Geothermal energy, long known for the tourist attractions it provided at Yellowstone National Park, is now being used for generating commercial amounts of electricity, for space heating, and as an aid to agriculture. Potential for development exists in many parts of the country in the form of geologic reservoirs of steam, hot water, warm water, hot dry rock, and deep geopressured reservoirs.

Areas with the most obvious potential are related to hot springs, active faulting, active seismicity, and recent volcanism. Heat-flow and subsurface-temperature data are particularly useful in localizing areas of geothermal activity. Tectonic setting and physiography, together with the above data, provide the basis for dividing the United States into a series of geothermal provinces. To date, the western provinces have shown the greatest potential for electric power development. The Geysers steam field has a generating capacity of 502 Mw with an estimated eventual capacity around 2,000 Mw. Several hot water fields are being developed for electric power generation in the Salton Trough of California, with an estimated total potential of 5,000 Mw. One hot-water discovery has been made in western Utah and three tests of possible power development potential are indicated in widely separated parts of the Great Basin and one discovery has been made in the Rio Grande Rift.

Research projects sponsored by the U.S. Department of Energy (DOE) are in progress to determine the feasibility of producing electricity from moderate temperature waters at Raft River, Idaho, and from hot dry rock at Fenton Hill, New Mexico. In the Gulf of Mexico province, DOE sponsored the testing of a well in Louisiana in 1977 and is currently drilling a 16,500-ft (5,029 m) test in Brazoria County, Texas, to evaluate geopressured reservoir potential.

Warm water has been used for space heating in Klamath Falls, Oregon, western South Dakota, and Boise, Idaho, for many years and other direct utilization projects are planned or in progress for use in space heating and agriculture. The search for warm water has been extended to the southeast Atlantic coastal plain where the DOE plans a series of shallow temperature holes to provide data to locate one or more deep tests.

Geothermal potential exists in broad areas of the western United States, where it appears capable of competing economically with other energy forms. Elsewhere the potential is lower but space heating and agricultural uses may become viable markets.