ABYSSAL PLAIN

flat seafloor area at an abyssal depth (3,000 to 6,000 m [10,000 to 20,000 feet]), generally adjacent to a continent. These submarine surfaces vary in depth only from 10 to 100 cm per kilometre of horizontal distance. Irregular in outline but generally elongate along continental margins, the larger plains are hundreds of kilometres wide and thousands of kilometres long. In the North Atlantic the Sohm Plain alone has an area of approximately 900,000 square km (350,000 square miles). The plains are largest and most common in the Atlantic Ocean, less common in the Indian Ocean, and even rarer in the Pacific, where they occur mainly as the small, flat floors of marginal seas or as the narrow, elongate bottoms of trenches. The plains are thought to be the upper surfaces of land-derived sediment that accumulates in abyssal depressions, thus smoothing out a preexisting hilly or otherwise irregular topography. Seismic profiles (cross sections) of abyssal plains reveal accumulations of sediment averaging one kilometre in thickness, deposited on undulating topography. Incomplete burial of preexisting relief may result in the presence of isolated volcanic hills or hill groups that rise abruptly out of some abyssal plains. Sediment from the continental margins accretes at steep continental slopes, and occasional submarine slumping of this coarse material creates dense, sediment-laden slurries, called turbidity currents, that flow down the slopes in obedience to gravity. Part of the turbidity-current sediment settles out at the bases of the continental slopes, creating continental rises of lesser gradient, but some of the coarse sediment reaches the abyssal depressions. Horizontal silty, sandy, and even gravelly beds that are fractions of a centimetre to several metres thick comprise 2 to 90 percent of abyssal-plain sediment. Many such layers demonstrably are of shallow-water organisms-e.g., the microscopic protozoan Foraminifera. An individual layer may be progressively finer grained from bottom to top; this grading reflects the bed's origin as the deposit of a single turbidity current. The coarse layers are interbedded with homogeneous deposits of fine-grained clay and the microscopic remains of organisms that inhabit the waters overlying the abyssal plains. Between turbidity-current episodes these fine-grained sediments are believed to fall through the water column particle by particle, accumulating at exceedingly slow rates (a millimetre to several centimetres per 1,000 years). Alternatively, it has been proposed that deep-sea clay deposits may be brought to abyssal plains continuously by slowly flowing, diffusely turbid bottom waters that originate in turbulent, shallow nearshore areas.