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Ocean habitats
Littoral zone
Intertidal zone
Neritic zone
Continental shelf
Kelp forests
Coral reefs
Ocean banks
Continental margin
Straits
Pelagic zone
Oceanic zone
Seamounts
Hydrothermal vents
Cold seeps
Demersal zone
Benthic zone
Aquatic layers
Lake stratification
Aquatic ecosystems
Wild fisheries
Land habitats
     The global continental shelf, highlighted in cyan

The continental shelf is the extended perimeter of each continent and associated coastal plain, and was part of the continent during the glacial periods, but is undersea during interglacial periods such as the current epoch by relatively shallow seas (known as shelf seas) and gulfs.

The continental rise is below the slope, but landward of the abyssal plains. Its gradient is intermediate between the slope and the shelf, on the order of 0.5-1°.[1] Extending as far as 500 km from the slope, it consists of thick sediments deposited by turbidity currents from the shelf and slope. Sediment cascades down the slope and accumulates as a pile of sediment at the base of the slope, called the continental rise.[2]

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. Such shores are also known as Territorial waters.

Contents

Geographical distribution

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).[3] 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 (66 ft).[4]

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.[5] 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.[6]

Topography

     Sediment     Rock      Mantle

The shelf usually ends at a point of decreasing slope (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 140 m (460 ft); this is likely a hallmark of past ice ages, when sea level was lower than it is now.[7]

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°.[8] The slope is often cut with submarine canyons. The physical mechanisms involved in forming these canyons was not well understood until the 1960s.[9]

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.[10]

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.[11] These shelf sediments accumulate at an average rate of 30 cm/1000 years, with a range from 15-40 cm.[12] Though slow by human standards, this rate is much faster than that for deep-sea pelagic sediments.

Biota

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.[13]

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.

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 350 nautical miles from the coast 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.[14]

See also

Notes

  1. ^ Pinet 37.
  2. ^ Pinet 39, Gross 45.
  3. ^ Pinet, 37.
  4. ^ Pinet 36-37.
  5. ^ Pinet 35-36.
  6. ^ Pinet 90-93.
  7. ^ Gross 43.
  8. ^ Pinet 36, Gross 43.
  9. ^ Pinet 98, Gross 44.
  10. ^ Pinet 84-86, Gross 43.
  11. ^ Gross 121-22.
  12. ^ Gross 127.
  13. ^ Pinet 316-17, 418-19.
  14. ^ http://untreaty.un.org/ilc/texts/instruments/english/conventions/8_1_1958_continental_shelf.pdf

References

  • Gross, Grant M. Oceanography: A View of the Earth. Englewood Cliffs: Prentice-Hall, Inc., 1972. ISBN 0-13-629659-9
  • Pinet, Paul R. (1996) Invitation to Oceanography. St. Paul, MN: West Publishing Co., 1996. ISBN 0-7637-2136-0 (3rd ed.)

External links

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1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

CONTINENTAL SHELF, the term in physical geography for the submerged platform upon which a continent or island stands in relief. If a coin or medal be partly sunk under water the image and superscription will stand above water and represent a continent with adjacent islands; the sunken part just submerged will represent the continental shelf and the edge of the coin the boundary between it and the surrounding deep, called by Professor H. K. H. Wagner the continental slope. If the lithosphere surface be divided into three parts, namely, the continent heights, the ocean depths, and the transitional area separating them, it will be found that this transitional area is almost bisected by the coast-line, that nearly one-half of it o,000,000 sq. m.) lies under water less than loo fathoms deep, and the remainder 12,000,000 sq. m. is under 600 ft. in elevation. There are thus two continuous plain systems, one above water and one under water, and the second of these is called the continental shelf. It represents the area which would be added to the land surface if the sea fell 600 ft. This shelf varies in width. Round Africa - except to the south - and off the western coasts of America it scarcely exists. It is wide under the British Islands and extends as a continuous platform under the North Sea, down the English Channel to the south of France; it unites Australia to New Guinea on the north and to Tasmania on the south, connects the Malay Archipelago along the broad shelf east .of China with Japan, unites north-western America with Asia, sweeps in a symmetrical curve outwards from north-eastern America towards Greenland, curving downwards outside Newfoundland and holding Hudson Bay in the centre of a shallow dish. In many places it represents the land planed down by wave action to a plain of marine denudation, where the waves have battered down the cliffs and dragged the material under water. If there were no compensating action in the differential movement of land and sea in the transitional area, the whole .of the land would be gradually planed down to a submarine platform, and all the globe would be covered with water. There are, however, periodical warpings of this transitional area by which fresh areas of land are raised above sea-level, and fresh continental coast-lines produced, while the sea tends to sink more deeply into the great ocean basins, so that the continents slowly increase in size. "In many cases it is possible that the continental shelf is the end of a low plain submerged by subsidence; in others a low plain may be an upheaved continental shelf, and probably wave action is only one of the factors at work" (H. R. Mill, Realm Nature, 1897).


<< Continent

Continued fractions >>


Simple English

A continental shelf is the part of the continent that is under water. The shelf was part of the land during the ice ages in the glacial periods, but under water in the interglacial periods. We are at present in an interglacial period.[1][2]

Every continent is in the sea, like an island. Most of the island is above the water line, and we see it as a continent. Some of it though, is below the water line. Beyond the continental shelf, the bottom goes down to much greater depths.


The continental shelf is a shallow ocean. It varies in depth, up to 140 metres deep. It varies greatly in its width. At the leading edge of a moving continental plate there will be little or no shelf. The western edge of the Americas are an example. The shelf on a passive edge of a plate will be wide and shallow. The widest shelf is the Siberian shelf in the Arctic Ocean: it is 1500km (930 miles) in width.

Contents

Inland seas

There have been, at some periods, shallow seas inside continents. These are called epicontinental seas. Much of present-day North America was covered by an epicontinental sea called the Sundance Sea during the Jurassic period.[3] In the Cretaceous an even larger area was covered by the Western Interior Seaway.

Ecology

File:Cwall99
Phytoplankton bloom off Cornwall, England. The light blue is caused by billions oof white coccolithophorid skeletons of Emiliana huxleyi.

Continental seas have the richest life of the oceans, more species and greater numbers than anywhere else. This is because it has both sunlight and nutrients for photosynthesis. This causes microscopic algae and cyanobacteria to bloom, and then animals eat the phytoplankton. Most of the nutrients in the oceans are washed down from the continents by rivers. In particular, the continents are almost the only source for some key elements such as iron. The lack of nutrients away from continents explains why so much of the Pacific Ocean is almost barren of life, and why the shelves are so rich in life.

Topography

The shelf usually ends at a point of decreasing slope (called the shelf break). The sea floor below the break is the continental slope. 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 140 m (460 ft); this is likely a hallmark of past ice ages, when sea level was lower than it is now.[1]p43

References

  1. 1.0 1.1 Gross, Grant M. 1972. Oceanography: a view of the Earth. Prentice-Hall, Englewood Cliffs N.J. ISBN 0-13-629659-9
  2. Pinet, Paul R. 1996. Invitation to oceanography. 3rd ed, West, St Paul, MN. ISBN 0-7637-2136-0
  3. Palaeos map http://www.palaeos.com/Mesozoic/Jurassic/MidJura.html#Geography

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