Geothermal energy is the heat that originates within the earth.

Many of the large-scale geological processes that have helped to form the earth’s surface features are powered by the flow of heat from inner regions of higher temperature to outer regions of lower temperature.

Generation of new oceanic crust at spreading centers such as the mid-Atlantic ridge, motion of the great lithosphere plates, uplifting of mountain ranges, release of stored strain energy by earthquakes and eruption of volcanoes are all powered by the outward transport of internal heat. Plastic, partially molten rock at estimated temperatures between 600°C and 1,200°C (1,100°F and 2,200°F) is postulated to exist everywhere beneath the earth’s surface at depths of 100 km (60 mi) or less.

By comparison, using present technology applied under favorable circumstances, holes can be drilled to depths of about 10 km (6.2 mi), where temperatures range upward from about 150°C (300°F) in average areas to perhaps 600°C (1,100°F) in exceptional areas.

Exploitable geothermal resources originate from transport of heat to the surface through several geological and hydrological processes. Geothermal resources commonly have three components: 1) a heat source, 2) relatively high permeability reservoir rock, and 3) water to transfer the heat.

In general, the heat source for most of the high-temperature resources (>150°C [300°F]) appears to be a molten or recently solidified intrusion, whereas many of the low-temperature (<100°C [212°F]) and moderate-temperature resources (between 100° and 150°C [212° and 300°F]) seem to result from deep circulation of meteoric water with heating due to the normal increase in temperature with depth.

A number of high-temperature resources also occur in the Basin and Range Province of the western U.S. as the result of deep circulation along major faults in a region of high heat flow.

In most geothermal systems, fracture permeability controls water movement, but inter-granular permeability is also important in some systems. Water is, of course, the ideal heat transfer fluid because it has a high heat capacity and high heat of vaporization, and can therefore transport more heat per unit volume that any other common fluid.

This table summarizes the way that geothermal resources are commonly classified. For the most part, only convective hydrothermal resources have been commercially developed. The other resource types will require new technology and/or higher energy prices in order to be more economically viable.

Geologists have discussed models for high-temperature convective hydrothermal systems. A body of molten, or recently solidified, hot rock (300°C to 1,200°C [570°F to 2,200°F]) presumably underlies higher-temperature hydrothermal resources. Interaction of this hot rock with ground water causes heating of the ground water, which then rises by buoyancy.

The bulk of the fluid in hydrothermal systems is derived from meteoric water, with the exception of those few systems where the fluids are derived from seawater or connate brines.

A free convective circulating system is set up with the heated water ascending in the center of the system along zones of permeability, spreading outward in the shallow subsurface or discharging to the surface, and with cool water descending along the margins and recharging the system. Rapid convection produces nearly uniform temperatures over large volumes of the reservoir.

The temperatures and pressures generally lie near the curve of boiling point versus depth for saline water, and sporadic boiling may occur. Whether or not steam actually exists in a hydrothermal resource depends, among other less important variables, on temperature and pressure conditions at depth.

Escape of hot fluids at the surface is often minimized by a near-surface, sealed zone or cap-rock formed of minerals precipitated from the geothermal fluids in fractures and pore spaces.