Volcano Formation Volcanoes form when hot material from below rises and leaks into the crust. Magma, rising from lower reaches, gathers in a reservoir, in a weak portion of the 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. White arrows in the picture show the volcano getting bigger as magma rises inside.
Scientist often monitor the changing shape of a volcano, especially prior to an eruption.
This is a drawing of magma rising up through the crust, causing the volcano to expand prior to eruption. Volcano Formation 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.