A volcano is a rupture in the Earth's crust, that allows (usually) hot,
molten rock (lava), ash, and gases to escape from deep within the Earth's
surface from a magma chamber. Volcano eruptions and volcanic activity involves
the accumulation of rock which is a driving force in geological formations such
as mountain and mountain ranges over long periods of time.
Volcanoes are generally found at the separation point or collision point of two or three tectonic plates. For instance, a mid-oceanic ridge, such as the Mid-Atlantic Ridge, offers an excellent example of volcanoes caused by "divergent tectonic plates" pulling apart. In contrast, the Pacific Ring of Fire has examples of volcanoes caused by "convergent tectonic plates" coming together.
Volcanoes do not usually form where two tectonic plates slide past one another (as in the San Andreas fault). Volcanoes can also form where there is stretching of the Earth's crust and where the crust grows thin (called "non-hotspot intraplate volcanism"), such as in the African Rift Valley or the European Rhine Graben with its Eifel volcanoes).
Volcanoes can be caused by "mantle plumes," so-called "hotspots;" these hotspots can occur far from plate boundaries, such as the Hawaiian Islands.