or vulcanism
Any of various processes and phenomena associated with the surface discharge of molten rock or hot water and steam, including geysers , and fumaroles .
Most active volcanoes occur where two plates converge and one overrides the other (see {{link=plate tectonics">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.