A supervolcano under a California lake has quietly bulged upward more than two feet since the 1970s

Beneath the placid waters and pine forests of eastern California sits one of North America’s largest volcanic systems, a broad basin known as the Long Valley Caldera. Since the late 1970s, the ground inside it has been slowly rising, lifting a central bulge by more than two feet in a process that scientists have monitored closely and that periodically stirs unease about what the magma below is doing.

The scar left by a colossal eruption

Long Valley is not a single peak but a depression roughly 20 miles long and 11 miles wide, carved out about 760,000 years ago when a catastrophic eruption emptied an enormous magma chamber. That event blasted out hundreds of cubic kilometers of material and blanketed much of the western and central United States in ash. When the emptied chamber roof collapsed, it left the sunken caldera that defines the landscape today, with the resort town of Mammoth Lakes on its rim.

The scale of that ancient eruption is what earns the system the informal “supervolcano” label, a term reserved for volcanoes capable of the very largest explosive events. The caldera has not produced anything remotely that size since, but it has never gone fully quiet either.

Decades of measured uplift

The current chapter of unrest began around 1978 to 1980, when a swarm of strong earthquakes struck the region and surveyors noticed the caldera floor had risen. Since then, a feature at the center of the basin called the resurgent dome has swelled upward. According to the U.S. Geological Survey, the dome has climbed by roughly 33 inches, or about 83 centimeters, between 1980 and the late 2010s, comfortably exceeding the two-foot threshold.

The rise has come in pulses rather than a steady creep. Some of the sharpest uplift arrived during episodes in the 1980s and again in the late 1990s, when the dome rose several inches in a single year and was accompanied by clusters of small earthquakes. More recent years have seen the swelling slow to a gentle pace measured in fractions of an inch annually.

What is pushing the ground up

Uplift of this kind generally reflects pressure building at depth, and the leading explanations involve the movement of molten rock, hot water and gases within the crust beneath the dome. Distinguishing among those causes matters, because rising magma poses a very different hazard than the migration of hydrothermal fluids or the release of trapped gas. Research summarized by the U.S. Geological Survey’s California Volcano Observatory has leaned toward the interpretation that much of the recent deformation is driven by fluids and gases cooling and moving through the system rather than a fresh surge of eruptible magma toward the surface.

That distinction has offered a measure of reassurance. A caldera that is releasing pressure through hydrothermal activity is behaving in a way consistent with a system that is gradually settling rather than charging toward an eruption, though monitoring continues precisely because the picture can change.

Signs at the surface

The unrest is not purely a matter of instruments detecting invisible motion. Long Valley and neighboring Mammoth Mountain have produced tangible signals, including persistent earthquake swarms and, notably, areas where trees have died off because carbon dioxide seeping up from magma at depth accumulated in the soil and suffocated their roots. Warm springs and steam vents scattered around the basin are further evidence of a hot system just below the surface.

These features make the caldera a natural laboratory. Sensors track ground deformation with satellite radar and GPS, seismometers log the frequent small quakes, and gas measurements watch for changes in what the system is exhaling, all feeding a hazard assessment that is updated as conditions evolve. The town of Mammoth Lakes even added an escape route years ago out of concern that a sudden eruption or gas release could trap residents, a rare example of a community building infrastructure around volcanic risk.

How worried residents should be

Slow uplift, frequent small earthquakes and gas emissions are characteristic of many large calderas around the world, and most such systems can persist in restless states for centuries or millennia without erupting. Scientists emphasize that the recent behavior at Long Valley, including a general slowing of the uplift, does not indicate an eruption is imminent. Should the balance shift, the dense monitoring network is designed to catch the escalation in seismicity, deformation and gas output that would likely precede any eruption, giving authorities time to respond.

For now, the story of Long Valley is one of patient observation. A landscape that looks serene from a car window conceals a system that has lifted itself more than two feet across four decades, a reminder that some of the planet’s most powerful forces operate on timescales that dwarf human attention spans while remaining very much alive.

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

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