<P> Existing rocks that come into contact with magma may be melted and assimilated into the magma . Other rocks adjacent to the magma may be altered by contact metamorphism or metasomatism as they are affected by the heat and escaping or externally - circulating hydrothermal fluids . </P> <P> Volcanism is not confined only to Earth, but is thought to be found on any body having a solid crust and fluid mantle . Evidence of volcanism should still be found on any body that has had volcanism at some point in its history . Volcanoes have indeed been clearly observed on other bodies in the Solar System--on some, such as Mars, in the shape of mountains that are unmistakably old volcanoes (most notably Olympus Mons), but on Io actual ongoing eruptions have been observed . It can be surmised that volcanism exists on planets and moons of this type in other planetary systems as well . In 2014, scientists found 70 lava flows which formed on the Moon in the last 100 million years . </P>

Where does most of earths volcanic activity occur