A volcano is any vent in the crust of the Earth or other planet or satellite, from which issue molten rock, pyroclastic debris, and steam. Volcanoes are closely associated with tectonic activity. Most of them occur on either the overriding or the diverging margins of the enormous lithospheric plates that make up the Earth's surface. The volcanoes of Japan provide an excellent example of the former, while those of the Mid-Atlantic Ridge the latter. Intraplate volcanoes such as those of the Hawaiian chain provide important evidence as to the direction and rate of plate motion.
Volcanoes affect humankind in many ways.
Their destructiveness is awesome, but the risk involved can be reduced by assessing volcanic hazards and forecasting volcanic eruptions. Volcanism provides fertile soils, valuable mineral deposits, and geothermal energy. This shaking, which accounts for the destructiveness of an earthquake, is caused by the passage of elastic waves through the Earth's rocks. These seismic waves are produced when some form of stored energy, such as elastic strain, chemical energy, or gravitational energy, is released suddenly. Few natural phenomena can wreak as much havoc as earthquakes.