- abyssal plain
Flat seafloor area at a depth of 10,000–20,000 ft (3,000–6,000 m), generally adjacent to a continent.The larger plains are hundreds of miles wide and thousands of miles long. The plains are largest and most common in the Atlantic Ocean, less common in the Indian Ocean, and even rarer in the Pacific Ocean, where they occur mainly as small, flat floors of marginal seas or as long, narrow bottoms of trenches. They are thought to be the upper surfaces of land-derived sediment that accumulates in abyssal depressions.
* * *▪ geologyflat 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.
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Abyssal plain — Abyssal plains are flat or very gently sloping areas of the deep ocean basin floor. They are among the Earth s flattest and smoothest regions and the least explored. Abyssal plains cover approximately 40% of the ocean floor and reach depths… … Wikipedia
abyssal plain — noun : any of the great flat sediment covered areas of ocean floor * * * abyssal plain, a deep, extremely level undersea plain, such as the plain lying about 750 miles to 1,000 miles off the eastern coast of North America, with deposits of clay,… … Useful english dictionary
abyssal plain — large, relatively flat areas of ocean floor found at 5,000 6,000m below sea level. If sediments are discharged from a river and deposit relatively quickly onto the plain they may form an abyssal fan … Geography glossary
abyssal plain — noun Date: 1954 any of the great flat sediment covered areas of ocean floor see continental shelf illustration … New Collegiate Dictionary
abyssal plain — the area of the generally flat ocean floor excluding ocean trenches below 2000 fathoms (3660 metres, presumably an older version based on fathoms) or 4000 metres … Dictionary of ichthyology
abyssal plain — noun a large expanse of very flat and smooth ocean floor, usually found at depths of 4,600 to 5,500 meters (15,000 to 18,000 feet) … Wiktionary
abyssal plain — /əbɪsəl ˈpleɪn/ (say uhbisuhl playn) noun a flat or very gently sloping area on the bottom of the abyssopelagic zone which is covered in sediment accumulated over millions of years … Australian English dictionary
Colombian Abyssal Plain — ▪ plain, Caribbean Sea submarine plain forming part of the floor of the south central Caribbean Sea, and the deepest and flattest portion of the Colombian Basin. It rises to the southeast to form the Caribbean coast of Colombia, joins the… … Universalium
Hatteras Abyssal Plain — ▪ submarine plain, Atlantic Ocean submarine plain forming the floor of the northwestern Atlantic Ocean. It lies east of the North American continental shelf between the southern United States and Bermuda, extending about 900 mi (1,450 km)… … Universalium
abyssal floor — abyssal plain … Dictionary of ichthyology