Images

Block diagram of a normal fault, where the footwall drops below the level of the headwall

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

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

Daniel O'Hara, Mendenhall Fellow at the USGS California Volcano Observatory. 

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.

USGS Mendenhall Postdoctoral Scholar Genna Chiaro specializes in petrology, geochronology, and volcanology. 

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.

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.

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.

The Salton Buttes volcanic field is the youngest and southernmost of the fields associated with the North American and Pacific plate boundary. Five obsidian rhyolite domes erupted on the southern shore of the Salton Sea between 6000 and 500 years ago. The area is active geothermally and seismically, with numerous hot springs and mudpots on the surface. 

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. 

USGS Hawaiian Volcano Observatory geologists collected a sample of the middle East Rift Zone Kīlauea eruption in Nāpau Crater, within a closed area Hawaiʻi Volcanoes National Park. Geologists put molten lava into a metal bucket and rapidly quench it with water.

This map shows the location of the June 24, 2024 earthquake swarm at the Lassen Volcanic Center relative to Lassen Peak and Growler & Morgan Hot Springs. Earthquakes are indicated by white, blue, and yellow circles, scaled to the earthquake size.

This digital helicorder record from station LSIB on the Northern California Seismic Network emphasizes the dozens of tiny earthquakes in Lassen's June 24 swarm. Earch line of the helicorder shows 15 minutes of time, with the entire record covering 24 hours.

Mono Craters consists of a series of high-silica rhyolite lava domes, many erupted within the last 10,000 years. The domes are often steep, glassy, and contain very few phenocrysts.

Microscopic view of different groundmass textures in rocks. On the left, this groundmass is a good choice for argon dating, as it consists of abundant interconnected crystals. On the right, the groundmass consists predominantly of glass (black because it does not transmit cross-polarized light) and is a poor choice for argon dating.

A mass spectrometer is used to measure the ratio of atoms with different masses—in this case, the different isotopes of argon gas, which can be used to determine the age of a volcanic rock. Left: a side view of a mass spectrometer at the USGS Argon Geochronology Laboratory in Moffett Field, CA. Right: a close-up view of the sample chamber in this mass spectrometer.

Islay Hills is a chain of peaks also known as the Nine Sisters (or “Morros”).