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The Big One: When Will the Yellowstone Supervolcano Erupt?
Yellowstone is an active volcano. Surface features such as geysers and hot springs are direct results of the region's underlying volcanism.
Credit: National Park Service

It's no mere doomsday pseudoscience: The Yellowstone supervolcano really could be the end of us all. When the Yellowstone Caldera — the name of the national park's geographic structure, which roughly translates as "caldron" — blows its lid, much of the continental United States will get covered in a blanket of ash. That ash will clog the atmosphere enough to block out the sun, disrupting the global climate enough to cause food shortages and mass famine.

The last full-scale eruption of that kind occurred 640,000 years ago, and the ones prior to that occurred 1.3 and 2.1 million years ago. Interspersed with the big ones have been smaller-scale but still major eruptions, most recently 70,000 years ago.

At the Yellowstone Volcano Observatory (YVO), an outpost run by the U.S. Geological Survey in conjunction with Yellowstone National Park and the University of Utah, a team of volcanologists continuously monitors the sleeping giant's tectonic activity. They listen to its rumblings (which are streamed online in real time) for clues as to what's brewing below the surface. Jacob Lowenstern, scientist-in-charge at YVO, told us what they're listening for and what they know so far about the next "big one."

Always astir

"Earthquake swarms" (that is, series of quakes), ground deformation, and hydrothermal (steam) explosions can all signal impending volcanic activity, Lowenstern said. All three are common at Yellowstone — the area has a history of earthquake swarms and uplift/subsidence cycles and is practically always astir — but for now, they aren't intense enough to warrant concern about an impending volcanic eruption. [Read: How Hot Is Lava? ]

"It is clear from geological studies that the kind of activity we see at Yellowstone  has been occurring for a very long time, and that such activity does not imply that an eruption is coming anytime soon," Lowenstern wrote in an email. "Given that one hasn't happened at Yellowstone for 70,000 years, and given that we know there are lots of earthquake swarms and episodes of ground deformation, it is clear that it takes quite a bit to cause Yellowstone to erupt."

During the five years since the YVO team began posting monthly volcano alert levels, the level has stayed at "normal." That will change, Lowenstern explained, only if an intense swarm of more than 500 earthquakes, some with magnitudes greater than 4.5, is accompanied by either a rapid change in ground displacement — for example, a rise or fall in the Earth's crust of more than 2 inches (5 cm) in 30 days — or a large hydrothermal explosion.

Before upping the alert level, "in general, we'd need to see more going on than what we've seen, and we'd want to see deformation and earthquakes happening simultaneously and in some abundance," Lowenstern said. Even those warning signs wouldn't necessarily mean a massive eruption, the scientists are quick to note. [Read: Which U.S. Volcanoes Are Most Dangerous Right Now? ]

Seeing the future

Until the moment the YVO team sees signs that an eruption is months or weeks away, there is no trustworthy way to predict when one will occur. "Scientists are reasonably good at short-term forecasts of volcanic activity but cannot look long into the future," Lowenstern told Life's Little Mysteries.

Earth's tectonic plates are too complex for their future to be knowable. Thus, in lieu of predicting exactly when specific earthquakes and volcanic eruptions will happen, geologists "typically rely on probabilistic evaluations that are based on how often volcanoes have erupted in the past," Lowenstern explained.

"At Yellowstone," he continued, "activity is clearly episodic. There was a very long period of volcanic activity between 170,000 years ago until about 70,000 years ago. Many tens of lava flows erupted during that time, though none were nearly as explosive as the supereruptions that are so oft-discussed in the press. Since 70,000 years ago, there have been no volcanic eruptions at Yellowstone.  Nearly all geologists I know expect that Yellowstone will experience future volcanic eruptions, but we honestly cannot state when they will occur, nor do we know if there are any more supereruptions in Yellowstone's future.

"It could still be tens of thousands of years before the next eruption. Having said that, it is always possible that things could change... and that's why we keep a close watch," he wrote.

Follow Natalie Wolchover on Twitter @nattyover.