{"id":133,"date":"2018-08-24T14:48:55","date_gmt":"2018-08-24T18:48:55","guid":{"rendered":"https:\/\/opentextbc.ca\/physicalgeologyh5p\/chapter\/4-3-geological-renaissance-of-the-mid-20th-century-2\/"},"modified":"2023-07-04T12:46:14","modified_gmt":"2023-07-04T16:46:14","slug":"geological-renaissance-of-the-mid-20th-century","status":"publish","type":"chapter","link":"https:\/\/opentextbc.ca\/physicalgeologyh5p\/chapter\/geological-renaissance-of-the-mid-20th-century\/","title":{"raw":"4.3 Geological Renaissance of the Mid-20th Century","rendered":"4.3 Geological Renaissance of the Mid-20th Century"},"content":{"raw":"Two key areas of research ultimately led to the acceptance of continental drift, and the formulation of plate tectonic theory.\u00a0 One was the study of <strong>paleomagnetism<\/strong>, the record of Earth's magnetic field through time.\u00a0 The other was exploration of the ocean floor.\r\n<h1>Paleomagnetism (Remnant Magnetism)<\/h1>\r\n[caption id=\"attachment_119\" align=\"alignright\" width=\"228\"]<img class=\"wp-image-119\" src=\"https:\/\/opentextbc.ca\/kzlab\/wp-content\/uploads\/sites\/360\/2018\/08\/image0151.png\" alt=\"Figure 4.6 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. [SE]\" width=\"228\" height=\"243\" \/> <strong>Figure 4.9<\/strong> 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. <a href=\"https:\/\/opentextbc.ca\/geology\/wp-content\/uploads\/sites\/110\/2015\/07\/image0151.png\">Image source.<\/a>[\/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.\u00a0 This happens to the mineral magnetite (Fe<sub>3<\/sub>O<sub>4<\/sub>) when it crystallizes from magma.\u00a0 Once the rock cools the crystals are locked in place.\u00a0 This means that if the rock moves, the crystals can't realign themselves, and they retain a <strong>remnant magnetism<\/strong>. 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.\r\n\r\nRocks 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.\u00a0 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.\r\n\r\nBy 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.\u00a0 Remember that the vertical component of the magnetic field points more sharply downward the closer it is to the magnetic north pole.\u00a0 Figure 4.9 shows the vertical component of remnant magnetism in a sequence of rocks.\u00a0 Notice that the arrow starts out at 500 Ma pointing slightly upward.\u00a0 This means that the rocks were in the southern hemisphere.\u00a0 As the rocks get younger, the arrow tilts toward horizontal, and then points downward.\u00a0 This indicates that the rocks were getting progressively closer to the north magnetic pole.\r\n<h2>Apparent Polar Wandering Paths<\/h2>\r\nIn 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]\u00a0and several others, started looking at the remnant magnetism of Phanerozoic British and European volcanic rocks, and collecting <strong>paleomagnetic<\/strong> 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 <strong>polar wandering paths<\/strong>, rather than staying close to the geographic north pole as it does today.\u00a0 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.\r\n\r\n[caption id=\"attachment_120\" align=\"aligncenter\" width=\"650\"]<img class=\"wp-image-120\" src=\"https:\/\/opentextbc.ca\/kzlab\/wp-content\/uploads\/sites\/360\/2021\/08\/image0131.png\" alt=\"image\" width=\"650\" height=\"323\" \/> <strong>Figure 4.10<\/strong> 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. <a href=\"https:\/\/opentextbc.ca\/geology\/wp-content\/uploads\/sites\/110\/2015\/07\/image0131.png\">Image source.<\/a>[\/caption]\r\n\r\nRuncorn and colleagues extended their paleomagnetic studies to North America, and began to realize that their initial conclusion had a problem.\u00a0 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).\u00a0 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.\u00a0 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.\r\n\r\nThe polar wandering paths were not actually records of the pole moving, they just looked that way, so the paths are now referred to as <em>apparent<\/em> polar wandering paths (APWP).\u00a0 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.\u00a0 There was simply no other way to explain the data.\r\n\r\nThis 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.\r\n<div class=\"textbox shaded\">\r\n\r\n<strong>Concept Check: Clues from Paleomagnetism on Wandering Continents\r\n<\/strong>\r\n\r\n<div class=\"offline\">\r\n\r\n<strong>Write the words into the correct boxes to complete this summary.<\/strong>A history of Earth's ancient magnetic field, determined through the study of\u00a0<span style=\"text-decoration: underline;\" aria-label=\"blank\">\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0<\/span>, is preserved in rocks as <span style=\"text-decoration: underline;\" aria-label=\"blank\">\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0<\/span>. Magnetic minerals align with <span style=\"text-decoration: underline;\" aria-label=\"blank\">\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0<\/span>, 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.\r\n\r\nA key discovery was that magnetic minerals on different continents show different <span style=\"text-decoration: underline;\" aria-label=\"blank\">\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0<\/span> paths. This doesn't make sense, because there can only be one north pole at a time. The only explanation is that the <span style=\"text-decoration: underline;\" aria-label=\"blank\">\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0<\/span> were moving. This is why these paths are now called <span style=\"text-decoration: underline;\" aria-label=\"blank\">\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0<\/span> paths.\r\n\r\nFill-in-the-blank options:\r\n<ul class=\"threecolumn\">\r\n \t<li>continents<\/li>\r\n \t<li>remnant magnetism<\/li>\r\n \t<li>paleomagnetism<\/li>\r\n \t<li>polar wandering<\/li>\r\n \t<li>magnetic north<\/li>\r\n \t<li>apparent polar wondering<\/li>\r\n<\/ul>\r\n<strong>To check your answers, navigate to the below link to view the interactive version of this activity.<\/strong>\r\n\r\n<\/div>\r\n[h5p id=\"48\"]\r\n<\/div>\r\n<h1>Ocean Basin Geology and Geography<\/h1>\r\nDuring 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.\r\n<h2>Acoustic Depth Sounding<\/h2>\r\nUp 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!\r\n\r\nThe voyage of the <em>Challenger<\/em> 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.\r\n\r\nFollowing development of acoustic depth sounders\u00a0(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).\r\n\r\n[caption id=\"attachment_121\" align=\"aligncenter\" width=\"600\"]<img class=\"wp-image-121\" src=\"https:\/\/opentextbc.ca\/kzlab\/wp-content\/uploads\/sites\/360\/2021\/08\/image0171.png\" alt=\"image\" width=\"600\" height=\"273\" \/> <strong>Figure 4.11<\/strong> 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. <a href=\"https:\/\/opentextbc.ca\/geology\/wp-content\/uploads\/sites\/110\/2015\/07\/image0171.png\">Image source.<\/a>[\/caption]\r\n\r\n[caption id=\"attachment_122\" align=\"aligncenter\" width=\"600\"]<img class=\"wp-image-122\" src=\"https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/bathymetry.png\" alt=\"\" width=\"600\" height=\"720\" \/> <strong>Figure 4.12<\/strong> 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. <a href=\"http:\/\/opentextbc.ca\/geology\/wp-content\/uploads\/sites\/110\/2015\/07\/image0371.png\">Image source.<\/a> Basemap after NOAA (2006), Public Domain. <a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Elevation.jpg\">Image source.<\/a>[\/caption]\r\n\r\nThe important physical features of the ocean floor are:\r\n<ul>\r\n \t<li>Extensive linear ridges (commonly in the central parts of the oceans) at depths of 2,000 to 3,000 m (Figure 4.12, a)<\/li>\r\n \t<li>Fracture zones perpendicular to the ridges (Figure 4.12, a)<\/li>\r\n \t<li>Deep-ocean plains at depths of 4,000 to 5,000 m (Figure 4.12, a and d)<\/li>\r\n \t<li>Relatively flat and shallow continental shelves with depths under 500 m (Figure 4.12, b)<\/li>\r\n \t<li>Deep trenches (up to 11,000 m deep), most near the continents (Figure 4.12, c)<\/li>\r\n \t<li>Seamounts and chains of seamounts (Figure 4.12, d)<\/li>\r\n<\/ul>\r\n<h2>Seismic Reflection Sounding<\/h2>\r\n<strong>Seismic reflection sounding<\/strong> involves transmitting high-energy sound bursts and then measuring the echoes with a series of receivers called <strong>geophones<\/strong> 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<em> through<\/em> the sea-floor sediments and map the bedrock topography and crustal thickness. This allowed sediment thicknesses to be mapped (Figure 4.13).\r\n\r\n[caption id=\"attachment_123\" align=\"aligncenter\" width=\"1500\"]<img class=\"wp-image-123 size-full\" src=\"https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/sedthick9.jpg\" alt=\"Figure 4.10 Map of global sediment thickness. [Source: NOAA, http:\/\/1.usa.gov\/1Ywxxz6]\" width=\"1500\" height=\"1391\" \/> <strong>Figure 4.13<\/strong> Map of global sediment thickness. Source: NOAA (2003), Public Domain. <a href=\"https:\/\/www.ngdc.noaa.gov\/mgg\/image\/sedthick9.jpg\">Image source.<\/a>[\/caption]It was soon discovered that although the sediments were up to several 1000s of m thick near the continents, they were relatively thin \u2014 or even non-existent \u2014 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.\r\n\r\n[caption id=\"attachment_124\" align=\"aligncenter\" width=\"650\"]<img class=\"wp-image-124\" src=\"https:\/\/opentextbc.ca\/kzlab\/wp-content\/uploads\/sites\/360\/2021\/08\/image023_2.png\" alt=\"image\" width=\"650\" height=\"129\" \/> <strong>Figure 4.14<\/strong> 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. <a href=\"http:\/\/opentextbc.ca\/geology\/wp-content\/uploads\/sites\/110\/2015\/07\/image023_2.png\">Image source.<\/a>[\/caption]\r\n<h2>Heat Flow Rates<\/h2>\r\nIn the early 1950s, Edward Bullard\u2014who spent time at the University of Toronto but is mostly associated with Cambridge University\u2014developed 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.\r\n<h2>Earthquake Belts<\/h2>\r\nWith developments of networks of seismographic stations in the 1950s, it became possible to plot the locations <em>and<\/em> 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).\r\n\r\n[caption id=\"attachment_125\" align=\"aligncenter\" width=\"1024\"]<img class=\"wp-image-125 size-large\" src=\"https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0251-1024x250.png\" alt=\"image\" width=\"1024\" height=\"250\" \/> <strong>Figure 4.15<\/strong> 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. <a href=\"http:\/\/opentextbc.ca\/geology\/wp-content\/uploads\/sites\/110\/2015\/07\/image0251.png\">Image source.<\/a>[\/caption]\r\n<h2>Magnetic Stripes on the Sea Floor<\/h2>\r\n[caption id=\"attachment_126\" align=\"alignright\" width=\"255\"]<img class=\"wp-image-126 size-full\" src=\"https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0271.png\" alt=\"\" width=\"255\" height=\"222\" \/> <strong>Figure 4.16<\/strong> 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. <a href=\"https:\/\/geomaps.wr.usgs.gov\/parks\/noca\/nocageol4c.html\" rel=\"noopener\">Image source.<\/a> (Adapted from Raff and Mason, 1961).[\/caption]\r\n\r\nIn the 1950s, scientists from the Scripps Oceanographic Institute in California persuaded the United States Coast Guard to include <strong>magnetometer<\/strong> 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 \u2014 not even the scientists who collected them. Many 1000s of km of magnetic surveys were conducted over the next several years.\r\n\r\nThe 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 <strong>plate tectonics<\/strong>. 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\u2014except internally to the U.S. Navy (which funded his research)\u2014but presented them mostly in lectures and seminars.\r\n\r\nHess 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.\r\n\r\nHess's hypotheses formed the basis for our ideas on <strong>sea-floor spreading<\/strong> and <strong>continental drift,<\/strong> 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.\r\n\r\n[caption id=\"attachment_127\" align=\"aligncenter\" width=\"550\"]<img class=\"wp-image-127\" src=\"https:\/\/opentextbc.ca\/kzlab\/wp-content\/uploads\/sites\/360\/2021\/08\/image0291.png\" alt=\"image\" width=\"550\" height=\"218\" \/> <strong>Figure 4.17<\/strong> A representation of Harold Hess\u2019s model for sea-floor spreading and subduction. Source: Steven Earle (2015), CC BY 4.0. <a href=\"http:\/\/opentextbc.ca\/geology\/wp-content\/uploads\/sites\/110\/2015\/07\/image0291.png\">Image source.<\/a>[\/caption]\r\n\r\nCollection 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.\r\n\r\nAt 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\u2019s 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.\r\n\r\nThe 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).\r\n\r\nIn 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 <em>normal<\/em> 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 <em>reversed<\/em> 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 <em>Nature<\/em> and <em>The Journal of Geophysical Research<\/em> were rejected. The idea is sometimes referred to as the Vine-Matthews-Morley (VMM) hypothesis.\r\n\r\nVine, 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.\r\n<div class=\"textbox shaded\">\r\n\r\n<strong>Concept Check: The Meaning of Magnetic Stripes<\/strong>\r\n<div class=\"offline\"><img class=\"alignnone size-full wp-image-1873\" src=\"https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2018\/08\/file-611065a9618f5.png\" alt=\" A diagram of the ocean floor showing symmetrical stripes on either side of progressively widening lithosphere.\" width=\"1024\" height=\"724\" \/>Magnetic stripes record Earth\u2019s magnetic field switching between\u00a0<span style=\"text-decoration: underline;\" aria-label=\"blank\">\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0<\/span> polarity (today a compass needle points <span style=\"text-decoration: underline;\" aria-label=\"blank\">\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0<\/span>) to <span style=\"text-decoration: underline;\" aria-label=\"blank\">\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0<\/span> polarity (a compass needle would point <span style=\"text-decoration: underline;\" aria-label=\"blank\">\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0<\/span>). The stripes are <span style=\"text-decoration: underline;\" aria-label=\"blank\">\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0<\/span> on either side of volcanic ocean ridges, and get <span style=\"text-decoration: underline;\" aria-label=\"blank\">\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0<\/span> away from the ridge. This indicated that new was forming along the ridges, now referred to as seafloor <span style=\"text-decoration: underline;\" aria-label=\"blank\">\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0<\/span>.\r\n\r\nFill-in-the-blank options:\r\n<ul class=\"threecolumn\">\r\n \t<li>spreading centres<\/li>\r\n \t<li>north<\/li>\r\n \t<li>south<\/li>\r\n \t<li>normal<\/li>\r\n \t<li>crust<\/li>\r\n \t<li>reversed<\/li>\r\n \t<li>older<\/li>\r\n \t<li>symmetrical<\/li>\r\n<\/ul>\r\nTo check your answers, navigate to the below link to view the interactive version of this activity.\r\n\r\n<\/div>\r\n[h5p id=\"49\"]\r\n\r\n<\/div>\r\n<h2>Chains of Islands Progressively Aging Islands<\/h2>\r\nIn 1963, J. Tuzo Wilson of the University of Toronto proposed the idea of a <strong>mantle plume<\/strong> or <strong>hot spot <\/strong>\u2014 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.\r\n\r\n[caption id=\"attachment_128\" align=\"aligncenter\" width=\"550\"]<img class=\"wp-image-128\" src=\"https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0312.png\" alt=\"Figure 4.15 The ages of the Hawaiian Islands and the Emperor Seamounts in relation to the location of the Hawaiian mantle plume [SE. Basemap from the National Geophysical Data Centre, accessed at: http:\/\/en.wikipedia.org\/wiki\/Hotspot_(geology)#\/ media\/File:Hawaii_hotspot.jpg.]\" width=\"550\" height=\"402\" \/> <strong>Figure 4.18<\/strong> 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. <a href=\"https:\/\/opentextbc.ca\/geology\/wp-content\/uploads\/sites\/110\/2015\/07\/image0312.png\">Image source.<\/a> Base map from the National Geophysical Data Centre\/USGS (2005), Public Domain. <a href=\"http:\/\/en.wikipedia.org\/wiki\/Hotspot_(geology)#\/media\/File:Hawaii_hotspot.jpg\">Image source.<\/a>[\/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).\r\n\r\nThere 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.\r\n\r\n[caption id=\"attachment_129\" align=\"aligncenter\" width=\"550\"]<img class=\"wp-image-129\" src=\"https:\/\/opentextbc.ca\/kzlab\/wp-content\/uploads\/sites\/360\/2021\/08\/image035.jpg\" alt=\"image\" width=\"550\" height=\"341\" \/> <strong>Figure 4.19<\/strong> Mantle plume locations.\u00a0 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 W\u00f6lbern (2007), Public Domain. <a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Hotspots.jpg\">Image source.<\/a>[\/caption]\r\n<h2>Transform Faults<\/h2>\r\nOceanic 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 <strong>transform faults<\/strong>.\r\n\r\n[caption id=\"attachment_130\" align=\"aligncenter\" width=\"550\"]<img class=\"wp-image-130\" src=\"https:\/\/opentextbc.ca\/kzlab\/wp-content\/uploads\/sites\/360\/2021\/08\/tranforms-2-1024x401.png\" alt=\"\" width=\"550\" height=\"215\" \/> <strong>Figure 4.20 <\/strong>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.\u00a0Source: Steven Earle (2015), CC BY 4.0. <a href=\"https:\/\/opentextbc.ca\/physicalgeologyearle\/wp-content\/uploads\/sites\/145\/2016\/03\/tranforms-2.png\" rel=\"noopener\">Image source.<\/a>[\/caption]\r\n\r\nIn 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 <strong>plate tectonics<\/strong>.\r\n<div class=\"textbox shaded\">\r\n\r\n<strong>Paper Transform Fault Model<\/strong>\r\n\r\nJ. 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\u2019re done, you should have two folds of paper extending downward as in Figure 4.22.\r\n\r\n[caption id=\"attachment_131\" align=\"aligncenter\" width=\"1024\"]<img class=\"wp-image-131 size-full\" src=\"https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/Transform-model-1024x405-1.png\" alt=\"\" width=\"1024\" height=\"405\" \/> <strong>Figure 4.21<\/strong> Transform fault model. Source: Steven Earle (2015), CC BY 4.0. <a href=\"https:\/\/opentextbc.ca\/geology\/wp-content\/uploads\/sites\/110\/2015\/07\/image039.png\" rel=\"noopener\">Image source.<\/a> Modified after Stewart (1990).[\/caption]\r\n\r\n[caption id=\"attachment_132\" align=\"aligncenter\" width=\"400\"]<img class=\"wp-image-132 size-full\" src=\"https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0411.jpg\" alt=\"\" width=\"400\" height=\"266\" \/> <strong>Figure 4.22<\/strong> Use of the transform fault model. Source: Steven Earle (2015), CC BY 4.0. <a href=\"https:\/\/opentextbc.ca\/geology\/wp-content\/uploads\/sites\/110\/2015\/07\/image0411.jpg\" rel=\"noopener\">Image source.<\/a>[\/caption]\r\n\r\nFind 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.\r\n\r\nThere are other versions of this model available at <a href=\"https:\/\/web.viu.ca\/earle\/transform-model\/\">Paper models of transform faults by Steven Earle<\/a>. For more information see Earle (2004).\r\n\r\n<\/div>\r\n<h4>References<\/h4>\r\n<p class=\"hanging-indent\">Earle, S. (2004). A simple paper model of a transform fault at a spreading ridge. <em>Journal of Geoscience Education, 52<\/em>, 391-392.<\/p>\r\n<p class=\"hanging-indent\">Raff, A., &amp; Mason, R. (1961) Magnetic survey off the west coast of North America, 40\u02da N to 52\u02da N latitude. <em>Geological Society of America Bulletin, 72<\/em>, 267-270.<\/p>\r\n<p class=\"hanging-indent\"><span class=\"name\">Stewart, J. A. (1990). <em>Drifting continents and colliding paradigms.<\/em>\u00a0Indiana University Press.<\/span><\/p>\r\n<p class=\"hanging-indent\"><span class=\"name\">Tarduno, J. A., <\/span><span class=\"name\">Duncan, R. A., Scholl, D. W., Cottrell, R. D., <\/span><span class=\"name\">Steinberger, B., <\/span><span class=\"name\">Thordarson, T., Kerr, B. C., Neal, C. R., Frey, F. A., Torii, M., and Carvallo, C. (2003). <\/span>The Emperor Seamounts: Southward Motion of the Hawaiian Hotspot Plume in Earth\u2019s Mantle. <em>Science, 301<\/em>(5636), 1064\u20131069. DOI: 10.1126\/science.1086442<\/p>\r\n<p class=\"hanging-indent\">Wilson, J. T. (1965). A new class of faults and their bearing on continental drift. <em>Nature, 207<\/em>, 343-347.<\/p>","rendered":"<p>Two key areas of research ultimately led to the acceptance of continental drift, and the formulation of plate tectonic theory.\u00a0 One was the study of <strong>paleomagnetism<\/strong>, the record of Earth&#8217;s magnetic field through time.\u00a0 The other was exploration of the ocean floor.<\/p>\n<h1>Paleomagnetism (Remnant Magnetism)<\/h1>\n<figure id=\"attachment_119\" aria-describedby=\"caption-attachment-119\" style=\"width: 228px\" class=\"wp-caption alignright\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-119\" src=\"https:\/\/opentextbc.ca\/kzlab\/wp-content\/uploads\/sites\/360\/2018\/08\/image0151.png\" alt=\"Figure 4.6 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. [SE]\" width=\"228\" height=\"243\" srcset=\"https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2018\/08\/image0151.png 544w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2018\/08\/image0151-282x300.png 282w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2018\/08\/image0151-65x69.png 65w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2018\/08\/image0151-225x239.png 225w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2018\/08\/image0151-350x372.png 350w\" sizes=\"auto, (max-width: 228px) 100vw, 228px\" \/><figcaption id=\"caption-attachment-119\" class=\"wp-caption-text\"><strong>Figure 4.9<\/strong> Rock layers recording remnant magnetism. The red arrows represent the direction of the vertical component of Earth&#8217;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. <a href=\"https:\/\/opentextbc.ca\/geology\/wp-content\/uploads\/sites\/110\/2015\/07\/image0151.png\">Image source.<\/a><\/figcaption><\/figure>\n<p>When rocks form, some of the minerals that make them up can become aligned with the Earth&#8217;s magnetic field, just like a compass needle pointing to north.\u00a0 This happens to the mineral magnetite (Fe<sub>3<\/sub>O<sub>4<\/sub>) when it crystallizes from magma.\u00a0 Once the rock cools the crystals are locked in place.\u00a0 This means that if the rock moves, the crystals can&#8217;t realign themselves, and they retain a <strong>remnant magnetism<\/strong>. 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.<\/p>\n<p>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.\u00a0 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&#8217;s magnetic field as the sediments are deposited.<\/p>\n<p>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&#8217;s formation, but also the latitude where the rock formed relative to magnetic north.\u00a0 Remember that the vertical component of the magnetic field points more sharply downward the closer it is to the magnetic north pole.\u00a0 Figure 4.9 shows the vertical component of remnant magnetism in a sequence of rocks.\u00a0 Notice that the arrow starts out at 500 Ma pointing slightly upward.\u00a0 This means that the rocks were in the southern hemisphere.\u00a0 As the rocks get younger, the arrow tilts toward horizontal, and then points downward.\u00a0 This indicates that the rocks were getting progressively closer to the north magnetic pole.<\/p>\n<h2>Apparent Polar Wandering Paths<\/h2>\n<p>In the early 1950s, a group of geologists from Cambridge University, including Keith Runcorn, Ted Irving,<a class=\"footnote\" title=\"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.\" id=\"return-footnote-133-1\" href=\"#footnote-133-1\" aria-label=\"Footnote 1\"><sup class=\"footnote\">[1]<\/sup><\/a>\u00a0and several others, started looking at the remnant magnetism of Phanerozoic British and European volcanic rocks, and collecting <strong>paleomagnetic<\/strong> 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&#8217;s magnetic pole had moved around significantly over time along <strong>polar wandering paths<\/strong>, rather than staying close to the geographic north pole as it does today.\u00a0 At the time, geophysical models suggested that the magnetic poles did not need to be aligned with the rotational poles, so this wasn&#8217;t an unreasonable conclusion, given what was known.<\/p>\n<figure id=\"attachment_120\" aria-describedby=\"caption-attachment-120\" style=\"width: 650px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-120\" src=\"https:\/\/opentextbc.ca\/kzlab\/wp-content\/uploads\/sites\/360\/2021\/08\/image0131.png\" alt=\"image\" width=\"650\" height=\"323\" srcset=\"https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0131.png 1061w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0131-300x149.png 300w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0131-1024x509.png 1024w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0131-768x381.png 768w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0131-65x32.png 65w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0131-225x112.png 225w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0131-350x174.png 350w\" sizes=\"auto, (max-width: 650px) 100vw, 650px\" \/><figcaption id=\"caption-attachment-120\" class=\"wp-caption-text\"><strong>Figure 4.10<\/strong> 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. <a href=\"https:\/\/opentextbc.ca\/geology\/wp-content\/uploads\/sites\/110\/2015\/07\/image0131.png\">Image source.<\/a><\/figcaption><\/figure>\n<p>Runcorn and colleagues extended their paleomagnetic studies to North America, and began to realize that their initial conclusion had a problem.\u00a0 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).\u00a0 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.\u00a0 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.<\/p>\n<p>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 <em>apparent<\/em> polar wandering paths (APWP).\u00a0 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.\u00a0 There was simply no other way to explain the data.<\/p>\n<p>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.<\/p>\n<div class=\"textbox shaded\">\n<p><strong>Concept Check: Clues from Paleomagnetism on Wandering Continents<br \/>\n<\/strong><\/p>\n<div class=\"offline\">\n<p><strong>Write the words into the correct boxes to complete this summary.<\/strong>A history of Earth&#8217;s ancient magnetic field, determined through the study of\u00a0<span style=\"text-decoration: underline;\" aria-label=\"blank\">\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0<\/span>, is preserved in rocks as <span style=\"text-decoration: underline;\" aria-label=\"blank\">\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0<\/span>. Magnetic minerals align with <span style=\"text-decoration: underline;\" aria-label=\"blank\">\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0<\/span>, 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&#8217;s magnetic field.<\/p>\n<p>A key discovery was that magnetic minerals on different continents show different <span style=\"text-decoration: underline;\" aria-label=\"blank\">\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0<\/span> paths. This doesn&#8217;t make sense, because there can only be one north pole at a time. The only explanation is that the <span style=\"text-decoration: underline;\" aria-label=\"blank\">\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0<\/span> were moving. This is why these paths are now called <span style=\"text-decoration: underline;\" aria-label=\"blank\">\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0<\/span> paths.<\/p>\n<p>Fill-in-the-blank options:<\/p>\n<ul class=\"threecolumn\">\n<li>continents<\/li>\n<li>remnant magnetism<\/li>\n<li>paleomagnetism<\/li>\n<li>polar wandering<\/li>\n<li>magnetic north<\/li>\n<li>apparent polar wondering<\/li>\n<\/ul>\n<p><strong>To check your answers, navigate to the below link to view the interactive version of this activity.<\/strong><\/p>\n<\/div>\n<div id=\"h5p-48\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-48\" class=\"h5p-iframe\" data-content-id=\"48\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Summary of evidence for continental motion from apparent polar wandering\"><\/iframe><\/div>\n<\/div>\n<\/div>\n<h1>Ocean Basin Geology and Geography<\/h1>\n<p>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.<\/p>\n<h2>Acoustic Depth Sounding<\/h2>\n<p>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!<\/p>\n<p>The voyage of the <em>Challenger<\/em> 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.<\/p>\n<p>Following development of acoustic depth sounders\u00a0(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&#8217;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).<\/p>\n<figure id=\"attachment_121\" aria-describedby=\"caption-attachment-121\" style=\"width: 600px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-121\" src=\"https:\/\/opentextbc.ca\/kzlab\/wp-content\/uploads\/sites\/360\/2021\/08\/image0171.png\" alt=\"image\" width=\"600\" height=\"273\" srcset=\"https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0171.png 1117w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0171-300x137.png 300w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0171-1024x467.png 1024w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0171-768x350.png 768w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0171-65x30.png 65w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0171-225x103.png 225w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0171-350x159.png 350w\" sizes=\"auto, (max-width: 600px) 100vw, 600px\" \/><figcaption id=\"caption-attachment-121\" class=\"wp-caption-text\"><strong>Figure 4.11<\/strong> 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. <a href=\"https:\/\/opentextbc.ca\/geology\/wp-content\/uploads\/sites\/110\/2015\/07\/image0171.png\">Image source.<\/a><\/figcaption><\/figure>\n<figure id=\"attachment_122\" aria-describedby=\"caption-attachment-122\" style=\"width: 600px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-122\" src=\"https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/bathymetry.png\" alt=\"\" width=\"600\" height=\"720\" srcset=\"https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/bathymetry.png 1028w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/bathymetry-250x300.png 250w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/bathymetry-853x1024.png 853w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/bathymetry-768x922.png 768w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/bathymetry-65x78.png 65w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/bathymetry-225x270.png 225w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/bathymetry-350x420.png 350w\" sizes=\"auto, (max-width: 600px) 100vw, 600px\" \/><figcaption id=\"caption-attachment-122\" class=\"wp-caption-text\"><strong>Figure 4.12<\/strong> 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. <a href=\"http:\/\/opentextbc.ca\/geology\/wp-content\/uploads\/sites\/110\/2015\/07\/image0371.png\">Image source.<\/a> Basemap after NOAA (2006), Public Domain. <a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Elevation.jpg\">Image source.<\/a><\/figcaption><\/figure>\n<p>The important physical features of the ocean floor are:<\/p>\n<ul>\n<li>Extensive linear ridges (commonly in the central parts of the oceans) at depths of 2,000 to 3,000 m (Figure 4.12, a)<\/li>\n<li>Fracture zones perpendicular to the ridges (Figure 4.12, a)<\/li>\n<li>Deep-ocean plains at depths of 4,000 to 5,000 m (Figure 4.12, a and d)<\/li>\n<li>Relatively flat and shallow continental shelves with depths under 500 m (Figure 4.12, b)<\/li>\n<li>Deep trenches (up to 11,000 m deep), most near the continents (Figure 4.12, c)<\/li>\n<li>Seamounts and chains of seamounts (Figure 4.12, d)<\/li>\n<\/ul>\n<h2>Seismic Reflection Sounding<\/h2>\n<p><strong>Seismic reflection sounding<\/strong> involves transmitting high-energy sound bursts and then measuring the echoes with a series of receivers called <strong>geophones<\/strong> 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<em> through<\/em> the sea-floor sediments and map the bedrock topography and crustal thickness. This allowed sediment thicknesses to be mapped (Figure 4.13).<\/p>\n<figure id=\"attachment_123\" aria-describedby=\"caption-attachment-123\" style=\"width: 1500px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-123 size-full\" src=\"https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/sedthick9.jpg\" alt=\"Figure 4.10 Map of global sediment thickness. [Source: NOAA, http:\/\/1.usa.gov\/1Ywxxz6]\" width=\"1500\" height=\"1391\" srcset=\"https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/sedthick9.jpg 1500w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/sedthick9-300x278.jpg 300w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/sedthick9-1024x950.jpg 1024w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/sedthick9-768x712.jpg 768w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/sedthick9-65x60.jpg 65w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/sedthick9-225x209.jpg 225w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/sedthick9-350x325.jpg 350w\" sizes=\"auto, (max-width: 1500px) 100vw, 1500px\" \/><figcaption id=\"caption-attachment-123\" class=\"wp-caption-text\"><strong>Figure 4.13<\/strong> Map of global sediment thickness. Source: NOAA (2003), Public Domain. <a href=\"https:\/\/www.ngdc.noaa.gov\/mgg\/image\/sedthick9.jpg\">Image source.<\/a><\/figcaption><\/figure>\n<p>It was soon discovered that although the sediments were up to several 1000s of m thick near the continents, they were relatively thin \u2014 or even non-existent \u2014 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.<\/p>\n<figure id=\"attachment_124\" aria-describedby=\"caption-attachment-124\" style=\"width: 650px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-124\" src=\"https:\/\/opentextbc.ca\/kzlab\/wp-content\/uploads\/sites\/360\/2021\/08\/image023_2.png\" alt=\"image\" width=\"650\" height=\"129\" srcset=\"https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image023_2.png 1428w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image023_2-300x60.png 300w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image023_2-1024x204.png 1024w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image023_2-768x153.png 768w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image023_2-65x13.png 65w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image023_2-225x45.png 225w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image023_2-350x70.png 350w\" sizes=\"auto, (max-width: 650px) 100vw, 650px\" \/><figcaption id=\"caption-attachment-124\" class=\"wp-caption-text\"><strong>Figure 4.14<\/strong> 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. <a href=\"http:\/\/opentextbc.ca\/geology\/wp-content\/uploads\/sites\/110\/2015\/07\/image023_2.png\">Image source.<\/a><\/figcaption><\/figure>\n<h2>Heat Flow Rates<\/h2>\n<p>In the early 1950s, Edward Bullard\u2014who spent time at the University of Toronto but is mostly associated with Cambridge University\u2014developed 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.<\/p>\n<h2>Earthquake Belts<\/h2>\n<p>With developments of networks of seismographic stations in the 1950s, it became possible to plot the locations <em>and<\/em> 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).<\/p>\n<figure id=\"attachment_125\" aria-describedby=\"caption-attachment-125\" style=\"width: 1024px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-125 size-large\" src=\"https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0251-1024x250.png\" alt=\"image\" width=\"1024\" height=\"250\" srcset=\"https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0251-1024x250.png 1024w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0251-300x73.png 300w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0251-768x188.png 768w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0251-1536x376.png 1536w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0251-65x16.png 65w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0251-225x55.png 225w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0251-350x86.png 350w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0251.png 1901w\" sizes=\"auto, (max-width: 1024px) 100vw, 1024px\" \/><figcaption id=\"caption-attachment-125\" class=\"wp-caption-text\"><strong>Figure 4.15<\/strong> 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. <a href=\"http:\/\/opentextbc.ca\/geology\/wp-content\/uploads\/sites\/110\/2015\/07\/image0251.png\">Image source.<\/a><\/figcaption><\/figure>\n<h2>Magnetic Stripes on the Sea Floor<\/h2>\n<figure id=\"attachment_126\" aria-describedby=\"caption-attachment-126\" style=\"width: 255px\" class=\"wp-caption alignright\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-126 size-full\" src=\"https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0271.png\" alt=\"\" width=\"255\" height=\"222\" srcset=\"https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0271.png 255w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0271-65x57.png 65w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0271-225x196.png 225w\" sizes=\"auto, (max-width: 255px) 100vw, 255px\" \/><figcaption id=\"caption-attachment-126\" class=\"wp-caption-text\"><strong>Figure 4.16<\/strong> 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. <a href=\"https:\/\/geomaps.wr.usgs.gov\/parks\/noca\/nocageol4c.html\" rel=\"noopener\">Image source.<\/a> (Adapted from Raff and Mason, 1961).<\/figcaption><\/figure>\n<p>In the 1950s, scientists from the Scripps Oceanographic Institute in California persuaded the United States Coast Guard to include <strong>magnetometer<\/strong> 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 \u2014 not even the scientists who collected them. Many 1000s of km of magnetic surveys were conducted over the next several years.<\/p>\n<p>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 <strong>plate tectonics<\/strong>. He maintained some uncertainty about his proposal however, and in order to deflect criticism from mainstream geologists, he labelled it &#8220;geopoetry.&#8221; In fact, until 1962, Hess didn&#8217;t even put his ideas in writing\u2014except internally to the U.S. Navy (which funded his research)\u2014but presented them mostly in lectures and seminars.<\/p>\n<p>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.<\/p>\n<p>Hess&#8217;s hypotheses formed the basis for our ideas on <strong>sea-floor spreading<\/strong> and <strong>continental drift,<\/strong> 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.<\/p>\n<figure id=\"attachment_127\" aria-describedby=\"caption-attachment-127\" style=\"width: 550px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-127\" src=\"https:\/\/opentextbc.ca\/kzlab\/wp-content\/uploads\/sites\/360\/2021\/08\/image0291.png\" alt=\"image\" width=\"550\" height=\"218\" srcset=\"https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0291.png 1286w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0291-300x119.png 300w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0291-1024x406.png 1024w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0291-768x305.png 768w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0291-65x26.png 65w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0291-225x89.png 225w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0291-350x139.png 350w\" sizes=\"auto, (max-width: 550px) 100vw, 550px\" \/><figcaption id=\"caption-attachment-127\" class=\"wp-caption-text\"><strong>Figure 4.17<\/strong> A representation of Harold Hess\u2019s model for sea-floor spreading and subduction. Source: Steven Earle (2015), CC BY 4.0. <a href=\"http:\/\/opentextbc.ca\/geology\/wp-content\/uploads\/sites\/110\/2015\/07\/image0291.png\">Image source.<\/a><\/figcaption><\/figure>\n<p>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.<\/p>\n<p>At the same time, other researchers led by groups in California and New Zealand were studying the phenomenon of reversals in Earth&#8217;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\u2019s 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.<\/p>\n<p>The time scale of magnetic reversals is irregular. The present &#8220;normal&#8221; 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).<\/p>\n<p>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 <em>normal<\/em> 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 <em>reversed<\/em> event would have polarity opposite Earth&#8217;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&#8217;s papers submitted earlier in 1963 to <em>Nature<\/em> and <em>The Journal of Geophysical Research<\/em> were rejected. The idea is sometimes referred to as the Vine-Matthews-Morley (VMM) hypothesis.<\/p>\n<p>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&#8217;s magnetic field reversals.<\/p>\n<div class=\"textbox shaded\">\n<p><strong>Concept Check: The Meaning of Magnetic Stripes<\/strong><\/p>\n<div class=\"offline\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone size-full wp-image-1873\" src=\"https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2018\/08\/file-611065a9618f5.png\" alt=\"A diagram of the ocean floor showing symmetrical stripes on either side of progressively widening lithosphere.\" width=\"1024\" height=\"724\" srcset=\"https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2018\/08\/file-611065a9618f5.png 1024w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2018\/08\/file-611065a9618f5-300x212.png 300w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2018\/08\/file-611065a9618f5-768x543.png 768w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2018\/08\/file-611065a9618f5-65x46.png 65w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2018\/08\/file-611065a9618f5-225x159.png 225w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2018\/08\/file-611065a9618f5-350x247.png 350w\" sizes=\"auto, (max-width: 1024px) 100vw, 1024px\" \/>Magnetic stripes record Earth\u2019s magnetic field switching between\u00a0<span style=\"text-decoration: underline;\" aria-label=\"blank\">\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0<\/span> polarity (today a compass needle points <span style=\"text-decoration: underline;\" aria-label=\"blank\">\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0<\/span>) to <span style=\"text-decoration: underline;\" aria-label=\"blank\">\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0<\/span> polarity (a compass needle would point <span style=\"text-decoration: underline;\" aria-label=\"blank\">\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0<\/span>). The stripes are <span style=\"text-decoration: underline;\" aria-label=\"blank\">\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0<\/span> on either side of volcanic ocean ridges, and get <span style=\"text-decoration: underline;\" aria-label=\"blank\">\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0<\/span> away from the ridge. This indicated that new was forming along the ridges, now referred to as seafloor <span style=\"text-decoration: underline;\" aria-label=\"blank\">\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0<\/span>.<\/p>\n<p>Fill-in-the-blank options:<\/p>\n<ul class=\"threecolumn\">\n<li>spreading centres<\/li>\n<li>north<\/li>\n<li>south<\/li>\n<li>normal<\/li>\n<li>crust<\/li>\n<li>reversed<\/li>\n<li>older<\/li>\n<li>symmetrical<\/li>\n<\/ul>\n<p>To check your answers, navigate to the below link to view the interactive version of this activity.<\/p>\n<\/div>\n<div id=\"h5p-49\">\n<div class=\"h5p-iframe-wrapper\"><iframe id=\"h5p-iframe-49\" class=\"h5p-iframe\" data-content-id=\"49\" style=\"height:1px\" src=\"about:blank\" frameBorder=\"0\" scrolling=\"no\" title=\"Summary of evidence for plate tectonics from magnetic stripes\"><\/iframe><\/div>\n<\/div>\n<\/div>\n<h2>Chains of Islands Progressively Aging Islands<\/h2>\n<p>In 1963, J. Tuzo Wilson of the University of Toronto proposed the idea of a <strong>mantle plume<\/strong> or <strong>hot spot <\/strong>\u2014 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&#8217;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&#8217;i itself, which consists of rock that is almost all younger than 1 Ma.<\/p>\n<figure id=\"attachment_128\" aria-describedby=\"caption-attachment-128\" style=\"width: 550px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-128\" src=\"https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0312.png\" alt=\"Figure 4.15 The ages of the Hawaiian Islands and the Emperor Seamounts in relation to the location of the Hawaiian mantle plume [SE. Basemap from the National Geophysical Data Centre, accessed at: http:\/\/en.wikipedia.org\/wiki\/Hotspot_(geology)#\/ media\/File:Hawaii_hotspot.jpg.]\" width=\"550\" height=\"402\" srcset=\"https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0312.png 784w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0312-300x219.png 300w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0312-768x561.png 768w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0312-65x48.png 65w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0312-225x164.png 225w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0312-350x256.png 350w\" sizes=\"auto, (max-width: 550px) 100vw, 550px\" \/><figcaption id=\"caption-attachment-128\" class=\"wp-caption-text\"><strong>Figure 4.18<\/strong> The ages of the Hawai&#8217;ian Islands and the Emperor Seamounts in relation to the location of the Hawai&#8217;ian mantle plume. Source: Steven Earle (2015), CC BY 4.0. <a href=\"https:\/\/opentextbc.ca\/geology\/wp-content\/uploads\/sites\/110\/2015\/07\/image0312.png\">Image source.<\/a> Base map from the National Geophysical Data Centre\/USGS (2005), Public Domain. <a href=\"http:\/\/en.wikipedia.org\/wiki\/Hotspot_(geology)#\/media\/File:Hawaii_hotspot.jpg\">Image source.<\/a><\/figcaption><\/figure>\n<p>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&#8217;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).<\/p>\n<p>There is evidence of many such mantle plumes around the world (Figure 4.19). Most are within ocean basins, including places like Hawai&#8217;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.<\/p>\n<figure id=\"attachment_129\" aria-describedby=\"caption-attachment-129\" style=\"width: 550px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-129\" src=\"https:\/\/opentextbc.ca\/kzlab\/wp-content\/uploads\/sites\/360\/2021\/08\/image035.jpg\" alt=\"image\" width=\"550\" height=\"341\" srcset=\"https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image035.jpg 927w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image035-300x186.jpg 300w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image035-768x476.jpg 768w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image035-65x40.jpg 65w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image035-225x140.jpg 225w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image035-350x217.jpg 350w\" sizes=\"auto, (max-width: 550px) 100vw, 550px\" \/><figcaption id=\"caption-attachment-129\" class=\"wp-caption-text\"><strong>Figure 4.19<\/strong> Mantle plume locations.\u00a0 Selected Mantle plumes: 1: Azores, 3: Bowie, 5: Cobb, 8: Eifel, 10: Galapagos, 12: Hawai&#8217;i, 14: Iceland, 17: Cameroon, 18: Canary, 19: Cape Verde, 35: Samoa, 38: Tahiti, 42: Tristan, 44: Yellowstone, 45: Anahim. Source: Ingo W\u00f6lbern (2007), Public Domain. <a href=\"https:\/\/commons.wikimedia.org\/wiki\/File:Hotspots.jpg\">Image source.<\/a><\/figcaption><\/figure>\n<h2>Transform Faults<\/h2>\n<p>Oceanic spreading ridges appear to be curved features on Earth&#8217;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 <strong>transform faults<\/strong>.<\/p>\n<figure id=\"attachment_130\" aria-describedby=\"caption-attachment-130\" style=\"width: 550px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-130\" src=\"https:\/\/opentextbc.ca\/kzlab\/wp-content\/uploads\/sites\/360\/2021\/08\/tranforms-2-1024x401.png\" alt=\"\" width=\"550\" height=\"215\" srcset=\"https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/tranforms-2-1024x401.png 1024w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/tranforms-2-300x117.png 300w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/tranforms-2-768x301.png 768w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/tranforms-2-65x25.png 65w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/tranforms-2-225x88.png 225w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/tranforms-2-350x137.png 350w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/tranforms-2.png 1178w\" sizes=\"auto, (max-width: 550px) 100vw, 550px\" \/><figcaption id=\"caption-attachment-130\" class=\"wp-caption-text\"><strong>Figure 4.20 <\/strong>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.\u00a0Source: Steven Earle (2015), CC BY 4.0. <a href=\"https:\/\/opentextbc.ca\/physicalgeologyearle\/wp-content\/uploads\/sites\/145\/2016\/03\/tranforms-2.png\" rel=\"noopener\">Image source.<\/a><\/figcaption><\/figure>\n<p>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 <strong>plate tectonics<\/strong>.<\/p>\n<div class=\"textbox shaded\">\n<p><strong>Paper Transform Fault Model<\/strong><\/p>\n<p>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\u2019re done, you should have two folds of paper extending downward as in Figure 4.22.<\/p>\n<figure id=\"attachment_131\" aria-describedby=\"caption-attachment-131\" style=\"width: 1024px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-131 size-full\" src=\"https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/Transform-model-1024x405-1.png\" alt=\"\" width=\"1024\" height=\"405\" srcset=\"https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/Transform-model-1024x405-1.png 1024w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/Transform-model-1024x405-1-300x119.png 300w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/Transform-model-1024x405-1-768x304.png 768w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/Transform-model-1024x405-1-65x26.png 65w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/Transform-model-1024x405-1-225x89.png 225w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/Transform-model-1024x405-1-350x138.png 350w\" sizes=\"auto, (max-width: 1024px) 100vw, 1024px\" \/><figcaption id=\"caption-attachment-131\" class=\"wp-caption-text\"><strong>Figure 4.21<\/strong> Transform fault model. Source: Steven Earle (2015), CC BY 4.0. <a href=\"https:\/\/opentextbc.ca\/geology\/wp-content\/uploads\/sites\/110\/2015\/07\/image039.png\" rel=\"noopener\">Image source.<\/a> Modified after Stewart (1990).<\/figcaption><\/figure>\n<figure id=\"attachment_132\" aria-describedby=\"caption-attachment-132\" style=\"width: 400px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-132 size-full\" src=\"https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0411.jpg\" alt=\"\" width=\"400\" height=\"266\" srcset=\"https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0411.jpg 400w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0411-300x200.jpg 300w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0411-65x43.jpg 65w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0411-225x150.jpg 225w, https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-content\/uploads\/sites\/360\/2021\/08\/image0411-350x233.jpg 350w\" sizes=\"auto, (max-width: 400px) 100vw, 400px\" \/><figcaption id=\"caption-attachment-132\" class=\"wp-caption-text\"><strong>Figure 4.22<\/strong> Use of the transform fault model. Source: Steven Earle (2015), CC BY 4.0. <a href=\"https:\/\/opentextbc.ca\/geology\/wp-content\/uploads\/sites\/110\/2015\/07\/image0411.jpg\" rel=\"noopener\">Image source.<\/a><\/figcaption><\/figure>\n<p>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.<\/p>\n<p>There are other versions of this model available at <a href=\"https:\/\/web.viu.ca\/earle\/transform-model\/\">Paper models of transform faults by Steven Earle<\/a>. For more information see Earle (2004).<\/p>\n<\/div>\n<h4>References<\/h4>\n<p class=\"hanging-indent\">Earle, S. (2004). A simple paper model of a transform fault at a spreading ridge. <em>Journal of Geoscience Education, 52<\/em>, 391-392.<\/p>\n<p class=\"hanging-indent\">Raff, A., &amp; Mason, R. (1961) Magnetic survey off the west coast of North America, 40\u02da N to 52\u02da N latitude. <em>Geological Society of America Bulletin, 72<\/em>, 267-270.<\/p>\n<p class=\"hanging-indent\"><span class=\"name\">Stewart, J. A. (1990). <em>Drifting continents and colliding paradigms.<\/em>\u00a0Indiana University Press.<\/span><\/p>\n<p class=\"hanging-indent\"><span class=\"name\">Tarduno, J. A., <\/span><span class=\"name\">Duncan, R. A., Scholl, D. W., Cottrell, R. D., <\/span><span class=\"name\">Steinberger, B., <\/span><span class=\"name\">Thordarson, T., Kerr, B. C., Neal, C. R., Frey, F. A., Torii, M., and Carvallo, C. (2003). <\/span>The Emperor Seamounts: Southward Motion of the Hawaiian Hotspot Plume in Earth\u2019s Mantle. <em>Science, 301<\/em>(5636), 1064\u20131069. DOI: 10.1126\/science.1086442<\/p>\n<p class=\"hanging-indent\">Wilson, J. T. (1965). A new class of faults and their bearing on continental drift. <em>Nature, 207<\/em>, 343-347.<\/p>\n<hr class=\"before-footnotes clear\" \/><div class=\"footnotes\"><ol><li id=\"footnote-133-1\">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. <a href=\"#return-footnote-133-1\" class=\"return-footnote\" aria-label=\"Return to footnote 1\">&crarr;<\/a><\/li><\/ol><\/div>","protected":false},"author":123,"menu_order":3,"template":"","meta":{"pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-133","chapter","type-chapter","status-publish","hentry"],"part":108,"_links":{"self":[{"href":"https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-json\/pressbooks\/v2\/chapters\/133","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-json\/wp\/v2\/users\/123"}],"version-history":[{"count":12,"href":"https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-json\/pressbooks\/v2\/chapters\/133\/revisions"}],"predecessor-version":[{"id":1900,"href":"https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-json\/pressbooks\/v2\/chapters\/133\/revisions\/1900"}],"part":[{"href":"https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-json\/pressbooks\/v2\/parts\/108"}],"metadata":[{"href":"https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-json\/pressbooks\/v2\/chapters\/133\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-json\/wp\/v2\/media?parent=133"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-json\/pressbooks\/v2\/chapter-type?post=133"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-json\/wp\/v2\/contributor?post=133"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/opentextbc.ca\/physicalgeologyh5p\/wp-json\/wp\/v2\/license?post=133"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}