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Mid-Atlantic Ridge:see Atlantic OceanAtlantic Ocean
[Lat.,=of Atlas], second largest ocean (c.31,800,000 sq mi/82,362,000 sq km; c.36,000,000 sq mi/93,240,000 sq km with marginal seas). Physical Geography
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the largest mountain system on the floor of the Atlantic Ocean, one of the links in the system of mid-oceanic ridges. The Mid-Atlantic Ridge is more than 18,000 km long and includes several smaller ridges. The Knipovich Ridge is situated in the basins of the Norwegian and Greenland seas, from 81° to 73° N lat. Mohns Ridge is located to the south, and the Jan Mayen Ridge even further south. The Reykjanes Ridge and the Northern Mid-Atlantic Ridge are south of Iceland, the former extending up to 52° N lat. The Southern Mid-Atlantic Ridge is situated south of the equator.
The transverse fractures of the Mid-Atlantic Ridge break the ridge into segments displaced by as much as 300–600 km from one another. Data from deepwater drilling, seismic profiling, and dredging done in the rift zone have shown that the geological cross section of the ridge is composed of two complexes: an upper complex consisting of tholeiite basalt with interlayers of carbonate sedimentary rocks, and a lower complex composed of am-phibolites and ophiolites (from anorthosites to ultrabasites). The rocks of the upper complex, which date to the Oligocene-Anthro-pogene, are broken into numerous blocks by young faults. The rocks of the lower complex, belonging to the Jurassic-Oligocene, are dislocated and metamorphosed regionally; the age of the ser-pentinized ultrabasites and gabbros that frequently occur in the metamorphic complex as flakes and protrusions may be Precam-brian. In the southern hemisphere the Mid-Atlantic Ridge is more monolithic; its slopes include a series of underwater volcanoes, some of whose peaks form islands (for example, Ascension Island and St. Helena). There is an active volcano in the Tristan da Cunha island group.
The meridional strike of the Mid-Atlantic Ridge changes to sublatitudinal near Bouvet Island (approximately 55° S lat.); the eastern segment is called the African-Antarctic Ridge.
REFERENCESHazen, B., M. Tarf, and M. Ewing. Dno Atlanticheskogo okeana. Moscow, 1962. (Translated from English.)
Leont’ev, O. K. Dno okeana. Moscow, 1968.
Il’in, A. V. “Osnovnye cherty geomorfologii dna Atlanticheskogo okeana.” In Usloviia sedimentatsii v Atlanticheskom okeane. Moscow, 1971.
O. K. LEONTEV