The term volcano refers to the opening or vent through which the molten rock and associated gases are expelled. Volcanoes are mountains, but they are very different from other mountains; they are not formed by folding and crumpling or by uplift and erosion. Instead, volcanoes are built by the accumulation of their own eruptive products. A volcano can also be defined as a liquid rock plumbing system which extends from several 10's of kilometers depth to the earth's surface, and includes the near vent deposits of eruptions. Geologists have generally grouped volcanoes into four main kinds: cinder cones, composite volcanoes, shield volcanoes, and lava domes.
Deep inside Earth, between the molten iron core and the thin crust at the surface, there is a solid body of rock called the mantle. When rock from the mantle melts, moves to the surface through the crust, and releases pent-up gases, volcanoes erupt(explode). Extremely high temperature and pressure cause the rock to melt and become liquid rock or magma. When a large body of magma has formed, it rises through the denser rock layers toward Earth's surface. Magma that has reached the surface is called lava. Lava, ash, and debris flows are the most common and serious volcanic hazards, but others do exist.
Obviously, there is no way to stop an eruption. It is possible, however, to attempt to reduce the eruption's effects by reinforcing structures. For example, strengthening roofs to support the weight of tephra - the material blown out of the volcanic vent when an explosion occurs- deposits or by building protective works (such as walls to deflect lava flows away from developed areas). Such efforts can be and have been successful, but are of limited use in a large-scale eruption.
There are more than 500 active volcanoes (those that have erupted at least once within recorded history) in the world, not counting those that lie beneath the sea. Fifty of them are in the United States (Hawaii, Alaska, Washington, Oregon, and California).