The ski town of Mammoth Lakes, California, sits next to a volcanic depression that the U.S. Geological Survey rates at its highest tier of threat. Long Valley Caldera measures 16 by 32 km, which converts to a 20-by-10-mile footprint east of the central Sierra Nevada. Its floor rose nearly 3 feet between 1980 and 2021, and then began to sink.
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The rating does not predict an eruption. It describes how much volcanic hazard and exposure surround a vent system, and Long Valley scores high on both counts because a town, a ski resort and a main highway corridor share the ground with a magma system that has stirred repeatedly in living memory.
A 20-by-10-mile basin with a 760,000-year-old origin
According to the USGS profile of Long Valley Caldera, the basin formed about 760,000 years ago in the Bishop Tuff eruption, an event the agency calls a supereruption. The collapse that followed left the elliptical depression visible today. The most recent eruption in the broader volcanic field occurred 16,000 to 17,000 years ago, and the system remains thermally active, with hot springs and fumaroles dotting the landscape. Mammoth Lakes is the nearby town that gives the area its resort identity, with Mammoth Mountain rising just beyond it.
The agency’s National Volcano Early Warning System ranks Long Valley Caldera as Very High threat potential. The California Volcano Observatory page places it alongside Lassen Volcanic Center and Mount Shasta in that top group, while the Mono-Inyo Chain sits one step lower and Mammoth Mountain itself is listed at Moderate.
That distinction matters in practice. Mammoth Mountain is a separate volcanic cluster on the caldera’s rim, and its unrest has historically looked different, with volcanic gas seeping through the soil rather than the broad ground swelling seen in the caldera’s center. The USGS says both Long Valley and Mammoth Mountain have gone through episodes of heightened unrest in recent decades, involving earthquakes, ground uplift or gas emissions.
Such a ranking is built from two ingredients, how hazardous a volcano is and how many people and assets lie within reach of it. A remote volcano that erupts violently can rank below a quieter one near a town, and Long Valley earns its place from the combination of a large, restless magma system and a community of year-round residents plus a steady flow of winter visitors. The rating therefore speaks to preparedness rather than to any near-term forecast, which is why the weekly observatory notices still read Normal.
Nearly 3 feet of uplift from 1980 to 2021
The raised ground is the part of the story that gives the area its reputation. In a January 2026 observatory article, A quiet exhalation at Long Valley, USGS geophysicist Joshua Crozier and volcano seismologist Alicia Hotovec-Ellis wrote that the ground rose nearly 3 feet, about 1 meter, between 1980 and 2021. That swelling is the episode behind the label of recent unrest that the agency attaches to the caldera.
The same article reports a reversal. Measured at the GPS stations KRAC and KRAK, the center of the caldera has sunk about 2.4 inches, or 6 cm, since 2021, the largest subsidence recorded there since detailed measurements began in the 1980s. The authors call the process a “volcanic exhalation,” in line with the idea that calderas breathe, inflating and deflating over time without erupting. Seismicity has also been quiet for the past few years, with a decline in earthquakes beginning around 2022.
Their caution is the notable part. The scientists say they cannot predict whether the deflation will last months, decades or longer, and the sinking has undone only a small share of the earlier rise. The likely source of the shrinking sits beneath the resurgent dome, in about the same place where inflation originated.
California Volcano Observatory instruments around the caldera
Monitoring combines seismometers, GPS, tiltmeters, strainmeters and satellite imagery, and the observatory publishes a weekly status. The weekly update of July 21 kept every monitored California volcano at an alert level of Normal and an aviation color code of Green. For the week of July 13 to 19 it counted three earthquakes above magnitude 1 in Long Valley Caldera, the largest a magnitude 1.8, and one above magnitude 1 in the Sierra Nevada block to the south.
A newer field campaign is probing what lies underneath. A USGS article by Crozier dated Aug. 4 describes the most extensive geodetic survey of the caldera since the late 1990s, using temporary GPS stations accurate to the centimeter, gravimeters that sense changes in gravity from water, gas or magma, and measurements of the water table in existing wells. Computer simulations will use the data to identify the fluids behind the current subsidence.
The unresolved question is why a system that lifted its floor by a meter over four decades has begun to settle. The survey results, not yet published in the sources read, are designed to answer it.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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