Although geothermal power has a range of applications, this paper primarily focuses on utility-scale use for geothermal power plants, and does not address smaller scale geothermal heat pumps like those used to heat and cool buildings.
Geothermal energy comes from deep earth heat that remains from the formation of the planet, as well as ongoing radioactive decay inside the earth’s mantle. The high temperatures form magma, which heats surrounding rock in the crust. This natural heat of the earth is most easily available for use when it comes closer to the surface as hot springs, geysers, lava pits, underground hot water or steam reservoirs, and some high temperature dry rock reservoirs.
These kinds of conditions are most often found in tectonic plate boundary regions, like the Western United States, as seen in Figure 1 below, or in geologic “hot spots” of geothermal activity like Hawaii.