There are hot spots in the Earth's crust where heat melts underground rock.
As the rock melts, gases are released and mix with molten rock called
magma. The magma bubbles and rises,
cutting through the rock, creating a channel. It then accumulates in a magma
chamber deep underground.
Over time, high pressure caused by both the
magma and gases build up in the chamber, forcing the magma to rise and
causing the volcano to form as a mountain. Then it violently explodes
out the top -- and even the sides of the volcano! Fiery rocks and ash rain
down from out of the sky after an eruption. But sometimes, lava, which is
what the magma is called after it comes out of the volcano, quietly
oozes out of one or several holes (
vents ) with no explosion at all.