A California supervolcano has bulged upward about two and a half feet since 1978

In the eastern shadow of California’s Sierra Nevada lies one of the largest volcanic systems in North America, a broad depression capable in its distant past of eruptions vast enough to blanket much of the continent in ash. For decades the floor of that depression, the Long Valley Caldera, has been slowly rising, swelling upward by roughly two and a half feet since the late 1970s. The steady uplift has drawn close scientific attention, though the leading interpretation of what it means may be more reassuring than alarming.

The scar of an ancient super-eruption

The Long Valley Caldera formed about 760,000 years ago in a cataclysmic eruption that ejected an enormous volume of material, on the order of hundreds of cubic kilometers of magma, in one of the most powerful volcanic events in the geologic record of the western United States. The eruption emptied the underground magma reservoir so violently that the ground above collapsed, leaving an oval basin roughly 20 miles long and 11 miles wide. Ash from that blast, preserved in rock layers known as the Bishop Tuff, has been found across much of the continent. The U.S. Geological Survey classifies the site through its monitoring of Long Valley Caldera as one of a small number of American systems large enough to produce a super-eruption, the reason it is sometimes called a supervolcano.

The dome that keeps rising

The recent uplift is concentrated in a central area called the resurgent dome, a section of the caldera floor that has been pushed upward from below. Precise measurements show that since around 1978 to 1980 the dome has risen roughly 2.5 feet, or about 80 centimeters, a slow but persistent swelling that instruments track through satellite radar and ground-based surveying. Uplift of this kind is caused by pressure increasing beneath the surface, typically from the movement of magma or the hot fluids and gases that magma releases as it cools.

The rise has not been smooth or constant. It has come in episodes, with periods of faster inflation punctuated by pauses and even slight subsidence, the kind of pulsing behavior common in restless calderas. Each episode of renewed uplift has prompted heightened monitoring, because a sudden acceleration could, in principle, signal magma pushing closer to the surface.

The unrest of the 1980s

The current era of watchfulness traces to a dramatic burst of activity in 1980, when a series of strong earthquakes, including four of magnitude 6, struck beneath the caldera within a couple of days. That seismic jolt, combined with the onset of measurable uplift, put the region on the scientific map as an active volcanic system rather than a dormant relic. Authorities established dedicated monitoring, and the area has been watched continuously ever since through the California Volcano Observatory, part of the USGS network of volcano observatories.

Another sign of the system’s activity appeared near Mammoth Mountain, on the caldera’s southwestern rim, where trees began dying in patches during the 1990s. The culprit was carbon dioxide seeping up from below in volumes high enough to suffocate tree roots in the soil, a phenomenon linked to the movement of volcanic gases. Those dead-tree zones became a visible, ground-level marker of the invisible processes stirring beneath the surface.

Why the swelling may signal cooling, not eruption

Despite the uplift and the periodic earthquakes, the prevailing scientific view is not that Long Valley is building toward an eruption. Research examining the caldera’s structure and behavior has increasingly pointed toward a system that is gradually cooling and winding down rather than recharging. In that interpretation, the ground rises because a body of magma at depth is slowly crystallizing and releasing hot water and gas, which pressurize the rock above and lift the surface, even as the eruptible molten material dwindles.

That distinction matters. Uplift driven by fresh magma intruding toward the surface would be a warning sign, whereas uplift driven by an aging magma body releasing its fluids as it solidifies is consistent with a volcano settling into a quieter old age. The USGS, which assesses hazards through its volcano hazards program, has kept the caldera’s alert level at normal, its lowest designation, reflecting the judgment that no eruption is imminent even as the ground continues its slow rise.

Living on a restless giant

The caldera is far from a barren hazard zone. The resort town of Mammoth Lakes sits within it, and the region draws skiers, hikers, and hot-spring visitors year-round, many of them unaware that the scenic landscape is the collapsed throat of an ancient super-eruption. The hot springs themselves are a direct benefit of the underlying heat, and the same geology that fuels concern also powers geothermal energy production nearby.

Scientists continue to watch the caldera with a dense array of instruments precisely because large volcanic systems can behave unpredictably over long spans. The two-and-a-half-foot rise since the late 1970s is a genuine measurement of an active, restless system, but read alongside the broader evidence it tells a story less of a supervolcano awakening than of one slowly exhaling the last of its heat. For the communities living on top of it, that combination, constant monitoring paired with a reassuring scientific consensus, is what allows daily life to continue over ground that is, quite literally, still moving.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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