At one time the sea-bed was thought to be flat and featureless and it is only in relatively recent times that its mountainous nature has been revealed. The system of oceanic mountain ridges is vast and its extent was not fully charted until the 1950s and 1960s. Full details of all the known ridges, together with ocean trenches, depth contours and other physical features, are shown in a series of maps in The Times Atlas and Encyclopedia of the Sea (Couper, 1989).
One part of the submarine ridge system forms a barrier separating the deep levels of the Arctic basin from those of the Atlantic. Much of the crest of this ridge is within 500 m of the surface, extending from the north of Scotland and the Orkneys and Shetlands to Rockall and the Faroes (the Wyville-Thompson ridge), and then to Iceland (the Iceland-Faroes rise), and across to Greenland and Labrador (the Greenland-Iceland rise).
The bottom of the Atlantic is divided into two basins by the mid-Atlantic ridge which extends from 70°N to 55°S. It runs from the Arctic through Iceland, and then follows a roughly S-shaped course from north to south, touching the surface at the islands of the Azores, St Paul, Ascension, Tristan da Cunha and Bouvet. A branch of the mid-Atlantic ridge, the Walvis ridge, extends from Tristan da Cunha to Walvis Bay on the west coast of Africa. South of South Africa the mid- Atlantic ridge trends eastwards and links with a north-south submarine ridge, the mid-Indian ridge, bisecting the Indian Ocean between Antarctica and the Indian and Arabian Peninsulas, and extending into the Arabian Sea (the Carlsberg ridge).
In the Pacific a very broad submarine plateau extends in a north-easterly direction from Antarctica to the west coast of North and Central America, its peaks forming some of the East Pacific Islands. The more numerous islands of the Central and West Pacific appear mainly to be of separate volcanic origin. A peculiar feature of the Pacific Ocean is the large number of flat-topped, underwater hills known as guyots. Although the summits of some of them now lie beneath as much as 800 m of water, they have the appearance of having been worn flat by wave erosion. It seems likely that at some earlier time these volcanic mounds reached above the surface, but have subsequently subsided.
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