We who live along Montana's Yellowstone River are downstream from a simmering caldera, a geologic hot spot that has become especially active recently. Indeed, Yellowstone National Park contains the floor of a gigantic volcanic cauldron, one that rises and sinks with the forces that lie beneath hence the picturesque geysers and steam holes. But a wave of recent earthquake activity is raising fears that have their origins 642,000 years ago, when a Yellowstone "supervolcano" exploded so violently that it created the caldera itself. Today, such an explosion 1,000 times more powerful than the explosion of Mount St. Helens in 1980 would not only cover most of the U.S. with ash but also throw so much dust into the atmosphere that the world's climate could change.
Could the current activity be the warning signs of another such apocalypse? Or just a large but not world-ending earthquake, like the 7.5-magnitude temblor that happened on a summer night in 1959 and caused a mountain to slide down into a campground, killing 28 people and damming the Madison River?
Last week, geologists at the Yellowstone Volcano Observatory (YVO) announced they had recorded a "notable swarm of earthquakes under way since Dec. 26 beneath Yellowstone Lake." The strongest tremor among the hundreds in the past week measured 3.9 on Dec. 27; most of the readings above 2.8 were felt by park employees and visitors around the lake area. The activity relaxed in magnitude early this week but then flexed upward again to top 3.0 by early New Year's Eve. "This December 2008 earthquake sequence is the most intense in this area for some years," YVO reported, "and is centered on the east side of the Yellowstone Caldera," the ancient collapsed volcano beneath Yellowstone Lake. The scientists said they cannot immediately "identify any causative fault or other feature without further analysis." (See a gallery of recent volcanic eruptions.)
This activity could have a whole range of consequences. In a study released last year, the United States Geological Survey (USGS) said possible hazards could include hydrothermal explosions, when steam breaks through the surface and forms a crater. That has happened 26 times in the park's 127 years of record-keeping. The USGS discounted chances for cataclysmic eruption of the caldera, noting that the hot, active magma chamber below Yellowstone has turned into "largely crystallized mush." But the same study also said: "Depending on the nature and magnitude of a particular hazardous event and the particular time and season when it might occur, 70,000 to more than 100,000 persons could be affected; the most violent events could affect a broader region or even continent-wide areas."
Jake Lowenstern, Ph.D., YVO's chief scientist, who also is part of the USGS Volcano Hazards Team, told TIME that a supervolcano event does not appear to be imminent. "We don't think the amount of magma exists that would create one of these large eruptions of the past," he said. "It is still possible to have a volcanic eruption comparable to other volcanoes. But we would expect to see more and larger quakes, deformation and precursory explosions out of the lake. We don't believe that anything strange is happening right now." Last summer YVO installed new instrumentation in boreholes 500 to 600 ft. deep to better detect ground deformation. Says Lowenstern: "We have a lot more ability to look at all the data now." (See an interactive graphic depicting how scientists monitor volcanoes.)
The Yellowstone Caldera formed by the massive upheaval 642,000 years ago that spread airborne debris all the way to the Gulf of Mexico is nowhere close to being extinct. Areas of the park's topography inflate like a bellows because of magma infusing into volcanic chambers about 6 miles below the surface. About 1,000 to 2,000 tremors a year (mostly small) have been recorded since 2004, when interpretation of satellite imagery with GPS readings indicated the caldera had been rising as much as 3 in. a year. The past week's number of tremors about 400 is considered unusual.
The 1991 explosion of Mount Pinatubo in the Philippines was the largest eruption in recent memory, and its effects on the atmosphere are still being measured. The 1883 eruption of Indonesia's Krakatoa led to a global cooling and the deadly winter of 1886-87 that wiped out the short-lived open-range cattle bonanza of Montana Territory. In 2000, Ken Wohletz, a scientist at Los Alamos National Laboratory, postulated that an even bigger Krakatoa eruption in 6th century A.D. may have sent a tall plume of vaporized seawater into the atmosphere, causing the formation of stratospheric ice clouds with superfine hydrovolcanic ash, which literally cast a pall over much of the world at the beginning of what became known as the Dark Ages.
YVO's alert code for the Yellowstone Caldera stands at green, but if it ever elevates to yellow or red based on seismic readings, Lowenstern says, "Ultimately it's my responsibility to put out alerts. The National Park Service and local officials would be responsible for civil defense measures and evacuation plans. For now, life goes on. The system is generally automated, and a seismologist at the University of Utah is on call to make sure it's a real event should it be anything unusual."
This time of year, Yellowstone is a land of dramatic fire-and-ice contrasts, with hissing, boiling water heated in chambers far below shooting out clouds of steam over a subzero, snowy alpine landscape, where bison and elk find warm patches of open ground to browse. We who live here, it has been said, do so at the mercy of geology. In much of the West, with its long seismic faults and Yellowstone-centered hot spots, it is for humans a sublimely tenuous coexistence with the earth's fickle tectonic temperaments.