Ask Smithsonian: Could the Volcano Beneath Yellowstone National Park Ever Erupt?

The good news is that an eruption there is highly unlikely, but the bad news is that it would be huge

There are certainties and uncertainties in geology, but one definite is that if the massive volcano that lies beneath Yellowstone National Park ever blows, it will likely be the biggest explosion ever recorded by humans.

That’s the bad news. But geologists say that such an explosion—which would be near-apocalyptic for all life within hundreds of miles and would also deeply impact the global climate—is not likely. And on a geologic timeframe, it’s not likely any time in the near future—like maybe not for 100,000 years. At least that’s the conclusion based on all current evidence.

Yellowstone, which occupies 28,000 square miles of the northwestern corner of Wyoming, is primarily a caldera—a boiling cauldron formed from a collapsed volcano, in this case, a supervolcano. That term was coined by non-scientists, but is gaining use among geologists, perhaps for the image it so readily conveys.

A supervolcano is capable of spewing out 240 cubic miles (1,000 cubic kilometers) of debris and is rated an 8 on the 8-point volcanic explosivity index (VEI), said Sally Sennert, a geologist with the Global Volcanism Program at the Smithsonian Institution’s National Museum of Natural History.

There are close to two dozen supervolcanoes around the world, but Yellowstone is the largest. The biggest Yellowstone explosion, about two million years ago, was an 8, making it something like 10,000 times as big as the 1980 Mount St. Helens eruption in Washington, which instantly destroyed 150 square miles of forest and killed 57 people, but was only rated a 4.

There were two more VEI 8 eruptions in Yellowstone: one just over a million years ago, and the other about 640,000 years ago. The last big volcanic burst—a lava flow—occurred 70,000 years ago. But Yellowstone’s surface continues to boil, bubble and spew with the evidence of what lies beneath. There are more than 10,000 hydrothermal features, including mudpots, hot springs and half the world’s active geysers—about 300 of the shooting plumes.

All that activity is both awe-inducing and unsettling to the park’s three million annual visitors. It also causes endless speculation, including about when or if there will be another big blow.

“Yellowstone is such a hot topic all the time,” —pun intended—said Sennert.

The park’s fiery flows are being fueled by a giant pulsating chamber of magma—molten and semi-molten rock and dissolved gases. The size of that chamber is almost incomprehensible—55 miles by 20 miles, and 6 miles deep, according to the latest measurements in 2013. That was twice as big as previously thought, but the University of Utah scientists who did the analysis said that the chamber was probably not growing—a relief, considering that it could signal a more-imminent eruption. More likely, they’d been able to better measure the boundaries.

The Yellowstone caldera is diligently monitored, most closely by the Yellowstone Volcano Observatory, which is funded by the U.S. Geological Survey, the University of Utah and the park.  The Observatory tracks earthquake activity, uplift and subsidence in the Earth’s crust, and hydrothermal changes. It’s an early-warning system, providing real-time, on-line data to help assess every possible hint of a problem.

There are 1,000 to 3,000 earthquakes in the park every year, but most are small and insignificant—they aren’t precursors to volcanic activity. In 2010, there was cause for a bit of alarm, as a “swarm” of 2,300 quakes were recorded in a single month, all about 10 miles northwest of Old Faithful, the park’s main attraction.

Those swarms are usually associated with uplift and subsidence, and in the case of Yellowstone, there’s a lot of rising and falling going on, driven by that big magma chamber. Earthquake swarms can also be the lead-up to an eruption, however. 

The oft-used analogy is that a magma chamber is like a bottle of carbonated liquid—shake it up and you get a pressurized container. The container can swell with the pressure and blow its top, or it can shrink without consequence.

Geologists figure that Yellowstone’s magma pocket has swelled and released once every 730,000 years or so. The last explosion was 640,000 years ago, which would put it on track for another catastrophic bang in about 90,000-to-100,000 years. Or, it might come sooner. Or, it might never happen. It’s hard to know for sure, since the caldera is almost like a living, breathing being.

Sennert said she’d feel plenty comfortable buying real estate in the nearby Jackson Hole area. But, she wouldn’t necessarily guarantee a lava- or ash-free yard.

“Volcanologists are very wary of ever telling somebody that a volcano is safe, because they are so unpredictable,” said Sennert.

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