This is a drawing of magma percolating up through the crust, causing the volcano to expand prior to eruption.
Click on image for full size
Volcanoes form when hot material from below rises and leaks into the crust. This hot material, called magma, comes either from a melt of subducted crustal material, which is light and buoyant after melting, or it may come from deeper in the interior of a planet and is light and buoyant because it is *very* hot.
Magma, rising from lower reaches, gathers in a reservoir, in a porous region of overlying rock called the magma chamber. Eventually, but not always, the magma erupts onto the surface. Strong earthquakes accompany rising magma, and the volcanic cone may swell in appearance, just before an eruption, as illustrated in this picture. Scientist often monitor the changing appearance of a volcano, especially prior to an eruption.
The different reasons why a volcano forms are
- via plumes or hot spots in the lithosphere
- as a result of subduction of the nearby lithosphere
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