Chapter 9: Geothermal Engineering

The word “geothermal” comes from the combination of the Greek words gê, meaning Earth, and thérm, meaning heat. Quite literally, geothermal energy is the heat of the Earth. Geothermal resources are concentrations of the Earth’s heat, or geothermal energy, that can be extracted and used economically now or in the reasonable future.

Spatial variations of the thermal energy within the deep crust and mantle of the Earth give rise to concentrations of thermal energy near the surface of the Earth which can be used as an energy resource. Heat is transferred from the deeper portions of the Earth by conduction through rocks, by movement of hot magma toward the surface, and by deep circulation of water. Most high-temperature geothermal resources are associated with concentrations of heat caused by the movement of magma (melted rock) to near-surface positions where the heat is stored. Because rocks have relatively small thermal conductivity, very large intrusions of magma may take millions of years to cool. ¹