ABYSSAL HILL

small, topographically well-defined submarine hill that may rise from several metres to several hundred metres above the abyssal seafloor, in water 3,000 to 6,000 m (10,000 to 20,000 feet) deep. Typical hills have diameters of several to several hundred metres. Abyssal-hill provinces, areas of abyssal seafloor occupied exclusively by such hills, characteristically occur seaward of the smooth abyssal plains at the bases of continental rises. Isolated hills and groups of hills also protrude from abyssal-plain surfaces, and the base of an abyssal-plain accumulation of sediment, as revealed by sub-bottom seismic profiling, generally matches the undulating topography and relief of abyssal-hill provinces. Abyssal hills, although generally covered with marine sediments, probably are identical in composition and origin to the extrusive basaltic prominences on the upper flanks of mid-oceanic ridges and rises. Thus, it is believed that abyssal hills underlie most of the ocean floor, locally buried by accumulations of abyssal sediment. In the Atlantic Ocean, long abyssal-hill provinces parallel both flanks of the Mid-Atlantic Ridge along most of its length. The Pacific Ocean has a smaller supply of continental sediment than the Atlantic Ocean, and numerous trenches and local rises separate the main ocean floor from the continents, preventing the seaward transport of sediment; consequently, between 80 and 85 percent of the Pacific abyssal floor is occupied by abyssal hills.