To understand how electricity is produced at The Geysers, it is helpful to know how geothermal energy begins. Molten rock, or magma, exists very deep in the earth where temperatures are extremely high. However, magma is relatively close to the surface in a few areas, particularly the volcanically active areas around the Pacific Rim. At The Geysers, the magma is believed to exist at least four miles below the earth's surface. The heat from the magma radiates to the layers of rock above heating water in the pores and fractures of the hot rock. A small portion of the heated water may rise to the surface, creating hot springs, geysers and fumaroles. At The Geysers, the reservoir water boils to steam and is trapped by an overlying layer of tight, unfractured rock, called cap rock.
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