A California supervolcano has bulged upward more than two feet since 1978

Beneath the scenic Long Valley region of eastern California sits one of the country’s largest volcanic systems, and it has been slowly swelling for decades. Since a period of unrest began in the late 1970s, the floor of the Long Valley Caldera has risen by more than two feet, lifted from below by pressure that scientists attribute to magma and hot fluids deep underground.

The uplift sounds ominous, and the caldera’s history certainly is: it was born in a colossal eruption hundreds of thousands of years ago. But geologists who monitor it closely stress that steady bulging is not the same as an approaching eruption, and the system is among the most heavily watched volcanoes in the United States.

The eruption that formed the caldera

The Long Valley Caldera is a broad depression that formed when an enormous volume of magma erupted and the ground above it collapsed into the emptied chamber. That catastrophic event left behind a basin more than 10 miles across, and later volcanic activity built domes and flows across the surrounding area. It is this deep-seated magmatic system that continues to make the region geologically restless today.

Calderas of this scale are sometimes labeled supervolcanoes because their largest past eruptions dwarf anything witnessed in recorded history. The label describes the potential of the system over geologic time, not a prediction about what it will do next.

Unrest that began in 1978

The current episode of activity traces to 1978, when a magnitude 5.4 earthquake struck a few miles southeast of the caldera. That was followed by more intense seismic swarms beginning in 1980, including several strong magnitude 6 shocks. In the wake of those earthquakes, scientists from the U.S. Geological Survey detected dome-like uplift near the center of the caldera.

Measured over the following years, the center of the caldera rose by roughly two and a half feet, with the deformation affecting an area of more than 100 square miles. The swelling did not proceed at a steady pace; it came in pulses, quickening during periods of earthquake activity and slowing at other times, a pattern typical of a caldera responding to shifting pressures at depth.

What is pushing the ground up

The favored explanation for the uplift is the movement of molten rock and hot fluids beneath the caldera. As magma or pressurized fluid accumulates several miles down, it inflates the crust above like air filling a balloon, raising the surface and straining the surrounding rock enough to trigger earthquakes. Research using seismic imaging has found evidence of significant melt beneath the caldera, confirming that the system still holds molten material.

Importantly, the presence of magma at depth is normal for a young volcanic system and does not by itself signal an imminent eruption. Many calderas around the world hold molten rock for tens of thousands of years between eruptions, deforming and quieting repeatedly without ever reaching the surface.

Why monitoring, not alarm, is the response

Because of its history, Long Valley is instrumented with a dense network of seismometers, ground-deformation sensors, and gas monitors that let scientists track the system in near real time. The goal is to distinguish routine unrest, which the caldera produces often, from the specific escalation that would precede an eruption, such as rapidly accelerating uplift, swarms of shallow earthquakes, and surging volcanic gases.

Guidance from the agency’s long-running assessment of the restless caldera emphasizes that living near an active volcanic system means learning to interpret its signals rather than fearing every tremor. Decades of swelling have produced no eruption, and the same monitoring that recorded the two-foot rise would be expected to give warning, likely weeks or more, before magma reached the surface. For residents and visitors to the eastern Sierra, the caldera’s slow bulge is a reminder that the landscape is alive, and a demonstration of how modern science keeps watch over a giant that stirs but has not woken.

Signs of the caldera at the surface

The unrest below ground has visible expressions above it. Long Valley and the adjacent Mono Basin are dotted with hot springs, steam vents, and areas where volcanic gases seep from the soil, all evidence of heat and fluids moving through the crust. In some locations, carbon dioxide rising from depth has killed patches of trees by displacing oxygen in the root zone, a phenomenon scientists monitor as one indicator of the system’s activity. These features make the region a natural laboratory for studying how a large caldera behaves between eruptions.

They also draw visitors, since the same geothermal heat that signals unrest feeds popular hot springs and supports geothermal energy production nearby. That coexistence of recreation, industry, and hazard is part of what makes clear public communication about the caldera so important.

How calderas are ranked for risk

Long Valley is classified among the higher-threat volcanic systems in the United States, a ranking that reflects both its potential for large eruptions and the population and infrastructure that could be affected. That status is what justifies its dense monitoring network and the steady stream of research aimed at understanding its plumbing. The classification is a measure of what to watch, not a forecast that an eruption is near. By keeping close track of ground movement, earthquakes, and gas emissions, scientists aim to ensure that if the caldera ever does move toward an eruption, the shift from routine unrest to genuine warning signs would not go unnoticed.

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

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