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continental shelf

 
Dictionary: continental shelf
continental shelf
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continental shelf
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n.
A submerged border of a continent that slopes gradually and extends to a point of steeper descent to the ocean bottom.


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Britannica Concise Encyclopedia: continental shelf
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The broad, gentle pitch of the continental shelf gives way to the relatively steep continental …
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The broad, gentle pitch of the continental shelf gives way to the relatively steep continental … (credit: Encyclopædia Britannica, Inc.)
Broad, relatively shallow submarine platform that forms a border to a continent, typically extending from the coast to depths of 330 – 660 ft (100 – 200 m). Continental shelves average about 40 mi (65 km) in width. Almost everywhere they are simply a continuation of the continental landmass: narrow, rough, and steep off mountainous coasts but broad and comparatively level offshore from plains. Continental shelves are usually covered with a layer of sand, silts, and silty muds. Their surfaces feature small hills and ridges that alternate with shallow depressions and valley-like troughs. In a few cases, steep-walled V-shaped submarine canyons cut deeply into both the shelf and the slope below.

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Geography Dictionary: continental shelf
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The gently sloping submarine fringe of a continent. This is ended by a steep continental slope which occurs at around 150 m below sea level. The UN Convention on the Continental Shelf of 1958 granted stated the right to mineral exploitation up to a depth of 200 m in their coastal waters, together with permission to authorize the construction of drilling rigs and the like, although such structures were not to be considered as islands. All this changed when the 1982 UN Convention on the Law of the Sea came into force; the continental shelf has a legal limit of up to 200 nautical miles from the coastline, and states have exclusive rights to all natural resources within that limit.

Science Dictionary: continental shelf
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The region adjoining the coastline of a continent, where the ocean is no more than a few hundred feet deep. The shelf is built up from sediments washed down to the sea by rivers.

  • The continental shelves are often valuable because of the mineral resources and abundant marine life found there. (See offshore drilling.)
  • Wikipedia: Continental shelf
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    Ocean habitats
    Littoral zone
    Intertidal zone
    Neritic zone
    Continental shelf
    Kelp forests
    Coral reefs
    Fishing banks
    Continental margin
    Pelagic zone
    Straits
    Seamounts
    Hydrothermal vents
    Cold seeps
    Demersal zone
    Benthic zone
    Aquatic ecosystems
    Aquatic layers
    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. See the Territorial waters page for more details.

    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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    Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.  Read more
    Britannica Concise Encyclopedia. Britannica Concise Encyclopedia. © 2006 Encyclopædia Britannica, Inc. All rights reserved.  Read more
    Geography Dictionary. A Dictionary of Geography. Copyright © Susan Mayhew 1992, 1997, 2004. All rights reserved.  Read more
    Science Dictionary. The New Dictionary of Cultural Literacy, Third Edition Edited by E.D. Hirsch, Jr., Joseph F. Kett, and James Trefil. Copyright © 2002 by Houghton Mifflin Company. Published by Houghton Mifflin. All rights reserved.  Read more
    Wikipedia. This article is licensed under the Creative Commons Attribution/Share-Alike License. It uses material from the Wikipedia article "Continental shelf" Read more