Geothermal Energy

Groundwater can be heated by a body of cooling magma or by penetrating deeply into the earth's crust along faults and being heated by the increased geothermal gradient.

Hot springs and geysers. Heated groundwater rises to the surface as hot springs and geysers. A hot spring consists of water 6 to 9 degrees centigrade warmer than the mean annual air temperature for the locality where it occurs. Hot‐spring pools are often steaming and actively forming new minerals. The hot springs in Yellowstone National Park and Nevada are good examples of groundwater heated by igneous activity.

A geyser is a more explosive hot spring that periodically erupts scalding water and steam. Fumaroles are vents from which steam and other gases escape. Geyser eruptions result from newly formed mineral deposits that clog the throat of the vent or from accumulations of vapor bubbles that increase the internal pressure. Water temperatures are generally near boiling. The hot water rapidly cools at the surface and precipitates new minerals around the geyser vents.

Typically composed of calcite or silica, the build‐up of these ledge‐like layers is called a sinter around a hot spring and geyserite around a geyser. A mudpot is a vent that produces thick, boiling mud and sulfurous gases. Rich precious‐metal deposits such as gold and silver are often associated with hot spring activity.

The use of geothermal energy. Geothermal energy is the energy produced when heated groundwater is tapped by wells and used to generate electricity. Although geothermal energy is one of the cleanest forms of energy, hydrogen sulfide gases and other toxic compounds may be associated with it. Compared to that of other sources of energy, the use of geothermal energy is not widespread; it can, however, be locally important, as it is in Iceland.