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California Volcano Observatory images.

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A 3D block diagram showing a cross-section of geologic layers being split by a fault. The block on the right side of the diagram is being dropped down along an incline below the block on the left.
Normal fault
Normal fault
Normal fault

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

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

A 3D block diagram showing a cross-section of geologic layers being split by a fault. The block on the right side of the diagram is moving away from the viewer past the block on the left, splitting a surface stream.
Transform fault
Transform fault
Transform fault

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

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

A 3D block diagram showing a cross-section of geologic layers being split by a fault. The block on the right side of the diagram is being pushed up along an incline over the block on the left.
Reverse fault
Reverse fault
Reverse fault

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

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

A man in a white t-shirt with curly blonde hair and a beard smiles in front of a vegetation-covered rock face.
Daniel O'Hara
Daniel O'Hara
Daniel O'Hara

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

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

A row of 5 spectrograms show time on the horizontal and frequency on the vertical. Earthquakes are visible as bright colors.
Spectrogram of Mammoth Mountain earthquake swarm, November 28, 2024
Spectrogram of Mammoth Mountain earthquake swarm, November 28, 2024
Spectrogram of Mammoth Mountain earthquake swarm, November 28, 2024

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.

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 young female geologist stands on a volcanic crater rim with her hands on her hips, wearing a gray shirt and green hat.
Genna Chiaro
Genna Chiaro
Genna Chiaro

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

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

A small outcropping of rough gray boulders sits in the middle of a field of dry, tan-colored grass. A metal clipboard leans on one rock to provide scale, and a low hill with a stand of oak trees is visible in the background.
Burdell Mountain andesite lava flow
Burdell Mountain andesite lava flow
Burdell Mountain andesite lava flow

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.

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.

Viewed from a rocky ledge, a brown-and-green grassy valley filled with eroded, rolling hills slopes towards a body of water at the lower right. Behind them, more hills covered in oak trees and a line of distant mountains form the horizon.
Quien Sabe Volcanic Field
Quien Sabe Volcanic Field
Quien Sabe Volcanic Field

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

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

A small outcropping of rough gray boulders sits in the middle of a field of dry, tan-colored grass. A metal clipboard leans on one rock to provide scale, and a low hill with a stand of oak trees is visible in the background.
Vitrophyre breccia in the Tolay Volcanic Field
Vitrophyre breccia in the Tolay Volcanic Field
Vitrophyre breccia in the Tolay Volcanic Field

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.

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.

Side-view diagram of a thin oceanic layer of the Earth’s crust diving beneath a thick layer of Continental crust
Subduction zone
Subduction zone
Subduction zone

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 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 side-view diagram showing a cone of mantle rising underneath a layer of oceanic crust with the ocean on top
Divergent spreading center
Divergent spreading center
Divergent spreading center

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.

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.

A conical, gray, rocky hill rises above a wetland and waterway dotted with bright white wading birds
Salton Buttes
Salton Buttes
Salton Buttes

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. 

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. 

An oblique block diagram shows two tectonic plates sliding laterally past each other while sitting atop a section of mantle.
Transform boundary
Transform boundary
Transform boundary

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.

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.

 A line map of California, Nevada, Oregon, and Idaho is overlaid with tectonic plate boundaries
Mendocino Triple Junction
Mendocino Triple Junction
Mendocino Triple Junction

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

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

A scientist in a long silver heat-resistant coat, gas mask, and helicopter helmet inspects a just-quenched sample of lava
Sampling molten lava from the middle East Rift Zone Kīlauea eruption in Nāpau Crater, September 17, 2024
Sampling molten lava from the middle East Rift Zone Kīlauea eruption in Nāpau Crater, September 17, 2024
Sampling molten lava from the middle East Rift Zone Kīlauea eruption in Nāpau Crater, September 17, 2024

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.

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.

Shaded relief map showing Lassen Volcanic National Park, with Lassen Peak labeled in the northern portion of the map, Growler & Morgan hot springs to the southwest, and the June 24 2024 swarm to the southeast
Lassen Volcanic Center earthquake swarm of June 24, 2024
Lassen Volcanic Center earthquake swarm of June 24, 2024
Lassen Volcanic Center earthquake swarm of June 24, 2024

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 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.

Helicorder record showing dozens of tiny earthquake traces occurring over 12 hours, with each line comprising 15 minutes and earthquakes looking like drum cymbals turned on their sides.
LSIB helicorder trace from the Lassen Volcanic Center on June 24, 2024.
LSIB helicorder trace from the Lassen Volcanic Center on June 24, 2024.
LSIB helicorder trace from the Lassen Volcanic Center on June 24, 2024.

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.

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.

Viewed from overhead, the gray and rubbly rhyolite lava domes of the Mono-Inyo Craters are interspersed with smooth patches of volcanic ash and scoria. In the distance, a snow-capped mountain range surrounds a broad lake.
Rhyolite lava domes of the Mono Craters
Rhyolite lava domes of the Mono Craters
Rhyolite lava domes of the Mono Craters

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.

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
Microscopic view of different groundmass textures in rocks
Microscopic view of different groundmass textures in rocks
Microscopic view of different groundmass textures in rocks

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.

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.

Mass spectrometer, used to measure the ration of atoms with different masses, in the USGS laboratory at Moffett Field, California
Mass spectrometer, used to measure the ration of atoms with different masses, in the USGS laboratory at Moffett Field, California
Mass spectrometer, used to measure the ration of atoms with different masses, in the USGS laboratory at Moffett Field, California
Mass spectrometer, used to measure the ration of atoms with different masses, in the USGS laboratory at Moffett Field, California

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.

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.

A rocky hill covered in chaparral rises above the gray roof of a motel. A fire road cuts a reddish path across the top.
Cerro San Luis Obispo, Islay Hills volcanic complex
Cerro San Luis Obispo, Islay Hills volcanic complex
Cerro San Luis Obispo, Islay Hills volcanic complex

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

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

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