Though geothermal resources lie beneath all of the United States, they are easier to reach near active faults. The Mountain West, populated with active faults and plagued by tectonic activity, has the highest underground temperatures near the surface. Thus, it shows most of the installed capacity for geothermal energy. Some pockets in the Gulf of Mexico region and the areas around oil and gas production also feature high subterranean temperatures with relatively easy access.
The National Renewable Energy Laboratory ranks areas of the United States by favorability of Deep Enhanced Geothermal Systems (EGS) based on data from the U.S. Geological Survey. This map roughly corresponds with deep subterranean temperatures across the country.
Outside of the Class V deep geothermal injection wells, geothermal heat pumps are used in some residential use cases. Ground temperature remains constant year-round independent of seasonal variations in air temperature. Heat pumps leverage this constant temperature and trade warmth in summer for the cooler underground temperatures. The reverse occurs in winter when the unit exchanges cold with the relatively warmer underground. Most heat pumps are closed systems of pipes and coils, but the open loop system uses a well or body of surface water as the heat exchange fluid. Wells that return water to the groundwater source (typically an underground source of drinking water) bear Class V classification.
Images: “Earth power” by xavierarnau via iStock; “Geothermal Resource of the United States” by USNREL; “geothermal_heatpump” by Coleman Tharpe