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ocean floor[′ō·shən ′flȯr]
one of the principal elements of the relief and geological structure of the bottom of the world ocean. It covers the abyssal part of the ocean bottom except for the midocean ridges and is characterized by the development of typical oceanic crust. It occupies the lowest hypsometric level of the earth’s surface (average depth, 4,000 m; maximum depth, 7,000 m). Area, more than 185 million sq km (more than 50 percent of the total area of the bottom of the world ocean).
The largest elements of the relief of the ocean floor and its tectonic structure are ocean basins and the various types of oceanic rises that separate them. The basins occupy the largest part of the ocean floor and have average depths of about 5,000 m. Abyssal-type rolling relief occupies up to 80 percent of the bottom of the basins (relative variations in depth of the order of 500–1,000 m); the remainder is flat and sloping abyssal plains. It is believed that the abyssal rolling relief is the initial tectonic and volcanogenic relief of the surface of the earth’s crust which has been covered with blanket-like, shallow (up to 1 km) sediment. The flat abyssal plains occupy less than 10 percent of the surface of the ocean floor and are usually located on the margins of basins that lie close to continents. They form as the result of complete burial of the initial rolling relief owing to the intensified influx of sedimentary material from the continent. The sloping plains are also located along the margins of basins and gradually give way to the plains of the continental rise. They are also accumulative formations that occur owing to the intensive supply of sediment from the shelf and continental slope. A variety of such plains in the Pacific Ocean is the “train of archipelagoes” that rims the largest underwater volcanic chains and is composed of volcanogenic products. There are many separate underwater mountains (volcanoes, judging by their morphology) in the flat abyssal plains and abyssal hills. Some of them, called guyots, have flattened tops which are evidence that the latter were once at the surface of the ocean and later subsided. In tropical waters the tops of many underwater mountains have coral structures (atolls).
The mountain relief of the ocean floor consists of block or arched uplifts with linear and isometric outlines; often these are ridges that have chains of volcanoes on their crests. Uplifts with isometric outlines are also called oceanic plateaus. Some oceanic plateaus have a more level relief than the bottom of the basins. This is because of the greater speed of accumulation of sediments on the oceanic plateaus. The ocean floor also includes the marginal elevations that stretch along the oceanic side of the deep-water trenches.
Fractures play an important role in the structure of the relief of the ocean floor. The system of sublatitudinal oceanic fractures in the northeastern basin of the Pacific Ocean is the most immense. These fracture zones are usually represented in the relief of the bottom by complexes of grabens and horst uplifts that often stretch for thousands of kilometers.
REFERENCESShepard, F. P. Morskaia geologiia. Leningrad, 1967. (Translated from English.)
Leont’ev, O. K. Dno okeana. Moscow, 1968.
Udintsev, G. B. Geomorfologiia i tektonika dna Tikhogo okeana. Moscow, 1972. (Tikhii okean [vol. 5].)
The Sea, vol. 3. New York-London, 1963; vol. 4, parts 1–2, 1970.