Oceanic Ridge

Oceanic Ridge, term usually taken to mean mid-ocean ridges, which are broad undersea mountain ranges typically occurring far from land. There are, however, many topographic features on the ocean floor that could be described as ridges.

The mid-ocean ridge system constitutes the largest mountain chain on Earth. It can be traced from the Siberian continental shelf across the Arctic Ocean (the Nansen Ridge) and through the Norwegian Sea to Iceland, where it can be identified on land. From Iceland it continues generally southwards through the mid-Atlantic as the Mid-Atlantic Ridge, around the Cape of Good Hope into the Indian Ocean. It then splits between Madagascar and India, one branch running north-west to the Gulf of Aden, where it again divides—part of it continuing to the Red Sea, and the other becoming a terrestrial mountain range as the African rift-valley system. The second branch continues to the south of Australia and New Zealand, across the South Pacific and on to the Gulf of California. In the eastern Pacific the ridge, here called the East Pacific Rise, is not in mid-ocean but gradually approaches the eastern shore of the Pacific Ocean.

A typical east-west echo-sounding crossing of an ocean basin like the North Atlantic Ocean shows a more or less central mid-ocean ridge rising about 3,000 m (9,840 ft) above the typical ocean floor at a depth of 4,000 to 5,000 m (13,120 to 16,400 ft). The ridge is in very rugged relief, with broad flanks, while its crest is often marked by a deep longitudinal cleft—a rift valley—along which there are numerous shallow earthquakes, and volcanism that pours out lavas of basalt. To each side of the ridge the depth of the volcanic crust increases gradually and the thickness of sediment increases; the seismic activity dies out more rapidly. Away from the ridge crest there are only scattered volcanoes, forming seamounts. The crest of the mid-ocean ridge may be offset laterally for large distances by fracture zones. In the equatorial Atlantic a series of semiparallel fracture zones offset the Mid-Atlantic Ridge by about 3,500 km (2,170 mi), so maintaining its median position.

Mid-ocean ridges are centres of seafloor spreading, the process by which new ocean crust is formed. They occur at the boundaries between two plates of oceanic crust where hot-molten lava rises to the surface, cools, and solidifies, with older crust moving away to either side of the ridge. Points on the Mid-Atlantic ridge move apart at about 2 cm (0.78 in) a year, while on the East Pacific Rise they separate more quickly, at about 14 cm (5.5 in) a year. The gradual change in the submerged volume of the mid-ocean ridges causes the very slow changes in sea level over geological time.

The crests of the spreading ridges are also the regions where hydrothermal vents are found, from which hot (up to 350° C) mineral-rich water vapour emerges from cracks in the seafloor. This water deposits pillar-like structures of metallic sulphides, supporting colonies of unusual animals. The chemicals coming from the hydrothermal vents play an important part in maintaining the composition of seawater.

The main mid-ocean ridge system is fundamental to our knowledge of plate tectonics. However, not all ridges in the ocean are seismically active, for example, the Ninety East Ridge and the Mascarene Plateau in the Indian Ocean. The origins of these features appear to differ. The Ninety East Ridge was probably an exposed volcanic ridge which sank at about the same rate as the crust of the Indian Plate. The Mascarene Plateau is thought to be an isolated continental fragment—a microcontinent.

See also Continental Drift.