VOLCANISM


Meaning of VOLCANISM in English

also spelled Vulcanism, any of various processes and phenomena associated with the surficial discharge of molten rock or hot water and steam, including volcanoes, geysers, and fumaroles. Although volcanism is best known on the Earth, there is evidence that it has been important in the development of other planets such as Mercury, Venus, and Mars, as well as certain satellites. On the Earth, volcanism occurs in several distinct geological settings. Most of these, however, are associated with the boundaries of the enormous, rigid plates that make up the lithospherethe crust and upper mantle. See also plate tectonics. The majority of active terrestrial volcanoes (roughly 80 percent) and related phenomena occur where two lithospheric plates converge and one overrides the other, forcing it down into the mantle to be reabsorbed. Long curved chains of islands known as island arcs form at such subduction zones. Volcanoes of the explosive type make up many of the islands of a single arc or the inner row of islands of a double arc. All such islands, which border the Pacific Basin, are built up from the sea floor, usually by the extrusion of basaltic and andesitic magmas. A second major site of active volcanism is along the axis of the mid-ocean ridge system, where the plates move apart on both sides of the ridge, and magma wells up from the mantle, creating new ocean floor along the trailing edges of both plates. Virtually all of this volcanic activity occurs under water. In a few places the mid-ocean ridges stand sufficiently high from the deep ocean floor that they break the surface, and sub-aerial volcanism occurs. Iceland is the best-known example. The magmas that are erupted along the mid-ocean ridges are basaltic in composition. A relatively small number of volcanoes occur within plates far from their margins. Some, as exemplified by the volcanic islands of Hawaii that lie in the interior of the Pacific Plate, are thought to occur because of plate movement over a hot spot from which magmas can penetrate to the surface. These magmas characteristically generate a chain of progressively older volcanoes, which mark the direction of past motion of the plate over a particular hot spot. The highly active volcanoes of the East African Rift Valley also occur within a plate (the African plate) but appear to result from a different mechanismpossibly an extremely slow form of plate spreading.

Britannica English vocabulary.      Английский словарь Британика.