Images

This seismic station in the Lassen Volcanic Center was damaged during the Dixie Fire of 2021, which burned huge swathes of forest in the Lassen Volcanic National Park. Above-ground equipment, including the tree that held the antenna and solar panel above winter snows, was destroyed; belowground seismic sensors were recoverable. USGS photo by Ryan Presser, NCSN.

The California Volcano Observatory was created in 2012. Its logo shows Mount Shasta with a California grizzly bear (extinct) in the foreground.

The cryogenic magnetometer within the shielded room in the U.S. Geological Survey Paleomagnetics Laboratory. Major features of the cryogenic magnetometer are labeled. Picture by A. Pivarunas (U.S. Geological Survey)

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.

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 footwall drops below the level of the headwall

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. 

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.

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.

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.

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

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.

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.

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.

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 spreading center is formed where two tectonic plates are moving away from each other. Magma from the mantle upwells to fill the space made by the diverging plates, and erupts at the boundary to form new crust.