A volcano is a geological landform usually generated by the eruption through a planet's surface of magma, molten rock welling up from the planet's interior. Other forms of volcano include ice volcanoes (particularly on some moons of Jupiter, Saturn and Neptune) and mud volcanoes. On Earth, volcanoes tend to occur near the boundaries of crustal plates. Important exceptions exist in hotspot volcanoes, which occur at locations far from plate boundaries; hotspot volcanoes are also found elsewhere in the solar system, especially on its rocky planets and moons.
Like most of the interior of the earth, the movements and dynamics of magma are poorly understood. However, it is known that an eruption may follow movement of magma upwards into the solid layer (the earth's crust) beneath a volcano and occupying a magma chamber. Eventually, magma in the chamber is forced upwards and flows out across the planet's surface as lava, or the rising magma can heat water in the surrounding landform and change the water into steam, creating great pressure. As a result, explosive eruptions will occur. Such explosive eruptions either this or escaping gases from the magma can produce a wide range of volcanic debris such as volcanic ash (also known as tephra), volcanic bombs, which can be large enough to kill people and animals. Eruptions can vary from effusive to extremely explosive.
Many volcanoes are formed at destructive plate margins: where oceanic crust is forced below the continental crust because oceanic crust is denser than continental crust - this process is called subduction. As the oceanic crust is subducted, it descends into the mantle where temperatures are generally higher than near the surface of the planet. Increases in temperature and pressure with depth cause water trapped in the descending oceanic crust to escape from minerals in the crust. This process is called dehydration, commonly occurs at depths of about 100 km (62 miles) and can also be a source of very deep earthquakes due to an associated change in volume of the dehydrating rock mass (such as the 2001 Nisqually Earthquake in Washington State, USA). The water that escapes from the dehydrating oceanic crust migrates into the surrounding mantle which has a different composition than the descending crust. At ambient conditions in the mantle at 100km depth, water will induce partial melting of the mantle. This melt is less dense than the surrounding mantle and will consequently rise though the mantle to the overlying crust. As the magma (melt) rises through the crust it may melt and assimilate some of the surrounding crust, it may cool and begin to grow crystals, and it may exsolve gas.