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Sediment Rock Mantle
The global continental shelf, highlighted in cyan
The continental shelf is the extended perimeter of each continent and associated coastal plain, which is covered during interglacial periods such as the current epoch by relatively shallow seas (known as shelf seas) and gulfs.
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The 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 1500 kilometers (930 miles) 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 80 km (50 mi). The depth of the shelf also varies, but is generally limited to water shallower than 150 m (490 ft).Pinet, 37. The slope of the shelf is usually quite low, on the order of 0.5°; vertical relief is also minimal, at less than 20 m (65 ft).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, comprised 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.
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 shelf sediments accumulate at an average rate of 30 cm/1000 years, with a range from 15-40 cm.Gross 127. Though slow by human standards, this rate is much faster than that for deep-sea pelagic sediments.
Combined with the sunlight available in shallow waters, the continental shelves teem with life compared 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 in the sedimentary deposits prevail, the shelves may in geologic time become sources of fossil fuels.
The relatively accessible continental shelf is the best understood part of the ocean floor. Most commercial exploitation from the sea, such as oil and gas extraction, takes place on the continental shelf. Sovereign rights over their continental shelves 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 partly superseded by the 1982 United Nations Convention on the Law of the Sea.http://untreaty.un.org/ilc/texts/instruments/english/conventions/8_1_1958_continental_shelf.pdf
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