Yellowstone National Park is a land of many perils. Occasionally, one of the three million yearly visitors strolls up to a 2,000-pound bison and is gored. Others eat poisonous plants, snowmobile on avalanche-prone slopes, or plunge off a cliff on that last step backward to frame the perfect photograph. And at Yellowstone's 10,000 volcanically driven hot springs, geysers, bubbling mud pots and fumaroles—earth's largest concentration of hydrothermal features—about two dozen people have been boiled alive after falling or jumping in.
"People do a lot of crazy things," says Lisa Morgan, a volcanologist with the U.S. Geological Survey (USGS) who conducts research in the park for a few weeks every summer. She is trying to protect the sandal-clad innocents not from these mishaps, however, but from the ultimate "thermal accident": hydrothermal explosions. They can happen when magma-heated water and steam build up in underground pockets. This pressure causes parts of the landscape to rise and fall like merry-go-round horses. Usually they settle back down again harmlessly. But now and then, things blow up.
One of Morgan's best guesses for the next big blowout—maybe the biggest in 3,000 years—is a 2,100-foot-wide, 100-foot-high swelling on the bed of Yellowstone Lake. No one has observed any of the park's ground movements long enough to say which ones signal danger, but she says the lake bed could conceivably burst open. If so, lakeside picnickers could see a tsunami or truck-size rocks heading their way. "I wouldn't want to be here," says Morgan. Then she thinks of the spectacle. "Well, maybe in an airplane."
The park sits on a still-active 30- by 45-mile caldera, a depression created when a volcano erupted 640,000 years ago. Chances of a lava eruption in the next 10,000 years appear remote, but magma simmering four to five miles beneath the caldera's trapped groundwater drives the park's hydrothermal convulsions. Geysers like Old Faithful release pressure, but it can build to the breaking point when heated fluids get sealed in by shifts in rock structures, clogged vents or overlying sediment and mineral deposits.
In and around Yellowstone Lake—which lies near the caldera's center—Morgan and colleagues have identified several areas heavily pocked by past hydrothermal explosions. The pits, which appear to untrained eyes as ponds or depressions in the ground, are a few yards to hundreds of yards wide. Along the lake, in eroded beach cliffs and creek banks, Morgan has found layers of sand and sharp-angled rock up to three feet thick; the debris was hurled as far as three and a half miles by past explosions in the lake bed. Arrowheads jumbled in lakeshore deposits suggest unlucky prehistoric Native Americans were around for some of the explosions.
Major ones occurred from 3,000 to 14,000 years ago, according to radiocarbon dating of wood fragments mingled with the deposited rock and soil debris. Since people started keeping track, in 1872, there have been at least 20 minor blowouts at sites around the park, including several at favorite tourist spots such as Biscuit Basin and Norris Geyser Basin. The last notable one was in 1989, when the throat of Norris Basin's Pork Chop Geyser apparently clogged with minerals. When it burst, boulders rained down near tourists more than 200 feet away. (They were unscathed.)
Only recently did scientists realize the entire park was heaving up and down. In the 1970s, geophysics professor Robert Smith of the University of Utah compared new scientific surveys of ground elevations with surveys made for road building in the 1920s. He found the caldera's center had risen nearly three feet. It kept rising until 1985, when a series of earthquakes rocked the park. Scientists speculate that the tremors coincided with the sideways escape of pent-up gases. Afterward, the caldera began deflating by three-quarters of an inch a year. In 1995, some parts of it reversed direction and started reinflating, until stopping in 2002. In the meantime, a previously undetected 25-mile-wide swelling began outside the caldera, near Norris Basin, surrounded by smaller swellings one to three miles in diameter.
Though no one is sure what all of this heaving means, it's given researchers a sense of urgency about understanding the park's contortions. "Protecting visitors is our No. 1 concern," says park geologist Hank Heasler, who is working with other scientists to come up with a threat-assessment plan.
New problem spots are popping up all the time as well. In March 2003, fourteen new steam vents opened along a 230-foot line north of Norris Basin, releasing plumes of dense water vapor and powdered glass shards in a tremendous roar. Then, last July, geysers began erupting at odd times. The park had to close off much of Norris when ground temperatures shot up in places from 80 degrees Fahrenheit to 200, and the earth near a boardwalk became more acidic and began to dissolve. The basin has since calmed down, and rangers have reopened most of it, but scientists are monitoring trailside areas with thermometers stuck in the ground, seismographs peppering strategic hills, and radar images taken from satellites. "Yellowstone is like a medical patient, but we haven't studied it long enough to know its normal pulse or respiration rate," says Heasler, standing half a mile from the new steam vents.
Morgan is still tracking the dome on the Yellowstone Lake floor called the "inflated plain." She first spotted it in 1999, while she and colleagues were mapping the lake bottom. The rise, she says, is apparently the result of steam or carbon dioxide building up under the lake bed, sealed in by sediments and overlying water pressure. The swelling seems to have grown in the 1990s and is suspiciously close in size to major blowout craters nearby. In fact, it lies along a nearby fissure, a crack that forms the bed of curiously straight Weasel Creek and continues through the lake bed itself. Morgan says the fissure may have been formed by the caldera's rise and fall, like the crack atop a loaf of bread rising in the oven.