Volcanism is part of the process of bringing material from the deep interior of a planet and spilling it forth on the surface. Eruptions also eject new molecules into the atmosphere. Volcanism is part of the process by which a planet cools off. Even though they are not volcanoes, geysers and hot springs are also part of the volcanic process, involving water and hydrothermal activity. Some planetary bodies, like Jupiter's moon Europa, exhibit icy volcanism, which is another form of volcanism involving water.
There are several ways in which a volcano forms, just as there are several different kinds of volcanoes. On Earth, the most general cause of volcanism is caused by subduction of the Earth's crust.
There are quite a few other planets which have volcanoes on the surface, including Venus, Mars, and Jupiter's moon Io. Other planets exhibit the results of volcanic activity. These include Mercury, the Earth's Moon, Jupiter's moon Europa, and perhaps Neptune's moon Triton.
