Volcanism, also called vulcanicity, is the processes whereby magma, a complex of molten silicates containing water and other volatiles in solution, rises toward Earth's surface, there forming igneous rocks. These may be extruded on the Earth's surface (see lava; volcano) or intruded into subsurface rock layers as, for example, dikes, sills, and laccoliths,

On icy moons the phenomenon is sometimes called cryovolcanism to distinguish it from silicate or sulfur volcanism.