Superheated water and steam trapped in rock strata in areas characterized by volcanic activity or by intrusions of molten magma. Associated temperatures range from 150 to 300C. It escapes either from natural surface vents (geysers, fumaroles, hot springs) or from bore holes drilled through the strata. A contributing source of heat is the natural radioactivity of rocks in the earth's upper mantle. Power was produced in Italy from geothermal sources as long ago as 1913; since then geothermal power plants have been installed in Iceland, New Zealand, France, Hungary, Japan, Mexico, and El Salvador. Many nonelectrical uses have been developed for home heating and industrial purposes, especially in Iceland. The chief source of geothermal energy in the continental U.S. is the California-Nevada area. The geysers in central California have been generating electric power from steam in substantial amounts for some years. The same is true of Hawaii. Geological formations appropriate for geothermal heat are so few that this form of energy will always have limited potential.
