Any of various processes and phenomena associated with the surface discharge of molten rock or hot water and steam, including volcanoes, geysers, and fumaroles. Most active volcanoes occur where two plates converge and one overrides the other (see plate tectonics). Volcanism can also occur along the axis of an oceanic ridge, where the plates move apart and magma wells up from the mantle. A few volcanoes occur within plates, far from margins. Some of these are thought to occur as a plate moves over a “hot spot” from which magma can penetrate to the surface; others appear to result from an extremely slow form of plate spreading.
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