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Learn More About Geothermal

Geothermal Energy as a Natural Resource (from USGS Circular 1249)

Geothermal energy is present everywhere beneath the Earth’s surface, although the highest temperature, and thus the most desirable, resources are concentrated in regions of active or geologically young volcanoes. Though the resource is thermal energy rather than a physical substance such as gold or coal, many aspects of geothermal energy are analogous to characteristics of mineral and fossil-fuel resources. Geothermal energy also has some unique, desirable attributes.

Map showing Earth’ s main lithospheric plates and some of the world’s active volcanoes (triangles). (Larger Image)

Global Distribution

Measurements made in drill holes, mines, and other excavations demonstrate that temperature increases downward within the Earth. The rate at which the temperature increases (temperature gradient or geothermal gradient) is proportional to the rate at which heat is escaping to the surface through the Earth’s crust (heat flow). Thus, zones of higher-than-average heat flow are the most likely places for encountering high temperatures at shallow depth, perhaps shallow enough to favor exploitation of geothermal energy. The average rate at which heat escapes through the Earth’s crust accounts for a prodigious amount each year, but local heat flow can vary widely from region to region.

Large quantities of heat that are economically extract-able tend to be concentrated in places where hot or even molten rock (magma) exists at relatively shallow depths in the Earth’s outermost layer (the crust). Such “hot” zones generally are near the boundaries of the dozen or so slabs of rigid rock (called plates) that form the Earth’s lithosphere, which is composed of the Earth’s crust and the uppermost, solid part of the underlying denser, hotter layer (the mantle). According to the now widely accepted theory of plate tectonics, these large, rigid lithospheric plates move relative to one another, at average rates of several centimeters per year, above hotter, mobile mantle material (the asthenosphere). High heat flow also is associated with the Earth’s “hot spots” (also called melting anomalies or thermal plumes), whose origins are somehow related to the narrowly focused upward flow of extremely hot mantle material from very deep within the Earth. Hot spots can occur at plate boundaries (for example, beneath Iceland) or in plate interiors thou-sands of kilometers from the nearest boundary (for example, the Hawaiian hot spot in the middle of the Pacific Plate). Regions of stretched and fault-broken rocks (rift valleys) within plates, like those in East Africa and along the Rio Grande River in Colorado and New Mexico, also are favorable target areas for high concentrations of the Earth’s heat at relatively shallow depths.

Zones of high heat flow near plate boundaries are also where most volcanic eruptions and earthquakes occur. The magma that feeds volcanoes originates in the mantle, and considerable heat accompanies the rising magma as it intrudes into volcanoes. Much of this intruding magma remains in the crust, beneath volcanoes, and constitutes an intense, high-temperature geothermal heat source for periods of thousands to millions of years, depending on the depth, volume, and frequency of intrusion. In addition, frequent earthquakes—produced as the tectonic plates grind against each other—fracture rocks, thus allowing water to circulate at depth and to transport heat toward the Earth’s surface. Together, the rise of magma from depth and the circulation of hot water (hydrothermal convection) maintain the high heat flow that is prevalent along plate boundaries.

Accordingly, the plate-boundary zones and hot spot regions are prime target areas for the discovery and development of high-temperature hydrothermal-convection systems capable of producing steam that can drive turbines to generate electricity. Even though such zones constitute less than 10 percent of the Earth’s surface, their potential to affect the world energy mix and related political and socioeconomic consequences is substantial, mainly because these zones include many developing nations. An excellent example is the boundary zone rimming the Pacific Plate—called the “Ring of Fire” because of its abundance of active volcanoes—that contains many high-temperature hydrothermal-convection systems. For the developing countries within this zone, the occurrence of an indigenous energy source, such as geothermal, could substantially bolster their national economies by reducing or eliminating the need to import hydro-carbon fuels for energy. The Philippines, Indonesia, and several countries in Central America already benefit greatly from geothermally generated electricity; additional projects are underway and planned. Of course, the use of geothermal energy already contributes to the economies of industrialized nations along the circum-Pacific Ring of Fire, such as the United States, Japan, New Zealand, and Mexico.

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Types of Geothermal Systems

1. Magmatic
   • Vapor-Dominated (The Geysers, Lardarello)
   • Liquid-Dominated (Salton Sea, Valles Caldera, Long Valley, Coso)
2. Amagmatic or “Deep Circulation”
   • High Heat Flow Settings (Dixie Valley, other Great Basin)
3. Temperature Classification
   • High Temperature >150 ^oC
   • Moderate Temperature 90 to 150 ^oC
   • Low Temperature <90 ^oC

   Crustal Cross-Section: Faults, fault rocks, stress, & fluid flow. (Larger Image)

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