Images

Roughly a third of the total erupted volume of the Clear Lake volcanic field is represented by the ~ 35 km³ of rocks comprising Mt. Konocti and nearby hills. The mountain itself is over 1200 m (~4000 ft) high and is comprised primarily of a series of dacitic lava domes – Buckingham Peak, Wright Peak, and South Peak, and Howard Peak are all dacites.

Pillow lavas like these form when a lava flow erupts under a significant body of water (including oceans, lakes, and rivers). Lobes of lava crystallize immediately in the water, forming crusts over molten interiors which continue to inflate. New breakouts of lava repeat the process and stack onto each other.

Just as the Bartlett Springs and Maacama faults are the major bounding faults of the Clear Lake volcanic field, the Collayami (purple) and Maacama (green) faults bound the Geysers Geothermal Field.

The Clear Lake volcanic field erupted in association with and within the San Andreas Fault Zone. Although the San Andreas fault is the hallmark fault associated with the transform tectonic boundary between the Pacific and North American plates, the “boundary” between plates is more accurately described by a zone of faulting than by a single fault.

At over 4700ft (1440 m), Cobb Mountain is the tallest peak in the Mayacamas Mountains. Cobb Mountain is comprised entirely of ~ 1 million-year-old silica-rich volcanic rocks related to the Clear Lake volcanic field.

CalVO geologist Seth Burgess looking across Clear Lake at Mount Konocti, a prominent volcanic dome complex within the Clear Lake volcanic field. Photo courtesy of Alexander Rubin.

Block diagram of a normal fault, where the hanging wall drops relative to the footwall.

Block diagram of a reverse fault, where the headwall is being pushed above the level of the footwall

Block diagram of a lateral (strike-slip) fault, where two blocks are moving sideways past each other without vertical motion. 

Back-scattered electron images of olivine and chromium spinel crystals from primitive Clear Lake Volcanic Field samples. The labeled colored dots on the crystals show the locations where they were analyzed with an electron microprobe for geochemical information. Figure by Dawnika Blatter.

USGS CalVO Research Geologist Seth Burgess collecting a sample of old, primitive lava in the Clear Lake Volcanic Field for geochemical analysis. USGS photo by Dawnika Blatter.

This map, taken from a 2024 journal article by Furlong et al. 2024, shows the location and magnitude of seismic events (earthquakes) and rock type on a basemap of a portion of the west coast of North America.

The Electron Probe MicroAnalyzer (EPMA) at CalVO's Moffett Field laboratory combines multiple instruments and detectors, optimized for high spatial resolution, quantitative, geochemical analysis.

These spectrograms of seismic stations near Long Valley's Mammoth Mountain show the brightly-colored punctuated spasms of small earthquakes happening within a 10 minute period.

The Operations (Ops) Room at the California Volcano Observatory serves as a hub for evaluating the data received from volcano monitoring networks in California and Nevada. USGS photo by J. Ball.

The Burdell Mountain volcanics are flow-banded porphyritic andesite, volcanic breccia, volcanic mudflow deposits, and minor flow-banded dacite which can be found west of the Petaluma Valley. Dated at about 11 million years old, they are related to the Quien Sabe Volcanics to the southeast and were displaced by the Hayward-Calaveras fault system.

A view of the southern Quien Sabe Range from high on Basalt Hill in Merced County. The Quien Sabe range is comprised of igneous intrusions, from >9 Ma to >11 Ma, predecessors to the volcanics of the Berkeley Hills and the southeastern equivalent of the Burdell Mountain lavas near Petaluma. Photo courtesy of Stephen W. Edwards

This exposure of volcanic breccia is a volcanic rock comprised of broken pieces of vitrophyre, a welded volcanic glass. The breccia is found in the Tolay Volcanics, a sequence of rhyolite, andesite, and basalt at least 1220 m thick. The 9 Ma Tolay Volcanics are equivalent to the Berkeley Hills Volcanics. Photo courtesy of Ross Wagner.

Transform boundaries are formed where two tectonic plates pass laterally by one another. These boundaries are commonly defined by a series of faults, each of which accommodates some of the translational movement between passing plates. Small bends in these boundaries led to the formation of mountains and valleys.

The Mendocino Triple Junction is a tectonic boundary where three plates (the Pacific, North American, and Juan De Fuca) meet. 

A subduction zone is formed where two tectonic plates come together and one plate overrides the other. The plate with lower density, usually comprised of continental crust, stays on top while the denser plate, usually made of oceanic crust, is pushed and pulled beneath, into Earth’s mantle.