The volcanoes
erupt because of density and pressure. Magma has lower density then the
surrounding rock that causes the magma to rise. The magma will rise to
the surface or to the depth that is determined by the density of the magma
and the weight of the rocks around it. As the magma rises, bubbles start
to form from the gas dissolved in the magma. The gas bubbles exert tremendous
pressure. This pressure helps to bring the magma to the surface and forces
it into the air.