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Geology & History

The Coso volcanic field is located east of the Sierra Nevada between Owens Valley and the Garlock fault in southern California within the present-day Basin and Range province. Volcanism began in the Coso Range approximately 6 million years ago, and the earliest basaltic lava flows are tilted, indicating that the start of volcanism in the area preceded the onset of faulting associated with Basin and Range extension.

Three periods of volcanic activity have occurred within the Coso Range. Basalt was the primary magma type erupted during the oldest episode, which started 6 million years ago. The most volcanically active period occurred during the Pliocene between about 4 and 2.5 million years ago producing 30 km3 (7.2 mi3) of the total 35 km3 (8.4 mi3) erupted material found in the Coso Range. Volcanic rocks age dated to this time period include basalt, andesite, dacite, rhyodacite and high-silica rhyolite. The third period of activity occurred during the Pleistocene between about 1.1 and 0.04 million years ago. The region is most well known for the 38 high-silica rhyolite domes and lava flows that erupted during this time period. These well-known domes were accompanied by the eruption of 14 basaltic centers.

The Coso volcanic field is also known for its geothermal activity and was long recognized as a potential geothermal resource, because it contains boiling mud pots and fumaroles within an area of many little-eroded volcanoes. During the mid 1970’s, a team of government and university scientists carried out a variety of field studies that suggested a potential geothermal resource of several hundred megawatts electric. Subsequent drilling has resulted in the production of about 270 megawatts of electricity, and power plants in the region continue to develop technology to ensure optimal energy output from the active region.