Clear Lake Volcanic Field

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, this volcanic field is 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. The latest eruptive activity, forming maars and cinder cones along the shores of Clear Lake, continued until about 9,000 years ago. A large silicic magma body provides the heat source for the Geysers, a geothermal field with a complex of electrical power plants.