Heat is constantly flowing from the Earth's interior to the surface. U.S. geothermal resources alone are estimated at 70 million quads, equivalent to a 750,000-year supply of energy for the entire nation at current rates of consumption. The geothermal energy potential in the uppermost six miles of the Earth's crust amounts to 50,000 times the energy of all oil and gas resources in the world.
Several types of geothermal resources are under development, although only two can be harnessed by technologies that are commercially available today:
Hydrothermal fluid resources
These reservoirs of steam or hot water are formed by water seeping into the Earth, collecting in, and being heated by, fractured or porous hot rock. Some of these reservoirs have an existing path to the surface, emerging as hot springs. Others are tapped by drilling wells to deliver hot water to the surface for generation of electricity or direct use. In the United States, the hottest (and currently most valuable) resources are located in the western states, Alaska, and Hawaii.
Shallow ground energy
The upper 10 feet of the Earth has a nearly constant temperature between 50°F and 60°F. This rock, soil, and water form the resource tapped by geothermal heat pumps. The ground acts as a heat source in the winter and absorbs heat for cooling in the summer. All areas of the United States have nearly constant shallow-ground temperatures and are suitable for geothermal heat pumps.
For more information about geothermal resources, pursue one of the following links:
State-by-state resource maps — Detailed geothermal resource maps for 11 western states, Alaska, and Hawaii.
Tribe-specific maps — Detailed tribal maps based on the state-by-state maps.
Geothermal Resource Information Clearinghouse — Provides links to several sources of information about geothermal resources.