The continental shelf

Close to land the sea is mostly shallow, the bottom shelving gradually from the shore to a depth of about 200 m. This coastal ledge of shallow sea bottom is the continental shelf (see Figure 1.2). About 8 per cent of the total sea area lies above it. Its seaward margin is termed the continental edge, beyond which the water becomes much deeper. The steeper gradient beyond the continental edge is termed the continental slope.

The width of the continental shelf varies very much in different parts of the world from less than 100 metres to more than 100 kilometres, with an average of about 65 km. It is extensive around the British Isles, where the continental edge runs to the west of Ireland and the north of Scotland. The English Channel, Irish Sea and almost the entire North Sea lie above the shelf. The shelf is also broad beneath the China Sea, along the Arctic coast of Siberia, under Hudson Bay, and along the Atlantic coast of Patagonia where the shelf extends out to the Falkland Islands.

Many of the shelf areas are of special economic importance because geographically the major fisheries are concentrated here. Northern hemisphere temperate and sub-polar continental shelves are particularly important in this respect. Shelf areas are also widely exploited as sources of oil and gas.

Several processes contribute to the formation of the continental shelf. It is formed partly by wave erosion cutting back the coastline. It may be extended seawards by accumulations of material eroded from the coast, or by river-borne silt deposited on the continental slope. Parts of the shelf appear to consist largely of material held against the continents by underwater barriers formed by reef-building organisms or by tectonic folding. In other places the shelf has been formed chiefly by sinking and inundation of the land; for example, under the North Sea. It is possible that in some regions the shelf has been broadened by increments of materials thrust up the continental slope by pressures between the continental blocks and the deep ocean floor.

Figure 1.1 Areas and mean depths of major oceans and seas.
Figure 1.2 Terms applied to parts of the sea bottom.

i Submarine ridges i—1 Ocean r—i Continental ^ Ocean i and plateaux '—1 basins '—' shelf trenches

Iff)' 1ffy 140° 120° 100°_80°_60" 49°_20°

Iff)' 1ffy 140° 120° 100°_80°_60" 49°_20°

180° 160° 140° 120° 100° 80° 60° 40° 20° 0°

Figure 1.3 Main areas of continental shelf, submarine ridges and plateaux, and ocean trenches.

180° 160° 140° 120° 100° 80° 60° 40° 20° 0°

Figure 1.3 Main areas of continental shelf, submarine ridges and plateaux, and ocean trenches.

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