Clear Lake volcanic field lies in the northern Coast Ranges, California, ca. 135 km north of San Francisco. The volcanic field consists of lava dome complexes, cinder cones, and maars of basaltic-to-rhyolitic composition. Mount Konocti, a dacitic lava dome on the south shore of Clear Lake, is the largest volcanic feature.
The area has intense gethermal activity, caused by a large, still hot silicic magma chamber about 14 km wide and 7 km beneath the surface. It provides the heat source for the Geysers, the world's largest producing geothermal field on the SW side of the volcanic field. Its geothermal power plants can generate approximately 2000 megawatts, enough to power two cities the size of San Francisco.
The latest volcanic activity happened about 10,000 years ago and formed maars and cinder cones along the shores of Clear Lake, the larges natural freshwater lake in California. Volcanism around Clear Lake is related to the complex San Andreas transform fault system.
Volcanic activity has been largely non-explosive, with the exception of one major explosive eruption that must have occurred and deposited a airfall tuff. Most lavas found here range from basalt through rhyolite in composition, dacite being the most dominant type.
Eruptions at Clear Lake volcanic field:
4 eruptive episodes separated by time gaps of ca. 150-200,000 years have been recognized: 2.1-1.3 million years ago (m.y.), 1.1-0.8 m.y., 0.65-0.30 m.y., and 100,000-10,000 years ago. The total volume of about 100 individual eruptions exceeds 70 cubic kilometers.
Eruptive products from the first activity episode are found in the east of the field. The second activity episode constructed Cobb Mountain (1 million years, Ma) and Mount Hannah (0.9 Ma). The third episode of activity was at the Mount Konocti–Thurston Lake area, the most voluminous dacite and rhyolite feature of the Clear Lake volcanics.
The most recent activity, up to about 10,000 years ago were small mostly basaltic and andesitic eruptions in the north of the field.
Sources: GVP, NCGS (Northern California Geological Society)
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