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