Beyond the continental edge the gradient of the sea bottom becomes steeper (the continental slope) and descends to the floor of the ocean basins, often reaching a depth of 3000-6000 m and even deeper in some places. The angle and extent of the continental slope vary with locality, averaging a gradient of about 7 per cent (a drop of 70 m in 1 km horizontal distance), but may be as steep as 50 per cent. The slope is seldom an even descent, and is much fissured by irregular gullies and steep-sided submarine canyons.
At the bottom of the continental slope the gradient becomes less steep due to the accumulation of sediment. This zone is termed the continental rise. It merges gradually with the deep ocean floor, which in some areas may be virtually flat over great areas, forming abyssal plains, extending for hundreds of miles with only slight changes of level. But in places the ocean floor rises to form ranges of submarine mountains with many summits ascending to within 2000-4000 m of the surface and the highest peaks breaking the surface as oceanic islands. These submarine oceanic ridges and plateaux are a major feature of the earth's crust, covering an area approximately equal to that of the continents. There are other parts where the ocean floor is furrowed by deep troughs, the ocean trenches, in which the bottom descends to depths of 7000-11 000 m.
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