Continental Shelf

The continental shelf is the part of the continent that extends below the sea to where there is a marked increase in angle—the beginning of the continental slope. This generally occurs at about 130 m, but it can be as little as 50 m or as much as 550 m. Passive continental margins are broad—some as much as 500 km (such as the shelf off Newfoundland) or even up to 1,500 m (for example, the Siberian shelf in the Arctic Ocean). Active continental margins, for example, like parts of the Pacific coast of the United States, may be as narrow as a few tens of meters. Continental shelves on passive sides are flat and have a gentle seaward tilt, about 0.1 degree—a drop of about 2 m for every kilometer.

Many broad shelves contain incised valleys that were cut when sea level was lower, several times during the last glacial period. These valleys are usually extensions of rivers on land and continue across the shelves and become deep canyons where they intersect the continental slope. The Hudson, Ganges, and Congo rivers are examples. Narrow shelves found along the active margins of continents have rocky shorelines and plunge downward after a short distance into trenches.

Wide or narrow, most shelves are covered with relatively young sediments that contribute to their flatness and hide the complex geological structure beneath them— which, for the most part, is the same as that of the adjacent land. The sediments, derived from the continents, are sandy near the shore and muddy in deeper water, but the outer part of the shelf may be covered with coarser sediment deposited when it was near the shore, when sea level was lower. Currents move the sediments around, and underwater exploration has revealed large sand waves on the surface of the sand. When sea level is high, estuaries and barrier beaches develop and trap sediment, reducing the amount that reaches the open sea. More sediment is deposited within the Hudson estuary than is deposited in the adjacent sea.

Shelves in tropical regions may be covered with carbonate biogenic deposits (the skeletons of plants and animals) as well as carbonates that are direct precipitates from seawater. Some shelves are swept clean by fast currents, their sediments carried off and deposited in deeper water.

Even though shelves represent only 8 percent of the entire ocean area, they are most important to people as the main source of fish and other sea life. But, incredibly, they are also the dumping site for all kinds of waste: sewage, garbage, construction debris, chemical by-products, and the like. In addition, the continental shelves are the major sites for oil and gas extraction. They are also the source of sand, the dredging of which disturbs the bottom habitats of plants and animals. Each year in the United States, millions of tons of sand are dredged for beach replenishment.

—Sidney Horenstein

See also: Abyssal Floor; Beaches; Continental Slope and Rise; Oceans

Bibliography

Montgomery, Carla W. 1996. Fundamentals of Geology, 3d ed. New York: McGraw Hill; Siebold, E., and W. H. Beyer. 1993. The Sea Floor: An Introduction to Marine Geology. Berlin: Springer Verlag; Walsh, J. J. 1989. On the Nature of Continental Shelves. New York: Academic.

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