9. Geothermal

B. Geothermal Resources

Geothermal energy resources result from complex geologic processes that lead to heat concentration at accessible depths.(7) The different forms of geothermal energy resources--hydrothermal, hot dry rock, geopressured, magma, and earth heat--all result from this concentration of Earth's heat in discrete regions of the subsurface. Temperature within the Earth increases with increasing depth (Figure 19). Highly viscous or partially molten rock(8) at temperatures between 1,200 and 2,200°F (650 to 1,200°C) is postulated to exist everywhere beneath the Earth's surface at depths of 50 to 60 miles (80 to 100 kilometers), and the temperature at the Earth's center, nearly 4,000 miles (6,400 kilometers) deep, is estimated to be 7,200°F (4,000°C) or higher. Heat flows constantly from its sources within the Earth to the surface.

Three sources of internal heat are most important: (1) heat released from decay of naturally radioactive elements; (2) heat of impact and compression released during the original formation of the Earth by accretion of in-falling meteorites; and (3) heat released from the sinking of abundant heavy metals (iron, nickel, copper) as they descended to form the Earth's core.An estimated 45 to 85 percent of the heat escaping from the Earth originates from radioactive decay of elements concentrated in the crust.(9),(10) The remainder results from slow cooling of the Earth, with heat being brought up from the core by convection in the viscous mantle.(11)

The different forms of geothermal resources have different characteristics that are important to geothermal energy development:

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