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A geologist crouches next to a rhyolite outcrop and points to an egg-shaped mass of radiating pink minerals about the size of a cantaloupe. The rest of the rock is pocked with other egg to grapefruit sized pink crystal masses, interspersed with chunky fragments of black glassy lava.
Large spherulites in Hot Creek rhyolite lava in the Long Valley Caldera
Large spherulites in Hot Creek rhyolite lava in the Long Valley Caldera
Large spherulites in Hot Creek rhyolite lava in the Long Valley Caldera

The presence of spherulites indicates that a lava flow cooled quickly, and their mineralogy holds clues to its precise cooling history. USGS photo by Jessica Ball

The presence of spherulites indicates that a lava flow cooled quickly, and their mineralogy holds clues to its precise cooling history. USGS photo by Jessica Ball

A broad, flat plain covered in dull green sagebrush stretches away from the viewer in this panorama. The photo is being taken from a rocky hill, with snow-capped, sharp peaks on the far side of the plain and low raised hills on the right side of the photo. One geologist is sitting on the rocks at lower right and one geologist is standing and giving a peace sign.
Panoramic view of the Long Valley Caldera from its north side
Panoramic view of the Long Valley Caldera from its north side
Panoramic view of the Long Valley Caldera from its north side

This panorama of the Long Valley Caldera, looking from north to south, shows its broad central plain, post-caldera rhyolite flows and uplift on the right, and eastern Sierra Nevada in the background. USGS photo by Jessica Ball.

This panorama of the Long Valley Caldera, looking from north to south, shows its broad central plain, post-caldera rhyolite flows and uplift on the right, and eastern Sierra Nevada in the background. USGS photo by Jessica Ball.

In front of an outcrop of matte grey rock splotched with lichens, a young man in an orange safety vest gives a happy thumbs up and an older man sits writing notes in a small notebook and smiles at the camera. The rocks are perched on a moderate slope covered in brushy manzanita and scraggly pine trees.
CalVO geologists take paleomag samples from a basaltic andesite lava flow on Ash Creek Butte
CalVO geologists take paleomag samples from a basaltic andesite lava flow on Ash Creek Butte
CalVO geologists take paleomag samples from a basaltic andesite lava flow on Ash Creek Butte

Andy Calvert and Tony Pivarunas drill into the face of a lava flow on Ash Creek Butte, a Pleistocene shield volcano northeast of Mount Shasta. USGS photo by Dawnika Blatter

Andy Calvert and Tony Pivarunas drill into the face of a lava flow on Ash Creek Butte, a Pleistocene shield volcano northeast of Mount Shasta. USGS photo by Dawnika Blatter

A digital elevation map of Yellowstone Caldera
A digital elevation map of Yellowstone Caldera
A digital elevation map of Yellowstone Caldera
A digital elevation map of Yellowstone Caldera

Rhyolite lavas in the Yellowstone Caldera younger than 631,000 years

Mount Konocti as seen from the town of Clear Lake.
Mount Konocti as seen from the town of Clear Lake.
Mount Konocti as seen from the town of Clear Lake.
Thurston Lake and Mount Konocti, Clear Lake Volcanic Field
Thurston Lake and Mount Konocti, Clear Lake Volcanic Field
Thurston Lake and Mount Konocti, Clear Lake Volcanic Field
Thurston Lake and Mount Konocti, Clear Lake Volcanic Field

Thurston Lake and Mount Konocti, Clear Lake Volcanic Field, California.