Long Valley Caldera’s odds of erupting run about one in a few hundred every year

Every year that the ground beneath Long Valley Caldera stays quiet, the odds of an eruption somewhere along its volcanic chain sit at roughly one in a few hundred, a figure the U.S. Geological Survey considers directly comparable to the odds of a great magnitude 8 earthquake striking coastal California in that same year. Neither number is small enough to dismiss, and neither is large enough to justify alarm on its own. The comparison itself is the point: emergency planners already treat the California quake odds as a serious, standing risk, and the survey wants the caldera’s numbers read the same way.

That yearly figure describes the eastern California region holding both Long Valley Caldera itself and the younger Mono-Inyo Craters volcanic chain running north from it toward Mono Lake. Absent any signs of unrest, meaning no earthquake swarms, no ground deformation, no unusual gas emissions and no fumarole activity, the region’s own geologic record sets the baseline odds where they stand. Unrest, when it happens, does not leave the number fixed.

How the 1980 Earthquakes Turned Into a Standing Watch

The modern monitoring effort traces to a specific sequence of shaking. A magnitude 5.4 earthquake in 1978 ended two decades of relative quiet in eastern California, and then, in May 1980, four strong magnitude 6 earthquakes struck the region, three of them on the same day. USGS scientists reexamining the caldera afterward found something new: the ground at its center had begun rising, measurably, for the first time since monitoring began.

David P. Hill, the USGS seismologist who led the survey’s long-running fact sheet on the caldera, documented that the center of Long Valley rose almost a foot in the year after the 1980 shocks and had swelled by nearly two and a half feet by the time of a later revision of the report. That swelling, still continuing today across more than 100 square miles, is what convinced the survey to install an expanded seismometer network in 1982 and to formalize the color-coded alert system now used to communicate changes in the caldera’s behavior to California’s emergency services.

Most of the Risk Runs Along the Mono-Inyo Chain

Future eruptions, according to the survey, are considerably more likely to break out somewhere along the Mono-Inyo Craters chain than from the resurgent dome or the south moat area inside the caldera proper. That distinction matters for how the odds get applied locally: the one-in-a-few-hundred figure describes the chain as a whole, not any single point along it.

A separate, smaller number covers that narrower question. The survey puts the odds that a small eruption anywhere on the chain would seriously affect one specific location along it at roughly one in a thousand in a given year, a full order of magnitude lower than the odds of an eruption happening somewhere on the chain at all.

Unrest Moves the Number, But Only So Far

Earthquake swarms, ground deformation, gas emissions and fumarole activity can all push the eruption odds higher, though the survey is careful to note the increase depends heavily on the nature, intensity and location of whatever unrest is underway. Background activity since 1980 has typically included up to twenty small earthquakes a day and occasional swarms carrying a magnitude 4 event roughly once a year; a swarm that size is treated as routine, not as a warning.

Even sustained unrest rarely translates into an eruption. Large calderas worldwide that show episodes of unrest culminate in actual eruptions only about one time in six, by the survey’s own accounting. Carbon dioxide seeping up from magma beneath Mammoth Mountain has been killing trees there since the early 1990s, a visible sign of the same underlying gas emissions the odds calculation accounts for, and it has not, on its own, shifted the caldera into a higher alert category.

Small Eruptions Are the Likely Future, Not Ash Columns

When the region does erupt, the survey expects something explosive but modest, not a repeat of the eruption that carved the caldera itself roughly seven hundred sixty thousand years ago, an event more than two thousand times larger than the 1980 eruption of Mount St. Helens. For the Mono-Inyo chain, any eruption would probably combine explosive and effusive behavior; for the Long Valley and Mammoth area specifically, the most likely outcome is a lava-fountaining eruption that builds a scoria cone and sends flows down nearby slopes.

No evidence currently points toward anything resembling that caldera-forming event happening again soon. Hill’s fact sheet states plainly that the same magma heating the region’s hot springs and steam vents also drives three geothermal power plants generating a combined 40 megawatts, evidence the system is active without suggesting it is building toward anything catastrophic.

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

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