Volcanism is the process in which molten rock, ash, and gases erupt from below the surface of the Earth (or other planets and satellites). Previous volcanism is evidenced not only in volcanic mountains, craters, and plateaus, but more broadly through the underlying dikes, sills and magma chambers that may become exposed through erosion, through widely dispersed ashfall, through sediments eroded from volcanoes and deposited elsewhere, and through the effects of volcanism on the atmosphere, oceans and climate.

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