Each of the three largest eruptions of Yellowstone's magma chamber resulted in the collapse of the chamber's roof rock, resulting in a circular caldera tens of kilometers wide. (by Arlene Ducao for American Museum of Natural History)

Yellowstone, we feel, is a very, very safe place to visit,” says Hank Heasler, one of two park geologists at Yellowstone. It’s true that acrid, piping-hot groundwater flows just under the park’s rocky plateau, forming a landscape bubbling, steaming, and spraying with hydrothermal activity. It’s also true that three of the most astonishing volcanic eruptions in the geologic record—each hundreds to thousands of times the volume of 1980’s Saint Helens eruption—occurred around what is now Yellowstone National Park, which includes parts of Wyoming, Montana, and Idaho. Over three million visitors step onto this charged volcanic landscape every year. Yet the geologists that monitor it are unconcerned about a large, imminent eruption. Far more unnerving is an encounter with one of the park’s wolves or bears.

A Restless History

“To the public, an active volcano is one that’s erupting now,” says United States Geological Survey geologist Jake Lowenstern, who heads the Yellowstone Volcano Observatory. Yellowstone is not erupting, but it is active. About 400 km below it, in the Earth’s upper mantle, lies a hot spot: a fixed region of partly molten rock far from any tectonic plate boundary. “You can think of the hot spot as a blowtorch,” explains Lowenstern. “It's creating melt in the mantle, and that melt is rising and melting the continental crust above it.” At the moment, a 50 km wide chamber of molten rock—magma—sits about 8 km beneath Yellowstone. When the crust above the chamber no longer can withstand the upward pressure of the swelling magma chamber, it fractures and the magma erupts.

The first of Yellowstone’s three big eruptions was 2.1 million years ago, the next was 1.3 million years ago, and the last was 640,000 years ago. During each event, gas-laden magma erupted explosively like an uncorked champagne bottle. The explosions shattered magma and overlying rocks into fragments and ash particles. Fluid magma exploded through the fractures and paved the Yellowstone soil.

Only about 10 percent of the magma chamber exploded in each “supervolcano” event; still, that amounted to more than a thousand cubic kilometers of material per eruption. “Two of the three eruptions put out enough volcanic ash to spread a cloud all the way to the Mississippi River and the Gulf of Mexico,” says Heasler. This blocked the Sun’s rays and cooled the Earth’s atmosphere, which took years to recover. After each eruption, the roof of the partially emptied magma chamber collapsed, forming an enormous surface depression called a caldera. When the magma chamber filled again to a pressure point, it erupted in a slightly different location. Remnants of the clifflike walls of Yellowstone’s three calderas are still visible. (See part of one caldera in this interactive.)

Basic math on Yellowstone's eruption cycle (one event every 600,000 to 800,000 years) seems to suggest a fourth event, well, about now. Heasler demurs. ”Three data points do not make a compelling argument for almost anything in science,” he says. Geologists are uncertain whether Yellowstone is winding down from the third eruption or ramping up to a fourth.

Technically, the next eruption could happen anytime. However, catastrophic eruptions occur so infrequently in the geologic record that it is statistically not likely anytime soon. More importantly, if Yellowstone were preparing to blow another big one, its heavily monitored signs of unrest would also clearly indicate imminent eruption. (They don’t.) More likely hazards are localized lava flows and hydrothermal explosions, which are just symptoms of the park’s volcanic underbelly.

What Dangers Await?

“If a lava flow were to occur here today, it certainly would have an effect,” says Lowenstern. “But it wouldn’t cause many, if any, deaths.” Lava is what magma is called after it breaches Earth’s surface. About 80 lava flows since Yellowstone’s last big eruption 640,000 years ago have filled in much of the three calderas, so that their entire circumferences are only detectable with careful fieldwork. Lava erupting from existing or new cracks at Yellowstone would likely be thick and viscous and have little gas left in it. Thus, it would ooze, not explode, and be unable to flow long distances easily. “Tourists just wouldn’t be allowed in certain areas,” says Lowenstern.

The remaining molten rock in Yellowstone’s collapsed magma chamber is now cooling. It donates heat to the water table above it, which creates Yellowstone’s more than 10,000 hydrothermal features. The hot groundwater can flash as steam in geysers like Old Faithful or belch through cauldronlike mud pots. The water also collects in pools, some of which are acidic, near boiling, blue-green with minerals and microbes, and reeking of rotten-eggy hydrogen sulfide. (“The smell of life,” Lowenstern calls it.) An unanticipated hydrothermal explosion could scald or severely injure park visitors and staff.

Still, a lava flow or a hydrothermal explosion does not herald a new catastrophic eruption. A surer sign would be a dramatic shift in the ground level at Yellowstone, a hint that the magma chamber was moving upward or significantly refilling. Scientists would also look for serious “swarms” of earthquake activity, which would suggest the malleable magma chamber was rupturing the brittle rock above it. Recent monitoring has detected both ground level rises (8 to 10 cm in the past 19 months) and seismic signals, but they’re not dramatic enough to warrant worry. They simply remind geologists that Yellowstone is naturally a place of change.

“Thermal features can change over one day,” says Heasler. “Yellowstone is an interesting place where we can see geologic processes changing on a day-to-day basis rather than a million-year-by-million-year basis.” Monitoring this change is the essence of the geologic work ongoing at Yellowstone. It is the key to predicting the park’s next big moment.