continental shelf

A CONTINENTAL SHELF is the submerged top of the continent's edge, lying between the shoreline and the continental slope that forms a border to a continent. In other words, the surface of the Earth lies at two general levels: a lower, which is the floor of the ocean basins, and an upper, the parts of which are the continents. Between these two levels is a comparatively narrow slope. The volume of ocean water is, however, a little too great to be entirely contained in the ocean basins, and so it must lap over somewhat on the lower, outer edges of the continental platforms. Such a submerged outer edge is called the continental self. The shelf is made shallower by deposition of material eroded from the land. The shelf has a gentle slope and is the shallowest portion of the ocean. Usually, a shelf is less than 650 ft (200 m) deep; in ANTARCTICA the continental shelf averages 1,650 ft (500 m).

The continental shelves are the regions of the oceans best known and the most exploited commercially. It is this region where virtually all of the petroleum, commercial sand and gravel deposits, and fishery resources are found. It is also the locus of waste dumping. Changes in sea level have alternately exposed and inundated portions of the continental shelf. A continental shelf typically extends from the coast to depths of 330 to 600 ft (100 to 200 m). 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 the section of ocean floor called the continental rise at a depth of roughly 13,000 to 16,000 ft (4,000 to 5,000 m).

The shelf varies greatly in width, but it averages about 40 mi or 65 km. Almost everywhere it represents simply a continuation of the land surface beneath the ocean margins; hence, it is broad and relatively level offshore from plains, and narrow, rough, and steep off mountainous coasts. For example, the shelf along the mountainous western coast of the United States is narrow, measuring only about 20 mi or 32 km wide, whereas that fringing the eastern coast extends more than 75 mi or 120 km in width. Exceptionally broad shelves occur off northern AUSTRALIA and ARGENTINA.

The continental slopes begin at the shelf break and plunge downward to the great depths of the ocean basin proper. Deep submarine CANYONs, some comparable in size to the GRAND CANYON of the Colorado River, are sometimes found cutting across the shelf and slope, often extending from the mouths of terrestrial rivers. The CONGO, AMAZON, GANGES, and HUDSON rivers all have submarine canyon extensions. It is assumed that submarine canyons on the continental shelf were initially carved during periods of lower sea level in the course of the ice ages. Their continental slope extensions were carved and more recently modified by turbidity currents—subsea “landslides” of a dense slurry of water and sediment.

Some parts of the world's continental shelves are extremely level (for example, the parts off the Arctic coast of SIBERIA), but more commonly, they exhibit some relief. Close to the coast of New England are submerged glacial deposits. In places, ridges or cliffs can be traced from the land onto the continental shelf. Usually continental shelves are covered with a layer of sand, silts, and mud. In a few cases, steep-walled Vshaped submarine canyons cut deeply into both the shelf and the slope below. Some of them connect with a system of land valleys, but their origin is one of the great scientific puzzles.

Many continental slopes end in gently sloping, smooth-surfaced features called continental rises. The continental rises usually have an inclination of less than half a degree. They have been found to consist of thick deposits of sediment, presumably deposited as a result of slumping and turbidity currents carrying sediment off the shelf and slope. The continental shelf, slope, and rise together are called the continental margin.

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 theory. 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 contractedand subsided, large amounts of sand, slits and mud from the continents settled and accumulated along the seaward side.

The Outer Continental Shelf (OCS) consists of the submerged lands, subsoil, and seabed, lying between the seaward extent of the states' jurisdiction and the seaward extent of federal jurisdiction. The continental shelf is the gently sloping undersea plain between a continent and the deep ocean. The United States OCS has been divided into four leasing regions. They are the Gulf of Mexico OCS Region, the Atlantic OCS Region, the Pacific OCS Region, and the Alaska OCS Region. In the United States in 1953, Congress designated the secretary of the interior to administer mineral exploration and development of the entire OCS through the Outer Continental Shelf Lands Act (OCSLA).