Radioactivity in the Earth's interior is the ultimate cause
of terrestrial volcanoes. As heat radiates outward, it melts
some of the rocks in the upper mantle. This molten rock,
also known as magma,
is less dense than surrounding rock and will "float" upward—into
the earth's crust along fissures, or weak areas in the crust.
Eventually, magma accumulates in a chamber 2-4 km below the Earth's
surface. As the chamber fills, it pushes the earth's surface
outward, forming additional fissures. When the pressure
in the chamber is great enough, magma may force its way to
the surface through smaller fissures and cracks. Wherever these
cracks lead to a vent, or opening, in the earth's surface, the magma
will escape as a lava flow or other type of volcanic eruption.
