This is a drawing of magma making it's way 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.
The hot material, called magma, rising from lower ground, gathers in a reservoir called the magma chamber. Eventually, but not always, the magma erupts onto the surface and forms lava. Strong earthquakes accompany rising lava, and the volcano may swell just before an eruption, as shown in this animation.
White arrows in the picture show the volcano getting bigger as the lava rises inside. Scientist measure the swelling of a volcano before an eruption.
The different reasons why a volcano forms are
- when bubbles of material rise to the surface
- when the crust is forced to go down
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