Chapter 4 Volcanism
4.3 Types of Volcanoes
There are numerous types of volcanoes or volcanic sources; some of the more common ones are summarized in Table 4.1.
| [Skip Table] | ||||
| Type | Tectonic Setting | Size and Shape | Magma and Eruption Characteristics | Example |
|---|---|---|---|---|
| Cinder cone | Various; some form on the flanks of larger volcanoes | Small (10s to 100s of metres) and steep (Greater than 20°) | Most are mafic and form from the gas-rich early stages of a shield- or rift-associated eruption | Eve Cone, northern B.C. |
| Composite volcano | Almost all are at subduction zones | Medium size (1000s of metres high and up to 20 km across) and moderate steepness (10° to 30°) | Magma composition varies from felsic to mafic, and from explosive to effusive | Mount St. Helens |
| Shield volcano | Most are at mantle plumes; some are on spreading ridges | Large (up to several 1,000 metres high and up to 200 kilometres across), not steep (typically 2° to 10°) | Magma is almost always mafic, and eruptions are typically effusive, although cinder cones are common on the flanks of shield volcanoes | Kilauea, Hawaii |
| Large igneous provinces | Associated with “super” mantle plumes | Enormous (up to millions of square kiometres) and 100s of metres thick | Magma is always mafic and individual flows can be 10s of metres thick | Columbia River basalts |
| Sea-floor volcanism | Generally associated with spreading ridges but also with mantle plumes | Large areas of the sea floor associated with spreading ridges | Pillows form at typical eruption rates; lava flows develop if the rare of flow is faster | Juan de Fuca ridge |
| Kimberlite | Upper-mantle sourced | The remnants are typically 10s to 100s of metres across | Most appear to have had explosive eruptions forming cinder cones; the youngest one is dated at about 10 ka, and all others are at least 30 Ma | Lac de Gras Kimberlite Field, N.W.T. |
The sizes and shapes of typical shield, composite, and cinder-cone volcanoes are compared in Figure 4.3.1, although, to be fair, Mauna Loa is the largest shield volcano on Earth; all others are smaller. Mauna Loa rises from the surrounding flat sea floor, and its diameter is in the order of 200 km. Its elevation is 4,169 m above sea level. Mount St. Helens, a composite volcano of average size, rises above the surrounding hills of the Cascade Range. Its diameter is about 6 km, and its height is 2,550 m above sea level. Cinder cones are much smaller. On this drawing, even a large cinder cone is just a dot.
Cinder Cones
, like Eve Cone in northern B.C. (Figure 4.3.2), are typically only a few hundred metres in diameter, and few are more than 200 m high. Most are made up of fragments of mafic rock (scoria) that were expelled as the magma boiled when it approached the surface, creating fire fountains. In many cases, these later became the sites of effusive lava flows when the gases were depleted. Most cinder cones are , meaning that they formed during a single eruptive phase that might have lasted weeks or months. Because cinder cones are made up almost exclusively of loose fragments, they have very little strength. They can be easily, and relatively quickly, eroded away.
Composite Volcanoes
, like Mount St. Helens in Washington State (Figure 4.3.3), are almost all associated with subduction at convergent plate boundaries—either ocean-continent or ocean-ocean boundaries (Figure 4.1.2b). They can extend up to several thousand metres from the surrounding terrain, and, with slopes ranging up to 30˚ They can be up to about 20 km across. At many such volcanoes, magma is stored in a magma chamber in the upper part of the crust. For example, at Mount St. Helens, there is evidence of a magma chamber that is approximately 1 kilometre wide and extends from about 6 km to 14 km below the surface (Figure 4.3.4). Systematic variations in the composition of volcanism over the past several thousand years at Mount St. Helens imply that the magma chamber is zoned, from more felsic at the top to more mafic at the bottom.
Mafic eruptions (and some intermediate eruptions), on the other hand, produce lava flows; the one shown in Figure 4.3.5b is thick enough (about 10 m in total) to have cooled in a pattern (Figure 4.3.7). Lava flows both flatten the profile of the volcano (because the lava typically flows farther than pyroclastic debris falls) and protect the fragmental deposits from erosion. Even so, composite volcanoes tend to erode quickly. Patrick Pringle, a volcanologist with the Washington State Department of Natural Resources, describes Mount St. Helens as a “pile of junk.” The rock that makes up Mount St. Helens ranges in composition from rhyolite (Figure 4.3.5a) to basalt (Figure 4.3.5b); this implies that the types of past eruptions have varied widely in character. As already noted, felsic magma doesn’t flow easily and doesn’t allow gases to escape easily. Under these circumstances, pressure builds up until a conduit opens, and then an explosive eruption results from the gas-rich upper part of the magma chamber, producing debris, as shown on Figure 4.3.5a. This type of eruption can also lead to rapid melting of ice and snow on a volcano, which typically triggers large mudflows known as (Figure 4.3.5a). Hot, fast-moving pyroclastic flows and lahars are the two main causes of casualties in volcanic eruptions. Pyroclastic flows killed approximately 30,000 people during the 1902 eruption of Mount. Pelée on the Caribbean island of Martinique. Most were incinerated in their homes. In 1985 a massive lahar, triggered by the eruption of Nevado del Ruiz, killed 23,000 people in the Colombian town of Armero, about 50 km from the volcano.
In a geological context, composite volcanoes tend to form relatively quickly and do not last very long. Mount St. Helens, for example, is made up of rock that is all younger than 40,000 years; most of it is younger than 3,000 years. If its 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.
Exercise 4.3 Volcanoes and Subduction
The map shown here illustrates the interactions between the North America, Juan de Fuca, and Pacific Plates off the west coast of Canada and the United States. The Juan de Fuca Plate is forming along the Juan de Fuca ridge, and is then subducted beneath the North America Plate along the red line with teeth on it (“Subduction boundary”).
- Using the scale bar in the lower left of the map, estimate the average distance between the subduction boundary and the Cascadia composite volcanoes.
- If the subducting Juan de Fuca Plate descends 40 km for every 100 km that it moves inland, what is its likely depth in the area where volcanoes are forming?
See Appendix 3 for Exercise 4.3 answers.
Shield Volcanoes
Most are associated with mantle plumes, although some form at divergent boundaries, either on land or on the sea floor. Because of their non-viscous mafic magma they tend to have relatively gentle slopes (2 to 10˚) and the larger ones can be over 100 km in diameter. The best-known shield volcanoes are those that make up the Hawaiian Islands, and of these, the only active ones are on the big island of Hawaii. Mauna Loa, the world’s largest volcano and the world’s largest mountain (by volume) last erupted in 1984. Kilauea, arguably the world’s most active volcano, has been erupting, virtually without interruption, since 1983. Loihi is an underwater volcano on the southeastern side of Hawaii. It is last known to have erupted in 1996, but may have erupted since then without being detected.
All of the Hawaiian volcanoes are related to the mantle plume that currently lies beneath Mauna Loa, Kilauea, and Loihi (Figure 4.3.8). In this area, the Pacific Plate is moving northwest at a rate of about 7 centimetres (cm) per year. This means that the earlier formed — and now extinct — volcanoes have now moved well away from the mantle plume. As shown on Figure 4.3.8, there is evidence of crustal magma chambers beneath all three active Hawaiian volcanoes. At Kilauea, the magma chamber appears to be several kilometres in diameter, and is situated between 8 km and 11 km below surface.[1]
Although it is not a prominent mountain (Figure 4.3.2), Kilauea volcano has a large in its summit area (Figure 4.3.9). A caldera is a volcanic that is more than 2 km in diameter; this one is 4 km long and 3 km wide. It contains a smaller feature called Halema’uma’u crater, which has a total depth of over 200 m below the surrounding area. Most volcanic craters and calderas are formed above magma chambers, and the level of the crater floor is influenced by the amount of pressure exerted by the magma body. During historical times, the floors of both Kilauea caldera and Halema’uma’u crater have moved up during expansion of the magma chamber and down during deflation of the chamber.
One of the conspicuous features of Kilauea caldera is rising water vapour (the white cloud in Figure 4.3.9) and a strong smell of sulphur (Figure 4.3.10). As is typical in magmatic regions, water is the main volatile component, followed by carbon dioxide and sulphur dioxide. These, and some minor gases, originate from the magma chamber at depth and rise up through cracks in the overlying rock. This degassing of the magma is critical to the style of eruption at Kilauea, which, for most of the past 35 years, has been effusive, not explosive.
The Kilauea eruption that began in 1983 started with the formation of a cinder cone at Pu’u ’O’o, approximately 15 km east of the caldera (Figure 4.3.11). The magma feeding this eruption flowed along a major conduit system known as the East Rift, which extends for about 20 km from the caldera, first southeast and then east. Lava fountaining and construction of the Pu’u ’O’o cinder cone (Figure 4.3.12a) continued until 1986 at which time the flow became effusive. From 1986 to 2014, lava flowed from a gap in the southern flank of Pu’u ’O’o down the slope of Kilauea through a (Figure 4.3.12d), emerging at or near the ocean. During 2014 and 2015, the lava flowed northeast toward the community of Pahoa (see Exercise 4.4). In May of 2018 a new eruption started another 15 km east of the 2014/15 flow in the area known as Leilani Estates. The Lower East Rift Flow was active for 6 months. During that time, 35 km2 of existing land was covered in lava and 3.5 km2 of new land was created (Figure 4.3.11), about 48 km of road were covered in lava and 716 dwellings were destroyed (see USGS Overview of Kilauea Volcanoe’s 2018 eruption[PDF]). Volcanic activity on the East Rift ceased in August 2018, and there has been no activity on Kilauea since then. This appears to mark the end of the eruption cycle that lasted—with only a few short interruptions—for 35 years. Kilauea will almost certainly erupt again within years or decades.
The two main types of textures created during effusive subaerial eruptions are pahoehoe and aa. , ropy lava that forms as non-viscous lava, flows gently, forming a skin that gels and then wrinkles because of ongoing flow of the lava below the surface (Figure 4.3.12b, and “lava flow video”). , or blocky lava, forms when magma is forced to flow faster than it is able to (down a slope for example) (Figure 4.3.12c). (lava fragments) is produced during explosive eruptions, and accumulates in the vicinity of cinder cones.
Figure 4.3.12d is a view into an active lava tube on the southern edge of Kilauea. The red glow is from a stream of very hot lava (~1200°C) that has flowed underground for most of the 8 km from the Pu’u ’O’o vent. Lava tubes form naturally and readily on both shield and composite volcanoes because flowing mafic lava preferentially cools near its margins, forming solid that eventually close over the top of the flow. The magma within a lava tube is not exposed to the air, so it remains hot and fluid and can flow for tens of km, thus contributing to the large size and low slopes of shield volcanoes. The Hawaiian volcanoes are riddled with thousands of old lava tubes, some as long as 50 km.
Kilauea started forming at approximately 300 ka, while neighbouring Mauna Loa dates back to 700 ka and nearby Mauna Kea ito around 1 Ma. If volcanism continues above the Hawaii mantle plume in the same manner that it has since 85 Ma, it is likely that Kilauea will continue to erupt for at least another 500,000 years. By that time, its neighbour, Loihi, 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 (Figure 4.3.8).
Exercise 4.4 Kilauea’s 2014 lava flow
The U.S. Geological Survey Hawaii Volcano Observatory (HVO) map shown here, dated January 29, 2015, shows the outline of lava that started flowing northeast from Pu’u ’O’o on June 27, 2014 (the “June 27th Lava flow,” a.k.a. the “East Rift Lava Flow”). The flow reached the nearest settlement, Pahoa, on October 29, 124 days later. After damaging some infrastructure west of Pahoa, the flow stopped advancing. A new outbreak occurred November 1, branching out to the north from the main flow.
What is the average rate of advance of the flow front from June 27 to October 29, 2014, in metres per day and metres per hour?
See Appendix 3 for Exercise 4.4 Answers.
Large Igneous Provinces
While the Hawaii mantle plume has produced a relatively low volume of magma for a very long time (~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 is thought that the volcanism leading to (LIP) 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 (CRGB), which extends across Washington, Oregon, and Idaho (Figure 4.3.14). This volcanism, which covered an area of about 160,000 square kilometres (km2) with basaltic rock up to several hundred metres thick, took place between 17 and 14 Ma.
Most other LIP eruptions are much bigger. The Siberian Traps (also basalt), which erupted at the end of the Permian period at 250 Ma, are estimated to have produced approximately 40 times as much lava as the CRBG.
The mantle plume that is assumed to be responsible for the CRBG 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 cubic kilometres (km3) of felsic magma, about 900 times the volume of the 1980 eruption of Mount St. Helens, but only 5% of the volume of mafic magma in the CRBG.
Sea-Floor Volcanism
Some LIP eruptions occur on the sea floor, the largest known being the one that created the Ontong Java plateau in the western Pacific Ocean at around 122 Ma. But most sea-floor volcanism originates at divergent boundaries and involves relatively low-volume eruptions. Under these conditions, hot lava that oozes out into the cold seawater quickly cools on the outside and then behaves a little like toothpaste. The resulting blobs of lava are known as , and they tend to form piles around a sea-floor lava vent (Figure 4.3.15). In terms of area, there is very likely more pillow basalt on the sea floor than any other type of rock on Earth.
Kimberlites
While all of the volcanism discussed so far is thought to originate from partial melting in the upper mantle or within the crust, there is a special class of volcanoes called that have their origins much deeper in the mantle, at depths of 150 km to 450 km. During a kimberlite eruption, material from this depth may make its way to surface quickly (hours to days) with little interaction with the surrounding rocks. As a result, kimberlite eruptive material is representative of mantle compositions: it is ultramafic.
Kimberlite eruptions that originate at depths greater than 200 km, within areas beneath old thick crust (), traverse the region of stability of diamond in the mantle, and in some cases, bring diamond-bearing material to the surface. All of the diamond deposits on Earth are assumed to have formed in this way; an example is the rich Ekati Mine in the Northwest Territories (Figure 4.3.16).
The kimberlites at Ekati erupted between 45 and 60 Ma. Many kimberlites are older, some much older. There have been no kimberlite eruptions in historic times. The youngest known kimberlites are in the Igwisi Hills in Tanzania and are only about 10,000 years old. The next youngest known are dated to about 30 Ma.
How frequently do volcanoes erupt?
The Smithsonian Institution maintains a comprehensive catalogue of the world’s volcanoes, with information and eruptive history for nearly 2700 volcanic sites. If you spend some time looking around that site you’ll discover the frequency of eruptions at different volcanoes is enormously variable, although we can make some generalizations. Focusing just on shield volcanoes and composite volcanoes some of the data are as follows:
| Composite volcanoes | Shield volcanoes |
|---|---|
| Avachinsky (Russia): 5 eruptions over the past 7000 years | Fernandina (Galapagos): 31 eruptions over the past 1000 years |
| Pinatubo (Philippines): 4 eruptions over the past 9000 years | Kilauea (Hawaii): 62 eruptions over the past 250 years |
| Adams (Oregon, USA): 6 eruptions over the past 7000 years | Nyamuragira (Congo): 48 eruptions over the past 154 years |
Based only on these numbers it is evident that, in general, shield volcanoes are much more active than composite volcanoes, but there are many exceptions to this trend. Some composite volcanoes are nearly as active as the shield volcanoes listed here, and some shield volcanoes that are still considered to be “active” are almost as inactive as the composite volcanoes listed here.
Image Descriptions
Figure 4.3.4 image description: Mount St. Helens rises over 2.5 kilometres above sea lever and consists mostly of rock less than 3,000 years old. Underneath the mountain is older volcanic rock. Just below sea level is a small magma chamber, which is a probable reservoir for 1981 and later eruptions. Down 5 to 14 kilometres below sea level is the main magma chamber. Variations in the composition of the erupted magma imply this chamber is stratified, with more magma at the bottom. [Return to Figure 4.3.4]
Figure 4.3.5 image description: Image (A) shows a cliff wall with grey/brown and orange horizontal layers. The sides look soft like they would be easily worn away. The grey/brown layers are lahar deposits and the orange layers are felsic pyroclastic deposits. Image (B) shows a columnar basalt lava flow which looks like a rocky, stone cliff with vertical layers. [Return to Figure 4.3.5]
Figure 4.3.13 image description: The U.S. Geological Survey Hawaii Volcano Observatory (HVO) map, dated January 29, 2015, shows the outline of lava that started flowing northeast from Pu’u ’O’o on June 27, 2004 (the “June 27th Lava flow,” a.k.a. the “East Rift Lava Flow”). The flow reached the nearest settlement, Pahoa, on October 29, after covering a distance of 20 km in 124 days. After damaging some infrastructure west of Pahoa, the flow stopped advancing. A new outbreak occurred November 1, branching out to the north from the main flow about 6 km southwest of Pahoa. [Return to Figure 4.3.13]
Figure 4.3.14 image description: The Columbia River Basalt Group covers most of south eastern Washington state and stretches along the borders between Washington, Idaho, and Oregon. The columnar basalts shown in the photo are in in eastern Washington. They rise up out of a flat valley as tall cliffs. [Return to Figure 4.3.14]
Media Attributions
- Figure 4.3.1: © Steven Earle. CC BY.
- Figure 4.3.2: Eve Cone © nass5518. CC BY.
- Figure 4.3.3: © Steven Earle. CC BY.
- Figure 4.3.4: Original image © Pringle, 1993. Modified by Steve Earle.
- Figure 4.3.5: © Steven Earle. CC BY.
- Figure 4.3.6: © Steven Earle. CC BY.
- Figure 4.3.7: © Steven Earle. CC BY.
- Figure 4.3.8: “Hawaii hotspot cross-sectional diagram” by USGS. Public domain.
- Figure 4.3.9: “Kilauea ali 2012 01 28” by NASA. Public domain.
- Figure 4.3.10: © Steven Earle. CC BY.
- Figure 4.3.11: Island of Hawai’i – Landsat mosaic by NOAA. Public domain. Modified by Steven Earle.
- Figure 4.3.12: © Steven Earle. CC BY.
- Figure 4.3.13: Image from USGS. Public domain.
- Figure 4.3.14: © Steven Earle. CC BY.
- Figure 4.3.15 (Left): Pillow Basalt Crop by NOAA. Public domain.
- Figure 4.3.15 (Right): © Steven Earle. CC BY.
- Figure 4.3.16: “Ekati mine” © Jason Pineau. CC BY-SA.
- 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. V. ↵
As we discussed in Chapter 10, oceanic crust is formed at sea-floor spreading ridges from magma generated by decompression melting of hot upward-moving mantle rock (Figure 10.4.3). About 10% of the mantle rock melts under these conditions, producing mafic magma. This magma oozes out onto the sea floor to form pillow basalts (Figure 18.0.1), breccias (fragmented basaltic rock), and flows, interbedded in some cases with limestone or chert. Beneath the volcanic rock are layers with gabbroic sheeted dykes (which sometimes extend up into the pillow layer), gabbroic stocks, and finally layered peridotite (ultramafic rock) at the base. The ultramafic rock of the mantle lies below that. Over time, the igneous rock of the oceanic crust gets covered with layers of sediment, which eventually become sedimentary rock, including limestone, mudstone, chert, and turbidites. The lithologies of the layers of the oceanic crust are shown in Figure 18.2.1.
The age of the oceanic crust has been determined by systematic mapping variations in the strength of the Earth’s magnetic field across the sea floor and comparing the results with our understanding of the record of Earth’s magnetic field reversal chronology for the past few hundred million years. The ages of different parts of the crust are shown in Figure 18.2.2. The oldest oceanic crust is around 280 Ma in the eastern Mediterranean, and the oldest parts of the open ocean are around 180 Ma on either side of the north Atlantic. It may be surprising, considering that parts of the continental crust are close to 4,000 Ma old, that the oldest sea floor is less than 300 Ma. Of course, the reason for this is that all sea floor older than that has been either subducted or pushed up to become part of the continental crust. For example, there are fragments of sea floor in British Columbia that date back to around 380 and 220 Ma, and there are similar rocks in the Canadian Shield that are older than 3 Ga.
As one would expect, the oceanic crust is very young near the spreading ridges (Figure 18.2.2), and there are obvious differences in the rate of sea-floor spreading along different ridges. The ridges in the Pacific and southeastern Indian Oceans have wide age bands, indicating rapid spreading (approaching 10 centimetres per year (cm/y) on each side in some areas), while those in the Atlantic and western Indian Oceans are spreading much more slowly (less than 2 cm/y on each side in some areas).
This map shows the magnetic patterns on the Juan de Fuca plate. The coloured bands represent periods of normal magnetism, while the white bands represent reversed magnetism. A magnetic-reversal time scale is also shown.
- How old is the oldest part of the Juan de Fuca Plate that is subducting along the Cascadia subduction boundary?
- How old is the youngest part of the Juan de Fuca Plate that is subducting?
The magnetic patterns and chronology shown here have been colour-coded to make them easy to interpret, but on most such maps the magnetic patterns are shown only as black and white stripes, making it much more difficult to interpret the ages of the sea floor. Magnetic-reversal patterns that have no context (such as the 0 age along the spreading ridge in this case) are very difficult to interpret.
As is evident from Figures 18.1.1 and 18.1.2, the sea floor is dotted with chains of seamounts, isolated seamounts, and ocean islands. Almost all of these features are volcanoes, and most are much younger than the oceanic crust on which they formed. Some seamounts and ocean islands are formed above mantle plumes, the best example being Hawaii. The oldest of the Hawaiian/Emperor seamounts is dated at around 80 Ma; it is situated on oceanic crust aged around 90 to 100 Ma. The youngest of the Hawaiian lavas—at Kilauea Volcano on the island of Hawaii—is now more than a year old (last eruption was April 30th 2018). The island is surrounded by oceanic crust that is around 85 Ma old. All of the mantle-plume-derived volcanic islands are dominated by mafic rocks.
Many seamounts are related to subduction along ocean-ocean convergent boundaries. These include the Aleutians, extending from Alaska to Russia, and the Lesser Antilles in the eastern part of the Caribbean.
Some of the linear belts of volcanoes in the Pacific Ocean do not show age-distance relationships like the volcanoes of the Hawaii-Emperor chain or the Galapagos Islands. For example, the Line Islands, which spread out over more than 1,000 kilometres south of the Hawaiian chain, were all formed between 70 and 85 Ma and are interpreted to be related to rifting.
Most tropical islands have associated carbonate reefs, in some cases, as fringes right around the island, and in some cases, as barriers some distance away. In many cases, the reef is there, but the island that is assumed to have led to its formation is gone. The formation of , , and is illustrated in Figure 18.2.4.
The key factor in this process is sea-level change, either because of post-glacial sea-level rise, or because of subsidence of a volcano — as it is moved away from a spreading ridge — or both. If the rate of sea-level change is slow enough (e.g., less than 1 cm/year), a reef can keep up and maintain its position at sea level long after its parent volcanic island has disappeared beneath the waves.
Image Descriptions
Figure 18.2.3 image description: The Juan de Fuca plate lies between the Pacific Plate and the North America Plate along the west coast of Vancouver Island and Washington State. The Juan de Fuca Plate is subducting under the North America Plate along the Cascadia subduction boundary. The Juan de Fuca Plate is youngest along the Juan de Fuca ridge at the Pacific Plate and is older as it moves east. The magnetic time scales shows periods of magnetic reversal, and the ages of the parts of the Juan de Fuca plate that are subducting range from just over 0 Ma in the northwest corner of the plate to over 8 Ma in the southeast corner of the plate. [Return to Figure 18.2.3]
Figure 18.2.4 image description: A volcanic island forms and a fringing reef develops around it in the water. It becomes a barrier reef as the volcanic island subsides and water is able to pool between the island and the reef to form a lagoon. An atoll is formed when the volcano subsides enough that it no longer breaches the ocean surface but the reef remains to form a pool. [Return to Figure 18.2.4]
Media Attributions
- Figures 18.2.1, 18.2.3, 18.2.4: © Steven Earle. CC BY.
- Figure 18.2.2: "Age of oceanic lithosphere" © National Oceanic and Atmospheric Administration. Adapted by Steven Earle. CC BY-SA.
Except within a few kilometres of a ridge crest, where the volcanic rock is still relatively young, most parts of the sea floor are covered in sediments. This material comes from several different sources and is highly variable in composition, depending on proximity to a continent, water depth, ocean currents, biological activity, and climate. Sea-floor sediments (and sedimentary rocks) can range in thickness from a few millimetres to several tens of kilometres. Near the surface, the sea-floor sediments remain unconsolidated, but at depths of hundreds to thousands of metres (depending on the type of sediment and other factors) the sediment becomes lithified.
The various sources of sea-floor sediment can be summarized as follows:
- sediment is derived from continental sources transported by rivers, wind, ocean currents, and glaciers. It is dominated by quartz, feldspar, clay minerals, iron oxides, and terrestrial organic matter.
- Pelagic carbonate sediment is derived from organisms (e.g., ) living in the ocean water (at various depths, but mostly near surface) that make their shells (a.k.a. ) out of carbonate minerals such as calcite.
- Pelagic silica sediment is derived from marine organisms (e.g., and ) that make their tests out of silica (microcrystalline quartz).
- Volcanic ash and other volcanic materials are derived from both terrestrial and submarine eruptions.
- Iron and manganese nodules form as direct precipitates from ocean-bottom water.
The distributions of some of these materials around the seas are shown in Figure 18.3.1. Terrigenous sediments predominate near the continents and within inland seas and large lakes. These sediments tend to be relatively coarse, typically containing sand and silt, but in some cases even pebbles and cobbles. Clay settles slowly in nearshore environments, but much of the clay is dispersed far from its source areas by ocean currents. Clay minerals are predominant over wide areas in the deepest parts of the ocean, and most of this clay is terrestrial in origin. Siliceous oozes (derived from radiolaria and diatoms) are common in the south polar region, along the equator in the Pacific, south of the Aleutian Islands, and within large parts of the Indian Ocean. Carbonate oozes are widely distributed in all of the oceans within equatorial and mid-latitude regions. In fact, clay settles everywhere in the oceans, but in areas where silica- and carbonate-producing organisms are prolific, they produce enough silica or carbonate sediment to dominate over clay.
Carbonate sediments are derived from a wide range of near-surface pelagic organisms that make their shells out of carbonate (Figure 18.3.2). These tiny shells, and the even tinier fragments that form when they break into pieces, settle slowly through the water column, but they don’t necessarily make it to the bottom. While calcite is insoluble in surface water, its solubility increases with depth (and pressure) and at around 4,000 metres, the carbonate fragments dissolve. This depth, which varies with latitude and water temperature, is known as the , or CCD. As a result, carbonate oozes are absent from the deepest parts of the ocean (deeper than 4,000 metres), but they are common in shallower areas such as the mid-Atlantic ridge, the East Pacific Rise (west of South America), along the trend of the Hawaiian/Emperor Seamounts (in the northern Pacific), and on the tops of many isolated seamounts.
Exercise 18.3 What type of sediment?
The diagram shows the sea floor in an area where there is abundant pelagic carbonate sediment. There is a continent within 100 kilometres of this area, to the right. What type of sediment (coarse terrigenous, clay, siliceous ooze, or carbonate ooze) would you expect at find at locations a, b, c, and d?
See Appendix 3 for Exercise 18.3 answers.
All terrestrial erosion products include a small proportion of organic matter derived mostly from terrestrial plants. Tiny fragments of this material plus other organic matter from marine plants and animals accumulate in terrigenous sediments, especially within a few hundred kilometres of shore. As the sediments pile up, the deeper parts start to warm up (from geothermal heat), and bacteria get to work breaking down the contained organic matter. Because this is happening in the absence of oxygen (a.k.a. conditions), the by-product of this metabolism is the gas methane (CH4). Methane released by the bacteria slowly bubbles upward through the sediment toward the sea floor.
At water depths of 500 metres to 1,000 metres, and at the low temperatures typical of the sea floor (close to 4°C), water and methane combine to create a substance known as . Within a few metres to hundreds of metres of the sea floor, the temperature is low enough for methane hydrate to be stable and hydrates accumulate within the sediment (Figure 18.3.4). Methane hydrate is flammable because when it is heated, the methane is released as a gas (Figure 18.3.4). The methane within sea-floor sediments represents an enormous reservoir of fossil fuel energy. Although energy corporations and governments are anxious to develop ways to produce and sell this methane, anyone that understands the climate-change implications of its extraction and use can see that this would be folly. As we’ll see in the discussion of climate change in Chapter 19, sea-floor methane hydrates have had significant impacts on the climate in the distant past.
Image Descriptions
Figure 18.3.3 image description: A. is farthest from the continent. D is closest to the continent.
- A depth of 4.5 kilometres.
- A depth of 3.5 kilometres.
- A depth of 5 kilometres.
- A depth of 1 kilometre, close to the edge of a continent.
Media Attributions
- Figure 18.3.1, 18.3.2, 18.3.3: © Steven Earle. CC B
- Figure 18.3.4 (Left): "Gashydrat im Sediment" © Wusel007. CC BY-SA.
- Figure 18.3.4 (Right): "Burning Gas Hydrates" by J. Pinkston and L. Stern (USGS). Public domain.
referring to sedimentary particles that originated on a continent
a single-celled protist with a shell that is typically made of CaCO3
the shell-like hard parts (either silica or carbonate) of small organisms such as radiolarian and foraminifera
photosynthetic algae that make their tests (shells) from silica
microscopic (0.1 to 0.2 millimetres) marine protozoa that produce silica shells
the depth in the ocean (typically around 4000 metres) below which carbonate minerals are soluble
a combination of water ice and methane in which the methane is trapped inside “cages” in the ice
Except within a few kilometres of a ridge crest, where the volcanic rock is still relatively young, most parts of the sea floor are covered in sediments. This material comes from several different sources and is highly variable in composition, depending on proximity to a continent, water depth, ocean currents, biological activity, and climate. Sea-floor sediments (and sedimentary rocks) can range in thickness from a few millimetres to several tens of kilometres. Near the surface, the sea-floor sediments remain unconsolidated, but at depths of hundreds to thousands of metres (depending on the type of sediment and other factors) the sediment becomes lithified.
The various sources of sea-floor sediment can be summarized as follows:
- sediment is derived from continental sources transported by rivers, wind, ocean currents, and glaciers. It is dominated by quartz, feldspar, clay minerals, iron oxides, and terrestrial organic matter.
- Pelagic carbonate sediment is derived from organisms (e.g., ) living in the ocean water (at various depths, but mostly near surface) that make their shells (a.k.a. ) out of carbonate minerals such as calcite.
- Pelagic silica sediment is derived from marine organisms (e.g., and ) that make their tests out of silica (microcrystalline quartz).
- Volcanic ash and other volcanic materials are derived from both terrestrial and submarine eruptions.
- Iron and manganese nodules form as direct precipitates from ocean-bottom water.
The distributions of some of these materials around the seas are shown in Figure 18.3.1. Terrigenous sediments predominate near the continents and within inland seas and large lakes. These sediments tend to be relatively coarse, typically containing sand and silt, but in some cases even pebbles and cobbles. Clay settles slowly in nearshore environments, but much of the clay is dispersed far from its source areas by ocean currents. Clay minerals are predominant over wide areas in the deepest parts of the ocean, and most of this clay is terrestrial in origin. Siliceous oozes (derived from radiolaria and diatoms) are common in the south polar region, along the equator in the Pacific, south of the Aleutian Islands, and within large parts of the Indian Ocean. Carbonate oozes are widely distributed in all of the oceans within equatorial and mid-latitude regions. In fact, clay settles everywhere in the oceans, but in areas where silica- and carbonate-producing organisms are prolific, they produce enough silica or carbonate sediment to dominate over clay.
Carbonate sediments are derived from a wide range of near-surface pelagic organisms that make their shells out of carbonate (Figure 18.3.2). These tiny shells, and the even tinier fragments that form when they break into pieces, settle slowly through the water column, but they don’t necessarily make it to the bottom. While calcite is insoluble in surface water, its solubility increases with depth (and pressure) and at around 4,000 metres, the carbonate fragments dissolve. This depth, which varies with latitude and water temperature, is known as the , or CCD. As a result, carbonate oozes are absent from the deepest parts of the ocean (deeper than 4,000 metres), but they are common in shallower areas such as the mid-Atlantic ridge, the East Pacific Rise (west of South America), along the trend of the Hawaiian/Emperor Seamounts (in the northern Pacific), and on the tops of many isolated seamounts.
Exercise 18.3 What type of sediment?
The diagram shows the sea floor in an area where there is abundant pelagic carbonate sediment. There is a continent within 100 kilometres of this area, to the right. What type of sediment (coarse terrigenous, clay, siliceous ooze, or carbonate ooze) would you expect at find at locations a, b, c, and d?
See Appendix 3 for Exercise 18.3 answers.
All terrestrial erosion products include a small proportion of organic matter derived mostly from terrestrial plants. Tiny fragments of this material plus other organic matter from marine plants and animals accumulate in terrigenous sediments, especially within a few hundred kilometres of shore. As the sediments pile up, the deeper parts start to warm up (from geothermal heat), and bacteria get to work breaking down the contained organic matter. Because this is happening in the absence of oxygen (a.k.a. conditions), the by-product of this metabolism is the gas methane (CH4). Methane released by the bacteria slowly bubbles upward through the sediment toward the sea floor.
At water depths of 500 metres to 1,000 metres, and at the low temperatures typical of the sea floor (close to 4°C), water and methane combine to create a substance known as . Within a few metres to hundreds of metres of the sea floor, the temperature is low enough for methane hydrate to be stable and hydrates accumulate within the sediment (Figure 18.3.4). Methane hydrate is flammable because when it is heated, the methane is released as a gas (Figure 18.3.4). The methane within sea-floor sediments represents an enormous reservoir of fossil fuel energy. Although energy corporations and governments are anxious to develop ways to produce and sell this methane, anyone that understands the climate-change implications of its extraction and use can see that this would be folly. As we’ll see in the discussion of climate change in Chapter 19, sea-floor methane hydrates have had significant impacts on the climate in the distant past.
Image Descriptions
Figure 18.3.3 image description: A. is farthest from the continent. D is closest to the continent.
- A depth of 4.5 kilometres.
- A depth of 3.5 kilometres.
- A depth of 5 kilometres.
- A depth of 1 kilometre, close to the edge of a continent.
Media Attributions
- Figure 18.3.1, 18.3.2, 18.3.3: © Steven Earle. CC BY.
- Figure 18.3.4 (Left): "Gashydrat im Sediment" © Wusel007. CC BY-SA.
- Figure 18.3.4 (Right): "Burning Gas Hydrates" by J. Pinkston and L. Stern (USGS). Public domain.
As everyone knows, seawater is salty. It is that way because the river water that flows into the oceans contains small amounts of dissolved ions, and for the most part, the water that comes out of the oceans is the pure water that evaporates from the surface. Billions of years of a small amount of salt going into the ocean—and none coming out (most of the time)—has made the water salty. The salts of the ocean (dominated by sodium, chlorine, and sulphur) (Figure 18.4.1) are there because they are very soluble and they aren’t consumed by biological processes (most of the calcium, for example, is used by organisms to make carbonate minerals). If salts are always going into the ocean, and never coming out, one might assume that the oceans have been continuously getting saltier over geological time. In fact this appears not to be the case. There is geological evidence that Earth’s oceans became salty early during the Archaean, and that at times in the past, they have been at least half again as salty as they are now. This implies that there must be a mechanism to remove salt from the oceans, and that mechanism is the isolation of some parts of the ocean into seas (such as the Mediterranean) and the eventual evaporation of those seas to create salt beds that become part of the crust. The Middle Devonian Prairie Evaporite Formation of Saskatchewan and Manitoba is a good example of this.
The average salinity of the oceans is 35 g of salt per litre of water, but there are significant regional variations in this value, as shown in Figure 18.4.2. Ocean water is least salty (around 31 g/L) in the Arctic, and also in several places where large rivers flow in (e.g., the Ganges/Brahmaputra and Mekong Rivers in southeast Asia, and the Yellow and Yangtze Rivers in China). Ocean water is most salty (over 37 g/L) in some restricted seas in hot dry regions, such as the Mediterranean and Red Seas. You might be surprised to know that, in spite of some massive rivers flowing into it (such as the Nile and the Danube), water does not flow out of the Mediterranean Sea into the Atlantic. There is so much evaporation happening in the Mediterranean basin that water flows into it from the Atlantic, through the Strait of Gibraltar.
In the open ocean, salinities are elevated at lower latitudes because this is where most evaporation takes place. The highest salinities are in the subtropical parts of the Atlantic, especially north of the equator. The northern Atlantic is much more saline than the north Pacific because the Gulf Stream current brings a massive amount of salty water from the tropical Atlantic and the Caribbean to the region around Britain, Iceland, and Scandinavia. The salinity in the Norwegian Sea (between Norway and Iceland) is substantially higher than that in other polar areas.
Exercise 18.4 Salt chuck
How salty is the sea? If you’ve ever had a swim in the ocean, you’ve probably tasted it. To understand how salty the sea is, start with 250 mL of water (1 cup). There is 35 g of salt in 1 L of seawater so in 250 mL (1/4 litre) there is 35/4 = 8.75 or ~9 g of salt. This is just short of 2 teaspoons, so it would be close enough to add 2 level teaspoons of salt to the cup of water. Then stir until it’s dissolved. Have a taste!
Of course, if you used normal refined table salt, then what you added was almost pure NaCl. To get the real taste of seawater you would want to use some evaporated seawater salt (a.k.a. sea salt), which has a few percent of magnesium, sulphur, and calcium plus some trace elements.
See Appendix 3 for Exercise 18.4 answers.
Not unexpectedly, the oceans are warmest near the equator—typically 25° to 30°C—and coldest near the poles—around 0°C (Figure 18.4.4). (Sea water will remain unfrozen down to about -2°C.) At southern Canadian latitudes, average annual water temperatures are in the 10° to 15°C range on the west coast and in the 5° to 10°C range on the east coast. Variations in sea-surface temperatures (SST) are related to redistribution of water by ocean currents, as we’ll see below. A good example of that is the plume of warm Gulf Stream water that extends across the northern Atlantic. St. John’s, Newfoundland, and Brittany in France are at about the same latitude (47.5° N), but the average SST in St. John’s is a frigid 3°C, while that in Brittany is a reasonably comfortable 15°C.
Currents in the open ocean are created by wind moving across the water and by density differences related to temperature and salinity. An overview of the main ocean currents is shown in Figure 18.4.5. As you can see, the northern hemisphere currents form circular patterns (gyres) that rotate clockwise, while the southern hemisphere gyres are counter-clockwise. This happens for the same reason that the water in your northern hemisphere sink rotates in a clockwise direction as it flows down the drain; this is caused by the Coriolis effect.
Because the ocean basins aren’t like bathroom basins, not all ocean currents behave the way we would expect. In the North Pacific, for example, the main current flows clockwise, but there is a secondary current in the area adjacent to our coast—the Alaska Current—that flows counter-clockwise, bringing relatively warm water from California, past Oregon, Washington, and B.C. to Alaska. On Canada’s eastern coast, the cold Labrador Current flows south past Newfoundland, bringing a stream of icebergs past the harbour at St. John’s (Figure 18.4.7). This current helps to deflect the Gulf Stream toward the northeast, ensuring that Newfoundland stays cool, and western Europe stays warm.
Exercise 18.5 Understanding the Coriolis effect
The Coriolis effect has to do with objects that are moving in relation to other objects that are rotating. An ocean current is moving across the rotating Earth, and its motion is controlled by the Coriolis effect.
Imagine that you are standing on the equator looking straight north and you fire a gun in that direction. The bullet in the gun starts out going straight north, but it also has a component of motion toward the east that it gets from Earth’s rotation, which is 1,670 kilometres per hour at the equator. Because of the spherical shape of Earth, the speed of rotation away from the equator is not as fast as it is at the equator (in fact, the Earth’s rotational speed is 0 kilometres per hour at the poles) so the bullet actually traces a clockwise curved path across Earth’s surface, as shown by the red arrow on the diagram. In the southern hemisphere the Coriolis effect is counterclockwise (green arrow).
The Coriolis effect is imparted to the rotations of ocean currents and tropical storms. If Earth were a rotating cylinder, instead of a sphere, there would be no Coriolis effect.
See Appendix 3 for Exercise 18.5 answers.
The currents shown in Figure 18.4.5 are all surface currents, and they only involve the upper few hundred metres of the oceans. But there is much more going on underneath. The Gulf Stream, for example, which is warm and saline, flows past Britain and Iceland into the Norwegian Sea (where it becomes the Norwegian Current). As it cools down, it becomes denser, and because of its high salinity, which also contributes to its density, it starts to sink beneath the surrounding water (Figure 18.4.8). At this point, it is known as North Atlantic Deep Water (NADW), and it flows to significant depth in the Atlantic as it heads back south. Meanwhile, at the southern extreme of the Atlantic, very cold water adjacent to Antarctica also sinks to the bottom to become Antarctic Bottom Water (AABW) which flows to the north, underneath the NADW.
The descent of the dense NADW is just one part of a global system of seawater circulation, both at surface and at depth, as illustrated in Figure 18.4.9. 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 in the Indian Ocean between Africa and India, and in the Pacific Ocean, north of the equator.
The thermohaline circulation is critically important to the transfer of heat on Earth. It brings warm water from the tropics to the poles, and cold water from the poles to the tropics, thus keeping polar regions from getting too cold and tropical regions from getting too hot. A reduction in the rate of thermohaline circulation would lead to colder conditions and enhanced formation of sea ice at the poles. This would start a positive feedback process that could result in significant global cooling. There is compelling evidence to indicate that there were major changes in thermohaline circulation, corresponding with climate changes, during the Pleistocene Glaciation.
Image Descriptions
Figure 18.4.5 image description: The currents of the world's oceans work together to form a number of general patterns. Currents flow into each other to form larger currents. Groups of currents in the northern hemisphere flow clockwise. This includes groups of currents in the North Pacific Ocean and the North Atlantic Ocean. Currents in the southern hemisphere flow counter-clockwise. This includes groups of currents in the South Pacific Ocean, the South Atlantic Ocean, and the Indian Ocean. Currents flowing towards the equator are colder than the surrounding water. Currents flowing away from the equator are warmer than the surrounding water. Currents below 60° South flow from east to west (or west to east) around Antarctica. Currents along the Equator also flows east to west (or west to east). Currents flowing from east to west (or west to east) are the same temperature as the surrounding water. For a more detailed description of specific currents, refer to the following table, which describes 26 major currents, including their location, direction of flow, and relationship to surrounding currents. They are arranged in alphabetical order. Or, you can [Return to Figure 18.4.5].
| Name of Current | Temperature of current compared to the surrounding water | Direction of flow | Relationship to nearby currents |
|---|---|---|---|
| Agulhas | A warm current | The Agulhas current flows south from the Arabian peninsula down the east coast of Africa. | The Agulhas current joins with the Mozambique current, which also flows south. |
| Alaska | A warm current | The Alaska current flows north up the west coast of the United States and Canada before circling to the west once it reaches Alaska | The Alaska current flows into the Oyashio current |
| Antarctic Circumpolar | No temperature difference | The Antarctic Circumpolar current flows east to circle around Antarctica. | The Antarctic Circumpolar flows east above the Antarctic Subpolar, which flows west. |
| Antarctic Subpolar | No temperature difference | The Antarctic Subpolar current flows west along the coast of Antarctica. | The Antarctic Subpolar current flows west below the Antarctic Circumpolar current, which flows east. |
| Benguela | A cold current | The Benguela current flows north along the south west coast of Africa. | The Benguela current flows into the South Equatorial current and is fed by the South Atlantic current. |
| Brazil | A warm current | The Brazil current flows south along the east coast of South America. | The Brazil current flows into the South Atlantic current and is fed by the south branch of the South Equatorial current. |
| California | A cold current | The California current flows south from the southwest coast of the United States down along the west coast of Mexico. | The California current flows into the North Equatorial current and is fed by the North Pacific current. |
| Canary | A cold current | The Canary current flows from south along the north west coast of Africa from Morocco to Sengal. | The Canary current flows into the North Equatorial current and is fed by the North Atlantic Drift. |
| East Australian | A warm current | The East Australian currents flow from the equator and south, past the east coast of Australia. One flows between New Zealand and Australia, and the other flows past the east side of New Zealand. | The East Australian currents flow into the South Pacific current and they are fed by the South Equatorial current. |
| East Greenland | A cold current | The East Greenland current flows south along the east coast of Greenland. | The East Greenland current flows into the Labrador current. |
| Equatorial Counter | No temperature difference. | The Equatorial Counter current flows east along the equator. It is broken up into three sections: One in the Pacific Ocean, one in the Atlantic Ocean, and one in the Indian Ocean. | The Equatorial Counter current flows east between the North Equatorial and the South Equatorial currents, which both flow west. |
| Gulf Stream | A warm current | The Gulf Stream flows north from the Caribbean along the east coast of the United States. | The Gulf Stream flows into the North Atlantic Drift current and is fed by the North Equatorial current. |
| Kuroshio | A warm current | The Kuroshio current flows north along the east coast of the Philippines and Japan. | The Kuroshio current flows into the North Pacific current and is fed by the North Equatorial current. |
| Labrador | A cold current | The Labrador current flows south along the eastern coast of Canada to the northern United States. | The Labrador current is partially fed by the East Greenland current. Once it reaches the northern United States, it flows past the Gulf Stream. |
| Mozambique | A warm current | The Mozambique current flows south along the east coast of Madagascar and into the Southern Ocean. | The Mozambique current flows into the South Indian current and is fed by the South Equatorial current. |
| North Atlantic Drift | No temperature difference | The North Atlantic Drift current flows east across the Atlantic Ocean from the north coast of the United States to the south coast of Spain. | The North Atlantic Drift current splits to flow north into the Norwegian current and to flow south into the Canary current. It is fed by the Gulf Stream. |
| North Equatorial | No temperature difference. | The North Equatorial current flows west just above the equator. It is broken up into three sections: One in the Pacific Ocean, one in the Atlantic Ocean, and one in the Indian Ocean. | The North Equatorial current in the Pacific Ocean flows into the Kuroshio current and is fed by the California current. The North Equatorial current in the Atlantic Ocean flows into the Gulf Stream and is fed by the Canary current. The North Equatorial current in the Indian Ocean turns at Africa to join the Equatorial Counter current. |
| North Pacific | No temperature difference | The North Pacific current flows west across the Pacific Ocean from Japan to the south coast of the United States. | The North Pacific current flows into the California current and is fed by the Kuroshio current. |
| Norwegian | A warm current | The Norwegian current flows north from the north coast of the United Kingdom to along the coast of Norway. | This current is fed by the northern branch of the North Atlantic Drift current and flows into the Arctic Ocean. |
| Oyashio | A cold current | The Oyashio current flows south along the east coast of Russia. | The Oyashio current clashes with the Kuroshio current, which flows north into the North Pacific current. |
| Peru | A cold current | The Peru current flows north along the central west coast of South America. | The Peru current flows into the South Equatorial current and is fed by the South Pacific current. |
| South Atlantic | No temperature difference | The South Atlantic current flows from the south tip of South America to towards the south tip of Africa. | The South Atlantic current flows into the Benguela current and is fed by the Brazil current. |
| South Equatorial | No temperature difference | The South Equatorial current flows west just below the equator. It is broken up into three sections: One in the Pacific Ocean, one in the Atlantic Ocean, and one in the Indian Ocean. | The South Equatorial current in the Pacific Ocean flows into the East Australian currents and is fed by the Peru current. The South Equatorial current in the Atlantic Ocean flows north and south: North along the north east coast of South America and south into the Brazil current. It is fed by the Benguela current. The South Equatorial current in the Indian Ocean flows into the Mozambique current and is fed by the West Australian current. |
| South Indian | No temperature difference | The South Indian current flows from the southern part of the Indian ocean towards the south west coast of Australia. | The South Indian current flows into the West Australian current and is fed by the Mozambique current. |
| South Pacific | No temperature difference | The South Pacific current flows east from the south east coast of Australia to the south west coast of South America. | The South Pacific current flows into the Peru current and is fed by the East Australian current. |
| West Australian current | A cold current | The West Australian current flows north along the west coast of Australia. | The West Australian current flows into the South Equatorial current and is fed by the South Indian current. |
Media Attributions
- Figure 18.4.1: © Steven Earle. CC BY.
- Figure 18.4.2: "WOA09 sea-surf SAL AYool" © Plumbago. CC BY-SA.
- Figure 18.4.3: © Steven Earle. CC BY.
- Figure 18.4.4: "WOA09 sea-surf TMP AYool" © Plumbago. CC BY-SA.
- Figure 18.4.5: "Corrientes Oceanicas" by Dr. Michael Pidwirny. Public domain.
- Figure 18.4.6: "Newfoundland Iceberg just off Exploits Island" © Shawn. CC BY-SA.
- Figures 18.4.7, 18.4.7: © Steven Earle. CC BY.
- Figure 18.4.9: "Thermohaline Circulation" by NASA. Public domain.
a closed circular ocean current
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
(NADW) deep Atlantic Ocean water that has descended in the far north of the basin in the area between Scandinavia and Greenland
(AABW) water at abyssal depths in the ocean that forms from the sinking of dense cold water adjacent to Antarctica
As everyone knows, seawater is salty. It is that way because the river water that flows into the oceans contains small amounts of dissolved ions, and for the most part, the water that comes out of the oceans is the pure water that evaporates from the surface. Billions of years of a small amount of salt going into the ocean—and none coming out (most of the time)—has made the water salty. The salts of the ocean (dominated by sodium, chlorine, and sulphur) (Figure 18.4.1) are there because they are very soluble and they aren’t consumed by biological processes (most of the calcium, for example, is used by organisms to make carbonate minerals). If salts are always going into the ocean, and never coming out, one might assume that the oceans have been continuously getting saltier over geological time. In fact this appears not to be the case. There is geological evidence that Earth’s oceans became salty early during the Archaean, and that at times in the past, they have been at least half again as salty as they are now. This implies that there must be a mechanism to remove salt from the oceans, and that mechanism is the isolation of some parts of the ocean into seas (such as the Mediterranean) and the eventual evaporation of those seas to create salt beds that become part of the crust. The Middle Devonian Prairie Evaporite Formation of Saskatchewan and Manitoba is a good example of this.
The average salinity of the oceans is 35 g of salt per litre of water, but there are significant regional variations in this value, as shown in Figure 18.4.2. Ocean water is least salty (around 31 g/L) in the Arctic, and also in several places where large rivers flow in (e.g., the Ganges/Brahmaputra and Mekong Rivers in southeast Asia, and the Yellow and Yangtze Rivers in China). Ocean water is most salty (over 37 g/L) in some restricted seas in hot dry regions, such as the Mediterranean and Red Seas. You might be surprised to know that, in spite of some massive rivers flowing into it (such as the Nile and the Danube), water does not flow out of the Mediterranean Sea into the Atlantic. There is so much evaporation happening in the Mediterranean basin that water flows into it from the Atlantic, through the Strait of Gibraltar.
In the open ocean, salinities are elevated at lower latitudes because this is where most evaporation takes place. The highest salinities are in the subtropical parts of the Atlantic, especially north of the equator. The northern Atlantic is much more saline than the north Pacific because the Gulf Stream current brings a massive amount of salty water from the tropical Atlantic and the Caribbean to the region around Britain, Iceland, and Scandinavia. The salinity in the Norwegian Sea (between Norway and Iceland) is substantially higher than that in other polar areas.
Exercise 18.4 Salt chunk
How salty is the sea? If you’ve ever had a swim in the ocean, you’ve probably tasted it. To understand how salty the sea is, start with 250 mL of water (1 cup). There is 35 g of salt in 1 L of seawater so in 250 mL (1/4 litre) there is 35/4 = 8.75 or ~9 g of salt. This is just short of 2 teaspoons, so it would be close enough to add 2 level teaspoons of salt to the cup of water. Then stir until it’s dissolved. Have a taste!
Of course, if you used normal refined table salt, then what you added was almost pure NaCl. To get the real taste of seawater you would want to use some evaporated seawater salt (a.k.a. sea salt), which has a few percent of magnesium, sulphur, and calcium plus some trace elements.
See Appendix 3 for Exercise 18.4 answers.
Not unexpectedly, the oceans are warmest near the equator—typically 25° to 30°C—and coldest near the poles—around 0°C (Figure 18.4.4). (Sea water will remain unfrozen down to about -2°C.) At southern Canadian latitudes, average annual water temperatures are in the 10° to 15°C range on the west coast and in the 5° to 10°C range on the east coast. Variations in sea-surface temperatures (SST) are related to redistribution of water by ocean currents, as we’ll see below. A good example of that is the plume of warm Gulf Stream water that extends across the northern Atlantic. St. John’s, Newfoundland, and Brittany in France are at about the same latitude (47.5° N), but the average SST in St. John’s is a frigid 3°C, while that in Brittany is a reasonably comfortable 15°C.
Currents in the open ocean are created by wind moving across the water and by density differences related to temperature and salinity. An overview of the main ocean currents is shown in Figure 18.4.5. As you can see, the northern hemisphere currents form circular patterns () that rotate clockwise, while the southern hemisphere gyres are counter-clockwise. This happens for the same reason that the water in your northern hemisphere sink rotates in a clockwise direction as it flows down the drain; this is caused by the .
Because the ocean basins aren’t like bathroom basins, not all ocean currents behave the way we would expect. In the North Pacific, for example, the main current flows clockwise, but there is a secondary current in the area adjacent to our coast—the Alaska Current—that flows counter-clockwise, bringing relatively warm water from California, past Oregon, Washington, and B.C. to Alaska. On Canada’s eastern coast, the cold Labrador Current flows south past Newfoundland, bringing a stream of icebergs past the harbour at St. John’s (Figure 18.4.7). This current helps to deflect the Gulf Stream toward the northeast, ensuring that Newfoundland stays cool, and western Europe stays warm.
Exercise 18.5: Understanding the Coriolis effect
The Coriolis effect has to do with objects that are moving in relation to other objects that are rotating. An ocean current is moving across the rotating Earth, and its motion is controlled by the Coriolis effect.
Imagine that you are standing on the equator looking straight north and you fire a gun in that direction. The bullet in the gun starts out going straight north, but it also has a component of motion toward the east that it gets from Earth’s rotation, which is 1,670 kilometres per hour at the equator. Because of the spherical shape of Earth, the speed of rotation away from the equator is not as fast as it is at the equator (in fact, the Earth’s rotational speed is 0 kilometres per hour at the poles) so the bullet actually traces a clockwise curved path across Earth’s surface, as shown by the red arrow on the diagram. In the southern hemisphere the Coriolis effect is counterclockwise (green arrow).
The Coriolis effect is imparted to the rotations of ocean currents and tropical storms. If Earth were a rotating cylinder, instead of a sphere, there would be no Coriolis effect.
See Appendix 3 for Exercise 18.5 answers.
The currents shown in Figure 18.4.5 are all surface currents, and they only involve the upper few hundred metres of the oceans. But there is much more going on underneath. The Gulf Stream, for example, which is warm and saline, flows past Britain and Iceland into the Norwegian Sea (where it becomes the Norwegian Current). As it cools down, it becomes denser, and because of its high salinity, which also contributes to its density, it starts to sink beneath the surrounding water (Figure 18.4.8). At this point, it is known as (NADW), and it flows to significant depth in the Atlantic as it heads back south. Meanwhile, at the southern extreme of the Atlantic, very cold water adjacent to Antarctica also sinks to the bottom to become (AABW) which flows to the north, underneath the NADW.
The descent of the dense NADW is just one part of a global system of seawater circulation, both at surface and at depth, as illustrated in Figure 18.4.9. 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 in the Indian Ocean between Africa and India, and in the Pacific Ocean, north of the equator.
The thermohaline circulation is critically important to the transfer of heat on Earth. It brings warm water from the tropics to the poles, and cold water from the poles to the tropics, thus keeping polar regions from getting too cold and tropical regions from getting too hot. A reduction in the rate of thermohaline circulation would lead to colder conditions and enhanced formation of sea ice at the poles. This would start a positive feedback process that could result in significant global cooling. There is compelling evidence to indicate that there were major changes in thermohaline circulation, corresponding with climate changes, during the Pleistocene Glaciation.
Image Descriptions
Figure 18.4.5 image description: The currents of the world's oceans work together to form a number of general patterns. Currents flow into each other to form larger currents. Groups of currents in the northern hemisphere flow clockwise. This includes groups of currents in the North Pacific Ocean and the North Atlantic Ocean. Currents in the southern hemisphere flow counter-clockwise. This includes groups of currents in the South Pacific Ocean, the South Atlantic Ocean, and the Indian Ocean. Currents flowing towards the equator are colder than the surrounding water. Currents flowing away from the equator are warmer than the surrounding water. Currents below 60° South flow from east to west (or west to east) around Antarctica. Currents along the Equator also flows east to west (or west to east). Currents flowing from east to west (or west to east) are the same temperature as the surrounding water. For a more detailed description of specific currents, refer to the following table, which describes 26 major currents, including their location, direction of flow, and relationship to surrounding currents. They are arranged in alphabetical order. Or, you can [Return to Figure 18.4.5].
| Name of Current | Temperature of current compared to the surrounding water | Direction of flow | Relationship to nearby currents |
|---|---|---|---|
| Agulhas | A warm current | The Agulhas current flows south from the Arabian peninsula down the east coast of Africa. | The Agulhas current joins with the Mozambique current, which also flows south. |
| Alaska | A warm current | The Alaska current flows north up the west coast of the United States and Canada before circling to the west once it reaches Alaska | The Alaska current flows into the Oyashio current |
| Antarctic Circumpolar | No temperature difference | The Antarctic Circumpolar current flows east to circle around Antarctica. | The Antarctic Circumpolar flows east above the Antarctic Subpolar, which flows west. |
| Antarctic Subpolar | No temperature difference | The Antarctic Subpolar current flows west along the coast of Antarctica. | The Antarctic Subpolar current flows west below the Antarctic Circumpolar current, which flows east. |
| Benguela | A cold current | The Benguela current flows north along the south west coast of Africa. | The Benguela current flows into the South Equatorial current and is fed by the South Atlantic current. |
| Brazil | A warm current | The Brazil current flows south along the east coast of South America. | The Brazil current flows into the South Atlantic current and is fed by the south branch of the South Equatorial current. |
| California | A cold current | The California current flows south from the southwest coast of the United States down along the west coast of Mexico. | The California current flows into the North Equatorial current and is fed by the North Pacific current. |
| Canary | A cold current | The Canary current flows from south along the north west coast of Africa from Morocco to Sengal. | The Canary current flows into the North Equatorial current and is fed by the North Atlantic Drift. |
| East Australian | A warm current | The East Australian currents flow from the equator and south, past the east coast of Australia. One flows between New Zealand and Australia, and the other flows past the east side of New Zealand. | The East Australian currents flow into the South Pacific current and they are fed by the South Equatorial current. |
| East Greenland | A cold current | The East Greenland current flows south along the east coast of Greenland. | The East Greenland current flows into the Labrador current. |
| Equatorial Counter | No temperature difference. | The Equatorial Counter current flows east along the equator. It is broken up into three sections: One in the Pacific Ocean, one in the Atlantic Ocean, and one in the Indian Ocean. | The Equatorial Counter current flows east between the North Equatorial and the South Equatorial currents, which both flow west. |
| Gulf Stream | A warm current | The Gulf Stream flows north from the Caribbean along the east coast of the United States. | The Gulf Stream flows into the North Atlantic Drift current and is fed by the North Equatorial current. |
| Kuroshio | A warm current | The Kuroshio current flows north along the east coast of the Philippines and Japan. | The Kuroshio current flows into the North Pacific current and is fed by the North Equatorial current. |
| Labrador | A cold current | The Labrador current flows south along the eastern coast of Canada to the northern United States. | The Labrador current is partially fed by the East Greenland current. Once it reaches the northern United States, it flows past the Gulf Stream. |
| Mozambique | A warm current | The Mozambique current flows south along the east coast of Madagascar and into the Southern Ocean. | The Mozambique current flows into the South Indian current and is fed by the South Equatorial current. |
| North Atlantic Drift | No temperature difference | The North Atlantic Drift current flows east across the Atlantic Ocean from the north coast of the United States to the south coast of Spain. | The North Atlantic Drift current splits to flow north into the Norwegian current and to flow south into the Canary current. It is fed by the Gulf Stream. |
| North Equatorial | No temperature difference. | The North Equatorial current flows west just above the equator. It is broken up into three sections: One in the Pacific Ocean, one in the Atlantic Ocean, and one in the Indian Ocean. | The North Equatorial current in the Pacific Ocean flows into the Kuroshio current and is fed by the California current. The North Equatorial current in the Atlantic Ocean flows into the Gulf Stream and is fed by the Canary current. The North Equatorial current in the Indian Ocean turns at Africa to join the Equatorial Counter current. |
| North Pacific | No temperature difference | The North Pacific current flows west across the Pacific Ocean from Japan to the south coast of the United States. | The North Pacific current flows into the California current and is fed by the Kuroshio current. |
| Norwegian | A warm current | The Norwegian current flows north from the north coast of the United Kingdom to along the coast of Norway. | This current is fed by the northern branch of the North Atlantic Drift current and flows into the Arctic Ocean. |
| Oyashio | A cold current | The Oyashio current flows south along the east coast of Russia. | The Oyashio current clashes with the Kuroshio current, which flows north into the North Pacific current. |
| Peru | A cold current | The Peru current flows north along the central west coast of South America. | The Peru current flows into the South Equatorial current and is fed by the South Pacific current. |
| South Atlantic | No temperature difference | The South Atlantic current flows from the south tip of South America to towards the south tip of Africa. | The South Atlantic current flows into the Benguela current and is fed by the Brazil current. |
| South Equatorial | No temperature difference | The South Equatorial current flows west just below the equator. It is broken up into three sections: One in the Pacific Ocean, one in the Atlantic Ocean, and one in the Indian Ocean. | The South Equatorial current in the Pacific Ocean flows into the East Australian currents and is fed by the Peru current. The South Equatorial current in the Atlantic Ocean flows north and south: North along the north east coast of South America and south into the Brazil current. It is fed by the Benguela current. The South Equatorial current in the Indian Ocean flows into the Mozambique current and is fed by the West Australian current. |
| South Indian | No temperature difference | The South Indian current flows from the southern part of the Indian ocean towards the south west coast of Australia. | The South Indian current flows into the West Australian current and is fed by the Mozambique current. |
| South Pacific | No temperature difference | The South Pacific current flows east from the south east coast of Australia to the south west coast of South America. | The South Pacific current flows into the Peru current and is fed by the East Australian current. |
| West Australian current | A cold current | The West Australian current flows north along the west coast of Australia. | The West Australian current flows into the South Equatorial current and is fed by the South Indian current. |
Media Attributions
- Figure 18.4.1: © Steven Earle. CC BY.
- Figure 18.4.2: "WOA09 sea-surf SAL AYool" © Plumbago. CC BY-SA.
- Figure 18.4.3: © Steven Earle. CC BY.
- Figure 18.4.4: "WOA09 sea-surf TMP AYool" © Plumbago. CC BY-SA.
- Figure 18.4.5: "Corrientes Oceanicas" by Dr. Michael Pidwirny. Public domain.
- Figure 18.4.6: "Newfoundland Iceberg just off Exploits Island" © Shawn. CC BY-SA.
- Figures 18.4.7, 18.4.7: © Steven Earle. CC BY.
- Figure 18.4.9: "Thermohaline Circulation" by NASA. Public domain.
the deeper parts of the ocean, between 4000 and 6000 metres
The topics covered in this chapter can be summarized as follows:
| Section | Summary |
|---|---|
| 18.1 The Topography of the Sea Floor | The oceans are about 4,000 metres deep on average, but they also have a wide range of topographical features, including shallow continental shelves, continental slopes, continuous ridges related to plate divergence, numerous isolated seamounts, and deep submarine canyons at subduction zones. |
| 18.2 The Geology of the Oceanic Crust | Most oceanic crust forms during sea-floor spreading and is characterized by pillow basalts, sheeted dykes, gabbro bodies, layered gabbro, and layered ultramafic rock. The oldest parts of the sea floor are older than 200 Ma, but most of the sea floor is younger than 100 Ma. Seamounts are common and almost all are volcanoes, related to mantle plumes, subduction, or other processes. In tropical regions, ocean islands tend to be surrounded by carbonate reefs. |
| 18.3 Sea-Floor Sediments | Almost all of the sea floor is covered by young sediments and sedimentary rocks, derived either from erosion of continents or from marine biological processes. Clastic sediments, some quite coarse, predominate on shelves and slopes. Terrigenous clays are distributed across the sea floor, but in areas where either carbonate- or silica-forming organisms thrive, the sediments are likely to be dominated by carbonate or silica oozes. Methane hydrates, derived from bacterial decomposition of organic matter, form within sediments on shelves and slopes. |
| 18.4 Ocean Water | Average ocean water has about 35 g/L of salt, mostly made up of chlorine and sodium, but also including magnesium, sulphur, and calcium. Salinity levels are highest in the tropics where evaporation is greatest. Sea-surface temperatures range from less than 0°C at the poles to over 25°C in equatorial regions. Open-ocean currents, which generally rotate clockwise in the northern hemisphere and counter-clockwise in the south, are critically important in redistributing heat on Earth. Deep-ocean currents, driven by density differences, are another key part of the heat redistribution system. Changes to current patterns or intensity have significant implications for global climate. |
Questions for Review
- What is the origin of the sediments that make up continental shelves? Why are the shelves on the eastern coast of North America so much wider than those along the west coast?
- The ocean trenches at some subduction zones are relatively shallow. What is one explanation for this?
- What are the main lithological components of oceanic crust, and how does this rock form?
- Referring to Figure 18.8, determine the age of the oldest sea floor in the Indian Ocean.
- Explain why relatively coarse terrigenous sediments (e.g., sand) tend to accumulate close to the continents, while terrigenous clay is dispersed all across the ocean floor.
- Although clay is widely dispersed in the oceans, in some areas, deep-sea sediments are dominated by clay, while in others they are dominated by carbonate or silica ooze. Why do these differences exist?
Figure A - Explain why carbonate sediments are absent from the deepest parts of the oceans.
- What is the source of the carbon that is present in sea-floor methane hydrate deposits?
- Where are the saltiest parts of the oceans? Why?
- Explain why sea-surface water with the greatest density is found in the north Atlantic, as shown on Figure A.
- What type of ocean currents result from the relatively dense water in the north Atlantic?
- How do the open-ocean currents affect the overall climate patterns on Earth?
Questions for Review
Media Attributions
- Figure A: “Sea Surface Density.” Adapted by Steven Earle.
The topics covered in this chapter can be summarized as follows:
| Section | Summary |
|---|---|
| 18.1 The Topography of the Sea Floor | The oceans are about 4,000 metres deep on average, but they also have a wide range of topographical features, including shallow continental shelves, continental slopes, continuous ridges related to plate divergence, numerous isolated seamounts, and deep submarine canyons at subduction zones. |
| 18.2 The Geology of the Oceanic Crust | Most oceanic crust forms during sea-floor spreading and is characterized by pillow basalts, sheeted dykes, gabbro bodies, layered gabbro, and layered ultramafic rock. The oldest parts of the sea floor are older than 200 Ma, but most of the sea floor is younger than 100 Ma. Seamounts are common and almost all are volcanoes, related to mantle plumes, subduction, or other processes. In tropical regions, ocean islands tend to be surrounded by carbonate reefs. |
| 18.3 Sea-Floor Sediments | Almost all of the sea floor is covered by young sediments and sedimentary rocks, derived either from erosion of continents or from marine biological processes. Clastic sediments, some quite coarse, predominate on shelves and slopes. Terrigenous clays are distributed across the sea floor, but in areas where either carbonate- or silica-forming organisms thrive, the sediments are likely to be dominated by carbonate or silica oozes. Methane hydrates, derived from bacterial decomposition of organic matter, form within sediments on shelves and slopes. |
| 18.4 Ocean Water | Average ocean water has about 35 g/L of salt, mostly made up of chlorine and sodium, but also including magnesium, sulphur, and calcium. Salinity levels are highest in the tropics where evaporation is greatest. Sea-surface temperatures range from less than 0°C at the poles to over 25°C in equatorial regions. Open-ocean currents, which generally rotate clockwise in the northern hemisphere and counter-clockwise in the south, are critically important in redistributing heat on Earth. Deep-ocean currents, driven by density differences, are another key part of the heat redistribution system. Changes to current patterns or intensity have significant implications for global climate. |
Questions for Review
- What is the origin of the sediments that make up continental shelves? Why are the shelves on the eastern coast of North America so much wider than those along the west coast?
- The ocean trenches at some subduction zones are relatively shallow. What is one explanation for this?
- What are the main lithological components of oceanic crust, and how does this rock form?
- Referring to Figure 18.8, determine the age of the oldest sea floor in the Indian Ocean.
- Explain why relatively coarse terrigenous sediments (e.g., sand) tend to accumulate close to the continents, while terrigenous clay is dispersed all across the ocean floor.
- Although clay is widely dispersed in the oceans, in some areas, deep-sea sediments are dominated by clay, while in others they are dominated by carbonate or silica ooze. Why do these differences exist?
Figure A - Explain why carbonate sediments are absent from the deepest parts of the oceans.
- What is the source of the carbon that is present in sea-floor methane hydrate deposits?
- Where are the saltiest parts of the oceans? Why?
- Explain why sea-surface water with the greatest density is found in the north Atlantic, as shown on Figure A.
- What type of ocean currents result from the relatively dense water in the north Atlantic?
- How do the open-ocean currents affect the overall climate patterns on Earth?
Media Attributions
- Figure A: “Sea Surface Density.” by NASA. Adapted by Steven Earle. Public domain.
