This is, in fact, the seaward extension of the continent from the shoreline to the continental edge marked, approximately, by the 100 fathom (600 feet) isobath (isobaths are contours marking depths below sea level). The continental shelf is thus a shallow platform whose width varies greatly, from a few miles in the north Pacific off the continent of North America , to over 100 miles off north-west Europe. In some places where the coasts are extremely mountainous, such as the Rocky Mountain and Andean coasts, the continental shelf may be entirely absent. Off broad lowland coasts like those of Arctic Siberia, a maximum width of 750 miles has been recorded! A width of 20 to 100 miles is generally encountered. The angle of the slope is also variable, and is normally least where the continental shelf is widest. A gradient of 1 in 500 is common to most continental shelves.
Many regard the continental shelf as part of the continent submerged due to a rise in sea level, e.g. at the close of the Ice Age, when the ice in the termperate latitudes melted and raised the sea level by several hundred feet. Some smaller continental shelves could have been caused by wave erosion where the land is being eroded by the sea. Conversely such shelves might have been formed by the deposition of land-derived or river-borne materials on the off-shore terrace.
The continental shelves are of great geographical significance for the following reasons :-
Many regard the continental shelf as part of the continent submerged due to a rise in sea level, e.g. at the close of the Ice Age, when the ice in the termperate latitudes melted and raised the sea level by several hundred feet. Some smaller continental shelves could have been caused by wave erosion where the land is being eroded by the sea. Conversely such shelves might have been formed by the deposition of land-derived or river-borne materials on the off-shore terrace.
- Their shallowness enables sunlight to penetrate through the water, which encourages the growth of minute plants and other microscope organisms. They are thus rich in plankton on which millions of surface and bottom-feeding fishes thrive. The continental shelves are therefore the richest fishing grounds in the world, e.g. the grand banks off Newfoundland, The North Sea and the Sunda shelf.
- Their limited depth and gentle slope keep out cold under-currents and increase the height of tides. This sometimes hinders shipping and other marine activities since ships can only entre and leave port on the tide. Most of the world's greatest seaports including Southampton, London, Hamburg, Rotterdam, Hong kong and Singapore are located on continental shelves.
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