" width="550"] Figure 15.15 The motion of unconsolidated sediments in an area of slumping. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

This slump in the Lethbridge area of Alberta (Figure 15.16) has likely been active for many decades, and moves a little more whenever there are heavy spring rains and snowmelt runoff. The toe of the slump is being eroded by the small stream at the bottom. The basal material (material at the toe of the slope) forms the support for the overlying mass of material in the slope, so erosion of that material contributes to slumping. This means the slumping will likely continue.

[caption id="attachment_625" align="aligncenter" width="550"] Figure 15.16 A slump along the banks of a small coulee near Lethbridge, Alberta. The main head-scarp is clearly visible at the top, and a second smaller one is visible about a quarter of the way down the slope. The toe of the slump is being eroded by the seasonal stream that created the coulee. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Mudflows and Debris Flows

[caption id="attachment_626" align="alignright" width="258"] Figure 15.17 A slump (left) and an associated mudflow (centre) at the same location as Figure 15.16, near Lethbridge, Alberta. Source: Steven Earle (2015), CC BY 4.0. Image source.
[/caption]

When a mass of sediment becomes completely saturated with water, the mass loses strength, to the extent that the grains may be pushed apart and flow, even on a gentle slope. This can happen during rapid spring snowmelt or heavy rains and is also relatively common during volcanic eruptions because of rapid melting of snow and ice. If the material involved is primarily sand-sized or smaller, it's a mudflow (e.g., Figure 15.17).

If the material involved is gravel sized or larger, it is known as a debris flow. It takes more gravitational force to overcome friction and move larger particles, so debris flows typically form in areas with steeper slopes and higher water pressure. In many cases, a debris flow takes place within a steep stream channel and is triggered by the collapse of bank material into the stream. This may create a temporary dam followed by a major flow of water and debris when the dam finally bursts. This is the situation that led to the fatal debris flow at Johnsons Landing, BC, in 2012. A mudflow or debris flow on a volcano or during a volcanic eruption is called a lahar.

A typical west-coast debris flow is shown in Figure 15.18. This event took place in November 2006 in response to very heavy rainfall. There was enough energy in the flow to move large boulders and to knock over large trees.

[caption id="attachment_627" align="aligncenter" width="550"] Figure 15.18 The lower part of debris flow within a steep stream channel near Buttle Lake, B.C., in November 2006. Note the trees along the edges of the stream that have been damaged by the rocks in the debris flow. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Practice with Types of Mass Wasting

[caption id="attachment_1724" align="aligncenter" width="1280"] Columns of basalt have tumbled from an exposed lava flow in Iceland.[/caption] This is a (hint: this type of mass wasting involves a rapid vertical descent) with a (hint: the term for accumulated debris from mass wasting of a steep slope or rock face) of basalt columns at the bottom. [caption id="attachment_1727" align="aligncenter" width="604"] The thin soil on this steep slope in Iceland has a wrinkled texture.[/caption] This is an example of . [caption id="attachment_1728" align="aligncenter" width="1224"] This grassy hillside is slipping along a curved failure surface.[/caption] This type of mass wasting is a . A distinctive (hint: A vertical face of exposed sediment, two words) is visible at the top, and the (hint: the advancing part of the sliding mass) bulges out toward the bottom. [caption id="attachment_1730" align="aligncenter" width="1280"] Cleanup after a devastating storm.[/caption] This material is here because of a (hint: mass wasting involving water-bourne particles in a range of sizes, two words). [caption id="attachment_1733" align="aligncenter" width="1280"] A massive slab of rock is slowly inching its way down this mountain.[/caption] This is a (hint: also called a sackung). A similar slope failure is present along the Revelstoke Reservoir. [caption id="attachment_1735" align="aligncenter" width="2411"] A mountainside gave way, and crumbled into pieces.[/caption] This (hint: you might also have seen snow involved in this type of slope failure) slid far from its origin, behaving like a turbulent fluid. This event originated with failure along bedding planes oriented (hint: parallel or perpendicular) to the slope. [caption id="attachment_1740" align="aligncenter" width="856"] Southern California community inundated with mud after heavy rains.[/caption] This happened when fine-grained sediments were washed down from hillsides recently deforested by wildfires. To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="167"]

15.3 Preventing, Delaying, Monitoring, and Mitigating Mass Wasting

kqzheng — Mon, 20 Nov 2017 19:54:17 +0000

Mass wasting is unavoidable, but in many situations there are actions we can take to reduce or mitigate the damaging effects of mass wasting on people and infrastructure. Where we can neither delay nor mitigate mass wasting, we may consider trying to initiate the slope failure in a controlled manner. In areas prone to mass wasting that can't be controlled or mitigated, we can minimize risk by not building in these areas at all.

Preventing and Delaying Mass Wasting

Some effects of mass wasting can be prevented by mechanical means, at least temporarily. Examples are the rock bolts in the road cut at Porteau Cove on the Sea-toSky highway in BC (Figure 15.11) or the drill holes used to drain water out of the slope at the Downie Slide (Figure 15.10), or the building of physical barriers, such as retaining walls along highway roadcuts.

These preventative measures are not permanent though, and are subject to degradation over time. The rock bolts in the road cut at Porteau Cove will corrode over time, and within a few decades many of them will begin to lose their strength. Unless they're replaced, they will no longer support the slope. Likewise, drainage holes at the Downie Slide will eventually become plugged with sediment and chemical precipitates, and unless they're periodically unplugged, their effectiveness will decrease. Eventually, unless new holes are drilled, the drainage will be compromised, and the slide will start to move again. This is why careful slope monitoring by geological and geotechnical engineers is important at major mass wasting sites such as the Downie Slide and along the Sea-to-Sky highway. Our efforts to control mass wasting are only as good as our efforts to maintain the preventive measures.

Delaying mass wasting is a worthy endeavour because during the time that the measures are still effective, they can save lives and reduce damage to property and infrastructure such as homes and roads. But we must be careful to avoid activities that could make mass wasting more likely. One of the most common anthropogenic causes of mass wasting is road construction, and this applies both to remote gravel roads built for forestry and mining, and large urban and regional highways.

Road construction is a potential problem for two reasons. First, creating a flat road surface on a slope inevitably involves creating a cut bank that's steeper than the original slope. This might also involve creating a filled bank that is both steeper and weaker than the original slope (Figure 15.19). Second, roadways typically cut across natural drainage features, and unless great care is taken to reroute the runoff water, oversaturation of slope material can occur, contributing to mass wasting. [caption id="attachment_630" align="aligncenter" width="550"] Figure 15.19 An example of a road constructed by cutting into a steep slope and the use of the cut material as fill. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Apart from saturation and water pressure considerations, engineers building roads and other infrastructure on bedrock slopes have to carefully consider the geology, and especially any weaknesses or discontinuities in the rock related to bedding, fracturing, or foliation. If possible, situations like that at Porteau Cove (Figure 15.11) should be avoided—by building somewhere else—rather than trying to stitch the slope back together with rock bolts.

It's widely believed that construction of buildings above steep slopes can contribute to the instability of the slope. This is likely true, but probably not because of the weight of the building. A typical house isn't heavier than the excavated ground that was removed to build the house. A more likely contributor to instability of the slopes below buildings is changes to the water drainage and to the saturation of the slope. Water can be collected by roofs, go into downspouts, and form concentrated flows that are directed onto or into the slope. Likewise, drainage from nearby access roads, lawn irrigation, leaking pools, and septic systems can all alter the surface and groundwater flow in a slope.

Monitoring Mass Wasting

Warning systems are helpful in some areas where there is a risk of mass wasting. They let us know if conditions have changed at a known slide area, or if a rapid failure, such as a debris flow, is on its way downslope. The Downie Slide above the Revelstoke Reservoir is continuously-monitored with a range of devices, such as inclinometers (slope-change detectors), bore-hole motion sensors, and GPS survey instruments. A simple mechanical device for monitoring the nearby Checkerboard Slide (which is also above the Revelstoke Reservoir) is shown in Figure 15.20. Both of these slides are very slow-moving, but it's important to be able to detect changes in their rates of motion. A rapid failure would result in large bodies of rock plunging into the reservoir and sending a wall of water over the Revelstoke Dam, potentially destroying the nearby town of Revelstoke.

[caption id="attachment_631" align="aligncenter" width="450"] Figure 15.20 Part of a motion-monitoring device at the Checkerboard Slide near Revelstoke, BC. The lower end of the cable (extending out from the top of the device to the right) is attached to a block of rock that is unstable. Any incremental motion of this block will move the cable, which will be detectable by this device. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Mt. Rainier, a glacier-covered volcano in Washington State (15.21), could produce massive mudflows or debris flows (lahars) with or without a volcanic eruption. Over 100,000 people in the Tacoma, Puyallup, and Sumner areas are at risk because they currently reside on deposits from past lahars and future lahars would likely also follow these paths (Figure 15.21). In 1998, a network of acoustic monitors was established around Mt. Rainier. The monitors are embedded in the ground adjacent to expected lahar paths. These monitors will provide warnings to emergency officials in the event of a lahar. When a lahar is detected, the residents of the area will have between 40 minutes and three hours to get to safe ground.

[caption id="attachment_632" align="aligncenter" width="550"] Figure 15.21 Mt. Rainier from Seattle, WA, USA. Source: Accozzaglia dot ca (2010), CC-BY-NC-ND 2.0. Image source.[/caption] [caption id="attachment_633" align="aligncenter" width="450"] Figure 15.22 Major pathways of Mt Rainier lahars over the past 10,000 years, Washington State, USA. Source: USGS (2005) Public Domain view source, modified from Driedger et al (2005) Public Domain. Image source[/caption]

Mitigating the Impacts of Mass Wasting

In situations where we cannot predict, prevent, or delay mass-wasting hazards, some effective measures can be taken to minimize the associated risk. For example, many highways in BC and western Alberta have avalanche shelters like the one shown in Figure 15.23. In some parts of the world, similar structures have been built to protect infrastructure from other types of mass wasting.

[caption id="attachment_634" align="aligncenter" width="550"] Figure 15.23 A snow avalanche shelter on the Coquihalla Highway (bottom centre of the image). The expected path of the avalanche is the steep and treeless slope above. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Debris flows are inevitable, unpreventable, and unpredictable in many parts of BC, but nowhere more so than along the Sea-to-Sky Highway between Horseshoe Bay and Squamish. The results have been deadly and expensive many times in the past. It would be very expensive to develop a new route in this region, so provincial authorities have taken steps to protect residents and traffic on the highway and railway. Debris flow defensive structures have been constructed in several drainage basins, as shown in Figure 15.24. One strategy is to allow the debris flow to flow quickly through to the ocean along a smooth channel. Another is to capture the debris within a constructed basin that allows the excess water to continue through.

[caption id="attachment_635" align="aligncenter" width="1024"] Figure 15.24 Two strategies for mitigating debris flows on the Sea-to-Sky Highway. Left: A concrete–lined channel on Alberta Creek allows debris to flow quickly through to the ocean. Right: A debris flow catchment basin on Charles Creek. In 2010, a debris flow filled the basin to the level of the dotted white line. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] Finally, in situations where we cannot do anything to delay, predict, contain, or mitigate slope failures, the responsible and ethical thing to do is to avoid building in or using the risky area. Sometimes this may require relocating a community after discovering a previously-unidentified risk. There is a famous example of this in BC at a site known as Garibaldi, 25 km south of Whistler. In the early 1980s the village of Garibaldi had a population of about 100, with construction underway on some new homes, and plans for many more. In the months that followed the deadly 1980 eruption of Mt. St. Helens in Washington State, the BC Ministry of Transportation commissioned a geological study to assess risks along their highways. The study revealed that a steep cliff known as The Barrier (Figure 15.25) had collapsed in 1855, leading to a large rock avalanche, and that it was likely to collapse again unpredictably, putting the village of Garibaldi at extreme risk. In an ensuing court case, it was ruled that the Garibaldi site was not a safe place for people to live. Those who already had homes there were compensated, and everyone was ordered to leave. [caption id="attachment_636" align="aligncenter" width="550"] Figure 15.25 The Barrier, south of Whistler, BC, was the site of a huge rock avalanche in 1855, which extended from the cliff visible here 4 km down the valley and across the current location of the Sea-to-Sky Highway and the Cheakamus River. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Chapter 15 Summary & Key Term Check

kqzheng — Mon, 20 Nov 2017 19:54:19 +0000

Chapter 15 Main Ideas

15.1 Factors That Control Stability on Slopes

Slope stability is controlled by the slope angle and the strength of the material on the slope. Slopes are a product of tectonic uplift, and their strength is determined by the type of material on the slope and its water content. Rock strength varies widely and is determined by internal planes of weakness and their orientation with respect to the slope. In general, the more water contained by the slope material, the greater the likelihood of failure. This is especially true for unconsolidated sediments, where excess water pushes against the grains. Addition of water is the most common trigger of mass wasting and can come from storms or rapid snow melt.

Practice Again

What controls slope stability?

15.2 Classification of Mass Wasting

The key criteria for classifying mass wasting are the nature of the movement that takes place, the type of material, and the speed of the material movement. Mass wasting events can be a precipitous fall of rock through the air, material sliding as a solid mass along either a plane or a curved surface, or internal flow of material as a viscous fluid. The type of material influences the mass movement, specifically whether it is solid rock or unconsolidated sediments. Slope failures can have translational (planar) or rotational (curved) rupture surfaces. The important types of mass wasting are creep, slump, slide, fall, and debris flow or mudflow.

15.3 Preventing, Delaying, and Mitigating Mass Wasting

We cannot prevent mass wasting, but we can delay it through efforts to strengthen the materials on slopes. Strategies include adding mechanical devices such as rock bolts or ensuring that water in the slope materials can easily drain away. Such measures are never permanent but may be effective for decades or even centuries. We can also avoid practices that make matters worse, such as cutting into steep slopes or impeding proper drainage. In some situations, the best approach is to mitigate the risks associated with mass wasting by constructing shelters or diversionary channels. In other cases, where slope failure is inevitable, we should simply avoid building in that location.

Key Term Check

What key term from Chapter 15 is each card describing? Turn the card to check your answer. [h5p id="168"]

18.1 Metal Deposits

kqzheng — Sat, 10 Feb 2018 00:02:29 +0000

Mining In Canada

Mining has always been a major part of Canada’s economy. Canada has some of the largest mining districts and deposits in the world, and for the past 150 years, we have been one of the most important suppliers of metals. Extraction of Earth’s resources goes back a long way in Canada. For example, the First Nations of British Columbia extracted obsidian from volcanic regions for tools and traded it up and down the coast. In the 1850s, gold was discovered in central British Columbia, and in the 1890s, even more gold was discovered in the Klondike area of Yukon. These two events were critical to the early development of British Columbia, Yukon, and Alaska. Canada’s mining sector had revenues in the order of $37 billion in 2013. The majority of that was split roughly equally among gold, iron, copper, and potash, with important but lesser amounts from nickel and diamonds (Figure 18.3). Revenues from the petroleum sector are significantly higher, at over $100 billion per year. [caption id="attachment_753" align="aligncenter" width="550"] Figure 18.3 The value of various Canadian mining sectors in 2013 Source: Steven Earle (2015), CC BY 4.0. Image source. Data from NRCAN (2013).[/caption]

Metal Deposits

A metal deposit is a body of rock in which one or more metals are sufficiently concentrated to be economically viable for recovery. Some background levels of important metals in average rocks are shown on Table 18.1, along with the typical grades necessary to make a viable deposit, and the corresponding concentration factors. In the case of copper, average rock has around 40 ppm (parts per million) of copper, but a grade of around 10,000 ppm or 1% is necessary to make a viable copper deposit. In other words, copper ore has about 250 times as much copper as typical rock. For the other elements in the list, the concentration factors are much higher. For gold, it’s 2,000 times and for silver it’s around 10,000 times.

Table 18.1 Typical background and ore levels of some important metals Source: Steven Earle (2015), CC BY 4.0. 20.1 Metal Deposits
Metal	Typical Background Level	Typical Economic Grade [footnote]It’s important to note that the economic viability of any deposit depends on a wide range of factors including its grade, size, shape, depth below the surface, and proximity to infrastructure, the current price of the metal, the labour and environmental regulations in the area, and many other factors.[/footnote]	Concentration Factor
Copper	40 ppm	10,000 ppm (1%)	250 times
Gold	0.003 ppm	6 ppm (0.006%)	2,000 times
Lead	10 ppm	50,000 ppm (5%	5,000 times
Molybdenum	1 ppm	1,000 ppm (0.1%)	1,000 times
Nickel	25 ppm	20,000 ppm (2%)	800 times
Silver	0.1 ppm	1,000 ppm (0.1%)	10,000 times
Uranium	2 ppm	10,000 ppm (1%)	5,000 times
Zinc	50 ppm	50,000 ppm (5%)	1,000 times

It's clear that some very significant concentration must take place to form a mineable deposit. This concentration could happen during the formation of the host rock, or after the rock forms, through a number of different types of processes. There's a wide variety of ore-forming processes, and hundreds of types of mineral deposits. The origins of a few of them are described below.

Types of Metal Deposits

Magmatic Nickel Deposits

A magmatic deposit is one in which metal is concentrated primarily at the same time as the formation and emplacement of the magma. Most of the nickel mined in Canada comes from magmatic deposits such as those in Sudbury (Ontario), Thompson (Manitoba) (Figure 18.4), and Voisey’s Bay (Labrador). These deposits form from mafic or ultramafic magmas derived from the mantle, and thus have relatively high nickel and copper contents to begin with (as much as 100 times more than normal rocks in the case of nickel). These elements may be further concentrated within the magma if sulphur is added from partial melting of the surrounding rocks. The heavy nickel and copper sulphide minerals are concentrated even more by gravity segregation, when crystals settle toward the bottom of the magma chamber. In some cases, there are significant concentrations of platinum-bearing minerals. [caption id="attachment_754" align="aligncenter" width="550"] Figure 18.4 The nickel smelter at Thompson, Manitoba. Source: Timkal (2008), CC BY-SA 3.0. Image source.[/caption] Most of these types of deposits around the world are Precambrian in age. This is probably because the mantle was much hotter at that time, and the necessary mafic and ultramafic magmas were more likely to be emplaced in the continental crust.

Volcanogenic Massive Sulphide Deposits

Much of the copper, zinc, lead, silver, and gold mined in Canada comes from volcanic-hosted massive sulphide (VHMS) deposits associated with submarine volcanism (VMS deposits). They're called massive sulphide deposits because the sulphide minerals (including pyrite (FeS₂) , sphalerite (ZnS), chalcopyrite (CuFeS₂), and galena (PbS)) are generally present in high concentrations, sometimes making up the majority of the rock. Examples are the deposits at Kidd Creek, Ontario, Flin Flon on the Manitoba-Saskatchewan border, Britannia on Howe Sound, and Myra Falls (within Strathcona Park) on Vancouver Island. VMS deposits form from water discharged at high temperatures (250° to 300°C) at ocean-floor hydrothermal vents, primarily in areas of subduction-zone volcanism. The environment is comparable to that of modern-day black smokers (Figure 18.5), where hot metal- and sulphide-rich water issues from the sea floor. [caption id="attachment_755" align="aligncenter" width="650"] Figure 18.5 Left: A black smoker on the Juan de Fuca Ridge off the west coast of Vancouver Island. Right: A model of the formation of a volcanogenic massive sulphide deposit on the sea floor. Source: left: NOAA, Public Domain. Image source. Right: Steven Earle (2015), CC BY 4.0. Image source.[/caption]The metals and the sulphur are leached out of the sea-floor rocks by convecting groundwater driven by the volcanic heat, and then quickly precipitated when the hot water enters the cold seawater, causing it to cool suddenly and change chemically. The volcanic rock that hosts the deposits is formed in the same area and at the same general time as the accumulation of the ore minerals.

Porphyry Deposits

Porphyry deposits are the most important source of copper and molybdenum in British Columbia, the western United States, and Central and South America. Most porphyry deposits also host some gold, and in rare cases it can be the primary commodity. B.C. examples include several large deposits within the Highland Valley mine (Figure 18.1) and numerous other deposits scattered around the central part of the province. Porphyry deposits form around a cooling felsic stock in the upper part of the crust. They are called “porphyry” because upper crustal stocks are typically porphyritic in texture, the result of a two-stage cooling process. Metal enrichment results in part from convection of groundwater related to the heat of the stock, and also from metal-rich hot water expelled by the cooling magma (Figure 18.6). [caption id="attachment_756" align="aligncenter" width="550"] Figure 18.6 A model for the formation of a porphyry deposit around an upper-crustal porphyritic stock and associated vein deposits. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]The host rocks, which commonly include the stock itself and the surrounding country rocks, are normally highly fractured. During the ore-forming process, some of the original minerals in these rocks are altered to potassium feldspar, biotite, epidote, and various clay minerals. The important ore minerals include chalcopyrite (CuFeS₂), bornite (Cu₅FeS₄), and pyrite in copper porphyry deposits, or molybdenite (MoS₂) and pyrite in molybdenum porphyry deposits. Native gold is present as minute flakes. Theenvironment around and above an intrusive body is also favourable for the formation of other types of deposits, particularly vein-type gold deposits (epithermal deposits). Some of the gold deposits of British Columbia (such as in the Eskay Creek area adjacent to the Alaska panhandle), and many of the other gold deposits situated along the western edge of both South and North America are of the vein type shown in Figure 18.6, and are related to nearby magma bodies.

Banded Iron Formation

Most of the world’s major iron deposits are of the banded iron formation type (classified as a type of chemical sedimentary rock), and most of these formed during the initial oxygenation of Earth’s atmosphere between 2,400 and 1,800 Ma. At that time, iron that was present in dissolved form in the ocean as Fe²⁺ became oxidized to its insoluble form, Fe³⁺, and accumulated on the sea floor, mostly as hematite interbedded with chert (Figure 18.7). Unlike many other metals, which are economically viable at grades of around 1% or even much less, iron deposits are only viable if the grades are in the order of 50% iron and if they are very large. [caption id="attachment_757" align="aligncenter" width="550"] Figure 18.7 Banded Iron Formation at Fortescue Falls in western Australia. Source: Graeme Churchard (2013), CC BY 2.0. Image source.[/caption]

Unconformity-Type Uranium Deposits

There are several different types of uranium deposits, but some of the largest and richest are those within the Athabasca Basin of northern Saskatchewan. These are called unconformity-type uranium deposits because they are all situated very close to the unconformity between the Proterozoic Athabasca Group sandstone and the much older Archean sedimentary, volcanic, and intrusive igneous rock (Figure 18.8). [caption id="attachment_758" align="aligncenter" width="550"] Figure 18.8 Model of the formation of unconformity-type uranium deposits of the Athabasca Basin, Saskatchewan Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]The origin of unconformity-type U deposits is not perfectly understood, but it's thought that two features are particularly important: (1) the relative permeability of the Athabasca Group sandstone, and (2) the presence of graphitic schist within the underlying Archean rocks. The permeability of the sandstone allowed groundwater to flow through it and leach out small amounts of U, which stayed in solution in the oxidized form U⁶⁺. The graphite (C) created a reducing environment (non-oxidizing) that converted the U from U⁶⁺ to insoluble U⁴⁺, at which point it was precipitated as the mineral uraninite (UO₂).

Practice with Types of Metal Deposits [h5p id="183"]

Write the words into the correct blanks to complete the descriptions of types of metal deposits.

In a , metal is concentrated during emplacement of mafic or ultramafic magma, then concentrated even more from of surrounding rocks and . Hot metal-rich water discharged at ocean-floor hydrothermal vents preciptates the minerals found in , where the "S" stands for , and the "V" refers to heat that drives the process. A forms around cooling felsic intrusions where convection of and release of hot water from concentrate metals. In this setting, vein-type gold deposits also form. Deposits of the type originated during the oxygenation of Earth's atmosphere. The richest uranium deposits are the in northern Saskatchewan's Athabasca Basin. They're so named because they're in Proterozoic situated on top of Archean rocks. Fill-in-the-blank options:

banded iron formation
sandstone
epithermal
magmatic deposit
gravity segregation
cooling magma
sulphide
partial melting
volcanic
porphyry deposit
groundwater
unconformity-type deposits
VMS deposits

Mining and Mineral Processing

Metal deposits are mined in a variety of different ways depending on their depth, shape, size, and grade. Relatively large deposits that are close to the surface and somewhat regular in shape are mined using open-pit mine methods (Figure 18.1). Creating a giant hole in the ground is generally cheaper than making an underground mine, but it's also less precise, so it's necessary to mine a lot of waste rock along with the ore. Relatively deep deposits or those with elongated or irregular shapes are typically mined from underground with deep vertical shafts, declines (sloped tunnels), and levels (horizontal tunnels) (Figures 18.9 and 18.10). In this way, it's possible to focus the mining on the ore body itself. With relatively large ore bodies, it may be necessary to leave some pillars to hold up the roof. [caption id="attachment_759" align="aligncenter" width="500"] Figure 18.9 Underground at the Myra Falls Mine, Vancouver Island. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption][caption id="attachment_760" align="aligncenter" width="500"] Figure 18.10 Schematic cross-section of a typical underground mine. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]In many cases, the near-surface part of an ore body is mined with an open pit, while the deeper parts are mined underground (Figures 18.10 and 18.11). [caption id="attachment_761" align="aligncenter" width="550"] Figure 18.11 Entrance to an exploratory decline (arrow) for the New Afton Mine situated in the side of the open pit of the old Afton Mine, near Kamloops, B.C. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] A typical metal deposit might contain a few percent of ore minerals (e.g., chalcopyrite or sphalerite), mixed with the minerals of the original rock (e.g., quartz or feldspar). Other sulphide minerals are commonly present within the ore, especially pyrite. When ore is processed (typically very close to the mine), it is ground to a fine powder and the ore minerals are physically separated from the rest of the rock to make a concentrate. At a molybdenum mine, for example, this concentrate may be almost pure molybdenite (MoS₂). The rest of the rock is known as tailings. It comes out of the concentrator as a wet slurry and must be stored near the mine, in most cases, in a tailings pond. The tailings pond at the Myra Falls Mine on Vancouver Island is shown in Figure 18.12, and the settling ponds for waste water from the concentrator are shown in Figure 18.13. The tailings are contained by an embankment. Also visible in the foreground of Figure 18.12 is a pile of waste rock, which is non-ore rock that was mined in order to access the ore. Although this waste rock contains little or no ore minerals, at many mines it contains up to a few percent pyrite. The tailings and the waste rock at most mines are an environmental liability because they contain pyrite plus small amounts of ore minerals. When pyrite is exposed to oxygen and water, it generates sulphuric acid—also known as acid rock drainage (ARD). Acidity itself is a problem to the environment, but because the ore elements such as copper or lead are more soluble in acidic water than neutral water, ARD is also typically quite rich in metals, many of which are toxic. [caption id="attachment_762" align="aligncenter" width="550"] Figure 18.12 The tailings pond at the Myra Falls Mine on Vancouver Island. The dry rock in the middle of the image is waste rock. The structure on the right is the headframe for the mine shaft. Myra Creek flows between the tailings pond and the headframe. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption][caption id="attachment_763" align="aligncenter" width="550"] Figure 18.13 The tailings pond (lower left) at Myra Falls Mine with settling ponds (right) for processing water from the concentrator. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]Tailings ponds and waste-rock storage piles must be carefully maintained to ensure their integrity, and monitored to ensure that acidic and metal-rich water is not leaking out. In August 2014, the tailings pond at the Mt. Polley Mine in central B.C. failed and 10 million cubic metres of waste water along with 4.5 million cubic metres of tailings slurry was released into Polley Lake, Hazeltine Creek, and Quesnel Lake (Figure 18.14, a and b). As of July 2015, the environmental implications of this event are still not fully understood. [caption id="attachment_764" align="aligncenter" width="550"] Figure 18.14a The Mt. Polley Mine area prior to the dam breach of August 2014. The tailings were stored in the area labelled “retention basin.” Source: Source: NASA (2014), Public Domain. Image source.[/caption][caption id="attachment_765" align="aligncenter" width="550"] Figure 18.14b The Mt. Polley Mine area after the tailings dam breach of August 2014. The water and tailings released flowed into Hazeltine Creek, and Polley and Quesnel Lakes. Source: NASA (2014,) Public Domain. Image source.[/caption]Most mines have concentrators on site because it's relatively simple to separate ore minerals from non-ore minerals and thus significantly reduce the costs and other implications of transportation. But separation of ore minerals is only the preliminary stage of metal refinement. For most metals the second stage involves separating the actual elements within the ore minerals. For example, the most common ore of copper is chalcopyrite (CuFeS₂). The copper needs to be separated from the iron and sulphur to make copper metal, and that involves complicated and very energy-intensive processes that are done at smelters or other types of refineries. Because of their cost and the economies of scale, there are far fewer refineries than there are mines. There are several metal refineries (including smelters) in Canada; some examples are the aluminum refinery in Kitimat, B.C. (which uses ore from overseas); the lead-zinc smelter in Trail, B.C.; the nickel smelter at Thompson, Manitoba; numerous steel smelters in Ontario, along with several other refining operations for nickel, copper, zinc, and uranium; aluminum refineries in Quebec; and a lead smelter in New Brunswick.

Practice with Mineral Processing

Write the words into the correct blanks to complete the summary. Relatively large, irregular, near-surface deposits are mined using methods. Deeper, elongated, and/or irregularly-shaped deposits are typically mined from deep vertical , (sloped tunnels), and (horizontal tunnels). Sometimes a combination of these methods is used. When ore is processed, a is made by physically separating ore minerals from the rest of the rock. The remaining are usually stored nearby in a pond. If wastes contain the mineral , exposing them to oxygen and water will generate . After ore minerals are isolated, the next step is extracting the actual elements of interest from those minerals. This happens at and other types of refineries. Fill-in-the-blank options:

concentrate
acid rock drainage
pyrite
open-pit mine
smelters
shafts
declines
levels
tailings

To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="184"]

18.2 Industrial Minerals

kqzheng — Sat, 10 Feb 2018 00:02:30 +0000

Metals are critical for our technological age, but other not-so-shiny materials are also needed to facilitate our way of life. For everything made out of concrete or asphalt, we need sand and gravel. To make the cement that holds concrete together, we also need limestone. For the glass in our computer screens and for glass-sided buildings, we need silica sand plus sodium oxide (Na₂O), sodium carbonate (Na₂CO₃), and calcium oxide (CaO). Potassium is an essential nutrient for farming in many areas, and we also need various types of clay for a wide range of applications (e.g., ceramics and many industrial processes).

Aggregate

The best types of aggregate (sand and gravel) resources are those that have been sorted by streams, and in Canada the most abundant and accessible fluvial deposits are associated with glaciation. That doesn’t include till, because it has too much silt and clay, but it does include glaciofluvial outwash (deposits from glacier-derived rivers), which is present in thick deposits in many parts of the country (Figure 18.15). [caption id="attachment_768" align="aligncenter" width="550"] Figure 18.15 Sand and gravel in an aggregate pit near Nanaimo, BC. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] In a typical gravel pit, materials are graded on-site according to size. They are then used in a wide range of applications from constructing huge concrete dams to filling children’s sandboxes. Sand is also used to make glass, but for most types of glass, it has to be at least 95% quartz (which the sandy layers shown in Figure 18.15 are definitely not), and for high-purity glass and the silicon wafers used for electronics, the source sand has to be over 98% quartz.

Limestone for Concrete

Approximately 80 million tonnes of concrete are used in Canada each year—a little over 2 tonnes per person. The cement used for concrete is made from approximately 80% calcite (CaCO₃) and 20% clay. This mixture is heated to 1450°C to produce the required calcium silicate compounds (e.g., Ca₂SiO₄). The calcite typically comes from limestone quarries like the one on Texada Island, B.C. (Figure 18.16). Limestone is also used as the source material for many other products that require calcium compounds, including steel and glass, pulp and paper, and plaster products for construction. [caption id="attachment_769" align="aligncenter" width="550"] Figure 18.16 Triassic Quatsino Formation limestone being quarried on Texada Island, B.C. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Evaporite Minerals

Sodium is required for a wide range of industrial processes, and the most convenient source is sodium chloride (the mineral halite, also called rock salt), which is mined from evaporite beds in various parts of Canada. The largest salt mine in the world is at Goderich, Ontario, where salt is recovered from the 100 m thick Silurian Salina Formation. The same formation is mined in the Windsor area. Rock salt is also used as a source of sodium and chlorine in the chemical industry to melt ice on roads, as part of the process of softening water, and as a seasoning. Under certain conditions, the mineral sylvite (KCl) accumulates in evaporite beds, and this rock is called potash. This happened across the Canadian prairies during the Devonian, creating the Prairie evaporite formation. Potassium is used as a crop fertilizer, and Canada is the world’s leading supplier, with most of that production coming from Saskatchewan. Another evaporite mineral, gypsum (CaSO₄.2H₂0), is the main component of plasterboard (drywall) that is widely used in the construction industry. One of the main mining areas for gypsum in Canada is in the Milford Station area of Nova Scotia, site of the world’s largest gypsum mine.

Building Materials

Rocks are quarried or mined for many different uses, such as building facades (Figure 18.17), countertops, stone floors, and headstones. In most of these cases, the favoured rock types are granitic rocks, slate, and marble. Quarried rock is also used in some applications where rounded gravel isn’t suitable, such as the ballast (road bed) for railways, where crushed angular rock is needed. [caption id="attachment_770" align="aligncenter" width="550"] Figure 18.17 Slate used as a facing material on a concrete building column in Vancouver Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Do You Know Where to Find the Materials You Need?

Which mineral do you need?

For high-purity glass and silicon wafers used in electronics, you need...
For the cement in concrete you need...
For sodium oxide for making glass windows, you need...
To add drywall to your home renovation project, you need...
For fertilizer, you need...

Fill-in-the-blank options:

halite
gypsum
calcite
sylvite
quartz

To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="185"]

18.3 Fossil Fuels

kqzheng — Sat, 10 Feb 2018 00:02:32 +0000

There are many types of fossil fuels, but all derive from the storage of organic matter in sediments or sedimentary rocks—hence the term "fossil". Fossil fuels are rich in carbon, almost all of which ultimately came from CO₂ taken out of the atmosphere during photosynthesis. That process, driven by solar energy, involves reduction (the opposite of oxidation) of the carbon, resulting in it being combined with hydrogen instead of oxygen. The resulting organic matter is made up of complex and varied carbohydrate molecules. Most organic matter is oxidized back to CO₂ relatively quickly (within weeks or years in most cases), but any of it that gets isolated from the oxygen of the atmosphere (in settings such as the deep ocean or in a stagnant bog) may last long enough to be buried by sediments. If that happens, it may be preserved for tens to hundreds of millions of years. Under natural conditions, that means it will be stored until those rocks are eventually exposed at the surface and weathered. In this section, we’ll discuss the origins and extraction of the important fossils fuels, including coal, oil, and gas.

Coal

Coal was the first fossil fuel to be widely used. You can read more in section 9.3 Organic Sedimentary Rocks about how coal forms where vigorous growth of vegetation in swampy areas leads to an abundance of organic matter that accumulates within stagnant water. The chemical nature of the water—it being acidic and having little to no oxygen—means there is little decay of the organic matter. If a thick layer of organic matter is accumulated and then buried, the organic matter begins to change as it is compressed and heated. This situation, where the dead organic matter is submerged in oxygen-poor water, must be maintained for centuries to millennia in order for enough material to accumulate to form a thick layer. Over time, as the organic matter is heated and compressed more and more, the carbon within it becomes concentrated, and it is increasingly able to provide energy when burned. Figure 18.18 shows the classification system for different grades of coal. Increasing pressure and temperature means proceeding clockwise through the diagram, starting with lignite (the lowest grade), then bituminous coal, and finally anthracite, the highest grade. [caption id="attachment_773" align="aligncenter" width="480"] Figure 18.18 Coal ranking system used by the United States Geological Survey (USGS). As vegetative organic matter is buried deeper, and experiences higher pressures and temperatures, it progresses clockwise through the diagram, beginning with lignite. Additional heat and pressure result in the coal having a higher concentration of carbon (the vertical axis), and producing more energy (the horizontal axis). Source: USGS (2009), Public Domain. Image source.[/caption] There are significant coal deposits in many parts of Canada, including the Maritimes, Ontario, Saskatchewan, Alberta, and British Columbia. In Alberta and Saskatchewan, much of the coal is used for electricity generation. Coal from the Highvale Mine (Figure 18.19), Canada’s largest, is used to feed the Sundance and Keephills power stations west of Edmonton. Almost all of the coal mined in British Columbia is exported for use in manufacturing steel. [caption id="attachment_774" align="aligncenter" width="550"] Figure 18.19 The Highvale Mine (background) and the Sundance (right) and Keephills (left) generating stations on the southern shore of Wabamun Lake, Alberta. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Oil and Gas

While almost all coal forms on land from terrestrial vegetation, most oil and gas is derived primarily from marine micro-organisms that accumulate within sea-floor sediments. In areas where marine productivity is high, dead organic matter is delivered to the sea floor fast enough that some of it escapes oxidation. This material accumulates in the muddy sediments, and becomes buried by other sediments. [caption id="attachment_775" align="alignright" width="400"] Figure 18.20 The depth and temperature limits for biogenic gas, oil, and thermogenic gas. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]As the depth of burial increases, so too (due to the geothermal gradient) does the temperature. Gradually the organic matter within the sediments is converted to hydrocarbons (Figure 18.20). The first stage is the biological production (involving anaerobic bacteria) of methane. Most of this escapes back to the surface, but some is trapped in methane hydrates near the sea floor. At depths beyond about 2 km, and at temperatures ranging from 60° to 120°C, the organic matter is converted by chemical processes to oil. This depth and temperature range is known as the oil window. Beyond 120°C most of the organic matter is chemically converted to methane. The rock with organic matter in which the formation of gas and oil takes place is called the source rock. Both liquid oil and gaseous methane are lighter than water, so as liquids and gases form, they tend to move slowly toward the surface, out of the source rock and into reservoir rocks. Reservoir rocks are typically relatively permeable because that allows migration of the fluids from the source rocks, and also facilitates recovery of the oil or gas. In some cases, the liquids and gases make it all the way to the surface, where they are oxidized, and the carbon is returned to the atmosphere. But in other cases, they are contained by overlying impermeable rocks (e.g., mudrock) in situations where anticlines, faults, stratigraphy changes, and reefs or salt domes create traps (Figure 18.21). [caption id="attachment_776" align="aligncenter" width="550"] Figure 18.21 Migration of oil and gas from source rocks into traps in reservoir rocks. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]The liquids and gases that are trapped within reservoirs become separated into layers based on their density, with gas rising to the top, oil below it, and water underneath the oil. The proportions of oil and gas depend primarily on the temperature in the source rocks. Some petroleum fields, such as many of those in Alberta, are dominated by oil, while others, notably those in northeastern B.C., are dominated by gas. In general, petroleum fields are not visible from the surface, and their discovery involves the search for structures in the subsurface that have the potential to form traps. Seismic surveys are the most commonly used tool for early-stage petroleum exploration, as they can reveal important information about the stratigraphy and structural geology of subsurface sedimentary rocks. An example from the Gulf of Mexico south of Texas is shown in Figure 18.22. In this area, a thick evaporite deposit (“salt”) has formed domes because salt is lighter than other sediments and tends to rise slowly toward the surface; this has created traps. The sequence of deformed rocks is capped with a layer of undeformed rock. [caption id="attachment_777" align="aligncenter" width="650"] Figure 18.22 Seismic section through the East Breaks Field in the Gulf of Mexico. The dashed red line marks the approximate boundary between deformed rocks and younger undeformed rocks. The wiggly arrows are interpreted migration paths. The total thickness of this section is approximately 5 km. Source: Steven Earle (2015), CC BY 4.0. Image source. Modified after AAPG.org (2014), CC BY-SA 3.0. Image source.[/caption]

Try It! The cross-section shown here is from a ship-borne seismic survey in the Bering Sea off the west coast of Alaska. As a petroleum geologist, it’s your job to pick two separate locations to drill for oil or gas. Which locations would you choose? [caption id="attachment_1105" align="aligncenter" width="500"] Source: USGS, Public Domain. Image source.[/caption] Click to see some possible drill locations.

What Is Unconventional Oil and Gas?

The type of oil and gas reservoirs illustrated in Figures 18.21 and 18.22 are described as conventional reserves. Some unconventional types of oil and gas include oil sands, shale gas, and coal-bed methane.

Oil Sands

Oil sands are important because the reserves in Alberta are so large (the largest single reserve of oil in the world), but they are very controversial from an environmental and social perspective. They are “unconventional” because the oil is exposed near the surface and is highly viscous because of microbial changes that have taken place at the surface. The hydrocarbons that form this reserve originated in deeply buried Paleozoic rocks adjacent to the Rocky Mountains and migrated up and toward the east (Figure 18.23). [caption id="attachment_779" align="aligncenter" width="550"] Figure 18.23 Schematic cross-section of northern Alberta showing the source rocks and location of the Athabasca Oil Sands. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] The oil sands are controversial primarily because of the environmental cost of their extraction. Since the oil is so viscous, it requires heat to make it sufficiently liquid to process. This energy comes from gas; approximately 25 m³ of gas is used to produce 0.16 m³ (one barrel) of oil. The energy equivalent of the required gas is about 20% of the energy that can be derived from the produced oil. The other environmental cost of oil sands production is the devastation of vast areas of land where strip-mining is taking place and tailings ponds are constructed, and the unavoidable release of contaminants into the groundwater and rivers of the region. At present, most oil recovery from oil sands is achieved by mining the sand and processing it on site. Exploitation of oil sand that is not exposed at the surface depends on in situ processes, an example being the injection of steam into the oil-sand layer to reduce the viscosity of the oil so that it can be pumped to the surface.

Shale Gas

Shale gas is gas that is trapped within rock that is too impermeable for the gas to escape under normal conditions, and it can only be extracted by fracturing the reservoir rock using water and chemicals under extremely high pressure. This procedure is known as hydraulic fracturing or “fracking.” Fracking is controversial because of the volume of water used, and because, in some jurisdictions, the fracking companies are not required to disclose the nature of the chemicals used. Although fracking is typically done at significant depths, there is always the risk that overlying water-supply aquifers could be contaminated (Figure 18.24). Fracking also induces low-level seismicity (earthquakes). [caption id="attachment_780" align="aligncenter" width="550"] Figure 18.24 Depiction of the process of directional drilling and fracking to recover gas from impermeable rocks. The light blue arrows represent the potential for release of fracking chemicals to aquifers. Source: Steven Earle (2015, CC BY 4.0. Image source. Modified after Mike Norton (2013), CC BY-SA 3.0. Image source.[/caption]

Coal-Bed Methane

During the process that converts organic matter to coal, some methane is produced, which is stored within the pores of the coal. When coal is mined, methane is released into the mine where it can become a serious explosion hazard. Modern coal-mining machines have methane detectors on them and actually stop operating if the methane levels are dangerous. It's possible to extract the methane from coal beds without mining the coal; gas recovered this way is known as coal-bed methane.

Practice with Fossil Fuel Types

Write the words into the correct blanks to complete this summary of fossil fuel types. forms in swampy areas where plant matter accumulates in stagnant water. Organic matter is transformed over time with heating and compression. are derived from marine micro-organisms that accumulate within sea-floor sediments with little decomposition. As the organic matter is heated, is produced first. The occurs at depths beyond ~2 km, and temperatures ranging from 60° to 120°C. With additional heating, is produced. The rock containing organic matter that eventually becomes oil and gas is called the . Oil and gas migrate into more permeable . occur where oil is exposed near the surface and micro-organisms make it more viscous. accumulates in impermeable rocks. It's extracted by using to make cracks in the rock. Another source of fossil fuel is the gas trapped in coal deposits. This is called coal-bed . Fill-in-the-blank options:

methane
oil window
reservoir rocks
hydraulic fracturing
source rock
methane
oil sands
coal
oil
oil and gas
shale gas

To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="186"]

18.4 Diamonds

kqzheng — Sat, 10 Feb 2018 00:02:33 +0000

Although Canada’s diamond mining industry didn’t get started until 1998, diamonds are currently the sixth most valuable product mined in the country (Figure 18.3), and Canada ranks sixth in the world in diamond production. Diamonds form deep in the mantle, at approximately 200 km to 250 km depth. Under very specific pressure and temperature conditions, carbon that's naturally present in mantle rock (not coal) can be transformed into diamonds. If pressures and temperatures change to be outside the ideal window for diamond formation, diamonds can be turned into graphite. Diamond-bearing rock is brought to the surface by kimberlite volcanism. This type of volcanism is extremely rare. The most recent kimberlite eruption is thought to have been 10,000 years ago and prior to that at around 30 Ma. You can read more about the volcanology of kimberlites in Section 11.3 Types of Volcanoes. All of the world’s kimberlite diamond deposits are situated within ancient shield areas called cratons in Africa, Australia, Russia, South America, and North America. It has long been known that diamonds could exist within the Canadian Shield, but up until 1991, exploration efforts had been unsuccessful. In 1980 two geologists, Chuck Fipke and Stu Blusson, started searching in the Northwest Territories by sampling glacial sediments looking for some of the minerals that are normally quite abundant within kimberlites: chromium-bearing garnet, chromium-bearing pyroxene, chromite (Cr₂O₃), and ilmenite (FeTiO₃). These distinctive minerals are used for this type of exploration because they are many times more abundant in kimberlite than diamond is. After more than a decade of exploration, Fipke and Blusson finally focused their search on an area 250 km northeast of Yellowknife, and in 1991, they announced the discovery of a diamond-bearing kimberlite body at Lac de Gras. That discovery is now the Diavik Mine, and there is another diamond mine—Ekati—25 km to the northwest (Figure 18.25). There are two separate mines at Diavik accessing three different kimberlite bodies, and there are five at Ekati. There are six operating diamond mines in Canada: four in the Northwest Territories (including Diavik and Ekati), and one each in Nunavut and Ontario. [caption id="attachment_783" align="aligncenter" width="550"] Figure 18.25 Diamond mines in the Lac de Gras region, Nunavut. The twin pits of the Diavik Mine are visible in the lower right on an island within Lac de Gras. The five pits of the Ekati mine are also visible, on the left and the upper right. The two main mine centres are 25 km apart. Source: NASA (2013), Public Domain. Image source.[/caption]

Putting It Together

Why don't diamonds form from coal? Fill in the blanks to find out.Diamonds require special pressure and temperature conditions deep within the (hint: crust, mantle, or core?). Coal forms from plant matter in (hint: a type of wetland), which are largely terrestrial ecosystems, and therefore on (hint: continental or oceanic?) lithosphere. Because of (hint: the balance between the weight of lithosphere and its buoyancy in the mantle), there's no way to get the coal down to where diamonds form. To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="187"]

3.1 Earth’s Layers: Crust, Mantle, and Core

kqzheng — Thu, 08 Mar 2018 20:07:27 +0000

Earth consists of three main layers: the crust, the mantle, and the core (Figure 3.3). The core accounts for almost half of Earth's radius, but it amounts to only 16.1% of Earth's volume. Most of Earth's volume (82.5%) is its mantle, and only a small fraction (1.4%) is its crust. [caption id="attachment_76" align="aligncenter" width="1024"] Figure 3.3 Earth's interior. Right- crust, mantle, and outer and inner core to scale. Left- Cutaway showing continental and ocean crust, and upper mantle layers. The lithosphere is the crust plus the uppermost layer of the mantle. Source: Karla Panchuk (2018) CC BY 4.0. Click for more attributions.[/caption]

Crust

The Earth's outermost layer—its crust—is rocky and rigid. There are two kinds of crust: continental crust, and ocean crust. Continental crust is thicker, and predominantly felsic in composition, meaning that it contains minerals that are richer in silica. The composition is important because it makes continental crust less dense than ocean crust. Ocean crust is thinner, and predominantly mafic in composition. Mafic rocks contain minerals with less silica, but more iron and magnesium. Mafic rocks (and therefore ocean crust) are denser than the felsic rocks of continental crust. The crust floats on the mantle. Continental crust floats higher in the mantle than ocean crust because of the lower density of continental crust. An important consequence of the difference in density is that if tectonic plates happen to bring ocean crust and continental crust into collision, the plate with ocean crust will be forced down into the mantle beneath the plate with continental crust.

Concept Check: Continental Crust vs. Oceanic Crust

Write the words into the correct boxes to explain the difference between continental and oceanic crust.The difference between continental crust, the crust that makes up continents, and oceanic crust, the crust making up the ocean floor is an essential feature of Earth's plate tectonic processes. The crust floats on the mantle, and the density of the crust determines how high or low it floats. Continental crust makes up continents because it has a and than oceanic crust. Oceanic crust makes up ocean floors because it has a and in the mantle, making regions of low elevation.The distinction between continental and oceanic crust influences the interactions between tectonic plates that collide. When plates collide, a plate boundary with will sink beneath a plate boundary with . Fill-in-the-blank options:

higher density
continental crust
oceanic crust
lower density
thin
floats higher
thick
sinks deeper

To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="36"]

Mantle

The mantle is almost entirely solid rock, but it is in constant motion, flowing very slowly. It is ultramafic in composition, meaning it has even more iron and magnesium than mafic rocks, and even less silica. Although the mantle has a similar chemical composition throughout, it has layers with different mineral compositions and different physical properties. It's possible for rocks to have different mineral compositions and still be the same in chemical composition because the increasing pressure deeper in the mantle causes mineral structures to be reconfigured. Rocks higher in the mantle are typically composed of peridotite, a rock dominated by the minerals olivine and pyroxene. The Tablelands rock in Figure 3.2 is a type of peridotite. Lower in the mantle, extreme pressures transform minerals and create rocks like eclogite (Figure 3.4), which contains garnets. [caption id="attachment_77" align="aligncenter" width="650"] Figure 3.4 Eclogite from the Swiss-Italian Alps. Reddish brown spots are garnets. Source: James St. John (2014), CC BY 2.0. Image source.[/caption]

Lithosphere

The lithosphere can't be classified neatly as either crust or mantle because it consists of both: it's formed from the crust as well as the uppermost layer of the mantle, which is stuck to the underside of the crust. Tectonic plates are fragments of lithosphere.

Asthenosphere

Beneath the lithosphere is the asthenosphere. Tiny amounts of melted rock dispersed through the otherwise solid asthenosphere make the asthenosphere weak compared to the lithosphere. The weakness of the asthenosphere is important for plate tectonics because it deforms as fragments of lithosphere move around over and through it. Without a weak asthenosphere, plates would be locked in place, unable to move as they do now. Note that even though the asthenosphere does have tiny amounts of melt, it's still essentially solid.

D"

The D" (dee double prime) layer is a mysterious layer beginning approximately 200 km above the boundary between the core and mantle (referred to as the core-mantle boundary). We know it exists because of how seismic waves change speed as they move through it, but it isn't clear why it's different from the rest of the mantle. One idea is that pressure and temperature conditions are causing minerals to undergo yet another transition, similar to the transition between the upper and lower mantle. Other ideas are that small pools of melt are present, or that the differences in seismic properties are due to subducted slabs of lithosphere resting on the core-mantle boundary.

Concept Check: Mantle vs. Lithosphere

[h5p id="37"]

Is the lithosphere part of the mantle? Yes and no. The lithosphere contains some mantle material but isn't entirely part of the mantle. Lithosphere consists of crust with material from the uppermost mantle attached to its base.

Core

The core is primarily composed of iron, with lesser amounts of nickel. Lighter elements such as sulfur, oxygen, or silicon may also be present. The core is extremely hot: ~3500° C to more than 6000°C. Yet despite the fact that the boundary between the inner and outer core is approximately as hot as the surface of the sun, only the outer core is liquid. The inner core is solid because the pressure at that depth is so high that it keeps the core from melting. (Note: At these temperatures, if you could somehow bring a sample of the core to Earth's surface without it cooling off, it would vapourize instantly and eventually rain solid droplets of very hot metal onto you. So don't do that.)

Practice with Earth's Layers [h5p id="38"]

Extra Challenging: Earth's Layers Analogy [h5p id="39"]

3.2 Imaging Earth’s Interior

kqzheng — Sun, 11 Mar 2018 20:44:51 +0000

Seismology is the study of vibrations within Earth. You may have heard of seismology in the context of detecting and studying earthquakes, but vibrations can also come from extraterrestrial impacts, explosions, storm waves hitting the shore, and tides. Seismic waves travel through different materials at different speeds. By measuring how long it takes for seismic waves to travel from their source to a recording station, and applying knowledge of how they interact with different materials, we can figure out where Earth's layers are, and what they're like. This is similar to the way ultrasound is used to image the human body. Another feature of seismic waves is that some, called P-waves, can travel rapidly though both liquids and solids, but others, called S-waves, can only travel though solids, and are slower than P-waves. This is handy because observing where P-waves travel, and S-waves do not, allows us to identify regions within Earth that are melted.

Seismic Wave Paths

Seismic waves travel in all directions from their source, but it's more convenient to imagine the path traced by one point on the wave front, and represent that path as a seismic ray (heavy arrows, Figure 3.5). [caption id="attachment_80" align="aligncenter" width="650"] Figure 3.5 Seismic waves and seismic rays. The paths of seismic waves can be represented as rays. Seismic ray paths are refracted (bent) when they enter a rock layer with a different seismic velocity. Dashed arrows on the right show the direction the ray would have travelled without refraction. Source: Karla Panchuk (2021), CC BY 4.0. Updated. View original.[/caption] When seismic waves encounter a different rock layer, some might bounce off the layer, or reflect. But some waves will travel through the layer. If the wave travels at a different speed in the new layer, its path will be bent, or refracted, as it crosses into the new layer. If the wave can travel faster in the new layer, it will be bent slightly toward the slower layers. In Figure 3.5, this bending causes the ray to go at a shallower and shallower angle on the way down, and then at progressively steeper angles on the way up. Seismic velocities are higher in more rigid layers, and higher pressures tend to make layers more rigid. Pressure increases with depth within the Earth, so broadly speaking, seismic waves can gofaster deeper within the Earth. Over all, seismic rays tend to take curved paths through the Earth because refraction bends their path until they're reflected and directed upward again, as in Figure 3.5.

Discoveries with Seismic Waves

The Moho: Where Crust Meets Mantle

In the early 1900s, Croatian seismologist Andrija Mohorovičić (pronounced Moho-ro-vi-chich) made one of the first seismology-related discoveries about Earth’s interior. He noticed that sometimes, seismic waves were detected at seismic stations (measuring locations) farther from an earthquake before they were detected at stations closer to the earthquake. He reasoned that the waves that traveled farther were faster because they bent down deep enough to get into different rocks where they could travel much faster (those of the mantle) before being bent upward back into the crust (Figure 3.6). An analogy would be hiking in the wilderness and deciding that instead of taking a long winding trail, you'll take a shortcut through a swamp. Except you underestimate how slow going it will be through the very muddy swamp, and it turns out you would have completed the hike faster if you'd stuck with the longer trail. [caption id="attachment_81" align="aligncenter" width="650"] Figure 3.6 Depiction of seismic waves emanating from an earthquake (red star). Some waves travel through the crust to the seismic station (at ~6 km/s), while others go down into the mantle (where they travel at ~8 km/s) and are bent upward toward the surface, reaching the station before the ones that travelled only through the crust. Source: Steven Earle (2016) CC BY 4.0 view source[/caption] The boundary between the crust and the mantle is now known as the Mohorovičić discontinuity (or Moho). Its depth is between 60 – 80 km beneath major mountain ranges, 30 – 50 km beneath most of the continental crust, and 5 – 10 km beneath ocean crust.

Concept Check: Where Is the Moho?

Where is the Moho located? Choose one option.

Within the lithosphere, and therefore within tectonic plates
At the base of tectonic plates, between the lithosphere and asthenosphere
It marks the sharp contrast between the weak asthenosphere and the rigid part of the upper mantle immediately beneath it.

To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="40"]

The Core-Mantle Boundary

Arguments for a liquid outer core were supported by a distinctive signature in the global distribution of seismic waves from earthquakes. When an earthquake occurs, there is a zone on the opposite side of Earth where S-waves are not measured. This S-wave shadow zone begins 103° on either side of the earthquake (Figure 3.7, left). There is also a P-wave shadow zone on either side of the earthquake, from 103° to 150° (Figure 3.7, right). [caption id="attachment_82" align="aligncenter" width="650"] Figure 3.7 Patterns of seismic wave travel through Earth’s mantle and core. S-waves can't travel through the liquid outer core, so they leave a shadow on Earth’s far side. P-waves do travel through the core, but P-wave refraction bends seismic waves away from P-wave shadow zones. Source: Karla Panchuk (2021), CC BY 4.0. Modified after Steven Earle (2016) CC BY 4.0. Image source.[/caption] The S-wave shadow zone occurs because S-waves can't t travel through the liquid outer core. The P-wave shadow zone occurs because seismic velocities are much lower in the liquid outer core than in the overlying mantle, so the P-waves are refracted in a way that leaves a gap. Not only do the shadow zones tell us that the outer core is liquid, the size of the shadow zones allows us to calculate the size of the core, and the location of the core-mantle boundary.

Concept Check: Liquid Cores of Other Planets

How big are the cores of Planet A and Planet B? Bigger or smaller than Earth's core? Use the S-wave shadow zone example for Earth to figure it out. Look through the following images to see the answers. Explanation The bigger the shadow zone, the larger diameter of the core. The starting point of the shadow zone depends on how close the core is to the source of the seismic signal. The closer the core, the sooner the S-waves will run into liquid, limiting how deep the S-waves can go, and thus how far around the planet they can be detected.

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Seismic Portrait of Earth's Layers

The change seismic wave velocity with depth in Earth (Figure 3.8) has been determined over the past several decades by analyzing seismic signals from large earthquakes all around the world. Earth's layers are detectable as changes in velocity with depth. Key features are the following:

The asthenosphere is visible as a low velocity zone within the upper mantle.
There is an abrupt increase in P-wave velocity at 420 km, showing the depth at which minerals transform into structures that are more stable at higher pressures and temperatures.
The boundary between the upper and lower mantle is visible at 670 km as a sudden change from rapidly increasing P- and S-wave velocities to slow or no change in P-wave and S-wave velocities.
The core-mantle boundary is apparent as a sudden drop in P-wave velocities, where seismic waves move from solid mantle to liquid outer core.
The boundary between the outer core and inner core is marked by a sudden increase in P-wave velocity after 5000 km, where seismic waves move from a liquid back into a solid again.

[caption id="attachment_83" align="aligncenter" width="1687"] Figure 3.8 P-wave and S-wave velocity variations with depth from the crust through the upper mantle (left) and from the crust through to the core (right). Source: Karla Panchuk (2021), CC BY 4.0. Modified after Steven Earle (2016), CC BY 4.0 Image source.[/caption]

Concept Check: Seismic Velocity Changes

[h5p id="42"]

Where are the most abrupt changes in seismic velocity in Earth's interior, and what causes them? The most abrupt changes happen at the top and bottom boundaries of the outer core. Velocities drop suddenly going from the solid mantle to the liquid outer core, then increase suddenly moving back into the solid material of the inner core.

Seismic Images of Plate Tectonic Structures

Using data from many seismometers and hundreds of earthquakes, it's possible to create images from the seismic properties of the mantle. This technique is known as seismic tomography. Tomography can be used to map out slabs of lithosphere that are entering the mantle, or have disappeared within it. Those slabs are cooler, and therefore more rigid than surrounding mantle rocks, so seismic waves travel through them faster. In Figure 3.9, higher-than-average seismic velocities in cool slabs are indicated in dark blue. [caption id="attachment_84" align="aligncenter" width="1024"] Figure 3.9 P-waves and S-waves used to map out the location of the Cocos slab of lithosphere. The slab appears in dark blue, indicating higher than average seismic wave velocities. Left- Tomograms showing seismic wave anomalies for a 1290 km surface. Right- Cross-sections along the transect marked X-Y on the globe. Source: Karla Panchuk (2018). CC BY 4.0. Modified after van der Meer et al. (2018), CC BY 4.0. Image source.[/caption] Thanks to the tomograms, we can see that the Cocos plate—which is colliding with Central America—is part of a much larger slab of lithosphere that has already settled onto the mantle. Tomograms representing a surface at 1290 km depth (Figure 3.9, left) show that at that level, the Cocos slab is beneath the Caribbean Sea. The tomograms on the right show a vertical view along the line X-Y marked on the globe. The vertical tomograms show us that the Cocos slab extends all the way down to the core-mantle boundary.

Visit the Underworld

What is the Atlas of the Underworld?

The Atlas of the Underworld is a catalog of more than 90 slabs of lithosphere that have been imaged within the mantle using seismic tomography. The Atlas includes tomographic images, locator maps, and geological histories for each slab. The catalog can be searched online at Atlas of the Underworld or viewed in the original publication by van der Meer et al. (2018). The Atlas of the Underworld is an open-access resource. Visit the Atlas of the Underworld

The HADES Underworld Explorer

Create your own tomographic cross-sections for locations anywhere in the world by using this intuitive drag-and-drop tool. Visit the HADES Underworld Explorer

References

van der Meer, D.G., van Hinsbergen, D.J.J., and Spakman, W., (2018). Atlas of the Underworld: slab remnants in the mantle, their sinking history, and a new outlook on lower mantle viscosity. Tectonophysics, 723, 309-448. https://doi.org/10.1016/j.tecto.2017.10.004

3.3 Earth's Interior Heat

kqzheng — Sun, 11 Mar 2018 03:32:06 +0000

Earth Is Hot Inside

How hot? At the base of the crust, it's approximately 1000°C. At the base of the mantle, temperatures are around 3500°C. Earth's centre is more than 6000°C. Earth's heat comes from two main sources: physical processes early in its formation, and radioactive decay.

Physical Processes that Heated Earth

Formation processes contributed heat in the following ways:

Heat came from the thermal energy already contained within the objects that accreted to form Earth.
Heat came from collisions. When objects hit Earth, some of the energy from their motion went into deforming Earth, and some of it was transformed into heat. Clap your hands vigorously to experience this on a much smaller (and safer!) scale.
As Earth became larger, its gravitational force became stronger. This increased Earth’s ability to draw objects to it, but it also caused the material making Earth to be compressed, rather like Earth giving itself a giant gravitational hug. Compression causes materials to heat up.

Heating had an important consequence for Earth’s structure, because it eventually permitted differentiation to take place. Earth got so hot that iron and nickel mixed in with silicate minerals melted, and tricked toward Earth's centre to form its core. Differentiation itself heated Earth even more due to friction from metal melts moving through Earth.

Atoms Break Apart and Release Energy

A major source of Earth's heat is radioactivity, the energy released when the unstable atoms decay (their nucleus breaks apart, or they lose particles from their nucleus). The main source of the radiation heating Earth is the decay of the radioactive isotopes uranium-235 (²³⁵U), uranium-238 (²³⁸U), potassium-40 (⁴⁰K), and thorium-232 (²³²Th) in Earth's mantle. Radioactive decay produced more heat early in Earth's history than it does today, because the more decay that happened, the fewer radioactive atoms were left to decay in the future. Heat contributed by radioactivity today is roughly a quarter what it was when Earth formed (Figure 3.10). [caption id="attachment_87" align="aligncenter" width="400"] Figure 3.10 Production of heat within the Earth over time by radioactive decay of uranium, thorium, and potassium. Heat production has decreased over time as the abundance of radioactive atoms has decreased. Source: Steven Earle (2015), CC BY 4.0. Image source. Modified after Arevalo et al. (2009).[/caption]

Earth Gets Hotter the Deeper You Go

Earth’s temperature increases with depth, but not uniformly (Figure 3.11). Earth's geothermal gradient is 15° to 30°C/km within the crust. It then drops off dramatically through the mantle, increases more quickly at the base of the mantle, and then increases slowly through the core. [caption id="attachment_88" align="aligncenter" width="1024"] Figure 3.11 Geothermal gradient (change in temperature with depth). Left- Geothermal gradient in the crust and upper mantle. The geothermal gradient remains below the melting temperature of rock, except in the asthenosphere. There, temperatures are high enough to melt some of the minerals. Right- Geothermal gradient throughout Earth. Rapid changes occur in the uppermost mantle, and at the core-mantle boundary. Source: Karla Panchuk (2018), CC BY 4.0. Modified after Steven Earle (2016), CC BY 4.0. Image source.[/caption] The temperature gradient within the lithosphere varies depending on the plate tectonic setting. Gradients are lowest in the central parts of continents. They are higher where plates collide, and higher still at boundaries where plates are moving away from each other. In spite of high temperatures within Earth, mantle rocks are almost entirely solid. This is because high pressures keep them from melting. The red dashed line in Figure 3.11 (right) shows the minimum temperature at which dry mantle rocks will melt. Rocks at temperatures to the left of the line will remain solid. In rocks at temperatures to the right of the line, some minerals will begin to melt. Notice that the red dashed line goes further to the right for greater depths, and therefore greater pressures. Now compare the geothermal gradient with the red dashed line. The geothermal gradient is to the left of the red line, except in the asthenosphere, where small amounts of melt are present.

Convection Helps to Move Heat Within Earth

The fact that the temperature gradient is much lower in the main part of the mantle than in the lithosphere has been interpreted as evidence of convection in the mantle. When the mantle convects, heat is transferred through the mantle by physically moving hot rocks. Mantle convection is the result of heat transfer from the core to the base of the lower mantle. As with a pot of soup on a hot stove (Figure 3.12), the material near the heat source (the soup at the bottom of the pot) becomes hot and expands, making it less dense than the material above. Buoyancy causes it to rise, and cooler material flows in from the sides. Of course, convection in the soup pot is much faster than convection in the mantle. Mantle convection occurs at rates of centimetres per year. [caption id="attachment_89" align="aligncenter" width="400"] Figure 3.12 Convection in a pot of soup on a hot stove (left). As long as heat is being transferred from below, the liquid will convect. If the heat is turned off (right), the liquid remains hot for a while, but convection will cease. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] Convection carries heat to the surface of the mantle much faster than heating by conduction. Conduction is heat transfer by collisions between molecules, and is how heat is transferred from the stove to the soup pot. A convecting mantle is an essential feature of plate tectonics, because the higher rate of heat transfer is necessary to keep the asthenosphere weak. Earth’s mantle will stop convecting once the core has cooled to the point where there is not enough heat transfer to overcome the strength of the rock. This has already happened on smaller planets like Mercury and Mars, as well as on Earth’s moon. When mantle convection stops, the end of plate tectonics will follow.

Models of Mantle Convection

In the soup pot example, convection moves hot soup from the bottom of the pot to the top. Some geologists think that Earth's convection works the same way—hot rock from the base of the mantle moves all the way to the top of the mantle before cooling and sinking back down again. This view is referred to as whole-mantle convection (Figure 3.13, left). Other geologists think that the upper and lower mantle are too different to convect as one. They point to slabs of lithosphere that are sinking back into the mantle, some of which seem to perch on the boundary between the upper and lower mantle, rather than sinking straight through. They also note chemical differences in magma originating in different parts of the mantle—differences that are not consistent with the entire mantle being well stirred. They argue that double-layered convection is a better fit with the observations (Figure 3.13, right). Still others argue that there may be some locations where convection goes from the bottom of the mantle to the top, and some locations where it doesn't (Figure 3.13, middle). [caption id="attachment_90" align="aligncenter" width="1024"] Figure 3.13 Models of mantle convection. Left- whole mantle convection. Rocks rise from the core-mantle boundary to the top of the mantle, then sink to the bottom again. Right- Two-layer convection, in which upper and lower mantle convect at different rates. Middle- Convection paths vary depending on the circumstances. Source: Karla Panchuk (2018) CC BY 4.0[/caption]

Putting It Together: Lord Kelvin and the Age of the Earth

Lord Kelvin (of temperature fame) is known for getting Earth’s age wrong. Being a physicist, he decided that thermodynamics would be a nice, simple starting point, and came up with a method that involved assuming something was spherical (mostly justified in this case). He thought along the lines of imagining Earth starting out as a red-hot iron cannonball, and then being plunged into an icebox as cold as space (−270° C). The cannonball would lose heat very quickly at first, but then more slowly once the outsides cooled off. At that point, heat would be slowly leaking out from the centre of the cannonball and through to the outside. If you know how to do thermodynamics, "very quickly at first" and "more slowly once the outsides cooled off" are things you can quantify, and—most important for our story—tie to how long the cannonball has been cooling. Lord Kelvin's plan was to estimate Earth's heat flow, and tie that to how long Earth had been cooling. The answer he got was between 20 and 40 million years, after accounting for uncertainties. As you may know, Lord Kelvin's estimate was off by at least 99%, so saying he wasn't even close is still being very generous. It is perhaps a good illustration of the great age of the Earth that if we round up, his answer was completely wrong. Lord Kelvin didn't make an error in his calculation. Where he fudged up was more basic than that. Given what you know about where Earth’s heat comes from, what were two reasons for fudging up? Hint: He assumed Earth cooled like a solid iron sphere. To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="43"]

References

Arevalo, R., McDonough, W., & Luong, M. (2009). The K/U ratio of earth: Insights into mantle composition, structure and thermal evolution. Earth and Planetary Science Letters, 278(3-4), 361-369. https://doi.org/10.1016/j.epsl.2008.12.023

3.4 Earth’s Magnetic Field

kqzheng — Wed, 14 Mar 2018 19:26:37 +0000

Similar to the mantle, Earth's liquid outer core convects because it's heated from beneath by the inner core. What's different is that because it's made of iron and conducts electricity (even when molten), the motion of the outer core generates a magnetic field. Earth’s magnetic field is defined by north and south poles representing lines of magnetic force flowing into Earth in the northern hemisphere, and out of Earth in the southern hemisphere (Figure 3.14). Because of the shape of the field lines, the magnetic force is oriented at different angles to the surface in different locations. The tilt, or inclination of magnetic field lines is represented by the tilt of compass needles in Figure 3.14. At the north and south poles, the force is vertical. The force is horizontal at the equator. Everywhere in between, the magnetic force is at an intermediate angle to the surface. [caption id="attachment_93" align="aligncenter" width="626"] Figure 3.14 Earth’s magnetic field depicted as the field of a bar magnet coinciding with the core. The south pole of the magnet points to Earth’s magnetic north pole. The red and white compass needles represent the orientation of the magnetic field at various locations on Earth’s surface. Source: Karla Panchuk (2018). CC BY-SA 4.0. Modified after Steven Earle (2015; CC BY-SA 4.0, view source), and T. Stein (2008; CC BY-SA 3.0 view source).[/caption]

Practice: Magnetic Inclination Use Figure 3.14 as a guide to help you complete this exercise.

Write in the geographic locations to match up with the correct magnetic inclination (the up-and-down orientation of a compass needle).

Straight down- Quttinirpaaq National Park, Ellesmere Island. This is the current location of the .
Down at a steep angle- Santa's workshop on a specially-designed ice breaker anchored in the Arctic Ocean (the location of the .)
Up at a steep angle- McMurdo Station, which is on the coast of Antarctica near the .
Parallel to flat ground- Damba Island, Uganda, which straddles the .

Fill-in-the-blank options:

geographic north pole
south pole
magnetic north pole
equator

To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="44"]

Polarity Reversals

Instability in Earth's Magnetic Field

Earth’s magnetic field is generated mostly within the outer core by the convective movement of liquid iron, but although convection is continuous, the magnetic field is not stable. Periodically, the magnetic field decays, then re-estabilshes. When it does re-establish, the polarity may have reversed. In other words, your compass needle would point south rather than north. Changes in Earth’s magnetic field have been studied using mathematical models that simulate convection in the outer core (Figure 3.15). Reversals happened spontaneously when the model was run to simulate a period of several hundred thousand years. Spontaneous reversals can happen because convection doesn't occur in an orderly way, in spite of what the bar magnet analogy may suggest. [caption id="attachment_94" align="aligncenter" width="650"] Figure 3.15 Computer simulations showing Earth's "normal" magnetic field (top), and the magnetic field as polarity flips from reversed on the left to normal on the right. Notice how the magnetic field lines become disorganized, then converge on a more orderly arrangement. Source: Karla Panchuk (2021) CC BY 4.0. Click for more attributions.[/caption] The solid inner core also convects, with many small-scale variations in convection patterns, but it does so more slowly than the liquid outer core. Yet Earth's magnetic field is the sum of all of those variations—both inner and outer—and for a polarity reversal to "take," a reversal must happen in the magnetic fields of both parts of the core. If the inner core weren't solid, magnetic reversals would happen far more frequently.

How Often Do Polarity Reversals Happen?

Over the past 250 Ma, there have been hundreds of magnetic field reversals, and their timing has been anything but regular. The shortest ones that geologists have been able to identify lasted only a few thousand years, and the longest one was more than 30 million years, during the Cretaceous Period (Figure 3.16). [caption id="attachment_95" align="aligncenter" width="1024"] Figure 3.16 Magnetic field reversal chronology for the past 170 Ma. Black stripes mark times when the magnetic field was oriented the same as today. Source: Steven Earle (2015). CC BY 4.0. Image source. Modified after AnomieX (2010), Public Domain. Image source.[/caption]

Concept Check: Polarity Reversals

Magnetic polarity reversals happen because:

Earth flips over so that it's geographic poles change place.
A new pattern develops in how Earth's core churns itself.
Earth's core switches to rotating in the opposite direction within Earth.
Earth switches to rotating in the opposite direction on its axis.

To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="45"]

References

British Geological Survey, Natural Environment Research Council (n.d.). Reversals: Magnetic flip. http://www.geomag.bgs.ac.uk/education/reversals.html

Glatzmaier, G. A. (n.d.) The geodynamo. https://websites.pmc.ucsc.edu/~glatz/geodynamo.html

Glatzmaier, G. A., & Roberts, P.H. (1995). A three-dimensional self-consistent computer simulation of a geomagnetic field reversal. Nature, 377, 203-209.

3.5 Isostasy

kqzheng — Thu, 15 Mar 2018 20:02:41 +0000

Lithospheric Plates Float on the Mantle

The mantle is able to convect because it can deform by flowing over very long timescales. This means that tectonic plates are floating on the mantle, like a raft floating in the water, rather than resting on the mantle like a raft sitting on the ground. How high the lithosphere floats will depend on the balance between gravity pulling the lithosphere down, and the force of buoyancy as the mantle resists the downward motion of the lithosphere. Isostasy is the state in which the force of gravity pulling the plate toward Earth's centre is balanced by the resistance of the mantle to letting the plate sink.

A Messy Example

To see how isostasy works, consider the rafts in Figure 3.17. The raft on the right is sitting on solid concrete. The raft will remain at the same elevation whether there are two people on it, or four, because the concrete is too strong to deform. In contrast, isostasy is in play for the rafts on the left, which are floating in a swimming pool full of peanut butter. With only one person on board, the raft floats high in the peanut butter, but with three people, it sinks dangerously low. We have filled the swimming pool with peanut butter rather than water because the viscosity of peanut butter (its stiffness or resistance to flowing) more closely represents the relationship between the tectonic plates and the mantle. Although peanut butter has a similar density to water, it's higher viscosity means that if a person is added to a raft, it will take longer for the raft to settle lower into the peanut butter that it would take the raft to sink into water. [caption id="attachment_98" align="aligncenter" width="650"] Figure 3.17 Illustration of isostatic relationships between rafts and peanut butter (left), and a non-isostatic relationship between a raft and solid ground (right). Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] The relationship of Earth’s crust to the mantle is similar to the relationship of the rafts to the peanut butter. The raft with one person on it floats comfortably high. Even with three people on it. the raft is less dense than the peanut butter, so it floats, but it floats uncomfortably low for those three people.

Mountain-Building Example

The lithosphere, with an average density of ~2.7 g/cm³, is less dense than the mantle (average density ~3.4 g/cm³ near the surface, but greater at depth), and so the lithosphere is floating on the mantle. When weight is added to the lithosphere through the process of mountain building, the lithosphere slowly sinks deeper into the mantle, and the mantle material that was there is pushed aside (Figure 3.18, left). When erosion removes material from the mountains over tens of millions of years, decreasing the weight, the crust rebounds and the mantle rock flows back (Figure 3.18, right). [caption id="attachment_99" align="aligncenter" width="650"] Figure 3.18 Isostatic relationship between the crust and the mantle. Mountain building adds mass to the crust, and the thickened crust sinks down into the mantle (left). As the mountain chain is eroded, the crust rebounds (right). Green arrows represent slow mantle flow. Source: Karla Panchuk (2018), CC BY 4.0. Modified after Steven Earle (2016), CC BY 4.0. Image source.[/caption]

Isostasy and Glacial Rebound

How Glacial Rebound Works

The crust and mantle respond in a similar way to glaciation. Thick accumulations of glacial ice add weight to the crust, and the crust subsides, pushing the mantle out of the way. The Greenland ice sheet, at over 2,500 m thick, has depressed the crust below sea level (Figure 3.19a). When the ice eventually melts, the crust and mantle will slowly rebound (Figure 3.19b), but full rebound will likely take more than 10,000 years (3.19c). [caption id="attachment_100" align="aligncenter" width="540"] Figure 3.19 Cross-section through the crust in the northern part of Greenland. a) Up to 2,500 m of ice depresses the crust downward (red arrows) and below sea level. b) After complete melting. Isostatic rebound would be slower than the rate of melting, leaving central Greenland at or below sea level for thousands of years. c) Complete rebound after ~10,000 years raises central Greenland above sea level again. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Where Is Glacial Rebound Happening Today?

Large parts of Canada are still rebounding as a result of the loss of glacial ice over the past 12,000 years, as are other parts of the world (Figure 3.20). The highest rate of uplift is in a large area west of Hudson Bay, where the Laurentide Ice Sheet was the thickest, at over 3,000 m. Ice finally left this region around 8,000 years ago, and the crust is currently rebounding at nearly 2 cm/year. Strong isostatic rebound is also occurring in northern Europe where the Fenno-Scandian Ice Sheet was thickest, and in the eastern part of Antarctica, which also experienced significant ice loss in the last 11,000 years. Glacial rebound in one location means subsidence (sinking) in surrounding areas (Figure 3.20, yellow through red regions). Regions surrounding the former Laurentide and Fenno-Scandian Ice Sheets that were lifted up when mantle rock was forced aside and beneath them are now subsiding as the mantle rock flows back. [caption id="attachment_101" align="aligncenter" width="630"] Figure 3.20 Current rates of post-glacial isostatic uplift (green, blue, and purple shades) and subsidence (yellow and orange). Subsidence is taking place where the mantle is slowly flowing back toward areas that are experiencing post-glacial uplift. Source: Steven Earle (2015), CC BY 4.0. Image source. Modified after Erik Ivins, JPL (2010). Public Domain. Image source.[/caption]

How Can the Mantle Be Both Solid and Plastic?

You might be wondering how it's possible that Earth’s mantle is solid, rigid rock, and yet it convects and flows like a very viscous liquid. The explanation is that the mantle behaves as a non-Newtonian fluid, meaning that it responds differently to stresses depending on how quickly the stress is applied. A good example of non-Newtonian behaviour is the deformation of Silly Putty, which can bounce when it is compressed rapidly when dropped, will break if you pull on it sharply, but will deform in a liquid manner if stress is applied slowly. The force of gravity applied over a period of hours can cause it to deform like a liquid, dripping through a hole in a glass tabletop (Figure 3.21). Similarly, the mantle will flow when placed under the slow but steady stress of a growing (or melting) ice sheet. [caption id="attachment_102" align="aligncenter" width="272"] Figure 3.21 Silly Putty exhibiting plastic behavior when acted upon by gravity over several hours. Source: Erik Skiff (2006), CC BY-SA. Image source.[/caption]

Put It Together: Isostasy and Sea-Level Rise

[h5p id="46"]

Question: The Greenland ice sheet has a sea-level equivalent of 7.42 m. This is determined by measuring the volume of the Greenland ice sheet, then figuring out what depth that amounts to if it was water instead of ice, and was distributed across the oceans.If the Greenland ice sheet melted completely, does that mean sea level would rise globally by 7.42 m?Hint: Both water and ice can be pretty heavy. Answer: No, it's more complicated than simply topping up the oceans by 7.42 m everywhere. Locations very near the (former) Greenland ice sheet could actually experience a decrease in sea level because removing the ice from Greenland would allow it to bob up higher on the mantle. Locations affected by subsidence could experience an increase in sea level greater than 7.42 m. Subsidence happens in regions around the glacier as the bulge of mantle material that the glacier pushed out starts to settle back under Greenland. Subsidence can also happen if the weight of water forces the lithosphere to flex downward into the mantle, deepening basins further.

Chapter 3 Summary & Key Term Check

kqzheng — Fri, 16 Mar 2018 05:10:26 +0000

Chapter 3 Main Ideas

3.1 Earth's Layers

Earth is divided into a rocky crust and mantle, and a core consisting largely of iron. The crust and the uppermost mantle form the lithosphere, which is broken into tectonic plates. The next layer, the asthenosphere, allows the plates to move because it deforms by flowing.

3.2 Imaging Earth's Interior

Seismic waves that travel through Earth are either P-waves or S-waves. P-waves are faster than S-waves, and can pass through fluids. Earth's layers can be identified by looking at changes in the velocity of seismic waves. Seismic wave shadow zones contributed to knowledge of the depth of the core-mantle boundary, and the knowledge that the outer core is liquid. Plate tectonic structures within Earth can also be mapped using the seismic waves generated by earthquakes.

3.3 Earth's Interior Heat

Earth’s temperature increases with depth (to around 6000°C at the centre), but the rate of increase is not the same everywhere. In the lithosphere, thickness and plate tectonic setting are are factors. Deeper within the mantle, convection currents are more important.

3.4 Earth's Magnetic Field

Earth's magnetic field is generated by convection of the liquid outer core. The magnetic field is similar to that of a bar magnet, and has force directions that vary with latitude. The polarity of the field is not constant, meaning that the positions of the north and south magnetic poles have flipped from “normal” (as it is now) to reversed and back many times in Earth's history.

3.5 Isostasy

The plastic nature of the mantle, which allows for mantle convection, also determines the nature of the relationship between the lithosphere and the mantle. The lithosphere floats on the mantle in an isostatic relationship. Where the lithosphere becomes thicker and heavier because of mountain building, it pushes farther down into the mantle. Oceanic crust, being denser than continental crust, floats lower on the mantle than continental crust.

Key Term Check

What key term from Chapter 3 is each card describing? Turn the card to check your answer. [h5p id="47"]

5.1 Atoms

kqzheng — Tue, 20 Mar 2018 20:21:11 +0000

Protons Are What Make Elements Distinct

All matter, including mineral crystals, is made up of atoms. All atoms are made up of three main particles: protons, neutrons, and electrons. Protons have a positive charge, neutrons have no charge, and electrons have a negative charge. Protons and neutrons have approximately the same mass, but electrons have a mass that is 10,000 times smaller. The element hydrogen (H) has the simplest atoms. Most hydrogen atoms have just one proton and one electron. The proton forms the nucleus (the centre of the atom), while the electron orbits around it (Figure 5.5, left). All other elements have more than one proton in their nucleus. [caption id="attachment_160" align="aligncenter" width="648"] Figure 5.5 Atomic structure of hydrogen and helium showing protons (p+), neutrons (n), and electrons (e-). Source: Bruce Blaus (2014), CC BY 3.0. Image source.[/caption] Protons repel each other because they are positively charged, but it's possible to have more than one proton in a nucleus because neutrons hold them together. The next most complex atom, helium (He) has two protons and two neutrons in its nucleus, in its most common form. Some atoms of the same element can have different numbers of neutrons. For example, forms of hydrogen exist with one and two neutrons, and a tiny fraction of He atoms have only one neutron. Forms of an element with different numbers of neutrons are called isotopes. The number of protons in an atom determines what element it will be, so the number of protons is called the atomic number of that element. The total number of protons and neutrons in the nucleus is the mass number. The mass number distinguishes between isotopes of an element. Isotopes of an element are denoted by putting the mass number as a subscript in front of the symbol for that element. For example, the isotopes of hydrogen are ¹H (1 proton), ²H (1 proton + 1 neutron), and ³H (1 proton + 2 neutrons). For most of the 16 lightest elements (up to oxygen) the number of neutrons is equal to the number of protons. For most of the remaining elements, there are more neutrons than protons. This is because the more protons that are concentrated in a small space, the more neutrons are needed to keep the nucleus together. The most common isotope of uranium (U), for example, is ²³⁸U. It has 92 protons, but requires 146 neutrons to keep them together. The neutrons are only partly successful: uranium is radioactive, meaning that its nucleus will eventually split apart and release energy. What remains of the nucleus has fewer protons, so after decay the atom is a different element.

Did You Get All of That? Let's See!

Match each answer to the correct box. A proton has a charge of , and an electron has a charge of . Neutrons have a charge of . Protons have the same mass as but both are much heavier than . In the nucleus, repel each other because they have the same charge, but keep the nucleus together. An element's refers to the number of protons it contains. The is the total number of protons and neutrons in the nucleus. Atoms of the same element always have the same but isotopes of that element will have a different . Fill-in-the-blank options:

+ 1
atomic number
neutrons
mass number
neutrons
electrons
0
protons
mass unmber
− 1
atomic number

To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="52"]

Electrons Are What Control How Atoms Interact

Electrons orbiting around the nucleus of an atom are arranged in shells (also called energy levels). The first shell can hold only two electrons (as in H and He in Figure 5.5), but the next shell holds up to eight electrons. An atom can have many shells of electrons, but there are never more than 8 outermost electrons interacting with surrounding atoms. The outermost electrons determine how atoms can be bonded together. Elements that have a full outer shell (e.g., neon, Figure 5.6 right) are inert because they do not react with other elements to form compounds. These are the noble gases (including helium, argon, krypton, and radon, in addition to neon) in the far-right column of the periodic table. For elements that do not have a full outer shell (e.g., lithium, Figure 5.6 left), the outermost electrons can interact with the outermost electrons of nearby atoms to create chemical bonds. [caption id="attachment_161" align="aligncenter" width="648"] Figure 5.6 The number of electrons in an atom's outermost shell (or energy level) determine whether it will bond to other atoms, and how it will bond. Right- Neon has a completely filled outer shell with 8 electrons. It does not bond with other atoms. Left- Lithium has only one electron in its outer shell. It bonds with other atoms. Source: Bruce Blaus (2014), CC BY 3.0. Image source.[/caption] The electron shell configurations for 29 of the first 36 elements are listed in Table 5.1. Note that some of the shells in the table below have more than 8 electrons. This is because they contain subshells. For example, the third shell can hold up to 18 electrons because it contains one subshell that can hold 2 electrons, and two subshells that can hold 8 electrons each.

Table 5.1 Electron shell configurations (number of electrons in each shell) of some of the elements up to krypton. Inert elements (those with filled outer shells) are shaded.
Element	Symbol	Atomic Number	First	Second	Third	Fourth
Hydrogen	H	1	1
Helium	He	2	2
Lithium	Li	3	2	1
Beryllium	Be	4	2	2
Boron	B	5	2	3
Carbon	C	6	2	4
Nitrogen	N	7	2	5
Oxygen	O	8	2	6
Fluorine	F	9	2	7
Neon	Ne	10	2	8
Sodium	Na	11	2	8	1
Magnesium	Mg	12	2	8	2
Aluminum	Al	13	2	8	3
Silicon	Si	14	2	8	4
Phosphorus	P	15	2	8	5
Sulphur	S	16	2	8	6
Chlorine	Cl	17	2	8	7
Argon	Ar	18	2	8	8
Potassium	K	19	2	8	8	1
Calcium	Ca	20	2	8	8	2
Scandium	Sc	21	2	8	9	2
Titanium	Ti	22	2	8	10	2
Vanadium	V	23	2	8	11	2
Chromium	Cr	24	2	8	13	1
Manganese	Mn	25	2	8	13	2
Iron	Fe	26	2	8	14	2
.	.	.	.	.	.	.
Selenium	Se	34	2	8	18	6
Bromine	Br	35	2	8	18	7
Krypton	Kr	36	2	8	18	8

Do You Understand Why Elements React?

Helium and neon are non-reactive for the same reason. Which of the elements below are also non-reactive? Select all that apply.

Argon
Nitrogen
Krypton
Potassium
Vanadium

To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="53"]

5.2 Bonding and Lattices

kqzheng — Wed, 21 Mar 2018 00:17:35 +0000

Atoms seek to have a full outer shell. For hydrogen and helium, a full outer shell means two electrons. For other elements, it means 8 electrons. Filling the outer shell is accomplished by transferring or sharing electrons with other atoms in chemical bonds. The type of chemical bond is important for the study of minerals because the type of bond will determine many of a mineral's physical and chemical properties.

Ionic Bonds

Consider the example of halite again, which is made up of sodium (Na) and chlorine (Cl). Na has 11 electrons: two in the first shell, eight in the second, and one in the third (Figure 5.7, top). Na readily gives up the third shell electron so it can have the second shell with 8 electrons as its outermost shell. When it loses the electron, the total charge from the electrons is -10, but the total charge from the protons is +11, so it is left with a +1 charge over all. [caption id="attachment_164" align="aligncenter" width="400"] Figure 5.7 Electron configuration of sodium and chlorine atoms (top). Sodium gives up an electron to become a cation (bottom left) and chlorine accepts an electron to become an anion (bottom right). Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] Chlorine has 17 electrons: two in the first shell, eight in the second, and seven in the third. Cl readily accepts an eighth electron to fill its third shell, and therefore becomes negatively charged because it has a total charge of -18 from electrons, and a total charge of +17 from protons. In changing their number of electrons, these atoms become ions—the sodium loses an electron to become a positive ion or cation,[footnote]You can remember that a cation is positive by remembering that a cat has paws (paws sounds like "pos" in "positive"). You could also think of the "t" in "cation" as a plus sign.[/footnote] and the chlorine gains an electron to become a negative ion or anion (Figure 5.7, bottom). Because negative and positive charges attract, sodium and chlorine ions stick together, creating an ionic bond. In an ionic bond, electrons can be thought of as having transferred from one atom to another.

Cation or Anion? [h5p id="54"]

Covalent Bonds

Some elements can also form bonds by sharing electrons rather than giving away or receiving them. This will happen when electrons are held especially tightly by their atoms. Chlorine gas (Cl₂, Figure 5.9) is formed when chlorine atoms share two outer-shell electrons so that each has a complete outer shell. This is called a covalent bond. [caption id="attachment_165" align="aligncenter" width="400"] Figure 5.9 A covalent bond between two chlorine atoms. The electrons are black in the left atom, and blue in the right atom. Two electrons are shared (one black and one blue) so that each atom appears to have a full outer shell. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] Carbon is another atom that participates in covalent bonding. An uncharged carbon atom has six protons and six electrons. Two of the electrons are in the inner shell and four are in the outer shell (Figure 5.10, left). Carbon would need to gain or lose four electrons to have a filled outer shell, and this would create too great a charge imbalance. Instead, carbon atoms share electrons to create covalent bonds (Figure 5.10, right). [caption id="attachment_166" align="aligncenter" width="434"] Figure 5.10 The electron configuration of carbon (left) and the sharing of electrons in covalent C bonding (right). The electrons shown in blue are shared between adjacent C atoms. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] In the mineral diamond (Figure 5.11, left), carbon atoms are linked in a three-dimensional framework, where each carbon atom is bonded to four other carbon atoms. Every bond is a very strong covalent bond. [caption id="attachment_167" align="aligncenter" width="450"] Figure 5.11 Covalently bonded structures. Left: Diamond with three-dimensional structure of covalently bonded carbon. Right: Graphite with covalently bonded sheets of carbon. Sheets are held together by weaker van der Waals forces. Source: Karla Panchuk (2018), CC BY 4.0. Modified after Materialscientist (2009), CC BY-SA 3.0. Image source.[/caption]

Other Types of Bonds

Most minerals are characterized by ionic bonds, covalent bonds, or a combination of the two, but there are other types of bonds that are important in minerals.

Van der Waals Forces and Hydrogen Bonds

Consider the mineral graphite (Figure 5.11, right): carbon atoms are linked together in sheets or layers in which each carbon atom is covalently bonded to three others. Graphite-based compounds are strong because of the covalent bonding between carbon atoms within each layer, which is why they're used in high-end sports equipment such as ultralight racing bicycles. Graphite itself is soft, however, because the layers themselves are held together by relatively weak Van der Waals forces. Van der Waals forces, like hydrogen bonds, work because molecules can be electrostatically neutral, but still have an end that is slightly more positive and an end that is slightly more negative. In water molecules (Figure 5.12, left), the bent shape puts the hydrogen atoms on one side of the molecule, and the oxygen atom, with more electrons, on the other. The charge is distributed asymmetrically across the water molecule. Contrast this with the straight carbon dioxide molecule (Figure 5.12, right). The slightly more negative oxygen atoms on the ends are distributed symmetrically on either side of the carbon atom. [caption id="attachment_168" align="aligncenter" width="500"] Figure 5.12 Hydrogen bonding. Water molecules (left) are polar molecules (their charge is distributed asymmetrically). Slightly negative parts of the molecule are attracted to slightly positive parts of other water molecules. Carbon dioxide (right) is a non-polar molecule. The slightly negative oxygen atoms are distributed symmetrically on either side of the carbon atom. Source: Karla Panchuk (2018), CC BY-SA 4.0. Modified after Querter (2011, CC BY-SA 3.0; view source) and Jynto (2011, CC0 1.0; view source).[/caption]

Metallic Bonds

Metallic bonding occurs in metallic elements because they have outer electrons that are relatively loosely held. (The metals are highlighted on the periodic table in Appendix 1.) When bonds between such atoms are formed, the dissociated electrons can move freely from one atom to another. This feature accounts for two very important properties of metals: their electrical conductivity and their malleability (they can be deformed and shaped). [caption id="attachment_169" align="aligncenter" width="475"] Figure 5.13 Metallic bonding. Dissociated electrons (grey dots) move between metal atoms. Source: Karla Panchuk (2018), CC BY-SA 4.0. Nucleus by Fornax (2010), CC BY-SA 3.0. Image source.[/caption]

Chemical Bond Types

Fill in the missing words to complete the definitions of the different types of chemical bonds. In bonding, atoms share electrons. Diamond, one of the hardest naturally occurring substances, is formed from this type of bond. bonding is what allows copper to be drawn into a wire instead of breaking. Halide elements, such as chlorine, which have space for one additional electron in their outer shells, will undergo bonding with elements like sodium than have an electron to give up. When weak bonds connect strongly bonded layers, the mineral will come apart in sheets. To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="55"]

5.3 Mineral Groups

kqzheng — Sun, 25 Mar 2018 05:44:18 +0000

Minerals are organized according to the anion or anion group (a group of atoms with a net negative charge, e.g., SO₄^2–) they contain, because the anion or anion group has the biggest effect on the properties of the mineral. Silicates, with the anion group SiO₄^4-, are by far the most abundant group in the crust and mantle. (They will be discussed in Section 5.4). The different mineral groups along with some examples of minerals in each group are summarized below.

Oxide Minerals: O^2- Anion

Oxide minerals (Figure 5.14) have oxygen (O^2–) as their anion. They don't include anion groups with other elements, such as the carbonate (CO₃^2–), sulphate (SO₄^2–), and silicate (SiO₄^4–) anion groups. The iron oxides hematite and magnetite are two examples that are important ores of iron. Corundum is an abrasive, but can also be a gemstone in its ruby and sapphire varieties. If the oxygen is also combined with hydrogen to form the hydroxyl anion (OH^–), the mineral is known as a hydroxide. Some important hydroxides are limonite and bauxite, which are ores of iron and aluminum, respectively. [caption id="attachment_172" align="aligncenter" width="650"] Figure 5.14 Oxide minerals include metal ore minerals, industrial minerals, and gemstones. Source: Karla Panchuk (2018), CC BY-NC-SA 4.0. Component photographs by Roger Weller/Cochise College.[/caption]

Sulphide Minerals: S^2- Anion

Sulphide minerals (Figure 5.15) include galena, sphalerite, chalcopyrite, and molybdenite, which are the most important ores of lead, zinc, copper, and molybdenum, respectively. Some other sulphide minerals are pyrite, bornite, stibnite, and arsenopyrite. Sulphide minerals tend to have a metallic sheen. [caption id="attachment_173" align="aligncenter" width="650"] Figure 5.15 Sulphide minerals often have a metallic lustre and include metal ores. Source: Karla Panchuk (2018) CC BY-NC-SA 4.0. Photos by R. Weller/ Cochise College. Click for more attributions.[/caption]

Sulphate Minerals: SO₄^2- Anion Group

Many sulphate minerals form when sulphate-bearing water evaporates. A deposit of sulphate minerals may indicate that a lake or sea has dried up at that location. Sulphates with calcium include anhydrite, and gypsum (Figure 5.16). Sulphates with barium and strontium are barite and celestite, respectively. In all of these minerals, the cation has a +2 charge, which balances the –2 charge on the sulphate ion. [caption id="attachment_174" align="aligncenter" width="650"] Figure 5.16 Sulphate minerals. Source: Karla Panchuk (2018), CC BY-NC-SA 4.0. Click for more attributions.[/caption]

Halide Minerals: Anions from the Halogen Group

The anions in halides are the halogen elements including chlorine, fluorine, and bromine. Examples of halide minerals are cryolite, fluorite, and halite (Figure 5.17). Halide minerals are made of ionic bonds. Like the sulphates, some halides also form when mineral-rich water evaporates. [caption id="attachment_175" align="aligncenter" width="650"] Figure 5.17 Halide minerals. Source: Karla Panchuk (2018), CC BY-NC-SA 4.0. Click for more attributions.[/caption]

Carbonate Minerals: CO₃^2- Anion Group

The carbonate anion group combines with +2 cations to form minerals such as calcite, magnesite, dolomite, and siderite (Figure 5.18). The copper minerals malachite and azurite are also carbonates. The carbonate mineral calcite is the main component of rocks formed in ancient seas by organisms such as corals and algae. [caption id="attachment_176" align="aligncenter" width="650"] Figure 5.18 Carbonate minerals. Source: Karla Panchuk (2018), CC BY-SA 4.0. Photos by Rob Lavinsky, iRocks.com, CC BY-SA 3.0. Click for more attributions.[/caption]

Phosphate Minerals: PO₄^3- Anion

The apatite group of phosphate minerals (Figure 5.19, left) includes hydroxyapatite, which makes up the enamel of your teeth. Turquoise is also a phosphate mineral (Figure 5.19, right). [caption id="attachment_177" align="aligncenter" width="650"] Figure 5.19 Phosphate minerals. Source: Karla Panchuk (2018), CC BY-NC-SA 4.0. Photos by R. Weller/ Cochise College. Click for more attributions.[/caption]

Silicates (SiO₄⁴^–)

The silicate minerals include the elements silicon and oxygen in varying proportions . These are discussed at length in Section 5.4 Silicate Minerals.

Native Element Minerals

These are minerals made of a single element, such as gold, copper, silver, or sulphur (Figure 5.20). [caption id="attachment_178" align="aligncenter" width="650"] Figure 5.20 Native element minerals are made up of a single element. Source: Karla Panchuk (2018), CC BY-SA 4.0. Click for more attributions.[/caption]

Beginner Practice with Anionic Groups Minerals are grouped according to the anion part of the mineral formula, and mineral formulas are always written with the anion part last. For example, in pyrite (FeS₂), Fe_2⁺ is the cation and S^– is the anion. This helps us to know that it’s a sulphide, but it isn't always that obvious... Hematite (Fe₂O₃) is an oxide; that’s easy, but anhydrite (CaSO₄) is a sulphate because SO₄^2– is the anion, not O. Similarly, calcite (CaCO₃) is a carbonate, and olivine (Mg₂SiO₄) is a silicate. Minerals with only one element (such as S) are native minerals, while those with an anion from the halogen column of the periodic table (Cl, F, Br, etc.) are halides. [h5p id="56"]

Anionic Groups: Level Up!

Select all of the oxide minerals (O₂²⁻anion).
- Feldspar KAlSi₃O₈
- Magnetite Fe₃O₄
- Pyroxene MgSiO₃
- Anglesite PbSO₄
- Ilmenite FeTiO₃
- Xenotime YPO₄
- Siderite FeCO₃
Which is the sulphide mineral (S^2- anion)?
1. Anglesite PbSO₄
2. Sphalerite ZnS
3. Sulphur S
Select all of the halide minerals.
- Fluorite CaF₂
- Cinnabar HgS
- Avogadrite (K,Cs)BF
- Sylvite KCl
- Bromargyrite AgBr

To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="57"]

13.2 Folds

kqzheng — Wed, 21 Feb 2018 23:22:07 +0000

Folds are a type of ductile deformation. They form when rocks bend in response to stress. The sides of a fold are its limbs (Figure 13.10). The limbs meet in a region of curvature called the hinge zone. A fold's axial surface is an imaginary surface that runs along the hinge zone and cuts the fold in half. The line that forms when the axial surface intersects another surface, such as the top of a bed, is called the axial trace. Axial traces are sometimes marked on geological maps to show the location of the fold's hinge zone. [caption id="attachment_529" align="aligncenter" width="650"] Figure 13.10 The parts of a fold. A fold consists of limbs that meet at the hinge zone. An axial surface bisects the fold along the hinge zone. The axial trace is where the axial surface intersects another surface, such as the top of a bed. Source: Karla Panchuk (2018), CC BY-NC-SA 4.0. Photo: Ron Schott (2009), CC BY-NC-SA 2.0. Image source.[/caption]

Fold Classification

Synclines and Anticlines

Folds can be classified according to the whether the limbs slope toward or away from the hinge zone. If the limbs slope toward the hinge zone (i.e., the hinge zone points downward), as in the fold in the left of Figure 13.11, the fold is called a syncline. If the limbs slope away from the hinge zone (i.e., the hinge zone points upward), the fold is called an anticline. There is an anticline on the right side of Figure 13.11. The fold in Figure 13.10 is also an anticline. Sometimes an anticline or a syncline will occur by itself, but they can also occur in a series of alternating synclines and anticlines, similar to the way the anticline and syncline share a limb in Figure 13.11. A sequence of linked anticlines and synclines is called a fold train. [caption id="attachment_530" align="aligncenter" width="650"] Figure 13.11 An asymmetrical syncline linked to an anticline on a beach in Cornwall, United Kingdom. The beds slope toward the hinge at different angles on either side of the axial surface. Source: Karla Panchuk (2018), CC BY-NC-SA 4.0. Photo: Harry Soar (2014), CC BY-NC-SA 2.0. Image source.[/caption]

Symmetrical, Asymmetrical, Overturned, and Recumbent

In a symmetrical fold, the limbs slope at approximately the same angle on either side of the axial surface. The fold in Figure 13.10 is symmetrical. In an asymmetrical fold, the limbs slope at different angles on either side of the axial surface. The syncline in Figure 13.11 is asymmetrical. The limb on the left side of the syncline slopes toward the hinge at a steeper angle than the limb on the right. If the fold is sufficiently tilted that the beds on one side have been tilted past vertical, and are sloping in the same direction, the fold is overturned (Figure 13.12). [caption id="attachment_531" align="aligncenter" width="550"] Figure 13.12 Overturned folds in Andalusia in southern Spain. Some limbs have been overturned far enough to be sloping in the same direction on either side of the axial trace. Source: Karla Panchuk (2018), CC BY-NC-SA 2.0. Photo: Ignacio Benvenuty Cabral (2017), CC BY-NC-SA 4.0. Image source.[/caption] It is possible for rocks to be folded so tightly that the fold limbs are nearly parallel. Folds with parallel limbs are called isoclinal folds. A recumbent fold is an isoclinal fold that has been overturned to the extent that the limbs are horizontal (Figure 13.13). [caption id="attachment_532" align="aligncenter" width="550"] Figure 13.13 A recumbent fold has limbs that are nearly parallel, and an axial trace that is nearly horizontal. Source: Karla Panchuk (2018), CC BY-NC-SA 4.0. Photo: Ignacio Benvenuty Cabral (2017), CC BY-NC-SA 4.0. Image source.[/caption]

Folds in the Landscape

Folds can be of any size, and it’s very common to have smaller folds within larger folds (Figure 13.14). Large folds can extend over 10s of kilometres, and very small ones might only be visible under a microscope. [caption id="attachment_533" align="aligncenter" width="550"] Figure 13.14 Folded limestone (grey) and chert (rust-coloured) in rocks of the Triassic Quatsino Formation on Quadra Island, British Columbia. The image is about 1 m across. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] When folded rocks are weathered and eroded, they can alter the landscape by forming long ridges and valleys (Figure 13.15). Ridges and valleys curve into V-shapes if the hinge of the fold is not horizontal. A fold with a hinge that slopes downward is called a plunging fold (Figure 13.16). [caption id="attachment_534" align="aligncenter" width="550"] Figure 13.15 Ridges and valleys in central Pennsylvania formed from weathered and eroded folds. The V-shapes indicate the folds are plunging. Source: NASA on the Commons (2001), Public Domain. Image source.[/caption] [caption id="attachment_535" align="aligncenter" width="550"] Figure 13.16 Plunging folds have sloping hinges. Plunging folds are described in terms of the plunge angle, the angle the hinge makes with a horizontal line. Inset- When a plunging fold intersects a surface, the result is a V-shaped pattern. Source: Karla Panchuk (2018), CC BY-SA 4.0. Photo: Dieter Mueller (2004), CC BY-SA 3.0. Image source.[/caption] Folds can create landforms, but anticlines are not necessarily expressed as ridges in the terrain. Likewise, synclines do not necessarily appear as valleys. When folded rocks erode, the landform that results depends how resistant individual layers are to erosion. For example, if the rocks in the interior of an anticline are more resistant to weathering than the surrounding rocks, a ridge will result (e.g., the low hill represented by units 4 and 5 in Figure 13.17, top). On the other hand, if rocks in the interior of the anticline are weaker, a valley will result (Figure 13.17, bottom, units d¹ and d²). Similarly, a syncline with stronger rocks in the interior will weather to form a ridge, and a syncline with weaker rocks in the interior will weather to form a valley. [caption id="attachment_536" align="aligncenter" width="550"] Figure 13.17 Cross-sections of eroded folds expressed as hills and valleys, from an early study on the geology of Wales, Devon, and Cornwall. Top- An anticline in Shropshire, England. Beds in the interior of the anticline form a gentle hill. Bottom- An anticline in Herefordshire, England in which beds in the interior of an anticline weathered to form a valley. Source: Symonds (1872), Public Domain. Image source: Top / Bottom[/caption]

Practice with Types of Folds

What kind of folds are these? These are folded and (hint: at least one limb is tilted beyond vertical, and both limbs tilt in the same general direction), but not , because the limbs aren't parallel. They are (hint: not a mirror image on either side of the hold axis) because the limbs tilt away from the fold axes at different angles. What's going on here? This V-shaped structure is a (hint: horizontal or plunging) fold. It was formed by (hint: compression or tension) stress. We know the stress came from the north (hint: east or west) and the south (hint: east or west) based on the orientation of the fold axis. Could this structure have originated right at Earth's surface? To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="156"]

References

Symonds, W. S. (1872). Records of the rocks; or, Notes on the geology, natural history, and antiquities of North & South Wales, Devon, & Cornwall. London: J. Murray. https://archive.org/details/recordsofrocksor00symoiala

13.3 Fractures, Joints, and Faults

kqzheng — Sat, 24 Feb 2018 00:54:56 +0000

When rocks break in response to stress, the resulting break is called a fracture. If rocks on one side of the break shift relative to rocks on the other side, then the fracture is a fault. If there is no movement of one side relative to the other, and if there are many other fractures with the same orientation, then the fractures are called joints. Joints with a common orientation make up a joint set (Figure 13.19). [caption id="attachment_539" align="aligncenter" width="550"] Figure 13.19 Joint sets have broken these siltstone and shale beds into long rectangular planks. Source: Michael C. Rygel (2008), CC BY-SA 3.0. Image source.[/caption]

Jointing

Most joints form when the overall stress regime is one of tension (pulling apart) rather than compression. The tension can be from a rock contracting, such as during the cooling of volcanic rock (Figure 13.9, upper left). It can also be from a body of rock expanding. Exfoliation joints, which make the rock appear to be flaking off in sheets (Figure 13.20), occur when a body of rock expands in response to reduced pressure, such as when overlying rocks have been removed by erosion. [caption id="attachment_540" align="aligncenter" width="550"] Figure 13.20 Half Dome at Yosemite National Park is an exposed granite batholith that displays exfoliation joints, causing sheets of rock to break off. Source: HylgeriaK (2010), CC BY-SA 3.0. Image source.[/caption] Nevertheless, it is possible for joints to develop where the overall regime is one of compression. Joints can develop where rocks are being folded, because the hinge zone of the fold is under tension as it stretches to accommodate the bending (Figure 13.21). [caption id="attachment_541" align="aligncenter" width="371"] Figure 13.21 Joints developed in the hinge zone of folded rocks. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] Joints can also develop in a rock a rock under compression as a way to accommodate the change in shape (Figure 13.22). The joints accommodate the larger compression stress (larger red arrows) by allowing the rock to stretch in the up-down direction (along the green arrows). [caption id="attachment_542" align="aligncenter" width="369"] Figure 13.22 Joints developing to accommodate the larger horizontal component of compression (large red arrows). Source: Steven Earle, CC BY 4.0. Image source.[/caption]

Faulting

A fault is a boundary between two bodies of rock along which there has been relative motion (e.g., Figure 13.23). Some large faults, like the San Andreas fault in California or the Tintina fault, extending from northern British Columbia through central Yukon and into Alaska, show evidence of hundreds of kilometres of motion. Other faults show only centimetres of movement. In order to estimate the amount of motion on a fault, it is necessary to find a feature that shows up on both sides of the fault, and has been offset by the fault. This could be the edge of a bed or dike as in Figure 13.23, or it could be a landscape feature, such as a fence or a stream. [caption id="attachment_543" align="aligncenter" width="550"] Figure 13.23 View looking down on a fault (white dashed line) in intrusive rocks on Quadra Island, British Columbia. The pink dyke has been offset approximately 10 cm by the fault (length of the white arrow). Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Types of Faults

Different kinds of faults develop under different stress conditions. We describe faults in terms of how the rocks on one side of the fault move relative to the other.

Dip-Slip Faults

Dip-slip faults are so named because the dominant motion involves moving up or down the dipping (tilting) fault plane. In dip-slip faults we identify rock above the fault as the hanging wall, (or headwall) and the rock beneath as the footwall. These terms were originally used by miners to describe the rocks above and below an ore body (Figure 13.24). [caption id="attachment_544" align="aligncenter" width="650"] Figure 13.24 The hanging wall (or headwall) of a fault is the rock above the fault. The footwall is the rock below. These terms were originally used by miners to describe the rocks above and below an ore body. Source: Photo- Gold Hill Mine, Yukon Territory, by Eric A. Hegg (1898), Public Domain. Image source.. Diagram- Karla Panchuk (2018) CC BY 4.0.[/caption] Tension produces normal faults, in which the crust undergoes extension. This permits the hanging wall to slide down the footwall in response to gravity (Figure 13.25, left). Compression produces reverse faults, pushing the hanging wall up relative to the footwall. Reverse faults shorten and thicken the crust (Figure 13.25, right).

Strike-Slip Faults

Faults where the motion is mostly horizontal and along the “strike” or the length of the fault are called strike-slip faults (Figure 13.26 bottom). These happen where shear stress causes bodies of rock to slide sideways with respect to each other, as is the case along a transform boundary. If the far side moves to the right, as in Figures 13.23 and 13.26 (right), it is a right-handed, right-lateral,or dextral strike-slip fault. If the far side moves to the left it is a left-handed, left-lateral, or sinistral strike-slip fault. [caption id="attachment_545" align="aligncenter" width="650"] Figure 13.25 Dip slip faults. Normal faults are caused by tension, while reverse faults happen during compression. Source: Karla Panchuk (2018), CC BY-SA 4.0. Modifed after Woudloper (2010), CC BY-SA 3.0. Image source.[/caption] [caption id="attachment_546" align="aligncenter" width="562"] Figure 13.26 Strike-slip faults. Rocks on either side of the fault move parallel to the fault. In dextral strike-slip faults the far side moves to the right of the observer. In sinistral strike-slip faults the far side moves to the left of the observer. Source: Karla Panchuk (2018), CC BY 4.0.[/caption]

Different Tectonic Settings Have Distinct Types of Faults

Horst and Graben Structure

In areas that are characterized by extensional tectonics, and with many normal faults arranged side-by-side, some blocks may subside (settle downward) relative to neighbouring parts. This is typical in areas of continental rifting, such as the Great Rift Valley of East Africa or in parts of Iceland. In such situations, blocks that move down relative to the other blocks are graben, and elevated blocks with graben on either side are called horsts. There are many horsts and graben in the Basin and Range area of the western United States, especially in Nevada. Part of the Fraser Valley region of British Columbia, in the area around Sumas Prairie, is a graben. [caption id="attachment_547" align="aligncenter" width="650"] Figure 13.27 Graben and horst structures form where extension is happening. All of the faults are normal faults. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Thrust Faults

Thrust faults are a type of reverse fault with a very low-angle fault plane. The fault planes of thrust faults typically slope at less than 30°. Thrust faults are relatively common in mountain belts that were created by continent-continent collisions. Some represent tens of kilometres of thrusting, where thick sheets of sedimentary rock have been pushed up and over other layers of rock (Figure 13.28). [caption id="attachment_548" align="aligncenter" width="550"] Figure 13.28 A thrust fault. Top: prior to faulting. Bottom: after significant fault offset. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]There are numerous thrust faults in the Rocky Mountains, and a well-known example is the McConnell Thrust, along which a sequence of sedimentary rocks about 800 m thick has been pushed for about 40 km from west to east over underlying rock (Figure 13.29). The thrusted rocks range in age from Cambrian to Cretaceous, so in the area around Mt. Yamnuska Cambrian-aged rock (around 500 Ma) has been thrust over, and now lies on top of Cretaceous-aged rock (around 75 Ma) (Figure 13.30). [caption id="attachment_549" align="aligncenter" width="581"] Figure 13.29 The McConnell Thrust in the eastern part of the Rockies. The rock within the faded area has been eroded. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] [caption id="attachment_550" align="aligncenter" width="569"] Figure 13.30 The McConnell Thrust at Mt. Yamnuska near Exshaw, Alberta. Cambrian limestones have been thrust over top of Cretaceous mudstone. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Practice with Types of Faults

What kind of dip-slip fault is this? The hanging wall went relative to the footwall. (Hint: Up or down?) That makes this a fault (Hint: Normal, reverse, thrust, or strike-slip?), caused by stress. (Hint: Tension, compression, or shear?) What kind of dip-slip fault is this? The hanging wall went relative to the footwall. (Hint: Up or down?) The fault cuts the beds at a angle. (Hint: High or low?) This makes it a fault (Hint: Normal, reverse, thrust, or strike-slip?), caused by stress. (Hint: Tension, compression, or shear?) What kind of faults are these? The hanging wall goes relative to the footwall when each fault is considered individually. (Hint: Up or down?) That makes this -slip fault (Hint: Strike or dip?) a fault. (Hint: Normal, reverse, or thrust?) What kind of fault is this? This is a -slip fault (Hint: Strike or dip?), formed from stress. (Hint: Tension, compression, or shear?) From the perspective of the photographer, the beds on the opposite side of the fault are shifted to the . (Hint: Left or right?) This makes this fault . (Hint: Sinistral or dextral?) To check your answers, navigate to the below link to view the interactive version of this activity.

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13.4 Mountain Building

kqzheng — Sat, 24 Feb 2018 21:31:15 +0000

Some of Earth's mountains are entirely or almost entirely the result of volcanic activity. These include volcanic islands like the Hawai'ian hotspot volcanoes, and newly formed volcanic island arcs along subduction zones. But the majority of mountain building on Earth is the result of plate tectonic forces that deform Earth's crust through faulting and folding. Mountain building through folding and faulting may or may not be supplemented by volcanic activity.

Mountain Building Along Convergent Margins

Mountain building along convergent margins is referred to as orogeny, and the mountains that are built are called orogens.

Ocean-Continent Collision

In ocean-continent collision zones, folding and faulting of rocks combines with volcanism to build mountains. An example of mountains built this way is the Sierra Nevada mountain range in Utah and Nevada. The orogeny that formed the Sierra Nevada range began around 140 million years ago. The mountain range was built up by igneous intrusions and volcanic eruptions along a continental volcanic arc (Figure 13.32). The terrain was altered further inland as well. Sheets of rock were thrust on top of each other, and pushed inland along a detachment fault, similar to the example of the McConnell Thrust in Figure 13.29. [caption id="attachment_553" align="aligncenter" width="967"] Figure 13.32 Orogeny in an ocean-continent collision zone. Mountains form from subduction zone volcanism, and from large sheets of rock that are thrust inland and folded. Materials accumulating on the leading edge of the continent in an accretionary wedge are eventually smashed onto the continent, adding to continental crust. Source: Karla Panchuk (2018), CC BY 4.0. Modified after Ron Blakey, NAU Geology (n.d.). Click for terms of use.[/caption] Continental crust flexed downward because of the weight of the mountains, and this formed a fore arc basin seaward of the new mountain range. Sediments accumulated within that basin. The leading edge of the continent also collected sediments and igneous rock scraped off the subducting plate, forming an accretionary wedge. Over time, the force of the collision would smash the basin sediments and the accretionary wedge against the continent, turning it into new continental crust.

Continent-Continent Collision

When two continents collide, it means the closure of a subduction zone, and an end to volcanism. The Alleghenian Orogeny, which brought together North America and Africa, helping to form Pangea, is an example of mountain building in a continent-continent collision zone. Before the continents came into contact with each other, mountain building on the eastern coast of North America would have involved deformation from an ocean-continent collision, as with Figure 13.32. But as subduction proceeded, the subducting plate drew Africa closer and closer to North America. The gap between the two continents began to close, and fill with sediments (Figure 13.33, top). [caption id="attachment_554" align="aligncenter" width="883"] Figure 13.33 Orogeny by continent-continent collision. The formation of Pangea included the merging of Africa and North America. This closed an ocean basin and stopped subduction along the coast of North America. Volcanism ended with the closure of the ocean basin, but mountains continued to grow through folding and faulting. Source: Karla Panchuk (2018), CC BY 4.0. Modified after Ron Blakey, NAU Geology (n.d.). Image source. Click for terms of use.[/caption] While a subduction zone existed, the addition of water to the mantle permitted partial melting of mantle rocks, and thus volcanic activity. However, when the two continents collided, the subduction zone closed off and volcanism was no longer possible. As the continents smashed together, deep faults formed and stacked blocks of crust on top of each other. Old faults were reactivated. Rocks also began to shift along the boundary between an earlier orogen, the Taconic Orogen, and North America (Figure 13.33, bottom). When the continents had finally merged, Africa met North America along a suture zone with remnants of a continental volcanic arc on one side, and folded and faulted sedimentary rocks on the other.

Mountain Building Along Divergent Margins

When continents begin to split apart, normal faults form. This can lead to large blocks of crust that are tilted, raised, or lowered compared to adjacent blocks. Blocks that are elevated compared to adjacent blocks can form another type of mountain, called a fault-block mountain. Fault block mountains formed in eastern North America when Pangea began to split up, and Africa pulled away from North America (Figure 13.34). [caption id="attachment_555" align="aligncenter" width="856"] Figure 13.34 Fault-block mountains formed in a rift zone. Magma can move up along normal faults, resulting in igneous intrusions, or volcanic eruptions. Over time, valleys between elevated blocks will fill with sediment as the blocks erode. Source: Karla Panchuk (2018), CC BY 4.0. Modified after Ron Blakey, NAU Geology (n.d.). Image source. Click for terms of use.[/caption] Over time, elevated blocks erode, filling up valleys with sediment. The thinning of continental crust that occurs with rifting can decrease the pressure on mantle rocks enough to trigger partial melting. Magma can move up along the normal faults, forming igneous intrusions, or feeding volcanoes. The Palisades Sill in New York and New Jersey is a result of rift-zone magmatism. It is a cliff-like feature resulting from erosion that exposed the tip of a structure like the sills in Figure 13.34.

13.5 Measuring Geological Structures

kqzheng — Sun, 25 Feb 2018 05:27:04 +0000

Documenting the characteristics of geological structures is used to understand the geological history of a region. One of the key features to measure is the orientation, or attitude, of bedding. We know that sedimentary beds are deposited in horizontal layers, so if the layers are no longer horizontal, then we can infer that tectonic forces have folded or tilted them. The orientation of a planar feature, such as a bed of sedimentary rock, can be described with two values. The strike of the bed is the compass orientation of a horizontal line on the surface of the bed. The dip is the angle at which the surface tilts down from the horizontal (Figure 13.35). The dip is measured perpendicular to strike, otherwise the dip angle that is measured will be smaller than the actual tilt of the bed. [caption id="attachment_558" align="aligncenter" width="550"] Figure 13.35 Strike and dip for tilted sedimentary beds. Water provides a horizontal surface. The strike and dip symbol is a T with the long horizontal bar representing the strike direction, and the small tick mark indicating the dip direction. The dip angle is written next to the tick mark. Source: Karla Panchuk (2018), CC BY 4.0. Modified after Steven Earle (2015), CC BY 4.0. Image source.[/caption] It may help to imagine a vertical surface, such as a wall in your house. The strike is the compass orientation of the wall and the dip is 90˚ from horizontal. If you could push the wall so it is leaning over, but still attached to the floor, the strike direction would be the same, but the dip angle would be less than 90˚. If you pushed the wall over completely so it was lying on the floor, it would no longer have a strike direction because you could draw a horizontal line in any of an infinite number of directions on the horizontal surface of the wall. Its dip would be 0˚. When reporting the dip, include the direction. For example, if the strike runs north-south and the dip is 30˚, it would be necessary to specify “to the west” or “to the east.” Similarly if the strike is northeast-southwest and the dip is 60˚, it would be necessary to say “to the northwest” or “to the southeast.” In the case of the vertical wall with a dip angle of 90˚, there is no dip direction. The dip points straight down, not toward any compass direction. Measurement of geological features is done with a special compass that has a built-in clinometer, which is a device for measuring vertical angles. The strike is measured by aligning the compass along a horizontal line on the surface of the feature (Figure 13.36, left). The dip is measured by turning the compass on its side and aligning it along the dip direction (Figure 13.36, right). [caption id="attachment_559" align="aligncenter" width="648"] Figure 13.36 Measurement of strike (left) and dip (right) using a geological compass with a clinometer. Source: Steven Earle (2015), CC BY 4.0. Image source left/ right[/caption] Strike and dip are used to describe any other planar features, including joints, faults, dykes, sills, and even the foliation planes in metamorphic rocks. Figure 13.37 shows an example of how we would depict the beds that make up an anticline on a map. The beds on the west (left) side of the map are dipping at various angles to the west. The beds on the east side are dipping to the east. The beds in the middle are horizontal; this is denoted by a cross within a circle on the map. The dyke is dipping at 80˚ to the west. The hinge line of the fold is denoted with a dashed line on the map, with two arrows pointing away from it, indicating the general dip directions of the limbs. If it were a syncline, the arrows would point inward toward the line. [caption id="attachment_560" align="aligncenter" width="633"] Figure 13.37 A depiction of an anticline and a dyke in cross-section (looking from the side) and in map view (or plan view) with the appropriate strike-dip and anticline symbols. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Putting Strike and Dip on a Map This cross-section shows seven tilted sedimentary layers (a to g), a fault, and a steeply dipping dyke. [caption id="attachment_561" align="aligncenter" width="650"] Figure 13.38 Practice with strike and dip symbols. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Place strike and dip symbols on the map to indicate the orientations of the beds shown, the fault, and the dyke.
What type of fault is shown?
What kind of stress created the fault?

Chapter 13 Summary & Key Term Check

kqzheng — Sun, 25 Feb 2018 20:31:39 +0000

Chapter 13 Main Ideas

13.1 Stress and Strain

Stress within rocks—which includes compression, extension and shearing—originates from plate tectonic processes and the weight of overlying rocks. Rock that is stressed responds with either elastic or plastic strain, and may eventually break. The way a rock responds to stress depends on its composition and structure, the rate at which strain is applied, and also on the temperature, pressure, and the presence of fluid within the rock.

Practice Again

Types of deformation

13.2 Folding

Folding is generally a ductile response to compression, although some brittle behaviour can happen during folding. A fold with a hinge that points upward is an anticline. A fold with a hinge that points downward is a syncline. The axial surface of a fold can be vertical, inclined, or even horizontal. The landforms produced by folds will depend on the resistance to weathering of rock layers within the fold.

Practice Again

Types of folds

13.3 Fractures, Joints, and Faults

Joints typically form during extension, but can also form during compression. Faulting, which involves the displacement of rock, can take place during compression or extension, as well as during shearing at transform boundaries. Thrust faults are a type of reverse fault with a fault plane tilted at a low angle. Thrust faults are common in mountain belts formed by plate collisions.

Practice Again

Types of faults

13.4 Mountain Building

Mountain building in zones of plate collision is called orogeny. The mountains that form are orogens, and consist of crust thickened and deformed by folding and faulting, as well as the intrusion of igneous rocks. Orogens in ocean-continent collision zones have volcanoes. Mountains formed in rift zones are the result of tilting of normal-faulted blocks, or some normal-faulted blocks subsiding while others remain elevated.

13.5 Measuring Geological Structures

The strike and dip of planar surfaces, such as a bedding planes, fractures or faults are measured to help understand the geological history of a region. Special symbols are used to show the orientation of structural features on geological maps.

Key Term Check

What key term from Chapter 13 is each card describing? Turn the card to check your answer. [h5p id="158"]

17.1 Types of Glaciers

kqzheng — Tue, 06 Mar 2018 02:01:06 +0000

There are two main types of glaciers: continental glaciers and alpine glaciers. Latitude, topography, and global and regional climate patterns are important controls on the distribution and size of these glaciers.

Continental Glaciers

Continental glaciers cover vast areas of land. Today, continental glaciers are only present in extreme polar regions: Antarctica and Greenland (Figure 17.3). Historically, continental glaciers also covered large regions of Canada Europe, and Asia, and they are responsible for many distinctive topographic features in these regions (Section 17.2 and 17.3). Continent glaciers can form and grow when climate conditions in a region cool over extended periods of time. Snow can build up over time in regions that do not warm up seasonally, and if the snow accumulates in vast amounts, it can compact under its own weight and form ice. Earth’s two current continental glaciers, the Antarctic and Greenland Ice Sheets, comprise about 99% of Earth’s glacial ice, and approximately 68% of Earth’s fresh water. The Antarctic Ice Sheet is vastly larger than the Greenland Ice Sheet (Figure 17.4) and contains about 17 times as much ice. If the entire Antarctic Ice Sheet melted, sea level would rise by about 80 m and most of Earth’s major coastal cities would be submerged. [caption id="attachment_707" align="aligncenter" width="669"] Figure 17.4 Simplified cross-section profiles of the Antarctic and Greenland continental ice sheets. Both ice sheets are drawn to the same scale (exaggerated in the vertical direction). Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Continental glaciers generally cover areas that are flat, but the force of gravity still acts on them and causes them to flow. Continental glacier ice flows from the region where it is thickest toward the edges where it is thinner (Figure 17.5). In the central thickest parts, the ice flows almost vertically down toward the base, while at the edges of the glacier, it flows horizontally out toward the margins. In continental glaciers like the Antarctic and Greenland Ice Sheets, the thickest parts (4,000 m and 3,000 m thick, respectively) are the areas where the rate of snowfall, and therefore of ice accumulation, are greatest. In Antarctica, the ice sheet flows out over the ocean, forming ice shelves. Ice shelves can slow the flow of continental glaciers outward. Conversely, if ice shelves break down continental glacier flow can speed up.

[caption id="attachment_708" align="aligncenter" width="624"] Figure 17.5 Cross-section showing ice-flow in the Antarctic Ice Sheet. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Alpine Glaciers

Alpine glaciers (also called valley glaciers) originate high up in the mountains, mostly in temperate and polar regions (Figure 17.1), but also in tropical regions in high mountains (e.g. in the Andes Mountains of South America). The flow of alpine glaciers is driven by gravity, and primarily controlled by the slope of the ice surface (Figure 17.6). Alpine glaciers grow due to accumulation of snow over time. In the zone of accumulation, the rate of snowfall is greater than the rate of melting. In other words, not all of the snow that falls each winter melts during the following summer, and the ice surface in the zone of accumulation does not lose its annual accumulation of snow cover over the course of the year. In the zone of ablation, the rate of melting exceeds accumulation. The equilibrium line marks the boundary between the zones of accumulation (above) and ablation (below) (Figure 17.6). [caption id="attachment_709" align="aligncenter" width="439"] Figure 17.6 Schematic diagram illustrating alpine glacier ice-flow. Source: Steven Earle (2015) CC BY 4.0 Image source.[/caption] Above the equilibrium line of a glacier, winter snow will remain even after summer melting, so snow gradually accumulates on the glacier over time. The snow layer from each year is covered and compacted by subsequent snow, and it is gradually compressed and converted to firn (Figure 17.7). [caption id="attachment_710" align="aligncenter" width="510"] Figure 17.7 Steps in the process of formation of glacial ice from snow, granules, and firn. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] Firn is a form of ice that forms when snowflakes lose their delicate shapes and become granules due to compression. With more compression, the granules are squeezed together, and air is forced out. Eventually the granules are “welded” together to create glacial ice (Figure 17.7). Downward percolation and freezing of water from melting contributes to the process of ice formation.

The equilibrium line of a glacier near Whistler, BC, is shown in Figure 17.8. Below this line is the zone of ablation. In the zone of ablation, bare ice is exposed because the previous winter’s snow has all melted. Above this line the ice is still mostly covered with snow from the previous winter.

[caption id="attachment_711" align="aligncenter" width="623"] Figure 17.8 The approximate location of the equilibrium line (red) in September 2013 on the Overlord Glacier, near Whistler, B.C. Source: Steven Earle (2015), CC BY 4.0, after Isaac Earle (n.d.), CC BY 4.0. Image source.[/caption]

The position of the equilibrium line changes from year to year as a function of the balance between snow accumulation in the winter, and snow and ice melt during the summer. If there is more winter snow and less summer melting, this favours the advance of the equilibrium line down the glacier (and ultimately increases the size of the glacier). Between accumulation and melting, the summer melt matters most to a glacier’s ice budget. Cool summers promote an increase in glacier size, and thus lead to advance of the equilibrium line. Warm summers promote melting, and retreat of the equilibrium line.

Do You Know Your Glacier Types?

Fill in the missing words. The Antarctic Ice Sheet is an example of a . These cover vast areas of land. These glaciers are "self-flattening" in the sense that gravity causes ice to flow horizontally away from thickened toward the thinner even though the glacier is on flat terrain. You can find high in the mountains. Flow in these glaciers is controlled by the of the ice surface. In the zone of , more snow falls each year than melts. With added pressure, snow transforms to , then , then ice. In the zone of , there's more melting than accumulation. The marks the boundary between these zones. Fill-in-the-blank options:

middles
firn
ablation
accumulation
continental glacier
slope
granules
equilibrium line
alpine glaciers
edges

To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="176"]

Alpine glaciers move because they are heavy, and the force of gravity acts on the ice in the glacier to pull it down the slope of the mountains where they form. The movement of the glacier generates stress in the ice, which is proportional to the slope of the glaciers surface features of the underlying rock surface, and to the depth within the glacier.

As shown in Figure 17.9, the stresses are relatively small near the ice surface but much larger at depth. Stresses are greater in areas where the ice surface is relatively steep. [caption id="attachment_712" align="aligncenter" width="884"] Figure 17.9 Stress within an alpine glacier (red numbers) as determined from the slope of the ice surface and the depth within the ice. The ice will deform and flow where the stress is greater than about 100 kilopascals, and regions with higher rates of deformation are depicted by the red arrows. Any motion of the lower ice will be transmitted to the ice above it, so although the red arrows get shorter toward the top, the ice is still moving (blue arrows in centre of diagram inset illustrate rate of ice motion). The upper ice (above the red dashed line) does not flow plastically, but it is carried along with the lower ice. Source: Joyce McBeth (2018), CC BY 4.0. Modified after Steven Earle (2016), CC BY 4.0. Image source.[/caption] Like rock, ice behaves in a brittle fashion under low pressure conditions (shallow depths in the glacier), and plastically at higher pressures (deeper in the glacier). Stress also affects how ice deforms; at high stress ice will either break or deform plastically (ductile deformation) depending on the pressure conditions. Under brittle deformation conditions (low pressures, shallow depths in the glacier), stress is released when the ice cracks, so does not build up to high values. Within the upper 50 - 100 m of ice (above the dashed red line, in Figure 17.9), flow is brittle: the ice is rigid and will crack in response to stress. Under ductile deformation conditions (higher pressures deeper in the glacier), stress can accumulate, and the ice will flow plastically in response to that stress. Ice deforms plastically if deeper than about 100 m in the glacier, and in this region stress levels can accumulate to high values (100 kilopascals or greater, Figure 17.9). When the lower ice of a glacier flows, it moves the upper ice along with it. It may seem from the stress patterns (red numbers and arrows in Figure 17.9) that the lower ice moves more or faster than the upper ice, but this is not the case. The lower ice deforms (flows) and the upper part is carried along and deforms through brittle deformation if subjected to sufficient stress. The upper part of the glacier moves faster than the base of the glacier because there is friction between the base of the glacier and the surface beneath it that slows the movement of the ice at the base. The plastic lower ice of a glacier can flow over irregularities in the rocks under the glacier. However, the upper rigid ice cannot flow in this way, and because it is being carried along by the lower ice, it tends to crack in locations when the lower ice flows over changes in the topography below the glacier. This leads to formation of crevasses in areas where the rate of flow of the deeper, plastic ice is changing. In the area shown in Figure 17.10, for example, the glacier is accelerating over the steep terrain, and the rigid surface ice cracks to release stress that accumulates due to the change in velocity and tension in the ice. [caption id="attachment_713" align="aligncenter" width="650"] Figure 17.10 Crevasses in a glacier in Mount Cook National Park, New Zealand. Source: Bernard Spragg (2008), CC0 1.0. Image source.[/caption] In addition to deformation, another important aspect of glacier flow is basal sliding, which is sliding movement between the base of the glacier and the underlying material. The base of a glacier can be cold (below the freezing point of water) or warm (above the freezing point). If it is warm, a film of water can form between the ice and the material underneath, and the ice will be able to slide over this surface (Figure 17.11, left). If the base is cold, the ice will be frozen to the material underneath and it will be stuck — unable to slide along its base. In this case, all the movement of the ice will be by internal flow. [caption id="attachment_714" align="aligncenter" width="650"] Figure 17.11 Differences in glacial ice motion with basal sliding (left) and without basal sliding (right). The dashed red line indicates the upper limit of plastic internal flow. Source: Steven Earle (2016), CC BY 4.0. Image source.[/caption]

There are several factors that can influence warming of the ice and basal flow at the base of an alpine glacier. Friction between the base of the glacier and the surface underneath generates heat and can lead to melting of the ice at the base of the glacier. Rainwater and meltwater from upper regions of the glacier can percolate down and transfer heat to warm the base of the glacier and enhance basal sliding, particularly in warmer seasons. Geothermal heat from below also contributes to melting at the base of glaciers in regions with high heat flow due to volcanic activity.

Another factor that controls the temperature at the base of a glacier is the thickness of the ice. The force of gravity acting on thicker ice can enhance friction and melting at the base. Ice is also a good insulator so can prevent accumulated heat from escaping. The leading edge of an alpine glacier is typically relatively thin (see Figure 17.9), so it is common for this part to be frozen to its base while the rest of the glacier is still sliding. Since the leading edge of the glacier is frozen to the ground, and the rest of the glacier behind continues to slide forward, this causes the trailing ice to be pushed (or thrust) over top of the leading edge, forming thrust faults in the ice (Figure 17.12).

[caption id="attachment_715" align="aligncenter" width="550"] Figure 17.12 Thrust faults at the leading edge of the Byron Glacier, Portage Lake, Alaska, USA. The dark stripes are sediments that were entrained in the base of the glacier ice and transported up along the thrust faults. Source: Cindy Zackowitz (2011), CC BY-NC 2.0. Image source.[/caption] Just as the base of a glacier moves slower than the surface, the edges, which are more affected by friction along the channel walls, also move slower. If we were to place a series of markers across an alpine glacier and come back a year later, we would see that the ones in the middle had moved further forward than the ones near the edges (Figure 17.13). [caption id="attachment_716" align="aligncenter" width="582"] Figure 17.13 Markers on an alpine glacier move forward at different rates over a period of time. Source: Steven Earle (2016), CC BY 4.0. Image source.[/caption]

Alpine glacial ice continuously moves down the slope of the ice in response to gravity, but it may not appear to be moving because the front edge of a glacier is also continuously losing volume. It either melts or, if they glacier terminates at a lake or ocean, the front edge will calve into the water (break off pieces of the front edge of the glacier that become icebergs). If the rate of forward motion of the glacier is faster than the rate of ablation (melting), the leading edge of the glacier advances (moves forward). If the rate of forward motion is about the same as the rate of ablation, the leading edge remains stationary, and if the rate of forward motion is slower than the rate of ablation, the leading-edge retreats (moves backward).

Calving of icebergs is an important process for glaciers that terminate in lakes or oceans. An example of such a glacier is the Berg Glacier on Mt. Robson (Figure 17.14), which sheds small icebergs into Berg Lake. The Berg Glacier also lose mass by melting, evaporation, and sublimation. [caption id="attachment_717" align="aligncenter" width="623"] Figure 17.14 Mt. Robson, the tallest peak in the Canadian Rockies, hosts the Berg Glacier (centre), and Berg Lake. Although there were no icebergs visible when this photo was taken, the Berg Glacier loses mass by shedding icebergs into Berg Lake. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

How Alpine Glaciers Move

Fill in the missing words to complete this description of how stresses affect glaciers. When a glacier moves, it generates stress in the ice. How the ice responds depends on how deep in the glacier it is. Within the upper 50 – 100 m, ice responds to stress with behavior, by . Deeper down, the ice deforms by instead. If the deeper ice is flowing over irregularities in the rocks under the glacier, but the upper ice can't flow that way, it cracks and forms as it's carried along by the flowing ice. Ice can move faster by if its base is above the freezing point of water, and the ice isn't firmly attached to the rock beneath. Otherwise, it can only flow as fast as it can deform. With pressures under thick ice, this can happen even if temperatures are . An alpine glacier that's continuously flowing might not appear to move at all because the front edge is melting or else into water. Fill-in-the-blank options:

basal sliding
crevasses
lower
higher
plastically
breaking
brittle
calving
flowing

To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="177"]

17.2 Glacial Erosion

kqzheng — Tue, 06 Mar 2018 03:54:50 +0000

Glaciers are effective agents of erosion, especially in situations where the base of the glacier is not frozen to the underlying material and can therefore slide over the bedrock or other sediment. The ice itself is not particularly effective at erosion because it is relatively soft (Mohs hardness 1.5 at 0°C). Glacial erosion is primarily driven by abrasion of the underlying rocks by rock fragments embedded within the ice. These rocks are pushed down onto the underlying surfaces by the ice, and because they are hard they can gouge and grind down the materials beneath the glacier. An analogy for these processes is to compare the effect of a regular piece of paper being rubbed against a wooden surface (“ice eroding rock”) to rubbing a piece of sandpaper over the same surface (“ice with embedded rocks eroding rock”). The results of glacial erosion are different in areas with continental glaciation versus alpine glaciation.

Continental Glacial Erosion Features

Continental glaciation tends to produce relatively flat bedrock surfaces, especially where the rock beneath is uniform in strength. In areas where there are differences in the strength of rocks, a glacier tends to erode the softer and weaker rock more effectively than the harder and stronger rock. Much of central and eastern Canada, which was completely covered by the huge Laurentide Ice Sheet at various times during the Pleistocene Epoch, has been eroded to a relatively flat surface. Glacial deposits have created distinctive topographic features on the landscapes in these regions — such as drumlins, eskers, and moraines (Figure 17.16). These continental glacial features are deposits of glacial materials and are described further in Section 17.3. [caption id="attachment_720" align="aligncenter" width="442"] Figure 17.16 Landscape features associated with continental glaciation Source: Luis María Benítez (2005), CC BY 4.0. Image source.[/caption] In areas of continental glaciation, the lithosphere is depressed by the weight of glacial ice that is up to 4,000 m thick. Basins formed along the edges of continental glaciers and filled with glacial meltwater and layers of sediments. Many such lakes, some of them huge, existed at various times along the southern edge of the Laurentide Ice Sheet that once covered much of Canada (Section 17.4). One example of these lakes was Glacial Lake Missoula, which formed within Idaho and Montana, just south of the BC border with the United States. During the latter part of the last glaciation (30 ka to 15 ka), the ice holding back Lake Missoula retreated enough to allow some of the lake water to escape, which escalated into a voluminous and rapid outflow (over days to weeks). During this outflow, most of the lake drained into the Columbia River valley and flowed to the Pacific Ocean. It is estimated that this type of catastrophic outflow happened at least 25 times during this period, and in many cases, the rate of outflow was equivalent to the discharge of all of Earth’s current rivers combined.

Alpine Glacial Erosion Features

Alpine glaciers produce very different topography than continental glaciers. Alpine glaciers produce wide valleys with relatively flat bottoms and steep sides due to the erosion that occurs at the base and edges of the glaciers. These are known as U-shaped valleys (Figure 17.17). In contrast, unglaciated river valleys generally have a V shape. [caption id="attachment_721" align="aligncenter" width="400"] Figure 17.17 A depiction of a U-shaped valley occupied by a large glacier. Source: Steven Earle (2016), CC BY 4.0. Image source.[/caption] In coastal regions where the bottom of the valley is filled with water, the U-shaped valleys are called fjords. The coastal mountains of BC have many fine examples of U-shaped valleys and fjords. Howe Sound is a fjord that was once occupied by a large glacier. Howe Sound and most of its tributary valleys have pronounced U-shaped profiles due to glaciation (Figure 17. 18). [caption id="attachment_722" align="aligncenter" width="400"] Figure 17.18 The view down the U-shaped valley of Mill Creek valley toward the U-shaped valley of Howe Sound, with the village of Britannia on the opposite side. Source: Keefer4 (2005), CC BY-SA 2.5. Image source.[/caption] Several other topographic features derived from alpine glacial erosion are found in U-shaped valleys and their tributary valleys (Figure 17.19). Arêtes are sharp ridges formed between U-shaped glacial valleys. Cols are low points (saddles) along arêtes; they form passes (high points) between glacial valleys. Horns are steep peaks that have been eroded by glaciers and freeze-thaw activity on three or more sides. Cirques are bowl-shaped basins that form at the head of a glacial valley, and tarns are lakes that form when cirques are flooded. Hanging valleys form when U-shaped valleys of tributary glaciers connect with a larger U-shaped valley; the tributary valley hangs above the main valley because the larger main-valley glacier is eroded more deeply into the terrain. Truncated spurs (aka “spurs”) are features at the ends of arêtes where the rock is eroded into steep triangle-shaped cliffs by the glacier in the main valley. [caption id="attachment_723" align="aligncenter" width="388"] Figure 17.19 A diagram of some of the important alpine-glaciation erosion features. Source: Steven Earle (2015), CC BY 4.0. Image source. Modified after Luis María Benítez (2005), CC0 1.0. Image source.[/caption] Figure 17.20 shows examples of these features in the Swiss Alps. The area in the image was intensely glaciated during the past glacial maximum and still contains glaciers. The large U-shaped valley in the lower right was occupied by glacial ice historically, and all of the other glaciers shown here were longer and much thicker than they are now. But even at the peak of the Pleistocene glaciation, some of the higher peaks and ridges in this image would have been exposed and not directly affected by glacial erosion. A peak that extends above the surrounding glacier is called a nunatak. In these areas, and in the areas above the glaciers in the image today, most of the erosion is linked to freeze-thaw action. [caption id="attachment_724" align="aligncenter" width="650"] Figure 17.20 A view of the Swiss Alps from the International Space Station, taken in 2006. The region shown is in the area of the Aletsch Glacier. The prominent peaks labelled “Horn” are the famous mountain peaks the Eiger (left) and Wetterhorn (right). A variety of alpine glacial erosion features are labelled. Source: Steven Earle (2016), CC BY 4.0. Image source. Modified after NASA Earth Observatory (n.d.), Public Domain. Image source.[/caption] A roche moutonnée is a glacial erosion feature that forms when a glacier moves over an outcrop of bedrock. Roche moutonnées consist of a hill of rock, often with a smooth, often low angle slope on one side, and a steeper and jagged slope on the other side. The side that is smooth and relatively low angle is the side the glacier was flowing from, and the a steep and sometimes jagged side is the direction the ice was moving (Figure 17.21, left). [caption id="attachment_1095" align="aligncenter" width="640"] Figure 17.21 Roche moutonnée near Myot Hill, Scotland. Source: Chris Upson (2006), CC BY-SA 2.0. Image source.[/caption] Glacial grooves (tens of centimetres to metres wide) and glacial striae (millimetres to centimetres wide) are created by the erosion caused by fragments of rock embedded in the ice at the base of a glacier (Figure 17.22, left and right). Glacial striae are very common on rock surfaces eroded by both alpine and continental glaciers. Glacial polish occurs when the abrasion of the rock by the glacier renders the rock so smooth it reflects light. [caption id="attachment_725" align="aligncenter" width="650"] Figure 17.22 Examples of glacial striae from near Squamish, BC. Ice flow was from right to left in both images. Source: Steven Earle (2016), CC BY 4.0. Image source.[/caption]

Lakes are common features in glacial environments. A lake that is confined to a glacial cirque is known as a tarn (Figure 17.23). Tarns are common in areas of alpine glaciation because the ice that forms a cirque typically carves out a depression in bedrock that can then fill with water. Moraines, which are linear deposits of glacial sediments (till) left by the glacier along its edges, can form a dam at the end of a tarn. Often, a series of moraines will form as glaciers recede. These can act as water dams, and result in strings of lakes called rock basin lakes or paternoster lakes.

[caption id="attachment_726" align="aligncenter" width="550"] Figure 17.23 Lower Thornton Lake, a tarn, in the Northern Cascades National Park, Washington. Source: Jeff Pang (2007), CC BY 2.0. Image source.[/caption]

A lake that occupies a glacial valley is known as a finger lake. In some cases, a finger lake is confined by a dam formed by an end moraine, in which case it may be called a moraine lake (Figure 17.24). Another type of glacial lake is a kettle lake. These are discussed in section 17.4 in the context of glacial deposits.

[caption id="attachment_727" align="aligncenter" width="550"] Figure 17.24 Peyto Lake in the Alberta Rockies, is both a finger lake and a moraine lake, as it is flooding a glacial valley, and is dammed by an end moraine at right. Source: Jeff Hollet (2016), Public Domain. Image source.[/caption]

Practice with Erosional Glacial Landforms [h5p id="178"]

17.3 Glacial Deposits

kqzheng — Tue, 06 Mar 2018 22:25:17 +0000

Sediments transported and deposited during glaciations are abundant throughout Canada. They are important sources of aggregate for construction materials (sand, gravel), and are also important groundwater reservoirs. Because they're almost all unconsolidated, they have significant implications for slope stability and mass wasting. Figure 17.26 illustrates some of the ways that sediments are transported and deposited by alpine glaciers. The Bering Glacier is the largest glacier in North America, and although most of it is in Alaska, it flows from an icefield that extends into the southwestern Yukon Territory. The surface of the ice is partially, or in some cases completely, covered with rocky debris that has fallen onto the glacier from surrounding steep rock faces. There are muddy rivers issuing from the glacier in several locations, depositing sediment on land, into Vitus Lake, and directly into the ocean. Icebergs (portions of the glacier that have broken off and float away in a lake or ocean) are laden with glacial sediments, which are released and deposited as the icebergs melt. There are also sediments being moved along within and beneath the glacier itself. [caption id="attachment_730" align="aligncenter" width="550"] Figure 17.26 Part of the Bering Glacier in southeast Alaska, the largest glacier in North America. It is about 14 km in width in the centre of this view. Source: Roger Simmon, Landsat 7 Science Team, NASA (2002), Public Domain. Image source.[/caption] Sediments are formed and transported in several ways in glacial environments (Figure 17.27). There are many different kinds of glacial sediments, which are generally classified by whether they are transported on, within, or beneath the glacial ice. [caption id="attachment_731" align="aligncenter" width="624"] Figure 17.27 Various types of sediments associated with the Bering Glacier. The glacier is shown in cross-section. Source: Steven Earle (2016), CC BY 4.0. Image source.[/caption] Supraglacial (on top of the ice) and englacial (within the ice) sediments are released from the melting front of a stationary glacier. These sediments can form a ridge of unsorted sediments called an end moraine. The end moraine from the furthest advance of a glacier is called a terminal moraine. The general name for any sediments transported and deposited by glacial ice is till. Subglacial sediment (e.g., lodgement till) is material that has been eroded from the rock underlying the glacier by the ice and then transported by the ice. It has a wide range of grain sizes, including a relatively high proportion of silt and clay. The larger clasts (pebbles to boulders in size) tend to become partly rounded by abrasion. When a glacier eventually melts, the lodgement till is exposed as a sheet of well-compacted sediment ranging from several centimetres to many metres in thickness. Lodgement till is normally poorly sorted and does not contain bedding features like a lake or stream sediment (Figure 17.28). [caption id="attachment_732" align="aligncenter" width="650"] Figure 17.28 Examples of glacial till: a: Lodgement till from the front of the Athabasca Glacier, Alberta; b: Ablation till at the Horstman Glacier, Blackcomb Mountain, BC. Source: Steven Earle (2016), CC BY 4.0. Image source.[/caption] Supraglacial sediments are primarily derived from freeze-thaw eroded material that has fallen onto the ice from rocky slopes above. These sediments form lateral moraines (moraine deposits along the edges of the glacier, see Figure 17.1 for an example). Where two glaciers meet, the sediments form medial moraines (medial moraines are visible in Figure 17.20 and Figure 17.26.) Most of this material is deposited on the ground when the ice melts. This is called ablation till, a mixture of fine and coarse angular rock fragments, with much less sand, silt, and clay than lodgement till (Figure 17.28). When supraglacial sediments become incorporated into the body of the glacier, they are known as englacial sediments (Figure 17.27). Water flows on the surface, within, and at the base of a glacier, even in cold areas and even when the glacier is advancing. Depending upon its velocity, this water is able to transport sediments of various sizes, and discharges most of these sediments out of the lower end of the glacier, where they are deposited as outwash sediments. These sediments accumulate in a wide range of environments in the proglacial region (the area in front of a glacier). Most of the sediments accumulate in fluvial environments, but some are deposited in lacustrine and marine environments. Glaciofluvial sediments are similar to sediments deposited in normal fluvial environments, but are glacially-derived sediments, and are thus dominated by silt, sand, and gravel. The grains tend to be moderately well rounded and sorted, and the sediments have similar sedimentary structures (e.g., bedding, cross-bedding, clast imbrication [overlapping]) to those formed by non-glacial streams (Figure 17.29). [caption id="attachment_733" align="aligncenter" width="650"] Figure 17.29 Examples of glaciofluvial sediments: a: glaciofluvial cross-bedded sand of the Quadra Sand Formation at Comox, BC.; b: glaciofluvial gravel and sand, Nanaimo, BC. Source: Steven Earle (2016), CC BY 4.0. Image source.[/caption] A large proglacial plain of sediment is called a sandur (or outwash plain), and within this area, glaciofluvial deposits can be tens of metres thick. In situations where a glacier is receding, a block of ice might become separated from the main ice sheet and become buried in glaciofluvial sediments. When the ice block eventually melts, a depression forms, known as a kettle, and if this fills with water, it is known as a kettle lake (Figure 17.30, 17.32). Kettle lakes are also known as pothole lakes or prairie potholes. [caption id="attachment_734" align="aligncenter" width="650"] Figure 17.30 A kettle lake amid vineyards and orchards in the Osoyoos area of BC. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] A supraglacial, englacial, or subglacial stream will create its own channel within the ice, and sediments that are being transported and deposited by the stream will build up within that channel. When the ice melts, the sediment will be deposited upon the underlying ground surface to form a long sinuous ridge known as an esker. Eskers are most common in areas of continental glaciation. They can be several metres high, tens of metres wide, and tens of kilometres long (Figure 17.31). Eskers are commonly comprised of well-sorted sands. [caption id="attachment_735" align="aligncenter" width="369"] Figure 17.31 Part of an esker that formed beneath the Laurentide Ice Sheet in northern Canada. Source: Gord McKenna (1986), CC BY-NC-ND 2.0. Image source.[/caption] Drumlins are elongated, oval shaped ridges of englacial to subglacial sediments that form at the base of continental glaciers. They are often tens of metres high and hundreds of metres long, and often occur in clusters (“fields”) of tens to hundreds of drumlins (Figure 17.32). As the sediments are deposited, the glacier molds the drumlins’ shapes as the glacier moves over and around them. The long axis of a drumlin is aligned with the direction that the ice moved when the drumlin was deposited. [caption id="attachment_736" align="aligncenter" width="768"] Figure 17.32 Drumlins and kettle lakes viewed from the air near Fort St John, BC. There are numerous drumlins in the image; one is outlined in red. Can you spot the others? Note the alignment of the long axes of the drumlins. Source: Joyce McBeth (2002) CC-BY 4.0.[/caption] Glacial outwash streams commonly flow into proglacial lakes (lakes in front of glaciers) where glaciolacustrine sediments are deposited. These are dominated by silt- and clay-sized particles and are typically laminated (finely layered) on the millimetre scale. In some cases, varves develop. Varves are a series of beds with distinctive summer and winter layers: relatively coarse in the summer when melt discharge is high, and finer in the winter, when discharge is low. Icebergs are common in proglacial lakes, and most of them contain englacial sediments of various sizes. As the icebergs melt, the released clasts sink to the bottom and are incorporated into the glaciolacustrine layers as drop stones (Figure 17.33a). The processes that occur in proglacial lakes can also take place where a glacier terminates in the ocean. The sediments deposited there are called glaciomarine sediments (Figure 17.33b). [caption id="attachment_737" align="aligncenter" width="650"] Figure 17.33 Examples of glaciolacustrine and glaciomarine sedimentary structures. a: varved glaciolacustrine sediments containing a drop stone, Nanaimo, BC.; and b: a laminated glaciomarine sediment, Englishman River, BC. Source: Steven Earle (2016), CC BY 4.0. Image source.[/caption]

Practice with Glacial Deposits These exercises will help you to understand the difference between the different kinds of deposits and where they form. [h5p id="179"] Use these flashcards to practice identifying the structures formed by glacial deposits. [h5p id="180"]

17.4 Glaciations over Earth’s History

kqzheng — Wed, 07 Mar 2018 23:55:48 +0000

We are currently living in the middle of a glacial period, though it's less intense now than it was 20,000 years ago. This is not the only period of glaciation in Earth’s history; there have been many in the distant past (Figure 17.34). In general, however, over the course of Earth’s history the Earth’s surface has been warm and ice-free for longer periods than it has been cold and glaciated. [caption id="attachment_740" align="aligncenter" width="701"] Figure 17.34 The record of major past glaciations during Earth’s history. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Pre-Cenozoic Glaciations

The oldest known glacial period is the Huronian. Based on evidence of glacial deposits from the area around Lake Huron in Ontario and elsewhere, it's evident that the Huronian Glaciation lasted from approximately 2.4 to 2.1 Ga. Because rocks of that age are rare, we don't know much about the intensity or global extent of this glaciation. Late in the Proterozoic, Earth had the most intense time of glaciation it has ever experienced. The glaciations of the Cryogenian Period (cryo is Latin for icy cold) are also known as the “Snowball Earth” glaciations. Scientists have hypothesized that the entire planet was frozen at this time—even in equatorial regions—with ice on the oceans up to 1 km thick. A factor in the cause of these glaciations may have been the development of and increased activity by photosynthetic organisms drawing down atmospheric carbon dioxide. There were two main glacial periods within the Cryogenian, each lasting for about 20 million years: the Sturtian at around 700 Ma, and the Marinoan at 650 Ma. There is also evidence of some shorter glaciations both before and after these longer intervals. The end of the Cryogenian glaciations coincides with the evolution of relatively large and complex life forms on Earth. This started during the Ediacaran Period, and then continued with a diversification of life so sudden and dramatic that it's called the Cambrian Explosion. The changing environmental conditions of the Cryogenian may have been what triggered the evolution of large and complex life. There have been three major glaciations during the Phanerozoic (the past 541 Ma). These include the Andean/Saharan (recorded in rocks of South America and Africa), the Karoo (named for rocks in southern Africa), and the Cenozoic glaciations. The Karoo was the longest of the Phanerozoic glaciations, persisting for much of the time that the supercontinent Gondwana was situated over the South Pole (~360 to 260 Ma). Glaciers covered large parts of Africa, South America, Australia, and Antarctica. This widespread glaciation—across continents that are now far apart to explain the distribution of ice—was an important component of Alfred Wegener’s evidence for continental drift. Unlike the Cryogenian glaciations, the Andean/Saharan, Karoo, and Cenozoic glaciations only affected parts of Earth. During Karoo times, for example, what is now North America was near the equator and remained unglaciated. Earth was warm and essentially unglaciated throughout the Mesozoic. Although there may have been some alpine glaciers at this time, they've left no evidence in the geologic record. The dinosaurs, which dominated terrestrial habitats during the Mesozoic, did not have to endure icy conditions.

Cenozoic Glaciations

A warm climate persisted into much of the Cenozoic. In fact, the Paleocene (from about 50 to 60 Ma) may have been the warmest part of the Phanerozoic since the Cambrian (Figure 17.35). [caption id="attachment_741" align="aligncenter" width="766"] Figure 17.35 Global temperature trends over the past 65 Ma (the Cenozoic). From the end of the Paleocene to the height of the Pleistocene glaciation, global average temperature dropped by about 14°C. Source: Joyce McBeth (2018), CC BY 4.0. Modified after Steven Earle (2015), CC BY 4.0 (view source) and Makiko Sato & James Hansen (2012), including data from Zachos et al (2008).[/caption] A number of tectonic events during the Cenozoic have contributed to persistent and significant planetary cooling from 50 Ma to near the present.

Formation of the Tibeta Plateau

The collision of the Indian plate with the Eurasian plate formed the Himalayan mountain range and the Tibetan Plateau. Mountain building events such as this are followed by weathering and erosion of the uplifted rocks. Higher than normal global rates of silicate mineral weathering associated with mountain building, especially weathering of feldspar, leads to a decrease in carbon dioxide concentrations in the atmosphere. This contributes to global climate cooling. [caption id="attachment_742" align="alignright" width="283"] Figure 17.36 The Antarctic Circumpolar Current (red arrows) prevents warm water from the rest of Earth’s oceans from reaching Antarctica. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Opening the Drake Passage

At 40 Ma, ongoing plate motion widened the narrow gap between South America and Antarctica, resulting in the opening of the Drake Passage. This allowed for unrestricted west-to-east flow of water around Antarctica, called the Antarctic Circumpolar Current (Figure 17.36), which effectively isolated the Southern Ocean from the warmer waters of the Pacific, Atlantic, and Indian Oceans. The region cooled significantly, and by 35 Ma (Oligocene) glaciers had started to form on Antarctica.

Connecting North and South America

Global temperatures remained relatively steady during the Oligocene and early Miocene, and the Antarctic glaciation waned during that time. At around 15 Ma, subduction-related volcanism between central and South America created the land connection between North and South America, preventing water from flowing between the Pacific and Atlantic Oceans. This further restricted ocean currents that transfer heat from the tropics to the poles, leading to cooling and advance of the Antarctic glaciation.

Ice Feedbacks and the Pleistocene Glaciation

The expansion of the Antarctic ice sheet increased reflection of solar radiation at the Earth’s surface and promoted a positive feedback loop of further cooling: with more glacial ice reflecting sunlight, there was more cooling, leading to accumulation of more ice, and so on. By the Pliocene (~5 Ma) ice sheets had started to grow in North America and northern Europe. The most intense part of the current glaciation—and the coldest climate conditions of the current glaciation—has been during the past million years (the last third of the Pleistocene). The Pleistocene Epoch of the Cenezoic Era (2.58 Ma to 0.126 Ma), is also known as the Ice Age, Pleistocene Glaciation, or Quaternary Glaciation. The Pleistocene has been characterized by temperature fluctuations over a range of almost 10°C on time scales of 40,000 to 100,000 years (Figure 17.37). [caption id="attachment_743" align="aligncenter" width="732"] Figure 17.37 Foram oxygen isotope record for the past 5 million years based on O isotope data from sea-floor sediments Source: Steven Earle (2015), CC BY 4.0. Image source.. Data from Lisiecki and Raymo (2005). Access the data.[/caption] These temperature variations have corresponding with expansion and contraction of ice sheets. The temperature variations are attributed to subtle changes in Earth’s orbit, tilt, and wobble. These cyclical changes are called Milankovitch cycles. Over the past million years, the glaciation cycles have cycled over every 100,000 years, approximately. See if you can spot this trend in data from the last 500,000 years (Figure 17.38). [caption id="attachment_744" align="aligncenter" width="471"] Figure 17.38 Global mean temperatures over the last 500,000 years. The current interglacial period (Holocene) is marked with an H. Source: Steven Earle (2015), CC BY 4.0 (view source), using data from Lisiecki and Raymo (2005). Access the data.[/caption]

The Wisconsinian Glaciation

The Wisconsinan Glaciation was the last major continental glaciation in North America (from 150-50 ka). During the Wisconsinan, all of Canada and a small portion of the northern United States was covered with continental glaciers (Figure 17.39). The massive Laurentide Ice Sheet covered most of eastern Canada, as far west as the Rockies, and the smaller Cordilleran Ice Sheet covered most of the western region of present day BC and the Yukon Territory. At various other glacial peaks during the Pleistocene and Pliocene, the ice extent was similarly distributed over North America, and in some cases, was even more extensive. The combined Laurentide and Cordilleran Ice Sheets were comparable in volume to the current Antarctic Ice Sheet. [caption id="attachment_745" align="aligncenter" width="442"] Figure 17.39 Extent of northern hemisphere ice sheets near the peak of the Wisconsinan Glaciation (grey shading). Interglacial ice is shown in black. Source: Hannes Grobe (2008), CC BY 3.0. Image source.
[/caption]

Why Does Earth Have Ice Ages?

Fill in the missing words to complete this summary of why Earth has ice ages. The causes of glaciations can be divided into two main categories: changes in how heat is distributed on Earth's surface, and changes in how much heat Earth retains. The amount of heat Earth retains depends on greenhouse gases (like ) in the atmosphere. Anything that draws down greenhouse gas levels, such as of mountains or enhanced , can cause cooling. When Earth's orbit and rotation change periodically (known as ) this affects how directly sunlight falls on Earth's surface. If light is less direct, cooling happens. are the main way that heat is moved around the planet. That's why that alter ocean can adjust Earth's heat distribution enough to cause polar locations to cool down. Some of these mechanisms wouldn't be enough on their own to have a major impact, but that changes when Earth's surface becomes covered by , making it more . This reduces how much heat the surface retains, and can amplify cooling. Fill-in-the-blank options:

ice
photosynthesis
reflective
plate tectonic changes
weathering
Milankovitch cycles
ocean currents
carbon dioxide
circulation

To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="181"]

References

Hansen, J. E., and Sato, M. (2012). Climate sensitivity estimated from Earth's climate history. http://www.columbia.edu/~jeh1/mailings/2012/20120508_ClimateSensitivity.pdf

Lisiecki, L. E., and M. E. Raymo (2005). A Pliocene-Pleistocene stack of 57 globally distributed benthic d¹⁸O records. Paleoceanography, 20, PA1003. doi:10.1029/2004PA001071. http://www.lorraine-lisiecki.com/LisieckiRaymo2005.pdf

Zachos, J. C., Dickens, G. R., and Zeebe, R. E. (2008). An early Cenozoic perspective on greenhouse warming and carbon-cycle dynamics. Nature, 541, 279-283. doi:10.1038/nature06588

Chapter 17 Summary & Key Term Check

kqzheng — Thu, 08 Mar 2018 01:13:36 +0000

Chapter 17 Main Ideas

17.1 Types of Glaciers

The two main types of glaciers are continental glaciers, which are very large and cover large parts of continents (e.g. the Antarctic Ice Sheet), and alpine glaciers, which occupy mountainous regions. Ice accumulates at higher elevations—above the equilibrium line—where the snow that falls in winter doesn't all melt in summer. In continental glaciers, ice flows outward from where it is thickest. In alpine glaciers, ice also flows from thicker to thinner regions in the glacier, obeying the law of gravity. At depth in glacier ice, flow occurs through internal deformation, but glaciers that have liquid water at their base can also flow by basal sliding. Crevasses form in the rigid surface ice in places where the lower plastic ice is changing flow rate or shape as it moves over the underlying topography.

Practice Again

17.2 Glacial Erosion

Glaciers are important agents of erosion. Continental glaciers tend to erode land surface into flat plains, while alpine glaciers create a wide variety of different erosional features. The key feature of alpine glacial erosion is the U-shaped valley. Arêtes are sharp ridges that form between two valleys, and horns form where a mountain is glacially eroded on at least three sides. Since tributary glaciers do not erode as deeply as main-valley glaciers, hanging valleys exist where the two meet. On a smaller scale, both types of glaciers form roche moutonnées, glacial grooves, and striae.

Practice Again

Erosional glacial landforms

17.3 Glacial Deposits

Glacial deposits form as materials are transported and deposited in a variety of different ways in a glacial environment. Sediments that are moved and deposited directly by ice are known as till. Till deposits left at the edges of the glacier as it recedes are known as moraines. Till can also form features such as drumlins (oval-shaped elongated hills) and kettle lakes. Glaciofluvial sediments are deposited by glacial streams, either forming eskers or large proglacial plains known as sandurs. Glaciolacustrine and glaciomarine sediments originate within glaciers and are deposited in lakes and oceans, respectively.

Practice Again

17.4 Glaciations over Earth's History

There have been many glaciations in Earth’s past, the oldest known starting about 2.4 Ga. The late Proterozoic “Snowball Earth” glaciations were thought to be sufficiently intense to affect the entire planet. The Pleistocene Glaciation was a series of glacial events over the past 2.85 Ma. The periodicity of glaciations in the Pleistocene is related to subtle changes in Earth’s orbital characteristics (Milankovitch cycles), which are exaggerated by positive climate feedback processes. North America was most recently glaciated during the Wisconsinan Glaciation, from 150-50 ka.

Practice Again

Reasons for ice ages

Key Term Check

What key term from Chapter 17 is each card describing? Turn the card to check your answer. [h5p id="182"]

Chapter 18 Summary & Key Term Check

kqzheng — Sat, 10 Feb 2018 00:02:35 +0000

Chapter 18 Main Ideas

Introduction

Geological resources are critical to our way of life and important to the Canadian economy. Gold, iron, copper, nickel, and potash are Canada’s most valuable mined commodities.

18.1 Metal Deposits

The proportions of metals in mineral deposits are typically several thousand times higher than those in average rocks, and special processes are required to extract the valuable content. Some deposits form through processes within a magma chamber, others during volcanism or adjacent to a stock, and some are related to sedimentary processes. Mining involves both surface and underground methods, but in either case, rock is brought to surface that can react with water and oxygen to produce acid rock drainage and metal contamination.

Practice Again

18.2 Industrial Materials

Non-metallic materials are very important to infrastructure and agriculture. Some of the major industrial minerals include sand and gravel, limestone for cement and agriculture, salt for a range of applications, potash fertilizer, and decorative stone.

Practice Again

Minerals used for industry

18.3 Fossil Fuels

The main fossil fuels are coal, oil, and gas. Coal forms on land in wet environments where organic matter can remain submerged and isolated from oxygen for millennia before it's buried by more sediments. The depth of that burial influences the grade of coal produced. Oil and gas originate from organisms living in marine environments, and again, fairly rapid burial is required to preserve the organic matter on the sea floor. At moderate burial depth (2 km to 4 km), oil is produced, and at greater depth, gas is produced. Both oil and gas migrate toward the surface and can be trapped beneath impermeable rock layers in structural features, such as anticlines or faults. Some unconventional fossil fuel resources include oil sands, shale gas, and coal-bed methane.

Practice Again

Fossil fuel types

18.4 Diamonds

Diamonds originate in the mantle and are only brought to the surface by the very rare eruption of kimberlitic volcanoes. The relatively recent discovery of diamonds in Canada was based on the exhaustive search for diamond indicator minerals in glacial sediments. There are now six diamond mines in Canada.

Key Term Check

What key term from Chapter 18 is each card describing? Turn the card to check your answer. [h5p id="188"]

4.1 Alfred Wegener’s Arguments for Plate Tectonics

kqzheng — Fri, 24 Aug 2018 18:46:50 +0000

Alfred Wegener (1880-1930; Figure 4.2) earned a PhD in astronomy at the University of Berlin in 1904, but had a keen interest in geophysics and meteorology, and focused on meteorology for much of his academic career. [caption id="attachment_134" align="aligncenter" width="550"] Figure 4.2 Alfred Wegener during a 1912-1913 expedition to Greenland. Source: Alfred Wegener Institute (2008), Public Domain. Image source.[/caption]In 1911 Wegener happened upon a scientific publication that described matching Permian-aged terrestrial fossils in various parts of South America, Africa, India, Antarctica, and Australia. He concluded that because these organisms could not have crossed the oceans to get from one continent to the next, the continents must have been joined in the past, permitting the animals to move from one to the other (Figure 4.3). Wegener envisioned a supercontinent made up of all the present day continents, and named it Pangea (meaning “all land”). He described the motion of the continents reconfiguring themselves as continental drift. [caption id="attachment_110" align="aligncenter" width="577"] Figure 4.3 The distribution of several Permian terrestrial fossils that are present in various parts of continents now separated by oceans. During the Permian, the supercontinent Pangea included the supercontinent Gondwana, shown here, along with North America and Eurasia. Source: J.M. Watson, USGS (1999), Public Domain. Image source.[/caption] Wegener pursued his idea with determination, combing libraries, consulting with colleagues, and making observations in an effort to find evidence in support of it. He relied heavily on matching geological patterns across oceans, such as sedimentary strata in South America matching those in Africa, North American coalfields matching those in Europe, the mountains of Atlantic Canada matching those of northern Britain—both in structure and rock type—and comparisons of rocks in the Canadian Arctic with those of Greenland (Figure 4.4). [caption id="attachment_111" align="aligncenter" width="508"] Figure 4.4 Diagram from Alfred Wegener's book Die Entstehung der Kontinente und Ozeane comparing rock types on Canadian Arctic Islands and Greenland. Source: Karla Panchuk (2018) CC BY 4.0. Click for more attributions.[/caption] Wegener also called upon evidence for the Carboniferous and Permian (~300 Ma) Karoo Glaciation from South America, Africa, India, Antarctica, and Australia (Figure 4.5). He argued that this could only have happened if these continents were once all connected as a single supercontinent. He also cited evidence (based on his own astronomical observations) that showed that the continents were moving with respect to each other, and determined a separation rate between Greenland and Scandinavia of 11 m per year, although he admitted that the measurements were not accurate. (The separation rate is actually about 2.5 cm per year.) [caption id="attachment_112" align="aligncenter" width="462"] Figure 4.5 Carboniferous and Permian Karoo Glaciation in the southern hemisphere. Paleogeographic reconstruction for 306 million years ago. Source: Cropped from C. R. Scotese, PALEOMAP Project (www.scotese.com). Image source. Click for terms of use.[/caption] Wegener first published his ideas in 1912 in a short book called Die Entstehung der Kontinente (The Origin of Continents), and then in 1915 in Die Entstehung der Kontinente und Ozeane (The Origin of Continents and Oceans). He revised this book several times up to 1929. It was translated into French, English, Spanish, and Russian in 1924. The main criticism of Wegener's idea was that he could not explain how continents could move. Remember that, as far as anyone was concerned, Earth's crust was continuous, not broken into plates. Thus, any mechanism Wegener could think of would have to fit with that model of Earth's structure. Geologists at the time were aware that continents were made of different rocks than the ocean crust, and that the material making up the continents was less dense, so Wegener proposed that the continents were like icebergs floating on the heavier ocean crust. He suggested that the continents were moved by the effect of Earth's rotation pushing objects toward the equator, and by the lunar and solar tidal forces, which tend to push objects toward the west. However, it was quickly shown that these forces were far too weak to move continents, and without any reasonable mechanism to make it work, Wegener’s theory was quickly dismissed by most geologists of the day. Alfred Wegener died in Greenland in 1930 while carrying out studies related to glaciation and climate. At the time of his death, his ideas were tentatively accepted by a small minority of geologists, and firmly rejected by most. But within a few decades that was all to change.

Resources

On The Shoulders of Giants: Alfred Wegener

References

Wegener, A. (1920). Die entstehung der kontinente und ozeane. Friedr. Vieweg & Sohn. https://www.gutenberg.org/files/45460/45460-h/45460-h.htm

4.2 Global Geological Models of the Early 20th Century

kqzheng — Fri, 24 Aug 2018 18:47:40 +0000

The untimely death of Alfred Wegener did not solve any problems for those who opposed his ideas, because they still had some inconvenient geological truths to deal with. One of those was explaining the distribution of terrestrial species across five continents that are currently separated by hundreds or thousands of kilometres of ocean water, and another was explaining the origin of extensive fold-belt mountains, such as the Appalachians, the Alps, the Himalayas, and the Canadian Rockies. Before we continue, it is important to know what was generally believed about global geology before plate tectonics. At the beginning of the 20th century, geologists had a good understanding of how most rocks were formed and understood their relative ages through interpretation of fossils, but there was considerable controversy regarding the origin of mountain chains, especially fold-belt mountains. At the end of the 19th century, one of the prevailing views on the origin of mountains was the theory of contractionism — the idea that since Earth is slowly cooling, it must also be shrinking. In this scenario, mountain ranges had formed like the wrinkles on a dried-up apple. Oceans formed above parts of former continents that had settled downward and become submerged. While this hypothesis helped to address the dilemma of the terrestrial fossils by explaining how continents once connected could now be separated by oceans, it came with its own set of problems. One problem was that Earth wasn't cooling fast enough to create the necessary amount of shrinking. Another problem was the principle of isostasy (already understood for several decades; see Section 3.5 Isostasy for a review of isostasy), which wouldn’t allow blocks of continental crust to sink in the way necessary for oceans to form in accordance with contractionist theory. Another widely held view was permanentism, the idea that the continents and oceans have always been generally the same as they are today. This view incorporated a mechanism for creation of mountain chains known as the geosyncline theory. A geosyncline is a thick (potentially 1000s of metres) deposit of sediments and sedimentary rocks, typically situated along the edge of a continent, and derived from continental weathering (Figure 4.6). [caption id="attachment_115" align="aligncenter" width="400"] Figure 4.6 The development of a geosyncline along a continental margin. (Note that a geosyncline is not related to a syncline, which is a downward fold in rocks.) Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] The idea that geosynclines developed into fold-belt mountains originated in the middle of the 19th century. It was first proposed by James Hall and later elaborated upon by Dwight Dana, both of whom worked extensively in the Appalachian Mountains of the eastern United States. The process of converting a geosyncline into a mountain belt was believed to involve compression by forces pushing from either side, causing sedimentary layers within the geosyncline to fold up. In 1937, Philip Kuenen published a paper of experiments with layers of paraffin wax to test how this might work. He was able to cause layers within a geosyncline to fold up as the geosyncline deepened and became more tightly folded during the experiment (Figure 4.7). [caption id="attachment_116" align="aligncenter" width="522"] Figure 4.7 Simulation of mountain building within a geosyncline using layers of wax. Left- A sequence of photographs showing deformation in the wax layers as pistons apply increasing amounts of compression from the side. Right- Close-up view of slices through the wax layers at the end of the experiment, showing that stiffer white layers of wax folded in a way that resembled the folds in mountain belts. Source: Karla Panchuk (2018), CC BY 4.0. Photographs from Kuenen (1937), Public Domain. Image source..[/caption] The problem with the geosynclinal hypothesis for mountain building is that the lateral forces required to cause the compression were never adequately explained. Kuenen compressed the wax layers in his experiment by using pistons that pushed horizontally from either side, but described that mechanism as unrealistic. He explained that the pistons were just to get the process started within the experiment, and that in nature the main force was likely that of gravity pulling the geosyncline downward, drawing the sides together as it folded. When the sides were drawn together, this provided the compression to fold the sediments within the geosyncline. He couldn't specify how the process got started in nature, but suggested that there could be a variety of reasons for an irregularity in the crust to respond to forces in such a way that would trigger downward sagging and folding. Proponents of the geosyncline theory of mountain formation—and there were many well into the 1960s—also had the problem of explaining the intercontinental terrestrial fossil matchups. The explanation offered was that land bridges had once linked the continents, permitting animals and plants to migrate back and forth. One proponent of this idea was the American naturalist Ernest Ingersoll. Referring to evidence of past climate changes, Ingersoll contributed the following to the 1918-20 edition of the Encyclopedia Americana:

The most interesting feature of these changes, however, is that by which, now and again, the Old World was connected with the New by necks or spaces of land, known as “land-bridges”; especially as these permitted an interchange of plants and animals, giving to us many new ones from the other side of the ocean, including, finally, man himself.

A problem with the land-bridge hypothesis is that there is no evidence of land bridges that could account for the fossil distribution patterns. The world's oceans are approximately 4 km deep on average, so the underwater slopes leading up to a land bridge would have to have been at least 10s of km wide in most places, and many times that in others. Even if flooded, a land bridge of that size would still be visible in the shape of ocean-floor terrain. Isostasy would not permit such a land bridge to sink down without leaving a trace. We do know the locations of some past land bridges, but they were very different from the ones that would be required for this hypothesis. They are bridges such as the flooded Bering Strait land bridge, which is beneath only 30 to 50 m of water, and was exposed when sea levels were much lower because of water being locked in polar ice caps during the last major glaciation event. The narrowest point of the Bering Strait is 82 km wide. The shortest distance between South America and Africa is more than 2800 km.

References

Ingersoll, E. (1919). Land-bridges across the oceans. In The Encyclopedia Americana (Vol. XVI, pp. 692-694). Encyclopedia Americana Corporation. http://en.wikisource.org/wiki/The_Encyclopedia_Americana_(1920)/Land-Bridges_Across_the_Oceans

Kuenen, P. H. (1937) The negative isostatic anomalies in the East Indies (with Experiments). Leidse Geologische Mededelingen, 8(2), 169-214. http://www.repository.naturalis.nl/record/505693

4.3 Geological Renaissance of the Mid-20th Century

kqzheng — Fri, 24 Aug 2018 18:48:55 +0000

Two key areas of research ultimately led to the acceptance of continental drift, and the formulation of plate tectonic theory. One was the study of paleomagnetism, the record of Earth's magnetic field through time. The other was exploration of the ocean floor.

Paleomagnetism (Remnant Magnetism)

[caption id="attachment_119" align="alignright" width="228"] Figure 4.9 Rock layers recording remnant magnetism. The red arrows represent the direction of the vertical component of Earth's magnetic field. The oldest rock has a magnetic dip characteristic of the southern hemisphere, but over time the dip changes, indicating that the rocks moved toward magnetic north. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]When rocks form, some of the minerals that make them up can become aligned with the Earth's magnetic field, just like a compass needle pointing to north. This happens to the mineral magnetite (Fe₃O₄) when it crystallizes from magma. Once the rock cools the crystals are locked in place. This means that if the rock moves, the crystals can't realign themselves, and they retain a remnant magnetism. This would be like jamming your compass needle so that if you turned away from north, the needle would turn with you rather than continuing to point north. Rocks like basalt, which cool from a high temperature and commonly have relatively high levels of magnetite, are particularly susceptible to being magnetized in this way. However, even sediments and sedimentary rocks can take on remnant magnetism as long as they have small amounts of magnetic minerals, because the magnetic grains can gradually become lined up with Earth's magnetic field as the sediments are deposited. By studying both the horizontal and vertical components of the remnant magnetism, one can tell not only the direction to magnetic north at the time of the rock's formation, but also the latitude where the rock formed relative to magnetic north. Remember that the vertical component of the magnetic field points more sharply downward the closer it is to the magnetic north pole. Figure 4.9 shows the vertical component of remnant magnetism in a sequence of rocks. Notice that the arrow starts out at 500 Ma pointing slightly upward. This means that the rocks were in the southern hemisphere. As the rocks get younger, the arrow tilts toward horizontal, and then points downward. This indicates that the rocks were getting progressively closer to the north magnetic pole.

Apparent Polar Wandering Paths

In the early 1950s, a group of geologists from Cambridge University, including Keith Runcorn, Ted Irving,[footnote]Ted Irving later set up a paleomagnetic lab at the Geological Survey of Canada in Sidney BC, and did important work on understanding the geology of western North America.[/footnote] and several others, started looking at the remnant magnetism of Phanerozoic British and European volcanic rocks, and collecting paleomagnetic data. Using an analysis similar to that in Figure 4.9, they noticed that rocks of different ages sampled from the same general area showed very different magnetic pole positions (the green line in Figure 4.10). They assumed this meant that Earth's magnetic pole had moved around significantly over time along polar wandering paths, rather than staying close to the geographic north pole as it does today. At the time, geophysical models suggested that the magnetic poles did not need to be aligned with the rotational poles, so this wasn't an unreasonable conclusion, given what was known. [caption id="attachment_120" align="aligncenter" width="650"] Figure 4.10 Apparent polar-wandering paths (APWP) for Eurasia and North America. The view is from the geographic north pole (black dot) looking down. Dots along each path show the location of magnetic north as determined from paleomagnetic data. Left- Data from Eurasia and North America agree on the location of magnetic north today (time 0), but not at any time in the past. Right- Once continent motion has been accounted for, there is agreement in data from Eurasia and North America on the location of magnetic north over the past 500 million years. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] Runcorn and colleagues extended their paleomagnetic studies to North America, and began to realize that their initial conclusion had a problem. Notice that on the right of Figure 4.10 the polar wandering path for North America (in red) does not match the path for Eurasia (in green). For example, data from North America suggest that 200 Ma ago, magnetic north was somewhere in China, whereas data from Europe said it was in the Pacific Ocean. There could only have been one magnetic north pole position at 200 Ma, therefore the only way to explain the discrepancy was if Europe and North America moved along different paths during this time while the pole stayed in more or less the same location. The polar wandering paths were not actually records of the pole moving, they just looked that way, so the paths are now referred to as apparent polar wandering paths (APWP). Subsequent paleomagnetic work showed that unique apparent polar wandering paths can be derived from rocks in South America, Africa, India, and Australia. In 1956, Runcorn became a proponent of continental drift. There was simply no other way to explain the data. This paleomagnetic work of the 1950s was the first new evidence in favour of continental drift, and it led a number of geologists to start thinking that the idea might have some merit. Nevertheless, for a majority of geologists, this type of evidence was not sufficiently convincing to get them to change their views.

Concept Check: Clues from Paleomagnetism on Wandering Continents

Write the words into the correct boxes to complete this summary.A history of Earth's ancient magnetic field, determined through the study of , is preserved in rocks as . Magnetic minerals align with , and once a rock solidifies, the minerals are locked in place. This is like gluing a compass needle down. When the minerals (or your modified compass) are moved, they no longer respond to Earth's magnetic field. A key discovery was that magnetic minerals on different continents show different paths. This doesn't make sense, because there can only be one north pole at a time. The only explanation is that the were moving. This is why these paths are now called paths. Fill-in-the-blank options:

continents
remnant magnetism
paleomagnetism
polar wandering
magnetic north
apparent polar wondering

To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="48"]

Ocean Basin Geology and Geography

During the 20th century, our knowledge and understanding of the ocean basins and their geology increased dramatically. Before 1900 we knew virtually nothing about the bathymetry (the hills and valleys of the ocean floor) and geology of the oceans. By the end of the 1960s, we had detailed maps of the topography of the ocean floors, a clear picture of the geology of ocean floor sediments and the solid rocks beneath them, and almost as much information about the geophysical nature of ocean rocks as of continental rocks.

Acoustic Depth Sounding

Up until the 1920s, ocean depths were measured using weighted lines dropped overboard. In deep water this is a painfully slow process and the number of soundings in the deep oceans was probably fewer than 1,000. That is roughly one depth sounding for every 350,000 square kilometres of the ocean. To put that in perspective, it would be like trying to describe the topography of British Columbia with elevation data for only a half a dozen points! The voyage of the Challenger in 1872 and the laying of trans-Atlantic cables had shown that there were mountains beneath the seas, but most geologists and oceanographers still believed that the oceans were essentially vast basins with flat bottoms, filled with thousands of metres of sediments. Following development of acoustic depth sounders (Figure 4.11) in the 1920s, the number of depth readings increased by many orders of magnitude, and by the 1930s there was no doubt that major mountain chains ran through all of the world's oceans. During and after World War II, there was a well-organized campaign to study the oceans, and by 1959, sufficient bathymetric data had been collected to produce detailed maps of all the oceans (Figure 4.12). [caption id="attachment_121" align="aligncenter" width="600"] Figure 4.11 A ship-borne acoustic depth sounder. The instrument emits sound (black arcs) that reflects off the sea floor and returns to the surface (white arcs). The time interval between emitting the sound and detecting it on receivers on the ship is proportional to the water depth. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] [caption id="attachment_122" align="aligncenter" width="600"] Figure 4.12 Ocean floor bathymetry (and continental topography). Inset (a): the mid-Atlantic ridge, (b): the Newfoundland continental shelf, (c): the Nazca trench adjacent to South America, and (d): the Hawaiian Island chain. Source: Steven Earle (2015), CC BY 4.0. Image source. Basemap after NOAA (2006), Public Domain. Image source.[/caption] The important physical features of the ocean floor are:

Extensive linear ridges (commonly in the central parts of the oceans) at depths of 2,000 to 3,000 m (Figure 4.12, a)
Fracture zones perpendicular to the ridges (Figure 4.12, a)
Deep-ocean plains at depths of 4,000 to 5,000 m (Figure 4.12, a and d)
Relatively flat and shallow continental shelves with depths under 500 m (Figure 4.12, b)
Deep trenches (up to 11,000 m deep), most near the continents (Figure 4.12, c)
Seamounts and chains of seamounts (Figure 4.12, d)

Seismic Reflection Sounding

Seismic reflection sounding involves transmitting high-energy sound bursts and then measuring the echoes with a series of receivers called geophones towed behind a ship. The technique is related to acoustic sounding as described above, however, much more energy is transmitted and the sophistication of the data processing is much greater. As the technique evolved, and the amount of energy was increased, it became possible to see through the sea-floor sediments and map the bedrock topography and crustal thickness. This allowed sediment thicknesses to be mapped (Figure 4.13). [caption id="attachment_123" align="aligncenter" width="1500"] Figure 4.13 Map of global sediment thickness. Source: NOAA (2003), Public Domain. Image source.[/caption]It was soon discovered that although the sediments were up to several 1000s of m thick near the continents, they were relatively thin — or even non-existent — along ocean ridges (Figure 4.14). The seismic studies also showed that the crust is relatively thin under the oceans (5 km to 6 km) compared to the continents (30 km to 60 km) and geologically very consistent, composed almost entirely of basalt. [caption id="attachment_124" align="aligncenter" width="650"] Figure 4.14 Topographic section at an ocean ridge based on reflection seismic data. Sediments are not thick enough to be detectable near the ridge, but get thicker on either side. The diagram represents approximately 50 km width, and has a 10x vertical exaggeration. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Heat Flow Rates

In the early 1950s, Edward Bullard—who spent time at the University of Toronto but is mostly associated with Cambridge University—developed a probe for measuring the flow of heat from the ocean floor. Bullard and colleagues found higher than average heat-flow rates along the ridges, and lower than average rates in trenches. These data were interpreted as evidence of mantle convection, with areas of high heat flow corresponding to upward convection of hot mantle material, and areas of low heat flow corresponding to downward convection.

Earthquake Belts

With developments of networks of seismographic stations in the 1950s, it became possible to plot the locations and depths of both major and minor earthquakes with great accuracy. A remarkable correspondence was observed between earthquake locations and both the mid-ocean ridges and the deep ocean trenches. In 1954 Gutenberg and Richter showed that the ocean-ridge earthquakes were all relatively shallow, and confirmed what had first been shown by Benioff in the 1930s, that earthquakes in the vicinity of ocean trenches were both shallow and deep, but that the deeper ones were situated progressively farther inland from the trenches (Figure 4.15). [caption id="attachment_125" align="aligncenter" width="1024"] Figure 4.15 Aleutian Island subduction zone earthquakes. Left- Map view with earthquakes marked as dots. Red dots are the shallowest earthquakes and blue are the deepest. Quakes get deeper further inland from the trench. Right- Cross-section through a-b. Coloured dots show the depth of earthquakes. Colours correspond to dots in the left figure. Earthquake depth is related to the position of the Pacific plate as it travels beneath the North American plate. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Magnetic Stripes on the Sea Floor

[caption id="attachment_126" align="alignright" width="255"] Figure 4.16 Pattern of sea-floor magnetic field intensity off the west coast of British Columbia and Washington. Black regions have higher than average magnetic field instensity, and white regions have lower than average intensity. Source: Steven Earle (2015), CC BY-SA 4.0. Modified after U. S. Geological Survey (n.d.), Public Domain. Image source. (Adapted from Raff and Mason, 1961).[/caption] In the 1950s, scientists from the Scripps Oceanographic Institute in California persuaded the United States Coast Guard to include magnetometer readings on one of their expeditions to study ocean floor topography. The first comprehensive magnetic data set was compiled in 1958 for an area off the coasts of BC and Washington State. This survey revealed a bewildering pattern of low and high magnetic field intensity in sea-floor rocks (Figure 4.16). When the data were first plotted on a map in 1961, nobody understood them — not even the scientists who collected them. Many 1000s of km of magnetic surveys were conducted over the next several years. The wealth of new data from the oceans began to significantly influence geological thinking in the 1960s. In 1960, Harold Hess from Princeton University, advanced a hypothesis with many of the elements that we now accept as plate tectonics. He maintained some uncertainty about his proposal however, and in order to deflect criticism from mainstream geologists, he labelled it "geopoetry." In fact, until 1962, Hess didn't even put his ideas in writing—except internally to the U.S. Navy (which funded his research)—but presented them mostly in lectures and seminars. Hess proposed that new sea floor was generated from mantle material at the ocean ridges, and that old sea floor was dragged down at the ocean trenches and re-incorporated into the mantle. He suggested that the process was driven by mantle convection currents, rising at the ridges and descending at the trenches (Figure 4.17). He also suggested that the less-dense continental crust did not descend with oceanic crust into trenches, but that colliding landmasses were thrust up to form mountains. Hess's hypotheses formed the basis for our ideas on sea-floor spreading and continental drift, but did not go so far as to claim that the crust is made up of separate plates. The Hess model was not roundly criticized, but also not widely accepted, partly because evidence was still lacking. [caption id="attachment_127" align="aligncenter" width="550"] Figure 4.17 A representation of Harold Hess’s model for sea-floor spreading and subduction. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] Collection of magnetic data from the oceans continued in the early 1960s, but the striped patterns remained unexplained. Some assumed that, as with continental crust, the stripes were related to compositional variations in rock, such as variations in the amount of magnetite. The first real understanding of the significance of the striped anomalies was the interpretation by Fred Vine, a Cambridge graduate student. Vine was examining magnetic data from the Indian Ocean and, like others before him, noted that the magnetic patterns were symmetrical on either side of the ridge. At the same time, other researchers led by groups in California and New Zealand were studying the phenomenon of reversals in Earth's magnetic field. They were trying to determine when such reversals had taken place over the past several million years by analyzing the magnetic characteristics of hundreds of samples from basaltic flows. As discussed in Chapter 3, Earth’s magnetic field periodically weakens, then becomes virtually non-existent before becoming re-established with the reverse polarity. During periods of reversed polarity, a compass would point south instead of north. The time scale of magnetic reversals is irregular. The present "normal" event, known as the Bruhnes magnetic chron, has persisted for about 780,000 years. It was preceded by a 190,000-year reversed event; a 50,000-year normal event known as Jaramillo; and then a 700,000-year reversed event (see Figure 3.16). In a paper published in September 1963, Vine and his PhD supervisor Drummond Matthews proposed that the patterns associated with ridges were related to the magnetic reversals, and that oceanic crust created from cooling basalt during a normal event would have polarity aligned with the present magnetic field, and would produce a positive anomaly (a black stripe on the sea-floor magnetic map). Oceanic crust created during a reversed event would have polarity opposite Earth's present field and thus produce a negative magnetic anomaly (a white stripe). The same idea had been put forward a few months earlier by Lawrence Morley, of the Geological Survey of Canada. However, Morley's papers submitted earlier in 1963 to Nature and The Journal of Geophysical Research were rejected. The idea is sometimes referred to as the Vine-Matthews-Morley (VMM) hypothesis. Vine, Matthews, and Morley were the first to show this type of correspondence between the relative widths of the stripes and the durations of the magnetic reversals. The VMM hypothesis was confirmed within a few years when magnetic data were compiled from spreading ridges around the world. It was shown that the same general magnetic patterns were present straddling each ridge, although the widths of the anomalies varied according to the spreading rates characteristic of the different ridges. It was also shown that the patterns corresponded with the known timeline of Earth's magnetic field reversals.

Concept Check: The Meaning of Magnetic Stripes

Magnetic stripes record Earth’s magnetic field switching between polarity (today a compass needle points ) to polarity (a compass needle would point ). The stripes are on either side of volcanic ocean ridges, and get away from the ridge. This indicated that new was forming along the ridges, now referred to as seafloor . Fill-in-the-blank options:

spreading centres
north
south
normal
crust
reversed
older
symmetrical

To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="49"]

Chains of Islands Progressively Aging Islands

In 1963, J. Tuzo Wilson of the University of Toronto proposed the idea of a mantle plume or hot spot — a place where hot mantle material rises in a stationary and semi-permanent plume, and affects the overlying crust. He based this hypothesis partly on the distribution of the Hawai'ian and Emperor Seamount island chains in the Pacific Ocean (Figure 4.18). The volcanic rock making up these islands becomes progressively younger toward the southeast, culminating in the island of Hawai'i itself, which consists of rock that is almost all younger than 1 Ma. [caption id="attachment_128" align="aligncenter" width="550"] Figure 4.18 The ages of the Hawai'ian Islands and the Emperor Seamounts in relation to the location of the Hawai'ian mantle plume. Source: Steven Earle (2015), CC BY 4.0. Image source. Base map from the National Geophysical Data Centre/USGS (2005), Public Domain. Image source.[/caption]Wilson suggested that a stationary plume of hot upwelling mantle material is the source of the Hawaiian volcanism, and that the ocean crust of the Pacific Plate is moving toward the northwest over this hot spot. Near the Midway Islands, the chain makes a pronounced change in direction, from northwest-southeast for the Hawai'ian Islands, to nearly north-south for the Emperor Seamounts. This change has been ascribed to a change in direction of the Pacific Plate moving over the stationary mantle plume. An alternative hypothesis is that rather than the Pacific Plate having undergone a sudden change in motion, the plume itself has moved at least 2,000 km south over the period between 81 and 45 Ma (Tarduno et al., 2003). There is evidence of many such mantle plumes around the world (Figure 4.19). Most are within ocean basins, including places like Hawai'i, Iceland, and the Galapagos Islands, but some are under continents. One example is the Yellowstone hot spot in the west-central United States, and another is the one responsible for the Anahim Volcanic Belt in central British Columbia. It is evident that mantle plumes are very long-lived phenomena, lasting for at least tens of millions of years, and possibly for hundreds of millions of years in some cases. [caption id="attachment_129" align="aligncenter" width="550"] Figure 4.19 Mantle plume locations. Selected Mantle plumes: 1: Azores, 3: Bowie, 5: Cobb, 8: Eifel, 10: Galapagos, 12: Hawai'i, 14: Iceland, 17: Cameroon, 18: Canary, 19: Cape Verde, 35: Samoa, 38: Tahiti, 42: Tristan, 44: Yellowstone, 45: Anahim. Source: Ingo Wölbern (2007), Public Domain. Image source.[/caption]

Transform Faults

Oceanic spreading ridges appear to be curved features on Earth's surface, but the ridges are in fact composed of a series of straight-line segments, offset at intervals by faults perpendicular to the ridge (Figure 4.20). In a paper published in 1965, Tuzo Wilson termed these features transform faults. [caption id="attachment_130" align="aligncenter" width="550"] Figure 4.20 Part of the Mid-Atlantic ridge near the equator. Transform faults (red lines) are in between the ridge segments (double white lines), where the yellow arrows (indicating relative plate movement) point in opposite directions. Solid white lines are fracture zones. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] In the same 1965 paper, Wilson introduced the idea that the crust can be divided into a series of rigid plates, and is thus responsible for the term plate tectonics.

Paper Transform Fault Model J. Tuzo Wilson used a paper model similar to the one in Figure 4.21 to explain transform faults to his colleagues. To use this model, print Figure 4.21, cut around the outside, and then slice along the line A-B (the fracture zone) with a sharp knife. Fold down the top half where shown, and then pinch together in the middle. Do the same with the bottom half. When you’re done, you should have two folds of paper extending downward as in Figure 4.22. [caption id="attachment_131" align="aligncenter" width="1024"] Figure 4.21 Transform fault model. Source: Steven Earle (2015), CC BY 4.0. Image source. Modified after Stewart (1990).[/caption] [caption id="attachment_132" align="aligncenter" width="400"] Figure 4.22 Use of the transform fault model. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] Find someone else to pinch those folds with two fingers just below each ridge crest, and then gently pull apart where shown. As you do, the oceanic crust will emerge from the middle, and you will see that the parts of the fracture zone between the ridge crests will be moving in opposite directions (this is the transform fault) while the parts of the fracture zone outside of the ridge crests will be moving in the same direction. You will also see that the oceanic crust is being magnetized as it forms at the ridge. The magnetic patterns represent the last 2.5 Ma of geological time. There are other versions of this model available at Paper models of transform faults by Steven Earle. For more information see Earle (2004).

References

Earle, S. (2004). A simple paper model of a transform fault at a spreading ridge. Journal of Geoscience Education, 52, 391-392.

Raff, A., & Mason, R. (1961) Magnetic survey off the west coast of North America, 40˚ N to 52˚ N latitude. Geological Society of America Bulletin, 72, 267-270.

Stewart, J. A. (1990). Drifting continents and colliding paradigms. Indiana University Press.

Tarduno, J. A., Duncan, R. A., Scholl, D. W., Cottrell, R. D., Steinberger, B., Thordarson, T., Kerr, B. C., Neal, C. R., Frey, F. A., Torii, M., and Carvallo, C. (2003). The Emperor Seamounts: Southward Motion of the Hawaiian Hotspot Plume in Earth’s Mantle. Science, 301(5636), 1064–1069. DOI: 10.1126/science.1086442

Wilson, J. T. (1965). A new class of faults and their bearing on continental drift. Nature, 207, 343-347.

4.4 Plates, Plate Motions, and Plate-Boundary Processes

kqzheng — Fri, 24 Aug 2018 18:51:55 +0000

The ideas of continental drift and sea-floor spreading became widely accepted by 1965, and more geologists started thinking in these terms. By the end of 1967, Earth's surface had been mapped into a series of plates (Figure 4.23). The major plates are Eurasian, Pacific, Indian, Australian, North American, South American, African, and Antarctic plates. There are also numerous small plates (e.g., Juan de Fuca, Nazca, Scotia, Philippine, Caribbean), and many very small plates or sub-plates. The Juan de Fuca Plate is actually three separate plates (Gorda, Juan de Fuca, and Explorer), all moving in the same general direction but at slightly different rates. [caption id="attachment_976" align="aligncenter" width="1024"] Figure 4.23 A detailed map of Earth's tectonic plates. [NASA. View Source][/caption]Plate motions can be tracked using Global Positioning System (GPS) data from different locations on Earth's surface. Rates of motions of the major plates range from less than 1 cm/y to more than 10 cm/y. The Pacific Plate is the fastest, moving at more than 10 cm/y in some areas, followed by the Australian and Nazca Plates. The North American Plate is one of the slowest, averaging ~1 cm/y in the south up to almost 4 cm/y in the north. Plates move as rigid bodies, so it may seem surprising that the North American Plate can be moving at different rates in different places. The explanation is that plates rotate as they move; the North American Plate, for example, rotates counter-clockwise, while the Eurasian Plate rotates clockwise. Boundaries between the plates are of three types: divergent (moving apart), convergent (moving together), and transform (moving side by side). The plates are made up of crust and lithospheric mantle (Figure 4.24). Even though the plates are in constant motion, and move in different directions, there is never a significant amount of space between them. Plates move along the lithosphere-asthenosphere boundary, because the asthenosphere is relatively weak. It deforms as the plates move, rather than locking them in place. [caption id="attachment_136" align="aligncenter" width="550"] Figure 4.24 The crust and upper mantle. Tectonic plates consist of lithosphere, which includes the crust and the lithospheric (rigid) part of the mantle. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] At spreading centres, the lithospheric mantle is relatively thin. The upward convective motion of hot mantle material generates temperatures that are too high for the existence of a significant thickness of rigid lithosphere at the same time that the plates are falling away from each other (Figure 4.17). The fact that plates include both crustal material and lithospheric mantle material makes it possible for a single plate to be include both oceanic and continental crust. The North American Plate includes most of North America, plus half of the northern Atlantic Ocean. Similarly the South American Plate extends across the western part of the southern Atlantic Ocean, while the European and African plates each include part of the eastern Atlantic Ocean. The Pacific Plate is almost entirely oceanic, but it does include the part of California west of the San Andreas Fault.

Divergent Boundaries

Divergent boundaries are spreading boundaries, where new oceanic crust is created from magma derived from partial melting of the mantle. The partial melting happens when hot mantle rock is moved from deep within Earth where pressures are too high for it to be liquid, to shallower depths where the pressure is much lower (Figure 4.25, bottom left). The triangular zone of partial melting near the ridge crest is approximately 60 km thick and the proportion of magma is about 10% of the rock volume, thus producing crust that is about 6 km thick once the melt escapes from the rock in which it formed, and ascends. Most divergent boundaries are located in the oceans, and the crustal material created at a spreading boundary is always oceanic in character; in other words, it is mafic igneous rock (basalt or gabbro, with minerals rich in iron and magnesium). Spreading rates vary considerably, from 1 cm/y to 3 cm/y in the Atlantic, to between 6 cm/y and 10 cm/y in the Pacific. Some of the processes taking place in this setting include (Figure 4.25, top):

Melted rock (magma) from the mantle rising up to fill the voids left by divergence of the two plates
Pillow lavas forming where melted rock emerges on the ocean floor and is cooled by seawater (Figure 4.25, bottom right)
Vertical sheeted dykes intruding into cracks resulting from the spreading
Magma cooling more slowly in the lower part of the new crust, forming bodies of gabbro

[caption id="attachment_137" align="aligncenter" width="650"] Figure 4.25 Divergent boundary. Lower left- General processes taking place along divergent boundaries. Top- Expanded view of the white box showing divergent boundary processes and materials. Bottom right- Pillow basalts from the ocean floor of Hawai'i. Source: Lower left- Steven Earle (2015), CC BY 4.0. Image source.; Top- Steven Earle (2015), CC BY 4.0. Image source. Modified after Sinton and Detrick (1992). Lower right- NOAA (1988), Public Domain. Image source.[/caption] Spreading is thought to start with lithosphere being warped upward into a dome by buoyant material from an underlying mantle plume or series of mantle plumes. The buoyancy of the mantle plume causes the dome to fracture in a radial pattern, with three arms spaced at approximately 120° (Figure 4.26). [caption id="attachment_138" align="aligncenter" width="650"] Figure 4.26 Depiction of the process of dome and three-part rift formation (left) and of continental rifting between the African and South American parts of Pangea at around 200 Ma (right). Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] When a series of mantle plumes exists beneath a large continent, the resulting rifts may align and lead to the formation of a rift valley, such as the present-day Great Rift Valley in eastern Africa. This type of valley may eventually develop into a linear sea (such as the present-day Red Sea), and finally into an ocean (such as the Atlantic). It is likely that as many as 20 mantle plumes, many of which still exist, were responsible for the initiation of the rifting of Pangea along what is now the mid-Atlantic ridge (see the Atlantic Ocean mantle plume locations in Figure 4.19).

Convergent Boundaries

Convergent boundaries, where two plates are moving toward each other, are of three types, depending on whether ocean or continental crust is present on either side of the boundary. The types are ocean-ocean, ocean-continent, and continent-continent.

Ocean-Ocean Convergent Boundaries

At an ocean-ocean convergent boundary, a plate margin consisting of oceanic crust and lithospheric mantle is subducted, or travels beneath, the margin of the plate with which it is colliding (Figure 4.27). Often it is the older and colder plate that is denser and subducts beneath the younger and hotter plate. Ocean trenches commonly form along these boundaries. [caption id="attachment_139" align="aligncenter" width="550"] Figure 4.27 Configuration and processes of an ocean-ocean convergent boundary Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] As the subducting crust is heated and the pressure increases, water is released from within the subducting material. This water comes primarily from alteration of the minerals pyroxene and olivine to serpentine near the spreading ridge shortly after the rock’s formation. The water mixes with the overlying mantle, which lowers the melting point of mantle rocks, causing magma to form. This process is called flux melting or fluid-induced melting. The newly produced magma, which is lighter than the surrounding mantle rocks, rises through the mantle and sometimes through the overlying oceanic crust to the ocean floor where it creates a chain of volcanic islands known as an island arc. A mature island arc develops into a chain of relatively large islands (such as Japan or Indonesia) as more and more volcanic material is extruded and sedimentary rocks accumulate around the islands. The largest earthquakes occur near the surface where the subducting plate is still cold and strong. Examples of ocean-ocean convergent zones are subduction of the Pacific Plate south of Alaska (Aleutian Islands) and west of the Philippines, subduction of the Indian Plate south of Indonesia, and subduction of the Atlantic Plate beneath the Caribbean Plate.

Ocean-Continent Convergent Boundaries

At an ocean-continent convergent boundary, the oceanic plate is subducted beneath the continental plate in the same manner as at an ocean-ocean boundary. Rocks and sediment on the continental slope are thrust up into an accretionary wedge, and compression leads to faults forming within the continental plate (Figure 4.28). The mafic magma produced adjacent to the subduction zone rises to the base of the continental crust and leads to partial melting of the crustal rock. The resulting magma ascends through the crust, producing a mountain chain with many volcanoes. [caption id="attachment_140" align="aligncenter" width="550"] Figure 4.28 Configuration and processes of an ocean-continent convergent boundary Source: Karla Panchuk (2018), CC BY 4.0. Modified after Steven Earle (2015), CC BY 4.0. Image source.[/caption] Examples of ocean-continent convergent boundaries are subduction of the Nazca Plate under South America (which has created the Andes Range) and subduction of the Juan de Fuca Plate under North America (creating the mountains Garibaldi, Baker, St. Helens, Rainier, Hood, and Shasta, collectively known as the Cascade Range).

Continent-Continent Convergent Boundary

A continent-continent collision occurs when a continent or large island that has been moved along with subducting oceanic crust collides with another continent (Figure 4.29). The colliding continental material will not be subducted because it is not dense enough, but the root of the oceanic plate will eventually break off and sink into the mantle. There is tremendous deformation of the pre-existing continental rocks, and creation of mountains from that rock, as well as from any sediments that had accumulated along the shores of both continental masses, and commonly also from some ocean crust and upper mantle material. [caption id="attachment_141" align="aligncenter" width="550"] Figure 4.29 Configuration and processes of a continent-continent convergent boundary Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] Examples of continent-continent convergent boundaries are the collision of the India Plate with the Eurasian Plate, creating the Himalaya Mountains, and the collision of the African Plate with the Eurasian Plate, creating the series of ranges extending from the Alps in Europe to the Zagros Mountains in Iran. When a subduction zone is jammed shut by a continent-continent collision, plate tectonic stresses that are still present can sometimes cause a new subduction zone to develop outboard of the colliding plate.

Transform Boundaries

Transform boundaries exist where one plate slides past another without producing or destroying crust, except in the special case where the transform boundary has bends and jogs. There will be collisions and divergence on a small scale as the jogs crash into the bends, or open up small windows to deeper crust. Most transform faults connect segments of mid-ocean ridges and are thus ocean-ocean plate boundaries (Figure 4.20). Some transform faults connect continental parts of plates. An example is the San Andreas Fault, which connects the southern end of the Juan de Fuca Ridge with the northern end of the East Pacific Rise (a ridge) in the Gulf of California (Figures 4.30 and 4.31). The part of California west of the San Andreas Fault and all of Baja California are on the Pacific Plate. But transform faults do not just connect divergent boundaries; the Queen Charlotte Fault connects the north end of the Juan de Fuca Ridge, starting at the north end of Vancouver Island, to the Aleutian subduction zone. [caption id="attachment_142" align="aligncenter" width="400"] Figure 4.30 The San Andreas Fault extends from the north end of the East Pacific Rise in the Gulf of California to the southern end of the Juan de Fuca Ridge. All of the red lines on this map are transform faults. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] [caption id="attachment_143" align="aligncenter" width="400"] Figure 4.31 The San Andreas Fault at Parkfield in central California. The person with the orange shirt is standing on the Pacific Plate and the person at the far side of the bridge is on the North American Plate. The bridge is designed to slide on its foundation. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Practice with Plate Boundary Types [h5p id="50"]

Plate Tectonics and Supercontinent Cycles

The present continents were once all part of a supercontinent that Alfred Wegener named Pangea (all land). More recent studies of continental matchups and the magnetic ages of ocean-floor rocks have enabled us to reconstruct the history of the break-up of Pangea. Pangea began to rift apart along a line between Africa and Asia and between North America and South America at around 200 Ma (Figure 4.33). During the same period the Atlantic Ocean began to open up between northern Africa and North America, and India broke away from Antarctica. Between 200 and 150 Ma, rifting started between South America and Africa and between North America and Europe, and India moved north toward Asia. By 80 Ma, Africa had separated from South America, and most of Europe had separated from North America. By 50 Ma, Australia had separated from Antarctica, and shortly after that, India collided with Asia. [caption id="attachment_144" align="aligncenter" width="400"] Figure 4.33 Sequence of paleogeographic reconstructions showing the breakup of Pangea. Source: Karla Panchuk (2017) CC BY-NC-SA 4.0. Maps from C. R. Scotese, PALEOMAP Project (www.scotese.com). Click for map sources and terms of use.[/caption] Within the past few million years, rifting has occurred in the Gulf of Aden and the Red Sea, and also within the Gulf of California. Incipient rifting has begun along the Great Rift Valley of eastern Africa, extending from Ethiopia and Djibouti on the Gulf of Aden (Red Sea) all the way south to Malawi. Pangea was not the first supercontinent. It was preceded by Pannotia (600 to 540 Ma), Rodinia (1,100 to 750 Ma), and by others before that. In fact, in 1966, Tuzo Wilson proposed that supercontinents are part of an on-going cycle, which we now refer to as a Wilson cycle. In a Wilson cycle, continents break up, and fragments drift apart only to collide again and make a new continent. At present we are in the stages of a Wilson cycle where fragments are drifting and changing their configuration. North and South America, Europe, and Africa are moving with their respective portions of the Atlantic Ocean. The eastern margins of North and South America and the western margins of Europe and Africa are called passive margins because there is no subduction taking place along them. Because the oceanic crust formed by spreading along the mid-Atlantic ridge is not currently being subducted (except in the Caribbean), the Atlantic Ocean is slowly getting bigger, and the Pacific Ocean is getting smaller. This situation may not continue for too much longer, however. As the Atlantic Ocean floor gets weighed down around its margins by great thickness of continental sediments, it will be pushed farther and farther into the mantle, and eventually the oceanic lithosphere may break away from the continental lithosphere and begin to subduct (Figure 4.34). [caption id="attachment_145" align="aligncenter" width="550"] Figure 4.34 Development of a subduction zone at a passive margin. Times A, B, and C are separated by tens of millions of years. Once the oceanic crust breaks off and starts to subduct, the continental crust (North America in this case) may no longer be pushed to the west and could start to move east because the rate of spreading in the Pacific basin is faster than along the Mid-Atlantic Ridge. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] A subduction zone will develop, and the oceanic plate will begin to descend under the continent. Once this happens, the continents will no longer continue to move apart because the spreading at the mid-Atlantic ridge will be taken up by subduction. If spreading along the mid-Atlantic ridge continues to be slower than spreading within the Pacific Ocean, the Atlantic Ocean will start to close up, and eventually (in a 100 million years or more) North and South America will collide again with Europe and Africa. If this continues without changing for another few hundred million years, we will be back to where we started, with one supercontinent (Figure 4.35). [caption id="attachment_146" align="aligncenter" width="400"] Figure 4.35 Sequence of reconstructions showing the possible future configuration of land masses on Earth at 50, 150, and 250 million years from now. Movements culminate in the formation of a new supercontinent called Pangea Ultima. Source: Karla Panchuk (2017) CC BY-NC-SA 4.0. Maps from C. R. Scotese, PALEOMAP Project (www.scotese.com). Click for map sources and terms of use.[/caption] There is strong evidence around the margins of the Atlantic Ocean that this process has taken place before. There are roots of ancient mountain belts along the eastern margin of North America, the western margin of Europe, and the north-western margin of Africa, which show that these landmasses once collided with each other to form a mountain chain. The mountain chain might have been as big as the Himalayas. The apparent line of collision runs between Norway and Sweden, between Scotland and England, through Ireland, through Newfoundland and the Maritimes, through the north-eastern and eastern states, and across the northern end of Florida. When rifting of Pangea started at approximately 200 Ma, the fissuring was along a different line from the line of the earlier collision. This is why some of the mountain chains formed during the earlier collision can be traced from Europe to North America and from Europe to Africa. It is probably no coincidence that the Atlantic Ocean rift may have occurred in approximately the same place during two separate events several hundred million years apart. The series of hot spots that has been identified in the Atlantic Ocean may also have existed for several hundred million years, and thus may have contributed to rifting in roughly the same place on at least two separate occasions (Figure 4.36). [caption id="attachment_147" align="aligncenter" width="550"] Figure 4.36 A scenario for the Wilson cycle. The cycle starts with continental rifting above a series of mantle plumes (red dots, A). The continents separate (B), and then re-converge some time later, forming a fold-belt mountain chain. Eventually rifting is repeated, possibly because of the same set of mantle plumes (D), but this time the rift is in a different place. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

References

Sinton, J. M., and Detrick, R. S. (1992). Mid-ocean ridge magma chambers. Journal of Geophysical Research, 97(B1), 197-216.

4.5 Mechanisms for Plate Motion

kqzheng — Fri, 24 Aug 2018 18:53:48 +0000

Mantle convection is often said to be critical to plate tectonics. While this is certainly true, there is still debate about the actual forces that make the plates move. One side of the argument holds that the plates are only moved by the traction caused by mantle convection, and that friction between the asthenosphere and lithosphere pulls the lithosphere along as the mantle convects. The other side holds that traction plays only a minor role and that ridge-push and slab-pull are more important (Figure 4.37). Ridge-push refers to gravity causing lithosphere to slide downhill away from the elevated mid-ocean ridges. Slab-pull refers to the weight of subducting slabs dragging the rest of the plate down into the mantle. [caption id="attachment_150" align="aligncenter" width="550"] Figure 4.37 Models for plate motion mechanisms. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] Kearey and Vine (1996) have listed some compelling arguments in favour of the ridge-push/slab-pull model:

Plates that are attached to subducting slabs (e.g., Pacific, Australian, and Nazca Plates) move the fastest, and plates that are not (e.g., North American, South American, Eurasian, and African Plates) move significantly slower.

In order for the traction model to apply, the mantle would have to be moving about five times faster than the plates are moving because the coupling between the partially liquid asthenosphere and the plates is not strong. Such high rates of convection are not supported by geophysical models.

Although large plates have the potential for much higher convection traction, plate velocity is not related to plate area.

References

Kearey, P., & Vine, F. (1996). Global tectonics (2nd E.). Blackwell Science Ltd.

Chapter 4 Summary & Key Term Check

kqzheng — Fri, 24 Aug 2018 18:55:01 +0000

Chapter 4 Main Ideas

4.1 Alfred Wegener's Arguments for Plate Tectonics

The evidence for continental drift in the early 20th century included the matching of continental shapes on either side of the Atlantic, and the geological and fossil matchups between continents that are now thousands of kilometres apart.

4.2 Global Geological Models of the Early 20th Century

The established theories of global geology were permanentism and contractionism, but neither of these theories was able to explain some of the evidence that supported the idea of continental drift.

4.3 Geological Renaissance of the Mid-20th Century

Giant strides were made in understanding Earth during the middle decades of the 20th century, including discovering magnetic evidence of continental drift, mapping the topography of the ocean floor, describing the depth relationships of earthquakes along ocean trenches, measuring heat flow differences in various parts of the ocean floor, and mapping magnetic reversals on the sea floor. By the mid-1960s, the fundamentals of the theory of plate tectonics were in place.

4.4 Plates, Plate Motions, and Plate-Boundary Processes

Earth’s lithosphere is made up of over 20 plates that are moving in different directions at rates of between 1 cm/y to greater than 10 cm/y. The three types of plate boundaries are divergent (plates moving apart and new crust forming), convergent (plates moving together and one possibly being subducted), and transform (plates moving side by side). Divergent boundaries form where existing plates are rifted apart, and it is hypothesized that this is caused by a series of mantle plumes. Subduction zones can form where accumulation of sediment at a passive margin leads to separation of oceanic and continental lithosphere. Supercontinents form and break up through these processes.

Practice Again

Types of plate boundaries

4.5 Mechanisms for Plate Motion

It is widely believed that ridge-push and slab-pull are the main mechanisms for plate motion, as opposed to traction by mantle convection. Mantle convection is a key factor for producing the conditions necessary for ridge-push and slab-pull.

Key Term Check

What key term from Chapter 4 is each card describing? Turn the card to check your answer. [h5p id="51"]

5.4 Silicate Minerals

kqzheng — Mon, 09 Apr 2018 07:36:27 +0000

Silicon and oxygen bond covalently to create a silicate tetrahedron (SiO₄^4-), which is a four-sided pyramid shape with oxygen at each corner and silicon in the middle (Figure 5.21). This structure is the building block of many important minerals in the crust and mantle. Silicon has a charge of +4, and oxygen has a charge of -2, so the total charge of the silicate anion is -4. [caption id="attachment_181" align="aligncenter" width="305"] Figure 5.21 The silica tetrahedron is the building block of all silicate minerals. Source: Karla Panchuk (2018), CC BY-SA 4.0. Modified after Helgi (2013), CC BY-SA 3.0. Image source.[/caption] In silicate minerals, these tetrahedra are arranged and linked together in a variety of ways, from single units to chains, rings, and more complex frameworks. In the rest of this section we will look at the structures of the most common silicate minerals in Earth's crust and mantle.

Make Your Own Tetrahedron [caption id="attachment_182" align="alignleft" width="301"] Figure 5.22 Pattern for a tetrahedron. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] Download Tetrahedron [PDF] with the tetrahedron pattern shown here. Cut around the outside of the shape (solid lines and dotted lines), and then fold along the solid lines to form a tetrahedron. If you have glue or tape, secure the tabs to the tetrahedron to hold it together. If you don’t have glue or tape, make a slice along the thin grey line and insert the pointed tab into the slit. If you're feeling ambitious, make several tetrahedra and and use toothpicks through the corners to make the configurations discussed below.

Isolated Tetrahedra

The simplest silicate structure—that of the mineral olivine (Figure 5.23)—is composed of isolated tetrahedra bonded to iron and/or magnesium ions. In olivine, the –4 charge of each silica tetrahedron is balanced by two iron or magnesium cations, each with a charge of +2. [caption id="attachment_183" align="aligncenter" width="990"] Figure 5.23 Olivine is a silicate mineral made of isolated silica tetrahedra bonded to Fe and Mg ions (left). Source: Karla Panchuk (2021), CC BY-SA 4.0. Click for more attributions.[/caption] Olivine can be pure Mg₂SiO₄ or pure Fe₂SiO₄, or a combination of the two, written as (Mg,Fe)₂SiO₄. Magnesium and iron can substitute for each other because they both have a charge of +2, and they are similar in size: magnesium cations have a radius of 0.73 Å, and iron cations have a radius of 0.62 Å [footnote] Å stands for Ångstrom, a unit commonly used to express atomic-scale dimensions. One angstrom is 10^–10 m or 0.0000000001 m.[/footnote]. While iron and magnesium ions are similar in size, allowing them to substitute for each other in some silicate minerals, the common ions in silicate minerals come in a wide range of sizes (Figure 5.24). Ionic radii are critical to the composition of silicate minerals, because the structure of the silicate mineral will determine the size of spaces available. [caption id="attachment_184" align="aligncenter" width="650"] Figure 5.24 The ionic radii in angstroms of some of the common ions in silicate minerals. Radii shown to scale. Notice that iron appears twice with two different radii. This is because iron can exist as a +2 ion (if it loses two electrons when it becomes an ion) or a +3 ion (if it loses three). Fe²⁺ is known as ferrous iron. Fe³⁺ is known as ferric iron. Source: Karla Panchuk (2021), CC BY 4.0. Modified after Steven Earle (2015), CC BY 4.0 Image source.[/caption]

Chain Silicates

The mineral pyroxene is an example of a single-chain silicate (Figure 5.25), where one oxygen from each tetrahedron is shared with the next tetrahedron. Sharing means that fewer oxygens are needed to make the tetrahedra, so there's less oxygen in this structure over all compared to olivine. This can be expressed as a silicon-to-oxygen ratio (Si:O). In olivine, tetrahedra aren't connected directly to each other, so each silicon atom must have four oxygen atoms all to itself to complete a tetrahedron. That's a ratio of 1:4. In pyroxene, each silicon atom only needs three unique oxygen atoms because it can borrow one from a neighbour to have a tetrahedron. For pyroxene, the Si:O is 1:3. With one less oxygen in the mix per tetrahedron, the net charge per silicon atom that must be balanced by cations is lower (-2 instead of -4). [caption id="attachment_185" align="aligncenter" width="650"] Figure 5.25 Pyroxene (dark mineral in the photo) is a silicate mineral in which tetrahedra are linked in strings, with adjacent tetrahedra sharing an oxygen atom. Source: Karla Panchuk (2021), CC BY-SA 4.0. End-on view modified after Klein & Hurlbut (1993). Click for more attributions.[/caption] The way tetrahedra share oxygens in single-chain silicates is why, even though pyroxene is built out of silicate tetrahedra (with the silicate anion being SiO₄^4-), its formula has the tetrahedra represented as SiO₃(e.g., MgSiO₃, FeSiO₃, and CaSiO₃[footnote]The variation in composition can also be written as (Mg,Fe,Ca)SiO₃, where the elements in the brackets can be present in any proportion. [/footnote], or a multiple thereof). In other words, pyroxene has one cation for each silica tetrahedron (e.g., MgSiO₃) while olivine has two (e.g., Mg₂SiO₄). The structure of pyroxene is also more “permissive” than that of olivine, meaning cations with a wider range of ionic radii can fit into it. That’s why pyroxenes can have calcium cations (radius 1.00 Å) substitute for iron (0.63 Å) and magnesium (0.72 Å). The mineral amphibole is also a chain silicate, but the the silica tetrahedra are linked in a double chains (Figure 5.26). Amphibole has a silicon-to-oxygen ratio higher than that of pyroxene, and hence still fewer cations are necessary to balance the charge. Amphibole is even more permissive than pyroxene, and its compositions can be very complex. [caption id="attachment_186" align="aligncenter" width="565"] Figure 5.26 Amphibole (dark mineral in the photo) is a silicate mineral in which tetrahedra are linked in doubled-up strings. Tetrahedra share two oxygen atoms with adjacent tetrahedra. End-on view modified after Klein & Hurlbut (1993). Source: Karla Panchuk (2021), CC BY-SA 4.0. Click for more attributions.[/caption]

Can You Find the Silicon to Oxygen Ratios?

Two chain silicate structures are shown below. For both, the smallest segment possible that can be repeated to get the chain, is highlighted. Refer to the highlights to verify for yourself the Si:O ratio of pyroxene (single chain) and to figure out the Si:O ratio of amphibole (double chain).

Single chain: Pyroxene structure.
1. How may tetrahedra are highlighted?
2. How many oxygen atoms are highlighted?
3. This gives a Si:O ratio of : . Note: Remember to reduce your fraction. E.g., [latex]\tfrac{2}{4}[/latex] is the same as [latex]\tfrac{1}{2}[/latex].
B. Double chain: Amphibole structure.
1. How may tetrahedra are highlighted?
2. How many oxygen atoms are highlighted?
3. This gives a Si:O ratio of : .

To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="58"]

Sheet Silicates

In mica structures, the silica tetrahedra are arranged in continuous sheets (Figure 5.27), where each tetrahedron shares three oxygen anions with adjacent tetrahedra. Because even more oxygens are shared between adjacent tetrahedra, fewer charge-balancing cations are needed for sheet silicate minerals. Bonding between sheets is relatively weak, and this accounts for the tendency of mica minerals to split apart in sheets (Figure 5.27 bottom right). Two common micas in silicate rocks are biotite (Figure 5.27 bottom left), which contains iron and/or magnesium, making it a dark mineral; and muscovite (Figure 5.27 right), which contains aluminum and potassium, and is light in colour. All of the sheet silicate minerals have water in their structure, in the form of the hydroxyl (OH-) anion. [caption id="attachment_187" align="aligncenter" width="652"] Figure 5.27 Micas are sheet silicates and split easily into thin layers along planes parallel to the sheets. Biotite mica (lower left) is has Fe and Mg cations. Muscovite mica (lower right) has Al and K instead. The muscovite mica shows how thin layers can split away in a sheet silicate. Source: Karla Panchuk (2018), CC BY-NC-SA 4.0. Top left- Modified after Steven Earle (2015), CC BY 4.0. Top right- Modified after Klein & Hurlbut (1993). Click for more attributions.[/caption] Some sheet silicates typically occur in clay-sized fragments (i.e., less than 0.004 mm). These include the clay minerals kaolinite, illite, and smectite, which are important components of rocks and especially of soils.

Framework Silicates

In framework silicates, tetrahedra are connected to each other in three-dimensional structures rather than in two-dimensional chains and sheets.

Feldspar

Feldspars are a group of very abundant framework silicates in Earth's crust. They include alumina tetrahedra as well as silicate tetrahedra. In alumina tetrahedra, there is an aluminum cation at the centre instead of a silicon cation. Feldspars are classified using a ternary (3-fold) system with three end-members ("pure" feldspars). This system is illustrated with a triangular diagram that has each end-member at one corner (Figure 5.28). The distance along a side of the diagram represents the relative abundance of the composition of each end-member. [caption id="attachment_188" align="aligncenter" width="960"] Figure 5.28 Ternary diagram showing the feldspar group of framework silicate minerals. Alkali feldspars are those with compositions ranging between albite (with a Na cation) and orthoclase and its polymorphs (with a K cation. Plagioclase feldspars are those with compositions ranging between albite and anorthite (with a Ca cation). Source: Karla Panchuk (2018) CC BY-SA 4.0. Ternary diagram modified after Klein & Hurlbut (1993). Click for more attributions and a ternary diagram without mineral images.[/caption] One end-member is potassium feldspar (also referred to as K-feldspar), which has the composition KAlSi₃O₈. Depending on the temperature and rate of cooling, K-feldspar can occur as one of three polymorphs: orthoclase, sanidine, or microcline. Another end member is albite, which has sodium instead of potassium (formula NaAlSi₃O₈). As is the case for iron and magnesium in olivine, there is a continuous range of compositions (referred to as a solid-solution series) between albite and orthoclase. Feldspars in this series are referred to as alkali feldspars. Potassium cations are much larger than sodium cations (1.37 Å versus 0.99 Å, respectively), so high temperatures are required to form alkali feldspars with intermediate compositions. The third end-member is anorthite and it has calcium instead of potassium or sodium (formula CaAl₂Si₃O₈). Feldspars in the solid-solution series between albite and anorthite are called plagioclase feldspars. Calcium and sodium cations are nearly the same size (1.00 Å and 0.99 Å, respectively), so from that perspective it makes sense that they substitute readily for each other, and that any intermediate compositions between CaAl₂Si₃O₈ and NaAlSi₃O₈ can exist. However, calcium and sodium ions don’t have the same charge (Ca²⁺ versus Na+), making it surprising that they substitute so easily. The difference in charge is accommodated by substituting some Al³⁺ for Si⁴⁺. Albite has one Al and three Si in its formula, while anorthite is has two Al and two Si. Plagioclase feldspars of intermediate composition also have intermediate proportions of Al and Si.

Quartz

Quartz (SiO₂; Figure 5.29) contains only silica tetrahedra. In quartz, each silica tetrahedron is bonded to four other tetrahedra (with an oxygen shared at every corner of each tetrahedron), making a three-dimensional framework. As a result, the ratio of silicon to oxygen is 1:2. Because the one silicon cation has a +4 charge and the two oxygen anions each have a –2 charge, the charge is balanced. There is no need to add cations to balance the charge. The hardness of quartz and the fact that it breaks irregularly (notice the bottom of the crystal in Figure 5.29 right) and not along smooth planes result from the strong covalent/ionic bonds characteristic of the silica tetrahedron. [caption id="attachment_189" align="aligncenter" width="650"] Figure 5.29 Quartz is another silicate mineral with a three-dimensional framework of silica tetrahedra. Sometimes quartz occurs as well-developed crystals (left), but it also occurs in common rocks such as granite (right). In addition to quartz, the granite contains potassium feldspar, albite, and amphibole. Source: Karla Panchuk (2018) CC BY-NC-SA 4.0. Click for more attributions.[/caption]

Practice with Silicate Minerals & Their Structures Download a printable version of this exercise: Silicate Minerals & Their Structures [PDF]. The original version of this chapter contained H5P content. You may want to remove or replace this element.

References

Klein, C. & Hurlbut, C. S., Jr. (1993). Manual of mineralogy (after J. D. Dana). John Wiley & Sons, Inc.

5.5 How Minerals Form

kqzheng — Wed, 11 Apr 2018 21:05:18 +0000

The following criteria are required for mineral crystals to grow:

The elements needed to make the mineral crystals must be present in sufficient abundance and appropriate proportions.
The physical and chemical conditions must be favourable.
There must be sufficient time for the atoms to become arranged into a lattice.

Physical and chemical conditions include factors such as temperature, pressure, amount of oxygen available, pH, and the presence of water. The presence of water makes it easier for ions to move to where they're needed, and can lead to the formation of larger crystals over shorter time periods, as with the gypsum crystals at the beginning of this chapter. Time is one of the most important factors because it takes time for atoms to line themselves up into an orderly structure. If time is limited, the mineral grains may remain very small. Most of the minerals that make up the rocks in the crust and mantle formed through the cooling and freezing of melted rock (magma). At the high temperatures that exist deep within Earth, some geological materials are liquid. As magma rises up through the crust, either by volcanic eruption or by more gradual processes, it cools and minerals crystallize. When cooling is rapid and many crystals form at once, only small mineral grains will form before the rock becomes solid. The resulting rock will be fine-grained (i.e., crystals less than 1 mm). When cooling is slow, or when few crystals are growing at a time, relatively large crystals will develop. Minerals can also form in several other ways:

Precipitation from a solution (e.g., from hot water flowing underground, or when evaporation concentrates ions in a lake or inland sea)
Precipitation from a gas (e.g., from vents releasing volcanic gases)
Metamorphism: solid minerals react with each other under high pressures and temperatures, and new minerals are formed.
Weathering: minerals unstable at Earth’s surface are chemically altered by surface processes.
Organic formation: organisms build shells (primarily of calcite or aragonite), and teeth and bones (primarily of apatite).

Sulphur Mining at Ijen Crater: Humans Intervene in Mineral Formation

Ijen Crater is the crater of an active volcano located on the eastern end of the island of Java in Indonesia. The crater is notable for a beautiful turquoise lake that is actually sulphuric acid, but also because the volcano vents sulphur gas into the crater. Without human intervention, sulphur precipitates directly from a gas into a solid that covers everything with a layer of sulphur. But miners have modified the landscape to make sulphur collection more efficient. They've added ceramic pipes over the vents, channeling the sulphurous gas and causing it to cool and condense into a liquid. Red molten sulphur flows out of the pipes and onto the ground where it cools to the usual yellow colour. Then miners use pry bars to lift up chunks of the sulphur from the ground. They pack it into woven baskets that they carry on their shoulders, then walk the sulphur up the steep crater and 3 km down the mountainside to where it is weighed, and they are paid for how much they've carried. The images below show the Ijen crater and the mining work. The images also show that most miners don't have PPE (personal protective equipment) like heavy boots, gloves, or—most importantly—gas masks. Some make do by tying cloth in front of their faces, and others simply hold their breath for long intervals. Inhaling the gas damages soft tissues, but the issue the miners struggle with most immediately is that it also corrodes their teeth. Inexperienced visitors without proper equipment have become seriously ill after being exposed to the environment for only a few hours. While the miners aren't paid enough to afford proper protective attire, the job still pays more than many in the area. If they can make two trips per day, their pay for the day could be approximately what a minimum-wage earner gets in Canada for one hour of work. [gallery columns="2" size="medium" ids="1850,1851,1852,1853,1854,1855,1856,1857"] Given how the sulphur forms at Kawah Ijen, would you expect large sulphur crystals to form, or smaller ones?

Larger, because there's so much material available.
Smaller, because the crystals form quickly.

Answer: Smaller. The crystals form very rapidly when the sulphur precipitates from gas, and also rapidly when the molten sulphur freezes. The faster crystals form, the smaller they are. The large blocks of sulphur that are visible in the images are actually masses of very tiny crystals.

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5.6 Mineral Properties

kqzheng — Fri, 13 Apr 2018 07:22:54 +0000

Minerals are universal. A crystal of hematite on Mars will have the same properties as one on Earth, and the same as one on a planet orbiting another star. That’s good news for geology students who are planning interplanetary travel, because they can use the same properties to identify minerals anywhere. That doesn’t mean that it’s easy, however. Identification of minerals takes practice. Some of the mineral properties that are useful for identification are colour, streak, lustre, hardness, habit, cleavage or fracture, and density.

Colour

Some minerals have distinctive colours that useful as diagnostic criteria. The mineral sulphur (Figure 5.30 left) is always a characteristic bright yellow. For other minerals, colour might vary. Hematite is an example of a mineral for which colour is not necessarily diagnostic. In some forms hematite is a deep dull red (a fairly unique colour), but in others it is a metallic silvery black (5.30, right). [caption id="attachment_195" align="aligncenter" width="600"] Figure 5.30 Colour is a useful diagnostic property for sulphur (left) and for some types of hematite (right) because the yellow and dark red colours are unique to those minerals. In contrast, silvery metallic forms of hematite are similar in appearance to many other minerals. Source: Karla Panchuk (2018), CC BY-NC-SA 4.0. Click for more attributions.[/caption] For other minerals, the problem is that a single mineral can have a wide range of colours. The colour variations can be the result of varying proportions of trace elements within the mineral, or structural defects within the crystal lattice. In the case of quartz (Figure 5.31), milky quartz gets its white colour from millions of tiny fluid-filled cavities. Smoky quartz gets its grey colour from structural damage caused by natural radiation. Amethyst and citrine get their colours from trace amounts of iron, and rose quartz gets its pink hue from manganese. [caption id="attachment_196" align="aligncenter" width="650"] Figure 5.31 The many colours of quartz.Quartz can be colourless, milky, a greyish smoky colour, purple, yellow, and pink. Source: Karla Panchuk (2018), CC BY-NC-SA 4.0. Photos by R. Weller/ Cochise College. Click for more attributions.[/caption]

Streak

The colour of a mineral is what you see when light reflects off the surface of the sample. One reason that colour can be so variable is that the surface texture is variable. A way to get around this problem is to grind a small amount of the sample to a powder and observe the colour of the powder. This colour is the mineral's streak. The mineral can be powdered by scraping the sample across a piece of unglazed porcelain called a streak plate (Figure 5.32). In Figure 5.32, two samples of hematite have been scraped across the streak plate. Even though one sample is metallic and the other is deep red, both have a similar reddish-brown streak. [caption id="attachment_197" align="aligncenter" width="600"] Figure 5.32 Hematite leaves a distinctive reddish-brown streak whether the sample is metallic or deep red. Source: Karla Panchuk (2015) CC BY 4.0[/caption] Streak is an especially helpful property when minerals look similar. In Figure 5.33, there are four different minerals, but all are silvery-black in colour, with varying degrees of metallic sheen. The streaks of these minerals are much more distinctive, ranging from dark grey to yellowish brown. [caption id="attachment_198" align="aligncenter" width="500"] Figure 5.33 Similar dark-grey minerals with varying degrees of metallic sheen leave different colours of streaks. The minerals are from upper left clockwise: hematite (reddish brown streak), magnetite (grey streak), sphalerite (yellowish brown streak), and galena (darker grey streak). Source: Karla Panchuk (2015), CC BY 4.0.[/caption]

Lustre

Lustre is the way light reflects off the surface of a mineral, and the degree to which it penetrates into the interior. The main distinction is between metallic lustre and non-metallic lustre. Light doesn't pass through metals, and that's the main reason they look metallic (e.g., the hematite on left of Figure 5.32). Even a thin sheet of metal—aluminum foil, for example—will be not permit light to pass through it. Many non-metallic minerals may look as if light will not pass through them, but if you look closely at a thin slice of the mineral, you'll see that the mineral is translucent or transparent. If a non-metallic mineral has a shiny, reflective surface, it is said to have a glassy lustre. The quartz crystals in Figure 5.31 are examples of minerals with glassy lustre. If the mineral surface is dull and non-reflective, it has an earthy lustre (like the hematite on the right of Figure 5.32). Other types of non-metallic lustres are silky, pearly, and resinous (like amber or candied pineapple). Lustre is a good diagnostic property because most minerals will always appear either metallic or non-metallic, although as the two hematite samples in Figure 5.32 show, there are exceptions.

Practice with Lustre [h5p id="60"]

Hardness

One of the most important diagnostic properties of a mineral is its hardness. In practical terms, hardness determines whether or not a mineral can be scratched by a particular material. In 1812 German mineralogist Friedrich Mohs came up with a list of 10 minerals representing a wide range of hardness, and numbered them 1 through 10 in order of increasing hardness (Figure 5.34, horizontal axis). While each mineral on the list is harder than the one before it, the measured hardness (vertical axis) is not linear. Notice the difference in hardness between talc and gypsum, then compare that to the difference between corundum and diamond. [caption id="attachment_199" align="aligncenter" width="650"] Figure 5.34 Minerals and reference materials in the Mohs scale of hardness. Absolute hardness values are measured values. Mohs hardness values do not increase proportionately with absolute hardness. Source: Karla Panchuk (2021), CC BY 4.0.[/caption] Some commonly available reference materials are also shown on this diagram,and summarized in the table below. Note that fingernails are often included as a reference material for testing minerals, but this only applies to natural fingernails. Artificial fingernails may be much harder than natural fingernails. Some materials used for artificial nails are harder than quartz.

Table 5.1 Mohs Hardness of Scale
Mohs Hardness	Mineral	Common Reference Materials
1	Talc (softest)
2	Gypsum	2-2.5 - Natural fingernail
3	Calcite	3 - Copper
4	Fluorite	4 - Nail (steel)
5	Apatite	5-5.5 - Glass plate, knife blade
6	Orthoclase Feldspar	6.5-7 - Streak plates, hardened steel file
7	Quartz
8	Topaz
9	Corundum
10	Diamond (hardest)

Using these materials to determine hardness involves testing to see which material will or won't scratch the mineral so you can find upper and lower limits on hardness. For example, if you have a mineral that you can’t scratch with your fingernail, but you can scratch with a copper wire, then its hardness is between 2.5 and 3. A mineral with known hardness can be used to test other minerals.

Concept Check: Testing for Hardness [h5p id="61"]

Crystal Habit

When minerals form within rocks, there is a possibility that they will form in distinctive crystal shapes if they are not crowded out by other pre-existing minerals. Every mineral has one or more distinctive crystal habits determined by their atomic structure, although it is not that common in ordinary rocks for the shapes to be obvious. Quartz, for instance, will form six-sided prisms with pointed ends (Figure 5.35 left), but this typically happens only when it crystallizes from a hot water solution within a cavity in an existing rock. Pyrite can form cubic crystals (Figure 5.35 centre), but can also form crystals with 12 faces, known as dodecahedra. The mineral garnet also forms many-sided crystals with an over-all rounded shape (Figure 5.35 right). [caption id="attachment_200" align="aligncenter" width="650"] Figure 5.35 Hexagonal prisms of quartz (left), intergrown cubic crystals of pyrite (centre), and 24-sided crystals of garnet (right). Source: Karla Panchuk (2018) CC BY-NC-SA 4.0. Photos by R. Weller/ Cochise College. Click for more attributions.[/caption] Some of the terms that are used to describe habit include bladed, botryoidal (grape-like), dendritic (branched), drusy (an encrustation of crystals), equant (similar size in all dimensions), fibrous, platy, prismatic (long and thin), and stubby.

Cleavage and Fracture

Cleavage and fracture describe how a mineral breaks. These characteristics are the most important diagnostic features of many minerals, and often the most difficult to understand and identify. Cleavage is what we see when a mineral breaks along a plane or planes, while fracture is an irregular break. Some minerals tend to cleave along planes at various fixed orientations. Some, like quartz, do not cleave at all, only fracture. Minerals that have cleavage can also fracture along surfaces that are not parallel to their cleavage planes. The way minerals break is determined by the arrangement of atoms within them, and more specifically by the orientation of weaknesses within their crystal lattice. Graphite and mica break off in parallel sheets (Figure 5.36). [caption id="attachment_201" align="aligncenter" width="532"] Figure 5.36 One direction of cleavage (basal cleavage). Left: Schematic of basal cleavage. Right: Muscovite showing basal cleavage. The white dashed line marks the edge of the cleavage plane. Source: Karla Panchuk (2018) CC BY-SA 4.0. Cleavage diagram modified after M.C. Rygel (2010) CC BY-SA 3.0 view source[/caption] Other minerals have two directions of cleavage, classified as two directions at 90° (Figure 5.37 top) and two directions not at 90° (Figure 5.37 bottom). While the diagrams of planes on the left of Figure 5.37 make this difference clear, it may be less obvious in practice. The minerals in Figure 5.37 both have two planes of cleavage that are very close to 90°. The white dashed lines mark the edges of the planes, as with Figure 5.36. See if you can find the planes repeated in the images. The images are close-up views of the minerals, only a few cm across. Sometimes you must look very carefully to find cleavage planes. [caption id="attachment_202" align="aligncenter" width="520"] Figure 5.37 Two directions of cleavage. Top: Two directions at 90° in pyroxene. Bottom: two directions not at 90° in plagioclase feldspar. Edges of cleavage planes marked with dashed lines. Source: Karla Panchuk (2018) CC BY-SA 4.0. Cleavage diagrams modified after M.C. Rygel (2010) CC BY-SA 3.0 view source[/caption] Some minerals have many directions of cleavage. Figure 5.38 shows minerals with three directions of cleavage. Halite (Figure 5.38 top) has three directions at 90° and calcite (Figure 5.38 bottom) has three directions not at 90°. [caption id="attachment_203" align="aligncenter" width="473"] Figure 5.38 Three directions of cleavage. Top: Three directions at 90° in halite. Bottom: Three directions not at 90° in calcite. Source: Karla Panchuk (2018) CC BY-SA 4.0. Cleavage diagrams modified after M.C. Rygel (2010) CC BY-SA 3.0 view source[/caption] There are a few common difficulties that students encounter when learning to recognize and describe cleavage. One is that it might be necessary to look very closely at a sample to see mineral cleavage. The key features in Figure 5.37 are only cm or mm in scale. If crystals are very small, it may not be possible to see cleavage at all. Another issue is that sometimes cleavage is present, but it is poor, meaning the cleavage surface isn't perfectly flat. Finally it can be difficult to know whether a flat surface on a crystal is a cleavage plane, a crystal face, or simply a surface that happens to be flat. Cleavage planes tend to repeat themselves at different depths throughout the mineral, so if you are unsure whether the surface you are looking at is a cleavage plane, try rotating the mineral in bright light. If cleavage is present, you will generally find that, for a given cleavage direction, all of the cleavage surfaces will glint in the light simultaneously. Crystal faces will also glint in light, but they do not repeat themselves at depth throughout the mineral. The best way to overcome all of these problems is to look at lots of examples. It's worth it to be able to identify cleavage and fracture, because cleavage is a reliable diagnostic property for most minerals.

Can You Pick Out the Broken Piece?

Sometimes it's hard to tell whether a well-formed mineral crystal is displaying crystal habit (i.e., it grew that way), or whether it's a broken piece of a mineral with cleavage (i.e., it broke into a particular shape because of a specific pattern of weaknesses in its atomic structure).The image shows two samples for each of two minerals. Three of the samples are pieces broken from a bigger crystal. Can you figure out which of the larger samples (A or B) is also a broken piece? Which of the larger samples (A or B) was broken from a bigger crystal?

Olivine (the large green crystal) is the broken fragment.
Halite (the large clear crystal) is the broken fragment.

A. Olivine The crystal grew this way. The broken olivine crystals have irregular surfaces, meaning they are displaying fracture, not cleavage. Breaking would not have resulted in the smooth faces on the large crystal, so it must have grown that way. B. Halite This is the broken crystal. The broken halite crystals display 3 cleavage planes at 90° to each other, and the large crystal shows the same kinds of faces at 90° to each other. Note that halite crystals can also grow into this shape.

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Density

Density is a measure of the mass of a mineral per unit volume, and it is a useful diagnostic tool in some cases. Most common minerals, such as quartz, feldspar, calcite, amphibole, and mica, are of average density (2.6 to 3.0 g/cm³), and it would be difficult to tell them apart on the basis of their density. On the other hand, many of the metallic minerals, such as pyrite, hematite, and magnetite, have densities over 5 g/cm³. If you picked up a sample of one of these minerals, it would feel much heavier compared to a similarly sized sample of a mineral with average density. A limitation of using density as a diagnostic tool is that one cannot assess it in minerals that are a small part of a rock with other minerals in it.

Other properties

Several other properties are useful for identification of some minerals. Some of these are:

Calcite reacts with dilute acid and will give off bubbles of carbon dioxide.
Magnetite is strongly magnetic, and some other minerals are weakly magnetic.
Sphalerite ((Zn,Fe)S) gives off a smell of sulphur when drawn across a streak plate.
Halite tastes salty.
Talc feels soapy to the touch.
Plagioclase feldspar has striations (parallel razor-thin lines etched on the surface) and some varieties show a play of colours when light hits them at the right angle (see the labradorite in Figure 5.28).

Are You Ready to Test Your Understanding of Mineral Properties? [h5p id="3"]

Chapter 5 Summary & Key Term Check

kqzheng — Mon, 16 Apr 2018 06:08:01 +0000

Chapter 5 Main Ideas

5.1 Atoms

An atom is made up of protons and neutrons in the nucleus, and electrons arranged in energy shells around the nucleus. The first shell holds two electrons, and outer shells hold more. Atoms strive to have eight electrons in their outermost shell (or two for H and He). Atoms gain, lose, or share electrons to achieve this. In so doing they become either positively charged cations (if they lose electrons) or negatively charged anions (if they gain them).

5.2 Bonding and Lattices

The main types of bonding in minerals are ionic bonding (electrons transferred) and covalent bonding (electrons shared). Some minerals have metallic bonding or weak Van der Waals forces. Minerals form in three-dimensional lattices. The configuration of the lattices and the type of bonding within help determine mineral properties.

5.3 Mineral Groups

Minerals are grouped according to the anion part of their formula. Some common types are: oxides, sulphides, sulphates, halides, carbonates, phosphates, silicates, and native minerals.

5.4 Silicate Minerals

Silicate minerals are the most common minerals in Earth’s crust and mantle. They all have silica tetrahedra (four oxygens surrounding a single silicon atom) arranged in different structures (chains, sheets, etc).

Do You Know the Common Silicate Minerals? The most common rock-building minerals come up frequently in this textbook. Now is a good time to make sure you know what they are. [h5p id="63"]

5.5 How Minerals Form

Most minerals in the crust form from the cooling and crystallization of magma. Some form from hot water solutions, during metamorphism or weathering, or through organic processes. More rarely, minerals precipitate directly from a gas, such as at a volcanic vent.

5.6 Mineral Properties

Some of the important properties for mineral identification include hardness, cleavage/fracture, density, lustre, colour, and streak colour.

Key Term Check

What key term from Chapter 5 is each card describing? Turn the card to check your answer. [h5p id="64"]

7.1 Magma and How It Forms

kqzheng — Thu, 19 Apr 2018 06:57:55 +0000

[caption id="attachment_226" align="alignright" width="400"] Figure 7.2 Element abundance in Earth's crust. Source: Karla Panchuk (2021), CC BY 2.0. Modified after Steven Earle (2016), CC BY 4.0. Image source.[/caption] Igneous rocks form when melted rock cools, and melted rock originates within Earth as magma. Magma compositions vary, but will have eight main elements in different proportions. The most abundant elements are oxygen and silicon, followed by aluminum, iron, calcium, sodium, magnesium, and potassium. These eight elements are also the most abundant in Earth's crust (Figure 7.2). All magmas have varying proportions of lighter elements such as hydrogen, carbon, and sulphur. Lighter elements are converted into gases like water vapour, carbon dioxide, hydrogen sulphide, and sulphur dioxide as the magma cools.

What Controls Magma Composition?

Magma composition depends on two things:

The composition of the rocks that melted to form the magma.
The conditions under which the melting happened

Most igneous rock in Earth's crust comes from magmas that formed through partial melting of existing rock, either in the upper mantle or the crust. During partial melting, only some of the minerals within a rock melt. This happens because different minerals have different melting points (temperatures at which they melt). The melt is less dense than the surrounding rock, and will percolate upward without the source rock having melted completely. The result is magma with a different composition than the original rock. Partial melting produces melt that has more silica than the original rock, because minerals higher in silica have lower melting points. To see how partial melting works, consider the mix of materials in Figure 7.3A. It contains white blocks of candle wax, black plastic pipe, green beach glass, and pieces of aluminum wire. When the mixture is heated to 50 °C in a warm oven, the wax melts into a clear liquid (B), but the other materials remain solid. This is partial melting. [caption id="attachment_227" align="aligncenter" width="650"] Figure 7.3 Experiment to illustrate partial melting. (A) The original components are white candle wax, black plastic pipe, green beach glass, and aluminum wire. (B) After heating to 50˚C for 30 minutes, the wax has melted. (C) After heating to 120˚C for 60 minutes, the plastic has also melted and mixed with the melted wax. (D) The liquid has been poured off and allowed to cool, making a solid with a different overall composition from the original mixture. Source: Karla Panchuk (2021), CC BY-NC-SA 4.0. Modified after Steven Earle (2015), CC BY 4.0. Image source.[/caption] When the mixture is heated to 120 °C, the plastic melts and mixes with the wax, but the aluminum and glass still remain solid (C). This is still considered partial melting because solid materials remain. When the plastic and wax mixture is poured into a separate container and allowed to cool (D), the resulting solid has a very different composition from the original mixture. The plastic and wax are analogous to more silica-rich minerals with relatively lower melting points than other minerals in the same rock. Of course, partial melting in the real world isn’t as simple as the example in Figure 7.3. Many rocks are much more complex than the four-component system used here. Some mineral components of rocks may have similar melting temperatures, and begin to melt at the same time. The melting temperature of a mineral may change in the presence of other minerals. Also, when rocks melt, the process can take millions of years, unlike the 90 minutes required to melt the pipe and wax in the experiment.

Concept Check: Melting Point of a Rock

A rock contains three minerals, A, B, and C, with melting points of 700 °C, 900 °C, and 1300 °C, respectively. What is the melting point of the rock?

700 °C, the lowest melting point of the three minerals
1300 °C, the highest melting point of the three minerals
700 °C to 1300 °C, the range covering the melting points of all the minerals
967 °C, the average melting point of all three minerals

To check your answers, navigate to the below link to view the interactive version of this activity.

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Why Rocks Melt

The obvious answer to the question “Why do rocks melt?” is that rocks have been heated above the melting point of one or more of the minerals that make them up. Magma from partial melting of mantle rocks rises upward through the mantle, and may pool at the base of the crust, or rise through the crust. Moving magma carries heat with it, and some of that heat is transferred to surrounding rocks when the magma touches them. If the melting temperature of a rock is less than the temperature of the magma, the rock will begin to melt. This is melting by conduction. Heat transfer is not the only way of melting rocks, however, and it doesn't explain another conundrum: where melted mantle rocks come from. The rock that makes up the mantle begins to melt at approximately 1330 °C, and the mantle is almost entirely at temperatures above 1330 °C. Nevertheless, the mantle is in fact solid with the exception of small amounts of melt in its uppermost parts. The answer to this paradox is that higher pressures within the mantle increase the melting point of rock compared to what it would be at Earth’s surface. For example, at the depth where the mantle temperature is 2300 °C, the melting point of mantle rock is actually 3800 °C. Clearly, a lot of additional heat would have to be added to melt those rocks, but there are other ways that melting happens. One way is to remove some of the pressure. This is called decompression melting (Figure 7.4, left). [caption id="attachment_228" align="aligncenter" width="1024"] Figure 7.4 Melting triggers. Left- Decompression melting occurs when a parcel of rock rises or the overlying crust thins. Right- Flux-induced melting occurs when volatile compounds such as water are added. Source: Karla Panchuk (2018), CC BY 4.0. Modified after Steven Earle (2016), CC BY 4.0. Image source.[/caption] Another way to trigger melting is to disrupt the chemical environment by adding water. This is called flux-induced melting, or fluid-induced melting. When a substance such as water is added to hot rocks, the melting points of the minerals within those rocks decreases. If a rock is already close to its melting point, the effect of adding water can be enough to trigger partial melting. The added water is a flux, hence the name flux-induced melting. In Figure 7.4 (right), the parcel of rock represented by the dashed box is not hot enough to be right of the line where dry mantle rocks melt, but it is to the right of the line where wet mantle rocks melt.

Practice with Melting Types & Phase Diagrams

The image shown here is a phase diagram representing the pressure and temperature conditions under which granite and basalt will melt, with and without water. The questions that follow will help you get oriented with the phase diagram, and then test your understanding of decompression melting and fluid-induced (or flux) melting. Image description: The image shows a phase diagram with a vertical axis labelled "pressure increases" and a downward pointing arrow. The horizontal axis is labelled "temperature increases" with an arrow pointing to the right. The diagram has four lines labelled A through D in order from left to right. Lines A and B are curved upward to the right, and lines C and D tilt downard to the right. Lines are labelled as follows: A- wet granite melts; B- wet basalt melts; C- dry granite melts; D- dry basalt melts. There are three points labelled x, y, and z. Point x is between lines A ("wet granite melts") and B ("wet basalt melts"). Point y is directly across from x, and just to the left of line C, "dry granite melts." Point z is straight above point y, and between lines C and D ("dry basalt melts").

Which part of the diagram represents conditions at Earth's surface?
- The top of the diagram
- The bottom of the diagram
Which rock has the higher melting point?
- Basalt
- Granite
Which lines can tell you the temperature at which flux melting (or fluid-induced melting) will happen at different depths within Earth?
- A and B
- C and D
Which point on the diagram does not represent a temperature where flux melting could happen to basalt?
- x
- y
- z
Which path on the diagram represents decompression melting for granite?
- From x to y
- From y to x
- From y to z
- From z to y
For the range of pressures and temperatures in this diagram, can melting happen by decompression alone for wet granite?
- Yes
- No

To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="71"]

Melting and Plate Tectonics

Triggers for melting are directly related to plate tectonic processes. Decompression happens when the lithosphere is thinning and putting less pressure on the rocks below, such as along rift zones. Decompression also occurs when mantle rocks move upward due to convection, or rise as a plume within the mantle. Flux-induced melting happens in subduction zones. Minerals are transformed by chemical reactions under high pressures and temperatures, and a by-product of those transformations is water. Relatively little water is required to trigger partial melting. In laboratory studies of the conditions of partial melting in the Japanese volcanic arc, rocks with only 0.2% of their weight consisting of water melted by up to 25%. Conduction can occur where lower silica melts come into contact with higher silica rocks, such as those in continental crust. Very hot mantle plumes can also melt overlying rocks.

Practice: How is Melting Triggered in Different Plate Tectonic Settings?

Image description: The image shows a phase diagram with a vertical axis labelled "pressure increases" and a downward pointing arrow. The horizontal axis is labelled "temperature increases" with an arrow pointing to the right. The diagram has four lines labelled A through D in order from left to right. Lines A and B are curved upward to the right, and lines C and D tilt downard to the right. Lines are labelled as follows: A- wet granite melts; B- wet basalt melts; C- dry granite melts; D- dry basalt melts. There are three points labelled x, y, and z. Point x is between lines A ("wet granite melts") and B ("wet basalt melts"). Point y is directly across from x, and just to the left of line C, "dry granite melts." Point z is straight above point y, and between lines C and D ("dry basalt melts").

Which part of the diagram represents conditions at Earth's surface?
- The top of the diagram
- The bottom of the diagram
Which rock has the higher melting point?
- Granite
- Basalt
Which lines can tell you the temperature at which flux melting (or fluid-induced melting) will happen at different depths within Earth?
- A and B
- C and D
Which point on the diagram does not represent a temperature where flux melting could happen to basalt?
- x
- y
- z
Which path on the diagram represents decompression melting for granite?
- From x to y
- From y to x
- From y to z
- From z to y
For the range of pressures and temperatures in this diagram, can melting happen by decompression alone for wet granite?
- Yes
- No

To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="5"]

References

Kushiro, I. (2007). Origins of magmas in subduction zones: a review of experimental studies. Proceedings of the Japan Academy, Series B, Physical and Biological Sciences, 83(1), 1-15. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3756732/

7.2 Crystallization of Magma

kqzheng — Mon, 30 Apr 2018 04:44:33 +0000

The rock cycle—in spite of being referred to as a cycle—can be said to have a starting point. All of the rock that's undergoing processes within the rock cycle originated from magma that formed due to partial melting of mantle rocks. Mantle rocks are ultramafic in composition, meaning they're very high in iron and magnesium, and relatively low in silica. In spite of ultramafic rocks being the starting point, ultramafic magma is not encountered in modern volcanic environments, and ultramafic rocks are relatively rare at Earth’s surface. Partial melting is part of the reason we no longer see ultramafic magmas and lavas. Earth just isn't hot enough to melt the highest-temperature minerals in the mantle. Ultramafic volcanic rocks—called komatiites—exist, but with two notable exceptions, the youngest of these is 2 billion years old, from a time early in Earth's history when it was much hotter.[footnote]The komatiites of the Song Da zone in northwestern Vietnam are 270 million years old, and those on Gorgona Island, Columbia are 89 million years old. Exactly how they formed is still a bit of a mystery. See Table 1 of arXiv:physics/0512118v2 [physics.geo-ph] for a compilation of komatiite ages with references.[/footnote] Partial melting of mantle rocks means that even magmas derived directly from the mantle are already a slightly different composition, in the same way that the partial melting experiment in Figure 7.3 resulted in a melt that included only two of the four components in the original mixture. In the case of silicate minerals, those with more silica will melt before those with less silica. Quarts and feldspar, for example, are the "candle wax" of partial melting. Biotite is the plastic pipe. This means the partial melt will have more silica than the original rock taken as a whole. Partial melting is a way to affect the composition of an igneous rock during the melting stage, but there are a number of processes that affect the composition as melted rock moves within Earth and cools. These include:

Mixing in of magma of a different composition, including from rocks melted by heat from the original magma.
Fractional crystallization, when minerals with a higher melting point crystallizing early in the cooling process and settling out of the magma.
A series of chemical reactions called Bowen’s reaction series that happen as the melt cools.

Compositional Categories

Before we talk about how we get igneous rocks of different compositions, we need to cover the terms used to describe the general compositional categories. These terms are used frequently when discussing igneous rocks and volcanic processes, so it's important to know them. In order of increasing silica content after the ultramafic category are mafic, intermediate, then felsic (Figure 7.6). [caption id="attachment_231" align="aligncenter" width="600"] Figure 7.6 Chemical compositions of typical mafic, intermediate, and felsic magmas. Source: Karla Panchuk (2021), CC BY 4.0. Modified after Steven Earle (2016), CC BY 4.0. Image source.[/caption] Magma compositions are reported in terms of the fraction of mass of oxides (e.g., Al₂O₃ rather than just Al). On average, mafic magmas are approximately half SiO₂ (45% to 55%), about 25% FeO, MgO, and CaO combined. They have about 5% Na₂O + K₂O. Felsic magmas, on the other hand, have much more SiO₂ (65% to 75%) and Na₂O + K₂O (around 10%) and much less FeO, MgO, and CaO (about 5% combined). Magmas that fall between mafic and felsic magmas have an intermediate composition.

Quick Check

Can you fill in the missing words to complete these statements about magma compositional categories? magmas are characterized by minerals with MAgnesium and FerrIC (containing iron) components, such as olivine and pyroxene. magmas have minerals like potassium FELdspar and quartz, which are more SIliCa-rich. To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="72"]

Next Level: Can You Figure Out the Magma Compositions? [h5p id="6"]

Bowen's Reaction Series

The minerals that make up igneous rocks crystallize (solidify, freeze) at a range of different temperatures. This explains why cooling magma can have some crystals within it and yet remain predominantly liquid. The sequence in which minerals crystallize from a magma as it cools is known as Bowen's reaction series (Figure 7.7). [caption id="attachment_232" align="aligncenter" width="650"] Figure 7.7 Bowen's reaction series describe the sequence in which minerals form as magma cools. Source: Steven Earle (2016), CC BY 4.0. Image source.[/caption]

How Did We Get Bowen's Reaction Series?

Understanding how the reaction series was derived is key to understanding what it means. [caption id="attachment_233" align="alignleft" width="236"] Figure 7.8 Norman Bowen in his laboratory. Source: University of Chicago Photographic Archive, apf1-00841, Special Collections Research Center, University of Chicago Library. Click for source and terms of use.[/caption] Norman Levi Bowen (Figure 7.8) was born in Kingston Ontario. He studied geology at Queen’s University and then at Massachusetts Institute of Technology. In 1912 he joined the Carnegie Institution in Washington, D.C., where he carried out ground-breaking experiments into how magma cools. Working mostly with mafic magmas, he determined the order of crystallization of minerals as temperature drops. First, he melted the rock completely in a specially-made kiln. Then he allowed it to cool slowly to a specific temperature before quenching (cooling it quickly) so that no new minerals could form. The rocks that formed were studied under the microscope and analyzed chemically. This was done over and over, each time allowing the magma to cool to a lower temperature before quenching. The result of these experiments was the reaction series that—even a century later—is still an important basis for our understanding of igneous rocks.

Discontinuous and Continuous Series

Bowen’s reaction series (Figure 7.7) has two pathways for minerals to form as magma cools: on the left is the discontinuous series. This refers to the fact that one mineral is transformed into a different mineral through chemical reactions. On the right is the continuous series, where plagioclase feldspar goes from being rich in calcium to being rich in sodium.

Continuous Series

At about the point where pyroxene begins to crystallize, plagioclase feldspar also begins to crystallize. At that temperature, the plagioclase is calcium-rich (toward the anorthite end-member). As the temperature drops—and providing that there's sodium left in the magma—the plagioclase that forms is a more sodium-rich variety (toward the albite end-member). The series is continuous because the mineral is always plagioclase feldspar, but the series involves a transition from calcium-rich to sodium-rich. When cooling happens relatively quickly, instead of getting crystals of uniform composition, individual plagioclase crystals can be zoned. This means they have concentric layers that go from calcium-rich in the centre to more sodium-rich around the outside (Figure 7.9). This occurs because calcium-rich early-forming plagioclase crystals become coated with progressively more sodium-rich plagioclase as the magma cools. [caption id="attachment_234" align="aligncenter" width="441"] Figure 7.9 Plagioclase crystal exhibiting compositional zones. Source: Akademia Górniczo-Hutnicza w Krakowie Otwartych Zasobów Edukacyjnych (n.d.), CC BY-NC-SA.[/caption]

Discontinuous Series

Olivine begins to form at just below 1300°C, but as the temperature drops, olivine becomes unstable. The early-forming olivine crystals react with silica in the remaining liquid and are converted into pyroxene, something like this: As long as silica remains and cooling is slow, this process continues down the discontinuous branch: olivine reacts to form pyroxene, and pyroxene reacts to form amphibole. Under the right conditions amphibole will form to biotite. Finally, if the magma is quite silica-rich to begin with, there will still be some silica left at around 750 °C to 800 °C. From this last magma, potassium feldspar, quartz, and possibly muscovite mica will form. Notice that the sequence of minerals that form goes from isolated tetrahedra (olivine) toward increasingly complex arrangements of silica tetrahedra. Pyroxene consists of single chains, amphibole has double chains, mica has sheets of tetrahedra, and potassium feldspar and quartz at the bottom of the series have tetrahedra connected to each other in three dimensions. If the magma cools enough, the first minerals to form will be completely used up in later chemical reactions. This is why igneous rocks don't normally have both olivine (at the top of the series) and quartz (at the bottom). Exceptions can occur when rocks that crystallized early in the series come into contact with magmas representing compositions later in the series, such as with the dark green olivine-rich xenoliths included within the quartz- and feldspar-rich rock in Figure 7.10. The dark line around the xenoliths is amphibole, which formed as the olivine reacted with the melt. In some of the smaller xenoliths within this boulder, the olivine has been completely transformed into amphibole. [caption id="attachment_236" align="aligncenter" width="504"] Figure 7.10 Boulder with olivine-rich xenoliths surrounded by silica-rich rock. Black rims on the xenoliths are where the olivine has reacted with the silica-rich melt, forming amphibole. Right- Enlarged view of the amphibole reaction rim. Source: Karla Panchuk (2018), CC BY 4.0.[/caption]

The Original Magma Composition Matters

The composition of the original magma determines how far the reaction process can continue before all of the magma is used up. In other words, it determines which minerals will form. In mafic magmas, some of the silica combines with iron and magnesium to make olivine. As it cools further, much of the remaining silica goes into calcium-rich plagioclase, and any silica left may be used to convert some of the olivine to pyroxene. Soon after that, all of the magma is used up and no further changes takes place. The minerals present will be olivine, pyroxene, and calcium-rich plagioclase. Felsic magmas tend to be cooler than mafic magmas when crystallization begins, because they don’t have to be as hot to remain liquid. They may start out crystallizing pyroxene (not olivine) and plagioclase. As cooling continues, the various reactions on the discontinuous branch will proceed because silica is abundant, and eventually potassium feldspar and quartz will form. Tthe plagioclase will become increasingly sodium-rich. Even very felsic rocks may not have biotite or muscovite if they don't have enough aluminum or hydrogen to make the OH complexes that are necessary for mica minerals.

Practice with Bowen's Reaction Series [h5p id="73"]

Fractional Crystallization Also Makes Magma Richer In Silica

A magma chamber is a space within the Earth that's filled with molten rock. If the magma in a magma chamber has a low viscosity—meaning it flows easily, which is likely if the magma is mafic—the crystals that form early, such as olivine (Figure 7.11a), may slowly settle toward the bottom of the magma chamber (Figure 7.11b). This process is called fractional crystallization. [caption id="attachment_237" align="aligncenter" width="1024"] Figure 7.11 Formation of a zoned magma chamber. a- Olivine crystals form. b- Olivine crystals settle to the base of the magma chamber, leaving the upper part of the chamber richer in silica. c- Olivine crystals remelt, making magma at the base of the chamber more mafic. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] When olivine forms, it takes iron- and magnesium-rich components out of the magma, leaving the overall composition of the magma near the top of the magma chamber more felsic. The crystals that settle may form an olivine-rich layer near the bottom of the magma chamber. Alternatively, because the lower part of the magma chamber is likely to be hotter than the upper part, the crystals might remelt. If remelting happens, the magma at the bottom of the chamber will have the iron- and magnesium-rich components added back, making it more mafic than it was to begin with (Figure 7.11c).

Mixing In Other Material

Magma chambers aren't isolated from their surroundings. If the rock in which the magma chamber is located (called the country rock) is more felsic than the magma, the country rock may also melt, adding to the magma already in the magma chamber (Figure 7.12). Sometimes magma carries fragments of unmelted rock, called xenoliths, within it. Melting of xenoliths can also alter the composition of magma, as can re-melting of crystals that have settled out of the magma. [caption id="attachment_238" align="aligncenter" width="459"] Figure 7.12 The composition of magma in a magma chamber is affected by fractional crystallization within the magma chamber, but it can also be affected by partial melting of the rock surrounding the magma chamber, melting of xenoliths within the magma, or re-melting of crystals that have settled to the bottom of the magma chamber. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

7.3 Classification of Igneous Rocks

kqzheng — Tue, 05 Jun 2018 18:07:29 +0000

Igneous rocks are classified (named) based on two sets of characteristics:

The minerals they contain
Their grain size and texture

Classification By Mineral Abundance

Igneous rocks can be divided into four categories based on their chemical composition: felsic, intermediate, mafic, and ultramafic. The diagram of Bowen's reaction series (Figure 7.7) shows that differences in chemical composition correspond to differences in the types of minerals within an igneous rock. Igneous rock names are based in part on the minerals that igneous rocks contain, and on how abundant they are in a rock. Figure 7.13 is a diagram with the minerals from Bowen's reaction series, and it's used to decide which name to give an igneous rock. First, notice that the diagram has rows with different kinds of information, but is also organized in columns according to the four compositional categories. The top box shows the range of mineral proportions for each compositional category. An igneous rock can be represented as a vertical line drawn through the top box of the diagram, and the vertical scale—with the distance between each tick mark representing 10% of the minerals within a rock by volume—is used to break down the proportion of each mineral it contains. Consider the arrows in the mafic field of the diagram. They represent a rock containing 60% pyroxene and 40% pyroxene. An igneous rock at the boundary between the mafic and ultramafic fields (marked with a vertical dashed line) would have approximately 20% olivine, 60% pyroxene, and 20% Ca-rich plagioclase feldspar by volume. [caption id="attachment_241" align="aligncenter" width="1024"] Figure 7.13 Classification diagram for igneous rocks. Igneous rocks are classified according to the relative abundances of minerals they contain. A given rock is represented by a vertical line in the diagram. In the mafic field, the arrows represent a rock containing 60% pyroxene and 40% olivine. The name an igneous rock gets depends not only on composition, but on whether it is intrusive or extrusive. Source: Karla Panchuk (2018), CC BY-NC-SA 4.0. Modified after Steven Earle (2015), CC BY 4.0. and others. Click for attributions and more information.[/caption]

Classification By Grain Size

The lower two boxes of the diagram contain igneous rock names, and you'll notice that there are two igneous rock names for each compositional category. So which name do you use? The name to choose depends on whether the igneous rock cooled within Earth (whether it's an intrusive or plutonic igneous rock), or whether it cooled on the Earth's surface after erupting from a volcano (making it an extrusive or volcanic igneous rock). What this means is that two igneous rocks comprised of exactly the same minerals, and in the exactly the same proportions, can have different names. A felsic intrusive rock is called granite, whereas a felsic extrusive rock is called rhyolite. Granite and rhyolite have the same mineral composition, but their grain size gives each a distinct appearance. A rock of intermediate composition is diorite if it's course-grained, and andesite if it's fine-grained. A mafic rock is gabbro if it's course-grained, and basalt if fine-grained. The course-grained version of an ultramafic rock is peridotite, and the fine-grained version is komatiite. It makes sense to use different names because rocks of different grain sizes form in different ways and in different geological settings.

Try It Out!

An igneous rock is shown below, along with the abundances of minerals within it. Answer the questions to figure out what kind of rock it is.

This rock has quartz in it. Based on Bowen's reaction series only, which two compositional categories can you rule out for sure?
1. Felsic
2. Intermediate
3. Mafic
4. Ultramafic
Next, look at the mineral abundances more carefully. Is this rock felsic or intermediate? Hint: When you look at the compositional diagram, what do you notice about the type and abundance of feldspar in these categories?
Is this rock intrusive or extrusive? Hint: can you see individual crystals?
Now put it all together. Which of these is an intermediate intrusive igneous rock?
1. Andesite
2. Rhyolite
3. Diorite
4. Granite

To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="74"]

What Determines Grain Size?

The key difference between intrusive and extrusive igneous rocks—the size of crystals making them up—is related to how rapidly melted rock cools. The longer melted rock has to cool, the larger the crystals within it can become. Magma cools much slower within Earth than on Earth's surface because magma within Earth is insulated by surrounding rock. Notice that in Figure 7.13, the intrusive rocks have crystals large enough that you can see individual crystals—either by identifying their boundaries, or seeing light reflecting from a crystal face. A rock with individual crystals that are visible to the unaided eye has a phaneritic or coarse-grained texture. The extrusive rocks have much smaller crystals. The crystals are so small that the bulk of individual crystals cannot be distinguished, and the rock may look like a dull mass. A rock with crystals that are too small to see with the unaided eye has an aphanitic or fine-grained texture. Table 7.1 summarizes the key differences between intrusive and extrusive igneous rocks.

Table 7.1 Comparison of Intrusive and Extrusive Igneous Rocks
	Magma cools within Earth	Lava cools on Earth's surface
Terminology	Intrusive/ plutonic	Extrusive/ volcanic
Cooling rate	Slow: surrounding rocks insulate the magma chamber.	Rapid: heat is exchanged with the atmosphere.
Texture	Phaneritic (coarse-grained): individual crystals are large enough to see without magnification.	Aphanitic (fine-grained): crystals are too small to see without magnification.

Practice with Igneous Rock Names [h5p id="7"]

Does This Mean an Igneous Rock Can Only Have One Grain Size?

No. Something interesting happens when there is a change in the rate at which melted rock is cooling. If magma is cooling in a magma chamber, some minerals will begin to crystallize before others do. If cooling is slow enough, those crystals can become quite large. Now imagine the magma is suddenly heaved out of the magma chamber and erupted from a volcano. The larger crystals will flow out with the lava. The lava will then cool rapidly, and the larger crystals will be surrounded by much smaller ones. An igneous rock with crystals of distinctly different size (Figure 7.14) is said to have a porphyritic texture, or might be referred to as a porphyry. The larger crystals are called phenocrysts, and the smaller ones are referred to as the groundmass. [caption id="attachment_242" align="aligncenter" width="531"] Figure 7.14 Porphyritic rhyolite with quartz and potassium feldspar phenocrysts within a dark groundmass. Porphyritic texture (when different crystal sizes are present) is an indication that melted rock did not cool at a constant rate. Source: Karla Panchuk (2018), CC BY-NC-SA 4.0. Photo by R. Weller/Cochise College (2011). Image source.[/caption]

Which Phenocrysts Will Form?

As a magma cools below 1300°C, minerals start to crystallize within it. If the magma is then erupted, the rest of the liquid will cool quickly to form a porphyritic texture: the rock will have some relatively large crystals (phenocrysts) of the minerals that crystallized early, and the rest will be very fine-grained or even glassy.Refer to the diagram shown here, and use the checklists that follows to predict which phenocrysts might be present if the magma cooled as far as Line a before erupting, and which would be present if the magma cooled to Line b before escaping.

Checklist 1. Which phenocrysts might be present if magma cools as far as Line a? (Select all that apply.)
- Olivine
- Pyroxene
- Amphibole
- Biotite
- Potassium feldspar
- Muscovite
- Quartz
- Plagioclase feldspar
Checklist 2. Which phenocrysts might be present if magma cools as far as Line b? (Select all that apply.)
- Olivine
- Pyroxene
- Amphibole
- Biotite
- Potassium feldspar
- Muscovite
- Quartz
- Plagioclase feldspar

To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="75"]

Classifying Igneous Rocks According to the Proportion of Dark Minerals

If you're unsure of which minerals are present in an intrusive igneous rock, there's is a quick way to approximate the composition of that rock. In general, igneous rocks have an increasing proportion of dark minerals as they become more mafic (Figure 7.16). [caption id="attachment_243" align="aligncenter" width="628"] Figure 7.16 Simplified igneous rock classification according to the proportion of light and dark (or ferromagnesian) minerals. Source: Karla Panchuk (2018), CC BY 4.0. Modified after Steven Earle (2015), CC BY 4.0. Image source.[/caption] The dark-coloured minerals are those higher in iron and magnesium (e.g., olivine, pyroxene, amphibole, biotite), and for that reason they're sometimes referred to collectively as ferromagnesian minerals. By estimating the proportion of light minerals to dark minerals in a sample, it is possible to place that sample in Figure 7.16. Graphical scales are used to help visualize the proportions of light and dark minerals (Figure 7.17). [caption id="attachment_244" align="aligncenter" width="429"] Figure 7.17 A guide for estimating the proportion of dark minerals in an igneous rock. Source: Karla Panchuk (2018), CC BY 4.0. Modified after Steven Earle (2015), CC BY 4.0. Image source.[/caption] It's important to note that estimating the proportion of dark minerals is only approximate as a means for identifying igneous rocks. One problem is that plagioclase feldspar is light-coloured when it's sodium-rich, but can appear darker if it's calcium-rich. Plagioclase feldspar is not ferromagnesian, so it falls in the non-ferromagnesian (light minerals) region in Figure 7.16 even when it's dark.

Try It Out! [h5p id="76"]

Classifying Igneous Rocks When Individual Crystals Are Not Visible

The method of estimating the percentage of minerals works well for phaneritic igneous rocks, in which individual crystals are visible with little to no magnification. If an igneous rock is porphyritic but otherwise aphanitic (e.g., Figure 7.14), the minerals present as phenocrysts give clues to the identity of the rock. However, there are cases where mineral composition can't be determined by looking at visible crystals. These include volcanic rocks without phenocrysts, and glassy igneous rocks.

Volcanic Rocks Without Phenocrysts

In the absence of visible crystals or phenocrysts, volcanic rocks are be classified on the basis of colour and other textural features. As you may have noticed in Figure 7.13, the colour of volcanic rocks goes from light to dark as the composition goes from felsic to mafic. Rhyolite is often a tan or pinkish colour, andesite is often grey, and basalt ranges from brown to dark green to black (Figure 7.19). [caption id="attachment_245" align="aligncenter" width="551"] Figure 7.19 In volcanic igneous rocks, individual crystals are not visible. Colours change from light to dark as the composition of the rocks go from felsic to mafic. Vesicles and amygdules are common characteristics of basalt. Source: Karla Panchuk (2018), CC BY-NC-SA 4.0. Click for more attributions.[/caption] Basalt often shows textural features related to lava freezing around gas bubbles. When magma is underground, pressure keeps gases dissolved, but once magma has erupted, the pressure is much lower. Gases dissolved in the lava are released, and bubbles can develop. When lava freezes around the bubbles, vesicles are formed (circular inset in 7.19). If the vesicles are later filled by other minerals, the filled vesicles are called amygdules (box inset in Figure 7.19).

Glassy Volcanic Rocks

Crystal size depends on cooling rate. The faster magma or lava cools, the smaller the crystals it contains. It's possible for lava to cool so rapidly that no crystals can form. The result is volcanic glass. Volcanic glass can be smooth like obsidian or vesicular like scoria (mafic) and pumice (felsic; Figure 7.20). Pumice can float on water because of its low-density felsic composition and enclosed vesicles. [caption id="attachment_246" align="aligncenter" width="715"] Figure 7.20 Glassy volcanic rocks. Obsidian has a glassy lustre, but scoria and pumice are highly vesicular. Source: Karla Panchuk (2018), CC BY-NC-SA 4.0. Click for more attributions.[/caption]

Practice with Textures [h5p id="4"] How did you do with those flashcards? If you're confident you know those terms, give this next exercise a try.

This is a photo of basalt. It has olivine phenocrysts (pale green crystals marked with arrows) and lots of holes. Which terms on the checklist apply to the sample?

Coarse-grained
Vesicular
Porphyritic
Amygdaloidal
Aphanitic
Phaneritic
Fine-grained

To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="77"]

7.4 Intrusive Igneous Rocks

kqzheng — Wed, 06 Jun 2018 06:38:21 +0000

In most cases, a body of hot magma is less dense than the rock surrounding it, so it has a tendency to creep upward toward the surface. It does so in a few different ways:

Filling and widening existing cracks
Melting the surrounding rock
Breaking the rock
Pushing the rock aside (where the rock is hot enough and under enough pressure to deform without breaking)

When magma forces itself into cracks, breaks off pieces of rock, and then envelops them, this is called stoping. The resulting fragments are xenoliths[footnote]From the Greek words xenos, meaning "foreigner" or "stranger," and lithos for "stone."[/footnote]. Xenoliths may appear as dark patches within a rock (Figure 7.21). [caption id="attachment_249" align="aligncenter" width="500"] Figure 7.21 Xenoliths of mafic rock in granite, Victoria, B.C. The fragments of dark rock have been broken off and incorporated into the light-coloured granite. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] Some of the magma may reach the surface, resulting in volcanic eruptions, but most cools within the crust. The resulting body of rock is called a pluton.[footnote]After Pluto was demoted from planet status, astronomers tried to come up with a name for objects like Pluto. For a while they considered "pluton" however geologists rightly objected that they had first claim on the word. In the end the International Astronomical Union settled on "dwarf planet" instead.[/footnote] Plutons can have different shapes and different relationships with the surrounding country rock (Figure 7.22). These characteristics determine what name the pluton is given. Large, irregularly shaped plutons are called stocks or batholiths, depending upon their size. Tabular plutons are called dikes if they cut across existing structures, and sills if they are parallel to existing structures. Laccoliths are like sills, except they have caused the overlying rocks to bulge upward. Pipes are cylindrical conduits. [caption id="attachment_250" align="aligncenter" width="629"] Figure 7.22 Plutons can have a variety of shapes, and be positioned in a variety of ways relative to the surrounding rocks. They are named according to these characteristics. Source: Karla Panchuk (2018), CC BY 4.0.[/caption]

Types of Plutons

Stocks and Batholiths

Large irregular-shaped plutons are called either stocks or batholiths, depending on their area. If an irregularly shaped body has an area greater than 100 km², then it’s a batholith, otherwise it's a stock. Note that our knowledge of the size of a body can be limited to what we see at the surface. A body with an area of less than 100 km² exposed at the surface might in fact be much larger at depth. It might be classified as a stock initially, until someone is able to map out its true extent. Batholiths are typically formed when a number of stocks coalesce beneath the surface to create one large body. One of the largest batholiths in the world is the Coast Range Plutonic Complex (also referred to as the Coast Range Batholith), which extends all the way from the Vancouver region to southeastern Alaska (Figure 7.23). [caption id="attachment_251" align="aligncenter" width="328"] Figure 7.23 The Coast Range Plutonic Complex (also called the Coast Range Batholith) is the largest in the world. It is part of a chain of batholiths along the western coast of North America. Source: Karla Panchuk (2018), CC BY 4.0. Modified after Bally (1989).[/caption]

Tabular Intrusions

Tabular (sheet-like) plutons are classified according to whether or not they are concordant with (parallel to) existing layering (e.g., sedimentary bedding or metamorphic foliation[footnote]Sedimentary bedding refers to the layers in which sedimentary rocks form. Metamorphic foliation refers to the way minerals or other elements in a rock are aligned as a result of being deformed by heat and pressure. Bedding and foliation will be discussed in more detail in later chapters.[/footnote]) in the country rock. A sill is concordant with existing layering, and a dike is discordant. If the country rock has no bedding or foliation, then any tabular body within it is a dike. Note that the sill-versus-dike designation is not determined simply by the orientation of the feature: a dike could be horizontal and a sill could be vertical. It all depends on the orientation of features in the surrounding rocks. A laccolith is a sill-like body that has expanded upward by deforming the overlying rock. If a sill forms, but magma pools and sags downward, it creates a lopolith.

Pipes

A pipe, as the name suggests, is a cylindrical body with a circular, elliptical, or even irregular cross-section, that serves as a conduit (or pipeline) for the movement of magma from one location to another. Pipes may feed volcanoes, but pipes can also connect plutons.

Chilled Margins

Magma can alter the country rock around it, and the reverse is also true. The most obvious effect that country rock can have on magma is a chilled margin along the edges of the pluton (Figure 7.24). The country rock is much cooler than the magma, so magma that comes into contact with the country rock cools faster than magma toward the interior of the pluton. Rapid cooling leads to smaller crystals, so the texture along the edges of the pluton is different from that of the interior of the pluton, and the colour may be darker. [caption id="attachment_252" align="aligncenter" width="400"] Figure 7.24 A mafic dike with chilled margins within basalt at Nanoose, B.C. The coin is 24 mm in diameter. The dike is about 25 cm across and the chilled margins are 2 cm wide. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Practice with Plutons [h5p id="78"] Now that you've practiced on a diagram, try to identify plutons in their natural habitats. [h5p id="79"]

References

Bally, A. W. (1989). Plate 10. Selected distribution maps, rate of accumulation maps, and lithofacies maps—Phanerozoic, North America. In A. W. Bally & A. R. Palmer (Eds.), The Geology of North America—An Overview: Volume A. Boulder: Geological Society of America.

Chapter 7 Summary & Key Term Check

kqzheng — Fri, 08 Jun 2018 18:39:58 +0000

Chapter 7 Main Ideas

7.1 Magma and How It Forms

Magma is molten rock, and in most cases, it forms from partial melting of existing rock. The chemistry of magma depends on the original rock that's melting, as well as how much partial melting happens. Magma forms by decompression melting, flux-induced melting (fluid-induced melting), and conduction.

Practice Again

7.2 Crystallization of Magma

Magmas range in composition from ultramafic to felsic. Mafic rocks are rich in iron, magnesium, and calcium, and contain approximately 50% silica. Felsic rocks are richer in silica (~70%) and have lower levels of iron, magnesium, and calcium, and higher levels of sodium and potassium than mafic rocks. Bowen's reaction series allows us to predict the order of crystallization of magma as it cools. Magma can be modified by fractional crystallization (separation of early-forming crystals), by mixing in material from the surrounding rocks by partial melting, and by mixing with magmas of differing chemistry.

Practice Again

Extra!

Fill in the blanks.

1. What's the difference between the two branches of Bowen's reaction series? On the (continuous or discontinuous) branch, different minerals form through a sequence of chemical reactions. For example, the first mineral to form as ultramafic magma cools is (this mineral is green). That mineral then reacts with the melt to make the next mineral, (pyroxene, amphibole, or biotite?). In contrast, on the (continuous or discontinuous) branch, the mineral is always (potassium feldspar or plagioclase feldspar?) but its composition varies.
2. How does Bowen's reaction series explain zoned plagioclase feldspar crystals? Plagioclase that is (calcium right or sodium rich) forms early on in the cooling process of a magma, but as the temperature drops, a more (calcium right or sodium rich) variety forms around the existing crystals.
3. How does fractional crystallization change the composition of a magma chamber? In a mafic magma chamber where the mineral (olivine or quartz) forms first, crystals will sink to the bottom of the magma chamber. The magma that's left behind will be more (mafic, intermediate, or felsic) in composition. At the bottom of the chamber, if the crystals melt again, that magma will be more (mafic, intermediate, or felsic) in composition.

To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="80"]

7.3 Classification of Igneous Rocks

Igneous rocks are classified based on their mineral composition and texture. Felsic igneous rocks have less than 20% dark minerals (ferromagnesian silicates including amphibole and/or biotite) with varying amounts of quartz, both potassium and plagioclase feldspars, and sometimes muscovite. Mafic igneous rocks have more than 50% dark minerals (primarily pyroxene) plus plagioclase feldspar. Most intrusive igneous rocks are phaneritic (individual crystals are visible unmagnified). If there were two stages of cooling (slow then fast), the texture may be porphyritic (large crystals in a matrix of smaller crystals).

Practice Again

7.4 Intrusive Igneous Bodies

Magma intrudes into country rock by pushing it aside or melting through it. Intrusive igneous bodies tend to be irregular (stocks and batholiths), tabular (dikes and sills), or pipe-like. Batholiths have areas of 100 km² or greater, while stocks are smaller. Sills are parallel to existing layering in the country rock, while dikes cut across layering. A pluton that intruded into cold rock is likely to have a chilled margin.

Practice Your Plutons Again

Key Term Check

What key term from Chapter 7 is each card describing? Turn the card to check your answer. [h5p id="81"]

Igneous Rock Name Check

Which of the igneous rocks from Chapter 7 is each card describing? Turn the card to check your answer. [h5p id="82"]

8.1 Physical Weathering

kqzheng — Thu, 26 Jul 2018 23:21:48 +0000

Intrusive igneous rocks form at depths of 100s of metres to 10s of kilometres. Most metamorphic rocks are formed at depths of kilometres to 10s of kilometres. Sediments are turned into sedimentary rocks only when they're buried by other sediments to depths in excess of several 100s of metres. Weathering can't happen until these rocks are revealed at Earth’s surface by uplift and the erosion of overlying materials. Once the rock is exposed at the surface as an outcrop, weathering begins. The agents of physical weathering can be broadly classified into two groups: those that cause the outer layers of a rock to expand, and those that act like wedges to force the rock apart.

Physical Weathering By Expansion

Some processes at Earth's surface can cause a thin outer layer of a rock to expand. Deeper than the thin outer layer, the rock does not expand. The difference is accommodated by a crack developing between the outer and inner layers, breaking the outer layer off in slabs (Figures 8.2 and 8.3). When layers break off a rock in slabs or sheets, it is referred to as exfoliation. Granite tends to exfoliate parallel to the exposed surface because it doesn't have pre-existing planes of weakness to determine how it breaks. In contrast, sedimentary rocks tend to exfoliate along the contacts between different sedimentary layers, and metamorphic rocks tend to exfoliate parallel to aligned minerals. [caption id="attachment_259" align="aligncenter" width="609"] Figure 8.2 Close-up view of exfoliation of a granite dome in the Enchanted Rock State Natural Area, Texas, USA. Source: Wing-Chi Poon (2005), CC BY-SA 2.5. Image source.[/caption] [caption id="attachment_260" align="aligncenter" width="613"] Figure 8.3 View of exfoliation at a distance (centre of image) in granite exposed on the west side of the Coquihalla Highway north of Hope, B.C. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Why Do Rocks Expand?

A rock within the Earth has pressure exerted upon it by other rocks sitting above it. This is called confining pressure. When the overlying mass is removed by weathering, the confining pressure decreases, allowing the rock to expand. The cracking that results is sometimes called pressure-release cracking. Heating a rock can also cause it to expand. If the rock is heated rapidly, as during a wildfire, cracks can form. If it goes through large daily temperature swings (e.g., in the desert where it's very hot during the day but cold at night), cracking can also eventually result as the rock is weakened.

Physical Weathering by Wedging

In wedging, a pre-existing crack in a rock is made larger by forcing it open.

Frost Wedging

Frost wedging (or ice wedging) happens when water seeps into cracks, then expands upon freezing. The expansion enlarges the cracks (Figure 8.4). The effectiveness of frost wedging depends on how often freezing and thawing occur. Frost wedging won’t be as important in warm areas where freezing is infrequent, in very cold areas where thawing is infrequent, or in very dry areas, where there is little water to seep into cracks. [caption id="attachment_261" align="aligncenter" width="640"] Figure 8.4 A rock broken by ice wedging sits in a stream in Mount Revelstoke National Park, Canada. Rocks break apart when ice expands in pre-existing cracks. Source: Karla Panchuk (2018) CC BY 4.0.[/caption] Frost wedging is most effective in climates where for at least part of the year, temperatures swing between warm and freezing. In many parts of Canada, the temperature swings between freezing at night and thawing in the day tens to hundreds of times a year. Even in warm coastal areas of southern British Columbia, freezing and thawing transitions are common at higher elevations. A common feature in areas where frost wedging is important is a talus slope. It's a fan-shaped deposit of fragments that have been popped off of rocks above, and piled up on steep slopes below (Figure 8.5). [caption id="attachment_262" align="aligncenter" width="600"] Figure 8.5 An area with very effective frost wedging near Keremeos, BC. The fragments that were wedged away from the cliffs above have accumulated in a talus deposit at the base of the slope. The rocks in this area are variable in colour, which is reflected in the colours of the talus. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Salt Wedging

Salt wedging happens when saltwater seeps into rocks and then evaporates on a hot sunny day. Salt crystals grow within cracks and pores in the rock, and the growth of these crystals can push grains apart, causing the rock to weaken and break. There are many examples of this on the rocky shorelines of Vancouver Island and the Gulf Islands, where sandstone outcrops are common and salty seawater is readily available (Figure 8.6). The honeycomb structure of rounded holes, called tafoni, is related to the original roughness of the surface. Low spots collect salt water, causing the effect to be accentuated around existing holes. [caption id="attachment_263" align="aligncenter" width="600"] Figure 8.6 Tafoni (Honeycomb weathering) in sandstone on Gabriola Island, British Columbia. The holes are caused by crystallization of salt within rock pores. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Plant and Animal Activity

The effects of plants are significant in mechanical weathering. Roots can force their way into even the tiniest cracks. They exert tremendous pressure on the rocks as they grow, widening the cracks and breaking the rock. This is called root wedging (Figure 8.7). [caption id="attachment_264" align="aligncenter" width="601"] Figure 8.7 Root wedging along a quarry wall. Left: Rocks beneath the thick red beds have been split into sheets by tree roots. Right: A closer examination reveals that tree roots are working into vertical cracks as well. Source: Karla Panchuk (2018), CC BY 4.0.[/caption] Animals can excavate and remove huge volumes of soil, and thus expose the rock to weathering by other mechanisms. Humans modify vast tracts of land by excavation, and have a profound effect on accelerating physical weathering.

Practice with Types of Physical Weathering

Fill in the missing words in these definitions. is a physical weathering process where (hint: expansion or shrinking) of rock causes outer layers to break off. It is sometimes caused by (hint: increasing or decreasing) the pressure on a rock. Sometimes it's because of rapid changes in (hint: what happens in the desert?). is a physical weathering process where existing cracks in rock are forced wider. Trees can do this with their . It can also happen when crystals of ice or form (hint: derived from evaporating sea water). To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="8"] Now that you're warmed up, try this:

Here are three photographs of weathering taking place at Stawamus Chief Provincial Park in BC. Insert the correct term in each blank.

Name the physical weathering. (Hint: When this process is complete, how will this rock come apart?) Need help finding the weathering? See below:
These trees are growing in cracks along the cliff face. This type of physical weathering is . (Hint: There are different kinds of this type of weathering. Make sure you're specific.)
Slabs are coming off of this cliff without the help of trees. What kind of physical weathering is responsible? (Hint: Where does the water from precipitation go?) Need a closer look? See below:

To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="9"]

Erosion

Physical weathering is greatly facilitated by erosion. Erosion is the removal of weathering products, such as fragments of rock. This exposes more rock to weathering, accelerating the process. A good example of weathering and erosion working together is the talus shown in Figure 8.5. The rock fragments forming the talus piles were broken off the steep rock faces at the top of the cliff by ice wedging, and then removed by gravity. Gravity doesn't always work alone to remove weathering products. Other agents of erosion include water in streams, ice in glaciers, and waves on coasts.

Concept Check

What's the difference between erosion and weathering? breaks down rocks. removes the broken pieces. Fill-in-the-blank options:

Weathering
Erosion

To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="84"]

8.2 Chemical Weathering

kqzheng — Fri, 27 Jul 2018 01:54:14 +0000

Chemical weathering results from chemical changes to minerals that become unstable when they are exposed to surface conditions. The kinds of changes that take place are specific to the mineral and the environmental conditions. Some minerals, like quartz, are virtually unaffected by chemical weathering. Others, like feldspar, are easily altered.

Types of Chemical Weathering Reactions

Dissolution

Dissolution reactions produce ions, but no minerals, and are reversible if the solvent is removed. A household example would be dissolving a teaspoon of table salt (the mineral halite) in a glass of water. The halite will separate into Na⁺ and Cl^- ions. If the water in the glass is allowed to evaporate, eventually there won't be enough water molecules to hold the Na⁺ and Cl^- ions apart, and the ions will come together again to form halite. Gypsum and anhydrite are other minerals that will dissolve in water alone. (So don't rinse off the halite, gypsum, or anhydrite samples in your mineral collection.) Other minerals, such as calcite, will dissolve in acidic water. Acidic water is common in nature, because carbon dioxide (CO₂) in the atmosphere reacts with water vapour in the atmosphere, and with water on land and in the oceans to produce carbonic acid (Figure 8.9). [caption id="attachment_267" align="aligncenter" width="650"] Figure 8.9 Calcite weathering by dissolution. Top: Carbon dioxide reacts with water to make acid. Bottom: Acid reacts with calcite and produces ions. Source: Karla Panchuk (2018), CC BY-NC-SA 4.0. Modified after What-When-How. Molecules from JMSE Molecular Editor, Bienfait and Ertl (2013), with permission for CC BY-NC-SA use.[/caption] While rainwater and atmospheric CO₂ can combine to create carbonic acid, there's only enough CO₂ in the air to make very weak carbonic acid. In contrast, biological processes acting in soil can produce a much higher CO₂ concentration within the soil, as well as adding organic acids. Any water percolating through the soil can become significantly more acidic.

Dissolution and Sinkholes

Calcite is a major component (typically more than 95%) of the sedimentary rock called limestone. Acidic groundwater will dissolve limestone, and can eventually remove enough calcite to form caves. If dissolution of limestone or other materials removes enough rock to undermine support near the surface, the surface may collapse, creating a sinkhole such as the one in Figure 8.10, downstream of the Mosul Dam in Iraq. [caption id="attachment_268" align="aligncenter" width="503"] Figure 8.10 Sinkhole downstream of the Mosul Dam in Iraq. The sinkhole is a result of dissolution of gypsum and anhydrite layers. Source: U. S. Army Corps of Engineers (2007), Public Domain. Image source.[/caption] Although the sinkhole might appear minor, it indicates a serious problem for the dam. The dam itself is constructed on limestone supported by beds of gypsum and anhydrite. Gypsum and anhydrite are soluble in water, and the gypsum and anhydrite beneath the dam are rapidly dissolving away. This was the case prior to construction of the dam, but the dam was filled, the problem became even worse. The increased water pressure began to force water through the formations much faster, accelerating dissolution. Ongoing measures to fill gaps with grout are required, or else there is a grave risk of catastrophic failure, placing nearly 1.5 million people at risk.

Hydrolysis

The term hydrolysis combines the prefix hydro, referring to water, with lysis, which is derived from a Greek word meaning to loosen or dissolve. Thus, you can think of hydrolysis as a chemical reaction where water loosens the chemical bonds within a mineral. This might sound the same as dissolution but the difference is that hydrolysis produces a different mineral in addition to ions. An example of hydrolysis is when water reacts with potassium feldspar to produce clay minerals and ions. The results can be seen by comparing weathered and unweathered surfaces of the same sample of granite (Figure 8.11). On the recently broken unweathered surface (Figure 8.11, left) feldspar is visible as bright white crystals. On a weathered surface (right) the feldspar has been altered to the chalky-looking clay mineral kaolinite. [caption id="attachment_269" align="aligncenter" width="578"] Figure 8.11 A piece of granite with unweathered (left) and weathered (right) surfaces. On the unweathered surfaces the feldspars are still fresh and glassy looking. On the weathered surface there are chalky white patches where feldspar has been altered to the clay mineral kaolinite. Source: Karla Panchuk (2018), CC BY 4.0. Photos by Steven Earle (2015), CC-BY 4.0. Image source.[/caption] Silicate minerals other than feldspar can undergo hydrolysis, but with different end results. Pyroxene can be converted to the clay minerals chlorite or smectite. Olivine can be converted to the clay mineral serpentine.

Hydration

Hydration reactions involve water being added to the chemical structure of a mineral. An example of a hydration reaction is when anhydrite (CaSO₄) is transformed into gypsum (CaSO₄·2H₂O). A consequence of hydration is that the resulting mineral has a greater volume than the original mineral. In the case of the Mosul Dam, hydration of anhydrite has important consequences. The increase in volume applied force to an overlying limestone layer, breaking it into pieces. While unbroken limestone is a strong enough material upon which to build a foundation, broken limestone is too weak to provide a safe foundation.

Oxidation

Oxidation happens when free oxygen (i.e., oxygen not bound up in molecules with other elements) is involved in chemical reactions. Oxidation reactions provide valuable insight into Earth’s early surface conditions because there's a clear transition in the rock record from rocks containing no minerals that are products of oxidation reactions, to rocks containing abundant minerals produced by oxidation. This reflects a transition from an oxygen-free atmosphere to an oxygenated one. In iron-rich minerals such as olivine, the oxidation reaction begins with taking iron out of the mineral and putting it into solution as an ion. Olivine reacts with carbonic acid, leaving dissolved iron, bicarbonate, and silicic acid:

Fe₂SiO₄ + 4H₂CO₃→ 2Fe^₂+ + 4HCO₃^- + H₄SiO₄

Iron and oxygen dissolved in water react in the presence of bicarbonate to produce hematite and carbonic acid:

2Fe^₂+ + ½ O₂ + 2H₂O + 4HCO₃^- → Fe₂O₃+ 4H₂CO₃

When the olivine in basalt is oxidized, the basalt takes on a reddish colour that's very different from the dark grey or black of unweathered basalt (Figure 8.12). [caption id="attachment_270" align="aligncenter" width="500"] Figure 8.12 Basalt pillows in Andalusia, Spain, with reddish weathered surfaces. Where parts of the pillows have broken away, darker unweathered basalt is visible. Source: Ignacio Benvenuty Cabral (2011), CC BY-NC-SA 2.0. Image source.[/caption] The oxidation reaction would be similar for other iron-containing silicate minerals such as pyroxene, amphibole, and biotite. Iron in sulphide minerals such as pyrite (FeS₂) can also be oxidized in this way. Hematite is only one of may minerals that can result from oxidation. In granite, for example, biotite and amphibole can be altered to form the iron oxide and iron hydroxyoxide minerals that are referred to in combination as limonite (orange material in Figure 8.13). [caption id="attachment_271" align="aligncenter" width="505"] Figure 8.13 Biotite and amphibole in this granite have been altered by oxidation to limonite (orange-yellow coating), which is a mixture of iron oxide and iron hydroxyoxide minerals. Source: Steven Earle (2015) CC-BY 4.0 view source[/caption]

Oxidation Reactions and Acid Rock Drainage

Oxidation reactions can pose an environmental problem in areas where rocks have elevated levels of sulphide minerals such as pyrite. This is because when oxygen and water react with pyrite, sulphuric acid is produced:

2FeS₂ + 7O₂ + 2H₂O → 2FeSO₄ + 2H₂SO₄

The runoff from areas where this process is taking place is known as acid rock drainage (ARD), and even a rock with only 1% or 2% pyrite can produce significant ARD. Some of the worst examples of ARD are at metal mine sites, especially where pyrite-bearing rock and waste material have been mined from deep underground, and then piled up and left exposed to water and oxygen. In these cases the problem is referred to as acid mine drainage. One example is the Mt. Washington Mine near Courtenay on Vancouver Island (Figure 8.12), but there are many similar sites across Canada and around the world. [caption id="attachment_272" align="aligncenter" width="600"] Figure 8.14 Acid mine drainage. Left: Mine waste where exposed rocks undergo oxidation reactions and generate acid at the Washington Mine, BC. Right: An example of acid drainage downstream from the mine site. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] At many ARD sites, the pH of the runoff water is less than 4 (very acidic). Under these conditions, metals such as copper, zinc, and lead easily dissolve in water, which can be toxic to aquatic life and other organisms. For many years, the river downstream from the Mt. Washington Mine had so much dissolved copper in it that it was toxic to salmon. Remediation work has since been carried out at the mine and the situation has improved.

Practice with Chemical Weathering [h5p id="85"] Match the words into the correct boxes to complete the definitions. In reactions, minerals turn into ions. Under the right conditions, this reaction can go in the opposite direction and turn the ions back into minerals. In reactions, water also disrupts chemical bonds, but in this case a new mineral is produced as well as ions. Water is added to a mineral's structure in reactions. In , a mineral is transformed by chemical reactions with oxygen. Fill-in-the-blank options:

dissolution
oxidation
hydrolysis
hydration

Now that you're warmed up, try this:

Which type of chemical weathering—dissolution, oxidation, hydration, or hydrolysis—causes the transformations shown here? Fill in the blanks. If you get stuck, look at the hints.

Pyrite (FeS₂) → Hematite (Fe₂O₃) (Hint: What's the difference in the elements making up each mineral?)
Calcite (CaCO₃) → Calcium (Ca₂⁺) and bicarbonate ions (HCO₃⁻) (Hint: Does this transformation produce a mineral?)
Feldspar (KAlSi₃O₈) → Kaolinite clay (Al₂Si₂O₅(OH)₄) (Hint: Water is a disruptive influence in this transformation)
Olivine ((Mg,Fe)₂SiO₄) → Serpentine (Mg, Fe)₃Si₂O₅(OH)₄ (Hint: Water is a disruptive influence in this transformation)
Pyroxene ((Mg,Fe)SiO₃) → Limonite (FeO(OH)·nH₂O) (Hint: Limonite is an iron oxide)
Anhydrite (CaSO₄) → Gypsum (CaSO₄·2H₂O) (Hint: What gets added to anhydrite to make gypsum?)

To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="86"]

References

Bienfait, B., & Ertl P. (2013). JSME: a free molecule editor in JavaScript. Journal of Cheminformatics, 5(24). https://doi.org/10.1186/1758-2946-5-24

8.3 Controls on Weathering Processes and Rates

kqzheng — Thu, 23 Aug 2018 21:02:05 +0000

Weathering doesn't happen at the same rate in all environments. The same types of weathering don't happen in all environments. There are a variety of factors that determine what kinds of weathering will occur, and how fast the processes will proceed.

Climate

Water and temperature are key factors controlling both weathering rates and the types of weathering that occur:

Water is required for chemical weathering reactions to occur.
Water must be present for ice wedging to happen.
Higher temperatures speed up chemical reactions.
Climate will determine whether water is present mostly in liquid form, solid form (ice), or both.
Climate will determine what plant life is available to force rocks apart with their roots, and to contribute organic acids to soils to aid in chemical weathering.

This means, for example, that chemical weathering will be faster in tropical climates than in the Arctic, a cold desert. It means physical weathering will be the predominant form of weathering in the Arctic.

Climate Impacts on Weathering

How does climate relate to weathering?The images below show two different environments, including information about the climate of each location. [caption id="attachment_1794" align="alignnone" width="864"] Location 1: Ferns and bamboo in a rainforest in Peru. Inset map: Green shaded area shows the distribution of rainforest in Peru. A very thick layer of low-nutrient soil is present beneath the vegetation. Average annual temperature: 28 °C. Average annual precipitation: 2600 mm.[/caption] [caption id="attachment_1796" align="alignnone" width="864"] Location 2: A view of the city of Yellowknife, located amid boreal forests in the Canadian Shield. Average annual temperature: -4 °C. Average annual precipitation: 289 mm.[/caption]

At which location is physical weathering more important, and why?
At which location is chemical weathering more important?

Answers

Physical (mechanical) weathering is more important in Yellowknife. Chemical weathering would be limited by temperatures that are below freezing for a significant part of the year, as well as by smaller amounts of precipitation. The primary means of physical weathering would be ice wedging and root wedging.
Chemical weathering is more important in the Peruvian rainforest. Chemical reactions would occur more rapidly there because of abundant rainfall and warm temperatures throughout the year.

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Oxygen and Carbon Dioxide

The presence and abundance of oxygen and carbon dioxide affect chemical weathering rates. Surface environments on Earth almost all have some free oxygen available, permitting oxidation reactions to take place. Exceptions are in settings such as deep lakes or swamps where oxygen cannot easily mix into the water, and where biological processes consume the oxygen rapidly. Carbon dioxide, which acidifies water and contributes to chemical weathering, is more concentrated in some settings than others. For example, because of the activities of organisms, soils can have very high concentrations of carbon dioxide, whereas carbon dioxide concentrations will be lower on surfaces free of soils and exposed to the atmosphere.

Minerals

The minerals making up a rock will determine what kinds of chemical weathering reactions are possible, and how rapidly chemical weathering reactions occur. Under the same conditions, dissolution of the calcite making up limestone will occur more rapidly than hydrolysis reactions happening to feldspar in granite. Quartz is very resilient to chemical weathering, and will remain long after calcite and feldspar have been weathered away. A rock with grains cemented by calcite will weather faster than a rock with grains cemented by quartz. In general, differences in the rates of chemical weathering among minerals can be broken down as follows:

Minerals that weather by dissolution (e.g., halite, gypsum, calcite) are the easiest to weather.
Silicate minerals with lower silica to oxygen ratios (e.g., silicates made of isolated silica tetrahedra or single chains) are easier to weather than silicate minerals with higher ratios (e.g., those made of silica tetrahedra arranged sheets or frameworks).
Minerals that are by-products of chemical weathering are some of the most resistant to further chemical weathering, although they may be more prone to physical weathering (e.g., clay minerals).

Weathering Makes Weathering Go Faster

Weathering accelerates weathering. Physical weathering forms cracks and breaks rocks apart into smaller pieces. The smaller the pieces, the greater the surface area exposed to chemical weathering. When the newly exposed surfaces are exposed to chemical weathering, it weakens the rock even further, making it more susceptible to physical weathering processes.

Designing a Sculpture

You've just been tasked with creating a stone sculpture of your pharoah. It's very important that your sculpture is resistant to weathering so that your pharoah will still be remembered thousands of years from now. Which of the following choices would give your sculpture the best chance of surviving?

Make it out of marble, which is almost entirely calcite.
Make it out of gabbro (an igneous rock with significant amounts of pyroxene and olivine).
Make it out of granite that's mostly feldspar and quartz.
Make it out of a sandstone that consists of fine grains of quartz sand cemented together with calcite.

To check your answers, navigate to the below link to view the interactive version of this activity.

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Differential Weathering

When rocks in an outcrop weather at different rates, the result is called differential weathering. Differential weathering causes some beds in an outcrop to be recessed relative to the others, because beds that are slow to weather will take longer to recede than weaker beds (Figure 8.15). [caption id="attachment_275" align="aligncenter" width="427"] Figure 8.15 Differential weathering in an outcrop along the Blaeberry River near Golden BC. The recessed beds within the outcrop are weathering faster than the surrounding beds. Source: Karla Panchuk (2009), CC BY 4.0[/caption]

8.4 Weathering and Erosion Produce Sediments

kqzheng — Thu, 23 Aug 2018 23:53:58 +0000

The visible products of weathering and erosion are the unconsolidated materials that we find around us on slopes, beneath glaciers, in stream valleys, on beaches, and in deserts. The loose collection of material is referred to as sediment, and the individual pieces that make it up are called clasts. Clasts can be of any size: sand-sized and smaller (in which case they might be referred to as particles or grains), or larger than a house. Clasts can range widely in size and shape (Figure 8.16) depending on the processes involved in making and transporting them. If and when deposits like these are turned into sedimentary rocks, the mineralogy and textures of these rocks will vary significantly. Importantly, when we describe sedimentary rocks that formed millions of years in the past, we can study the mineralogy and textures to make inferences about the conditions that existed during the deposition of the sediment, and the later burial and formation of sedimentary rock. The properties we look at are composition, grain size, sorting, rounding, and sphericity. [caption id="attachment_278" align="aligncenter" width="615"] Figure 8.16 Products of weathering and erosion formed under different conditions. Source: Steven Earle (2016), CC BY 4.0. Image source.[/caption]

Composition

Composition refers to the mineral or minerals making up the clast. Small clasts might be single mineral grains, but larger ones can have several different mineral grains, or even several different pieces of rock within them. The composition can tell us about what rock the sediments came from, and about the geological setting from which the sediment was derived. Not all minerals have the same hardness and resistance to weathering, so as weathering and erosion proceed, some minerals become more abundant than others within sediments. Quartz is one example of a mineral that's more abundant. It's highly resistant to weathering by weak acids or reaction with oxygen. This makes it unique among the minerals that are common in igneous rocks. Quartz is also very hard, so it's resistant to physical weathering. In contrast, feldspar and iron- and magnesium-bearing minerals are not as resistant to weathering. As weathering proceeds, they are likely to be broken into small pieces and converted into clay minerals and dissolved ions. Ultimately this means that quartz, clay minerals, iron oxides, aluminum oxides, and dissolved ions are the most common products of weathering.

Grain Size

Whether a grain is large or small tells us about its journey from its source to where it was deposited. Physical weathering can break off large pieces from rock. Large pieces carried along by streams will bump into each other, causing smaller pieces to break off. Over time the grains get smaller and smaller still. If we find grains that are very small, we can conclude that they travelled over a long distance. Geologists have a specific set of definitions to describe the size of grains (Figure 8.17). This scale has some of the grain sizes listed in microns (µm). There are 1000 µm in 1 mm. [caption id="attachment_279" align="aligncenter" width="447"] Figure 8.17 Classification of grain sizes. Silt and clay are considered fine-grained particles, sand is medium-grained, and particles larger than sand are considered coarse-grained.. Source: Karla Panchuk (2016), CC BY 4.0. Click for a text version of this table.[/caption] The particles classified as sand are what you would intuitively think of as being sand-sized, so an easy way to remember the scale is that anything smaller than sand is fine-grained, and anything larger is coarse-grained. Fine sand grains are still easily discernible with the naked eye. Silt grains are barely discernible in rocks, and silty rocks feel gritty when rubbed. Clay grains are invisible to the naked eye, and rocks comprised of clay feel smooth when rubbed. One other thing to notice about this scale is that the finest-grained particle is referred to as clay. While a clay-sized particle could be composed of clay minerals (and often they are), it doesn’t have to be. Any particle of that size would be referred to as clay.

Grain Size and Transportation

The grain size of sediments is not just for purposes of description. It's also a valuable clue to the processes that have acted on those sediments, because the size of the clast determines how much energy is required to move it. Whether or not a medium such as water or air has the ability to move a clast of a particular size and keep it moving depends on the velocity of the flow. For the most part, the faster the medium flows, the larger the clasts that can be moved. Figure 8.18 shows a stream bed that now contains only a trickle of water—barely enough to move particles of sand or cool puppy feet. But the velocity of water in the stream changes from season to season, as does the volume of water. All of the clasts in the stream bed were transported there by water at some point. [caption id="attachment_280" align="aligncenter" width="500"] Figure 8.18 Ruby looks upstream in a channel near Golden BC. For much of the year the only water in the stream is the trickle in which Ruby stands, but in the spring the water can flow rapidly enough to carry boulders. Source: Karla Panchuk (2009), CC BY 4.0.[/caption] Very fine-grained particles are the exception to rule that the larger the clasts, the faster the water that is required to transport them. Clay and silt grains stick together, requiring higher water velocities to pick them up and move them than some larger particles. Water that flows fast enough to pick up sand would not be fast enough to pick up clay.

Sorting

Weathering can break off large fragments of rock, and erosion and transport can break these fragments down to smaller and smaller sizes. The extent to which the grains in sediment differ in size is described by sorting (Figure 8.19, top). [caption id="attachment_281" align="aligncenter" width="438"] Figure 8.19 Top: Sorting of grains, ranging from well sorted where the grains are similar in size, to poorly sorted, where the grains vary greatly in size. Bottom: Rounding refers to how smooth or rough the edges of a clast are. Clasts with sharp edges and corners are angular. Clasts with smooth surfaces are rounded. Clasts that fall in between are sub-angular or sub-rounded. Source: Reagan et al. (2015), CC BY 3.0. Image source.[/caption] If the grains in a sample of sediment are the same size or very nearly so, the sediment is said to be well sorted. If the grains vary substantially in size, the sediment is poorly sorted. Because grains become progressively smaller as they are transported, sorting improves the further the sediments are from their source.

Rounding

Rounding refers to whether clasts have sharp edges and corners or not (Figure 8.19, bottom). If the grains are rough, with lots of edges and corners, then they are angular. Grains with smooth surfaces are rounded. Grains in between can be sub-angular or sub-rounded. The farther sediments are transported, the rounder they become.

Sphericity

Sphericity describes whether a grain is elongated or not. Grains that are longer than they are wide (like an ellipse) have low sphericity, whereas grains that have the same diameter no matter where you measure it (like a sphere) have high sphericity. In the bottom row of boxes in Figure 8.19 the grains at the top of each box exhibit high sphericity, and the grains at the bottom exhibit low sphericity. Notice that a grain can be angular but still have high sphericity. It can be rounded, but still have low sphericity. Sphericity also increases the further the sediments are from their source.

Practice with Sedimentary Grain Characteristics

Three samples of sand-sized grains are shown in the images below. Answer the questions that follow to see if you understand the common terms used to describe sediments. Descriptions:

Sample A: Fragments of red coral, algae plates, and urchin needles from a shallow water area (~2 m depth) near a reef in Belize. The grains are between 0.1 and 1 mm across.
Sample B: Quartz and rock fragments from a glacial stream deposit near Osoyoos, BC. The grains are between 0.25 and 0.5 mm in diameter.
Sample C: Grains of olivine (green) and volcanic glass (black) from a beach on the big island of Hawai’i. The grains are approximately 1 mm in diameter.

Questions

Which sample has the lowest sphericity?
Which sample has the most angular grains?
Which sample has the best sorting?
Which sediments would be considered the most mature?

To check your answers, navigate to the below link to view the interactive version of this activity.

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Level Up: Using Sedimentary Grain Characteristics to Understand the Origins of Sedimentary Rocks [h5p id="90"]

References

Reagan, M.K., Pearce, J.A., Petronotis, K., and the Expedition 352 Scientists, (2015). Proceedings of the International Ocean Discovery Program, Volume 352, publications.iodp.org, doi:10.14379/iodp.proc.352.102.2015

8.5 Weathering and Soil Formation

kqzheng — Fri, 24 Aug 2018 04:05:33 +0000

Weathering is a key part of the process of soil formation, and soil is critical to our existence on Earth. In other words, we owe our existence to weathering, and we need to take care of the soil! Many people refer to any loose material on Earth’s surface as soil, but to scientists soil is the material that includes organic matter, forms within the top few tens of centimetres of the surface, and is important for sustaining plant growth. Soil is a complex mixture of minerals (~45%), organic matter (~5%), and empty space (~50%, filled to varying degrees with air and water). The mineral content of soil varies, but is dominated by clay minerals and quartz, along with minor amounts of feldspar and small fragments of rock. The types of weathering that take place within a region have a major influence on soil composition and texture. For example, in a warm climate where chemical weathering dominates, soils tend to be richer in clay. Soil scientists describe soil texture in terms of the relative proportions of sand, silt, and clay (Figure 8.21). Sand and silt components are dominated by quartz, with lesser amounts of feldspar and rock fragments. The clay component is dominated by clay minerals. [caption id="attachment_284" align="aligncenter" width="544"] Figure 8.21 Soil texture classification diagram. Textures are determined by the proportions of sand-, silt-, and clay-sized grains. Source: Mike Norton (2011), CC BY-SA 3.0. Image source.[/caption]

Concept Check: The Basics of Soil Composition

Drag the words to complete this description of soil composition. Soil is made up mostly of (50%) and (45%) Only 5% of it is . Clay and (hint: This mineral is very resistant to weathering.) are the most abundant minerals in soil. Minor components are and small fragments of rock. Fill-in-the-blank options:

minerals
empty space
organic matter
quartz
feldspar

To check your answers, navigate to the below link to view the interactive version of this activity.

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Factors Affecting How Soil Forms

Soil forms through the physical and chemical weathering of rocks and sediments, and the accumulation and decay of organic matter. The factors that affect the nature of soil and the rate of its formation include:

Climate, especially average temperature and precipitation amounts, and the consequent types of vegetation
The parent rock or sediment that was weathered to make the soil
The slope of the surface where soil is accumulating
How long soil has been forming at a location

Climate

Both the physical breakup of rocks and the chemical weathering of minerals contribute to soil formation. The downward percolation of water brings dissolved ions and also facilitates chemical reactions. Soil forms most readily under temperate to tropical conditions, and moderate precipitation. Temperature matters because chemical weathering reactions and those facilitated by organisms proceed fastest under warm conditions, and plant growth is enhanced in warm climates. Where the climate is cooler, the rates of chemical weathering reactions decrease, and when water is frozen, may cease entirely. Although water is needed for chemical weathering to take place, too much water can lead to soils that lack nutrients. In rain forests, for example, high rainfall contributes so much water that important nutrients are leached away, and acidic soils are left behind. In humid and poorly drained regions, swampy conditions may prevail, producing soil that is dominated by organic matter, but low in inorganic nutrients. Too little water (e.g., in deserts and semi-deserts) limits the rate of downward chemical transport, and it also means that salts and carbonate ions dissolved in upward-moving groundwater can precipitate and build up in sediments, hindering organic activity. These soils also lack organic matter (Figure 8.22). [caption id="attachment_285" align="aligncenter" width="466"] Figure 8.22 Soil consisting of wind-blown silt (loess) and little organic matter in an arid part of north-eastern Washington state. Source: Steven Earle (2016), CC BY 4.0. Image source.[/caption]

Parent Material

Parent material for soils can be any type of bedrock, and any type of unconsolidated sediment, such as glacial deposits and stream deposits. Soils are described as residual soils if they develop on bedrock, and transported soils if they develop on transported material such as glacial sediments. This doesn’t mean that the soils themselves have been transported, but that the soil developed on unconsolidated material rather than on bedrock. Sandy soils develop from quartz-rich parent material, such as granite, sandstone, or loose sand. Quartz-poor material, such as shale or basalt, generates soils with little sand. Parent materials provide important nutrients to residual soils. For example, a minor constituent of granitic rocks is the calcium-phosphate mineral apatite, which is a source of the important soil nutrient phosphorus. Basaltic parent material tends to generate very fertile soils because, in addition to phosphorus, it provides significant amounts of iron, magnesium, and calcium. The iron, magnesium, and calcium come from minerals such as olivine ((Mg,Fe)₂SiO₄) and plagioclase feldspar (CaAl₂Si₂O₈) in the basalt. Some unconsolidated materials, such as river-flood deposits, make for especially good soils because they tend to be rich in clay minerals. Clay minerals have large surface areas with negative charges that are attractive to positively charged elements like calcium, magnesium, iron, and potassium — important nutrients for plant growth.

Slope

Soil can only develop where surface materials remain in place and are not frequently washed away or lost to mass wasting (landslides). Soils cannot develop where the rate of soil formation is lower than the rate of erosion, so steep slopes tend to have little or no soil.

Time

Even under ideal conditions, soil takes thousands of years to develop. Virtually all of southern Canada was covered with glaciers up until 14,000 years ago, and most of the central and northern parts of BC, the Prairies, Ontario, and Quebec were still glaciated at 12,000 years ago. Glaciers remained in the central and northern parts of Canada until around 10,000 years ago, so conditions were still not ideal for soil development even in the southern regions. This means that soils in Canada, particularly in central and northern Canada, are relatively young and not well developed. The same applies to soils that are forming on newly created surfaces, such as recent deltas or sand bars, in areas of mass wasting, or where an area has been resurfaced by volcanic deposits. Because soil takes so long to form, human activities that damage soils have long-term consequences for ecosystems, and for the utility of the soil for food production.

Concept Check: Soil Formation Conditions

Which three items on this list will give you the best soil-forming conditions, and generate a soil rich in iron, magnesium, and calcium.

A gentle slope
A steep slope prone to landslides
Granite parent material
Basalt parent material
A cool and dry climate
A warm and humid climate

To check your answers, navigate to the below link to view the interactive version of this activity.

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Soil Horizons

When soils form, the downward movement of clay, water, and dissolved ions can lead to the development of chemically and texturally distinct layers known as soil horizons. In temperate climates, common soil horizons that develop are the following (Figure 8.23):

O horizon— A layer of organic matter
A horizon— Partially decayed organic matter mixed with mineral material
E horizon— The eluviated (leached) layer from which some of the clay and iron have been removed to create a pale layer that may be sandier than the other layers
B horizon— Where clay, iron, and other elements from the overlying soil accumulate
C horizon— Contains broken fragments of rock. Weathering of the underlying bedrock or sediments is not yet complete.

[caption id="attachment_286" align="aligncenter" width="432"] Figure 8.23 Typical horizons in a temperate soil, from Wales. Source: Karla Panchuk (2018), CC BY 4.0. Photograph: Richard Hartnup (2005), Public Domain. Image source.[/caption] Although rare in Canada, another type of layer that develops in hot arid regions is known as caliche (pronounced ca-lee-chee). It forms from the downward (or in some cases upward) movement of calcium ions, and the precipitation of calcite within the soil. When well developed, caliche cements the surrounding material together to form a layer that has the consistency of concrete.

Practice with Soil Horizons [h5p id="93"]

How Soil Is Lost

Like all geological materials, soil is subject to erosion. Under natural conditions on gentle slopes, the rate of soil formation either balances or exceeds the rate of erosion. However, human practices related to forestry and agriculture have significantly upset this balance. Soils are held in place by vegetation. When vegetation is removed, either through cutting trees or routinely harvesting crops and tilling the soil, this protection is lost. When soil is not protected, wind and water can easily erode it away. Water erosion is accentuated on sloped surfaces because fast-flowing water has greater eroding power than still water. Raindrops can disaggregate exposed soil particles, putting clay into suspension in the water. Sheetwash—unchannelled flow across a surface—carries suspended material away, and channels erode right through the soil layer, removing both fine and coarse material (Figure 8.24). [caption id="attachment_287" align="aligncenter" width="506"] Figure 8.24 Soil erosion by rain and unchanneled runoff in a field in Alberta. Source: Alberta Agriculture and Rural Development. Click for source information and terms of use.[/caption] Wind erosion is exacerbated by the removal of trees that act as wind breaks, and by agricultural practices that leave bare soil exposed (Figure 8.25). [caption id="attachment_288" align="aligncenter" width="500"] Figure 8.25 Soil erosion by wind in Alberta. Source: Alberta Agriculture and Rural Development. Click for source information and terms of use.[/caption] Tillage is also a factor in soil erosion, especially on slopes, because each time the soil is lifted by a cultivator, it is moved a few centimetres down the slope.

8.6 Soils of Canada

kqzheng — Fri, 24 Aug 2018 20:20:22 +0000

Until the 1950s, the classification of soils in Canada was based on the system used in the United States. However, it was long recognized that the U.S system did not apply well to many parts of Canada because of climate and environmental differences. The Canadian System of Soil Classification was first outlined in 1955 and has been refined and modified numerous times since then. There are 10 orders of soil recognized in Canada (Table 8.1), and you can explore the distribution of soils using Agriculture and Agri-Food Canada's interactive map (Figure 8.26). See the resources section at the bottom of the page for additional sources of information on Canadian soils, including videos.

Table 8.1 Canadian Soil Classification System
Order	Brief Description	Environment
Forests
Podzolic	Well-developed A and B horizons	Coniferous forests throughout Canada
Luvisolic	Clay-rich B horizon	Northern prairies and central BC, mostly on sedimentary rocks
Brunisolic	Poorly developed or immature soil, that does not have the well-defined horizons of podsol or luvisol	Boreal-forest soils in the discontinuous permafrost areas of central and western Canada, and also in southern BC.
Grasslands
Chernozemic	High levels of organic matter and an A horizon at least 10 cm thick	Southern prairies and parts of BC’s southern interior, in areas that experience summer water deficits
Solonetzic	A clay-rich B horizon, commonly with a salt-bearing C horizon	Southern prairies, in areas that experience water deficits during the summer
Glacial and tundra
Cryosolic	Poorly developed soil, mostly C horizon	Permafrost areas of northern Canada
Vertisolic	Clay-rich soils associated with glacial lake deposits	Southern prairies
Other
Organic	Dominated by organic matter; mineral horizons are typically absent	Wetland areas, especially along the western edge of Hudson Bay, and in the area between the prairies and the boreal forest
Regosolic	Does not have a B horizon (i.e., no accumulation of leached minerals)	Unstable sediments including steep slopes prone to landslides, shifting sand dunes, and floodplains where sediments are frequently moved by streams
Gleysolic	Colour patterns related to the absence of oxygen	Water-saturated soils

[caption id="attachment_291" align="aligncenter" width="624"] Figure 8.26 Distribution of soil orders in Canada. Click here to go to the interactive map. Source: Agrifood and Agriculture Canada. Contains information licensed under the Open Government License - Canada. Click for terms of use.[/caption] Processes of soil formation include downward transport of solid and dissolved materials, and the nature of those processes depends in large part on the climate. In Canada’s predominantly cool and humid climate—characteristic of most places other than the far north—podzolization is the norm. This involves downward transportation of hydrogen, iron, and aluminum from the upper part of the soil profile, and accumulation of clay, iron, and aluminum in the B-horizon. Most of the podzols, luvisols, and brunisols of Canada form through various types of podzolization. In the grasslands of the dry southern parts of the prairie provinces and in some of the drier parts of southern BC, dark brown organic-rich chernozem soils are dominant. In some cases, weak calcification takes place when calcium is leached from the upper layers and accumulates in the B-horizon. Development of caliche layers is rare in Canada. Organic soils form in areas with poor drainage and a rich supply of organic matter, such as in swamps. These soils have very little mineral matter. In the permafrost regions of the north, where glacial retreat was most recent, the time available for soil formation has been short and the rate of soil formation slow. The soils are called cryosols (the cryo prefix is used to indicate extreme cold). In permafrost areas, the freeze-thaw process churns the soil, resulting in limited soil horizon development.

Practice with Soils of Canada [h5p id="94"]

Resources

Soils of Canada (University of Saskatchewan) Soil Classification: Soil Orders of Canada Watch videos about each soil order.

8.7 Weathering and Climate Change

kqzheng — Sat, 25 Aug 2018 00:48:30 +0000

Scientists who study Earth's past climate tell us that today carbon dioxide is being added to the atmosphere faster than during some of the most extreme climate change events in Earth history. Eventually, higher carbon dioxide levels will accelerate chemical weathering, and that will help to remove some of the carbon dioxide from the atmosphere. The problem is that carbon cycling on Earth operates on different timescales depending on the components of the Earth system that are involved. Over the short term, biological processes are important. In particular, living organisms—mostly plants—consume carbon dioxide from the atmosphere to make their tissues. After they die, the carbon is released back into the atmosphere over years to decades as the plant matter decays. Over the longer term, geological processes drive the carbon cycle. Geological carbon-cycle processes operate very slowly, but they affect much more of Earth's carbon than the biological component. Carbon can move from the biological cycle to the geological cycle if it's buried in sedimentary rocks. The biological carbon could be fragments of plant material or organic molecules that are preserved as coal or in organic-rich shale. It could also be calcium carbonate body parts of marine organisms that are preserved in limestone. The geological component of the carbon cycle is shown in Figure 8.27. The various steps in the process (not necessarily in this order) are as follows:

Organic matter from plants is stored in peat, coal, and permafrost for thousands to millions of years.
Weathering of silicate minerals converts atmospheric carbon dioxide to dissolved bicarbonate, which is stored in the oceans for thousands to tens of thousands of years.
Dissolved carbon is converted by marine organisms to calcite, which is stored in carbonate rocks for tens of millions to hundreds of millions of years.
Organic carbon compounds are stored in sediments for tens to hundreds of millions of years; some end up in petroleum deposits.
Carbon-bearing sediments are transferred to the mantle, where the carbon may be stored for tens of millions to billions of years.
During volcanic eruptions, carbon dioxide is released back to the atmosphere, where it is stored for years to decades.

[caption id="attachment_294" align="aligncenter" width="1024"] Figure 8.27 The geological component of the carbon cycle includes: (a) organic carbon in peat, coal and permafrost, (b) weathering of silicate minerals converts atmospheric carbon dioxide to dissolved bicarbonate, (c) marine organisms convert dissolved carbon to calcium carbonate, (d) carbon compounds are stored in sediments, (e) carbon-bearing sediments are transferred to longer-term storage in the mantle, and (f) carbon dioxide is released back to atmosphere during volcanic eruptions. Source: Steven Earle (2016), CC BY 4.0. Image source.[/caption]

Weathering and Clues About Past Climate

The link between weathering and the carbon cycle means that geologists can use evidence of weathering in the rock record to help understand how the carbon cycle has changed in the past, and thus how Earth's climate has changed. At some times in Earth’s history, the geological carbon cycle has been balanced, with carbon being released to the atmosphere by some processes at approximately the same rate as other processes store it. Under these conditions, the climate can remain relatively stable. At other times, the balance is upset. Prolonged periods of greater than average volcanism can cause an imbalance. The eruption of the Siberian Traps at around 250 Ma warmed the climate significantly over a few million years, leading to a mass extinction. Mountain-building events may also cause an imbalance. The formation of the Himalaya range between about 40 Ma and 10 Ma ago exposed rocks to weathering over a large region. The over-all rate of weathering on Earth increased because the mountains were so high, and the range was so extensive. The weathering of these rocks—most importantly the hydrolysis of feldspar—consumed atmospheric carbon dioxide and transferred carbon to the oceans and to ocean-floor carbonate minerals. Decreasing carbon dioxide levels contributed to climate cooling that culminated in the Pleistocene glaciations. Today, burning fossil fuels is causing an imbalance in the carbon cycle. Burning coal, oil, and gas releases in a geological instant carbon that was stored by the biological carbon cycle over hundreds of millions of years. Weathering is a natural mechanism that is usually key to adjusting imbalances and removing large volumes of carbon from the atmosphere, but because it's part of the geological carbon cycle, it operates over long timescales. If humans stopped burning all fossil fuels today, it could still take thousands of years for balance to be restored.

Chapter 8 Summary & Key Term Check

kqzheng — Sat, 25 Aug 2018 02:13:00 +0000

Chapter 8 Main Ideas

8.1 Physical Weathering

Rocks weather when they are exposed to surface conditions. In most cases, conditions at Earth's surface are very different from the conditions under which the rocks formed. Physical weathering processes include exfoliation, freeze-thaw, salt crystallization, and the wedging effects of plant growth.

8.2 Chemical Weathering

Chemical weathering takes place when minerals within rocks are not chemically stable in their existing environment. Chemical weathering processes include hydrolysis of silicate minerals to form clay minerals, oxidation of iron in silicate and other minerals to form iron oxide minerals, and dissolution of calcite.

More Practice with Types of Physical and Chemical Weathering [h5p id="95"]

8.3 Controls on Weathering Processes and Rates

Chemical weathering is faster when temperatures are warmer and moisture is present. Physical weathering is more important in regions with frequent freeze-thaw cycles. Weathering rates can depend on the abundance oxygen and carbon, and will vary with the mineral composition of a rock. Weathering itself accelerates weathering by exposing more surface area to chemical reactions.

8.4 Weathering and Erosion Produce Sediments

Quartz grains are one of main products of weathering and erosion, because quartz is resistant to chemical and physical weathering. Clay minerals, iron oxide and iron hydroxide minerals, aluminum hydroxide minerals, and ions in solution are common products of chemical weathering. Particles produced by weathering can be described in terms of their composition, grain size, sorting, rounding, and sphericity.

More Practice with Grain Characteristics

Two sand samples are shown below, with detailed descriptions underneath. For each of the characteristics listed, note A or B to indicate which sample best exemplifies that characteristic. Descriptions:

Sample A: Sand with olivine grains from Marine d'Albo, Corsica, France. Grains are generally smooth. They come in many shapes (some almost circular and some very elongated) and compositions, and range in size from 0.5 to 2 mm.
Sample B: Sand from Qafsah, Tunesia. The sand is stained yellow, but is all quartz. Individual grains are all less than 0.5 mm. Grains are all approximately the same shape (generally blocky with sharp edges) and the same size.

Indicate whether the below describes Sample A or :

The sample that has the best sorting
The sample that has the lowest sphericity
The sample that has the best rounding (Remember: rounding is related to smoothness, not the overall shape of the grain)
The sample that is the most mature

To check your answers, navigate to the below link to view the interactive version of this activity.

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8.5 Weathering and Soil Formation

Soil is a mixture of fine mineral fragments (including quartz and clay minerals), organic matter, and empty spaces that may be partially filled with water. Soil formation is controlled by climate (especially temperature and humidity), the nature of the parent material, the slope (because soil can’t accumulate on steep slopes), and the amount of time available. Typical soils have layers called horizons, which form because of differences in the conditions with depth.

8.6 Soils of Canada

Canada has a range of soil types related to our unique conditions. The main types of soil form in forested and grassland regions, but there are extensive wetlands in Canada that produce organic soils, and large areas where soil development is poor because of cold conditions.

More Practice with Canadian Soil Types [h5p id="97"]

8.7 Weathering and Climate Change

Weathering is an important part of how imbalances in the carbon cycle are adjusted, but it's part of the geological component of the carbon cycle which acts over the long term. The carbon cycle includes the addition of carbon to the atmosphere by volcanic eruptions. Carbon is extracted from the atmosphere when silicate minerals are weathered, but also when carbon when it is transformed into organic matter by plants. Organic matter can be stored in soil, permafrost, and rocks. Burning of fossil fuels involves moving carbon from geological reservoirs to the atmosphere on timescales much faster than the geological carbon cycle operates.

Key Term Check

What key term from Chapter 8 is each card describing? Turn the card to check your answer. [h5p id="98"]

9.1 Clastic Sedimentary Rocks

kqzheng — Fri, 24 Aug 2018 19:07:56 +0000

How Clastic Sediments Become Sedimentary Rocks

Lithification (Figure 9.3) is the process of converting sediments into solid rock. Compaction is the first step. Sediments that have been deposited are buried when more and more sediments accumulate above them. The weight of the overlying sediments pushes the clasts together, closing up some of the pore spaces (the gaps between grains) and forcing them together. Pore spaces often contain water (although they can also contain air or even hydrocarbons), so the water is squeezed out. [caption id="attachment_302" align="aligncenter" width="1024"] Figure 9.3 Lithification turns sediments into solid rock. Lithification involves the compaction of sediments and then the cementation of grains by minerals that precipitate from groundwater in the spaces between these grains. Source: Karla Panchuk (2016), CC BY 4.0.[/caption] Cementation is the next step. Groundwater flowing through the remaining pore spaces contains ions, and these ions may precipitate, leaving behind minerals in the pore spaces. These minerals bind the grains together, and are referred to collectively as cement. Quartz and calcite are common cement minerals, but depending on pressure, temperature, and chemical conditions, cement might also include other minerals such as hematite and clay. Figure 9.4 shows cemented grains in sandstone viewed under a microscope. The grains are all quartz but they appear different shades of grey because they're being viewed through cross-polarized light. It's difficult to tell the grains from the cement in this case because both are made of quartz, but in the image on the right the more obvious grain boundaries are marked with dashed lines. Some of the cement is marked with blue shading. Using the image on the right, see if you can pick out the grain boundaries in the image on the left. [caption id="attachment_303" align="aligncenter" width="720"] Figure 9.4 Sandstone under a microscope. Grains and cement are quartz. Left- Original image. Right- Visible grain boundaries are marked with dashed lines, and some of the cement is shaded in blue. Source: Karla Panchuk (2018), CC BY 4.0. Modified after Woudloper, Public Domain. Image source.[/caption]

Concept Check: Lithification

Match the words into the correct boxes to explain the steps in making a clastic sedimentary rock. Once sediments are , can begin. The first step is , which puts the grains in closer contact with each other. The second step is , during which minerals glue the grains together. The most common mineral "glue" is (hint: The most common glue is also the one that holds up the best.), but can also bind the grains together. Fill-in-the-blank options:

deposited
compaction
quartz or calcite
hematite or clay
cementation
lithification

To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="100"]

Types of Clastic Sedimentary Rocks

Clastic sedimentary rocks are named according to the characteristics of clasts (rock and mineral fragments) that comprise them. These characteristics include grain size, shape, and sorting. The different types of clastic sedimentary rocks are summarized in Figure 9.5. [caption id="attachment_304" align="aligncenter" width="778"] Figure 9.5 Types of clastic sedimentary rocks. Source: Karla Panchuk (2018), CC BY-NC-SA 4.0. Photos by James St. John and R. Weller/ Cochise College. Click for more attributions.[/caption]

Coarse-Grained Clastic Rocks

Clastic sedimentary rocks with a significant fraction of clasts larger than 2 mm are known as conglomerate if the clasts are well rounded, or breccia if they are angular (Figure 9.5, top row). Conglomerates form in high-energy environments, such as fast-flowing rivers, where the particles can become rounded as they bump into each other while being carried along. Breccias typically form where the particles are not transported a significant distance, such as in alluvial fans and talus slopes.

Medium-Grained Clastic Rocks

Sandstone (Figure 9.5, middle row) is a very common sedimentary rock, and there are many different kinds of sandstone. It's worth knowing something about the different types because they're organized according to characteristics that are useful for the detective work of figuring out what conditions led to the formation of a particular sandstone. Broadly, sandstones can be divided into two groups: arenite and wacke (rhymes with tacky). Arenite is “clean” sandstone consisting mostly of sand-sized grains and cement, with less than 15% of fine-grained silt and clay in the matrix (the material between the sand-sized grains). Arenites are subdivided according to what the sand-sized grains are made of (Figure 9.6). If 90% or more of the grains are quartz, then the sandstone is called a quartz arenite (also called a quartz sandstone). If more than 10% of the grains are feldspar and more of the grains are feldspar than fragments of other rocks (lithic[footnote]“Lithic” means “rock.” Lithic clasts are rock fragments (multimineralic fragments), as opposed to single-mineral fragments.[/footnote] fragments) then the sandstone is called an arkosic arenite, or just arkose. If the rock has more than 10% rock fragments, and more rock fragments than feldspar, it is lithic arenite. [caption id="attachment_305" align="aligncenter" width="573"] Figure 9.6 A compositional triangle for arenite sandstones, with the three most common components of sand-sized grains: quartz, feldspar, and rock fragments. Arenites have less than 15% silt or clay. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] Wacke is a "dirty" sandstone, containing 15-75% fine-grained particles (clay, silt) in its matrix. A wacke can have more fine-grained particles than cement in its matrix, making for a crumbly rock. Wackes are subdivided in the same way that arenites are: quartz wacke, feldspathic wacke, and lithic wacke. Another name for a lithic wacke is greywacke. Figure 9.7 shows thin sections[footnote]Thin sections are slivers of rock sliced thinly enough so that light can pass through them, and they can be examined under a microscope.[/footnote] (microscopic views) of quartz arenite, arkose, and lithic wacke. In the images, quartz grains are marked Q, feldspar grains are marked F, and lithic fragments are marked L. Notice the relative abundances of each component in the three types of rocks. [caption id="attachment_306" align="aligncenter" width="1024"] Figure 9.7 Photos of thin sections of three types of sandstone. Some of the minerals are labelled: Q=quartz, F=feldspar and L= lithic (rock fragments). The quartz arenite and arkose have relatively little silt/clay matrix, while the lithic wacke has abundant matrix. Source: Steven Earle (2016), CC BY 4.0. Image source.[/caption]

Practice with Sandstone [h5p id="101"] Now that you're warmed up and have a completed diagram, use the diagram to help you identify the two samples in this next exercise:

Sample 1. Rounded sand-sized grains are approximately 99% quartz and 1% feldspar. Silt and clay make up less than 2% of the rock. What kind of sandstone is this? (Hint: The abundance of quartz is important here.) Sample 2. Angular sand-sized grains are approximately 70% quartz, 20% lithic, and 10% feldspar. Silt and clay make up about 20% of the rock. What kind of sandstone is this? (Hint: Notice that fine particles are important in this sample.) To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="102"]

Fine-Grained Clastic Rocks

Rock composed of at least 75% silt- and clay-sized clasts is called mudrock (Figure 9.5, bottom row). If a mudrock shows evidence of fine layers (laminations) and breaks into sheets, it's called shale. Otherwise, it's siltstone (dominated by silt), mudstone (a mix of silt and clay), or claystone (dominated by clay). The fine-grained nature of mudrocks tells us that they form in very low energy environments, such as lakes, flood plains, and the deep ocean. Clastic sediments are deposited in a wide range of environments, including from melting glaciers, slope failures, rivers (both fast and slow flowing), lakes, deltas, and ocean environments (both shallow and deep). Depending on the grain size in particular, they may eventually form into rocks ranging from mudstone to breccia and conglomerate. By examining clastic sedimentary rocks it is possible to translate the classification you have just learned into an interpretation of the environment in which the rocks were deposited.

Sediment Maturity

The concept of maturity is often used in discussions of sedimentary rocks, although it still refers to the maturity of the sediments themselves. Remember that a mature sediment is one that has become smaller, rounder, better sorted, and with the grains that are most resistant to physical and chemical weathering. On the spectrum of sediment maturity, quartz sandstone would be a mature sedimentary rock, and wacke or conglomerate would be immature rocks.

Practice with Sedimentary Rock Names [h5p id="103"] If you can match sedimentary rock names to descriptions, you're ready to try the next level and come up with the names themselves. This is more challenging, but can refer to your completed diagram above if you need help. [h5p id="104"]

9.2 Chemical and Biochemical Sedimentary Rocks

kqzheng — Fri, 24 Aug 2018 19:08:50 +0000

Chemical and biochemical sedimentary rocks are dominated by mineral components that have been transported as ions in solution (e.g., Na⁺, Ca²⁺, HCO_3^–, etc.). Clastic sedimentary rocks have particles that are cemented together by some of the same materials, but the difference is that in chemical and biochemical rocks, those cementing minerals are the rock, not just "glue."

Chemical vs. Biochemical

The difference between chemical and biochemical sedimentary rocks is that in biochemical sedimentary rocks, organisms play a role in turning the ions into sediment. This means the presence and nature of biochemical sedimentary rocks are linked to the life requirements of the organisms involved. In chemical sedimentary rocks, the process is inorganic, often resulting from a body of water evaporating and concentrating the ions. It's possible for one type of sedimentary rock to form through both chemical (inorganic) and biochemical (organically mediated) processes. Chemical and biochemical sedimentary rocks are classified based on the minerals they contain, and are frequently dominated by a single mineral. It's true that some clastic sedimentary rocks, such as quartz arenite, can also be dominated by a single mineral, but the reasons are different. A clastic sedimentary rock can contain whatever minerals were present in the parent rock. The minerals the clastic rock ends up containing will depend on how much “processing” the sediments undergo by physical and chemical weathering, and transport, before the sediment was cemented. On the other hand, chemical/biochemical sedimentary rocks are limited largely to those minerals that are highly soluble in water. Because mineral content is a defining characteristic of chemical and biochemical sedimentary rocks, we will use it to organize our discussion of these rocks.

Carbonate Rocks

In carbonate rocks, the dominant mineral contains the carbonate anion (CO₃^2-). The main carbonate minerals are calcite and aragonite. Both minerals have the formula CaCO₃ but they have different crystal structures. A less common carbonate mineral that's still important for forming carbonate rocks is dolomite, which has the formula CaMg(CO₃)₂. It's similar to calcite and aragonite, except that some of the calcium is replaced with magnesium. Dolomite is more common as a replacement mineral, which has replaced calcite in carbonate rocks.

Limestone

Limestone is comprised of calcite and aragonite. It can occur as a chemical sedimentary rock, forming inorganically due to precipitation, but most limestone is biochemical in origin. In fact, limestone is by far the most common biochemical sedimentary rock. Almost all limestone forms in marine (i.e., oceans or salty seas) environments, and most of that forms on the shallow continental shelves, especially in tropical regions with coral reefs. Today continental shelves are relatively narrow zones along the margins of continents, but for large parts of geologic history sea-level was much higher, and large parts of the interiors of continents were flooded. Reefs are highly productive ecosystems populated by a wide range of organisms, many of which use calcium and bicarbonate ions from seawater to make carbonate minerals (especially calcite) for their shells and other structures. These include corals as well as green and red algae, urchins, sponges, molluscs, and crustaceans. Some of micro-organisms use CaCO₃ to build tiny tests (shells) which accumulate on the ocean floor when these organisms die. Erosion can break all of these carbonate materials apart, scattering fragments throughout surrounding region (Figure 9.8). [caption id="attachment_309" align="aligncenter" width="500"] Figure 9.8 Various corals and green algae on a reef at Ambergris, Belize. The light-coloured sand consists of carbonate fragments eroded from the reef organisms. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] Figure 9.9 shows a cross-section through a typical reef environment in a tropical region (normally between 40 °N and 40 °S). Reefs tend to form near the edges of steep drop-offs because the reef organisms thrive on nutrient-rich upwelling currents. As the reef builds up, waves erode it. Currents carry carbonate sediments into the steep offshore fore-reef area and the shallower inshore back-reef area. Reef-derived sediments are dominated by reef-type carbonate fragments of all sizes, including mud. [caption id="attachment_310" align="aligncenter" width="605"] Figure 9.9 Cross-section through a typical tropical reef. Source: Steven Earle (2015) CC BY 4.0 view source[/caption] In many such areas, carbonate-rich sediments also accumulate in quiet lagoons, where mud and mollusc-shell fragments predominate (Figure 9.10, left) or in offshore areas with strong currents, where either foraminifera tests accumulate (Figure 9.10, middle) or calcite crystallizes inorganically to form ooids–spheres of calcite that form in shallow tropical ocean water with strong currents (Figure 9.10, right). [caption id="attachment_311" align="aligncenter" width="551"] Figure 9.10 Carbonate rocks and sediments. Left- Mollusc-rich limestone formed in a lagoon area at Ambergris, Belize. Middle- Foraminifera-rich sediment from a submerged carbonate sandbar near to Ambergris, Belize . Right- Ooids from a beach at Joulters Cay, Bahamas. Sources: Left, Middle- Steven Earle (2015), CC BY 4.0. Image source. Right- Mark Wilson (2010), Public Domain. Image source.[/caption] Limestone also accumulates in deeper water, from the steady settling out of the carbonate shells of tiny organisms that lived near the ocean surface. Processes on the ocean floor cause the water in the deepest parts of the ocean to become more acidic. This puts a lower limit on how deep in the ocean calcite and aragonite can accumulate, because they dissolve under acidic conditions.

Tufa and Travertine

Calcite can form chemical sedimentary rocks on land in a number of environments. Tufa forms at springs. The tufa towers in Figure 9.11 formed where spring water discharged into lake water. [caption id="attachment_312" align="aligncenter" width="500"] Figure 9.11 Tufa towers (made of calcium carbonate) in Mono Lake, California. Evaporation keeps the concentration of ions in the lake very high, allowing the calcium carbonate to precipitate. Source: Brocken Inaglory (2006), CC BY-SA 3.0. Image source.[/caption] Travertine (which is less porous) forms at hot springs. Similar material precipitates within limestone caves to form speleothems (mineral deposits in caves, Figure 9.12) such as stalactites and stalagmites. [caption id="attachment_313" align="aligncenter" width="500"] Figure 9.12 Speleothems in Cave Nefza in Tunisia Source: Badreddine Besbes (2015), CC BY-SA 3.0. Image source.[/caption]

Dolostone

Dolostone (also referred to as dolomite) is the carbonate rock made of the mineral dolomite (CaMg(CO₃)₂). Dolostone is quite common (there’s a whole Italian mountain range named after it), which is surprising because marine organisms don't precipitate dolomite. Dolomite forms through dolomitization, a process thought to involve chemical reactions between magnesium-rich water percolating through rocks, and sediments containing calcite. Calcite and dolomite can be distinguished from one another by applying a drop of weak acid to the rock; calcite will react with weak acid, whereas dolomite will not. Also, when dolomite weathers, it tends to turn buff (tan) in colour, whereas calcite tends toward grey and white.

Chert

Chert is made of silica (SiO₂). It has the same chemical formula as quartz, but is cryptocrystalline, meaning that the quartz crystals comprising chert are so small that it is difficult to see them even under a microscope. Chert can be a chemical sedimentary rock, often forming as beds within limestone (Figure 9.13), or as irregular lenses or blobs (nodules). It can also be biochemical. Some tiny marine organisms (e.g., diatoms and radiolaria) make their tests from silica. When they die their tiny shells settle slowly to the bottom of the lake or ocean, where they accumulate and are transformed into chert. [caption id="attachment_314" align="aligncenter" width="500"] Figure 9.13 Chert (brown layers) interbedded with limestone, Triassic Quatsino Fm, Quadra Island, BC. All of the layers have been folded, and the chert, being more resistant to weathering than limestone, stands out. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Banded Iron Formations (BIFs)

Some ancient chert beds—most dating to between 1800 and 2400 Ma—are also part of a rock known as a banded iron formation (BIF). It's a deep sea-floor deposit of iron oxide that is a common ore of iron. These rocks consist of alternating layers of dark iron oxide minerals (magnetite and hematite) and chert stained red by hematite (Figure 9.14). [caption id="attachment_315" align="aligncenter" width="500"] Figure 9.14 An example of a banded iron formation with dark iron oxide layers interspersed with chert stained red by hematite. This rock is 2.1 billion years old. Source: Andre Karwath (2005), CC BY-SA. Image source.[/caption] BIFs formed before Earth's atmosphere was fully oxygenated. At that time, seawater contained abundant soluble ferrous iron (Fe²⁺). However, once cyanobacteria began releasing oxygen into the atmosphere as a byproduct of photosynthesis, the iron in the seawater reacted with the oxygen, turning it into insoluble ferric iron (Fe³⁺). The result was that iron oxide minerals precipitated and sank to the ocean floor. The prevalence of BIFs in rocks dating from 2400 to 1800 Ma reflects a time when free oxygen was being added to the atmosphere, but removed just as quickly by chemical reactions. After 1800 Ma, little dissolved iron was left in the oceans so no more BIFs formed.

Evaporites

In arid regions, lakes and inland seas typically have no stream outlet, and the water that flows into them is removed only by evaporation. Under these conditions, the water becomes increasingly concentrated with dissolved salts, and eventually some of these salts may reach saturation levels and start to crystallize (Figures 9.15, 9.16). [caption id="attachment_316" align="aligncenter" width="600"] Figure 9.15 kłlilx’^w (Spotted Lake) near Osoyoos, BC in May. The water was relatively fresh because of winter rains. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] [caption id="attachment_317" align="aligncenter" width="600"] Figure 9.16 kłlilx’^w in mid-August, showing the surface encrusted with salt deposits. Source: Anthropodermic (2011), CC BY-SA 2.0. Image source. Note: The lake is a protected cultural heritage site of the Syilx Okanagan People. Anyone wishing to visit should seek permission.[/caption] Although all evaporite deposits are unique because of differences in the chemistry of the water, in most cases minor amounts of carbonates start to precipitate when the solution is reduced to about 50% of its original volume. Gypsum (CaSO₄·H₂O) precipitates at about 20% of the original volume, and halite (NaCl) precipitates at 10%. Other important evaporite minerals include sylvite (KCl) and borax (Na₂B₄O₇·10H₂O). Sylvite is mined as potash at numerous locations across Saskatchewan from evaporites that formed during the Devonian (~385 Ma) when an inland sea occupied much of the region.

Practice with Chemical and Biochemical Sedimentary Rocks

These structures are made of the (hint: chemical or biochemical?) sedimentary rock called . [caption id="attachment_1689" align="aligncenter" width="300"] Mineral deposits in Avshalom Cave in the Judean hills.[/caption]
Gypsum is an example of a (hint: chemical or biochemical?) sedimentary rock called a(n) . [caption id="attachment_1692" align="aligncenter" width="300"] A cliff of gypsum that was once at the bottom of an ocean basin. The ions that make the gypsum were concentrated over many cycles of seawater evaporating then refilling.[/caption]
Petrified wood is an example of the (hint: chemical or biochemical?) sedimentary rock called . [caption id="attachment_1693" align="aligncenter" width="300"] Petrified wood starts out as regular wood, but after the tree dies and is buried, silica seeps in and fills up the tree cells, preserving details even on a microscopic level. This process is entirely inorganic.[/caption]
This rock, also called a BIF for short, is a (hint: chemical or biochemical?) sedimentary rock with the full name . [caption id="attachment_1694" align="aligncenter" width="300"] Dark grey layers in this rock are rich in magnetite (Fe₂O₄), and reddish layeres have hematite (Fe₂O₃) and chert (SiO₂).[/caption]
This is a (hint: chemical or biochemical?) sedimentary rock called . [caption id="attachment_1695" align="aligncenter" width="300"] A carbonate rock made up of the calcite shells of marine organisms. Abundant fossils are visible.[/caption]
This is a (hint: chemical or biochemical?) sedimentary rock called . [caption id="attachment_1696" align="aligncenter" width="300"] These carbonate columns precipitated in a lake with a very high concentration of carbonate (CO₃²⁻) and bicarbonate (HCO₃⁻) ions in its waters.[/caption]

To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="105"]

Extra! Detective Work with Evaporite Rocks

[h5p id="106"]

After the Antarctic and Arctic Deserts, the Atacama Desert of South America is the driest place on Earth. It's so barren and hostile to life that NASA has used it to test methods for finding life on Mars. Researchers have used a variety of dating methods to get the ages of exposed gypsum beds. Those methods indicate that the gypsum is at least 14 million years old, and therefore that the driest regions of the Atacama desert may have been that dry for at least 14 million years. Why does the age of the gypsum tell researchers how long the desert has been dry? [caption id="attachment_1767" align="alignnone" width="641"] A “tree” of volcanic rock in the Atacama Desert, sculpted by wind-blown sand. Inset: Atacama Desert (yellow) and surrounding arid regions (orange).[/caption] Answer Gypsum is an evaporite, and therefore soluble in water. If gypsum has been present for that long, it means there hasn't been enough rain to dissolve it or wash it away in that time.

9.3 Organic Sedimentary Rocks

kqzheng — Fri, 24 Aug 2018 19:09:40 +0000

Organic sedimentary rocks are those containing large quantities of organic molecules. Organic molecules contain carbon, but here we're referring specifically to molecules with carbon-hydrogen bonds, such as materials from the soft tissues of plants and animals. In other words, the carbon in calcite (CaCO₃) wouldn’t make calcite an organic mineral because it isn’t bonded to hydrogen. An important organic sedimentary rock is coal. Most coal forms in swampy land adjacent to rivers and within deltas, and where climates are humid and tropical to temperate. The vigorous growth of vegetation leads to an abundance of organic matter that accumulates within stagnant, acidic water. This limits decay and oxidation of the organic material. If this situation—where the dead organic matter is submerged in oxygen-poor water—is maintained for centuries to millennia, a thick layer of material can accumulate. Limited decay will transform this layer into peat (Figure 9.17a, Figure 9.18 upper left). [caption id="attachment_320" align="aligncenter" width="655"] Figure 9.17 Formation of coal. (a) Accumulation of organic matter within a swampy area forms a layer of peat; (b) The organic matter is buried under sediment and is compressed; (c) With greater burial, lignite coal forms; (d) At even greater depths, bituminous and eventually anthracite coal form. Source: Steven Earle (2015), CC BY 4.0.[/caption] At some point the swamp deposit is covered with more sediment — typically because a river changes its course or sea level rises (Figure 9.17b). As more sediments are added, the organic matter is compressed and heated as temperatures increase with depth. This has the effect of concentrating the carbon within the coal. The amount of heating will determine how far this process progresses. The further the process does progress, the more the coal will go from having obvious pieces of plant material within it, to being a black, shiny mass. Low-grade lignite coal forms at depths between 100 m to 1,500 m and temperatures up to ~50°C (Figure 9.17c). This is still a relatively early stage in the coal formation process, so the lignite commonly displays plant fossils that have not yet been destroyed in the process of coalification (Figure 9.18 upper right). At between 1,000 m to 5,000 m depth and temperatures up to 150°C m, bituminous coal forms (Figure 9.17d, 9.18 lower right). At depths beyond 5,000 m and temperatures over 150°C, anthracite coal forms (Figure 9.18 lower left). In fact, as temperatures rise, the lower-grade forms of coal are actually being transformed from sedimentary to metamorphic rocks. [caption id="attachment_321" align="aligncenter" width="596"] Figure 9.18 The formation of coal begins when plant matter is prevented from decaying by accumulating in low-oxygen, acidic water. A layer of peat forms. Heating and compression of peat form lignite, bituminous coal, and finally anthracite, as pressure and temperature increases. Source: Karla Panchuk (2017), CC BY-NC-SA 4.0. Photos by R. Weller/ Cochise College and U. S. Geological Survey. Click for more attributions and terms of use.[/caption] The transition from peat to anthracite results in a progressive increase in the carbon concentration, in hardness, and in the amount of energy available to be released upon combustion.

Concept Check: Coal Formation

Question 1: Where does coal form? Fill in the blanks. Coal forms in (hint: swampy or dry?) conditions where there is lots of accumulated matter, and (hint: a gas that many decomposer organisms need to live)-poor conditions limit decay. Question 2: How does coal form? Drag the words into the correct boxes. The first material to form from the accumulating plant matter is . As sediments and then rocks accumulate above it, it's compressed and heated. If temperatures and pressures continue to increase over time, it can be transformed into , then , then . Early in its formation, rock-forming conditions are present, but as the process progresses, conditions change to . Fill-in-the-blank options:

bituminous coal
metamorphic
anthracite
lignite
sedimentary
peat

To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="107"]

9.4 Depositional Environments and Sedimentary Basins

kqzheng — Fri, 24 Aug 2018 19:10:35 +0000

The setting in which sediments are accumulated is called a depositional environment. (Some of the more important of these environments are illustrated in Figure 9.19.) Thus far you've seen that some types of sedimentary rocks—coal, and gypsum, for example—require very specific conditions to form because particular biological processes or chemical reactions are necessary. Different types of clastic sedimentary rocks also form in particular depositional environments. In the case of clastic rocks, the key environmental conditions are related to the amount of energy available to transport sediments, and how far the sediments get from their source before being deposited. Broadly, depositional environments can be said to be terrestrial, marine, or to reflect a transitional zone between the two. Terrestrial refers to depositional environments on land. These may be depositional environments such as deserts, found on dry land, but they could also be environments such as freshwater lakes or rivers. Marine refers to environments associated with saltwater seas and oceans. Transitional depositional environments include environments such as deltas, where freshwater rivers empty into saltwater seas or oceans. [caption id="attachment_324" align="aligncenter" width="1024"] Figure 9.19 Some of the important depositional environments for sediments and sedimentary rocks. Source: Karla Panchuk (2021) CC BY-SA 4.0. Modified after Mike Norton (2018) CC BY-SA 3.0. Image source.[/caption] Tables 9.1 and 9.2 provide a summary of the processes and sediment types that pertain to the various depositional environments illustrated in Figure 9.19. The types of sediments that accumulate in these environments are examined in more detail in the last section of this chapter.

Table 9.1 Terrestrial Depositional Environments. *Source: Karla Panchuk (2018), CC BY 4.0. Modified after Steven Earle (2015), CC BY 4.0. Data source.*
Environment	Key Transport Processes	Depositional Settings	Typical Sediments
Glacial	Gravity, moving ice, moving water	Valleys, plains, streams, lakes	Glacial till, gravel, sand, silt, clay
Alluvial	Gravity, moving water	Where steep-sided valleys meet plains	Coarse angular fragments
Fluvial	Moving water	Streams	Gravel, sand, silt, organic matter
Aeolian	Wind	Deserts and coastal regions	Sand, silt
Lacustrine	Moving Water	Lakes	Sand, silt, clay, organic matter
Evaporite	Still water	Lakes in arid regions	Salts, clay

Table 9.2 Marine & Transitional Depositional Environments *Source: Karla Panchuk (2018), CC BY 4.0. Modified after Steven Earle (2015), CC BY 4.0. Data source*
Environment	Key Transport Processes	Depositional Settings	Typical Sediments
Deltaic	Moving water	Deltas	Sand, silt, clay, organic matter
Beach	Waves, long-shore currents	Beaches, spits, sand bars	Gravel, sand
Tidal	Tidal currents	Tidal flats	Fine-grained sand, silt, clay
Reef	Waves, tidal currents	Reefs and adjacent basins	Carbonates
Shallow marine	Waves, tidal currents	Shelves, slopes, lagoons	Carbonates in tropical climates; sand/silt/clay elsewhere.
Lagoonal	Little transportation	Lagoon bottom	Carbonates in tropical climates, silt, clay
Submarine fan	Underwater gravity flows	Continental slopes, abyssal plains	Gravel, sand, silt, clay
Deep water	Ocean currents	Deep-ocean abyssal plains	Clay, carbonate mud, silica mud

Practice with Depositional Environments [h5p id="108"]

Sedimentary Basins Are Needed to Collect Sediment

Most of the sediments that you might see around you, including talus on steep slopes, sand bars in streams, or gravel in road cuts, will never become sedimentary rocks. This is because they've only been deposited relatively recently—perhaps a few centuries or millennia ago—and will be re-eroded before they are buried deep enough beneath other sediments to be lithified. In order for sediments to be preserved long enough to be turned into rock (a process that takes millions or tens of millions of years) they need to have been deposited in a basin in which sediments can be preserved for that long. Most such basins are formed by plate tectonic processes (Figure 9.20). [caption id="attachment_325" align="aligncenter" width="650"] Figure 9.20 Some types of tectonically produced basins: (a) trench basin, (b) forearc basin, (c) foreland basin, and (d) rift basin. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] Trench basins form where a subducting oceanic plate dips beneath the overriding continental or oceanic lithosphere. They can be several kilometres deep, and in many cases, host thick sequences of sediments from nearby eroding coastal mountains. There is a well-developed trench basin off the west coast of Vancouver Island. A forearc basin lies between the subduction zone and the volcanic arc, and may be formed in part by friction between the subducting plate and the overriding plate, which pulls part of the overriding plate down. The Strait of Georgia, the channel between Vancouver Island and the BC mainland, is a forearc basin. A foreland basin is caused by the mass of a mountain range depressing the crust. A rift basin forms where continental crust is being pulled apart, and the crust on both sides the rift subsides. If rifting continues this will eventually becomes a narrow sea, and then an ocean basin. The East African rift basin represents an early stage in this process.

Practice with Types of Sedimentary Basins

Fill in the missing words to complete the descriptions of the different types of sedimentary basins. One type of sedimentary basin forms when continental crust begins to split apart and sag downward. This is a basin. Other types of sedimentary basins are related to subduction zones. The deepest type, a basin, occurs right where subduction is happening. If a subducting plate drags down the front edge of the plate with which it's colliding, a basin could form between the subduction zone and a chain of volcanic mountains. Thanks to isostasy, the weight of the mountains themselves can flex the lithosphere downward into a basin. To check your answers, navigate to the below link to view the interactive version of this activity.

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9.6 Groups, Formations, and Members

kqzheng — Fri, 24 Aug 2018 19:11:49 +0000

Geologists who study sedimentary rocks need ways to divide them into manageable units, and they also need to give those units names so that they can easily be referred to and compared with other rocks deposited in other places. The International Commission on Stratigraphy (ICS) has established a set of conventions for grouping, describing, and naming sedimentary rock units. The main stratigraphic unit is a formation. A formation is a series of beds that is distinct from other beds above and below, and is thick enough to be shown on the geological maps that are widely used within the area in question. In most parts of the world, geological mapping is done at a relatively coarse scale, so most formations are on the order of a few hundred metres thick. At that thickness, a typical formation would appear on a typical geological map as an area that is at least a few millimetres thick. A series of formations can be classified together to define a group, which could be as much as a few thousand metres thick, and represents a series of rocks that were deposited within a single basin (or a series of related and adjacent basins) over millions to tens of millions of years. In areas where detailed geological information is needed (for example, within a mining or petroleum district) a formation might be divided into members, where each member has a specific and distinctive lithology (rock type). For example, a formation that includes both shale and sandstone might be divided into members, one of which is shale, and the other sandstone. If even more detail is required, members may be divided into beds, but that's only applicable to beds that have a special geological significance. Groups, formations, and members are typically named for the area where they're found.

The Nanaimo Group: An Example

The sedimentary rocks of the Nanaimo Group on Vancouver Island provide a useful example for understanding groups, formations, and members. During the latter part of the Cretaceous Period (from about 90 Ma to 65 Ma) a thick sequence of clastic rocks was deposited in a foreland basin between what is now Vancouver Island and the BC mainland (Figure 9.28). Nanaimo Group comprises a 5000 m thick sequence of conglomerate, sandstone, and mudstone layers. Coal was mined from the Nanaimo Group rocks from around 1850 to 1950 in the Nanaimo region, and is still being mined in the Campbell River area. [caption id="attachment_336" align="aligncenter" width="500"] Figure 9.28 The distribution of the Upper Cretaceous Nanaimo Group rocks on Vancouver Island, the Gulf Islands, and in the Vancouver area. Source: Steven Earle (2015), CC BY 4.0. Image source, modified after Mustard (1994).[/caption] The Nanaimo Group is divided into 11 formations (Table 9.3). In general, the boundaries between formations are based on major lithological differences. A wide range of depositional environments existed during the accumulation of the Nanaimo Group rocks, including:

Nearshore marine for the Comox and Haslam Formations
Fluvial and deltaic with backwater swampy environments for the coal-bearing Extension, Pender, and Protection Formations
A deep-water submarine fan environment for the upper six formations.

The differences in the depositional environments are probably a product of variations in tectonic-related uplift over time.

Table 9.3 Nanaimo Group Formations. *Source: Steven Earle, with data from Mustard (1994)*
Age (Ma)	Formation	Lithologies	Depositional Environment
~65-66	Gabriola	Sandstone with minor mudstone	Submarine fan, high energy
~66-67	Spray	Mudstone/ sandstone turbidites	Submarine fan, low energy
~67-68	Geoffrey	Sandstone and conglomerate	Submarine fan, high energy
~68-70	Northumberland	Mudstone turbidites	Submarine fan, low energy
~70	De Courcy	Sandstone	Submarine fan, high energy
~70-72	Cedar District	Mudstone turbidites	Submarine fan, low energy
~72-75	Protection	Sandstone and minor coal	Nearshore marine and onshore deltaic and fluvial
~75-80	Pender	Sandstone and minor coal	Nearshore marine and onshore deltaic and fluvial
~80	Extension	Conglomerate, with minor sandstone and some coal	Nearshore marine and onshore deltaic and fluvial
~80-85	Haslam	Mudstone and siltstone	Shallow marine
~85-90	Comox	Conglomerate, sandstone, mudstone (coal in the Campbell River area)	Nearshore fluvial and marine

The five lower formations of the Nanaimo Group are all exposed in the Nanaimo area, and were well studied during the coal-mining era between 1850 and 1950. With the exception of the Haslam formation, they were divided into members, because that was useful for understanding the rocks in the areas where coal was mined. There is much variety in the Nanaimo Group rocks, and it would take hundreds of photographs to illustrate all of the different types of rocks. Nevertheless, a few representative examples are shown in Figures 9.29-9.31. [caption id="attachment_337" align="aligncenter" width="500"] Figure 9.29 Nanaimo Group, Spray Formation. Turbidite layers on Gabriola Island. Each turbidite set consists of a lower sandstone layer (light colour) that grades upward into siltstone, and then into mudstone. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] [caption id="attachment_338" align="aligncenter" width="500"] Figure 9.30 Nanaimo Group, Pender Formation. Two separate layers of fluvial sandstone with a thin (approx. 75 cm) coal seam in between. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] [caption id="attachment_339" align="aligncenter" width="500"] Figure 9.31 Nanaimo Group, Comox Formation. The metal object is the end of a rock hammer that is 3 cm wide. Almost all of the clasts in this view are well-rounded basalt pebbles cobbles eroded from the Triassic Karmutsen Formation that makes up a major part of Vancouver Island. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Concept Check: Groups, Formations, and Members

Match the words into the correct boxes to complete this explanation. If you look at a geologic map, the most basic units visible (at least a few mm thick on the map) are . In real life, these units can be of metres thick. Those basic units are organized into , which are packages of rocks of metres thick that represent deposition in an entire basin, or in related basins. If more detail is needed, the basic units can be subdivided into , each with a specific rock type. In summary, are divided into , which can be further divided into . Fill-in-the-blank options;

members
thousands
members
groups
formations
hundreds
formations
groups

To check your answers, navigate to the below link to view the interactive version of this activity.

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References

Mustard, P. (1994). The upper cretaceous Nanaimo group, Georgia Basin. In J. Monger (Ed.), Geology and geological hazards of the Vancouver region. Geological Survey of Canada Bulletin, 481, 27-95.

Chapter 9 Summary & Key Term Check

kqzheng — Fri, 24 Aug 2018 19:12:43 +0000

Chapter 9 Main Ideas

9.1 Clastic Sedimentary Rocks

Clastic sedimentary rocks are formed from rock and mineral particles that are cemented together. The naming system for these rocks depends on grain size, sorting, composition, and shape. Five common types of clastic sedimentary rocks are conglomerate, breccia, sandstone, shale, and mudstone. Sandstones are further organized according to the abundance of fine particles they contain, and the composition of their sand-sized grains.

Practice Again

9.2 Chemical and Biochemical Sedimentary Rocks

Chemical and biochemical sedimentary rocks form from ions that were transported in solution, and then converted into minerals by chemical and/or biological processes. The most common biochemical rock, limestone, typically forms in shallow tropical marine environments, where biological activity is a very important factor. Chert and banded iron formations can be from deep-ocean environments. Evaporites form where the waters of lakes and inland seas become supersaturated due to evaporation.

Practice Again

Chemical and biochemical sedimentary rock types

Extra! Compare & Contrast Do you know your clastic, chemical, and biochemical rocks well enough to be able to tell them apart? How would you distinguish each of the following rocks from each other? Start by thinking about which general sedimentary rock type they are. When you have it figured out, click each link to check your answer.

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Conglomerate vs. breccia: Both conglomerate and breccia are coarse-grained and poorly sorted clastic sedimentary rocks. However, the grains in conglomerate are rounded whereas the grains in breccia are angular.
Sandstone vs. shale: These are both clastic rocks. Sandstone is medium-grained so you may be able to see individual sand grains. Shale is fine grained, so individual grains will be too small to see and it will feel smoother than the sandstone. Also, shale is distinctive in that it breaks apart in layers.
Travertine vs. limestone formed in a reef: Travertine and limestone are both made of the mineral calcite. One way to distinguish between these rocks is to look for fossils of reef-building organisms in the biochemical limestone. Travertine is a chemical sedimentary rock, so it won't be built from the shells of marine organisms.
Chert vs. gypsum, an evaporite rock: Both are chemical sedimentary rocks. Chert is made of silica (the same composition as quartz) and gypsum is a mineral as well as a rock. You could test the mineral properties of the samples (chert will scratch glass, but you can probably scratch gypsum with your fingernail), but you could also add water to the samples. Gypsum will dissolve but chert will not. (But don't do that if you want to keep your gypsum sample safe.)

9.3 Organic Sedimentary Rocks

Organic sedimentary rocks contain abundant organic carbon molecules (molecules with carbon-hydrogen bonds). An example is coal, which forms when dead plant material is preserved in stagnant swamp water, and later compressed and heated.

Practice Again

Coal formation

9.4 Depositional Environments and Sedimentary Basins

There is a wide range of depositional environments, both on land (including glaciers, lakes, and rivers) and in the ocean (including deltas, reefs, shelves, and the deep-ocean floor). In order to be preserved, sediments must accumulate in sedimentary basins, many of which form through plate-tectonic processes.

Practice Again

9.5 Sedimentary Structures and Fossils

Sedimentary rocks can have distinctive structures that are important in determining their depositional environments. Fossils are useful for determining the age of a rock, the depositional environment, and the climate at the time of deposition.

Practice Again

Types of sedimentary structures

Extra!

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Mysterious Rocks on Mars

The image below was taken by NASA’s Curiosity Rover (field of view is approximately 1.2 m across.) The image shows a slab of red stone surrounded by sand. The slab is covered in a complex network of interconnected lines. [caption id="attachment_1864" align="aligncenter" width="726"] Photograph: NASA/JPL-Caltech/MSSS (2017). Public Domain. View source.[/caption]

Which sedimentary structure might the lines represent?

The pattern of lines on the slab of rock is most likely from mud cracks.

What would that sedimentary structure imply about past environments on Mars?

The presence of mud cracks is evidence that Mars had abundant liquid water in the past.

9.6 Groups, Formations, and Members

Sedimentary sequences are classified into formations so that they can be mapped easily and without confusion. Formations can be combined into groups, or broken down into members for more detail.

Key Term Check

What key term from Chapter 9 is each card describing? Turn the card to check your answer. [h5p id="114"]

10.1 Controls on Metamorphic Processes

kqzheng — Fri, 24 Aug 2018 18:58:58 +0000

The main factors that control metamorphic processes are:

The chemical composition of the parent rock
The temperature at which metamorphism takes place
The pressure applied, and whether the pressure is equal in all directions or not
The amount and type of fluid (mostly water) that is present during metamorphism
The amount of time over which metamorphic conditions are sustained

Mineral composition

Parent rocks can be from any of the three rock types: sedimentary, igneous, or metamorphic. The critical feature of the parent rock is its mineral composition. This is because the stability of minerals—how they are influenced by changing conditions—is what determines which minerals form as metamorphism takes place. When a rock is subjected to increased temperatures and pressures, some minerals will undergo chemical reactions and turn into new minerals, while others might just change their size and shape.

Temperature

Temperature is a key variable in determining which metamorphic reactions happen because minerals are stable over a specific range of temperatures (dependent on pressure and the presence of fluids). Quartz, for example, is stable from surface temperatures up to approximately 1800°C. Under higher pressures, that upper limit will also be higher. If water is present, the limit will be lower. Most other common minerals have upper limits between 150°C and 1000°C. Some minerals will change their crystal structure depending on the temperature and pressure. Quartz has different polymorphs that are stable between 0°C and 1800°C, but the differences in quartz polymorphs aren't nearly as stunning as with the minerals kyanite, andalusite, and sillimanite. They are polymorphs with the composition Al₂SiO₅. These polymorphs are especially useful for studying metamorphic rocks, because their presence can be used to figure out what pressures and temperatures a metamorphic rock experienced (Figure 10.3). If a rock has more than one of these polymorphs, the pressure and temperature range can be narrowed down even further to the boundaries of the stability fields. [caption id="attachment_347" align="aligncenter" width="600"] Figure 10.3 The Al₂SiO₅ polymorphs andalusite, kyanite, and sillimanite, and their stability fields. Source: Karla Panchuk (2018), CC BY-SA 4.0. Click for more attributions.[/caption]

Pressure

Pressure has implications for mineral stability, and therefore the mineral content of metamorphic rocks, but it also determines the texture of metamorphic rocks. When directed pressure (or directed stress) acts on a rock, it means the stress on the rock is greater in one direction than another. In an experiment with cylinders of modeling clay stacked in a block (Figure 10.4, top), pushing down on the clay from above resulted in higher directed pressure in the up-down direction (larger arrows; downward from pushing on the clay, and upward from the force of the table beneath the clay) than in the sideways direction, where only air pressure was acting (small arrows). The clay cylinders became elongated in the direction of least pressure. [caption id="attachment_348" align="aligncenter" width="600"] Figure 10.4 Modelling clay experiments showing the effects of pressure on textures. Top: Directed pressure- clay was set on a flat surface and pushed down on from above (large arrows). Cylinders making up the clay block became elongated in the direction of least stress. Bottom: Shear stress applied to the top and bottom of a block of clay caused the interior to stretch. Note white dashed reference circles and elongated ellipses. Source: Karla Panchuk (2018), CC BY 4.0.[/caption] Rocks undergo shear stress when forces act parallel to surfaces. In another modelling-clay experiment, applying oppositely directed forces to the top and bottom of a block of clay (Figure 10.4, bottom) caused diagonal stretching within the block. Note the change in shape of the dashed white reference circles. In both experiments, parts of the clay became elongated in a particular direction. When mineral grains within a rock become aligned like this, it produces a fabric called foliation. Foliation is described in more detail later in this chapter.

Dance Break!

Image Description: Instructions for the moonwalk. Steps 2 and 4 involve brushing your foot backward on the floor. Steps 1 and 3 involve applying force straight up and down.

Fill in the dance-step numbers to complete these directions. The moonwalk is a dance move that gets its awesome sauce from the contrast between the dancer appearing to be doing a forward walking motion, but actually sliding smoothly backward. Using these simple instructions, you too can moonwalk. Just be sure to apply only directed pressure to the floor during Step and Step . Apply shear stress to the floor only during Step and Step as you drag your foot backward. To check your answers, navigate to the below link to view the interactive version of this activity.

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Fluids

Water is the main fluid present within rocks of the crust, and the only one considered here. The presence of water is important for two main reasons. First, water facilitates the transfer of ions between minerals and within minerals, and therefore can speed up metamorphic chemical reactions. Not only can metamorphism happen more rapidly, but processes can be completed that might not otherwise have time to occur. Secondly, water—especially hot water—can have elevated concentrations of dissolved substances, making it an important medium for moving ions from one place to another within the crust. Processes facilitated by hot water are called hydrothermal processes (hydro refers to water, and thermal refers to heat).

Time

Most metamorphic reactions are slow. It's estimated that when new minerals grow in a rock during metamorphism, they add about 1 mm to the outside of the mineral crystal for every million years. Very slow reaction rates make it hard to study metamorphic processes in a lab. While the rate of metamorphism is slow, the tectonic processes that lead to metamorphism are also very slow, so there is a good chance that metamorphic reactions will be completed. For example, an important setting for metamorphism is many kilometres deep within the roots of mountain ranges. A mountain range takes tens of millions of years to form, and tens of millions of years more to be eroded to the extent that we can see the rocks that were metamorphosed deep beneath it.

How Old Is This Rock?

The large reddish crystals in this metamorphic rock are garnet, and the surrounding light-coloured rock is dominated by muscovite mica. The largest of the garnets have diameters similar to that of the 23 mm Euro coin. Assume that the diameters of the garnets increased at a rate of 1 mm per million years. How old is this rock?

Much less than 23 million years old
About 23 million years old
A lot older than 23 million years

To check your answers, navigate to the below link to view the interactive version of this activity.

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10.3 Classification of Metamorphic Rocks

kqzheng — Fri, 24 Aug 2018 18:59:47 +0000

Metamorphic rocks are broadly classified based on whether or not they're foliated. Non-foliated metamorphic rocks don't have aligned mineral crystals because (unlike foliated rocks) they form where pressure is uniform, or else near the surface where pressure is very low. They can also form when the parent rock consists only of blocky minerals such as quartz and calcite, that don't have one dimension substantially longer than the other. (Note that the rule about crystal shape breaks down in zones of intense deformation, where even minerals like quartz can be squeezed into long stringers, much like squeezing toothpaste out of a tube; Figure 10.12). [caption id="attachment_360" align="aligncenter" width="600"] Figure 10.12 Rocks from the Western Carpathians mountain range without deformation (left) and after deformation (right). Scale bar: 1 mm. Left- An undeformed granitic rock containing the mica mineral biotite (Bt), plagioclase feldspar (Pl), potassium feldspar (Kfs), and quartz (Qtz). Right- A metamorphic rock (mylonite) resulting from extreme deformation of granitic rocks. Quartz crystals have been flattened and deformed. The other minerals have been crushed and deformed into a fine-grained matrix (Mtx). Source: Farkašovský et al. (2016), CC BY-NC-ND. Click to view the original figure captions and access the full text.[/caption]

Types of Foliated Metamorphic Rocks

Four common types of foliated metamorphic rocks, listed in order of metamorphic grade or intensity of metamorphism are slate, phyllite, schist (pronounced "shist"), and gneiss (pronounced "nice"). Each of these has a characteristic type of foliation

Slate

Slate (Figure 10.13) forms from the low-grade metamorphism of shale. (Metamorphic grade refers to the intensity of metamorphism.) Slate has microscopic clay and mica crystals that have grown perpendicular to the maximum stress direction. Slate tends to break into flat sheets or plates, a property described as slaty cleavage. [caption id="attachment_361" align="aligncenter" width="607"] Figure 10.13 Slate, a low-grade foliated metamorphic rock. Left- Slate fragments resulting from rock cleavage. Right- The same rock type in outcrop. Source: Karla Panchuk (2018), CC BY-SA 4.0. Click for more attributions.[/caption]

Phyllite

Phyllite (Figure 10.14) is similar to slate, but has typically been heated to a higher temperature. As a result, the micas have grown larger. They still are not visible as individual crystals, but the larger size leads to a satiny sheen on the surface. The cleavage of phyllite is slightly wavy compared to that of slate. [caption id="attachment_362" align="aligncenter" width="595"] Figure 10.14 Phyllite, a fine-grained foliated metamorphic rock. Left- A hand sample showing a satin texture. Right- The same rock type in outcrop in the city of Sopron, Hungary. Source: Karla Panchuk (2018), CC BY-SA 4.0. Click for more attributions.[/caption]

Schist

Schist (Figure 10.15) forms at higher temperatures and pressures and has mica crystals that are large enough to see without magnification. Individual crystal faces may flash when the sample is turned in the light, making the rock appear to sparkle. Other minerals such as garnet might also be visible, but it 's not unusual to find that schist consists predominantly of a single mineral. [caption id="attachment_363" align="aligncenter" width="403"] Figure 10.15 Schist, a medium- to high-grade foliated metamorphic rock. Top- Hand sample showing light reflecting off of mica crystals. Bottom- Close-up view of mica crystals and garnet. Source: Karla Panchuk (2018), CC BY-SA 4.0. Click for more attributions.[/caption]

Gneiss

Gneiss (Figure 10.16) forms at the highest pressures and temperatures, and has crystals large enough to see with the unaided eye. Gneiss features minerals that have separated into bands of different colours, and those bands are define foliation for gneiss. Sometimes the bands are very obvious and continuous (Figure 10.16, upper right), but sometimes they are more like lenses (upper left). Dark bands are largely amphibole while the light-coloured bands are feldspar and quartz. Most gneiss has little or no mica because it forms at temperatures higher than those under which micas are stable. [caption id="attachment_364" align="aligncenter" width="433"] Figure 10.16 Gneiss, a coarse-grained, high grade metamorphic rock, is characterized by colour bands. Top- Hand samples showing that colour bands can be continuous (left) or less so (right). Bottom- Gneiss in outcrop at Belteviga Bay, Norway. Notice the light and dark stripes on the rock. Source: Karla Panchuk (2018), CC BY-SA 4.0. Click for more attributions.[/caption] While slate and phyllite typically form only from mudrock protoliths, schist and especially gneiss can form from a variety of parent rocks, including mudrock, sandstone, conglomerate, and a range of both volcanic and intrusive igneous rocks. Schist and gneiss can be named on the basis of important minerals that are present: a schist derived from basalt is typically rich in the mineral chlorite, so we call it chlorite schist. One derived from shale may be a muscovite-biotite schist, or just a mica schist, or if there are garnets present it might be mica-garnet schist. Similarly, gneiss that originated as basalt and is dominated by amphibole, is an amphibole gneiss or amphibolite (Figure 10.17). [caption id="attachment_365" align="aligncenter" width="400"] Figure 10.17 Amphibolite in thin section (2mm field of view), derived from metamorphism of a mafic igneous rock. Green crystals are the amphibole hornblende, and colourless crystals are plagioclase feldspar. Note horizontal crystal alignment. Source: D.J. Waters, University of Oxford. view source/ view context. Click for original figure caption and terms of use.[/caption]

Types of Non-foliated Metamorphic Rocks

Metamorphic rocks that form under low-pressure conditions or under the effects confining pressure—which is equal in all directions—do not become foliated. In most cases, this is because they are not buried deeply enough, and the heat for the metamorphism comes from a body of magma that has moved into the upper part of the crust. Metamorphism that happens when rocks are heated by magma is called contact metamorphism. Some examples of non-foliated metamorphic rocks are marble, quartzite, and hornfels.

Marble

Marble (Figure 10.18) is metamorphosed limestone. When it forms, the calcite crystals recrystallize (re-form into larger blocky calcite crystals), and any sedimentary textures and fossils that might have been present are destroyed. If the original limestone is pure calcite, then the marble will be white. On the other hand, if it has impurities such as clay, silica, or magnesium, the marble could be “marbled” in appearance (Figure 10.18, bottom). [caption id="attachment_366" align="aligncenter" width="480"] Figure 10.18 Marble is a non-foliated metamorphic rock with a limestone protolith. Left- Marble made of pure calcite is white. Upper right- microscope view of calcite crystals within marble that are blocky and not aligned. Lower right- A quarry wall showing the "marbling" that results when limestone contains components other than calcite. Source: Karla Panchuk (2018), CC BY-NC-SA. Click for more attributions.[/caption]

Quartzite

Quartzite (Figure 10.19) is metamorphosed sandstone. It is dominated by quartz, and in many cases, the original quartz grains of the sandstone are welded together with additional silica. Sandstone often contains some clay minerals, feldspar or lithic fragments, so quartzite can also contain impurities.

[caption id="attachment_367" align="aligncenter" width="622"] Figure 10.19 Quartzite is a non-foliated metamorphic rock with a sandstone protolith. Left- Quartzite from the Baraboo Range, Wisconsin. Right- Photomicrograph showing quartz grains in quartzite from the Southern Appalachians. In the upper left half of the image, blocky quartz crystals show some evidence of alignment running from the upper right to the lower left. Source: Karla Panchuk (2018), CC BY-SA 4.0. Photomicrograph: Geologian (2011), CC BY-SA 3.0. Image source.[/caption] Even if formed under directed pressure, quartzite is generally not foliated because quartz crystals do not normally align with the directional pressure. On the other hand, any clay present in the original sandstone is likely to be converted to mica during metamorphism, and any such mica is likely to align with the directional pressure.

Hornfels

Hornfels is another non-foliated metamorphic rock that normally forms during contact metamorphism of fine-grained rocks like mudstone or volcanic rocks. Hornfels have different elongated or platy minerals (e.g., micas, pyroxene, amphibole, and others) depending on the exact conditions and the parent rock, yet because the pressure wasn't substantially higher in any particular direction, these crystals remain randomly oriented. The hornfels in Figure 10.20 (left) appears to have gneiss-like bands, but these actually reflect the beds of alternating sandstone and shale that were in the protolith. They are not related to alignment of crystals due to metamorphism. On the right of Figure 10.20 is a microscopic view of another sample of hornfels, also from a sedimentary protolith. The dark band at the top is from the original bedding. Here you can see that the brown mica crystals (biotite) are not aligned. [caption id="attachment_368" align="aligncenter" width="559"] Figure 10.20 Hornfels, a non-foliated metamorphic rock formed from a fine-grained protolith. Left- Hornfels from the Novosibirsk region of Russia from a sedimentary protolith. Dark and light bands preserve the bedding of the original sedimentary rock. The rock has been recrystallized during contact metamorphism and does not display foliation. (scale in cm). Right- Hornfels in thin section from a sedimentary protolith. Note that the brown mica crystals are not aligned. The dark band at the top reflects the layering within the sedimentary parent rock, similar to the way those layers are preserved in the sample on the left. Source: Left- Fedor (2006), Public Domain. Image source.; Right- D.J. Waters, University of Oxford view source/ view context. Click for terms of use.[/caption]

What Happens When Different Rocks Undergo Metamorphism?

The nature of the parent rock controls the types of metamorphic rocks that can form from it under differing metamorphic conditions (temperature, pressure, fluids). The kinds of rocks that can be expected to form at different metamorphic grades from various parent rocks are listed in Table 10.1. [caption id="attachment_369" align="aligncenter" width="640"] Source: Karla Panchuk (2018), CC BY 4.0., modified after Steven Earle (2015), CC BY 4.0 Image source. Click for a text version.[/caption] Some rocks, such as granite, don't change much at the lower metamorphic grades because their minerals are still stable up to several hundred degrees. Sandstone and limestone don’t change much either because their metamorphic forms (quartzite and marble, respectively) have the same mineral composition, but re-formed larger crystals. On the other hand, some rocks do change substantially. Mudrock (e.g., shale, mudstone) can start out as slate, then progress through phyllite, schist, and gneiss, with a variety of different minerals forming along the way. Schist and gneiss can also form from sandstone, conglomerate, and a range of both volcanic and intrusive igneous rocks.

Migmatite: Both Metamorphic and Igneous

If a metamorphic rock is heated enough, it can begin to undergo partial melting in the same way that igneous rocks do. The more felsic minerals (feldspar, quartz) will melt, while the more mafic minerals (biotite, hornblende) do not. When the melt crystallizes again, the result is light-coloured igneous rock interspersed with dark-coloured metamorphic rock. This mixed rock is called migmatite (Figure 10.21). Note that the foliation present in the metamorphic rock is no longer present in the igneous rock. Liquids cannot support a differential stress, so when the melt crystallizes, the foliation is gone. [caption id="attachment_370" align="aligncenter" width="409"] Figure 10.21 Migmatite photographed near Geirangerfjord in Norway. Source: Siim Sepp (2006), CC BY-SA 3.0. Image source.[/caption] A fascinating characteristic of migmatites is ptygmatic (pronounced "tigmatic") folding. These are folds look like they should be impossible because they are enveloped by rock that doesn't display the same complex deformation (Figure 10.22). How could those wiggly folds get in there without the rest of the rock being folded in the same way? [caption id="attachment_371" align="aligncenter" width="400"] Figure 10.22 Ptygmatic folding from Broken Hill, New South Wales, Australia. Ptygmatic folding happens when a stiff layer within a rock is surrounded by weaker layers. Folding causes the stiff layer to crinkle while the weaker layers deform around it. Source: Roberto Weinberg. Image source. Click for terms of use.[/caption] The answer to the ptygmatic fold mystery is that the folded layer is much stiffer than the surrounding layers. When the rock is squeezed, the stiff layer buckles but the weaker surrounding rock flows around buckling.

Practice with Types of Metamorphic Rocks

This is the (hint: foliated or non-foliated?) metamorphic rock called . [caption id="attachment_1704" align="aligncenter" width="300"] Platy minerals give this fine-grained rock a satin sheen. It comes apart in wavy layers.[/caption]
This is the (hint: foliated or non-foliated?) metamorphic rock called . [caption id="attachment_1705" align="aligncenter" width="300"]
Blocky calcite crystals make up this rock, although impurities are sometimes present.[/caption]
This is the (hint: foliated or non-foliated?) metamorphic rock called . [caption id="attachment_1706" align="aligncenter" width="300"] A rock made up of a single mineral (in this case biotite), with crystals large enough to see with the naked eye. It splits apart in wavy layers.[/caption]
This is the (hint: foliated or non-foliated?) metamorphic rock called . [caption id="attachment_1707" align="aligncenter" width="300"] This rock is made up of blocky quartz crystals. Its wavy surface preserves ripple marks from a 480 million year old streambed.[/caption]
This is the (hint: foliated or non-foliated?) metamorphic rock called . [caption id="attachment_1703" align="aligncenter" width="300"] A very fine-grained rock that breaks into thin sheets.[/caption]
This is the (hint: foliated or non-foliated?) metamorphic rock called . [caption id="attachment_1702" align="aligncenter" width="300"]
A coarse-grained rock with crystals large enough to see. Minerals are arranged in bands of light and dark colour.[/caption]
This is the (hint: foliated or non-foliated?) metamorphic rock called . [caption id="attachment_1679" align="aligncenter" width="300"] A fine-grained rock usually derived from contact metamorphism of fine-grained sedimentary or volcanic rocks.[/caption]

To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="120"]

References

Farkašovský, R., Bónová, K., & Košuth, M. (2016). Microstructural, modal and geochemical changes as a result of granodiorite mylonitisation – a case study from the Rolovská shear zone (Čierna hora Mts, Western Carpathians, Slovakia). Geologos, 22(3), 171-190. doi: 10.1515/logos-2016-0019

10.4 Types of Metamorphism and Where They Occur

kqzheng — Fri, 24 Aug 2018 19:00:42 +0000

The outcome of metamorphism depends on pressure, temperature, and the abundance of fluid involved, and there are many settings with unique combinations of these factors. Some types of metamorphism are characteristic of specific plate tectonic settings, but others are not.

Burial Metamorphism

Burial metamorphism occurs when sediments are buried deeply enough that the heat and pressure cause minerals to begin to recrystallize and new minerals to grow, but does not leave the rock with a foliated appearance. As metamorphic processes go, burial metamorphism takes place at relatively low temperatures (up to ~300 °C) and pressures (100s of m depth). To the unaided eye, metamorphic changes may not be apparent at all. Contrast the rock known commercially as Black Marinace Gold Granite (Figure 10.23)—but which is in fact a metaconglomerate—with the metaconglomerate in Figure 10.10. The metaconglomerate formed through burial metamorphism doesn't display any of the foliation that has developed in the metaconglomerate in Figure 10.10. [caption id="attachment_374" align="aligncenter" width="400"] Figure 10.23 Metaconglomerate formed through burial metamorphism. The pebbles in this sample are not aligned and elongated as in the metaconglomerate in Figure 10.10. Source: James St. John (2014), CC BY 2.0. Image source.[/caption]

A Note About Commercial Rock Names Names given to rocks that are sold as building materials, especially for countertops, may not reflect the actual rock type. It's common to use the terms granite and marble to describe rocks that are neither. While these terms might not provide accurate information about the rock type, they generally do distinguish natural rock from synthetic materials. An example of a synthetic material is the one referred to as quartz, which includes ground-up quartz crystals as well as resin. If you happen to be in the market for stone countertops and are concerned about getting a natural product, it's best to ask lots of questions.

Regional Metamorphism

Regional metamorphism is large-scale metamorphism, such as what happens to continental crust along convergent tectonic margins (where plates collide). The collisions result in the formation of long mountain ranges, like those along the western coast of North America. The force of the collision causes rocks to be folded, broken, and stacked on each other, so not only is there the squeezing force from the collision, but from the weight of stacked rocks. The deeper rocks are within the stack, the higher the pressures and temperatures, and the higher the grade of metamorphism that occurs. Rocks that form from regional metamorphism are likely to be foliated because of the strong directional pressure of converging plates. The Himalaya range is an example of where regional metamorphism is happening because two continents are colliding (Figure 10.24). Sedimentary rocks have been both thrust up to great heights—nearly 9 km above sea level—and also buried to great depths. Considering that the normal geothermal gradient (the rate of increase in temperature with depth) is around 30°C per kilometre in the crust, rock buried to 9 km below sea level in this situation could be close to 18 km below the surface of the ground, and it's reasonable to expect temperatures up to 500°C. Notice the sequence of rocks beginning with slate higher up where pressures and temperatures are lower, and ending in migmatite at the bottom where temperatures are so high that some of the minerals start to melt. These rocks are all foliated because of the strong compressing force of the converging plates. [caption id="attachment_375" align="aligncenter" width="501"] Figure 10.24 Regional metamorphism beneath a mountain range resulting from continent-continent collision. Arrows show the forces due to the collision. Dashed lines represent temperatures that would exist given a geothermal gradient of 30 ºC/km. A sequence of foliated metamorphic rocks of increasing metamorphic grade forms at increasing depths within the mountains. Source: Karla Panchuk (2018), CC BY 4.0. Modified after Steven Earle (2015,) CC BY 4.0. Image source.[/caption]

Seafloor (Hydrothermal) Metamorphism

At an oceanic spreading ridge, recently formed oceanic crust of gabbro and basalt is slowly moving away from the plate boundary (Figure 10.25). Water within the crust is forced to rise in the area close to the source of volcanic heat, drawing in more water from further away. This eventually creates a convective system where cold seawater is drawn into the crust, heated to 200 °C to 300 °C as it passes through the crust, and then released again onto the seafloor near the ridge. [caption id="attachment_376" align="aligncenter" width="563"] Figure 10.25 Seafloor (hydrothermal) metamorphism of ocean crustal rock on either side of a spreading ridge. Source: Karla Panchuk (2018), CC BY 4.0. Modified after Steven Earle (2015), CC BY 4.0. Image source.[/caption] [caption id="attachment_377" align="alignright" width="300"] Figure 10.26 Greenstone from the metamorphism of seafloor basalt that took place 2.7 billion years ago. The sample is from the Upper Peninsula of Michigan, USA. Source: James St. John (2012), CC BY 2.0. Image source.[/caption] The passage of this water through the oceanic crust at these temperatuers promotes metamorphic reactions that change the original olivine and pyroxene minerals in the rock to chlorite ((Mg₅Al)(AlSi₃)O₁₀(OH)₈) and serpentine ((Mg, Fe)₃Si₂O₅(OH)₄). Chlorite and serpentine are both hydrated minerals, containing water in the form of OH in their crystal structures. When metamorphosed ocean crust is later subducted, the chlorite and serpentine are converted into new non-hydrous minerals (e.g., garnet and pyroxene) and the water that's released migrates into the overlying mantle, where it contributes to melting. The low-grade metamorphism occurring at these relatively low pressures and temperatures can turn mafic igneous rocks in ocean crust into greenstone (Figure 10.26), a non-foliated metamorphic rock.

Subduction Zone Metamorphism

At subduction zones, where ocean lithosphere is forced down into the hot mantle, there is a unique combination of relatively low temperatures and very high pressures. The high pressures are to be expected, given the force of collision between tectonic plates, and the increasing lithostatic pressure as the subducting slab is forced deeper and deeper into the mantle. The lower temperatures exist because even though the mantle is very hot, ocean lithosphere is relatively cool, and a poor conductor of heat. That means it will take a long time to heat up, and can be several hundreds of degrees cooler than the surrounding mantle. In Figure 10.27, notice that the isotherms (lines of equal temperature, dashed lines) plunge deep into the mantle along with the subducting slab, showing that regions of relatively low temperature exist deeper in the mantle. [caption id="attachment_378" align="aligncenter" width="502"] Figure 10.27 Regional metamorphism of oceanic crust at a subduction zone occurs at high pressure but relatively low temperatures. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] A special type of metamorphism takes place under these very high-pressure but relatively low-temperature conditions, producing an amphibole mineral known as glaucophane (Na₂(Mg₃Al₂)Si₈O₂₂(OH)₂). Glaucophane is blue, and the major component of a rock known as blueschist. If you have never seen or even heard of blueschist, that's not surprising. What is surprising is that anyone has seen it! Most of the blueschist that forms in subduction zones continues to be subducted. It turns into eclogite at about 35 km depth, and then eventually sinks deep into the mantle, never to be seen again. In only a few places in the world, the subduction process was interrupted, and partially subducted blueschist returned to the surface. One such place is the area around San Francisco. The blueschist at this location is part of a set of rocks known as the Franciscan Complex (Figure 10.28). [caption id="attachment_379" align="aligncenter" width="602"] Figure 10.28 Franciscan Complex blueschist exposed north of San Francisco. The blue colour of the rock is due to the presence of the amphibole mineral glaucophane. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Contact Metamorphism

Contact metamorphism happens when a body of magma intrudes into the upper part of the crust. Heat is important in contact metamorphism, but pressure is not a key factor, so contact metamorphism produces non-foliated metamorphic rocks such as hornfels, marble, and quartzite. [caption id="attachment_380" align="alignright" width="290"] Figure 10.29 Schematic cross-section of the middle and upper crust showing two magma bodies. The upper body, which has intruded into cool unmetamorphosed rock, has created a zone of contact metamorphism. The lower body is surrounded by rock that is already hot (and probably already metamorphosed), and so it does not have a significant metamorphic aureole. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] Any type of magma body can lead to contact metamorphism, from a thin dyke to a large stock. The type and intensity of the metamorphism, and width of the metamorphic aureole that develops around the magma body, will depend on a number of factors, including the type of country rock, the temperature of the intruding body, the size of the body, and the volatile compounds within the body (Figure 10.29). A large intrusion will contain more thermal energy and will cool much more slowly than a small one, and therefore will provide a longer time and more heat for metamorphism. This will allow the heat to extend farther into the country rock, creating a larger aureole. Volatiles may exsolve from the intruding melt and travel into the country rock, facilitating heating and carrying chemical constituents from the melt into the rock. Thus, aureoles that form around "wet" intrusions tend to be larger than those forming around their dry counterparts. Contact metamorphic aureoles can be small—from just a few cm around small dykes and sills—to as much as 100 m around a large stock. Contact metamorphism can take place over a wide range of temperatures, from around 300 °C to over 800 °C. Different minerals will form depending on the exact temperature and the nature of the country rock. Although bodies of magma can form in a variety of settings, one place magma is produced in abundance, and where contact metamorphism can take place, is along convergent boundaries with subduction zones, where volcanic arcs form (Figure 10.30). Regional metamorphism also takes place in this setting, and because of the extra heat associated with the magmatic activity, the geothermal gradient is typically steeper in these settings (between ~40 and 50 °C/km). Under these conditions, higher grades of metamorphism can take place closer to surface than is the case in other areas. [caption id="attachment_381" align="aligncenter" width="600"] Figure 10.30 Contact metamorphism (yellow rind) around a high-level crustal magma chamber, and regional metamorphism in a volcanic-arc related mountain range. Dashed lines show isotherms. Source: Karla Panchuk (2018), CC BY 4.0. Modified after Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Shock Metamorphism

When extraterrestrial objects hit Earth, the result is a shock wave. Where the object hits, pressures and temperatures become very high in a fraction of a second. A "gentle" impact can hit with 40 GPa and raise temperatures up to 500 °C. For reference, pressures in the lower mantle start at 24 GPa (GigaPascals), and climb to 136 GPa at the core-mantle boundary, so the impact is like plunging the rock deep into the mantle and releasing it again within seconds. The sudden change associated with shock metamorphism makes it very different from other types of metamorphism that can develop over hundreds of millions of years, and that start and stop as tectonic conditions change. Two features of shock metamorphism are shocked quartz, and shatter cones. Shocked quartz (Figure 10.31 left) refers to quartz crystals that display damage in the form of parallel lines throughout a crystal. The quartz crystal in Figure 10.31 has two sets of these lines. The lines are small amounts of glassy material within the quartz, formed from almost instantaneous melting and resolidification when the crystal was hit by a shock wave. Shatter cones are cone-shaped fractures within the rocks, also the result of a shock wave (Figure 10.31 right). The fractures are nested together like a stack of ice-cream cones. [caption id="attachment_382" align="aligncenter" width="548"] Figure 10.31 Shock metamorphism features. Left- Shocked quartz with lines of glassy material, from the Suvasvesi South impact structure in Finland. Right- Shatter cones from the Wells Creek impact crater in the USA. Sources: Left- Martin Schmieder, CC BY 3.0. Image source.. Right- Zamphuor (2007), Public Domain. Image source..[/caption]

Dynamic Metamorphism

Dynamic metamorphism is the result of very high shear stress, such as occurs along fault zones. Dynamic metamorphism occurs at relatively low temperatures compared to other types of metamorphism, and consists predominantly of the physical changes that happen to a rock experiencing shear stress. It affects a narrow region near the fault, and rocks nearby may appear unaffected. At lower pressures and temperatures, dynamic metamorphism will have the effect of breaking and grinding rock, creating cataclastic rocks such as fault breccia (Figure 10.32). At higher pressures and temperatures, grains and crystals in the rock may deform without breaking into pieces (Figure 10.33, left). The outcome of prolonged dynamic metamorphism under these conditions is a rock called mylonite, in which crystals have been stretched into thin ribbons (Figure 10.33, right). [caption id="attachment_383" align="aligncenter" width="605"] Figure 10.33 Fault breccia, created when shear stress along a fault breaks up rocks. Left- close-up view of fault breccia clearly showing dark angular fragments. Right- A fault-zone containing fragments broken from the adjacent walls (dashed lines). Note that the deformation does not extend far past the margins of the fault zone. Source: Karla Panchuk (2018), CC BY 4.0. Click for more attributions.[/caption] [caption id="attachment_384" align="aligncenter" width="702"] Figure 10.34 Mylonite, a rock formed by dynamic metamorphism. Left- An outcrop showing the early stages of mylonite development, called protomylonite. Notice that the deformation does not extend to the rock at the bottom of the photograph. Middle- Mylonite showing ribbons formed of drawn-out crystals. Right- Microscope view of mylonite with mica (colourful crystals) and quartz (grey and black crystals). This is a case where the shape of quartz crystals is controlled more by stress than by crystal habit. Source: Karla Panchuk (2018), CC BY-SA 4.0. Click for more attributions.[/caption]

In Summary: Types of Metamorphism

Complete these statements by filling in the correct type of metamorphism. metamorphism occurs on a large scale when continents collide and mountain belts form. metamorphism is low grade metamorphism resulting from the weight of overlying rocks. metamorphism is high grade metamorphism that occurs in subducted slabs of ocean crust. Meteorite impacts cause metamorphism. Metamorphism resulting from rocks being “cooked” by magma is metamorphism. metamorphism is associated with superheated water along mid-ocean ridges. To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="121"]

References

Bucher, K., & Grapes, R. (2011) Petrogenesis of metamorphic rocks, 8th edition. Springer.

French, B.M. (1998). Traces of catastrophe: A handbook of shock-metamorphic effects in terrestrial meteorite impact structures. Lunar and Planetary Institute. https://www.lpi.usra.edu/publications/books/CB-954/CB-954.intro.html

10.5 Metamorphic Facies and Index Minerals

kqzheng — Fri, 24 Aug 2018 19:02:34 +0000

Metamorphic Facies

In any given metamorphic setting there can be a variety of protolith types exposed to metamorphism. While these rocks will be exposed to the same range of pressure and temperatures conditions within that setting, the metamorphic rock that results will depend on the protolith. A convenient way to indicate the range of possible metamorphic rocks in a particular setting is to group those possibilities into metamorphic facies. In other words, a given metamorphic facies groups together metamorphic rocks that form under the same pressure and temperature conditions, but which have different protoliths. Figure 10.34 shows the different metamorphic facies as patches of different colours. The axes on the diagram are temperature and depth; the depth within the Earth will determine how much pressure a rock is under, so the vertical depth axis is also a pressure axis. Each patch of colour represents a range of temperature and pressure conditions where particular types of metamorphic rocks will form. Metamorphic facies are named for rocks that form under specific conditions (e.g., eclogite facies, amphibolite facies etc.), but those names don't mean that the facies is limited to that one rock type. [caption id="attachment_387" align="aligncenter" width="650"] Figure 10.34 Metamorphic facies and types of metamorphism shown in the context of depth and temperature. The metamorphic rocks formed from a mudrock protolith under regional metamorphism with a typical geothermal gradient are listed. Letters correspond to the types of metamorphism shown in Figure 10.36. Source: Karla Panchuk (2018), CC BY 4.0. Modified after Steven Earle (2016), CC BY 4.0. Image source.[/caption]

Concept Check: Metamorphic Facies

Fill in the missing words to complete the summary. Metamorphic group together metamorphic rocks that form under (hint: the same or different?) pressure and temperature conditions, but which have (hint: the same or different?) parent rocks. The groups are named for a single metamorphic rock that forms under those specific conditions. They can include (hint: Many or no?) other metamorphic rocks. To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="122"]

Another feature to notice in the diagram are the many dashed lines. The yellow, green, and blue dashed lines represent the geothermal gradients in different environments. Recall that the geothermal gradient describes how rapidly the temperature increases with depth in Earth. In most areas (green dashed line), the rate of increase in temperature with depth is 30 °C/km. In other words, if you go 1,000 m down into a mine, the temperature will be roughly 30 °C warmer than the average temperature at the surface. In volcanic areas (yellow dashed line), the geothermal gradient is more like 40 to 50 °C/km, so the temperature increases much faster as you go down. Along subduction zones (blue dashed line), the cold ocean lithosphere keeps temperatures low, so the gradient is typically less than 10 °C/km. The yellow, green, and blue dashed lines in Figure 10.34 tell you what metamorphic facies you will encounter for rocks from a given depth in that particular environment. A depth of 15 km in a volcanic region falls in the amphibolite facies. Under more typical conditions, this is the greenschist facies, and in a subduction zone it is the blueschist facies. You can make the connection more directly between the metamorphic facies and the types of metamorphism discussed in the previous section by matching up the letters a through e in Figure 10.34 with the labels in Figure 10.35. [caption id="attachment_388" align="aligncenter" width="650"] Figure 10.35 Environments of metamorphism in the context of plate tectonics: (a) regional metamorphism related to mountain building at a continent-continent convergent boundary, (b) seafloor (hydrothermal) metamorphism of oceanic crust in the area on either side of a spreading ridge, (c) metamorphism of oceanic crustal rocks within a subduction zone, (d) contact metamorphism adjacent to a magma body at a high level in the crust, and (e) regional metamorphism related to mountain building at a convergent boundary. Source: Karla Panchuk (2018) CC BY 4.0. Modified after Steven Earle (2015), CC BY 4.0. Image source.[/caption] One other line to notice in Figure 10.34 is the red dashed line on the right-hand side of the figure. This line represents temperatures and pressures where granite will begin to melt if water is present. Migmatite is to the right of the line because it forms when some of the minerals in a metamorphic rock begin to melt, and then cool and crystallize again.

Practice with Metamorphic Facies and Geothermal Gradients Note: It's okay to peek at the metamorphic facies diagram (Figure 10.34) if you need to.

Match the words into the correct boxes. The geothermal gradient is in subduction zones, because temperatures are at depth than in other locations. Most subduction zone conditions fall within the facies, named for a uniquely coloured foliated rock. The geothermal gradient is in volcanic regions because temperatures get at shallower depths than in other locations. At the highest pressures and temperatures, the volcanic region geothermal gradient falls within the facies. Contact metamorphism falls within relatively pressure conditions in the facies. This is because as you go deeper, temperatures get too for there to be a big contrast between magma and other rocks. Fill-in-the-blank options:

hornfels
shallowest
low
steepest
hotter
blueschist
high
cooler
granulite

To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="123"]

Index Minerals

Some common minerals in metamorphic rocks are shown in Figure 10.36, arranged in order of the temperature ranges where they tend to be stable. The upper and lower limits of the ranges are intentionally vague because these limits depend on a number of different factors, such as the pressure, the amount of water present, and the overall composition of the rock. [caption id="attachment_389" align="aligncenter" width="550"] Figure 10.36 Metamorphic index minerals and approximate temperature ranges. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] Even though the limits of the stability ranges are vague, the stability range of each mineral is still small enough that the minerals can be used as markers for those metamorphic conditions. Minerals that make good markers of specific ranges of metamorphic conditions are called index minerals.

The Meguma Terrane of Nova Scotia: An Example of How Index Minerals Are Used

The southern and southwestern parts of Nova Scotia were regionally metamorphosed during the Devonian Acadian Orogeny (around 400 Ma), when a relatively small continental block—the Meguma Terrane (Figure 10.37 top )—collided with the existing eastern margin of North America. The clastic sedimentary rocks within this terrane were variably metamorphosed. [caption id="attachment_390" align="aligncenter" width="603"] Figure 10.37 Regional metamorphic zones in the Meguma Terrane of southwestern Nova Scotia. Top- Map of metamorphic zones. Bottom- Stability ranges for minerals within the Meguma Terrane. Source: Karla Panchuk (2017), CC BY 4.0. Modified after Steven Earle (2015, CC BY 4.0, view source), Keppie & Muecke (1979) and White & Barr (2012).[/caption] Index minerals have been used to map areas of higher or lower metamorphic intensity, called metamorphic zones. A metamorphic zone is a region bounded by the first appearance of one index mineral and the first appearance of the next. In the Meguma Terrane, the biotite zone (darker green) begins in the east and north with the first appearance of biotite. The biotite zone ends toward the south and west where garnet first appears. Because index minerals can have overlapping stability conditions, a lower-intensity index mineral can still be present in a higher-intensity metamorphic zone. Knowledge of metamorphic zones makes it possible to draw conclusions about the geological conditions in which metamorphic rocks formed. The highest-intensity metamorphism (highest metamorphic grade)—the sillimanite zone—is in the southwest. Progressively lower grades of metamorphism exist toward the east and north. The rocks of the sillimanite zone were likely heated to over 700 °C, and therefore must have been buried to depths between 20 km and 25 km. The surrounding lower-grade rocks were not buried as deeply, and the rocks within the peripheral chlorite zone were likely not buried to more than about 5 km depth. A probable explanation for this pattern is that the area with the highest-grade rocks was buried beneath the central part of a mountain range formed by the collision of the Meguma Terrane with North America. The collision caused rocks to be folded, and to be faulted and stacked on top of each other. These mountain-building processes thickened Earth's crust, and increased its mass locally as the mountains grew. The increased mass of the growing mountains caused the lithosphere to float lower in the mantle (Figure 10.38, left). As the mountains were eventually eroded over tens of millions of years, the crust floated higher and higher in the mantle, and erosion exposed metamorphic rocks that were deep within the mountains (Figure 10.38, right). [caption id="attachment_391" align="aligncenter" width="650"] Figure 10.38 Schematic cross-section through the Meguma Terrane. Left- Metamorphic zones and temperatures when mountain-building processes thickened the crust. Right- The mountains have been eroded, exposing metamorphic rocks that formed deep within the mountains. Source: Karla Panchuk (2018), CC BY 4.0. Modified after Steven Earle (2015), CC BY 4.0. Image source left/ right.[/caption] Building a narrative for the metamorphism in Nova Scotia’s Meguma Terrane is just one example of how index minerals can be used.

Try It Yourself: Meguma Terrane Index Minerals [h5p id="124"]

References

Keppie, D., & Muecke, G. (1979). Metamorphic map of Nova Scotia. (Nova Scotia Department of Mines and Energy, Map 1979-006).

White, C. E., & Barr, S. M. (2012) Meguma terrane revisited: Stratigraphy, metamorphism, paleontology and provenance. Geoscience Canada, 39(1). https://journals.lib.unb.ca/index.php/GC/article/view/19450/21005

10.6 Metamorphic Hydrothermal Processes and Metasomatism

kqzheng — Fri, 24 Aug 2018 19:03:25 +0000

The heat from a body of magma in the upper crust can create a very dynamic situation with geologically interesting and economically important implications. In the simplest cases, water does not play a big role, and the main process is heat transfer from the pluton to the surrounding rock, creating a zone of contact metamorphism (Figure 10.39a). In many cases, however, water is released from the magma body as crystallization takes place, and this water is dispersed along fractures in the surrounding rock (Figure 10.39b). [caption id="attachment_394" align="aligncenter" width="652"] Figure 10.39 Metamorphism and alteration around a pluton in the upper crust. (a) Thermal metamorphism only (within the purple zone); (b) Thermal metamorphism plus veining (white) related to dispersal of magmatic fluids into the overlying rock; (c) Thermal metamorphism plus veining from magmatic fluids plus alteration and possible formation of metallic minerals (hatched yellow areas) from convection of groundwater. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] The water released from a magma chamber is typically rich in dissolved minerals. As this water cools, it interacts with the surrounding rocks, changing both the chemistry of the water and the chemistry of the rocks. This can cause minerals to precipitate from the water. Minerals can also precipitate if the water boils because of a drop in pressure. Precipitated minerals form veins within fractures in the surrounding rock. Quartz veins are commonly formed in this situation, and can contain other minerals such as pyrite, hematite, calcite, and even silver and gold. Heat from the magma body will cause surrounding groundwater to expand and then rise toward the surface. In some cases, this may initiate a convection system where groundwater circulates past the pluton. Such a system could operate for thousands of years, resulting in the circulation of millions of tonnes of groundwater from the surrounding region past the pluton. Hot water circulating through the rocks and interacting chemically with them can lead to significant changes in the mineralogy of the rock, including alteration of feldspars to clays, and deposition of quartz, calcite, and other minerals in fractures and other open spaces (Figure 10.40). Chemical change in rocks due to interaction with hot water is called hydrothermal alteration. [caption id="attachment_395" align="aligncenter" width="400"] Figure 10.40 White veins of calcite in limestone of the Comox Formation, Nanaimo BC. Quarter for scale. Source: Steven Earle (2016), CC BY 4.0. Image source.[/caption]

Metasomatism

Metamorphic reactions involve the release of fluids as minerals change, and chemical reactions with locally-derived fluids. However, if a large amount of externally-derived fluid—such that supplied by magma—is flushed through the system at the high pressures and temperatures characteristic of metamorphism, it can substantially alter the chemical composition of the rock. This type of hydrothermal alteration is called metasomatism. A special type of metasomatism takes place where a hot pluton intrudes into carbonate rock such as limestone. Magmatic fluids rich in silica, calcium, magnesium, iron, and other elements can dramatically change the chemistry of the limestone, forming minerals that would not normally exist in either the igneous rock or limestone. A rock called skarn results, containing minerals such as garnet, epidote, magnetite, and pyroxene, among others (Figure 10.41). [caption id="attachment_396" align="aligncenter" width="383"] Figure 10.41 Skarn from Mount Monzoni, Northern Italy, with recrystallized calcite (blue), garnet (brown), and pyroxene (green). The rock is 6 cm across. Source: Siim Sepp (2012), CC BY-SA 3.0. Image source.[/caption]

Practice with Contact Metamorphism and Metasomatism [h5p id="125"]

Chapter 10 Summary & Key Term Check

kqzheng — Fri, 24 Aug 2018 19:05:09 +0000

Chapter 10 Main Ideas

10.1 Controls on Metamorphic Processes

Metamorphism is controlled by five main factors: the composition of the parent rock, the temperature to which the rock is heated, the amount and direction of pressure, the volumes and compositions of fluids that are present, and the amount of time available for metamorphic reactions to take place.

10.2 Foliation and Rock Cleavage

When the pressure acting on a rock is not uniform in all directions, foliation can develop. Foliation may occur in the form of platy or elongated mineral crystals that have grown at right angles to the maximum pressure, or it may develop when crystals or clasts within a rock are deformed. Foliation causes crystals or clasts within a rock to become aligned. When metamorphic rocks break parallel to the direction of foliation, rock cleavage results.

Practice Again

Identifying the presence of foliation and cleavage

10.3 Classification of Metamorphic Rocks

Metamorphic rocks are classified on the basis of texture and mineral composition. Foliation is a key feature of metamorphic rocks formed under directed pressure; foliated metamorphic rocks include slate, phyllite, schist, and gneiss. Metamorphic rocks formed in environments without strong directed pressure include hornfels, marble, and quartzite.

Practice Again

Practice with types of metamorphic rocks

10.4 Types of Metamorphism and Where They Occur

Almost all regions that experience metamorphism are being acted upon by plate-tectonic processes. Oceanic crustal rock can be metamorphosed near the spreading ridge where it was formed. Regional metamorphism takes place in areas where mountain ranges are forming, which are most common at convergent boundaries. Contact metamorphism takes place around magma bodies in the crust, which are also most common above convergent boundaries. Shock metamorphism happens when extraterrestrial bodies impact Earth, and is unusual among metamorphic processes because it occurs in seconds or minutes, rather than taking millions of years. Dynamic metamorphism occurs when shear stress is applied to rocks, such as along faults.

Practice Again

Types of metamorphism

10.5 Metamorphic Facies and Index Minerals

Metamorphic facies are groups of metamorphic rocks that form under the same range of pressure and temperature conditions, but from different parent rocks. Geologists use index minerals such as chlorite, garnet, andalusite, and sillimanite to identify metamorphic zones. Index minerals tell us about the pressure and temperature conditions under which metamorphic rocks formed.

Putting It Together: Using Metamorphic Grade & Index Minerals to Solve a Mystery [h5p id="126"]

You have four samples and a mystery on your hands... You're so lucky! Your friend went on a hike through an eroded mountain range and brought some rocks back for you. Well, maybe not quite so lucky... your friend knows which locations he collected samples from, but didn't record which sample came from where. Now you have four samples and a mystery on your hands. Your friend provided you with a sketch of the sampling locations (below), and penciled in the original shape of the mountain range before erosion happened. The sketch shows that sample sites 1 through 4 start from the edge of the mountain range and move toward its core, in the direction of increasing metamorphic grade. Can you use this sketch and the sample descriptions to figure out which sample came from each location? Two reference diagrams are provided below, should you need them.

Write the correct sample location number in each box.

Schist consisting predominantly of biotite, with minor garnet Migmatite, a rock with dark and light layers folded and twisted into convoluted shapes Slate, a fine-grained rock with nearly planar rock cleavage Schist consisting predominantly of chlorite To check your answers, navigate to the above link to view the interactive version of this activity.

Mineral Stability Ranges

Metamorphic Facies

10.6 Metamorphic Hydrothermal Processes and Metasomatism

Contact metamorphism takes place around magma bodies that have intruded into cool rocks in the crust. Heat from magma is transferred to the surrounding country rock, resulting in mineralogical and textural changes. Hot water from a cooling body of magma, or from convection of groundwater driven by the heat of the pluton, can lead to hydrothermal alteration. When large volumes of fluid are flushed through rocks experiencing metamorphic pressures and temperatures, metasomatism results. Metasomatism can cause valuable metals to accumulate in the surrounding rocks.

Practice Again

Contact metamorphism and metasomatism

Key Term Check

What key term from Chapter 10 is each card describing? Turn the card to check your answer. [h5p id="127"]

16.1 What Is the Earth System?

kqzheng — Mon, 27 Aug 2018 00:18:23 +0000

Earth can be characterized in terms of its “spheres.” The atmosphere is the envelope of gas surrounding the planet. The hydrosphere is the water on the planet, whether in oceans, rivers, glaciers, or the ground. The biosphere comprises living organisms. The lithosphere is the rocky outer shell of the planet. Components of these spheres interact constantly, with processes occurring in one sphere having an impact in other spheres. Cycles such as the water cycle or the carbon cycle constantly move matter and energy between spheres. Taking an Earth-system approach—looking at how the spheres are connected—is a way to account for the web of interactions responsible for the “big picture” of the Earth that we know. The climate change related to the opening of the Drake Passage (Figure 16.2) is a good example of why a system of interactions is needed to understand how Earth works. The Drake Passage (bottom left map) is the gap between the southern-most tip of South America, and Antarctica. Prior to 40 million years ago, the Drake Passage did not exist (top left map), and neither did the Antarctic ice cap. The arm of land connecting South America and Antarctica allowed warm ocean currents (red arrows) to carry heat from the equator to Antarctica. When the gap opened up, a new cold-water current formed (blue arrows) that blocked warm water from reaching Antarctica. Without the warm current, Antarctica froze over. [caption id="attachment_643" align="aligncenter" width="864"] Figure 16.2 An example of Earth-system interactions. The opening of the Drake Passage changed how ocean currents moved heat around the planet, and cooled Antarctica until it froze over. Source: Karla Panchuk (2018) CC BY-NC-SA 4.0. Map: Modified after Woods Hole Oceanographic Institution. Click the image for more attributions and terms of use.[/caption] There were many interconnecting processes within the Earth-system (Figure 16.2, right) that drove glaciation in Antarctica. First, heat energy within Earth drove plate tectonics (lithosphere), making it possible for South America to separate from Antarctica. This impacted ocean currents (hydrosphere), and ultimately how water was stored on Antarctica (hydrosphere) by changing the climate of Antarctica. In the Earth system, nothing happens in isolation. The change in the climate of Antarctica had a global impact. The ice cap on Antarctica increased Earth’s albedo, the reflectiveness of Earth’s surface. The more reflective Earth’s surface, the more of the sun’s light is reflected back into space without heating Earth. This caused even more ice to form, and cooled the planet as a whole. When Earth cools, the change in temperature has a cascade of effects including changing precipitation patterns (hydrosphere), and changing the characteristics of habitats (biosphere). When habitats cool, organisms needing more warmth will migrate closer to the equator. This is true of plants as well as other forms of life. Ice is not the only type of land cover that affects albedo—forests do as well. Forests also increase local atmospheric moisture levels through transpiration, when they release water vapour into the atmosphere. Local temperature and moisture differences also affect rainfall patterns on top of larger-scale changes resulting from cooling. The chain of events in summarized in Figure 16.2 is only a broad overview of all of the consequences of opening a gap between South America and Antarctica. For example, it does not include the effects of what a change in the types of plants in a location does to local weathering and erosion. Trees can accelerate weathering, releasing more nutrients from rocks into runoff, which can affect algae blooms in water bodies, which in turn reduces oxygen levels in the water, which affects organisms living in the water that rely on oxygen. Trees growing along a river can also slow the rate of erosion, reducing the amount of sediment in the river, and ultimately the rate of development of a delta at the river’s mouth. Deltas undergo subsidence as accumulated sediments are compressed, so if the sediment supply is reduced, parts of the delta may become flooded, changing the extent of wetlands. Wetlands with waters depleted in oxygen can prevent plant material from decaying and releasing their carbon back into the carbon cycle as carbon dioxide. Changing atmospheric carbon dioxide levels alters the way energy moves through Earth’s atmosphere, and affects Earth’s surface temperatures. The short version of why it’s important to look at Earth as a system is that everything is connected, so that a change in one part of the system can ripple through the rest of the system and have effects well beyond any one location or time.

Feedbacks Amplify or Diminish Earth-System Change

The web of interactions in the Earth system is complex, but there is yet another level of complication. Sometimes a change in the Earth system can trigger other changes that have the effect of amplifying the original change, or diminishing it. The series of interactions that amplify or diminish a change are called feedbacks. A feedback that amplifies change is called a positive feedback. A feedback that diminishes the size of a change is called a negative feedback. In the events related to the glaciation of Antarctica, the formation of ice is an example of a positive feedback. Ice formation was caused by cooling, but it triggered even more cooling by reflecting sunlight away from Earth's surface. This is called ice albedo feedback. An example of a negative feedback is plant growth. Plants need CO₂ to make food, so as long as the plants have enough nutrients and water, and temperatures are still suitable, increasing CO₂ in the atmosphere could increase plant growth. Plant growth would draw down atmospheric CO₂, so that there would be less warming than would otherwise be expected from the initial rise in atmospheric CO₂ levels.

Misconceptions About Feedbacks

There are two common misconceptions about feedbacks. One misconception is that positive feedbacks result in changes that are good, and negative feedbacks result in changes that are bad. In fact, whether a feedback is positive or negative is unrelated to whether or not the change would be considered a good thing. For example, if a feedback accelerates warming and makes an ecosystems uninhabitable for animals that used to live there, it would still be a positive feedback even though it had a negative impact on the animals in that ecosystem. A feedback that slowed the rate of warming and gave the animals time to adapt would still be considered a negative feedback even though it helped the animals to survive. Another misconception is that a positive feedback always results in some value increasing (e.g., a rise in temperature), and a negative feedback results in a decrease in that value. Positive feedbacks can cause a value to decrease (e.g., as ice forms more sunlight is reflected, leading to decreased temperatures), and negative feedbacks can cause a value to increase. What matters is whether the initial change is amplified or reduced, not which way the numbers are changing.

Feedbacks and Instability in the Earth System

The potential for sudden extreme changes in the Earth system depends on what feedbacks are available. At times when Earth's climate was much warmer than today, no glaciers were present. When the climate is much cooler, a relatively small decrease in temperature could be enough to start the formation of ice and trigger the ice albedo feedback. However, if the climate is much warmer, the same decrease in temperature would not cool Earth enough to trigger the ice albedo feedback, and further climate cooling would be avoided. The reverse is also true- if warming occurs in a climate that is cold enough for glaciers to form, some of that ice might melt, reducing the albedo of Earth's surface, and permitting even more warming. On the other hand, if the climate is already too warm for ice to exist, a small amount of warming won't be amplified in the same way. The albedo effect is not the only feedback that can make cooler climates less stable. Melting of permafrost (sediment that remains frozen year round) can also have an impact. Frozen soil contains trapped organic matter that is converted by micro-organisms to CO₂ and methane (CH₄) when the soil thaws. Both these gases contribute to warming when they accumulate in the atmosphere. Additional warming can cause even more permafrost to melt, permitting even more activity by micro-organisms, and releasing more CO₂ and CH₄. Either of these feedbacks is enough on their own to accelerate climate change, but when they are both present together, the effect is even stronger. What this means is that the conditions in the Earth system before a change happens—called the initial conditions—play an important role in determining the impact of any changes that occur. A change that would have little impact under one set of initial conditions could have far reaching effects under another. Thinking of Earth as a system is a way to factor in the initial conditions. Otherwise we would be very puzzled why a small rise in global temperatures at one time in Earth history could have almost no discernible effect, but the same rise in temperatures at another time could lead to profound change.

Concept Check: Why Study Earth as a System?

Fill in the missing words to complete this summary.Earth's “spheres" include the (the envelope of gas surrounding the planet), the (water on the planet, whether in oceans, rivers, glaciers, or the ground), the (comprises living organisms) and the (the rocky outer shell of the planet). Taking an Earth- approach to studying events on Earth means looking at the processes that connect the spheres. This includes processes that amplify changes through feedbacks, and processes that diminish changes through feedbacks. To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="169"]

16.2 Causes of Climate Change

kqzheng — Sun, 26 Aug 2018 21:39:23 +0000

What Is Climate?

Our day-to-day experience of the Earth system is in the form of the conditions we experience at Earth's surface. The daily conditions that we think of as weather—the temperature, presence or absence of precipitation, winds, humidity, and so on—are a snapshot of the state of the Earth system at a particular instant in time and in a particular location. The weather that we get is variable, but in Saskatchewan most people would not be surprised to experience summer days with temperatures of 20 °C to 30 °C, and winter days with temperatures between –20 °C to –30 °C. Our notion of what summers and winters are generally like reflects our understanding of Saskatchewan's climate. If we get a day in July with a daytime high of 10 °C, that would seem like unusually cold weather because we know it is uncharacteristic of the climate over all. We characterize the climate by collecting data about the weather every day, and then calculating the average conditions over a period of decades. The Government of Canada provides averages for the periods 1961 to 1990, 1971 to 2000, and 1981 to 2010 in an online database that is searchable by geographic location or station. Data measured at Saskatoon's Diefenbaker International Airport show that the average annual temperature from 1981 to 2010 is 0.6 °C higher than the annual average from 1961 to 1990, due warmer conditions in the winter and early spring (Figure 16.3). [caption id="attachment_646" align="aligncenter" width="720"] Figure 16.3 Average temperatures for the periods 1961 to 1990, and 1981 to 2010 measured at Saskatoon's Diefenbaker International Airport (YXE). Source: Karla Panchuk (2018), CC BY 4.0. View YXE data for 1961 to 1990. View YXE data for 1981 to 2010.[/caption] The climate as represented by the 1961 to 1990 interval was slightly cooler than the climate represented by the 1981 to 2010 interval. People who lived in Saskatoon between 1961 and 2010 may or may not have a sense that the weather they experienced from day to day was different for those intervals. In fact, some may have the record high of 35.3 °C on September 4, 1978 seared into their memory, and feel that Septembers just aren't as hot as they used to be. They would be correct that as of 2017, there are no September temperatures recorded at the Diefenbaker International Airport weather station with a daytime high greater than 35.3 °C. But if that gave them the impression that Septembers are cooler on average today than in the past, that would not be consistent with the data.

Climate-Forcing Mechanisms

A climate-forcing mechanism is a process that causes climate to change. Climate forcings work by initiating changes in how heat energy moves into, through, and out of the Earth system. When we discuss a particular climate change event, the climate-forcing mechanism is what initiated the change. Feedbacks also alter climate, but we want to know what triggered the feedbacks in the first place.

Climate Forcing by Changes in Insolation

Insolation, or incident solar radiation, refers to how much of the sun’s energy reaches Earth’s surface in a given period of time. Insolation is measured in Watts per square meter (W/m²).

Long-term Solar Evolution

Over the long term (billions of years), stars like our sun become larger, brighter, and hotter (Figure 16.4). Earth receives 40% more heat from the sun today than it did 4.5 billion years ago. In Figure 16.4, the blue Now arrow shows the sun’s current point in its life history. Although the blue arrow appears to indicate an instant in time, the time interval reflecting the duration of human existence on Earth is but a tiny fraction of the width of the line. As far as human experience is concerned, the long-term evolution of the sun is so slow that it has made no difference at all on insolation for the entire time humans have existed. [caption id="attachment_647" align="aligncenter" width="720"] Figure 16.4 The life history of a star, from condensation of a nebula, to expansion to a red giant, and ending as a white dwarf. Source: Karla Panchuk (2017), CC BY 4.0. Modified after Oliver Beatson (2009), Public Domain. Image source.[/caption]

Orbital Cycles

Insolation is also affected by cyclical changes in Earth’s orbit and rotation. Over intervals of approximately 100,000 years, the eccentricity of Earth’s orbit changes. Eccentricity is a measure of how elliptical a circle is. Higher eccentricity means that the orbit is more elliptical (Figure 16.5, left, blue orbit), whereas lower eccentricity means the orbit is more circular (Figure 16.5, left, red orbit). Eccentricity is important because when it is high, the Earth-sun distance varies more from season to season than it does when eccentricity is low. [caption id="attachment_648" align="aligncenter" width="650"] Figure 16.5 Cycles in Earth’s orbit. Left: The shape of Earth’s orbit (its eccentricity) changes over 100,000 year cycles from more circular to more elliptical. Middle: Over 41,000 year periods, Earth’s axis of rotation nods toward and away from the sun. Right: Over 21,000 year cycles, Earth wobbles on its axis of rotation. Source: Karla Panchuk (2017), CC BY 4.0. Modified after Steven Earle (2015), CC BY 4.0 Image source. Click for more attributions.[/caption] Over intervals of approximately 41,000 years, the obliquity of Earth’s axis of rotation changes (Figure 16.5, middle). This results in a nodding motion that alters how directly the sun shines on Earth’s poles. When the angle is at its maximum (24.5°), Earth’s seasonal differences are accentuated. When the angle is at its minimum (22.1°), seasonal differences are minimized. Cycles of precession happen over intervals of approximately 20,000 years, causing Earth’s axis of rotation to wobble (Figure 16.5, right). This means that although the North Pole is presently pointing to the star Polaris (the pole star), in 10,000 years it will point to the star Vega. The importance of eccentricity, tilt, and precession to Earth’s climate cycles (now known as Milanković Cycles) was first pointed out by Yugoslavian engineer and mathematician Milutin Milanković in the early 1900s. Milanković recognized that although the variations in the orbital cycles did not affect the total amount of insolation that Earth received, it did affect where on Earth that energy was strongest. Glaciations are most sensitive to the insolation received at latitudes of approximately 65°. As continents are configured today, this is most significant at 65° N, because there is almost no land at 65° S. The most important factors are whether the northern hemisphere is pointing toward or away from the sun at its closest or farthest approach, and how eccentric the sun’s position is in Earth’s orbit. For example, if the northern hemisphere is at it farthest distance from the sun during summer (Figure 16.6, top), this means cooler summers. If the northern hemisphere is at its closest distance to the sun during summer (Figure 16.6, bottom), this means hotter summers. Cool summers — as opposed to cold winters — are the key factor in the accumulation of glacial ice, so the upper scenario in Figure 16.6 is the one that promotes glaciation. This factor is greatest when eccentricity is high. [caption id="attachment_649" align="aligncenter" width="650"] Figure 16.6 Effect of precession on insolation in the northern hemisphere summers. In (a) the northern hemisphere summer takes place at greatest Earth-sun distance, so summers are cooler. In (b) (10,000 years or one-half precession cycle later) the opposite is the case, so summers are hotter. The red dashed line represents Earth’s path around the Sun. Source: Steven Earle (2015), CC BY 4.0 Image source.[/caption] The effects of all three cycles are evident in geochemical climate data. Figure 16.7 shows the “signals” for obliquity (A), eccentricity (B), and precession (C) over a period from 800,000 years in the past, to 800,000 years in the future. The vertical black line running down the middle of the diagram marks the present day. When the insolation from all three signals is determined, the result is a more complex waveform (D) with times of low variation in insolation, and times with higher variation in insolation. [caption id="attachment_650" align="aligncenter" width="700"] Figure 16.7 Comparison of orbital cycles, insolation, and climate data for a 1.6 million year period. Source: Karla Panchuk (2017), CC BY-SA 4.0. Modified after Incredio (2009), CC BY 3.0. Image source. Click for more attributions.[/caption] The graphs E and F are climate information measured in microfossils dwelling at the ocean floor (E) and in water from ice cores (F). Peaks in temperature in F correspond to peaks in the oxygen isotope record in E, which indicate that it was warmer, there was less ice, and sea level was higher. Troughs are times when Earth was deep within an ice age. It was cooler, there was ice on land, and sea level was lower. The vertical dashed lines on the left-hand side of Figure 16.7 mark the times of peak warm temperatures and allow for comparison of the timing of the temperature peaks with the timing of the orbital cycles. Peak temperature events are approximately 100,000 years apart, suggesting that the eccentricity cycle might be the most important contributor. Indeed, in B most (but not all) of the peak temperature events correspond to a time when Earth’s orbit was at or near peak eccentricity for that cycle. It is tempting to conclude that eccentricity is the most important orbital cycle for climate change over all. However, this pattern only began a little over 1 million years ago. For 1.5 million years before that, the 41,000-year obliquity cycle seems to dominate insolation cycles. In general, times of warmest or coolest temperatures don’t line up perfectly with orbital cycles. There is no one orbital cycle that is most important for all of Earth history. It is also the case that changes in insolation due to orbital cycles are not sufficient to cause temperatures to change as much as the geological record says they have; feedbacks must be factored in to explain the observed temperature changes.

Sunspot Cycles

Sunspots are dark patches that appear on the surface of the sun as a result of intense local disturbances in the sun’s magnetic field (Figure 16.8, left). Loops of plasma (gas with electrical charge, Figure 16.8, right) follow along magnetic field lines from one sunspot to another. [caption id="attachment_651" align="aligncenter" width="550"] Figure 16.8 Sunspots. Left: Photograph of sunspots with dots representing the size of Earth and Jupiter for scale. Right: Plasma loops viewed in x-ray wavelengths jumping from one sunspot to another on the sun’s surface. Source: Left- NASA/Solar Dynamics Observatory (2012), Public Domain. Image source.. Right- NASA/Solar Dynamics Observatory (2015), Public Domain. Image source..[/caption] Sunspots appear dark because they are lower-temperature regions on the sun’s surface. For that reason you might think that more sunspots means a reduction in insolation. In fact, just the opposite is true, because sunspots are a side-effect of increased solar activity. Peaks in the number of sunspots counted annually since approximately 1870 (Figure 16.9, blue), coincide with peaks in measurements of solar energy output from the same time period (Figure 16.9, pink). [caption id="attachment_652" align="aligncenter" width="650"] Figure 16.9 Sunspot cycles. Peaks in the number of sunspots (blue) occur approximately every 11 years, and these correspond to peaks in solar energy output (pink). The influence of sunspot cycles is too small to have a clear impact on global average temperatures (grey). Source: Karla Panchuk (2017) CC BY-SA 4.0. Sunspot records modified after D. Bice (n.d.) CC BY-SA 3.0 view source. Global average temperature modified after Met Office (2015) Contains public sector information licensed under the Open Government Licence v1.0. view source[/caption] Sunspot cycles happen over approximately 11 year intervals, and the changes in insolation that occur during these cycles are relatively small. In the end the effect of sunspot cycles on climate can be lost amidst other factors. In Figure 16.9 there is no clear relationship between the sunspot cycles and the global average temperatures (in grey) reported for the same period.

Be Aware of Graph Scales Figure 16.9 shows three kinds of data: temperatures, sunspot numbers, and solar energy flow. Each of these data sets is a different type of information, so each needs its own vertical axis. The vertical axes are scaled so that the data fill the area of the graph as much as possible. Stretching the vertical scale to fit the full plotting area makes it easier to see how well the peaks and troughs in each record line up with each other. Unfortunately, this can also skew our impression of the data. For example, in the period from 1880 to 1920, all three records have a similar vertical distance from peak to trough. In other words, all three records have approximately the same size of wiggles. This does not mean that the change in insolation from sunspot cycles was big enough to cause all of the variation in the temperature record. From this graph alone, there's no way to tell how much the change in insolation due to sunspot cycles mattered to global temperatures during the period 1880 to 1920.

Concept Check: Climate Forcing by Insolation

Fill in the missing words to complete this summary. Insolation, or (hint: three words, beginning in in, sol, and ending in ation), refers to how much of the sun’s energy reaches Earth’s surface in a given period of time. Over the long term, the sun has (hint: increased or decreased?) brightness, but this process is so slow as to be insignificant over timescales of less than a (hint: thousand, million, or billion?)years. Orbital cycles include changes in (hint: precession, eccentricity, or obliquity?) (how elliptical Earth's orbit is), (hint: precession, eccentricity, or obliquity?) (how Earth nods back and forth on its axis), and (hint: precession, eccentricity, or obliquity?) (how Earth's axis wobbles in circles like a top). They are important because they control how directly the sun's energy hits Earth's high latitudes. are dark marks on the sun's surface caused by disturbances in the sun's magnetic field, and are more numerous when solar activity is more intense. These affect climate on (hint: decade, century, or millenium?)-long cycles. To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="170"]

Climate Forcing by Changes in Heat Transport

The ocean transports large amounts of heat around the Earth through a conveyor-belt-like system of currents. The ocean has surface currents that are driven by wind, but it also has deeper currents that are not wind-driven. The deeper currents behave like stacked rivers because they are different temperatures and have different salt contents, and therefore different densities. The differences in density between these water masses are what drive circulation. Circulation that is driven by density is called thermohaline circulation; thermo refers to heat and haline refers to salt. To see how this works, consider the warm and saline Gulf Stream current (Figure 16.10, top). It flows northward past Britain and Iceland into the Norwegian Sea, and cools as it moves north, becoming denser. Its high salinity contributes to its density, and it sinks, or downwells, deep beneath the surrounding water, forming the North Atlantic Deep Water (NADW) current that flows south. Meanwhile, at the southern extreme of the Atlantic, very cold water adjacent to Antarctica also sinks to the bottom to become the Antarctic Bottom Water (AABW) current. The AABW flows north, beneath the NADW. [caption id="attachment_653" align="aligncenter" width="550"] Figure 16.10 A simplified north-south cross-section through the Atlantic Ocean basin showing the different current layers. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] The water that sinks in the areas of deep water formation in the Norwegian Sea and adjacent to Antarctica moves very slowly at depth. It eventually resurfaces, or upwells, in the Indian Ocean between Africa and India, and in the Pacific Ocean, north of the equator (Figure 16.11). [caption id="attachment_654" align="aligncenter" width="550"] Figure 16.11 Global thermohaline circulation patterns. Red lines are surface currents, and blue lines are deep currents. Source: NASA Earth Observatory (2008), Public Domain. Image source.[/caption] Some ocean currents move warm water from the equator toward the poles. As in the example of the Drake Passage, the path of warm currents can have a significant impact on the climate of a region, and potentially of the planet as a whole. Processes that disrupt the density of seawater can slow or stop currents, preventing warm water from reaching higher latitudes. The recovery from the last ice age is characterized by sudden returns to glacial conditions over as little as 3 years. This is thought to be the result of enormous glacial lakes forming on continents as the glaciers melted, then being suddenly released into the ocean by a burst ice dam. The glacial water would be very cold, but it would also be fresh, making it less dense than the ocean water. The fresh glacial water would form a cap and slow the downwelling conveyor belt at high latitudes. Scientists are trying to determine the current and past state of the Atlantic-basin system of circulation, called the Atlantic Meridional Overturning Circulation (AMOC), to tell whether it is changing in response to warming and adding fresh water from melting ice sheets. The AMOC varies considerably on decadal cycles because of cycles in the wind patterns in the Atlantic, so it is important to distinguish these cyclical changes from any longer-term underlying changes. Because of these studies, we have an idea of what the physical properties of the Atlantic Ocean look like when circulation is stronger or weaker. When circulation slows, the density is lower in the downwelling regions (Figure 16.12, blue patch in the Labrador Sea). The density is higher south of this region, along the eastern coast of the United States and southern Canada (Figure 16.12, orange patch). Model simulations are used to confirm that the changes in density we observe are consistent with how we understand the circulation system to work. Measurements of the actual flow rate at depth in the Atlantic Ocean (Figure 16.12, right, blue dots) confirm that density decreases in downwelling regions (black and red lines) when circulation slows. A more recent study (Caesar et al., 2018) has shown that observed and model temperatures follow the same pattern, with cooling where the blue patches are in Figure 16.12, and warming in the region of the orange patches. This is to be expected because the slowdown in circulation affects how heat is moved northward. [caption id="attachment_655" align="aligncenter" width="650"] Figure 16.12 Using changing density to track circulation in the Atlantic Ocean. Left- Density calculated from measurements of temperature and salinity in a layer between 1,000 m and 2,500 m depth in the Labrador Sea (black box). Middle- Model results used to see what change in density can be expected. The model shows the same general relationship, with lower density in the Labrador Sea, and higher density to the south. Note that density units are in kg/m² rather than kg/m³ because they are integrated over the layer. Right- Changing density in the Labrador Sea over time. Red and black lines show changes in density from two different data sets. In general, the peaks and troughs of these data sets match up. Blue dots are measurements of the rate of circulation from a project that began collecting data in 2004. See the References section for more information about the relevant studies. Source: Karla Panchuk (2018), CC BY-SA 4.0. Modified after Jon Robson (2013), CC BY-SA 4.0. Image source.[/caption]

Is Atlantic Circulation Slowing Down More than Usual? Changes in the Atlantic meridional overturning circulation (AMOC) happen from decade to decade. To know whether circulation is changing compared to what is normal, it is necessary to get information about what circulation looked like in the past. This is difficult to do, because measurements of circulation rates, temperature, and salinity don't go back as far as we need them to. In a new study, Thornalley et al. (2018) have used geochemical analyses of microfossils to build a longer-term record of temperatures, then used that record to look for the temperature "fingerprint" of slowing AMOC, an increasing difference in the temperatures of surface waters compared to deeper waters in the downwelling zone (Figure 16.13, top). They observe a longer-term cooling trend beginning at the close of the Little Ice Age, suggesting that less heat is being moved toward the downwelling zone (labelled A on the globe in Figure 16.13). [caption id="attachment_656" align="aligncenter" width="1024"] Figure 16.13 Long-term record of Atlantic Meridional Overturning Circulation (AMOC). Top- Temperature fingerprint of circulation determined using geochemical analyses of marine microfossil shells. Results show the difference between temperatures measured at depth at A on the globe, and temperatures measured near the surface at B. Cooling at A relative to B is indicative of weakening AMOC. Bottom- Changes in silt grain-size used to show changes in the velocity of currents at depth at C on the globe. A smaller average grain size means a slower current, and weaker AMOC. Source: Karla Panchuk (2018) CC BY 4.0. Modified after Thornalley et al. (2018). Locator globe modified after Reisio (n.d.), Public Domain. Image source.[/caption] They also measured the size of silt grains on the sea floor, above which a southward-moving component of the AMOC system flows (location labelled C on the globe in Figure 16.13). Grain size is used as a substitute for a direct measure of ocean current velocity because the velocity determines what grain size can be carried. They identified a lower average grain size over the past ~150 years compared to the average grain size from earlier (Figure 16.13, bottom, dashed lines). The authors of the study note that the average grain size changes more during cold events in the Northern Hemisphere (the Dark Ages Cold Period and the Little Ice Age). Thornalley et al (2018) conclude that the AMOC has been weaker on average during the past ~150 years than during the previous ~1,500 years. However, they cannot say for sure how much of that change is from melting that occurred at the close of the Little Ice Age, from melting triggered by warming since the Industrial Revolution, or some combination of the two. Direct measurements of density and current velocity tell us that as of 2017, the AMOC continues to weaken.

Plate Tectonics and Heat Transport

The opening of the Drake Passage is one example of how plate tectonic changes can affect ocean heat transport, and therefore climate. Plate tectonic changes that build or break up continents also play a role. When continents become large, ocean currents warm their margins, but the interiors can be much cooler. Anyone living on the Canadian prairies who has shivered through -40 °C temperatures in the winter, while watching news reports of rain in Vancouver will be familiar with this effect. When the supercontinent Gondwana was over the south pole approximately 300 million years ago (Figure 16.14), this triggered an ice age. The build-up of ice was hastened by the ice albedo feedback effect. [caption id="attachment_657" align="aligncenter" width="550"] Figure 16.14 Glaciation on the supercontinent Gondwana. Paleogeographic reconstruction for 306 million years ago. Source: C. R. Scotese, PALEOMAP Project (www.scotese.com). Image source. Click for terms of use.[/caption]

Short-Term Cycles in Heat Transport: El Niño Southern Oscillation

The El Niño Southern Oscillation (ENSO) operates on a much shorter timescale than climate forcings driven by plate tectonics or orbital cycles, alternating between El Niño and La Niña events on timescales of between two and seven years (Figure 16.15). [caption id="attachment_658" align="aligncenter" width="550"] Figure 16.15 Variations in the ENSO index from 1950 to 2015. Source: Steven Earle (2015), CC BY 4.0. Image source.. Modified after Klaus Wolter/ NOAA (n.d.), Public Domain. Image source.[/caption] Under normal conditions, strong winds blowing westward across the Pacific cause water to pile up in the western Pacific. This forces deeper colder water to the surface in the eastern Pacific (Figure 16.16, left). During La Niña events, further intensification of winds causes even more cold water to upwell. During an El Niño event, the winds weaken, allowing water to flow back to the east (Figure 16.16, right). The cold water settles deeper once again, meaning that warmer water is present along the eastern margin of the Pacific Ocean. [caption id="attachment_659" align="aligncenter" width="650"] Figure 16.16 El Niño Southern Oscillation (ENSO) cycles are driven by changes in wind patterns that affect the distribution of warm and cold water in the Pacific Ocean. Source: Karla Panchuk (2018), CC BY 4.0. Modified after Fred the Oyster and NOAA/PMEL/TAO Project Office. View Normal Conditions/ El Niño[/caption] ENSO events affect weather on a global scale (Figures 16.17 and 16.18). In western Canada, El Niño years have warmer than average winters, whereas La Niña years have cooler than average winters. [caption id="attachment_660" align="aligncenter" width="550"] Figure 16.17 El Niño climate impacts. Source: NOAA Climate.gov (n.d.), Public Domain. Image source.[/caption] [caption id="attachment_661" align="aligncenter" width="550"] Figure 16.18 La Niña climate impacts. Source: NOAA Climate.gov (n.d.), Public Domain. Image source.[/caption]

Concept Check: Climate Forcing by Changes in Heat Transport

Fill in the missing words to complete the summary. circulation is ocean circulation driven by differences in the of seawater. Ocean currents transport heat from the . Changing the rate or path of currents affects changes how much heat reaches the . Warming and cooling cycles due to El Niño Southern Oscillation events are related to upwelling in the eastern . changes can both alter both the paths of currents, and make it harder to warm the of continents. Fill-in-the-blank:

plate tectonic
density
wind-driven
interiors
Thermohaline
poles
Pacific Ocean
equator

To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="171"]

Climate Forcing by Changes in the Atmosphere's Energy Budget

Earth’s atmosphere regulates climate by controlling how much energy from Earth’s surface escapes to space, and how much of the sun’s energy reaches Earth’s surface.

Albedo

Albedo is a measure of the reflectivity of a surface. Earth’s various surfaces have widely differing albedos, expressed as the percentage of light that reflects off a given material. This is important because most solar energy that hits a very reflective surface is not absorbed and therefore does little to warm Earth. Water in the oceans or on a lake is one of the darkest surfaces, reflecting less than 10% of the incident light. Clouds and snow or ice are among the brightest surfaces, reflecting 70% to 90% of the incident light (Figure 16.19). [caption id="attachment_662" align="aligncenter" width="650"] Figure 16.19 Typical albedo values for Earth surfaces. Surfaces with low values reflect less light than surfaces with high values. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Albedo, Feedbacks, and the Acceptance of Milanković Cycles as a Climate Forcing Mechanism

When Milanković published his hypothesis in 1924, it was widely ignored, partly because it was evident to climate scientists that the forcing produced by the orbital variations alone was not strong enough to drive the climate changes of the glacial cycles. Those scientists did not recognize the power of positive feedbacks. It wasn’t until 1973, 15 years after Milanković’s death, that sufficiently high-resolution data were available to show that the Pleistocene glaciations were indeed driven by the orbital cycles, and it became evident that the orbital cycles were just the first step, initiating a range of feedback mechanisms that made the climate change, many of which were related to albedo. Consider the following:

When large volumes of ice melt — such as the continental ice sheets of Antarctica and Greenland, as well as alpine glaciers— this decreases albedo. More solar energy is then absorbed by land, amplifying the increase in temperature.
When sea ice melts and exposes water, the albedo of the exposed area decreases drastically, from approximately 80% to less than 10%. Far more solar energy can be absorbed by the water compared to the previous ice cover, amplifying the temperature increase.
Sea level rises when ice and snow melt on land, and because seawater expands when heated. Higher sea level means a larger proportion of the planet is covered with water, which has a lower albedo than land. More heat is absorbed, amplifying the temperature increase. Since the last glaciation, a rise in sea level of approximately 125 m has flooded vast areas of land.

Albedo Impacts of Vegetation Changes Changes in climate can cause forests to be replaced by grasslands, which have higher albedo than dark forest cover. If deserts expand, vegetated areas can be replaced by higher-albedo sand. Many human activities affect albedo, including adding urban surfaces to an environment, and planting crops. Figure 16.20 shows a forest that has been clear-cut. If a clear-cut has an albedo similar to that of sand, how would clear cutting change the albedo of the area? [caption id="attachment_663" align="aligncenter" width="500"] Figure 16.20 A clear-cut near Eugene, Oregon. Source: Calibas (2011), CC BY-SA 3.0. Image source.[/caption] Note that trees cool their environment through transpiration, when they release water vapour from their leaves. Changes in local temperatures when trees are clear-cut also include the effects of reduced evaporative cooling. Changes in vegetative cover also affect the rates of CO₂ uptake by plants.

Greenhouse Gases (GHGs)

All molecules vibrate at various frequencies and in various ways, and some of those vibrations take place at frequencies within the range of the infrared radiation that is emitted by Earth’s surface. Gases with two atoms, such as O₂, can only vibrate by stretching (back and forth; Figure 16.21 top), and those vibrations are much faster than that of IR radiation. Gases with three or more atoms (such as CO₂) can vibrate in other ways, such as by bending (Figure 16.21 bottom). Those vibrations are slower and allow the molecules to absorb and release infrared radiation. [caption id="attachment_664" align="aligncenter" width="240"] Figure 16.21 Molecules with two atoms (top) vibrate differently from molecules with more than two (bottom), and this determines whether a gas will be a greenhouse gas or not. Source: Steven Earle (2016), CC BY 4.0. Image source.[/caption] When infrared radiation interacts with CO₂ or with one of the other GHGs, the molecular vibrations are enhanced because there is a match between the wavelength of the light and the vibrational frequency of the molecule. This makes the molecule vibrate more vigorously, heating the surrounding air in the process. These molecules also emit infrared radiation in all directions, some of which reaches Earth’s surface. Heating due to the vibrations of greenhouse gas molecules is called the greenhouse effect. Water molecules (H₂O), and methane molecules (CH₄) also interact with infrared radiation when they vibrate, so they are greenhouse gases as well. Ice core records show that over the last 800,000 years, rapid cycles into and out of glacial temperatures are associated with similarly-timed cycles in atmospheric CO₂ levels (Figure 16.21). [caption id="attachment_665" align="aligncenter" width="600"] Figure 16.22 Variations in atmospheric CO₂ levels and temperature over the last 800,000 years. Top- CO₂ concentration from ice core data in Lüthi et al (2008). The dashed line shows a recent measurement of atmospheric CO₂ levels from the Mauna Loa Observatory. Click to view the latest measurements. Bottom: Temperature record derived from oxygen isotope measurements of water in ice cores. Data from Jouzel et al (2008). Upper dashed line- global surface temperature for 2016 from NASA's Goddard Institute for Space Studies Reference line. Click to view the most recent anomaly. Lower dashed line: average temperature for the past 1000 years. Source: Karla Panchuk (2018), CC BY 4.0. Modified after National Research Council (2010). Click for terms of use.[/caption]

Earth-System Response Time

You might have noticed that for most of the 800,000-year record in Figure 16.22, there is a fairly consistent relationship between scale of change in atmospheric CO₂ levels and the resulting change in temperature. You might also have noticed that compared to most of the record, the rise in temperature since 1950 is unexpectedly small, given the increase in atmospheric CO₂ levels since that time. The reason for the relatively small temperature increase in response to the recent CO₂ increase is in large part because the recent rise in CO₂ is happening far more rapidly than other parts of the Earth system can respond. The ocean in particular is slowing down the response. The ocean takes up heat from the atmosphere, and thus helps to determine surface temperatures. A relatively cool ocean can take up more heat from the atmosphere, reducing warming. The fastest way for the ocean as a whole to take up heat is through the "stirring" that happens with ocean circulation, but circulation happens on thousand-year timescales. The slow rate of circulation means that centuries from now, there will still be cool water rising up from the deep ocean that has yet to be exposed to the warmer surface conditions. At other times in the 800,000-year record, changes in CO₂ levels happened on timescales much closer to those at which the ocean takes up heat.

Atmospheric Effects of Volcanic Eruptions

Volcanic eruptions don’t just involve lava flows and exploding rock fragments. Eruptions also release particles and gases into the atmosphere. Important volcanic gases include water vapour, CO₂, and sulphur dioxide (SO₂). Volcanic CO₂ emissions can contribute to climate warming if a greater-than-average level of volcanism is sustained over a long time. At the end of the Permian Period, the massive Siberian Traps were produced by eruptions lasting at least a million years. Large quantities of CO₂ were released, warming the climate and triggering a cascade of Earth-system responses. The end of the Permian Period at 252 Ma is marked by the greatest mass extinction in Earth history. Over the shorter term, however, volcanic eruptions can have the opposite effect, cooling the climate. SO₂ reacts with water in the atmosphere to make droplets of sulphuric acid. The sulphuric acid droplets scatter sunlight, reducing how much of the sun’s energy can reach Earth’s surface. They also affect cloud formation. The volcanic cooling effect is relatively short-lived, because the particles settle out of the atmosphere within a few years.

Climate Change at the end of the Cretaceous Period The large extraterrestrial impact at the end of the Cretaceous Period 66 Ma ago is thought to have produced a massive amount of dust, which may have remained in the atmosphere for several years. It may also have produced a great deal of CO₂. What do you think would have been the short-term and longer-term climate-forcing implications of these two factors?

Concept Check: Climate Forcing by Changes to the Atmosphere's Energy Budget

Fill in the missing words to complete the summary.

The of Earth's surface describes how much of the sun's energy is absorbed, and how much is reflected. Fresh snow and ice reflects of incident light, and water reflects , meaning that Earth absorbs more heat when it's ice-free.

The chemical characteristics of the atmosphere also determine how heat moves through it. Greenhouse gases cause the atmosphere to absorb heat by at the same frequency as (heat) coming from Earth and the sun. Volcanoes can climate drastically over the short term by emitting gas, but can climate over the long term with sustained emissions of gas. Fill-in-the-blank:

cool
carbon dioxide
less than 10%
70% to 90%
vibrating
sulphur dioxide
infrared radiation
warm
albedo

To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="172"]

References

Caesar, I., Rahmstorf, S., Robinson, A., Feulner, G., & Saba, V. (2018). Observed fingerprint of a weakening Atlantic Ocean overturning circulation. Nature, 556. 194-196. https://doi.org/10.1038/s41586-018-0006-5

Ingleby, B., & Huddleston, M. (2007). Quality control of ocean temperature and salinity profiles—Historical and real-time data. Journal of Marine Systems, 65, 158-175. doi:10.1016/j.jmarsys.2005.11.019

Jouzel, J., Masson-Delmotte, V., Cattani, O., Dreyfus, G., Falourd, S., Hoffmann, G., Minster, B., Nouet, J., Barnola, J.M., Chappellaz, J.A., Fischer, H., Gallet, J.C., Johnsen, S.J., Leuenberger, M., Loulergue, L., Luethi, D., Oerter, H., Parrenin, F., Raisbeck, G.M., Raynaud, D., Schilt, A., Schwander, J., Selmo, E., Souchez, R., Spahni, R., Stauffer, B., Steffensen, J.P., Stenni, B., Stocker, T.F., Tison, J.L., Werner, M., & Wolff, E.W. (2007). Orbital and millennial Antarctic climate variability over the past 800,000 years. Science, 317(5839), 793-797. https://www.ncdc.noaa.gov/paleo-search/study/6080

Lüthi, D., Le Floch, M., Bereiter, B.; Blunier, T., Barnola, J.M., Siegenthaler, U., Raynaud, D., Jouzel, J., Fischer, H., Kawamura, K., & Stocker, T.F. (2008). High-resolution carbon dioxide concentration record 650,000-800,000 years before present. Nature, 453, 379-382. https://www.ncdc.noaa.gov/paleo-search/study/6091

Met Office Hadley Centre (n.d.) EN3: Quality controlled subsurface ocean temperature and salinity data. https://www.metoffice.gov.uk/hadobs/en3/. Note: EN4 data set (new version) available.

RAPID-AMOC (n.d.) RAPID: Monitoring the Atlantic meridional overturning circulation at 26.5°N since 2004. http://www.rapid.ac.uk/

Robson, J., Hodson, D., Hawkins, E., & Sutton, R. (2014). Atlantic overturning in decline? Nature Geoscience, 7(2-3). https://doi.org/10.1038/ngeo2050 Open access version

Shaffrey, L.C., Stevens, I., Norton, W. A., Roberts, M. J., Vidale, P. L., Harle, J. D., Jrrar, A., Stevens, D. P., Woodage, M. J., Demory, M. E., Donners, J., Clark, D. B., Clayton, A., Cole, J. W., Wilson, S. S., Connelley, W. M., Davies, T. M., Iwi, A. M., Johns, T. C., King, J. D., New, A. L., Singlo, J. M., Slingo, A., Steenman-Clark, L., & Martin, G. M. (2009). U.K. HiGEM: The New U.K. High-Resolution Global Environment Model—Model Description and Basic Evaluation. Journal of Climate 22, 1861 - 1896. doi:10.1175/2008JCLI2508.1

Smith, D. M., & Murphy, J. M. (2007). An objective ocean temperature and salinity analysis using covariances from a global climate model. Journal of Geophysical Research, 112(C02022). doi:10.1029/2005JC003172

Thornalley, D. J. R., Oppo, D. W., Ortega, P., Robson, J. I., Brierley, C. M., Davis, R., Hall, I. R., Moffa-Sanchez, P., Rose, N. L., Spooner, P. T., Yashayaev, I., & Keigwin, L. D. (2018). Anomalously weak Labrador Sea convection and Atlantic overturning during the past 150 years. Nature 556, 227-230. https://doi.org/10.1038/s41586-018-0007-4

16.3 Methods for Studying Past Climate

kqzheng — Wed, 12 Sep 2018 00:55:13 +0000

Whereas weather refers to day-to-day variations in temperature, precipitation, winds, and so on, climate refers to long-term trends in weather patterns (over decades or more). The term paleoclimate refers to Earth’s climate in the past. The information we have about Earth’s past climates can be classified as direct data or proxy data. Direct data are information derived from first-hand observations of climate. Direct data can be instrumental data, derived from tools designed to quantify observations, or from qualitative descriptions. Proxy data are information derived from natural materials with characteristics that are affected by climate in a systematic way. This could also be said of some instrumental data: an alcohol thermometer uses the fact that the volume of alcohol changes in a consistent way in response to temperature. Proxy data rely on relationships that are also as systematic and consistent, but there are important differences:

Instruments are designed so that the response of the instrument reflects only one characteristic of the environment (e.g., an alcohol thermometer is sealed so that only temperature affects the level of alcohol, not air pressure or evaporation), but proxy data may need to be carefully analyzed to account for other processes.
Data from instruments are lost if observations are not made and recorded. A thermometer is constantly changing in response to temperature, and will not stay in a particular state once conditions change. Proxy records capture information about the environment in a way that can persist over the long term, for as long as the materials last in usable form. Some proxy records preserve information from billions of years ago.
Instruments transform characteristics of the environment into information that we can access immediately (e.g., you can read a temperature directly from a thermometer), but materials from which we derive proxy data require processing, often with specialized laboratory equipment, to get data we can use.
Instruments measure only what we design them to measure, and measure only as well as we design them to do so. Because proxy data come from materials that persist through time, it is possible to improve the techniques used to analyze materials, and redo earlier measurements. Discovering new proxies is an ongoing area of research, and regularly reveals ways to determine characteristics of past states of the Earth system that we didn’t think were knowable.
Instrumental records and other direct observations can be well constrained in time. In other words, we often know the exact dates the observations were made. Proxy data may come from materials that are difficult to pin down to an exact date, and additional information may be required to determine when the records were formed. Sometimes proxy data can only be extracted as an average over a number of years. The relevant question becomes whether the resolution of the record (how detailed it is) is high enough for the time frame of interest. For example, an annual average would be no use at all to understand monthly climate variations, but would provide exceptionally high resolution for geological processes operating over tens of thousands of years.

Types of Direct Data

Instrumental records of climate are those derived from tools such as thermometers, rain gauges, or satellite measurements of the extent of ice sheets. Instrumental records are a recent development, as the history of the Earth system goes. The oldest known temperature measurements cover the period from 1654 to 1670, and were made by monks and Jesuit priests who operated stations within a meteorological network supported by the Medici family of Florence. Non-instrumental historical records of climate also exist, and cover periods of human history prior to the development of the climate-measuring tools we have now. With detective work, these can be used to paint a detailed picture of past climates. Non-instrumental historical records include written records about how long ice and snow were present in a particular year, when harvests occurred, when floods happened, and shipping records that report the extent of sea ice. Paintings of alpine glaciers give information about how far the ice extended, and this can be used to reconstruct temperatures.

The Challenges of Getting Climate Information from Historical Records

In their paper Historical Climate Records in China and Reconstruction of Past Climates, Jiacheng Zhang and Thomas Crowley used official Chinese records extending as far back as 1000 CE to get a detailed picture of climate. This involved transforming descriptions of weather events into a systematic scale. One challenge is defining the scale, but another is deciding what individual accounts actually mean. The authors point out that records of rain or drought can reflect the perceptions and generalizations of the people who wrote about the weather, rather than what actually happened. Consider the following description of rainfall in 1644 from Diary of Qi Zongmin: “in Wyzhong there is no rain for six months from May” Does this mean there was no rain at all prior to May, or just very little? Was it actually six months since there had been rain, or is that an approximation? How do we reliably translate different systems of time measurement into durations and dates, like "six months" or "May?" How can we tell for sure where a location is if place names or political boundaries change? As the authors caution, "a great deal of cross-checking [is required] in order to arrive at a useful descriptive account of climate anomalies.”

Sources of Proxy Data

Tree Rings

The study of tree growth rings for the purpose of understanding past states of the Earth system is called dendroclimatology. Temperatures and a history of drought or wet periods can be reconstructed from the widths of tree rings. Because tree rings form annually, these records can also be well constrained in time. The widths of tree rings reflect how fast the tree grows in a given season. There are factors other than temperature and moisture that affect growth rate, so a sampling strategy must be carefully designed to ensure confidence in climate reconstructions.

Stable Isotopes

Atoms have a nucleus made of protons and neutrons. The number of protons in the nucleus determines what element the atom is, and will always be the same for a given element. In contrast, the number of neutrons can vary for an element. Versions an element having different numbers of neutrons are the isotopes of that element. Sometimes an atom has a number of neutrons that makes it unstable. Those atoms eventually break apart, releasing energy, and are called radioactive isotopes. The decay rate of radioactive isotopes is known, making it possible to use them to find the ages of natural materials. For example, carbon-14 dating makes use of the radioactive isotope of carbon, ¹⁴C, which has eight neutrons instead of the usual number, six (the 14 refers to 6 protons + 8 neutrons). For investigating Earth’s past climate, stable isotopes, which do not decay, are used instead. Stable isotopes of the same element are measured in natural materials, and their ratios compared. Both isotopes are involved in the same chemical reactions and physical processes, but the slight difference in mass caused by one or two extra neutrons means that those processes are more likely to take up the lighter isotope than the heavier one. Some processes do this in such a particular way that evidence of their occurrence is left behind as a distinctive fingerprint in the stable isotope composition of materials formed in their environment. The pair of isotopes used to reconstruct past temperatures are the oxygen isotopes ¹⁶O and ¹⁸O. The ratio of ¹⁸O to ¹⁶O in water is reflected in the calcium carbonate of shells that form in the water. The shells may remain in the geologic record long after the water is gone, making it possible to know the oxygen isotope compositions, and thus temperatures, of water bodies that existed in the distant past.

Ice Cores

The ice in polar glaciers and mountain glaciers preserves a detailed snapshot of Earth’s atmosphere and climate. A sample of Earth’s atmosphere, including gases and particles, is captured and held within the ice, and buried beneath subsequent ice layers. The annual layers in the ice can be used to determine a timescale for the data. The gases in air bubbles trapped within ice (Figure 16.23) are analyzed to determine the chemical composition of the atmosphere at the time the gasses were trapped. [caption id="attachment_668" align="aligncenter" width="450"] Figure 16.23 A researcher holds a fragment of ice from Antarctica. The dots in the fragment are air bubbles containing samples of Earth's past atmosphere. Source: Atmospheric Research, CSIRO (2000), CC BY 3.0. Image source.[/caption] Ice cores (Figure 16.24) are cylinders of ice retrieved using a specialized drill bit. The cores are carefully packaged and stored in specially designed facilities (Figure 16.25) until they are analyzed. [caption id="attachment_669" align="aligncenter" width="500"] Figure 16.24 A scientist weighs and measures a cylinder of core from the West Antarctic Ice Sheet before she packages it for transport. Source: NASA/Lora Koenig (2010), CC BY 2.0. Image source.[/caption] [caption id="attachment_670" align="aligncenter" width="500"] Figure 16.25 National Science Foundation Ice Core Facility in Lakewood, Colorado. Ice cores are housed in tubes 1 m long. The main storage facility is kept at -36 ºC. Fortunately, scientists can examine the cores under much warmer conditions in a nearby room maintained at -24 ºC. Source: U. S. Geological Survey/ Eric Cravens (n.d.), Public Domain. Image source.[/caption]

Rock and Fossil Distributions

Earth can be divided into six main climate zones (Figure 16.26). The zones run roughly along lines of latitude, so that the climate zone changes as you move north or south of the equator. When the climate warms, the zones shift away from the equator; an area now in the boreal climate zone might have been in the warm temperate climate zone when Earth’s climate was warmer. When the climate cools, the zones shift toward the equator. [caption id="attachment_671" align="aligncenter" width="600"] Figure 16.26 Six climate zones of the Köppen-Geiger classification. Source: Karla Panchuk (2018), CC BY-SA 4.0. Modified after LordToran (2007), CC BY-SA 3.0. Image source.[/caption] Some rock types are characteristic of particular climate zones. For example, coal deposits are characteristic of a subtropical climate. Limestone with coral reef fossils is characteristic of a tropical climate. If a rock type is found outside of its climate zone, that might indicate a change in climate. Coal can be found near Estevan, Saskatchewan, now in the warm temperate climate zone. This suggests that at one time, a warmer climate resulted in a northward shift of the subtropical climate zone. Some of the oil in western Canada is present in pore spaces within ancient coral reefs. The warm temperate climate zone cannot have been at its present location when those reefs formed. Fossils can be used similarly. If an organism lives in a habitat with a particular climate, then evidence that the organism has migrated away from the equator could indicate warming. Migration toward the equator could indicate cooling. The study of pollen and plant spores, called palynology, is very helpful for determining the distribution of plants when evidence of larger plant parts (e.g., fossil leaves and bark) is absent. Pollen and spores are very tough, and will survive in the environment when other plant materials do not. A detailed record of pollen and spores, and hence of the climate zones in a particular location, can be derived from lake sediments. Lakes in climates with strong seasonality (a distinct difference in temperature as seasons progress) can accumulate distinct annual sediment layers, called varves (Figure 16.27). Each year is represented by a light layer and a dark layer. The light layers consist of sand and silt from spring runoff. The darker layers include organic matter accumulated during the year. [caption id="attachment_672" align="aligncenter" width="550"] Figure 16.27 Varves in a core from Canoe Brook, Drummerston, Vermont. Each pair of light and dark layers represents one year. The top of the core is to the right. Source: Karla Panchuk (2017), CC BY-NC-SA 4.0 (labels added). Modified after Jack Ridge/ North American Glacial Varve Project (2008). Image source. Click for terms of use.[/caption] Varves can be counted to determine the age of the sediment, and the pollen and spores within the sediment can be extracted to see what types of vegetation were present at different times.

Concept Check: Paleoclimate Study Methods

Fill in the missing words to complete the summary. refers to day-to-day temperature, precipitation, and wind conditions. refers to long-term trends over decades and longer. is the study of what Earth's climate was like at different times in the past. only go as far back as humans were able to record observations. are required to study earlier times. These data come from that respond in a reliable way to climate conditions. In , climate data are collected by examining the thickness of growth rings in trees. In shells, can be used to determine temperatures. In , bubbles of gas contain samples of ancient atmospheres. Seasonal layers within lakes, called are helpful for studies with pollen. We can also use the geographic distribution of climate-sensitive rocks and fossils to see if have shifted over time. Fill-in-the-blank options:

ice cores
palynology
climate
stable isotopes
direct data
paleoclimatology
dendroclimatology
varves
proxy data
climate zones
natural materials
weather

To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="173"]

References

Ridge, J.C. (2008) The North American Glacial Varve Project. Retrieved from http://eos.tufts.edu/varves

Zhang, J., & Crowley, T. (1989). Historical Climate Records in China and Reconstruction of Past Climates. Journal of Climate 2(8), 833-849. https://doi.org/10.1175/1520-0442(1989)002<0833:HCRICA>2.0.CO;2

16.4 Computer Models of the Earth System

kqzheng — Wed, 12 Sep 2018 19:47:52 +0000

Earth-system interactions are so complex that it is next to impossible to follow all of the connections and implications without help. So, scientists use Earth-system computer models to assist. Earth-system models incorporate knowledge of the many components of the Earth system in a way that makes it possible to test how important any one change is. Earth-system models vary in how many aspects of the Earth system they include, and how detailed their representations of those aspects are. Models are designed to answer particular kinds of questions so their performance can be optimized; a study that is concerned only with large-scale global changes might not require a model with a highly detailed representation of Earth's coastlines. It takes more time to run a complicated model, so this saves on computing resources. When computer models are discussed, we acknowledge that there is a difference between measurements of the real world, and the output from the model. Modelers are careful to refer to measurements of the real word as data, and output from the model as results. This also helps to avoid confusion when comparing models to real-world measurements to gauge how realistic the model output is.

What Are Computer Models, Exactly?

Computer models describe natural phenomena using mathematical equations. On the most basic level, computer models take some quantity—whether heat, water, or the concentration of a pollutant—and calculate how it moves through a system. Sometimes they look only at how that quantity changes through time. A computer model of the water volume in a bathtub could be limited to looking at how rapidly water flows in through the tap, and how rapidly it flows out through the drain. But sometimes models look at how a quantity changes in space as well as through time. A study of wind-driven currents in a lake must include information about the shape and depth of the lake to capture how friction at the lake bottom and along the sides affects water flow (Figure 16.28). Data about the lake shape and depth (Figure 16.28, top) is translated to a model grid (Figure 16.28, bottom). Calculations are done to see how wind and friction control how water moves into and out of each cell in the grid. [caption id="attachment_675" align="aligncenter" width="501"] Figure 16.28 Set-up for a model of wind-driven current flow in Lake Ontario. Top: Map of Lake Ontario showing water depth and the location of current meters. Bottom: Grid used to translate water depth information for model calculations. Source: Karla Panchuk (2002), CC BY 4.0. Based on the exercise described in Chapter 10 of Slingerland & Kump (2011).
[/caption] The model produces information about wave height (Figure 16.29, left) and shows the direction and speed of water flow across the lake using arrows of different sizes (Figure 16.29, right). If scientists are interested in how a pollutant would move around the lake, they can include the location where the pollutant is added, and how rapidly it is added, and track how it moves. [caption id="attachment_676" align="aligncenter" width="650"] Figure 16.29 Height of the water surface (left) and current velocity (right) from a model of wind-driven flow in Lake Ontario. The length of current arrows shows the speed of the current, and the arrow points in the direction of flow. Source: Karla Panchuk (2002) CC BY 4.0. Based on the exercise described in Chapter 10 of Slingerland & Kump (2011).
[/caption] If the model is to be used to track the movement of a pollutant through the lake, it is important to know that it has done a good job of calculating the current velocity. In this case, data from current meters in the lake can be compared to the current velocities that the model calculates (Figure 16.30). The model captures the fact that flow is northward near the margins of the lake, and southward in the middle, but the model current velocities are not exactly the same. This means that the model would do a good job of predicting where the pollution went, but not as good a job at predicting how fast it got there. [caption id="attachment_677" align="aligncenter" width="500"] Figure 16.30 Comparison of model results with current meter data. Points plotted beneath the blue dashed line indicate northward flow. Points above indicate southward flow. Source: Karla Panchuk (2002) CC BY 4.0. Data from Simons and Schertzer (1989).[/caption] The model in this case had a relatively course representation of the lake geography, so a first step would be to make the grid cells smaller to do a better job of simulating the shape of the lake, then look at the flow in finer detail. Another step would be to represent the lake water using a vertical stack of several grid cells to better capture the extent to which bottom friction and wind force affect the lake water at different depths, and to do a better job of representing the water depth.

An Example of Using a Computer Model to Study Past Earth-System Change

The Paleocene-Eocene Thermal Maximum (PETM) was a sudden global warming event that happened approximately 56 million years ago. There was interest in studying this event because its suddenness was thought to be a good analogy for the rapid changes happening in the Earth system today. Cores from ocean-floor rocks show that the oceans became so acidified during the PETM that calcium carbonate sediments dissolved over vast areas of the ocean floor, vanishing entirely from some regions. The same cores also showed a shift in the carbon-isotope composition of calcium carbonate sediments. Both the acidification and the carbon-isotope shift indicated that a large amount of carbon was added to the Earth system to trigger the PETM. The problem was that there were a number of possible sources for the carbon, and thus a number of possible triggers for the event. Although scientists provided reasoned arguments for their favourite hypotheses, there was no way to know for sure which was the best answer. To solve this problem, an Earth-system model was used that could test which scenario could best account for the pattern of dissolving calcium carbonate. It took into account the shape of ocean basins, ocean current circulation, and carbonate system chemistry in ocean water and in sediments. It also took into account changes in sediments once they were deposited. The steps to using this model were the following:

A search was done to locate as many studies of ocean floor sampling sites as possible that had information about changes in the amount of calcium carbonate during the PETM.
The model was set up so that it did a good job of reproducing the distribution of calcium carbonate before the PETM happened. This was important to ensure that model scenarios began with a realistic set of conditions.
Each of the possible scenarios involved carbon coming from different sources in the Earth system, meaning that each scenario could be represented in the model by adding to the atmosphere different amounts of carbon with different carbon isotope compositions. The more carbon a scenario required, the more the calcium carbonate sediments would have dissolved in real life.
The model was run for each different amount of carbon. For each scenario, the pattern of calcium carbonate sediments that the model gave was compared to the actual distribution of calcium carbonate sediments known from the data collected in Step 1.

In the end, the model showed that some of the scenarios did not even come close to matching the observations, either dissolving way too much calcium carbonate, or far too little. The model showed that two scenarios did come close to reproducing the pattern of calcium carbonate, and that one did a better job of matching the observations than the other. When it came time to write a report about the experiments, the scientists learned that newly published measurements from another study supported the scenario that the model suggested was best. It would have been acceptable to write a paper describing the model results, and which scenario worked best. However, also being able to comment about new supporting data meant there was a better chance of convincing other scientists that the model results were meaningful.

Predicting the Future of the Earth System with Models

Using models to investigate the Earth system requires careful consideration of how to build the model and run experiments. But it also requires skillful use of real-life measurements to set up the model, and to interpret and evaluate its results. The PETM model study was an example of how a model can be used to test hypotheses about past behaviour of the Earth system. There were data from before, during, and after the event to help set up the model and gauge its effectiveness. Using Earth-system models to predict the future is a different kind of modeling challenge, because we don't already know what the right answer is. The situation being modeled hasn't happened yet. Scientists who try to predict the future of the Earth system have to do things a bit differently in order to have some confidence in the reliability of their model outcomes:

They must come up with reasonable forcing scenarios for the model. A model used to predict the future of Earth's climate will need input about what atmospheric greenhouse gas levels will be. That will depend on what actions humans take. Scientists deal with this unknown variable by testing multiple scenarios for greenhouse gas levels, such as what would happen if fossils fuels continue to be used as they have been, or alternatively, what would happen if we completely stopped using fossil fuels tomorrow. The scenarios they choose span a range of possibilities, including extreme cases, to make sure they understand what the possible range of outcomes could be.
Scientists use some of the data they have to set the model up so that it is a realistic representation of the Earth system at a particular time in history. They then test the model to see if it can reproduce a different set of data later in history. This is a way to see if the model can get the right answer for a time when we know the right answer. Finally, after determining that the model gives reasonable results for times when we know the right answer, it is run for future scenarios.
To be confident about predictions of future Earth-system change, scientists may collaborate to run their scenarios on many different Earth-system models designed by many different research groups at many different institutions. These models are set up with slightly different mathematical representations of processes, or different levels of detail in geographic representations. The scientists who built the various models might disagree on what numbers to assign some of the variables, or even which parts of the Earth system are necessary to include. If all the models produce similar results for a particular scenario in spite of representing a wide range of ideas about how such models should work, scientists can be more confident in those results.
Scientists report uncertainty with their model results. It is a common misconception that uncertainty means the same thing as in everyday language—that we just don't know something, or can't say for sure. But for models, uncertainty is a number that indicates the likelihood that a model result is within a certain range of values. It is determined using methods that are themselves the product of careful research. A meaningful discussion of uncertainty will concern a specific model or set of models, a specific variable, and include a specific range of values. It will also include information about how large the uncertainty is compared to the changes they are investigating. If these details are missing from the conversation, it's a clue that “uncertainty” is being used in a common-language way rather than the way that modellers use it. Note that reporting uncertainty is not exclusive to models predicting the future, but it is particularly important for those models because of the great scrutiny Earth-system models receive when they are used to investigate future climate change.

References

Simons, T. J., & Schertzer, W. M. (1989) The circulation of Lake Ontario during the summer of 1982 and the winter of 1982/83. Burlington, ON: Environment Canada. http://publications.gc.ca/site/eng/9.854046/publication.html

Slingerland, R., & Kump, L. (2011). Mathematical Modeling of Earth's Dynamical Systems: A Primer. Princeton University Press.

16.5 Humans in the Earth System

kqzheng — Sun, 26 Aug 2018 21:40:41 +0000

The Start of Human Influence on the Earth System

Anthropogenic change in the Earth system is change caused by humans. Many discussions of anthropogenic climate-change place the start of human impacts on the Earth system at the beginning of the industrial era, in the mid 18th century. The industrial era was when humans began to use fossil fuels—at the time, mostly coal—on a much larger scale than before to do things like run manufacturing machinery and trains. Some climate scientists place the first anthropogenic impacts much earlier, however. Some suggest that anthropogenic climate change began around 8,000 BCE when humans cleared land for agriculture in Europe and the Middle East. Clearing forests for crops is a type of climate forcing because the CO₂ storage capacity of the crops is generally lower than that of the trees they replace. Some climate scientists also point to the creation of wetlands to grow rice in Asia around 5,000 BCE. Creating wetlands is a type of climate forcing because the anaerobic bacterial decay of organic matter within wetlands produces CH₄. Whether anthropogenic climate change began with the Agricultural Revolution or the Industrial Revolution may be a matter for debate for some, but it is clear that Earth-system change accelerated once the Industrial Revolution began. Part of this is due to the fact that agricultural activities had to be scaled up to feed an ever-growing population. When humans first started growing crops, the world population was approximately 5 million (Figure 16.31), fewer people than live in Toronto today. The world population rose to approximately 18 million when wetland rice cultivation began (fewer people than live within the city limits of Beijing today), to over 800 million at the start of the Industrial Revolution. The world population was estimated at 7,600 million in 2018. [caption id="attachment_680" align="aligncenter" width="600"] Figure 16.31 World population growth over the past 12,000 years. Source: Steven Earle (2015), CC BY 4.0. Image source. Data from Roser and Ortiz-Ospina (2018). Image source/ view data file[/caption] The other reason humans accelerated Earth-system change after the start of the industrial era is that human activities required a source of energy, and fossil fuels such as coal and oil were that source. Fossil fuels are those derived largely from plant material that grew, died, and was partially preserved at various times throughout Earth history. The plants removed CO₂ from the atmosphere when they were alive, and stored it in organic compounds in their tissues. The materials accumulated over hundreds of millions of years in settings like swampy forests, shallow seas, and deltas. When fossil fuels are burned, the stored carbon is released back into the atmosphere as CO₂.

The Carbon-Isotope Fingerprints of Fossil Fuel

Carbon isotopes provide insights into the extent to which fossil fuels have impacted the Earth system, because fossil fuels have a unique carbon-isotope fingerprint that is detectable in the atmosphere and in geological materials.

Stable Carbon Isotopes (12-Carbon and 13-Carbon)

When plants transform CO₂ into tissues, the process imparts a unique carbon-isotope signature to the resulting organic matter. Plants preferentially take in CO₂ with the isotope ¹²C over CO₂ with isotope ¹³C. They do so in a consistent way, giving plant tissues a distinctive ratio of ¹³C to ¹²C. Fossil fuels are derived from plant materials, and they preserve this isotopic ratio. The ratio of ¹³C to ¹²C is commonly expressed relative to a standard to give numbers that are easy to work with and compare. The notation δ¹³C refers to the ratio of ¹³C to ¹²C in a sample compared to the ratio in a standard, and is expressed in parts per thousand (or per mil, ‰). The standard has a δ¹³C of 0‰. Carbon in plant tissues has a δ¹³C of -25‰ to -30‰, meaning it has a ¹³C to ¹²C ratio that is 25 to 30 parts per thousand lower than the standard. Burning fossil fuel releases CO₂ with that ratio into the atmosphere. For most of the past 1000 years, the atmosphere has had a δ¹³C of approximately -6.5‰. The carbon-isotope composition of organic matter is much lower than that of the atmosphere, so the mixing in of carbon from fossil fuels causes the over-all carbon-isotope composition of the atmosphere to decrease. An analogy for mixing low δ¹³C CO₂ into the atmosphere is rapidly adding cold water to a hot bathtub. The faster the cold water is added, the faster the bathwater will cool. The colder the water being added, the faster the bathwater will cool. In this analogy, the atmosphere is the bathtub, and fossil fuels are the water being added. The low δ¹³C value of fossil fuels (-25‰ to -30‰) is like very cold water being added. As we would expect, the carbon isotope composition of the atmosphere takes a sudden downward turn at the same time that humans undertake the Industrial Revolution, and begin burning large quantities of fossil fuels, adding CO₂ to the atmosphere at an accelerating rate (Figure 16.32). [caption id="attachment_681" align="aligncenter" width="600"] Figure 16.32 A 1000-year record of atmospheric CO₂ levels (blue circles) and carbon isotope composition (grey circles) measured in Antarctic ice cores. The Industrial Revolution (grey shading), marking the start of the industrial era and the large-scale use of fossil fuels by humans, coincides with a sudden rise in CO₂ levels, and a fall in the carbon-isotope composition of atmospheric CO₂. Source: Karla Panchuk (2018), CC BY 4.0. Data from Rubino et al (2013).[/caption] Scientists who study past climates on Earth are familiar with carbon-isotope records like this one, because such records are used to reconstruct major changes in the Earth system through their impact on the carbon cycle. In carbon-isotope records from the distant past, a shift of more than 1.5‰ would be enough to catch the attention of a researcher and make them wonder what could have happened. What is unusual about the 1.5‰ drop today in comparison to those observed in the geological record is how rapidly it is happening. It is more common to see such changes happen over millions of years, not hundreds of years. The rate at which atmospheric CO₂ δ¹³C is dropping is approximately 10 times faster than the carbon-isotope shift at the PETM, which is the fastest event ever documented in the rock record. Carbon dioxide in the atmosphere mixes into the oceans, where organisms take up carbonate ions to make calcium carbonate shells. The 1.5‰ drop has been imprinted in the calcium carbonate of marine organisms like sponges (Böhm et al, 2002), and will remain in the rock record globally, as evidence of human activity. Because of this, and because of many other such markers that are being left in the rock record by human activities (the presence of plastic, for example), some have suggested that it is time to define a new division of geological time, the Anthropocene Epoch. The start of the Anthropocene Epoch would mark the point at which human activities became evident in the geological record.

Radioactive Carbon (14-Carbon)

Carbon-14 dating relies on the fact that ¹⁴C decays to ¹⁴N at a known rate. By knowing the rate, and how much ¹⁴C and ¹⁴N are present, we can work out how long the decay has been happening. Knowledge of the decay rate of ¹⁴C also makes it useful to track fossil fuel additions to the atmosphere. The rate of decay of a radioactive isotope is expressed as a half-life, which in this case is the amount of time it would take half of the ¹⁴C atoms in a sample to decay to ¹⁴N. The half-life of ¹⁴C is 5,730 years. After 10 half-lives, or 57,300 years, there isn’t enough ¹⁴C left to do an age measurement. Fossil fuels are millions to hundreds of millions of years old, long enough for there to be none of the ¹⁴C originally contained by the plant material. There is a notation system for ¹⁴C similar to the δ¹³C notation system for the ratio of ¹³C to ¹²C, in which the amount of ¹⁴C is compared to a standard. Carbon-14 amounts are reported as Δ¹⁴C values in units of ‰. In that system, the atmosphere as a whole had a Δ¹⁴C of 45‰ in 2010, and fossil fuels have a Δ¹⁴C of -1000‰. Effectively, the atmosphere appears to be aging rapidly. In the bathtub analogy for carbon isotopes, adding CO₂ from fossil fuels is like dumping ice into the tub. The effects of fossil fuel CO₂ on atmospheric Δ¹⁴C levels must account for ¹⁴C being made through natural processes in the atmosphere, and decaying away; for the decay of a large pulse of ¹⁴C created by nuclear bomb tests; and for other sources of carbon with very low Δ¹⁴C values. Fortunately for scientists tracking fossil fuels by their impact on atmospheric Δ¹⁴C, the contribution of low Δ¹⁴C CO₂ from other sources is tiny compared to known rates of fossil fuel emissions, and the other quantities are also well known. Thus, they have been able to determine a decrease in Δ¹⁴C of 3‰ for every 1 ppm of CO₂ added from fossil fuels.

The Carbon Cycle and Change in Today's Earth System

Change in the Earth system is strongly driven by Earth’s carbon cycle, the interrelated materials and processes that change carbon from one form to another, and move it from one reservoir to another (Figure 16.33). The CO₂ in the atmosphere is just one part of the carbon cycle. Carbon in the atmosphere is taken in by marine and terrestrial plants, and released when they are decomposed. Microbial activities in the soil and respiration by plants release carbon. Carbon also moves into and out of the ocean through exchange processes at the ocean’s surface. [caption id="attachment_1090" align="aligncenter" width="711"] Figure 16.33 Flows of carbon in the Earth system. Numbers are rates in billions of tons of carbon (gigaton, Gt) per year. Yellow numbers are rates unrelated to human activity. Red numbers show the contribution of human activities as of 2012. Source: U.S. Department of Energy (2012), Public Domain. Image source.[/caption] In the carbon cycle today, natural processes as a whole comprise far more of the flow in the carbon cycle than human activities do. For comparison, the relative sizes of flows in Figure 16.33 are illustrated by the size of the arrows. As of 2012, human activities were responsible for approximately 9 billion tons (9 Gt) of carbon added to the atmosphere per year. A large part of the 9 Gt comes from burning oil, coal, and gas, and some from changes in how land is used (e.g., clearing forests to plant crops, Figure 16.34). Some comes from changes that humans have made that affect the ability of the Earth system to take up carbon. [caption id="attachment_683" align="aligncenter" width="1024"] Figure 16.34 Flow diagram illustrating the pathways through which human activities produce greenhouse gases. The diagram connects the items in each column with flows that ultimately lead to the type of fuel used, and the greenhouse gasses produced. The width of each band is proportional to the quantity flowing from one column to the next. Note that F-Gas refers to anthropogenic fluorinated gases, which are extremely powerful greenhouse gases. Source: Fischedick et al. (2014), Figure 10.1, based on Bajželj et al. (2013). Image source (p. 745). Click for terms of use.[/caption] The Earth system has accommodated the 9 billion tons by taking up an additional 3 billion tons per year in photosynthesis, and dissolving an additional 2 billion tons per year in the ocean. The remaining 4 billion tons accumulates in the atmosphere each year because the Earth system does not presently have the capacity to remove it. The fossil fuels added by humans are particularly problematic because burning them means releasing hundreds of millions of years worth of plant-stored carbon that would otherwise not have been an active part of the carbon cycle today. Contrast this with cutting down a tree and burning the wood. Burning the wood also releases CO₂ from carbon that was stored in plant tissues, but the difference is in timescale and quantity. If a tree grows for 50 years before it is used as fuel, then over a century there is effectively no change in atmospheric CO₂. What carbon the tree took out of the atmosphere decades before, burning and decomposition have returned. For fossil fuels, on the other hand, the carbon was removed from the atmosphere tens or even hundreds of millions of years ago. Trees draw down CO₂ before we burn them, balancing out the equation, but with fossil fuels there is no initial draw-down from our present atmosphere. Releasing the carbon stored in those fuels results in a net addition to the atmosphere. What makes this even worse is that because fossil fuels have been accumulating for so long, there is an enormous quantity that can be burned. Trees can only be burned as fast as they replace themselves, but with fossil fuels it is like accumulating trees for millions of years, then burning them all at once.

Signals of Present-Day Earth-System Change

Rising Temperatures

From studies of Earth's past climate history, it is clear what to expect as atmospheric CO₂ levels rise. Climate warming is one outcome. We know from ice core records that global average temperatures are warmer now than they have been for most of the last 800,000 years (Figure 16.22). Over the shorter term, direct measurements show that the climate has been on a warming trend after the start of the Industrial Revolution (Figure 16.9). Proxy data making up a revised version of the "hockey stick" diagram—so named because the shape reminded some people of a hockey stick laying on its side—take the record back to 1000 years ago, and show global average temperatures falling until the onset of the industrial era (Figure 16.35). [caption id="attachment_684" align="aligncenter" width="686"] Figure 16.35 Global average temperature change for the last 1000 years. Blue- The original "hockey stick" diagram showing a reconstruction of northern hemisphere temperatures using tree rings as a proxy. Red- Direct temperature measurements. Green dots- Global temperature reconstruction using a wide range of direct measurements, historical records, and proxies (sediments, ice cores, tree rings, corals, stalagmites, pollen). The original hockey stick diagram was the focus of much controversy because it was the first evidence of anthropogenic climate change that could be understood by the general public. The PAGES2K project sought to bring vast quantities of data to establish once and for all whether a global signal of warming could be reliably discerned. The result was very similar to the original hockey stick. Source: Karla Panchuk (2018) CC BY-SA 4.0. Modified after Klaus Bittermann (2013) CC BY-SA 4.0 view source. Learn more about PAGES2K and find data.[/caption]

Sea Level Change

As of April 2018, global sea level has risen approximately 28 cm since 1800. According to satellite data, the average rate of change since 1993 has been a rise of approximately 3 mm per year. Part of the rise is due to the expansion of seawater as it warms. Another part of the rise is from water added by melting glaciers and other year-round land-based snow and ice. Note that melting of sea ice—ice already floating in the ocean—does not contribute directly to sea-level rise because the ice is already floating in the ocean. Based on how much melting has occurred thus far, sea levels are projected to rise to between 47 cm and 130 cm above 1880 levels (Figure 16.36). However, there is some uncertainty about how melting rates will respond to changes in the Earth system that result from climate change, such as changes in currents, or seawater beneath the leading edge of melting ice sheets warming the ice from beneath. With that uncertainty factored in, sea level rise could be as low as 33 cm above 1880 levels, or more than 2 m higher. [caption id="attachment_685" align="aligncenter" width="650"] Figure 16.36 Measured and projected change in global average sea level. Data come from proxy records as well as from direct measurements from tidal gauges and satellite data. Projected sea level rise could be as little as 33 cm over 1800 levels, or as much as 206 cm. Source: Karla Panchuk (2018), CC BY 4.0. Modified after Steven Earle (2015,) CC BY 4.0 (view source) and J. Willis, Jet Propulsion Laboratory (2013). Image source and more information about this figure. Click for terms of use.[/caption] Keep in mind that the global average is indeed an average. Where ocean waters experience more warming, and thus more thermal expansion, sea level rise may be greater than elsewhere. Regions that are rebounding as ice melts could experience less sea level rise, or even a fall in sea level, because the elevation of the terrain is actually increasing over time. On the other hand, regions on the peripheral bulge around the margins of ice sheets could experience greater than average sea level rise because the terrain will subside at the same time that the oceans are gaining volume. Areas that become flooded could experience greater than average sea level rise, because the weight of water causes the land to subside further. In the aftermath of Hurricane Harvey in September of 2017, measurements were reported that showed subsidence of up to 1.5 cm in the region of Houston, Texas. In this case, some of the subsidence could have been from sediments being compressed under the weight of flood waters, however the weight of water, like the weight of ice, does cause the crust to float lower in the mantle.

Melting Ice Sheets

Keeping track of how rapidly ice sheets are melting is important both for being able to predict future sea level change, and for knowing in general how rapidly the Earth system is changing. In a recent study, Bamber et al. (2018) analyzed satellite measurements to determine how much mass had been lost from the Antarctic ice sheets, the Greenland Ice Sheet, and from other glaciers and ice caps around the world since 1992 (Figure 16.37). [caption id="attachment_686" align="aligncenter" width="605"] Figure 16.37 Large ice sheets of Antarctica and Greenland (blue) and glaciers and ice caps (yellow). Circles are proportional to the area of each region that is covered by glaciers. The green part of the circle indicates the proportion of the ice with margins resting on land, and the blue part indicates margins in the ocean. This difference is important in part because of the potential for faster melting when the base of an ice sheet is in contact with warming seawater. Source: Bamber et al. (2018), CC BY 4.0. Image source (see Fig. 1)[/caption] The study found that over all, the mass of ice in ice sheets, ice caps, and glaciers has been falling at an increasing rate since 1992, and therefore adding to sea level at an increasing rate (Figure 16.38). The exception is the East Antarctic Ice Sheet, which actually showed an increase in mass during the studied interval. This is because snowfall has increased in the East Antarctic, to the point where more snow is falling now than at any time in the past 2000 years (Medley et al., 2017). The East Antarctic is warming just as the West is, but the difference is that the winds that preferentially bring precipitation to the East rather than the West can carry more moisture because the air is warmer. [caption id="attachment_687" align="aligncenter" width="602"] Figure 16.38 Results of a study of the change in mass of ice on Earth's surface. Satellite data show that over all, melting has accelerated since 1992. Source: Karla Panchuk (2018) CC BY 4.0, modified after Bamber et al. (2018) CC BY 4.0. Image source (see Fig. 11)[/caption]

Concept Check: Evidence of Human Influence on the Carbon Cycle

Fill in the missing words to complete the summary. Human activities may have affected the carbon cycle as early as 8,000 BCE, but it wasn't until humans began to undertake activities that atmospheric levels began to rise rapidly. At the same time, the atmosphere began to take on the characteristics of derived from . This is the chemical fingerprint of . Although some flows of carbon in the carbon cycle are much larger than flows, the natural carbon cycle can't carbon fast enough to what humans are adding. Fill-in-the-blank options:

ancient carbon
agricultural
balance
natural
carbon isotope
carbon dioxide
anthropogenic
fossil fuel
remove
industrial
plant matter

To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="174"]

References

Bajželj, B., Allwood, J. M., and Cullen, J. M. (2013). Designing Climate Change Mitigation Plans That Add Up. Environmental Science & Technology 47, 8062-8069. doi: 10.1021/es400399h. http://pubs.acs.org/doi/pdf/10.1021/es400399h

Bamber, J. L., Westaway, R. M., Marzeion, B., & Wouters, B. (2018). The land ice contribution to sea level during the satellite era. Environmental Research Letters 13(2018). https://doi.org/10.1088/1748-9326/aac2f0

Böhm, F., Haase-Schramm, A., Eisenhauer, A., Dullo, W.-C., Joachimski, M. M., Lehnert, H., & Reitner, J. (2002). Evidence for preindustrial variations in the marine surface water carbonate system from coralline sponges. Geochem. Geophys. Geosyst., 3(3), 10.1029/2001GC000264. http://onlinelibrary.wiley.com/doi/10.1029/2001GC000264/epdf

Earth System Research Laboratory, Global Monitoring Division, NOAA (n.d.). The data: What 14C tells us. https://www.esrl.noaa.gov/gmd/outreach/isotopes/c14tellsus.html

Fischedick M., Roy, J., Abdel-Aziz, A., Acquaye, A., Allwood, J. M., Ceron, J.-P., Y. Geng, Y., Kheshgi, H., Lanza, A., Perczyk, D., Price, L., Santalla, E., Sheinbaum, C., and Tanaka, K. (2014). Industry. In: Climate change 2014: Mitigation of climate change. Contribution of working group III to the fifth assessment report of the intergovernmental panel on climate change. Edenhofer, O., Pichs-Madruga, R., Sokona, Y., Farahani, E., Kadner, S., Seyboth, K., Adler, A., Baum, I., Brunner, S., Eickemeier, P., Kriemann, B., Savolainen, J., Schlömer, S., von Stechow, C., Zwickel, T., and Minx, J. C. (eds.). Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. https://www.ipcc.ch/pdf/assessment-report/ar5/wg3/ipcc_wg3_ar5_chapter10.pdf

Madrigal, A. C. (2017, September 5) The Houston flooding pushed the Earth's crust down 2 centimeters. https://www.theatlantic.com/technology/archive/2017/09/hurricane-harvey-deformed-the-earths-crust-around-houston/538866/

Medley, B., McConnell, J. R., Neumann, T. A., Reijmer, C. H., Chellman, N., Sigl, M., & Kipfstuhl, S. (2018). Temperature and Snowfall in Western Queen Maud Land Increasing Faster Than Climate Model Projections. Geophysical Research Letters, 45(3), 1472-1480. https://doi.org/10.1002/2017GL075992

Rahmstorf, S. (2013). Most comprehensive paleoclimate reconstruction confirms hockey stick. https://thinkprogress.org/most-comprehensive-paleoclimate-reconstruction-confirms-hockey-stick-e7ce8c3a2384/

Roser, M., & Ortiz-Ospina E. (2018). World population growth. https://ourworldindata.org/world-population-growth

Rubino, M., Etheridge, D. M., Trudinger, C. M., Allison, C. E., Battle, M. O., Langenfelds, R. L., Steele, L. P., Curran, M., Bender, M., White, J. W. C., Jenk, T. M., Blunier, T., & Francey, R. J. (2013). A revised 1000 year atmospheric δ¹³C‐CO₂ record from Law Dome and South Pole, Antarctica. Journal of Geophysical Research: Atmospheres 118, 8482–8499. doi:10.1002/jgrd.50668

Authors

kqzheng — Thu, 19 Aug 2021 18:04:16 +0000

Cover

kqzheng — Thu, 19 Aug 2021 18:04:16 +0000

Book Information

kqzheng — Thu, 19 Aug 2021 18:04:16 +0000

How to Make the Most of This Interactive Book & Example Exercises

kqzheng — Tue, 10 Aug 2021 19:06:34 +0000

This section is to help learners make the most of the time they spend with this textbook, and to help teacherfolk understand the philosophy behind the exercises, what they're like, and how to use them for their own projects. What's in here:

How to Use the the Activities as a Learner
The Philosophy
Using These Exercises in Your Own Projects
Some Examples

How to Use the Activities as a Learner

Let's start with an example that will show you learners some tricks to look for with these activities, and you teacherfolk why I changed how I look at [pb_glossary id="830"]formative assessment[/pb_glossary]. This is my H5P version of an exercise from a language-learning app. The amazing thing about this exercise is that it's a test of Portuguese vocabulary, but a) you don't even need to know any Portuguese to get the right answer, and b) that's actually a good thing. (Teacherfolk, I struggled with this notion at first, believe me.) So here it is, and give it a try. You can get 100% regardless of how much Portuguese you know—I promise! Tip: If you're stuck, click on the little blue icons ("i" cons, get it?). You'll see the clues that describe how I reasoned though this exercise the first time I did it. [h5p id="1"] When I did this exercise the first time, I got "mulher" through process of elimination, but subsequently noticed that if you swapped out the l for a t, you get a word that sounds like "mother." It stuck in my head after that. Is this too easy to do with the tips? Then don't look at the tips, silly! Or only look at the ones you need to until you remember the words. And once you remember the words, you can come back to the exercise and test your memory without looking at the tips at all. You win whether you "tip" your way through it and learn some words by accident, or you use it to practice your recall. It doesn't really matter how you get the information in your head if it sticks there and makes sense. This exercise is just one type of a variety that I've included here (skip to the bottom if you'd like to see some examples right this minute), but a good approach with all of them is to do them in a way that works for where you are in that moment as a learner. These aren't tests so much as opportunities for you to build your own "tricks" for remembering, and then to practice your knowledge. You should try the activities even if you don't think you can get them exactly right. That's sort of the point.

The Philosophy

(For Teacherfolk Who Are Fans of Such Things or Learners Who Wonder What Teacherfolk Think About) After I came up with my trick for "mulher," the word stuck in my head like chewing gum on a shoe before a job interview. I became so fascinated by how this app worked that I tested out three other languages—Norwegian, Ukrainian, and Navajo—for several weeks before hitting a wall. It was with Navajo, and it was because I couldn't use my tricks anymore. Lightbulb!!! What I came to realize (and what I've done here) is that you can break activities into three categories:

Construction
Practice
Leveling Up

These represent progressive advancement of knowledge, but they're not necessarily meant to be linear. Construction is what you saw with "mulher." It's finding ways to build your knowledge, and scaffolding by whatever means necessary. Practice is just that—repeating skills and knowledge to become familiar and comfortable with them. If you can do practice and construction iteratively, or even simultaneously, you've got some powerful learning potential. You just need to tune out your inner womp-womp tuba that tells you to feel sad when you get the wrong answer, because it's all good! Leveling up is a synthesis element, where you can do something more complex with what you've acquired through construction and practice. And even then, it's possible to backstop complex and challenging activities with opportunities for additional construction and practice.

Using These Exercises in Your Own Projects

Licensing

Go ahead: borrow, modify, etc.! All of the H5P exercises in this book have a Creative Commons license, and with a few exceptions related to licensing of images, they are either Attribution (BY) or Attribution-ShareAlike (BY-SA). If you don't speak Creative Commons, that means you can remix and redistribute them as you like, but some do have restrictions as to how you can license derivative works, and whether commercial use is permitted. (Learn more about Creative Commons licenses here.) None of the activities require special permission to use or modify.

Modularity

I designed these exercises to be as modular as possible, meaning that they should be usable without having to grab contextual material from the textbook.

Ways to Deploy Activities

Use the Textbook or Grab a URL

If you're new to H5P, you don't have to do anything special to use these. You can share the URL of textbook pages with the activities, or you can get a URL for the activity itself. To get the URL, do the following:

Click on <>Embed at the bottom of the activity.
Grab the URL from within the embed code. It'll look like the link below, except the number at the end will vary.
Share this link with users to have them open the exercise in a new browser window. (Not fancy, but it works.)

Sample URL from embed code

https://opentextbc.ca/physicalgeologyh5p/wp-admin/admin-ajax.php?action=h5p_embed&id=1

Embed an Activity

You can use the full <>Embed code (it'll look like the code below) to put the activity directly into a different website, including into a learning management system. If you're using an LMS, there will be an "Embed" menu item somewhere that will give you a box to paste the code into.

Full embed code sample

Download the Activity

If you have access to a website with the H5P plugin, then you can download these activities and upload them to your own website. Downloading the activities is the way to go if you'd like to modify them. It's also a great way to "look under the hood" and get to know how to build them. To download an activity, click on ⟳Reuse at the bottom of the activity, and select Download as an .h5p file. The More Info link above the download option will take you to more detailed instructions, should you need them. Note that you won't be able to use these files outside of an H5P plugin. They won't open on your computer, for example.

Hot Tip: At the time of this writing, it's hard to put subscripts and superscripts in many of the activities. Fortunately, you can write H₂O instead of H2O by copying and pasting the relevant Unicode characters into the editor. Copying and pasting doesn't work with formatted superscripts and subscripts (i.e., if you copied and pasted this H₂O that I entered by using the subscript option built into the editor for this page, the subscript will revert on you).

Some Examples

Use these links to navigate, or just scroll down to explore. 1. What's A Mineral? A drag and drop exercise with a drop-down explanation box. 2. Finn's Messed-Up Collection. Help Finn put her mineral collection back together by using descriptions, mineral identification tables, and identifying minerals based on their properties. 3. Types of Igneous Textures. Flashcards to practice identifying igneous textures 4. Melting and Plate Tectonic Settings. Drag images of different plate tectonic settings to boxes that describe the type of melting. 5. Can You Figure Out the Magma Composition? A drag-and-drop based exercise to guide learners through classifying magma compositions based on oxide content 6. Crystalline Igneous Rocks. A drag-and-drop exercise with images of crystalline igneous rocks and a mineral component diagram. 7. Identifying Types of Physical Weathering. A two part exercise with a fill-in-the-blanks warm up, followed by flashcard-esque questions with tips. 8. Sedimentary Rock Murder Mystery Challenge. An exercise to synthesize rock cycle and sedimentary rock concepts and knowledge, but with just enough hints that even a beginner will benefit. 9. Distinguishing Foreshocks, Aftershocks, and Mainshocks. A quick concept check fill-in-the-blanks matched with an image. 10. Rock-Dating Scenario. A Multi-component scenario activity where learners apply radiometric and relative dating principles to answer questions.

1. What's A Mineral?

[h5p id="2"]

2. Finn's Messed-Up Collection

[h5p id="3"]

3. Types of Igneous Textures

[h5p id="4"]

4. Melting & Plate Tectonic Settings

[h5p id="5"]

5. Can You Figure Out the Magma Composition?

[h5p id="6"]

6. Crystalline Igneous Rocks

[h5p id="7"]

7. Identifying Types of Physical Weathering

Practice with Types of Physical Weathering [h5p id="8"] Now that you're warmed up, try this: [h5p id="9"]

8. Sedimentary Rock Murder Mystery Challenge

Can you combine what you've learned so far about the rock cycle, weathering, sediments, sedimentary rocks, and sedimentary structures to solve these cases? For each case you are given the evidence (details about a sedimentary rock). It's your job to figure out who the victim was (the source rock), the murder weapon (what kind of weathering transformed the source rock), the get-away car (how the sediment was transported), and the crime scene (where the sediment was eventually deposited and lithified). If you get stuck, click on the tips for additional clues. [h5p id="10"]

9. Distinguishing Foreshocks, Aftershocks, and Mainshocks

[h5p id="11"]

10.Rock-Dating Scenario

[h5p id="12"]

9.5 Sedimentary Structures and Fossils

kqzheng — Fri, 09 Nov 2018 23:46:28 +0000

So far you've learned that the characteristics of sedimentary rocks will depend on the environment in which they formed, because those environments determine what kinds of particles are available to make rocks, what ions are present, and how particles or ions move and accumulate. Geologists look very closely at sedimentary grains to determine their mineralogy or lithology (in order to make inferences about the type of source rock and the weathering processes), their degree of rounding, their sizes, and the extent to which they have been sorted by transportation and depositional processes. But depositional environments also put their unique footprint on sedimentary rocks by causing rocks to develop different sedimentary structures as they form. By understanding the origins of those structures, we can make some very useful inferences about the processes and depositional environment that ultimately resulted in the rocks that we are studying.

Types of Layering

Bedding refers to sedimentary layers that can be distinguished from one another on the basis of characteristics such as texture, composition, colour, or weathering characteristics (Figure 9.21). They may also be similar layers separated by partings, narrow regions marking weaker surfaces where erosion is enhanced. Bedding is an indication of changes in depositional processes that may be related to seasonal differences, changes in climate, changes in locations of rivers or deltas, or tectonic changes. Bedding can form in almost any depositional environment. [caption id="attachment_328" align="aligncenter" width="500"] Figure 9.21 Beds in the Triassic Sulphur Mt. Formation near Exshaw, Alberta. Bedding is defined by differences in colour and texture, and also by partings (darker lines) between beds that may otherwise appear to be similar. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] Cross-bedding is bedding that contains angled layers. It forms when sediments are deposited by flowing water or wind (Figure 9.22). Cross-beds in streams tend to be on the scale of cm to tens of cm, while those in aeolian (wind deposited) sediments can be on the scale of metres. [caption id="attachment_329" align="aligncenter" width="500"] Figure 9.22 Cross-bedded Jurassic Navajo Formation aeolian sandstone at Zion National Park, Utah. In most of the layers the cross-beds dip down toward the right, implying wind direction from right to left during deposition. One bed dips in the opposite direction, implying a different wind direction. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] Cross-beds form as sediments are deposited on the leading edge of an advancing ripple or dune. Each layer is related to a different ripple that advances in the flow direction, and is partially eroded by the following ripple (Figure 9.23). Cross-bedding is a very important sedimentary structure to recognize because it can provide information on the direction of current flows and, when analyzed in detail, on other features like the rate of flow and the amount of sediment available. [caption id="attachment_330" align="aligncenter" width="500"] Figure 9.23 Formation of cross-beds as a series of ripples or dunes that migrate with the flow. Each ripple advances forward (right to left in this view) as more sediment is deposited on its leading face. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] Ripples, generated by flowing water may be preserved on the surfaces of sedimentary beds. Ripples associated with the formation of cross-bedding under unidirectional flow can help to determine flow direction because they tend to have their steepest surface facing down-flow. Ripples can also form from back-and-forth sloshing flows. These are symmetrical, and are a sure sign of a beach. Imbrication is another sign of flowing water. In a stream environment, boulders, cobbles, and pebbles can become tilted in the same general direction. Clasts in streams tend to tilt with their upper ends pointing downstream, because this is the most stable position with respect to the stream flow (Figure 9.24). [caption id="attachment_331" align="aligncenter" width="500"] Figure 9.24 Imbrication of clasts in a fluvial environment. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] Graded bedding is characterized by a change in grain size from bottom to top within a single bed. “Normal” graded beds are coarse at the bottom and become finer toward the top (Figure 9.25), a product of deposition from a slowing current. Some graded beds are reversed (coarser at the top), and this normally results from deposition by a fast-moving debris flow. Most graded beds form in a submarine fan environment, where sediment-rich flows called turbidites descend periodically from a shallow marine shelf down a slope and onto the deeper sea floor. [caption id="attachment_332" align="aligncenter" width="500"] Figure 9.25 Graded bedding going from pebbles at the bottom to sand at the top. Source: James St. John (2018), CC BY 2.0. Image source.[/caption]

Other Structures

Mud cracks form when a shallow body of water (e.g., a tidal flat or pond), into which muddy sediments have been deposited, dries up and cracks (Figure 9.26). This happens because the clay in the upper mud layers shrinks upon drying. [caption id="attachment_333" align="aligncenter" width="511"] Figure 9.27 Mud cracks in a tidal flat in England. Source: Alan Parkinson (2000), CC BY-SA 2.0. Image source.[/caption]

A Note About Fossils

Fossils are not covered in detail in this book, but they are extremely important for understanding sedimentary rocks. Fossils can be used to date sedimentary rocks, but just as importantly, they tell us a great deal about the depositional environment of the sediments and the climate at the time. They can help to differentiate marine, aquatic, and terrestrial environments; estimate the depth of the water; detect the existence of currents; and even to estimate average temperature and precipitation.

Practice with Sedimentary Structures Can you name these structures? Turn the card to check your answer. [h5p id="110"]

Putting It All Together: Murder Mystery Challenge Can you combine what you've learned so far about the rock cycle, weathering, sediments, sedimentary rocks, and sedimentary structures to solve these cases? For each case you are given the evidence (details about a sedimentary rock). It's your job to figure out who the victim was (the source rock), the murder weapon (what kind of weathering transformed the source rock), the get-away car (how the sediment was transported), and the crime scene (where the sediment was deposited and lithified). If you get stuck, click on the tips for additional clues. [h5p id="10"]

10.2 Foliation and Rock Cleavage

kqzheng — Wed, 26 Sep 2018 01:51:49 +0000

How Foliation Develops

When a rock is acted upon by pressure that is not the same in all directions, or by shear stress (forces acting to "smear" the rock), minerals can become elongated in the direction perpendicular to the main stress. The pattern of aligned crystals that results is called foliation. Foliation can develop in a number of ways. Minerals can deform when they are squeezed (Figure 10.5), becoming narrower in one direction and longer in another. [caption id="attachment_351" align="aligncenter" width="550"] Figure 10.5 Foliation that develops when minerals are squeezed and deform by lengthening in the direction perpendicular to the greatest stress (indicated by black arrows). Left- before squeezing. Right- after squeezing. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] If a rock is both heated and squeezed during metamorphism, and the temperature change is enough for new minerals to form from existing ones, the new minerals can be forced to grow longer perpendicular to the direction of squeezing (Figure 10.6). If the original rock had bedding (represented by diagonal lines in Figure 10.6, right), foliation may obscure the bedding. [caption id="attachment_352" align="aligncenter" width="550"] Figure 10.6 Effects of squeezing and aligned mineral growth during metamorphism. Left: Protolith with diagonal bedding. Right: Metamorphic rock derived from the protolith. Elongated mica crystals grew perpendicular to the main stress direction. The original bedding is obscured. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] This is not always the case, however. The large boulder in Figure 10.7 in has strong foliation, oriented nearly horizontally in this view, but it also has bedding still visible as dark and light bands sloping steeply down to the right. [caption id="attachment_353" align="aligncenter" width="500"] Figure 10.7 A geologists sits on a rock that has foliation (marked by the dashed line that is nearly horizontal), and still retains evidence of the original bedding (steeply dipping dashed line). The rock has undergone a relatively low degree of metamorphism, which is why the bedding is still visible. Source: Karla Panchuk (2018), CC BY 4.0. Modified after Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Foliation and Crystal Habit

Most foliation develops when new minerals are forced to grow perpendicular to the direction of greatest stress. This effect is especially strong if the new minerals grow in platy or elongated shapes. The rock in the upper left of Figure 10.8 is foliated, and the microscopic structure of the same type of foliated rock is shown in the photograph beneath it. Over all, the photomicrograph shows that the rock is dominated by elongated crystals aligned in bands running from the upper left to the lower right. The stress that produced this pattern was greatest in the direction indicated by the black arrows, at a right angle to the orientation of the minerals. The aligned minerals are mostly mica, which has a platy crystal habit, with plates stacked together like pages in a book. [caption id="attachment_354" align="aligncenter" width="550"] Figure 10.8 A foliated metamorphic rock called phyllite (upper left). The satin sheen comes from the alignment of minerals. Lower left- a view of the same kind of rock under a microscope showing mica crystals (colourful under polarized light) aligned in bands. The region outlined in a red dashed line shows a lens of quartz crystals that do not display alignment. Upper right- stacks of platy mica crystals. Lower right- a blocky quartz crystal. Source: Karla Panchuk (2018), CC BY-SA 4.0. Click for more attributions.[/caption] The zone in the photomicrograph outlined with the red dashed line is different from the rest of the rock. Not only is the mineral composition different—it's quartz, not mica—but the crystals aren't aligned. The quartz crystals were subjected to the same stress as the mica crystals, but because quartz grows in blocky shapes rather than elongated ones, the quartz crystals didn't align themselves. Even though the quartz crystals themselves are not aligned, the mass of quartz crystals forms a lens that does follow the general trend of alignment within the rock. This happens because the stress can cause some parts of the quartz crystals to dissolve, and the resulting ions flow away at right angles to the greatest stress before forming crystals again. The effects of recrystallization in Figure 10.8 wouldn't be visible without a microscope, but when larger crystals or large clasts are involved, the effects can be visible as "shadows" or "wings" around crystals and clasts. The rock in Figure 10.9 had a quartz-rich conglomerate as a parent rock. Differential stress has caused quartz pebbles within the rock to become elongated, and it has also caused wings to form around some of the pebbles (see the pebble in the dashed ellipse). The location of the wings depends on the distribution of stress on the rock (Figure 10.9, upper right). [caption id="attachment_355" align="aligncenter" width="606"] Figure 10.9 Metaconglomerate with elongated of quartz pebbles. The pebbles have developed "wings" to varying degrees (e.g., white dashed ellipse). These are the result of quartz dissolving where stress is applied, and flowing away from the direction of maximum stress before recrystallizing (upper right sketch). Source: Karla Panchuk (2018) CC BY-NC-SA 4.0. Click for more attributions and terms of use.[/caption]

Foliation Controls How Rocks Break

Foliated metamorphic rocks have elongated crystals that are oriented in a preferred direction. This forms planes of weakness, and when these rocks break, they tend to break along surfaces that parallel the orientation of the aligned minerals (Figure 10.10). Breaks along planes of weakness within a rock that are caused by foliation are referred to as rock cleavage (or just cleavage if everyone in the conversation knows we're talking about rocks, not minerals). This is distinct from cleavage in minerals because mineral cleavage happens between atoms within a mineral, but rock cleavage happens between minerals. [caption id="attachment_356" align="aligncenter" width="555"] Figure 10.10 Close-up view of a metamorphic rock with aligned elongated crystals. The crystals control the shape of the break in the rock (black gap), resulting in breaks occurring along parallel surfaces. Source: Karla Panchuk (2018) CC BY 4.0[/caption] The mineral alignment in the metamorphic rock called slate is what causes it to break into flat pieces (Figure 10.11, left), and is why slate has been used as a roofing material (Figure 10.11, right). The tendency of slate to break into flat pieces is called slaty cleavage. [caption id="attachment_357" align="aligncenter" width="551"] Figure 10.11 Rock cleavage in the fine-grained metamorphic rock called slate results in breaks along relatively flat surfaces (left). This is why slate has been used for roofing material (right). Source: Left- Roger Kidd (2008), CC BY-SA 2.0. Image source; Right- Michael C. Rygel (2007), CC BY-SA 3.0. Image source.[/caption] Rock cleavage is what caused the boulder in Figure 10.7 to split from bedrock in a way that left the flat upper surface upon which the geologist is sitting.

Practice with Foliation

Which set of arrows shows the direction force was applied that aligned these minerals? [caption id="attachment_1674" align="aligncenter" width="300"] Foliated metamorphic rock amphibolite viewed in thin section under a microscope.[/caption]
1. Arrows A & B
2. Arrows X & Y
If this rock broke, where would the break be more likely to happen? [caption id="attachment_1674" align="aligncenter" width="300"] Foliated metamorphic rock amphibolite viewed in thin section under a microscope.[/caption]
1. Along a line between arrows A & B
2. Along a line between arrows X & Y
This rock is:
1. Foliated
2. Non-foliated
This metamorphic rock is:
1. Foliated
2. Non-foliated
This metamorphic rock is:
1. Foliated
2. Non-foliated

To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="118"]

Concept Check: Summary of Foliation and Rock Cleavage

Fill in the blanks to complete the explanations of cleavage and foliation. Rock cleavage is different from mineral cleavage. Cleavage in (hint: rocks or minerals?) refers to breaks that are controlled by the atomic arrangement within a crystal. In contrast, cleavage in (hint: rocks or minerals?) refers to breaks controlled by the arrangement of mineral crystals. Whether a metamorphic rock is foliated or non-foliated will depend in part on how pressure was applied to the rock. When pressure is (hint: the same or not the same?) in all directions, foliation can develop. Foliation also requires that minerals are (hint: blocky or platy?) or (hint: elongated or stubby?) so that they can be aligned along their lengths. You can use household objects to see for yourself why some crystal shapes permit foliation while others don't. You can't align the dog toys in Image (hint: Image A or B?) because none of them are longer in one direction than the other. On the other hand, the toys in Image (hint: Image A or B?) can be arranged to run more or less parallel to each other. To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="119"]

Chapter 16 Summary & Key Term Check

kqzheng — Mon, 17 Sep 2018 18:55:57 +0000

Chapter 16 Main Ideas

16.1 What Is the Earth System?

Viewing Earth as a system allows us to take into account the complex ways in which the atmosphere, hydrosphere, biosphere, and lithosphere interact. Positive feedbacks amplify changes in the Earth system, and negative feedbacks reduce them. The stability of the Earth system will depend on what feedbacks are available. The presence of ice sheets makes the Earth system less stable.

16.2 Causes of Climate Change

Weather describes day-to-day conditions, but climate refers to the long-term average conditions over decades or longer. Climate forcings alter climate. They include processes that change the rate and location of solar energy reaching Earth’s surface; processes that alter how ocean currents move heat around Earth’s surface; and processes that affect how heat moves into and out of the atmosphere. Climate forcings operate on a range of timescales, from billions of years to less than a decade. Changes in greenhouse gas concentrations and albedo are two climate forcings affected by human activities.

Practice Again

16.3 Methods for Studying Past Climate

Climate conditions for some of human history can be determined from direct measurements that have been recorded, but for studying paleoclimate it's often necessary to use proxy data. Proxy data come from natural materials that behave in a systematic way in response to climate conditions like temperature or precipitation. Proxies include tree ring data, stable isotopes, measurements of gas bubbles trapped in ice, and the geographic distribution of rocks and fossils.

Practice Again

Paleoclimatology methods summary

16.4 Computer Models of the Earth System

Earth-system models use mathematical equations to simulate Earth-system processes. Models are set up and checked using real-life measurements. Model uncertainty is a number that tells us the likelihood that a particular model result falls within a certain range of values. It is a way to evaluate whether results can be used to draw meaningful conclusions.

16.5 Humans in the Earth System

Data show recognizable anthropogenic influence on the Earth system beginning when humans began to use fossil fuels for industrial purposes. CO₂ in the atmosphere has the isotopic fingerprints of fossil fuels. The flow of anthropogenic carbon into the Earth system is relatively small compared to some natural flows, but natural processes do not remove all of what humans put in, causing CO₂ to accumulate in the atmosphere.

Practice Again

Signs of human influence in the climate system

Key Term Check

What key term from Chapter 16 is each card describing? Turn the card to check your answer. [h5p id="175"]

19.1 The Geological Timescale

kqzheng — Sat, 27 Oct 2018 00:51:34 +0000

Developing the Geological Timescale

James Hutton (1726-1797) was a Scottish geologist, and considered by some to be the originator of modern Geology. Hutton studied present-day processes and applied his observations to the rock record in order to understand what he saw there. Such a method is now encapsulated in the principle of uniformitarianism, which states that the present is the key to the past. Given how slowly geological processes happen, Hutton concluded that geological time must be very long indeed to account for the large changes apparent in the rock record. But this principle needs to be taken with a grain of salt: there are some processes that have happened in the past that are no longer occurring (e.g., eruption of ultramafic lavas), and some processes that occur so irregularly that we have yet to witness such an event in historic time (e.g., impact of a large asteroid with Earth). William Smith worked as a surveyor in the coal-mining and canal-building industries in southwestern England in the late 1700s and early 1800s. While doing his work, he had many opportunities to observe ancient sedimentary rocks in the region, and noticed that he could trace rocks from one area to the next based textural similarities and differences. He also noticed that fossils could be used to match up rocks of the same age. Smith is credited with formulating the principle of faunal succession, the concept that specific types of organisms lived during different time intervals. He used the principle of faunal succession to great effect in his monumental project to create a geological map of England and Wales, published in 1815. Inset into Smith’s great geological map is a small diagram showing a schematic geological cross-section extending from the Thames estuary of eastern England to the west coast of Wales. Smith showed the sequence of rocks, from the Paleozoic rocks of Wales and western England, through the Mesozoic rocks of central England, to the Cenozoic rocks of the area around London (Figure 19.2). [caption id="attachment_790" align="aligncenter" width="500"] Figure 19.2 William Smith’s “Sketch of the succession of strata and their relative altitudes,” an inset on his geological map of England and Wales (with era names added). Source: Steven Earle (2015), CC BY 4.0. Modified after William Smith (1815), Public Domain. View map..[/caption] Smith didn't put any dates on these rocks, because he didn’t know them. But he was aware of the principle of superposition, the idea developed much earlier by the Danish theologian and scientist Nicholas Steno, that young sedimentary rocks form on top of older ones. Therefore, Smith knew that this diagram represented a stratigraphic column. And since almost every period of the Phanerozoic is represented along this section through Wales and England, it is also a primitive geological timescale. Smith’s work set the stage for the naming and ordering of the geological time periods, which was initiated around 1820, first by British geologists, and later by other European geologists. Many of the periods are named for places where rocks of that age are found in Europe, such as Cambrian for Cambria in Wales, Devonian for Devon in England, Jurassic for the Jura Mountains in France and Switzerland, and Permian for the Perm region of Russia. Some are named for the type of rock that is common during that age, such as Carboniferous for the coal-bearing rocks of England, and Cretaceous for the chalks of England and France. The early time scales were only relative because 19th century geologists did not know the absolute ages of rocks (i.e., the age as a number). This information was not available until the development of isotopic dating techniques early in the 20th century.

Today's Geologic Timescale

Eons: The Longest Intervals

The geological timescale is currently maintained by the International Commission on Stratigraphy (ICS), which is part of the International Union of Geological Sciences. The timescale is continuously being updated as we learn more about the timing and nature of past geological events. View the ICS timescale. Geological time has been divided into four eons: Hadean, Archean, Proterozoic, and Phanerozoic (Figure 19.3). The first three of these eons represent almost 90% of Earth’s history. Rocks from the Phanerozoic (meaning “visible life”) are the most commonly exposed rocks on Earth, and they contain evidence of life forms with which we are familiar. [caption id="attachment_791" align="aligncenter" width="1024"] Figure 19.3 The eons of Earth's history. Source: Karla Panchuk (2018), CC BY 4.0. Modified after Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Practice with Eons

This exercise has three levels of difficulty.

Place the eons and eras in the correct order.

Today
- Era
- Era Eon
- Era
541 Ma
- Eon
2500 Ma
- Eon
4000 Ma
- Eon
4600 Ma

Fill-in-the-blank options:

Phanerozoic
Cenozoic
Hadean
Paleozoic
Proterozoic
Mesozoic
Archean

To check your answers or try a more difficult version of this question, navigate to the below link.

[h5p id="189"]

Divisions of the Phanerozoic Eon

The Phanerozoic — the past 541 Ma of Earth’s history — is divided into three eras: the Paleozoic (“early life”), the Mesozoic (“middle life”), and the Cenozoic (“new life”), and each era is divided into periods (Figure 19.4). Most of the organisms with which we share Earth evolved into familiar forms at various times during the Phanerozoic. [caption id="attachment_792" align="aligncenter" width="1024"] Figure 19.4 The eras (middle row) and periods (bottom row) of the Phanerozoic. Source: Karla Panchuk (2018), CC BY 4.0. Modified after Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Practice with Divisions of the Phanerozoic Eon This exercise has three levels of difficulty.

Place the eras and periods in the correct order.

Today
- Era
66 Ma
- Period
- Period Era
- Period
252 Ma
- Period
- Period
- Period
- Period
- Period Era
- Period
541 Ma

Fill in the blank options:

Mesozoic
Silurian
Cenozoic
Ordovician
Cambrian
Devonian
Triassic
Carboniferous
Jurassic
Permian
Paleozoic
Cretaceous

To check your answers or try a more difficult version of this question, navigate to the below link.

[h5p id="190"]

Divisions of the Cenozoic Era

The Cenozoic, representing the past 66 Ma, is divided into three periods: the Paleogene, Neogene, and Quaternary. The periods are subdivided into seven epochs (Figure 19.5). Non-avian dinosaurs became extinct at the start of the Cenozoic, after which birds and mammals radiated to fill the available habitats. Earth was very warm during the early Eocene, and has cooled steadily ever since. Glaciers first appeared on Antarctica in the Oligocene and then on Greenland in the Miocene. By the Pleistocene, glaciers covered much of North America and Europe. The most recent of the Pleistocene glaciations ended ~11,700 years ago. The current epoch is known as the Holocene. Epochs are further divided into ages. [caption id="attachment_793" align="aligncenter" width="1024"] Figure 19.5 The periods and epochs of the Cenozoic Era. Source: Karla Panchuk (2018), CC BY 4.0. Modified after Steven Earle (2015), CC BY 4.0. Image source.[/caption] Most of the boundaries between the periods and epochs of the geological timescale have been fixed on the basis of significant changes in the fossil record. For example, the boundary between the Cretaceous and the Paleogene coincides exactly with the extinction of the non-avian dinosaurs. Many other types of organisms went extinct at this time, and the boundary between the two periods marks the division between sedimentary rocks containing Cretaceous organisms below, and those containing Paleogene organisms above.

Practice with Divisions of the Cenozoic Era

This exercise has three levels of difficulty.

Place the periods and epochs of the Cenozoic Era in the correct order.

Today
- Epoch Period
- Epoch
2.6 Ma
- Epoch Period
- Epoch
23 Ma
- Epoch
- Epoch Period
- Epoch
66 Ma

Fill in the blank options:

Ecocene
Pleistocene
Paleocene
Paleogene
Miocene
Oligocene
Quaternary
Holocene
Pliocene
Neogene

To check your answers or try a more difficult version of this question, navigate to the below link.

[h5p id="191"]

References

Smith, W. (1815). A delineation of the strata of England and Wales with part of Scotland. [map].

19.2 Relative Dating Methods

kqzheng — Sat, 27 Oct 2018 01:22:49 +0000

Relative Dating Principles

The most basic way of dating geological features is to look at the relationships between them. There are a few simple rules for doing this, but caution must be taken because there may be situations in which the rules are not valid. Local factors must be understood before an interpretation can be made with confidence. These situations are generally rare, but they should not be forgotten when unraveling the geological history of an area. The principle of superposition states that sedimentary layers are deposited in sequence, and the layers at the bottom are older than those at the top. The exception is if the sequence of rocks has been flipped completely over by tectonic processes, or disrupted and re-stacked by faulting. The principle of original horizontality indicates that sediments are originally deposited as horizontal to nearly horizontal sheets. The exception happens on small scales, like cross-beds which form at an angle to the main bedding surfaces. The same is true of delta foreset beds (Figure 19.6). [caption id="attachment_1110" align="aligncenter" width="600"] Figure 19.6 A cross-section through a river delta forming in a lake. The delta foresets are labeled "Delta deposits" in this figure, and you can quickly see that the front face of the foresets are definitely not deposited horizontally. Source: AntanO (2017), CC BY 4.0 Image source.[/caption] The principle of lateral continuity states that sediments are deposited such that they extend sideways for some distance before thinning and pinching out at the edge of the depositional basin. But sediments can also terminate against faults or erosional features (see unconformities below), and so may be cut off by local factors. The main idea here is that sedimentary layers don't just stop abruptly for no reason. The principle of cross-cutting relationships states that any geological feature that cuts across or disrupts another feature must be younger than the feature that is disrupted. An example of this is given in Figure 19.7, which shows three different sedimentary layers. The lower sandstone layer is disrupted by two faults, so we can infer that the faults are younger than this layer. But the faults don't appear to continue into the coal seam, and they certainly don't continue into the upper sandstone. So we can infer that coal seam is younger than the faults (because the coal seam cuts across them). The upper sandstone is youngest of all, because it lies on top of the coal seam. [caption id="attachment_797" align="aligncenter" width="650"] Figure 19.7 Superposition and cross-cutting relationships in Cretaceous Nanaimo Group rocks in Nanaimo BC. The coal seam is about 50 cm thick. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] An example that violates this principle is a type of fault called a growth fault, where the fault continues to move as sediments are added on top of the hangingwall block. In this case, the lower portion of the fault that cuts the lower sediments may have originally formed before the uppermost sediments were deposited, despite the fault cutting through all of the sediments, and appearing to be entirely younger than all of the sediments. The next three principles could be considered specific varieties of the principle of cross-cutting relationships, because they involve one geological feature disrupting another in some way. The principle of inclusions states that any rock fragments that are included in a rock must be older than the rock in which they are included. For example, a xenolith in an igneous rock, or a clast in sedimentary rock must be older than the rock that includes it (Figure 19.8). [caption id="attachment_798" align="aligncenter" width="664"] Figure 19.8 Applications of the principle of inclusion. Left- A xenolith of diorite incorporated into a basalt lava flow, Mauna Kea volcano, Hawai'i. The lava flow took place some time after the diorite crystallized (hammer head for scale). Right- Rip-up clasts of shale embedded in Gabriola Formation sandstone, Gabriola Island, BC. The pieces of shale were eroded as the sand was deposited, so the shale is older than the sandstone. Source: Karla Panchuk (2018) CC BY 4.0. Photographs by Steven Earle (2015), CC BY 4.0. Image sources left/ right[/caption] A possible situation that would violate this principle is a scenario where an igneous dyke intrudes through a sequence of rocks, and hence is younger than these rocks (principle of cross-cutting relationships). Later deformation may cause the dyke to be pulled apart into small pieces, and surrounded by the host rocks. That would make the pieces of the dyke appear to be xenoliths, but they would be younger than the surrounding rock. The principle of baked contacts states that the heat of an intrusion will bake (metamorphose) the rocks in close proximity to the intrusion. Hence the presence of a baked contact indicates the intrusion is younger than the rocks around it. If an intrusive igneous rock is exposed via erosion, then later buried by sediments, the surrounding rocks will not be baked, as the intrusion was already cold at the time of sediment deposition. But baked contacts may be difficult to discern, or may be minimally developed to absent when the intrusive rocks are low in volume or felsic (relatively cool) in composition. The principle of chilled margins states that the portion of an intrusion that has cooled and crystallized next to cold surrounding rock will form smaller crystals than the portion of the intrusion that cooled more slowly deeper in the instrusion. Smaller crystals generally appear darker in colour than larger crystals, so a chilled margin appears as a darkening of the intrusive rock towards the surrounding rock. This principle can be used to distinguish between an igneous sill, which will have a chilled margin at top and bottom, and a lava flow, which will have a chilled margin only at the bottom.

Practice with Relative Dating Principles

Type the correct stratigraphic principle.

Sedimentary layers are deposited in sequence, so the oldest layers are on the bottom.
Sediments are originally deposited as horizontal to nearly horizontal sheets.
Sediments extend sideways for some distance unless they're cut off by something like a fault or erosion.
Xenoliths are always older than the rock containing them. (hint: this is a specific type of one of the other principles.)
A geological feature that disrupts another geological feature is the younger of the two.
If rocks around an intrusion are altered by contact metamorphism, the rocks are older than the intrusion. (hint: this is a specific type of one of the other principles.)
If the edges of an intrusion cool off quickly, the intrusion is younger than the rock to which it lost its heat. (hint: this is a specific type of the other principles.)

Fill-in-the-blank options:

horizontality
cross-cutting
superposition
inclusions
baked contacts
lateral continuity
chilled margins

To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="192"] Now that you're warmed up, try this:

[caption id="attachment_1700" align="aligncenter" width="696"] Enhanced-colour view of an outcrop from Horseshoe Bay[/caption]

This outcrop has three rock types. What principle will you use to figure out their relative ages? (Hint: Notice which layers are interrupted.)
List the rock types according to the principle of superposition.

Write down your answers before reading the explanation below:

Cross-cutting relationships tell us the relative ages of the three elements in this outcrop.
The basalt is cut by both the felsic intrusion and the quartz vein, so it has to be the oldest. If it's metamorphosed because of the felsic intrusion, that would be another way to help infer its relative age.
The felsic intrusion also cuts the quartz vein, so it had to come after the quartz formed.

[h5p id="193"]

Unconformities

An unconformity represents an interruption in the process of deposition of sediments. Recognizing unconformities is important for understanding time relationships in sedimentary sequences. An unconformity is visible in the Grand Canyon (Figure 19.9, white dashed line) above Proterozoic rocks that were tilted and then eroded to a flat surface prior to deposition of the younger Paleozoic rocks. The difference in time between the youngest of the Proterozoic rocks and the oldest of the Paleozoic rocks is nearly 300 million years. Tilting and erosion of the older rocks took place during this time, and if there were any deposition occurring in this area during this time, erosion removed those sediments. [caption id="attachment_799" align="aligncenter" width="605"] 19.9 Angular unconformity in the Grand Canyon in Arizona, with a sketch of rock orientations. The tilted rocks at the bottom are part of the Proterozoic Grand Canyon Group (aged 825 to 1,250 Ma). The flat-lying rocks at the top are Paleozoic (540 to 250 Ma). The boundary between the two (dashed white line) represents a time gap of nearly 300 million years. Source: Karla Panchuk (2018), CC BY 4.0. Photograph by Steven Earle (2015), CC BY 4.0. Image source.[/caption] There are four types of unconformities, reflecting different arrangements and types of rocks above and below the surface of non-deposition or erosion (Figure 19.10):

A nonconformity (Figure 19.10a) is a boundary with non-sedimentary rocks below, and sedimentary rocks above.
An angular unconformity (Figure 19.10b) is a boundary between two sequences of sedimentary rocks, where underlying units have been tilted or folded and eroded before the younger units were deposited.
A disconformity (Figure 19.10c) is a boundary between two sequences of sedimentary rocks, where the underlying units have been eroded but not tilted or deformed before the younger layers were deposited.
A paraconformity is a time gap in a sequence of sedimentary rocks related to non-deposition rather than erosion. The gap won't be visible as an angular unconformity or a disconformity.

[caption id="attachment_800" align="aligncenter" width="686"] 19.10 The four types of unconformities: (a) a nonconformity between non-sedimentary rock and sedimentary rock, (b) an angular unconformity, (c) a disconformity between layers of sedimentary rock, where the older rock has been eroded but not tilted, and (d) a paraconformity where there is a long period (millions of years) of non-deposition between two parallel layers. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Practice with Unconformities [h5p id="194"] Now that you're warmed up, try applying your knowledge of unconformities to this relative dating exercise. [h5p id="195"]

19.3 Dating Rocks Using Fossils

kqzheng — Tue, 30 Oct 2018 18:52:41 +0000

Geologists obtain a wide range of information from fossils:

Fossils help us to understand evolution, and life in general.
They provide critical information for understanding depositional environments and changes in Earth’s climate.
They can be used to date rocks.

Although the recognition of fossils goes back hundreds—if not thousands—of years, the systematic cataloguing and assignment of relative ages to different organisms from the distant past—the science of paleontology—only dates back to the earliest part of the 19th century. The oldest undisputed fossils are from rocks dated ~3.5 Ga, and although fossils this old are typically poorly preserved and are not useful for dating rocks, they can still provide important information about conditions at the time. The oldest well-understood fossils are from rocks dating back to ~600 Ma, and the sedimentary record from this time forward is rich in fossil remains that provide a detailed record of the history of life. However, as anyone who has gone hunting for fossils knows, this doesn't mean that all sedimentary rocks have visible fossils or that fossils are easy to find. Fossils alone can't give us numerical ages for rocks, but over the past century geologists have acquired enough isotopic dates from rocks associated with fossiliferous rocks (such as igneous dykes cutting through sedimentary layers) to be able to put specific time limits on most fossils.

A Brief History of Life

A selective history of life on Earth over the past 600 million years is provided in Figure 19.11.

The major groups of organisms that we are familiar with appeared between the late Proterozoic and the Cambrian (~600 Ma to ~541 Ma).
Plants, which originally evolved in the oceans as green algae, invaded land during the Ordovician (~450 Ma).
Insects, which evolved from marine arthropods, invaded land during the Devonian (400 Ma), and amphibians (i.e., vertebrates) invaded land about 50 million years later.
By the late Carboniferous, trees had evolved from earlier plants, and reptiles had evolved from amphibians.
By the mid-Triassic, dinosaurs and mammals had evolved from reptiles and reptile ancestors.
Birds evolved from dinosaurs during the Jurassic.
Flowering plants evolved in the late Jurassic or early Cretaceous.
The earliest primates evolved from other mammals in the early Paleogene, and the genus Homo evolved during the late Neogene (~2.8 Ma).

[caption id="attachment_803" align="aligncenter" width="1024"] Figure 19.11 A selective summary of life on Earth during the late Proterozoic and the Phanerozoic. The top row shows geological eras, and the lower row shows geological periods. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

If we understand the sequence of evolution on Earth, we can apply this knowledge to determining the relative ages of rocks. This is William Smith’s principle of faunal succession, although in spite of the name, it can apply to fossils of plants and simple organisms as well as to fauna (animals).

Mass Extinctions

The Phanerozoic Eon has witnessed five major extinctions (stars in Figure 19.11). The most significant of these was at the end of the Permian, which saw the extinction of over 80% of all species, and over 90% of all marine species. Most well-known types of organisms that survived were still severely impacted by this event. The second most significant extinction occurred at the Cretaceous-Paleogene boundary (K-Pg, also known the Cretaceous-Tertiary or K-T extinction). At that time, ~75% of marine species disappeared, as well as dinosaurs (except for birds) and pterosaurs. Other species were badly reduced but survived, and then flourished in the Paleogene. The K-Pg extinction may have been caused by the impact of a large asteroid (10 km to 15 km in diameter) and/or volcanic eruptions associated with the formation of the Deccan Traps. It's not a coincidence that the major extinctions all coincide with boundaries of geological periods and/or eras. Paleontologists have placed most of the divisions of the geological time scale at points in the fossil record where there are major changes in the type of fossils observed.

Fossil Ranges

If we can identify a fossil, and we know when the organism lived, we can assign a range of time to the formation of the sediments in which the organism was preserved when it died. This range might be several millions of years, because some organisms survived for a very long time. But If the rock we're studying has several types of fossils in it, and we can assign time ranges to all of these fossils, we may be able to narrow down the time range for the age of the rock considerably (Figure 19.12). [caption id="attachment_804" align="aligncenter" width="408"] Figure 19.12 Application of bracketing to constrain the age of a rock based on the presence of several fossils. The yellow bar represents the time range during which each of the four species (A – D) existed on Earth. Although each species lived for several millions of years, we can narrow down the age of the rock to a span of just 1.3 Ma during which all four species coexisted. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Index Fossils

Some organisms survived for a very long time, and are not particularly useful for dating rocks. Sharks, for example, have existed for over 400 million years, and the great white shark has survived for 16 million years so far. In contrast, organisms that lived for relatively short time periods are useful for dating rocks, especially if they were distributed over a wide geographic area and hence can be used to compare rocks from different regions. These are known as index fossils. There is no specific limit on how short the time span has to be for a fossil to qualify as an index fossil. Some such organisms lived for millions of years, and others for much less than a million years. Some well-studied groups of organisms qualify as biozone fossils because, although the genera and families lived over a long time, each species lived for a relatively short time and can be easily distinguished from others on the basis of specific features. Ammonites, for example, have a distinctive feature known as the suture line, where the internal shell layers that separate the individual chambers (septae) meet the outer shell wall (Figure 19.13). These suture lines are sufficiently variable to identify species that can be used to estimate the relative or absolute ages of the rocks in which they are found. [caption id="attachment_805" align="aligncenter" width="650"] Figure 19.13 The septum of an ammonite (white part, left), and the suture lines where the septae meet the outer shell (right). Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Foraminifera—small, calcium carbonate-shelled marine organisms that originated during the Triassic and are still alive today—are also useful biozone fossils. Numerous different foraminifera lived during the Cretaceous Period. Some lived for over 10 million years, but others lived for less than 1 million years (Figure 19.14). If the foraminifera in a rock can be identified to the species level, the rock's age can be determined.

[caption id="attachment_806" align="aligncenter" width="700"] Figure 19.14 Time ranges for Cretaceous foraminifera (left), and modern foraminifera from the Ambergris area of Belize (right). Source: Left- Steven Earle (2015), CC BY 4.0, from data in Scott (2014). Right- Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Try It!

This diagram shows the age ranges for some late Cretaceous inoceramid clams in the genus Mytiloides.

Image Description:

Species M. hattiru: 93.4 - 92.6 Ma
Species M. kossmati: 93.3 - 92.5 Ma
Species M. columbiarus: 93.2 - 92.5 Ma
Species M. subhercynius: 92.7 - 91.9 Ma
Species M. labiatus: 92.9 - 92.6 Ma

Questions:

Use the bracketing method to figure out the possible age range of a rock in which all five of these organisms were found.
How would the age range change if M. subhercynius were not present in the rock?

To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="196"]

Correlation

Geologists employ relative age dating techniques to correlate rocks between regions. Correlation seeks to relate the geological history between regions, by relating the rocks in one region to those in another. There are different techniques of correlation. The easiest technique is to correlate by rock type, or lithology, called lithostratigraphic correlation. In this method, specific rock types are related between regions. If a sequence of rocks at one site consists of a sandstone unit overlain by a limestone unit, then a unit of shale, and the exact same sequence of rocks—sandstone, limestone, shale—occurs at a nearby site, lithostratigraphic correlation means assuming that the rocks at both sites are in the same rocks. If you could see all of the rock exposed between the two sites, the units would connect with one another. The problem with this type of correlation is that some rocks may only have formed locally, or may pinch out between the two sites, and therefore not be present at the site to which a correlation is being attempted. Another technique, biostratigraphic correlation, involves correlation based on fossil content. This technique uses fossil assemblages (fossils of different organisms that occur together) to correlate rocks between regions. The best fossils to use are those that are widely spread, abundant, and lived for a relatively short period of time. Yet another technique, chronostratigraphic correlation, is to correlate rocks that have the same age. This can be the most difficult way to correlate, because rocks are generally diachronous. That is, if we trace a given rock unit across any appreciable lateral distance, the age of that rock actually changes. To give a familiar example, when you go to the beach, you know that the beach itself and the lake bottom in the shallow water is sandy. But if you swim out to deeper water and touch bottom, the bottom feels muddy. The difference in sediment type has to do with the energy of deposition, with the waves at and near the beach keeping any fine sediments away, only depositing them in deeper quieter waters. If you think of this example in time, you realize that the sand at and near the beach is being deposited at the same time as the mud in deeper water. But if lake levels drop, the beach sands will slowly migrate outwards and cover some of the deeper water muds. If lake levels rise, the deeper water muds will slowly migrate landwards and cover some of the shallower water sands. This is an example of Walther's Law, which states that sedimentary rocks that we see one on top of each other in the rock record actually formed adjacent to one another at the time of deposition. In order to correlate rock units in time, we need to find marker beds that formed instantaneously, such as an ash layer from the eruption of a volcano that blanketed an entire region in ash.

References

Harries, P.J., Kauffman, E.G., Crampton, J.S. (Redacteurs), Bengtson, P., Cech, S., Crame, J.A., Dhondt, A.V., Ernst, G., Hilbrecht, H., Lopez, Mortimore, G.R., Tröger, K.-A., Walaszcyk, I., & Wood, C.J. (1996). Mitteilungen aus dem Geologisch - Paläontologischen Museum der Universität Hamburg, 77, 641-671. http://www.fuhrmann-hilbrecht.de/Heinz/geology/InoIntro/InoIntro.html

Scott, R. (2014). A Cretaceous chronostratigraphic database: construction and applications, Carnets de Géologie, 14(2), 15-37. http://paleopolis.rediris.es/cg/1402/

19.4 Isotopic Dating Methods

kqzheng — Tue, 30 Oct 2018 23:04:58 +0000

Isotope Pairs

Isotopes of an element have different numbers of neutrons. Sometimes this just makes one of the isotopes slightly lighter or heavier than others, but other times it makes the element unstable, causing it to undergo radioactive decay. Unstable elements paired with the elements they decay to are used in isotopic dating methods. In most cases, we can't use isotopic techniques to directly date fossils or the sedimentary rocks in which they are found, but we can constrain their ages by dating igneous rocks that cut across sedimentary rocks, or volcanic ash layers that lie within sedimentary layers. Isotopic dating of rocks, or the minerals within them, is based upon the fact that we know the decay rates of certain unstable isotopes of elements, and that these decay rates have been constant throughout geological time. It is also based on the premise that when the atoms of an element decay within a mineral or a rock, they remain trapped in the mineral or rock, and don't escape.

How It Works: Potassium-Argon Dating

One of the isotope pairs commonly used to date rocks is the decay of ⁴⁰K to ⁴⁰Ar (potassium-40 to argon-40). ⁴⁰K is a radioactive isotope of potassium that is present in very small amounts in all minerals that contain potassium. It has a half-life of 1.3 billion years, meaning that over a period of 1.3 Ga one-half of the ⁴⁰K atoms in a mineral or rock will decay to ⁴⁰Ar, and over the next 1.3 Ga one-half of the remaining atoms will decay, and so forth (Figure 19.15). ⁴⁰K is called the parent isotope, and ⁴⁰Ar the daughter isotope, as the parent gives way to the daughter during radioactive decay. [caption id="attachment_809" align="aligncenter" width="577"] Figure 19.15 The decay of 40K over time. Each half-life is 1.3 billion years, so after 3.9 billion years (three half-lives) 12.5% of the original ⁴⁰K will remain. The red-blue bars represent ⁴⁰K and the green-yellow bars represent ⁴⁰Ar. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Misconception Alert When we look at radioactive decay, we're always thinking about half-life as it applies to the remaining atoms of a radioactive isotope, and not necessarily the original amount. In other words, after the second half life, another half of the 50% is remaining (i.e., 25%), rather than all of the remaining 50% being gone. That's why the dark coloured bars in Figure 19.15 don't go to zero for the second half-life.

In order to use the K-Ar dating technique, we need to have an igneous or metamorphic rock that includes a potassium-bearing mineral. In granite, we can use the mineral potassium feldspar (salmon-coloured crystals in Figure 19.16). When potassium feldspar forms, it has no argon. But over time, the ⁴⁰K in the feldspar decays to ⁴⁰Ar, and the atoms of ⁴⁰Ar remain embedded within the crystal, unless the rock is subjected to high temperatures after it forms. [caption id="attachment_810" align="aligncenter" width="491"] Figure 19.16 Crystals of potassium feldspar (salmon colour) in a granitic rock are candidates for isotopic dating using the K-Ar method because they contained potassium and no argon when they formed. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] The sample must be analyzed using a very sensitive mass-spectrometer, which can detect the differences between the masses of atoms, and can therefore distinguish between ⁴⁰K and the much more abundant ³⁹K. The minerals biotite and hornblende are also commonly used for K-Ar dating.

Other Isotope Pairs

There are many isotope pairs that can be employed in dating igneous and metamorphic rocks (Table 19.1), each with its strengths and weaknesses. In the above example, the daughter isotope ⁴⁰Ar is naturally a gas, and can escape the potassium feldspar quite easily if the feldspar is exposed to heating during metamorphism, or interaction with hydrothermal fluids. This means we have to examine the feldspar mineral closely first to see if there is any evidence of alteration. If some ⁴⁰Ar has been lost, but the sample is dated anyway, the age we get will be too young, because it will look like the argon has been accumulating for less time than it really has.

Table 19.1 Commonly used isotope systems for dating geological materials. Source: Steven Earle (2015), CC BY 4.0.
Isotope System	Half-life	Useful Range	Comments
Potassium-Argon	1.3 Ga	10 Ka - 4.57 Ga	Widely applicable because most rocks contain potassium
Uranium-Lead	4.5 Ga	1 Ma - 4.57 Ga	The rock must contain uranium-bearing minerals (felsic igneous rocks)
Rubidium-Strontium	47 Ga	10 Ma - 4.57 Ga	Less precision than other methods for old rocks
Carbon-Nitrogen (radiocarbon dating)	5,730 a	100 a to 60,000 a	Sample must contain wood, bone, or carbonate minerals; can be applied to young sediments

Each parent isotope has a certain half-life, which ranges from microseconds to billions of years, depending on the isotope. In dating rocks, we need to select an isotope pair with a parent isotope that has a reasonable half-life for our sample. If the half-life is too short, then most of the parent isotope will have decayed to form the daughter isotope. If we can't measure the amount of parent isotope very accurately, which will be impossible to do if there is only the tiniest amount of parent isotope left, our calculated age will have huge errors associated with it. The same applies if the half-life is too long. In this case, very little of the daughter isotope will have formed, and our inability to measure the small amount of daughter isotope accurately will again result in huge errors in our calculated age. Another complicating factor is whether the mineral of interest incorporated any of the daughter isotope into its structure at the time of formation. When we select a mineral and an isotope pair to date that mineral, we make the assumption that all of the daughter isotope we find in the mineral was produced in the mineral by radioactive decay of the parent isotope. But if the mineral formed with some daughter isotope already present in its structure, then the age we calculate will be too old. A more robust mineral to use to date certain types of igneous and metamorphic rocks is zircon. Zircon is a mineral that incorporates uranium into its structure at the time of formation. One of the isotopes of uranium decays to lead with a long half-life (4.5 Ga). Zircon is a mineral of choice for dating because it takes no lead into its structure when it forms, so any lead present is due entirely to the radioactive decay of the uranium parent. Another reason is because zircon is a very resistant mineral. It can handle exposure to hydrothermal fluids, and all but the highest grades of metamorphism, and not lose any of the parent or daughter isotopes. One drawback is that zircon tends to form only in felsic igneous rocks, so if we're trying to date a mafic rock, we need to use a different mineral.

The Meaning of a Radiometric Date

When we employ isotopic methods on minerals, we're measuring an age date. Generally, an age date refers to the time since a mineral crystallized from molten rock, when the elements that make up the mineral got locked into the mineral's structure. But as we've already seen, elevated temperatures can cause elements to escape from a mineral, without the mineral melting. This means that when we date a mineral, we might actually be dating the time since the mineral last experienced a period of heating above its Curie point, which is the temperature beyond which the mineral is able to lose (or gain) elements from its structure, without melting. So we have to know something about the rock before we forge ahead to measure an age. We may choose a mineral and isotope pair that are very resistant to metamorphism, so that we can "see through" the metamorphism, and determine the original age that the mineral crystallized from a melt. Or we may be interested in the age of the metamorphic event itself, so choose a mineral and isotope pair that is susceptible to resetting the isotopic clock during metamorphism (such as by losing all of the daughter isotope). Absolute age dating is a powerful tool for unraveling the geological history of a region, but we must ultimately rely upon igneous rocks (that may have later metamorphosed) for the minerals that we are able to date (more on issues with dating sedimentary rocks below). Another issue with absolute age dating is that it is expensive, and a single analysis can cost hundreds of dollars. This is why geologists never forget their relative age dating principles, and are always applying them in the field to determine the sequence of events that formed the rocks in a region.

Combining Absolute Ages with Relative Dating

The age dates for three igneous rock layers are given. Can you figure out the age ranges for the sets of sedimentary units A, B, and C? Important first step! We have to be sure that the igneous rock layers are so we can use the principle of . If they're the principle will apply instead, and this won't work as well.

cross-cutting
lava flows
superposition
sills

Fill in the ages in the blank spaces below. Let's get dating?

The oldest Layer A could be is Ma.
The youngest Layer A could be is Ma (Note: the stratigraphic column may not be complete).
The oldest Layer B could be is Ma.
The youngest Layer B could be is Ma.
The oldest Layer C could be is Ma.
The youngest Layer C could be is Ma (Note: This stratigraphic column may not be complete).

Oh dear. It looks like there was a mix-up at the lab, and you got bad info. The igneous layers are sills, not lava flows. Can you salvage this mess and still com eup with some age constraints on the sedimentary rock? When you have the answer, navigate to the link below to see if you're right. To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="197"]

Isotope Dating Techniques and Sedimentary Rocks

[caption id="attachment_812" align="alignright" width="300"] Figure 19.17 Conglomerate is a sedimentary rock consisting of large rounded clasts surrounded by finer-grained material. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] A clastic sedimentary rock (e.g., conglomerate, Figure 19.17) is made up of older rock and mineral fragments. These fragments were derived from weathering and erosion of pre-existing rocks. The process of forming a sedimentary rock from sediments generally occurs at low temperatures, so the minerals are not heated beyond their Curie points. This means the minerals still preserve their original ages (either igneous crystallization age, or a metamorphic age), but what does that actually say about the age of the sedimentary rock? In almost all cases, the fragments have come from a range of source rocks that all formed at different times. If we dated a number of individual grains in the sedimentary rock, we would likely get a range of different dates, all older than the age of the sedimentary rock. The most that such ages gleaned from a sedimentary rock can tell us is a maximum age of the sedimentary rock. It might be possible to date some chemical sedimentary rocks isotopically, but there are no useful isotopes that can be used on old chemical sedimentary rocks.

Radiocarbon Dating

Radiocarbon dating (using ¹⁴C) can be applied to many geological materials, including sediment and sedimentary rocks, but only if the materials in question are younger than ~60 ka, and contain organic material. Beyond this time, there is so little ¹⁴C left that it cannot be measured accurately, resulting in unreliable age dates. Fragments of wood incorporated into young sediment are good candidates for carbon dating, and this technique has been used widely in studies involving late Pleistocene glaciers and glacial sediments. Figure 19.18 shows radiocarbon dates from wood fragments in glacial sediments have been used to estimate the time of the last glacial advance along the Strait of Georgia. [caption id="attachment_813" align="aligncenter" width="443"] Figure 19.18 Radiocarbon dates on wood fragments in glacial sediments in the Strait of Georgia. Source: Steven Earle (2015), CC BY 4.0. Image source. Modified after Clague (1976).[/caption]

Putting It Together: Using Multiple Methods to Date a Sedimentary Layer

Well it's about time!You're studying an important fossil bed in Layer C of this cross-section and are desperate to figure out how old it is, because it might just be the discovery of the century. You sent two samples off to the lab for isotopic dating, and finally have some results. See your lab results below, then use the following reference materials to answer a few questions. Note: You might have to fill in some details on your own, because it looks like the lab didn't complete the reports. Lab Report 1. Sample: Wood fragment from Bed D. Result: 55% of parent isotope remaining. Lab Report 2. Sample: Potassium feldspar from Intrusion F. Result: 91% of parent isotope remaining. First things first: did you use the right graphs? Fill in the blanks: The wood sample from Layer D was dated using . The potassium feldspar crystal was dated using . Fill-in-the-blank options:

carbon-14
potassium-argon

What ages did you get? Fill in the blanks: The wood samples give an age of years. The potassium feldspar crystal is Ma old. To convince the world of your amazing discovery, you need to show that Layer C is very young (geologically speaking, at least). So far you have a wide range of possible ages, and that's not getting you any closer to world fame. But wait! You've just noticed that there is a (Hint: Type of unconformity) between Layer C and Intrusion F, which means that there's also a (Hint: Type of unconformity) between Layers C and B. You do some research and learn that these are the only features of this kind in your cross-section. This means you can conclude that of the layers you've had analyzed, C is much closer in age to (Hint: D or F?) than to (Hint: D or F?). To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="12"]

References

Clague, J. (1976). Quadra Sand and its relation to late Wisconsin glaciation of southeast British Columbia. Canadian Journal of Earth Sciences, 13, 803-815.

19.5 Other Dating Methods

kqzheng — Wed, 31 Oct 2018 00:37:42 +0000

There are numerous other techniques for dating geological materials, but we will examine just two of them here: dendrochronology—tree-ring dating—and dating based on the record of reversals of Earth’s magnetic field.

Dendrochronology

Dendrochronology can be applied to dating very young geological materials based on reference records of tree-ring growth going back many millennia. The longest such records can take us back over 25 ka, to the height of the last glaciation. One of the advantages of dendrochronology is that, providing reliable reference records are available, the technique can be used to date events to the nearest year. Dendrochronology has been used to date the last major subduction zone earthquake on the coast of B.C., Washington, and Oregon. When large earthquakes occur in this region, there's a tendency for some coastal areas to subside by one or two metres. Seawater then rushes in, flooding coastal flats and killing trees and other vegetation within a few months. There are at least four locations along the coast of Washington that have such dead trees, and probably many more in other areas. Wood samples from these trees have been studied and the ring patterns have been compared with patterns from old living trees in the region (Figure 19.19). [caption id="attachment_816" align="aligncenter" width="639"] Figure 19.19 Example of tree-ring dating of dead trees. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] [caption id="attachment_817" align="alignright" width="300"] Figure 19.20 Sites in Washington where dead trees are present in coastal flats. The outermost wood of eight trees was dated using dendrochronology, and of these, seven died during the year 1699, suggesting that the land was inundated by water at this time. Source: Steven Earle (2015), CC BY 4.0. Image source. From data in Yamaguchi et al. (1997).[/caption] At all of the locations studied, the trees were found to have died either in the year 1699, or very shortly thereafter (Figure 19.20). This led to the conclusion that a major earthquake took place in this region sometime between the end of growing season in 1699 and the beginning of the growing season in 1700. Evidence from a major tsunami that struck Japan on January 27, 1700, narrowed the timing of the earthquake to sometime in the evening of January 26, 1700. (For more information, see The 1700 Juan de Fuca Earthquake - Steven Earle - Malaspina University-College.)

Magnetic Chronology

Changes in Earth’s magnetic field can also be used to date events in geologic history. The magnetic field causes compass needles point toward the north magnetic pole, but this hasn’t always been the case. At various times in the past, Earth’s magnetic field has reversed its polarity, and during such times a compass needle would have pointed to the south magnetic pole. By studying magnetism in volcanic rocks that have been dated isotopically, geologists have been able to establish the chronology of magnetic field reversals going back for ~250 Ma. About 5 Ma of this record is shown in Figure 19.21, where the black bands represent periods of normal magnetism (“normal” meaning a polarity identical to the current magnetic field) and the white bands represent periods of reversed magnetic polarity. These periods of consistent magnetic polarity are given names to make them easier to reference. The current period of normal magnetic polarity, known as Brunhes, has lasted for the past 780,000 years. Prior to that there was a short reversed period and then a short normal period, the latter of which is known as Jaramillo. [caption id="attachment_818" align="aligncenter" width="1024"] Figure 19.21 The last 5 Ma of magnetic field reversals. Source: Steven Earle (2015), CC BY 4.0. Modified after U.S. Geological Survey (2007), Public Domain. Image source.[/caption] Oceanic crust becomes magnetized by the magnetic field that exists as the crust forms from magma at mid-ocean ridges. As magma cools, the magnetic fields of tiny crystals of magnetite that form within the magma become aligned with the existing magnetic field, and remain in this orientation, even if Earth’s magnetic field later changes polarity (Figure 19.22). Oceanic crust that is forming today is being magnetized in a “normal” sense, but crust that formed 780,000 to 900,000 years ago, in the interval between the Brunhes and Jaramillo normal periods, was magnetized in the “reversed” sense. [caption id="attachment_819" align="aligncenter" width="550"] Figure 19.22 Formation of magnetized oceanic crust at a spreading ridge. Coloured bars represent periods of normal magnetic polarity. Capital letters denote the Brunhes, Jaramillio, Olduvai, and Gauss normal magnetic periods. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] Magnetic chronology can be used as a dating technique because we can measure the magnetic field of rocks using a magnetometer, or of entire regions by towing a magnetometer behind a ship or an airplane. For example, the Juan de Fuca Plate, which lies off of the west coast of BC, Washington, and Oregon, is being and has been formed along the Juan de Fuca spreading ridge (Figure 19.23). The parts of the plate that are still close to the ridge exhibit normal magnetic polarity, while parts that are further away (and formed much earlier) have either normal or reversed magnetic polarity, depending upon when the rock formed. [caption id="attachment_820" align="aligncenter" width="400"] Figure 19.23 The pattern of magnetism within the area of the Juan de Fuca Plate, off the west coast of North America. Coloured bands represent parts of the sea floor with normal magnetic polarity, and the magnetic time scale is shown using these same colours. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] By carefully matching the sea-floor magnetic stripes with the known magnetic chronology, we can determine the age at any point on the plate. We can see that the oldest part of the Juan de Fuca Plate that has not yet subducted (off of the coast of Oregon) is just over 8 million years old, while the part that is subducting beneath Vancouver Island is between 0 and ~6 million years old.

Putting It Together: Using Fossils with Magnetic Polarity to Date Rocks [h5p id="198"]

References

Yamaguchi, D.K., Atwater, B.F., Bunker, D.E., Benson, B.E., & Reid, M. S. (1997). Tree-ring dating the 1700 Cascadia earthquake. Nature, 389, 922-923.

19.6 Understanding Geological Time

kqzheng — Wed, 31 Oct 2018 01:16:50 +0000

It's one thing to know the facts about geological time—how long it is, how we measure it, how we divide it into smaller time intervals, and what we call the various intervals— but it's quite another to really understand geological time. The problem is that our lives are short and our memories are even shorter. Our experiences span only a few decades, so we really don’t have a way of knowing what 11,700 years means. What’s more, it is hard for us to understand how 11,700 years differs from 65.5 Ma, or even from 1.8 Ga. It's not that we can't comprehend what the numbers mean, it's that we can't really appreciate how much time is involved. You may wonder why it's so important to understand geological time. There are some very good reasons. One is so that we can fully understand how geological processes that seem impossibly slow can produce anything of consequence. Consider driving from one major city to another, where a journey of several hours might occur at speeds of ~100 km/h. Continents move toward each other at rates of a fraction of a millimetre per day, a speed something on the order of 0.00000001 km/h (try walking at this speed!). And yet, at this impossibly slow rate, continents can move thousands of kilometres through geological time. Sediments typically accumulate at even slower rates—less than a millimetre per year—but are still thick enough to be thrust up into huge mountains or carved into breathtaking canyons. Another reason is to understand issues like extinction of endangered species, and human influence on climate. People who don't understand geological time are quick to say that the climate has changed in the past, and that what's happening now is no different. And climate certainly has changed in the past: from the Eocene (50 Ma) to the present day, Earth’s climate cooled by ~12°C on average. This is a huge change that ranks as one of the most important climate changes of Earth's past, and yet the rate of change over this time was only 0.000024 °C/century. Recent warming has occurred at a rate of ~1.1°C over the past 100 years (NASA GISS), 45,800 times faster than the rate of climate change since the Eocene. One way to wrap your mind around geological time is to put it into the perspective of single year. At this rate, each hour of the year is equivalent to approximately 500,000 years, and each day is equivalent to 12.5 million years. If all of geological time is compressed down into a single year, Earth formed on January 1, and the first life forms evolved in late March (~3,500 Ma). The first multicellular life forms appeared on November 13 (~600 Ma), plants appeared on land on November 24, and amphibians on December 3. Reptiles evolved from amphibians during the first week of December, and dinosaurs and early mammals evolved by December 13. Non-avian dinosaurs, which survived for 160 million years, went extinct on Boxing Day (December 26). The Pleistocene glaciation began at ~6:30 p.m. on New Year’s Eve, and the last glacial ice melted from southern Canada by 11:59 p.m. It's worth repeating: on this time scale, the earliest complex ancestors of the animals and plants we know today didn't appear on Earth until mid-November, the non-bird dinosaurs disappeared after Christmas, and most of Canada was periodically locked in ice from 6:30 to 11:59 p.m. on New Year’s Eve. As for people, the first to inhabit Canada arrived about one minute before midnight.

References

NASA Goddard Institute for Space Studies (n.d.). GLOBAL Station Temperature Index in 0.01 degrees Celsius base period: 1951-1980 [Data file]. http://data.giss.nasa.gov/gistemp/tabledata_v3/GLB.Ts.txt

Chapter 19 Summary & Key Term Check

kqzheng — Wed, 31 Oct 2018 19:29:47 +0000

Chapter 19 Main Ideas

19.1 The Geological Timescale

The work of William Smith was critical to the establishment of the first geological timescale early in the 19^th century, but it wasn’t until the 20^th century that geologists were able to assign reliable dates to the various time periods. The geological timescale is now maintained by the International Commission on Stratigraphy. Geological time is divided into eons, eras, periods, and epochs.

Practice Again

19.2 Relative Dating Methods

We can determine the relative ages of different rocks by observing and interpreting relationships among them, such as superposition, cross-cutting, and inclusions. Gaps in the geological record are represented by various types of unconformities.

Practice Again

19.3 Dating Rocks Using Fossils

Fossils are useful for dating rocks back to ~600 Ma. If we know the age range of a fossil, we can date rocks containing it, but some organisms lived for many millions of years. Index fossils represent shorter geological time spans, and if a rock has several different fossils with known age ranges, we can narrow the time during which the rock formed.

19.4 Isotopic Dating Methods

Radioactive isotopes decay at constant known rates, and can be used to date igneous and metamorphic rocks. Some commonly used isotope systems are potassium-argon, rubidium-strontium, uranium-lead, and carbon-nitrogen.

19.5 Other Dating Methods

There are many other methods for dating geological materials. Two that are widely used are dendrochronology and magnetic chronology. Dendrochronology, based on studies of tree rings, is widely applied to dating glacial events. Magnetic chronology is based on the known record of Earth’s magnetic field reversals.

19.6 Understanding Geological Time

While understanding geological time is relatively easy, actually comprehending the significance of the vast amounts of geological time is a great challenge. To be able to solve important geological problems and certain societal challenges, we need to really appreciate the vastness of geological time.

Key Term Check

What key term from Chapter 19 is each card describing? Turn the card to check your answer. [h5p id="199"]

formative assessment

kqzheng — Tue, 10 Aug 2021 19:07:46 +0000

Or informative assessment, as I like to call it. These are test-your-understanding types of activities not meant to count for a lot of points.

About BCcampus Open Education

kqzheng — Fri, 20 Aug 2021 20:07:32 +0000

Physical Geology - H5P Edition by Karla Panchuk was funded by BCcampus Open Education. BCcampus Open Education began in 2012 as the B.C. Open Textbook Project with the goal of making post-secondary education in British Columbia more accessible by reducing students’ costs through the use of open textbooks and other OER. BCcampus supports the post-secondary institutions of British Columbia as they adapt and evolve their teaching and learning practices to enable powerful learning opportunities for the students of B.C. BCcampus Open Education is funded by the Ministry of Post-Secondary Education and Future Skills and the Hewlett Foundation. Open educational resources (OER) are teaching, learning, and research resources that, through permissions granted by the copyright holder, allow others to use, distribute, keep, or make changes to them. Our open textbooks are openly licensed using a Creative Commons licence and are offered in various eBook formats free of charge, or as printed books that are available at cost. For more information about open education in British Columbia, please visit the BCcampus Open Education website. If you are an instructor who is using this book for a course, please fill out our Adoption of an Open Textbook form.

Accessibility Statement

hfriedman — Mon, 30 Jan 2023 20:34:40 +0000

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This statement was last updated on January 30th, 2023 The Accessibility Checklist table was adapted from one originally created by the Rebus Community and shared under a CC BY 4.0 License.

For Students: How to Access and Use this Textbook

hfriedman — Mon, 30 Jan 2023 20:35:56 +0000

This textbook is available in the following formats:

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For more information about the accessibility of this textbook, see the Accessibility Statement. You can access the online webbook and download any of the formats for free here: Physical Geology - H5P Edition. To download the book in a different format, look for the "Download this book" drop-down menu and select the file type you want.

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Style Chapter

jgray — Tue, 09 May 2023 15:55:47 +0000

Accordions

[h5p id="153"]

If the accordion only has one item, make the top level of the accordion bold.
If the accordion has multiple items, either structure it as a bulleted list or use headings.

HTML

[h5p id="5000"]

Can plastic deformation of Earth’s crust (deformation that is not reversible) cause an earthquake?

No. The vibration from earthquakes is a result of elastic rebound. This means that deformed rock has snapped back to its original shape. If deformation is not reversible, then elastic energy is not being stored, and rebound is not possible. Rocks may still be offset along a fault due to plastic deformation, but the offset happens as slow, creeping movement called aseismic slip (i.e., slip without seismicity/ shaking).

About

kqzheng — Thu, 19 Aug 2021 18:04:16 +0000

Buy

kqzheng — Thu, 19 Aug 2021 18:04:16 +0000

Access Denied

kqzheng — Thu, 19 Aug 2021 18:04:16 +0000

Chapter 1. Introduction to Geology

kqzheng — Wed, 10 May 2017 20:30:52 +0000

[caption id="attachment_20" align="aligncenter" width="700"] Figure 1.1 Badlands in southern Saskatchewan. Erosion has exposed layers of rock going back more than 65 million years. Source: Karla Panchuk (2017) CC BY-SA 4.0. Click the image for more attributions.
[/caption]

Chapter Goals

Complete this chapter so you can:

Explain what geology is, and why we study Earth.
Describe the kinds of work geologists do.
Explain what geological time is.
Explain how the principle of uniformitarianism allows us to translate observations about Earth today into knowledge about how Earth worked in the past.
Summarize the main idea behind the theory of plate tectonics.

Chapter 2. The Origin of Earth and the Solar System

kqzheng — Wed, 03 Jan 2018 00:39:42 +0000

[caption id="attachment_42" align="aligncenter" width="650"] Figure 2.1 Earthrise, October 12, 2015. The Lunar Reconnaissance Orbiter Camera captured images of the lunar surface with Earth in the background. Source: NASA Lunar Reconnaissance Orbiter Science Team (2015) Public Domain. view source[/caption] The story of how Earth came to be is a fascinating contradiction. On the hand, many things had to go just right for Earth to turn out the way it did, and for life to develop. On the other hand, the formation of planets similar to Earth is an entirely predictable consequence of the physical and chemical processes taking place around stars. In fact, it has happened more than once. This chapter starts Earth’s story from the beginning—the very beginning—to explain why, for billions of years, generations of stars had to be born and then die explosive deaths before Earth could exist. How stars form and burn, and affect the objects around them are fundamental to Earth's story, as is the rough neighbourhood in which Earth spent its early years.

Chapter Goals

Complete this chapter so you can:

Explain the big bang theory for the origin of the universe.
Explain how clouds of gas floating in space can turn into stars and planets.
Describe the types of objects that are present in our solar system, and why they exist where they do.
Explain how Earth got its layered structure, water, and atmosphere.
Explain how the moon formed.
Compare and contrast our solar system with other planetary systems.

Chapter 3. Earth’s Interior

kqzheng — Thu, 08 Mar 2018 04:15:59 +0000

[caption id="attachment_73" align="aligncenter" width="650"] Figure 3.1 The red rocks of the Tablelands in Gros Morne National Park are a sample of Earth's mantle. Top: Red Tablelands rocks on the right stand in contrast to the green surroundings. Bottom: A closer view of Tablelands terrain, showing rocks weathered red, and a near absence of plant life. Click for attributions.[/caption]

Chapter Goals

Complete this chapter so you can:

Explain the variations in the composition and characteristics of Earth’s different layers.
Explain how seismic data can be used to understand the structure of Earth's interior.
Describe the temperature variations within Earth and their implications for internal processes such as mantle convection.
Explain the origins of Earth’s magnetic field and the timing of magnetic field reversals.
Describe the isostatic relationship between the crust and the mantle, and the implications of that relationship for geological processes on Earth

Ways to Know About Earth's Interior

We need to know something about the inside of our planet—what it’s made of, and what happens within it—in order to understand how Earth works, especially the mechanisms of plate tectonics. Lucky for us, there are many ways for geologists gather information about Earth's deep interior, because the one thing they can't do is go down and look at it.

Happy Little Accidents of Plate Tectonics

The barren red rocks of the Tablelands stand in stark contrast to their lush green surroundings in Gros Morne National Park (Figure 3.1, top). If the Tablelands appear out of place, it's because they are. The Tablelands are one of few places on Earth where you can walk directly on the rocks of Earth's mantle, thanks to an accident of plate tectonics that happened hundreds of millions of years ago. The red colour comes from iron-bearing minerals reacting with oxygen. Unaltered, the rocks are dark green (Figure 3.2). The rocks lack vegetation because their chemical composition doesn't provide adequate nutrients for plants. [caption id="attachment_74" align="aligncenter" width="324"] Figure 3.2 Tablelands mantle rock with reddish weathering rind, and dark green fresh surface. Scale in cm. Source: Karla Panchuk (2017) CC BY 4.0.[/caption]

Space Smash-Ups

Meteorites derived from smashed differentiated bodies (e.g., asteroids large enough to have separated into mantle and core) are another way to infer the nature of Earth's interior. Asteroids that formed at a similar distance from the sun as Earth had a mineral composition akin to Earth's. When these objects were shattered in giant collisions, the result was stony meteorites from fragmented mantle rock, and iron meteorites from fragmented core.

Earthquakes

We also get information about the structure of Earth's interior by analyzing the speeds and paths of earthquake vibrations, called seismic waves.

Chapter 4. Plate Tectonics

kqzheng — Fri, 24 Aug 2018 18:45:09 +0000

[caption id="attachment_107" align="aligncenter" width="1024"] Figure 4.1 Iceland is known for its volcanoes, which are present because Iceland is located on the Mid-Atlantic Ridge, where the Atlantic Ocean is spreading apart and new crust is forming. In fact, Iceland exists because that volcanic activity has built up the island from the ocean floor. Iceland is cut by rift zones (white lines on the map at left) where the island is splitting apart along with the rest of the Atlantic Ocean. Rift zones are marked by belts of young volcanic rocks (dark green). You can stand on a rift zone if you visit Thingvellir National Park (right). Rifting has produced a valley where the crust has settled downward. The margins of the North American and Eurasian tectonic plates are visible as ridges on either side of the valley. The photographer was standing on a ridge on the North American side. Source: Karla Panchuk (2018), CC BY-SA 4.0. Photo: Ruth Hartnup (2005), CC BY 2.0. Image source. Click for more attributions.[/caption]

Chapter Goals

Complete this chapter so you can:

Discuss the early evidence for continental drift, and Alfred Wegener’s role in promoting this theory.
Describe other models that were used early in the 20th century to understand global geological features.
Summarize the geological advances that provided the basis for understanding the mechanisms of plate tectonics, and the evidence that plates and are constantly being created and destroyed.
Describe the seven major plates, including their size, motion, and the types of boundaries between them.
Describe the geological processes that take place at divergent and convergent plate boundaries, and explain why transform faults exist
Explain how supercontinents form and how they break apart.
Explain why tectonic plates move.

Introduction

Plate tectonics is the model or theory that we use to understand how our planet works: it explains the origins of continents and oceans, the origins of folded rocks and mountain ranges, the presence of different kinds of rocks, the causes and locations of earthquakes and volcanoes, and changes in the positions of continents over time. So... everything! The theory of plate tectonics was proposed to the geological community more than 100 years ago, so it may come as a surprise that an idea underpinning the study of Earth today did not become an accepted part of geology until the 1960s. It took many decades for this theory to become accepted for two main reasons. First, it was a radically different perspective on how Earth worked, and geologists were reluctant to entertain an idea that seemed preposterous in the context of the science of the day. The evidence and understanding of Earth that would have supported plate tectonic theory simply didn’t exist until the mid-twentieth century. Second, their opinion was affected by their view of the main proponent, Alfred Wegener. Wegener was not trained as a geologist, so he lacked credibility in the eyes of the geological community. Alfred Wegener was also German, whereas the geological establishment was centred in Britain and the United States- and Britain and the United States were at war with Germany in the first part of the 20th century. In summary, plate tectonics was an idea too far ahead of its time, and delivered by the wrong messenger.

References

Thordarson, T., and Larsen, G. (2007) Volcanism in Iceland in historical time: Volcano types, eruption styles and eruptive history. Journal of Geodynamics, 43, 118–152. https://doi.org/10.1016/j.jog.2006.09.005

Chapter 5. Minerals

kqzheng — Tue, 20 Mar 2018 01:50:28 +0000

[caption id="attachment_155" align="aligncenter" width="550"] Figure 5.1 Giant crystals of gypsum in the Naica Mine in Mexico. The crystals formed in volcanically heated water, and became accessible when the cave was drained as part of mining activities. The cave was very hot, making it fatal for visitors to enter without cooling equipment and respirators. When mining activities ceased, caverns were allowed to flood again. Source: Karla Panchuk (2019) CC BY-NC-SA 4.0. Photograph- Paul Williams (2009), CC BY-NC 2.0. Image source. Click for more attributions.[/caption]

Chapter Goals

Complete this chapter so you can:

List the criteria required for a substance to be considered a mineral.
Explain how atoms bond within minerals.
Explain how mineral lattices influence the properties of minerals.
Summarize the categories of minerals defined by anions or anionic groups.
Describe the types of configurations of silica tetrahedra found in silicate minerals.
Explain how minerals form.
Describe the key properties for identifying minerals.

What Is a Mineral?

Minerals are all around us: the graphite in your pencil, the salt on your table, the plaster on your walls, and the trace amounts of gold in your computer. Minerals can be found in a wide variety of consumer products such as paper, medicine, processed foods, and cosmetics. And of course, everything made of metal is also derived from minerals. A mineral is a naturally occurring solid made of specific elements, and arranged in a particular repeating three-dimensional structure. “Naturally occurring” means that minerals can be formed from substances and under conditions found in nature. Substances that can only be made by humans—classified as anthropogenic materials—do not count as minerals, nor do substances produced by natural processes acting upon anthropogenic materials. In the context of the definition of minerals, “solid" means solid at 25º C. There are some exceptions to this rule, made for substances defined as minerals before 1959, prior to strict procedures being established for determining what is or isn't a mineral. One example is ice, which is only solid at or below 0° C. Another is mercury, which is solid below -39º C. Mercury that is present in rocks at temperatures above -39º C appears as silvery blobs of liquid (Figure 5.2). [caption id="attachment_156" align="aligncenter" width="550"] Figure 5.2 Droplets of native mercury (pure mercury, Hg), also called quicksilver, amid waxy red crystals of cinnabar (HgS). Cinnabar is a mercury ore mineral. Source: Parent Géry (2012), CC BY-SA 3.0. Image source.[/caption] "Specific elements” means that minerals have a specific chemical formula or composition. The mineral pyrite, for example, is FeS₂ (two atoms of sulphur for each atom of iron), and any significant departure from that formula would make it a different mineral. Some minerals can have variable compositions within a specific range. The mineral olivine, for example, has a formula written as (Fe,Mg)₂SiO₄, because the composition of olivine can range all the way from Fe₂SiO₄ to Mg₂SiO₄,and have any proportion of iron and magnesium in between. This type of substitution is known as solid solution. Most important of all, the atoms within a mineral are arranged in a specific repeating three-dimensional structure or lattice. This regular structure means that all minerals are crystals. The mineral halite, which we use as table salt, has a relatively simple crystal lattice (Figure 5.3). Atoms of sodium (Na, purple) alternate with atoms of chlorine (Cl, green). The chemical bonds holding the Na and Cl atoms together are all at 90º to each other. Even tiny crystals, like the ones in your salt shaker, have lattices that extend in three dimensions for thousands of repetitions. Halite will always have this structure, and will always have the formula NaCl. [caption id="attachment_157" align="aligncenter" width="300"] Figure 5.3 Halite crystal lattice. Halite is the mineral in table salt. Source: Steven Earle (2015), CC BY 4.0.[/caption] Some mineral-like materials do not have a regular internal atomic arrangement. Opal (Figure 5.4) is one example. In many respects it fits the definition of a mineral: it has a specific chemical composition (SiO₂·nH₂O, where n means that there can be varying amounts of water in the structure), forms naturally through geological processes, and is solid at 25 ºC. However, the structure of opal consists of closely packed spheres (Figure 5.4, right) rather than a lattice like halite. Substances like opal, which are mineral-like, but which do not have a crystalline structure, are called mineraloids. [caption id="attachment_158" align="aligncenter" width="650"] Figure 5.4 Opal is mineral-like, but does not have a crystalline structure. Instead, it is made up of layers of closely packed spheres (right). Source: Left- James St. John (2016), CC BY 2.0. Image source; Right- Mineralogy Division, Geological and Planetary Sciences, Caltech (n.d.), CC BY-NC. Image source./ View context.[/caption]

Note: Element symbols such as Na and Cl are used extensively in this book. In Appendix A. List of Geologically Important Elements and the Periodic Table you can find a list of the symbols, the names of the elements common in minerals, and a copy of the periodic table of elements.

Concept Check: What Is a Mineral? [h5p id="2"]

References

Nickel, E. H. (1995). The definition of a mineral. The Canadian Mineralogist 33, 698-690. http://www.minsocam.org/msa/IMA/ima98(04).pdf

Williams, P. (2010, July 28). Deadliest place on Earth? Surviving Cueva de los Cristales - The Giant Crystal Cave. http://www.ironammonite.com/2009/12/surviving-cueva-de-los-cristales-giant.html

Chapter 6. The Rock Cycle

kqzheng — Wed, 10 May 2017 20:33:12 +0000

[caption id="attachment_208" align="aligncenter" width="998"] Figure 6.1 A petrified beach near Rock Springs, Wisconsin, U. S. A. The wrinkled face of this vertical cliff displays ripples from an ancient beach. Flowing water moved sand grains to form ripples, and over time the sand was transformed into a solid sedimentary rock. The petrified beach was buried deeper and deeper, and the higher pressures and temperatures caused the sand grains to lose their individual boundaries and merge together. Thus, the sedimentary rock was transformed into a different type of rock, called a metamorphic rock. Source: Karla Panchuk (2021), CC BY-SA 4.0. Click for more attributions.[/caption]

Chapter Goals

Complete this chapter so you can:

Explain what a rock is.
Summarize the main characteristics of igneous, sedimentary, and metamorphic rocks.
Describe the rock cycle and the types of processes that lead to the formation of igneous, sedimentary, and metamorphic rocks.
Explain why there is an active rock cycle on Earth.

Chapter 7. Igneous Rocks

kqzheng — Thu, 19 Apr 2018 04:51:59 +0000

[caption id="attachment_224" align="aligncenter" width="864"] Figure 7.1 Lava lake of Mount Nyiragongo, a volcano in the Democratic Republic of Congo. Igneous rocks form when melted rock freezes. Source: Karla Panchuk (2018), CC BY-NC-SA 4.0. Click for more attributions.[/caption]

Chapter Goals

Complete this chapter so you can:

Explain partial melting and the geological processes that lead to melting.
Describe the range of chemical compositions of magmas.
Discuss the processes that take place during magma cooling, and the order of crystallization in Bowen's reaction series.
Explain how fractional crystallization and partial melting alter magma composition.
Classify igneous rocks according to the proportions of minerals within them.
Describe the origins of aphanitic, phaneritic, and porphyritic textures
Classify plutons according to their shapes and relationships to surrounding rocks.
Explain how chilled margins form.

Two Main Types of Igneous Rocks

Igneous rocks form when molten (liquid) rock cools and solidifies. The characteristics of igneous rocks differ depending on whether the melt cools on Earth’s surface, or in Earth’s interior. For that reason, we use different terms to distinguish between the two cases. Melted rock on Earth’s surface is called lava. The rocks that form when lava solidifies are called extrusive igneous rocks or volcanic igneous rocks. The latter term highlights the fact that extrusive igneous rocks are associated with volcanic eruptions. Melted rock within the Earth is called magma. Rocks that form when magma solidifies are called intrusive igneous rocks or plutonic rocks.

Concept Test: Intrusive or Extrusive?

Bear Lodge (also known as Devils Tower) in Wyoming, USA, is thought to be a volcanic plug. This means it formed when melted rock froze inside of a volcano about 40 million years ago. The rock that formed was harder than the surrounding volcano, and remains long after the volcano itself was eroded away.Is the rock of Bear Lodge intrusive or extrusive? Answer Bear Lodge is an intrusive (also called plutonic) igneous rock. It's true that its formation is related to volcanic activity, but it cooled from magma that was never erupted from the volcano, not from lava that escaped the volcano.

[h5p id="69"]

Chapter 8. Weathering, Sediment, and Soil

kqzheng — Tue, 12 Jun 2018 07:13:47 +0000

[caption id="attachment_257" align="aligncenter" width="675"] Figure 8.1 The Hoodoos, near Drumheller, Alberta, have formed from the differential weathering (weaker rock weathering faster than stronger rock) of sedimentary rock. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Chapter Goals

Complete this chapter so you can:

Explain why rocks formed at depth in the crust are susceptible to weathering at the surface.
Describe the main processes of mechanical weathering, and the materials that are produced.
Describe the main processes of chemical weathering, and common chemical weathering products.
Explain the characteristics used to describe sediments, and what those characteristics can tell us about the origins of the sediments.
Discuss the relationships between weathering and soil formation, and the origins of soil horizons.
Describe and explain the distribution of Canadian soil types.
Explain how changing weathering rates affect the carbon cycle and the climate system.

What Is Weathering?

Weathering occurs when rock is exposed to the “weather”—to the forces and conditions that exist at Earth’s surface. Rocks that form deep within Earth experience relatively constant temperature, high pressure, have no contact with the atmosphere, and little or no interaction with moving water. Once overlying layers are eroded away and a rock is exposed at the surface, conditions change dramatically. Temperatures vary widely, and pressure is much lower. Reactive gases like oxygen and carbon dioxide are plentiful, and in many climates, water is abundant. Weathering can be characterized as mechanical (or physical), and chemical. In mechanical weathering, physical processes break rock into smaller pieces. In chemical weathering, chemical reactions change minerals into forms that are less affected by chemical reactions that occur at Earth's surface. Mechanical and chemical weathering reinforce each other, because mechanical weathering provides new fresh surfaces for attack by chemical processes, and chemical weathering weakens the rock so that it is more susceptible to mechanical weathering. Together, these processes create the particles and ions that can eventually become sedimentary rock. They also create soil, which is necessary for our existence on Earth.

Can You Tell the Difference Between Physical & Chemical Weathering?

Over millions of years, acidic rainwater has slowly dissolved this limestone, leaving behind a ghostly forest of stone pillars. Is this weathering physical or chemical? A rock lays shattered amid soft moss. It's the victim of water flowing into its cracks, only to freeze and pry the cracks apart. Is this weathering physical or chemical? To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="83"]

Chapter 10. Metamorphism and Metamorphic Rocks

kqzheng — Fri, 15 Jun 2018 06:17:37 +0000

[caption id="attachment_344" align="aligncenter" width="650"] Figure 10.1 Grey and white striped metamorphic rocks (called gneiss) at Pemaquid Point were transformed by extreme heat and pressure during plate tectonic collisions. Source: Karla Panchuk (2018), CC BY 4.0. Photos by Joyce McBeth (2009), CC BY 4.0. Click for more attributions.
[/caption]

Chapter Goals

Complete this chapter so you can:

Summarize the factors that influence the nature of metamorphic rocks.
Explain how foliation forms in metamorphic rocks.
Classify metamorphic rocks based on their texture and mineral content, and explain the origins of both.
Describe settings where metamorphic rocks form, and explain the links to plate tectonics.
Describe the different types of metamorphism, including burial metamorphism, regional metamorphism, seafloor metamorphism, subduction zone metamorphism, contact metamorphism, shock metamorphism, and dynamic metamorphism.
Explain how metamorphic facies and index minerals are used to characterize metamorphism in a region.
Explain why fluids are important for metamorphism, and describe what happens during metasomatism.

Metamorphism Occurs Between Diagenesis And Melting

Metamorphism is the change that takes place within a body of rock as a result of it being subjected to high pressure and/or high temperature. The parent rock or protolith is the rock that exists before metamorphism starts. New metamorphic rocks can form from old ones as pressure and temperature progressively increase. This means that the term parent rock is sometimes applied to an initial unmetamorphosed rock, but it could also be applied to a metamorphic rock before it undergoes even further metamorphic change. We don’t always know whether metamorphism occurred in an uninterrupted sequence or whether metamorphism stopped and started again for different reasons at different times. Metamorphic rocks form under pressures and temperatures that are higher than those experienced by sediments and sedimentary rocks during diagenesis (a blanket term for a range of low-temperature and low-pressure physical and chemical changes that happen to buried sediments and sedimentary rocks, including interactions with groundwater), but at temperatures too low to allow melting. Given that pressure and water content affect a rock's melting point, metamorphism can still be ongoing at higher temperatures for some kinds of rocks, whereas other rocks will begin to melt under these same conditions. Metamorphic rocks can have very different mineral assemblages and textures than their parent rocks (Figure 10.2), but their over-all chemical composition usually doesn't change very much. [caption id="attachment_345" align="aligncenter" width="600"] Figure 10.2 Shale is the parent rock of gneiss (pronounced "nice"). These rocks look very different, but gneiss can form when the atoms contained within the shale are re-arranged into new mineral structures. Source: Karla Panchuk (2018), CC BY-NC-SA 4.0. Click for photo sources.[/caption] Most metamorphism results from the burial of igneous, sedimentary, or pre-existing metamorphic rocks, to the point where they experience different pressures and temperatures than those at which they originally formed. Metamorphism can also take place if cold rock near the surface is intruded and heated by a hot igneous body. Metamorphism usually involves temperatures above 150 °C, but some types of metamorphism do occur at temperatures lower than those at which the parent rock formed.

Refresher: Controls on Melting Points of Rocks

The highest temperature at which metamorphism can take place is the temperature at which a rock begins to melt.

How does water affect the upper temperature limit of metamorphism?
How does the mineral composition of a rock affect the upper temperature limit?

To check your answers, navigate to the below link to view the interactive version of this activity.

[h5p id="115"]

Chapter 11. Volcanism

kqzheng — Wed, 10 May 2017 20:35:54 +0000

[caption id="attachment_401" align="aligncenter" width="720"] Figure 11.1 Mt. Garibaldi (in the background), near Squamish BC, is one of Canada’s most recently active volcanoes, last erupted approximately 10,000 years ago. It is also one of the tallest, at 2,678 m in height. Source: Karla Panchuk (2017), CC BY-SA 4.0. Photograph: Michael Scheltgen (2006), CC BY 2.0. view source Click for more attributions.[/caption]

Learning Objectives

After reading this chapter and answering the Questions For Review at the end, you should be able to:

Explain what a volcano is.
Describe the different kinds of materials produced by volcanoes.
Describe the structures of shield volcanoes, composite volcanoes, and cinder cones.
Explain how the style of a volcanic eruption is related to magma composition.
Describe the role of plate tectonics in volcanism and magma formation.
Summarize the hazards that volcanic eruptions pose to people and infrastructure.
Describe how volcanoes are monitored, and the signals that indicate a volcano could be ready to erupt.
Provide an overview of Canadian volcanic activity.

Chapter Goals

Complete this chapter so you can:

Explain what a volcano is.
Describe the different kinds of materials produced by volcanoes.
Describe the structures of shield volcanoes, composite volcanoes, and cinder cones.
Explain how the style of a volcanic eruption is related to magma composition.
Describe the role of plate tectonics in volcanism and magma formation.
Summarize the hazards that volcanic eruptions pose to people and infrastructure.
Describe how volcanoes are monitored, and the signals that indicate a volcano could be ready to erupt.
Provide an overview of Canadian volcanic activity.

Why Study Volcanoes?

Volcanoes are awe-inspiring natural events. They have instilled fear and fascination with their red-hot lava flows, and cataclysmic explosions. In his painting The Eruption of Vesuvius (Figure 11.2), Pierre-Jacques Volaire captured the stunning spectacle of the eruption on Mt. Vesuvius on 14 May 1771. He also captured some stunningly casual spectating being done by tourists and their dog (lower left). [caption id="attachment_402" align="aligncenter" width="944"] Figure 11.2 The Eruption of Vesuvius, by Pierre-Jacques Volaire (1771). Public Domain.[/caption] As Volaire's painting suggests, curiosity alone would be enough to make people want to learn why volcanoes happen and how they work. However, there are other reasons why we should know more about volcanoes. One reason is that studying volcanoes helps us understand the evolution of the Earth system—not just Earth's geological features, but past changes in climate, and even the causes of mass extinctions. Another reason is the critical need to understand the hazards posed by volcanoes to people and infrastructure. Over the past few decades, volcanologists have made great strides in their ability to forecast volcanic eruptions and predict the consequences, saving thousands of lives.

Chapter 12. Earthquakes

kqzheng — Wed, 08 Nov 2017 21:21:09 +0000

[caption id="attachment_470" align="aligncenter" width="720"] Figure 12.1 Demolition of a structure damaged when an earthquake of magnitude 6.3 struck Christchurch, New Zealand on February 22, 2011. Many structures had already sustained damage from an earthquake that struck six months earlier, in September of 2010. Collapsing structures and falling debris accounted for most of the 185 deaths. Source: Karla Panchuk (2017), CC BY-SA 4.0. Photograph: Terry Philpott (2012), CC BY-NC 2.0. Click for more attributions.[/caption]

Chapter Goals

Complete this chapter so you can:

Explain how elastic deformation of Earth's crust results in earthquakes.
Describe how the main shock and immediate aftershocks define the rupture surface of an earthquake, and explain how the transfer of stress to other parts of a fault is related to aftershocks.
Explain the process of episodic tremor and slip.
Explain how plate tectonic setting affects where earthquakes occur.
Distinguish between an earthquake's magnitude and its intensity, and explain how these are determined.
Describe the hazards caused by earthquakes, including ground shaking, fires, slope failures, liquefaction, and tsunami.
Explain how the risk of an earthquake can be assessed, and describe steps that governments and individuals can take to minimize the impacts of earthquakes.

Why Study Earthquakes?

On the morning of June 23, 1946, a magnitude 7.3 earthquake struck Vancouver Island. It caused substantial damage to structures, including the school shown in Figure 12.2, and resulting in one fatality. The shaking was so violent that the seismograph measuring the earthquake in Victoria was broken a few seconds after the earthquake started. [caption id="attachment_471" align="aligncenter" width="864"] Figure 12.2 Damage to an elementary school in Courtenay, British Columbia after a magnitude 7.3 earthquake on Sunday, June 23, 1946. Left: A hole left after the chimney collapsed through the roof. Right: Damage inside the school. In addition to damaging structures, the earthquake triggered numerous slope failures. Source: Photographs courtesy of Earthquakes Canada. Click for image sources and terms of use.[/caption] Even if the seismograph had survived, it wasn't sensitive enough to provide sufficiently detailed information about local earthquakes, and was unable to record some of them at all. There was only one monitoring station in British Columbia, so while it was possible to determine how far away the earthquake was from the station, geologists weren't able to say in which direction. In 1955, Canadian seismologist W. G. Milne wrote that "the 1946 earthquake indicated in a very forceful manner the need for better instruments for the study of earthquakes in British Columbia." A network of improved instruments was established as a direct result of the earthquake. Time and time again, earthquakes have caused massive damage and many, many casualties. Recording earthquakes and determining their location of origin is important for establishing what geological conditions are responsible for the earthquakes, and understanding the risk they pose. After new, more sensitive instruments were installed at the Dominion Astrophysical Observatory in Victoria, geologists quickly learned that they had underestimated earthquake activity in the area. Between June of 1948 and August of 1951, 224 local earthquakes were recorded! By studying earthquakes, geoscientists and engineers are making progress toward learning how to minimize earthquake damage, and how to reduce the number of people affected by earthquakes. This knowledge can be communicated to governments so they are aware of what's needed to keep the population safe. It can also be communicated to individuals, so they know what to expect and do in the event of an earthquake, and can be adequately prepared with emergency supplies.

Additional Resources

Lamontagne, M., Halchuk, S., Cassidy, J. F., and Rogers, G. C. (2008). Significant Canadian Earthquakes of the Period 1600 - 2008. Seismological Research Letters 79(2), 211 - 223. doi: 10.1785/gssrl.79.2.211

Detailed description of the effects of the 1946 earthquake: Hodgson, E. A. (1946). British Columbia earthquake, June 23, 1946. The Journal of the Royal Astronomical Society of Canada XL(8), 285 - 319.

References

Milne, W. G. (1955). Seismology in British Columbia. The Journal of the Royal Astronomical Society of Canada, XLIX(4), 141 - 150. https://pressbooks.bccampus.ca/knowinghome/wp-content/uploads/sites/1304/2021/03/1955JRASC__49__141M.pdf

Ministry for Culture and Heritage (2017). Christchurch earthquake kills 185. https://nzhistory.govt.nz/page/christchurch-earthquake-kills-185

Chapter 13. Geological Structures and Mountain Building

kqzheng — Wed, 31 Jan 2018 21:54:28 +0000

[caption id="attachment_517" align="aligncenter" width="567"] Figure 13.1 Folded rocks in the Cariboo Mountains of BC. Source: Karla Panchuk (2018), CC BY-NC-SA 4.0. Photograph by Drew Brayshaw (2009), CC BY-NC 2.0. Image source. Click for more attributions.[/caption]

Chapter Goals

Complete this chapter so you can:

Describe the types of stresses that affect rocks.
Explain how rocks respond to those stresses through brittle, elastic, or plastic deformation.
Explain how rocks become folded and know the terms used to describe fold characteristics.
Describe the conditions under which rocks fracture.
Describe the different types of faults (normal, reverse, thrust, strike-slip) and the stresses that create them.
Describe the different ways in which deformation of Earth's lithosphere builds mountains.
Measure the strike and dip of a geological feature, and plot the information on a map.

Introduction

Folds, like those in the centre of Figure 13.1, are a common feature of mountain belts. Have you ever wondered how something as hard as rock could flex and bend to make folds, and what forces are required? Geologists have. Observing and analyzing geological structures helps us to understand the kinds of forces that affect rocks, both on small scales, and on scales as large as tectonic plates. With that knowledge we can understand how plate tectonic processes change the shape of Earth's lithosphere. We can also piece together a history of past changes, including how mountain belts are formed. Structural geologists make careful observations of the orientations of breaks and bends in rocks, and can compile those measurements into maps of geological structures. These maps can be valuable tools for finding mineral resources.

Chapter 14. Streams and Floods

kqzheng — Wed, 10 May 2017 20:37:47 +0000

[caption id="attachment_566" align="aligncenter" width="650"] Figure 14.1 A small waterfall on Johnston Creek in Johnston Canyon, Banff National Park, AB Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Chapter Goals

Complete this chapter so you can:

Explain the hydrological cycle, its relevance to streams, and describe the residence time of water in these systems.
Describe what a drainage basin is, and explain the origins of the different types of drainage patterns.
Explain how streams become graded, and how certain geological and anthropogenic changes can result in a stream becoming ungraded.
Describe the formation of stream terraces.
Describe the processes that move sediments in streams, and how changes in stream velocity affect the types of sediments that are moved by the stream.
Explain the origin of natural stream levees.
Describe the process of stream evolution and the types of environments where one would expect to find straight-channel, braided, and meandering streams.
Describe the annual flow characteristics of typical streams in Canada and the processes that lead to flooding.
Describe some of the important historical floods in Canada.
Determine the probability of floods of various magnitudes, based on the flood history of a stream.
Explain some of the steps that we can take to limit damage from flooding.

Why Study Streams?

Streams are the most important agents of erosion and transportation of sediments on Earth’s surface at this time in Earth’s history. They are responsible for generating much of the topography on that land surfaces that we see around us. They're places of beauty and tranquility, and they provide much of the water that is essential to our existence. But streams are not always peaceful and soothing. During large storms and rapid snowmelts, they can become raging torrents capable of moving cars and houses, and destroying roads and bridges. When they spill over their banks, they can flood huge areas, devastating populations and infrastructure. Over the past century, many of the most damaging natural disasters in Canada have been floods.

Chapter 15. Mass Wasting

kqzheng — Wed, 10 May 2017 20:38:10 +0000

Adapted by Joyce M. McBeth, University of Saskatchewan from Physical Geology by Steven Earle

Chapter Goals

Complete this chapter so you can:

Explain how slope stability is related to slope angle
Summarize some of the factors that influence the strength of materials on slopes, including the type of rock, the presence and orientation of planes of weakness such as bedding planes or fractures, the type of unconsolidated material upon the slope, and the effects of water flowing upon or within the slope material
Explain what types of events can trigger mass wasting
Summarize the types of motion that can happen during mass wasting
Describe the difference between a translational and a rotational slope failure
Describe the main types of mass wasting — creep, slump, slide, fall, and debris or mud flow — in terms of the types of materials involved, the type of motion, and the likely rates of motion
Explain what steps we can take to delay mass wasting, and why we cannot prevent it permanently
Describe some of the measures that can be taken to mitigate the risks associated with mass wasting

Introduction

Mass movement, mass wasting, and slope failure are all terms used to refer to materials moving down a slope because of the force of gravity. The term landslide is also used colloquially to refer to mass movement. The types of materials that move can be anything from mud to house-sized slabs of rock, to snow. The materials may move very slowly over years so that change is barely perceptible, or the movement might take only seconds.

The Hope Slide: a Historic Canadian Example of a Mass Wasting Event

Early in the morning of January 9, 1965, 47 million cubic metres of rock broke away from the steep upper slopes of Johnson Peak (16 km southeast of Hope, B.C.) and tumbled 2,000 m down the mountain, gouging out the contents of a small lake at the bottom, and continuing a few hundred metres up the other side of the valley (Figure 15.1). Four people who had been stopped on the highway by a snow avalanche were killed. Many more people might have become victims, except that a Greyhound bus driver en route to Vancouver turned his bus around upon seeing the avalanche.

[caption id="attachment_606" align="aligncenter" width="650"] Figure 15.1 The site of the 1965 Hope Slide, photographed in 2014. The initial failure is thought to have taken place along foliation planes in the rock and a sill. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

The rock failed along foliation planes of the metamorphic rock on Johnson Peak, in an area that had been eroded into a steep slope by glacial ice. There's no evidence that it was triggered by any specific event, and there was no warning that it was about to happen. Even if there had been warning, nothing could have been done to prevent it. There are hundreds of similar sites throughout mountainous regions of British Columbia and elsewhere in Canada where large mass wasting events could occur.

What can we learn from the Hope Slide? In general, we cannot prevent most mass wasting events, and significant effort is required if an event is to be predicted with any level of certainty. Understanding the geology is critical to understanding mass wasting. Although slope failures are inevitable in a region with steep slopes, larger slope failures happen less frequently than smaller ones. The consequences of a large mass wasting event also vary depending on the downslope conditions, such as the presence of people, buildings, roads, or fish-bearing streams.

An important reason for learning about mass wasting is to understand the nature of how and why materials fail in mass wasting events. If we understand this better, we can use this knowledge to help minimize the risks from similar events in the future.

Chapter 17. Glaciation

kqzheng — Tue, 06 Mar 2018 01:37:53 +0000

Adapted by Joyce McBeth, University of Saskatchewan from Physical Geology by Steven Earle

[caption id="attachment_703" align="aligncenter" width="500"] Figure 17.1 Glaciers in the Alberta Rockies: Athabasca Glacier (centre left), Dome Glacier (right), and the Columbia Icefield (visible above both glaciers). The Athabasca Glacier has prominent lateral moraines on both sides. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Chapter Goals

Complete this chapter so you can:

Define, draw, and describe the major features of glaciers
Explain the differences between continental and alpine glaciation
Summarize how snow and ice accumulate above a glacier’s equilibrium line and are converted to ice
Explain how basal sliding and internal flow facilitates the movement of ice from the upper part to the lower part of an alpine glacier
Describe and identify the various landforms related to alpine glacial erosion, including U-shaped valleys, arêtes, cols, horns, hanging valleys, truncated spurs, drumlins, roche moutonnées, glacial grooves, and striae
Identify various types of glacial lakes, including tarns, finger lakes, moraine lakes, and kettle lakes
Describe the nature and origin of lodgement till and ablation till
Describe the nature and origin of glaciofluvial, glaciolacustrine, and glaciomarine sediments
Describe the timing and extent of Earth’s past glaciations, going as far back as the early Proterozoic
Describe the important geological events that led up to the Pleistocene glaciations
Explain how the Milankovitch orbital variations along with positive climate feedback mechanisms may have controlled the timing of the Pleistocene glaciations

Introduction

A glacier is a long-lasting (decades or more) body of ice that is large enough to move under its own weight. They are at least tens of metres thick and at least hundreds of metres in extent. About 10% of Earth’s land surface is currently covered with glacial ice, and although the vast majority of this is in Antarctica and Greenland, there are many glaciers in Canada, especially in the mountainous parts of BC, Alberta, and the Yukon, and in the far north (Figure 17.1). At various times during the past million years, glacial ice has been much more extensive, covering at least 30% of the Earth’s land surface at times. Glaciers currently represent the largest repository of fresh water on Earth (~69% of all fresh water). They are highly sensitive to changes in climate, and in recent decades have been melting rapidly worldwide (Figure 17.2). Although some of the larger glacial masses may still last for several centuries, smaller glaciers, including many in western Canada, may be gone within decades. For mountainous regions, glaciers are an important sources of drinking water. Rapid glacial melting is a troubling issue for western Canadians because glacial ice is an important part of the hydrologic cycle in glaciated regions. Irrigation systems in BC, and across Alberta and Saskatchewan, are replenished by meltwater originating from glaciers in the Coast Range and the Rocky Mountains. [caption id="attachment_704" align="aligncenter" width="500"] Figure 17.2 Example of rapid melting of a glacier over a 63-year period. Muir Glacier, Alaska. Source: NASA Climate 365 Project (n.d.), Public Domain. Image source.[/caption] [caption id="attachment_705" align="aligncenter" width="500"] Figure 17.3 Part of the continental ice sheet in Greenland, with some outflow alpine glaciers in the foreground. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Chapter 18. Geological Resources

kqzheng — Wed, 10 May 2017 20:39:23 +0000

Chapter Goals

Complete this chapter so you can:

Describe the importance of geological resources to our way of life.
Summarize the types of materials mined in Canada and explain some of the processes involved in the formation of metal deposits.
Explain how a metal deposit is developed into a mine.
Define acid rock drainage (ARD) and discuss why some mines can lead to ARD and contamination of the environment by metals.
Summarize some of the important industrial materials extracted in Canada and describe what they are used for.
Describe the processes that lead to the formation of coal deposits.
Explain the processes that lead to the formation of oil and gas, the distinction between source rocks and reservoir rocks, and the importance of traps.
Describe the origins and recovery of some of the unconventional fossil fuels.
Describe the origins, discovery, and extraction of diamonds in Canada.

If You Can't Grow It, You Have to Mine It

It's been said that “if you can’t grow it, you have to mine it,” meaning that anything we can’t grow we have to extract from Earth in one way or another. This includes water, our most important resource, but it also includes the materials that we need to construct things like roads, dams, and bridges, or manufacture things like plates, toasters, and telephones. Most of our energy resources come from the Earth, including uranium and fossil fuels, and much of the infrastructure of this electrical age depends on copper (Figure 18.1). [caption id="attachment_750" align="aligncenter" width="550"] Figure 18.1 The open pit (background) and waste-rock piles (middle) of the Highland Valley Copper Mine at Logan Lake, British Columbia. Source: Photo by Russell Hartlaub, used with permission. Image source.[/caption] Virtually everything we use every day is made from resources from Earth. Consider a tablet computer (Figure 18.2): [caption id="attachment_751" align="aligncenter" width="650"] Figure 18.2 The main components of a tablet computer. Source: Steven Earle (2015), CC BY 4.0. Image source. Base photograph: Zach Vega (2013), CC BY-SA 3.0. Image source.[/caption]

Most of the case is made of the plastic known as ABS, which is made from either gas or petroleum. Some tablets have a case made from aluminum.
The glass of a touch screen is made mostly from quartz combined with smaller amounts of sodium oxide (Na₂O), sodium carbonate (Na₂CO₃), and calcium oxide (CaO).
To make it work as a touch screen, the upper surface is coated with indium tin oxide. When you touch the screen you’re actually pushing a thin layer of polycarbonate plastic (made from petroleum) against the coated glass, completing an electrical circuit. The computer is then able to figure out exactly where you touched the screen.
Computer processors are made from silica wafers (more quartz) and also include a significant amount of copper and gold. Gold is used because it is a better conductor than copper and doesn’t tarnish the way silver or copper does.
Most computers have nickel-metal-hydride (NiMH) batteries, which contain cadmium, cobalt, manganese, aluminum, and the rare-earth elements lanthanum, cerium, neodymium, and praseodymium, in addition to nickel.
The processor and other electronic components are secured to a circuit board, which is a thin layer of fibreglass sandwiched between copper sheets coated with small amounts of tin and lead. Various parts are put together with steel screws that are made of iron and molybdenum.

That’s not everything that goes into a tablet computer, but to make just those components we need a pure-silica sand deposit, a salt mine for sodium, a rock quarry for calcium, an oil well, a gas well, an aluminum mine, an iron mine, a manganese mine, a copper-molybdenum-gold mine, a cobalt-nickel mine, a rare-earth element and indium mine, and a source of energy to transport all of the materials, process them, put them together, and finally transport the computer to your house or the store where you bought it.

H5P listing

bpayne — Thu, 19 Aug 2021 18:15:15 +0000

Chapter 9. Sedimentary Rocks

kqzheng — Fri, 24 Aug 2018 19:07:04 +0000

[caption id="attachment_299" align="aligncenter" width="605"] Figure 9.1 Cretaceous sedimentary rocks exposed along a road near Drumheller, Alberta, Canada. Sedimentary rocks form in layers called beds, and the planar boundaries that separate each bed are called contacts. Each bed tells a story about the conditions in which it formed. In this picture the beds are indicating that sea level repeatedly rose and fell. The black layer about halfway up the picture is a coal seam. It tells us that the environment at that time was swampy. Source: Karla Panchuk (2008), CC BY 4.0.[/caption]

Chapter Goals

Complete this chapter so you can:

Explain the differences between the four kinds of sedimentary rocks: clastic, chemical, biochemical, and organic.
Describe some of the specific kinds of rocks in each of the four categories, and the depositional environments in which they form.
Describe the various terrestrial and marine sedimentary depositional environments, and explain how the formation of sedimentary basins is related to plate tectonic processes.
Apply your understanding of the features of sedimentary rocks, including grain characteristics, sedimentary structures, and fossils, to the interpretation of past depositional environments and climates.
Explain what groups, formations, and members are in sedimentary rocks, and why such terminology is used.

Earth's Storybook

All three kinds of rocks—igneous, metamorphic, and sedimentary—can tell us parts of Earth’s story. It is sedimentary rocks, however, that are most akin to a geological storybook. They form one layer at a time, capturing information about conditions at Earth’s surface within each layer. The layers stack one on top of another, forming a chronological sequence like pages in a book.

Sedimentary Rocks Form From the Products of Weathering and Erosion

Weathering and erosion (Chapter 8) are the first two steps in the transformation of pre-existing rocks into sedimentary rocks. The remaining steps in the formation of sedimentary rocks are transportation, deposition, burial, and lithification. These steps are shown on the right-hand side of the rock cycle diagram below (Figure 9.2). [caption id="attachment_300" align="aligncenter" width="583"] Figure 9.2 The rock cycle. Processes related to sedimentary rocks are shown on the right-hand side. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption] Transportation is the movement of sediments or dissolved ions from the site of erosion to a site of deposition. This can be by wind, flowing water, glacial ice, or mass movement down a slope. Deposition takes place where the conditions change enough so that the sediments can no longer be transported. This could happen if the current slows down. Burial occurs when sediments are deposited upon existing sediments, covering and compacting them. Lithification is what happens when those compacted sediments become cemented together to form solid sedimentary rock. Lithification occurs at depths of hundreds to thousands of metres within Earth.

Four Types of Sedimentary Rocks

Sedimentary rocks can be divided into four main types: clastic, chemical, biochemical, and organic. Clastic sedimentary rocks are composed mainly of material that is transported as solid fragments (called clasts), and then cemented together by minerals that precipitated from solution. Chemical sedimentary rocks are composed mainly of material that is transported as ions in solution. Biochemical sedimentary rocks also form from ions in solution, but organisms play an important role in converting those ions into calcium carbonate or silica body parts. Organic sedimentary rocks contain large amounts of organic matter, such as from plant leaves and tree bark.

Concept Check: Sedimentary Rock Types [h5p id="99"]

Chapter 16. Earth-System Change

kqzheng — Sun, 26 Aug 2018 21:37:14 +0000

[caption id="attachment_641" align="aligncenter" width="650"] Figure 16.1 Antarctic Peninsula. Antarctica was not always covered by ice. A change in the Earth system triggered the onset of Antarctic glaciation approximately 40 million years ago. Source: Karla Panchuk (2018), CC BY-SA 4.0. Photo: Liam Quinn (2011), Image source. Click for more attributions.[/caption]

Chapter Goals

Complete this chapter so you can:

Explain why it is useful to think of Earth as a system.
Describe the effect of feedbacks on the climate system.
Explain the difference between weather and climate.
Describe the climate-forcing mechanisms related to insolation, heat transport over Earth's surface, and changes in the atmosphere's energy budget.
Explain the difference between direct and proxy data about Earth's climate, and give examples of each.
Describe the role of computer models for understanding the Earth system.
Summarize the history of human influence on the Earth system.
Explain how carbon isotopes link rising atmospheric CO₂ levels to fossil fuels burned by humans.
Describe how humans have affected the present-day carbon cycle and why the human contribution is significant.
Summarize the kinds of observations that show signals of present-day climate change.
Describe the current and projected state of the Earth system in the Anthropocene Epoch.

The Only Constant Is Change

If one thing has been constant about the Earth system over geological time, it is unceasing change. In the geological record of climate, sedimentary deposits provide evidence of glaciations in the distant past, and chemical characteristics of sea-floor sediments tell about periods of extreme warmth. The Earth-system, and thus Earth's climate, has not only changed frequently, but also with large temperature fluctuations. Today’s mean global temperature is approximately 16°C. During Snowball Earth episodes more than 600 million years ago, when Earth's surface was frozen from pole to pole (or nearly so), the global mean was as cold as -50°C. At various other times in Earth history, it has been close to 30°C. Part of this chapter addresses natural processes of climate change, how they work, and how we know what Earth's past climate was like. Geologists study those natural climate-change processes to understand how human-caused, or anthropogenic, changes to the Earth system might affect the climate in the future, and how much the climate has changed over the time that humans increased their influence on the Earth-system. The rest of the chapter addresses what has been learned by asking those questions.

Chapter 19. Measuring Geological Time

kqzheng — Sat, 27 Oct 2018 00:40:16 +0000

[caption id="attachment_788" align="aligncenter" width="650"] Figure 19.1 Arizona’s Grand Canyon is an icon for geological time; 1,450 million years are represented by this photo. The light-coloured layers of rocks at the top formed at ~ 250 Ma, and the dark ones at the bottom of the canyon at ~ 1,700 Ma. Source: Steven Earle (2015), CC BY 4.0. Image source.[/caption]

Chapter Goals

Complete this chapter so you can:

Apply basic geological principles to determine the relative ages of rocks.
Explain the difference between relative and absolute age-dating techniques.
Summarize the history of the geological time scale and the relationships between eons, eras, periods, and epochs.
Understand the importance and significance of unconformities.
Estimate the age of a rock based on the fossils that it contains.
Use isotopic data to estimate the absolute age of a rock.
Describe some applications and limitations of isotopic techniques for absolute geological dating.
Describe the techniques for dating geological materials using tree rings and magnetic data.
Explain why an understanding of geological time is critical to both geologists and the public in general.

Geological Time Is Vast

Time is the dimension that sets geology apart from most other sciences. Geological time is vast, and Earth has changed tremendously during this time. Even though most geological processes are very, very slow, the vast amount of time that has passed has allowed for the formation of extraordinary geological features, as the Grand Canyon (Figure 19.1). There are many ways to measure geological time. We can tell the relative ages of rocks (e.g., whether one rock is older than another) based on their spatial relationships, we can use fossils to date sedimentary rocks because we have a detailed record of the evolution of life on Earth, and we can use a range of isotopic techniques to determine the absolute ages (in millions of years) of igneous and metamorphic rocks. But just because we can measure geological time doesn’t mean that we understand it. One of the biggest hurdles faced by geology students—and geologists as well—in understanding geology is to really come to grips with the slow rates at which geological processes happen, and the vast amount of time involved.