CONTINENTAL SHELF

broad, relatively shallow submarine platform that forms a border to a continent. A continental shelf typically extends from the coast to depths of 100200 m (330660 feet). In nearly all instances, it ends at its seaward edge with an abrupt drop called the shelf break. Below this lies the continental slope, a much steeper zone that usually merges with a section of the ocean floor called the continental rise at a depth of roughly 4,000 to 5,000 m (13,000 to 16,500 feet). Continental shelves tend to vary greatly in width, but they average about 65 km (40 miles). Almost everywhere the shelves represent simply a continuation of the continental land mass beneath the ocean margins. Accordingly, they are narrow, rough, and steep off mountainous coasts but broad and comparatively level offshore from plains. The shelf along the mountainous western coast of the United States, for example, is narrow, measuring only about 32 km (20 miles) wide, whereas that fringing the eastern coast extends more than 120 km (75 miles) in width. Exceptionally broad shelves occur off northern Australia and Argentina. Continental shelves are usually covered with a layer of sand, silts, and silty muds. Their surfaces exhibit some relief, featuring small hills and ridges that alternate with shallow depressions and valleylike troughs. In a few cases, steep-walled V-shaped submarine canyons cut deeply into both the shelf and the slope below. Since the 1970s an increasing number of investigators have sought to explain the origin of continental shelves and their related structures in terms of plate tectonics. According to this theory, the shelves of the Pacific Ocean, for example, formed as the leading edges of continental margins on lithospheric plates that terminate either at fracture zones (sites where two such plates slide past each other) or at subduction zones (sites where one of the colliding plates plunges into the underlying partially molten asthenosphere and is consumed, while the overriding plate is uplifted). Shelves of such origin tend to be steep, deformed, and covered by a thin layer of erosional debris. The Atlantic continental shelves, on the other hand, show little or no tectonic deformation and bear a thick veneer of sedimentary material. They are thought to be remnants of the trailing edges of the enormous plates that split apart and receded many millions of years ago to form the Atlantic basin. As the edges of the plates gradually contracted and subsided, large amounts of sand, silts, and mud from the continents settled and accumulated along their seaward side.