Appendix G

Geological Processes and Geothermal Energy

The genesis of geothermal resources lies in the geological transport of anomalous amounts of heat close enough to the surface for access. Thus, the distribution of geothermal areas is not random but is governed by geological processes of global, regional and local scale. This fact is important in exploration for geothermal resources.

Geothermal resources commonly have three components: (1) an anomalous concentration of heat (i.e., a heat source); (2) fluid to transport the heat from the rock to the surface; and (3) permeability in the rock sufficient to form a plumbing system through which the water can circulate.

Heat Sources

The two most common sources of geothermal heat are: (1) intrusion of molten rock (magma) from great depth to high levels in the earth's crust; and (2) ascent of groundwater that has circulated to depths of 1 to 3 miles (1.6 to 5 km) and has been heated in the normal or enhanced geothermal gradient without occurrence of a nearby intrusion.

One geological process that generates shallow magmatic crustal intrusions in several different ways is known as plate tectonics.(1) As the laterally moving oceanic plates press against neighboring plates, some of which contain the imbedded continental land masses, the oceanic plates are thrust beneath the continental plates. These zones of under-thrusting, where crust is consumed, are called subduction zones.(2)

The subducted plate descends into the mantle and is heated by the surrounding warmer material and by friction. Temperatures become high enough to cause partial melting. Since molten or partially molten rock bodies (magmas) are lighter than solid rock, the magmas ascend buoyantly through the crust. Volcanos result if some of the molten material escapes at the surface, but the majority of the magma usually cools and consolidates underground. Crustal intrusion and volcanos occur on the landward side of oceanic trenches 30 to 150 miles (50 to 250 km) inland. The volcanos of the Cascade Range of California, Oregon, and Washington, for example, overlay the subducting Juan de Fuca plate and owe their origin to the process just described. The Pacific Ring of Fire, which extends around the margins of the Pacific basin, is composed of volcanos in the Aleutians, Japan, the Philippines, Indonesia, New Zealand, South America, and Central America, all of which are due to subduction.

Another important source of volcanic rocks are point sources of heat in the mantle. The mantle contains local areas of upwelling, hot material called plumes,(3) which have persisted for millions of years. As crustal plates move over these hot spots, a linear or arcuate chain of volcanos results, with young volcanic rocks at one end of the chain and older ones at the other end. The Hawaiian Island chain is an example. The thermal features of Yellowstone National Park are believed to be the result of an underlying mantle plume.


Geothermal resources require a fluid transport medium.(4) In the earth that medium is groundwater that circulates near or through the heat source. The groundwater can originate as connate water that was trapped in voids during the formation of the rock. But quite often the water is meteoric in origin, meaning that it percolated from the surface along pathways determined by geological structures such as faults and formation boundaries. The density and viscosity of water both decrease as temperature increases. Water heated at depth is lighter than cold water in surrounding rocks, and is therefore subject to buoyant forces that tend to push it upward. If heating is great enough for buoyancy to overcome the resistance to flow in the rock, heated water will rise toward the earth's surface. As it rises, cooler water moves in to replace it. In this way, natural convection is set up in the groundwater around and above a heat source such as an intrusion. Convection can bring large quantities of heat within reach of wells drilled form the surface.

Because of their varied origin and the reactivity inherent to heated water, geothermal waters exhibit a wide range of chemical compositions. Salinities can range from a few parts per million up to 30 percent; dissolved gases such as carbon dioxide and hydrogen sulfide are common. As a result, geothermal waters play an important role in crustal processes, not only in transporting heat, but also in altering the physicochemical properties of rock. Such fluids have produced many ore deposits of copper, lead, zinc, and other metals in proximity to heat sources.


Permeability is a measure of a rock's capacity to transmit fluid. The flow takes place in pores between mineral grains and in open spaces created by fractures and faults. Porosity is the term given to the amount of void space in a volume of rock. Interconnected porosity provides flow paths for the fluids, and creates permeability. The porosity of the reservoir rocks determines the total amount of fluid available, whereas the permeability determines the rate at which fluid can be produced. One must not envisage a large bathtub of hot water that can be tapped at any handy location. Both porosity and permeability vary over wide ranges at different points in the reservoir. Open fault zones, fractures and fracture intersections, contacts between different rock types and shattered zones produced by hydraulic fracturing, and mineral growth areas in rocks all lead to varying degrees of permeability.

Most geothermal systems are structurally controlled, i.e., the magmatic heat source has been emplaced along zones of structural weakness in the crust. Permeability may be increased around the intrusion from fracturing and faulting in response to stresses involved in the intrusion process itself and in response to regional stresses.

Hydrothermal Resources

A conceptual model of a hydrothermal system where steam is the pressure-controlling phase is a so-called vapor-dominated geothermal system.(5) The Geysers geothermal area in California, about 80 miles north of San Francisco, is a vapor-dominated resource. Steam is produced from depths of 3,000 to 10,000 feet (1 km to 3 km) and is used to run turbine engines which turn electrical generators. The Geysers is still the largest geothermal electric producing area in the world despite the continued drop in production and lack of adequate recharging of the required fluids. Other producing vapor-dominated resources occur at Larderello and Monte Amiata, Italy, and at Matsukawa, Japan.

In a high-temperature, liquid-dominated geothermal system(6),(7),(8),(9) groundwater circulates downward in open fractures and removes heat from deep, hot rocks as it rises buoyantly and is replaced by cool recharge water moving in from the sides. Rapid convection produces uniform temperatures over large volumes of the reservoir. There is typically an upflow zone at the center of each convection cell, an outflow zone or plume of heated water moving laterally away from the center of the system, and a downflow zone where recharge water is actively moving downward. Escape of hot fluids is often minimized by a near-surface sealed zone or caprock formed by precipitation of minerals in fractures and pore spaces.

Return to Table of Contents
* * *