DESCRIPTION:
Clear Lake Volcanic Field, California

The late-Pliocene to early Holocene Clear Lake volcanic field in the northern Coast Ranges, contains lava dome complexes, cinder cones, and maars of basaltic-to-rhyolitic composition. The westernmost site of Quaternary volcanism in California, the Clear Lake field is located far to the west of the Cascade Range in a complex geologic setting within the San Andreas transform fault system. Mount Konocti, a composite dacitic lava dome on the south shore of Clear Lake, is the largest volcanic feature. Volcanism has been largely non-explosive, with only one major airfall tuff and no ash flows. The latest eruptive activity, forming maars and cinder cones along the shores of Clear Lake, continued until about 10,000 years ago. A large silicic magma chamber provides the heat source for the Geysers, the world's largest producing geothermal field.

Some researchers (e.g. Christiansen and Lipman, 1972) have suggested that Sutter Buttes and the Sonoma and Clear Lake volcanics, south and southwest of Lassen, are older extensions of subduction-related Cascade volcanism. This seems unlikely. If Sutter Buttes were part of a series of older Cascade stratovolcanoes abandoned due to the northward migration of the south end of Juan de Fuca Plate, the "last Cascade volcano" hypothesis would be tenable. But northward, arc volcanoes are young and active. In fact, why do the Cascades have an abrupt southern termination?

The Clear Lake and Sonoma volcanics are the less-than-5-million-year old components of a northwesterly younging line of volcanic fields of Tertiary to Holocene age (Hearn, et.al., 1981). All these volcanics lie within the San Andreas fault system, which appears to have provided magma access to the surface. Hearn et.al. point out that the timing of the volcanism suggests that it follows termination of subduction, as the Mendocino triple junction migrated northward. They also propose that the volcano alignment reflects an underlying hot spot. That suggestion seems inconsistent with the northward movement of the Pacific Plate which most of the volcanics ride. These volcanics are among the closest to a subduction plate boundary of any in the world and will repay closer tectonic investigation. Similarly, a tiny sliver of basalt dated at 3.57 million years (Prowell, 1974, quoted in Luedke and Smith, 1981) occurs 45 kilometers east of Santa Cruz, Calfiornia near the Calaveras and Hayward faults. Apparently leakage of basalts along the San Andreas fault system has occurred repeatedly.

The Clear Lake volcanic field ( late Pliocene to Holocene) lies in a tectonically active, complex geologic setting within the San Andreas transform fault system in northern Coast Ranges of California. Clear Lake and the volcanic field are located within a fault-bounded, locally extensional basin. The lake is the largest freshwater lake entirely within California; it is probably volcano-tectonic in origin, but is not a caldera lake. The volcanic field is the northernmost of a series of young Cenozoic volcanic fields in the Coast Ranges. Within the field, eruptive loci have migrated northward through the last 2.1 million years. Eruptive centers are lacking. Volcanism appears to be related to extension in a pull-apart basin within the San Andreas fault system and is not directly related to subduction, which ceased off the California coast at this latitude around 3 million years ago.

The Clear Lake volcanics range from basalt through rhyolite in composition. Basalt is rare, and the dominant composition is dacite. Four eruptive episodes separated by time gaps of 0.15 - 0.2 million years were characterized by different compositional ranges. Total erupted volume probably exceeds 70 cubic kilometers. Volcanism has been largely non-explosive with only one major silicic air-fall tuff and no ash-flow tuffs. Numerous young locally vented deposits of palagonitic mafic tuff occur around the southeast shore of Clear Lake. ...

Clear Lake is a popular recreation center with resorts, camping, and boating facilities. State Highway 20 skirts the east side of the lake, and numerous smaller roads provide access to the volcanic features.

The late-Pliocene to early Holocene Clear Lake volcanic field in the northern Coast Ranges, contains lava dome complexes, cinder cones, and maars of basaltic-to-rhyolitic composition. The westernmost site of Quaternary volcanism in California, the Clear Lake field is located far to the west of the Cascade Range in a complex geologic setting within the San Andreas transform fault system. Mount Konocti, a composite dacitic lava dome on the south shore of Clear Lake, is the largest volcanic feature. Volcanism has been largely non-explosive, with only one major airfall tuff and no ash flows. The latest eruptive activity, forming maars and cinder cones along the shores of Clear Lake, continued until about 10,000 years ago. A large silicic magma chamber provides the heat source for the Geysers, the world's largest producing geothermal field.

... the 1-kilometer-high composite dacite volcano, Mount Konocti ... Mount Konocti, largest edifice of the Clear Lake volcanic field ...

Gravity and teleseismic studies suggest that a large silicic magma chamber around 14 kilometers in diameters, lies 7 kilometers and deeper beneath the volcanic field. This reservoir is thought to be the heat source for The Geysers geothermal field (on the southwest side of the volcanic field), which is the largest producing geothermal field in the world, with installed electrical generating capacity of around 2,000 megawatts in 1988, enough electricity for about two cities the size of San Francisco. Numerous thermal springs occur along northwest to north-northwest-trending faults that are subparallel to the main San Andreas fault. Associated epithermal deposits of mercury and gold include the Sulphur Bank Mine (still the site of active mercury deposition) and the McLaughlin Mine (a major disseminated gold deposit in an outlier of the Clear Lake volcanics), and associated hot springs deposits. ...

The Geysers geothermal field near Santa Rosa, in Northern California produces enough electricity to meet the power demands of San Francisco. The Geysers area is the largest geothermal development in the world.

• Clear Lake, Volcanic Field, 1,439 meters elevation
• Cobb Mountain, Cone, 1,439 meters elevation
• Mount Hannah, Dome, 1,212 meters elevation
• Mount Konocti, Dome
• Round Mountain, Cone
• Roundtop Mountain, Cone
• The Geysers, Thermal Feature

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