SEAMOUNT

large submarine volcanic mountain rising at least 1,000 m (3,300 feet) above the surrounding deep-sea floor; smaller submarine volcanoes are called sea knolls, and flat-topped seamounts are called guyots. Great Meteor Tablemount in the northeast Atlantic, standing more than 4,000 m (13,120 feet) above the surrounding terrain, with a basal diameter of up to 110 km (70 miles), illustrates the size that such features can attain. The sides of larger seamounts generally are concave upward and rarely slope more than 14; smaller seamounts lack this concavity and can have sides as steep as 35. In plan, seamounts tend to be elliptical or elongate, possibly because the lavas are extruded from linear rifts in the seafloor. Most material dredged from seamounts is microcrystalline, or glassy, oceanic basalt that probably formed as submarine lava flows. The summits and flanks of seamounts are generally covered with a thin layer of marine sediment. Seamounts are exceedingly abundant and occur in all major ocean basins. By the late 1970s more than 10,000 seamounts had been reported from the Pacific Ocean basin alone. Virtually every oceanographic expedition discovers new seamounts, and it is estimated that approximately 20,000 exist in the oceans of the world. A linear cluster of seamounts may result when several are fed by lava extruded from a single linear rift. Most Pacific seamounts occur in linear clusters or elongate groups of 10 to 100. The individual seamounts in a chain may share a common ridge connecting their bases, as in the Mid-Pacific Mountains. Seamount chains in the Pacific basin tend to be aligned northwesterly, and several chains are intimately associated with fracture zones; the Eltanin Fracture Zone in the southwestern Pacific is an example. At least one seamount chain, the New England Seamounts, lies in the northwestern Atlantic. No seamount chains have been reported from the Indian Ocean, possibly because that basin has been less extensively surveyed.