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.
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.
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.
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.
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.