Yellowstone sits atop a supervolcano scientists monitor around the clock

Millions of visitors walk past Yellowstone’s geysers and hot springs every year without thinking of the park as a volcano at all, let alone one of the few places on Earth to have produced an eruption large enough to earn the label “supervolcano.” A dedicated federal observatory has watched the ground beneath the park continuously for more than two decades, and its own public guidance is unusually blunt about both what the volcano has done in the deep past and what it is not currently doing now.

What Actually Makes a Volcano a “Supervolcano”

The term supervolcano refers specifically to a volcanic center that has produced an eruption rated magnitude 8 on the Volcanic Explosivity Index, meaning the eruption’s deposits measure more than 1,000 cubic kilometers, or about 240 cubic miles, according to the U.S. Geological Survey. Every eruption of that scale on record occurred tens of thousands to millions of years ago, which is why volume of deposits, rather than direct observation, is how scientists classify them. Yellowstone qualifies because of three such eruptions in its past, the oldest of which, the Huckleberry Ridge eruption, occurred about 2.1 million years ago. Other volcanic centers that meet the same threshold include Long Valley in eastern California, Toba in Indonesia and Taupo in New Zealand, whose most recent supervolcanic eruption, about 27,000 years ago, remains the most recent one on the planet. USGS geologist Bob Christiansen, who mapped Yellowstone’s three calderas largely through fieldwork rather than remote sensing, has pushed back on a popular claim that the calderas were only discovered from satellite photos; the boundaries were worked out on the ground with a hammer and hand lens in the 1960s and 1970s, and his resulting geologic maps were later used to check NASA’s orbital imagery rather than the other way around.

The Odds of Another Caldera-Forming Eruption

USGS scientists estimate the annual probability of another caldera-forming eruption at Yellowstone at roughly 1 in 730,000, or about 0.00014 percent, a figure the agency compares to the odds of a one-kilometer asteroid striking Earth in a given year. That number comes with an explicit caveat from USGS itself: it is derived simply by averaging the two time gaps separating Yellowstone’s three known giant eruptions, which the agency says is far too small a sample to treat as a precise forecast. It has now been roughly 640,000 years since the most recent of those three eruptions, and USGS states plainly that there is no evidence a catastrophic eruption is imminent, adding that such an event is considered unlikely even within the next few centuries. The agency is equally direct that a smaller lava eruption shows no signs of being imminent either, and it draws a firm line against the idea, popularized by a 2005 BBC and Discovery Channel docudrama depicting a future Yellowstone disaster, that the park is quietly building toward a catastrophe on any predictable timetable.

What Would Actually Happen: Ash Reaches Farther Than Lava

If Yellowstone erupts again, USGS hazard assessments say the far more probable outcome by orders of magnitude is a nonexplosive lava flow rather than a caldera-collapsing explosion. Roughly 80 such lava-flow eruptions have occurred in the 640,000 years since the last giant eruption, most recently at the Pitchstone Plateau about 70,000 years ago, and this remains the most likely type of future activity. Even a lava-flow eruption, however, could include explosive phases that produce significant ash and pumice, and USGS identifies ash and tephra fall as the single most widespread volcanic hazard tied to the system regardless of eruption type. In the specific case of another caldera-forming eruption, which the agency stresses is the least likely scenario of all, ashfall would extend across much of the United States, southern Canada and northern Mexico, and the associated volcanic gases could alter global weather patterns and disrupt agricultural production for one to two decades afterward. USGS notes that most of Yellowstone’s other geologic activity is far less dramatic: its geysers, hot springs and mud pots are generally benign features of the same hydrothermal system, though they can occasionally produce violent steam-driven hydrothermal explosions unrelated to magma movement, a distinct hazard from the ash and lava scenarios tied directly to the volcano itself.

How Yellowstone Volcano Observatory Watches for Warning Signs

The Yellowstone Volcano Observatory, a partnership between the USGS, Yellowstone National Park and the University of Utah, maintains real-time monitoring of earthquakes, ground deformation, streamflow and selected stream temperatures across the region. USGS scientists say the buildup to a large eruption would likely be detectable weeks to months or years in advance, with the more immediate precursors, intense earthquake swarms and rapid ground deformation, typically emerging just days to weeks before an actual eruption; small earthquakes and gentle ground movement on their own are common at Yellowstone and do not signal an eruption is approaching. Between 1,000 and 3,000 earthquakes strike the Yellowstone region in a typical year, the overwhelming majority too small to feel, though the area has recorded larger shocks, including the magnitude 7.5 Hebgen Lake earthquake in 1959, the largest in the U.S. intermountain region’s recorded history, and a magnitude 6.1 earthquake near Norris Geyser Basin in 1975. Neither that seismicity nor the thousands of years of steam explosions at Yellowstone’s hydrothermal features have triggered a volcanic eruption, and USGS notes that even drilling into the system to relieve pressure would be technically pointless, since any borehole would quickly seal itself as minerals crystallize out of fluids near magmatic temperatures and pressures.

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

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