The Geysers-Clear Lake area is located in the northern California Coast Ranges about 150 km (93 mi) north of San Francisco and about 50 km (31 mi) east of the San Andreas Fault.
The region is cut by many northwest- to north-trending faults, which are part of the broad San Andreas transform fault system that separates the North American and Pacific plates.
Subduction ended at the Clear Lake latitude about three million years ago as the San Andreas transform fault propagated northward behind the Mendocino triple junction. This left behind a 'slab window,' a ductile opening into the mantle through which mantle material rises and melts the Earth's crust. The resulting crustal-silicic melt began erupting as the Clear Lake Volcanic Field approximately a million years after subduction ceased in this region. Volcanism was initially widespread but in the past million years has focused near Clear Lake, following the alignment of existing faults. The tension on these faults forms the pull-apart (or transtensional) basin which holds Clear Lake and the volcanic field. Geophysical data suggest that there is still a large silicic magma body under the main Clear Lake Volcanic Field and The Geysers.
Clear Lake Volcanics
The Clear Lake Volcanics are the northernmost and youngest of several volcanic fields in the California Coast Ranges, which grow progressively older to the south. Clear Lake rocks date from 2.1 Ma to about 8.5 ka and have an estimated erupted volume of 100 km3. Silicic lavas are the dominant type, with the most voluminous rock type in the volcanic field being rhyodacite, but basaltic andesite, andesite, and dacite are also present. Volcanism has moved progressively northward within the volcanic field over time, after an early phase when predominantly basaltic andesite lava erupted as scattered centers over a wide area. Since about 1 Ma, volcanism has been localized south and east of the lake. The youngest volcanic deposits are tephra, bomb, and ash layers associated with a series of maar craters and scoria cones which probably erupted between 40,000 and 8,500 years ago.