The high pressure deep inside the earth pushes the magma to the surface of the earth's crust. It makes the volcano erupt, spewing hot lava, which is liquified magma, and other solid materials made up of ash and pumice. A volcano serves as a vent for the magma, and as a safety valve to release the pressure from deep below the earth's surface. There are many different kinds of volcanoes. Volcanologists, scientists who study volcanoes, classify them by their shape and type of eruption.
Composite volcanoes are the most common type and they can be extremely explosive and destructive.
Mount St. Helens, in the state of Washington, is a famous composite volcano. After sleeping for 123 years, it awoke with an explosion of ash and steam on 27 March 1980. A bulge had begun to grow on its north slope because highly pressurized magma was forcing its way into the volcano. Finally, on 18 May of the same year, a pillar of ash and pumice blasted out of the bulge, tearing the volcano apart. Volcanologists classify the Mount St. Helens eruption as a Plinian eruption.