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continental shelf

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continental shelf
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{{short description|A portion of a continent that is submerged under an area of relatively shallow water known as a shelf sea}}{{ocean habitat topics|image=Southeastern United States continental shelf.jpg|caption=Anatomy of a continental shelf of the south eastern coast of the United States}}A continental shelf is a portion of a continent that is submerged under an area of relatively shallow water known as a shelf sea. Much of the shelves were exposed during glacial periods and interglacial periods.The shelf surrounding an island is known as an insular shelf. The continental margin, between the continental shelf and the abyssal plain, comprises a steep continental slope followed by the flatter continental rise. Sediment from the continent above cascades down the slope and accumulates as a pile of sediment at the base of the slope, called the continental rise. Extending as far as 500 km (310 mi) from the slope, it consists of thick sediments deposited by turbidity currents from the shelf and slope.Pinet 39, Gross 45. The continental rise's gradient is intermediate between the slope and the shelf. Under the United Nations Convention on the Law of the Sea, the name continental shelf was given a legal definition as the stretch of the seabed adjacent to the shores of a particular country to which it belongs.

Geographical distribution

(File:Elevation.jpg|thumb|left|350px|{{legend|#03FCDE|The global continental shelf, highlighted in cyan}})Width of the continental shelf varies considerably – it is not uncommon for an area to have virtually no shelf at all, particularly where the forward edge of an advancing oceanic plate dives beneath continental crust in an offshore subduction zone such as off the coast of Chile or the west coast of Sumatra. The largest shelf – the Siberian Shelf in the Arctic Ocean – stretches to {{convert|1500|km|mi|sp=us}} in width. The South China Sea lies over another extensive area of continental shelf, the Sunda Shelf, which joins Borneo, Sumatra, and Java to the Asian mainland. Other familiar bodies of water that overlie continental shelves are the North Sea and the Persian Gulf. The average width of continental shelves is about {{convert|80|km|mi|abbr=on}}. The depth of the shelf also varies, but is generally limited to water shallower than {{convert|150|m|ft|abbr=on}}.Pinet, 37. The slope of the shelf is usually quite low, on the order of 0.5°; vertical relief{{clarify |date=October 2018 |reason=Define this term (vertical relief). It is not widely known with precision and it is not precisely understood directly from context. The cited source is not on line and thus not easy to refer to.}} is also minimal, at less than {{convert|20|m|ft|abbr=on}}.Pinet 36–37.Though the continental shelf is treated as a physiographic province of the ocean, it is not part of the deep ocean basin proper, but the flooded margins of the continent.Pinet 35–36. Passive continental margins such as most of the Atlantic coasts have wide and shallow shelves, made of thick sedimentary wedges derived from long erosion of a neighboring continent. Active continental margins have narrow, relatively steep shelves, due to frequent earthquakes that move sediment to the deep sea.Pinet 90–93.

Topography

(File:Continental shelf.png|350px|thumb) The shelf usually ends at a point of increasing slopeWEB,weblink shelf break – geology, Encyclopædia Britannica, (called the shelf break). The sea floor below the break is the continental slope. Below the slope is the continental rise, which finally merges into the deep ocean floor, the abyssal plain. The continental shelf and the slope are part of the continental margin. The shelf area is commonly subdivided into the inner continental shelf, mid continental shelf, and outer continental shelf, each with their specific geomorphology and marine biology. The character of the shelf changes dramatically at the shelf break, where the continental slope begins. With a few exceptions, the shelf break is located at a remarkably uniform depth of roughly {{convert|140|m|ft|abbr=on}}; this is likely a hallmark of past ice ages, when sea level was lower than it is now.Gross 43. The continental slope is much steeper than the shelf; the average angle is 3°, but it can be as low as 1° or as high as 10°.Pinet 36, Gross 43. The slope is often cut with submarine canyons. The physical mechanisms involved in forming these canyons were not well understood until the 1960s.Pinet 98, Gross 44.

Sediments

The continental shelves are covered by terrigenous sediments; that is, those derived from erosion of the continents. However, little of the sediment is from current rivers; some 60–70% of the sediment on the world's shelves is relict sediment, deposited during the last ice age, when sea level was 100–120 m lower than it is now.Pinet 84–86, Gross 43.Sediments usually become increasingly fine with distance from the coast; sand is limited to shallow, wave-agitated waters, while silt and clays are deposited in quieter, deep water far offshore.Gross 121-22. These accumulate 15–40 cm every millennium, much faster than deep-sea pelagic sediments.Gross 127.

Biota

Continental shelves teem with life, because of the sunlight available in shallow waters, in contrast to the biotic desert of the oceans' abyssal plain. The pelagic (water column) environment of the continental shelf constitutes the neritic zone, and the benthic (sea floor) province of the shelf is the sublittoral zone.Pinet 316-17, 418–19.Though the shelves are usually fertile, if anoxic conditions prevail during sedimentation, the deposits may over geologic time become sources for fossil fuels.

Economic significance

The relatively accessible continental shelf is the best understood part of the ocean floor. Most commercial exploitation from the sea, such as metallic-ore, non-metallic ore, and hydrocarbon extraction, takes place on the continental shelf. Sovereign rights over their continental shelves up to a depth of 200 metres or to a distance where the depth of waters admitted of resource exploitation were claimed by the marine nations that signed the Convention on the Continental Shelf drawn up by the UN's International Law Commission in 1958. This was partly superseded by the 1982 United Nations Convention on the Law of the Sea.WEB,weblink Treaty Series – Convention on the Continental Shelf, 1958
authorlink= date = 1958-04-29, United Nations, vol. 499, p. 311. which created the 200 nautical mile exclusive economic zone, plus continental shelf rights for states with physical continental shelves that extend beyond that distance.The legal definition of a continental shelf differs significantly from the geological definition. UNCLOS states that the shelf extends to the limit of the continental margin, but no less than 200 nautical miles and no more than 350 nm from the baseline. Thus inhabited volcanic islands such as the Canaries, which have no actual continental shelf, nonetheless have a legal continental shelf, whereas uninhabitable islands have no shelf.

See also

Notes

{{Reflist|2}}

References

  • BOOK, Gross, M. Grant, Oceanography: A View of the Earth,weblink 12 January 2016, 1972, Prentice-Hall, Englewood Cliffs, 978-0-13-629659-1,
  • BOOK, Pinet, Paul R., Invitation to Oceanography,weblink 13 January 2016, 2003, Jones & Bartlett Learning, Boston, 978-0-7637-2136-7,

External links

  • weblink" title="web.archive.org/web/20050217014801weblink">Office of Naval Research: Ocean Regions: Continental Margin & Rise
  • UNEP Shelf Programme
  • weblink" title="web.archive.org/web/20110720071445weblink">Anna Cavnar, Accountability and the Commission on the Limits of the Continental Shelf: Deciding Who Owns the Ocean Floor
{{physical oceanography|expanded=other}}{{coastal geography}}{{Authority control}}{{Use dmy dates|date=April 2017}}


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