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YELLOWSTONE NATIONAL PARK — While a full-scale volcanic eruption of a Yellowstone super volcano often grabs the headlines and shows up in disaster movies; full-scale eruptions are incredibly rare and unlikely to occur without advanced warning. Hydrothermal explosions, on the other hand, are far more common and more concerning than most people realize.
Speaking of the recent hydrothermal explosion at Biscuit Basin in July, Michael Poland, head scientist at the Yellowstone Volcano Observatory, said, "This was the thing that geologists were most worried about — an explosion like this during the day, during the summer, in a geyser basin that is heavily visited."
"It's always been sort of the most likely hazard to impact people in the park — that, and a big earthquake," he said. "And I don't think it was that well appreciated, prior to this event. But maybe it's better appreciated now."
Just a few months before the Biscuit Basin explosion, Poland said Yellowstone experienced another hydrothermal explosion in April, at Norris Geyser Basin. But because it occurred during the transition from winter to summer, no one was there to witness it in person. It was only discovered when geologist located a newly formed crater in May, and then verified the eruption using a U.S. Geological Survey monitoring station, which had been installed in 2023.
USGS modeling data suggests hydrothermal explosions occur every year in Yellowstone National Park but, compared to the Biscuit Basin explosion, which sent water, steam and debris as high as 500 feet to 600 feet into the air, most explosions are comparatively small, reaching heights between 6 and 20 feet.
Explosions on the scale of the July 23 Biscuit Basin event occur about every one or two decades, according to Poland. But those often occur in the backcountry, or in winter when no one is around to witness it, or be injured, consequently.
"There's an inverse relationship," he said, regarding the frequency of such explosions. "The small explosions happen all the time. And, the larger ones (like the Biscuit Bay explosion) are much, much rarer."
That offers some comfort, because Yellowstone has a somewhat terrifying history of colossal hydrothermal explosions that make the recent episode at Biscuit Basin seem like a mere firecracker.
East of Fishing Bridge, there's the unassuming Indian Pond, which you can see in this video:
Measuring 0.06 square miles, the explosion that carved Indian Pond would have dwarfed the most recent explosion at Biscuit Basin — but, even then, it was comparatively tiny.
Turbid Lake, a nearly mile-wide crater left by another hydrothermal explosion approximately 9,400 years ago, rests covertly in Yellowstone's backcountry, disguised as a tranquil waterscape.
But two of the largest hydrothermal explosions in Yellowstone's history are hidden in plain sight. These colossal craters illustrate just how staggering and destructive hydrothermal explosions can be.
Elliott's Crater, a half-mile-wide scar carved by a staggering explosion some 8,000 years ago, sits hidden deep beneath the surface of the massive Yellowstone Lake. It was only discovered after detailed bathymetric maps of the lake bottom were made sometime between 1999 and 2003.
To this day, the floor of Elliott's Crater remains hydrothermally active.
But even Elliot's Crater is far from the largest hydrothermal crater in the park. That title belongs to Mary Bay, on the northern shore of Yellowstone Lake. Mary Bay, which is visible in the video, isn't just the largest hydrothermal crater within the park. So far as geologists can tell, it's the largest hydrothermal crater on Earth.
Mary Bay is more than 1½ miles in diameter — basically, gigantic.
Tourists entering the park from Cody, Wyoming, actually drive through a portion of the crater as they pass by Mary Bay. But, even a visit to the northeastern shore of Yellowstone Lake only begins to tell the story of the titanic explosion that carved this mind-boggling crater.
It was a cataclysmic event. About 13,800 years ago, titanic ice sheets were retreating the thermal aqueducts on the northern reaches of Yellowstone Lake and ruptured in violent fashion, resulting in an explosion that released as much as 3.5 trillion kilojoules of energy. According to Poland, the explosion covered the entire surface of Yellowstone Lake and much of the surrounding area with debris, and rocketed a plume of water, steam and rock roughly 6,000 feet into the air — 10 times the height of the recent Biscuit Basin explosion — while leaving a crater that could swallow downtown Salt Lake City.
Should we expect another?
While explosions on the scale of what occurred at Biscuit Basin are the result of silica clogging thermal passage ways, mega explosions seem to be more complicated, according to Poland.
"In the case of Mary Bay, it is a cascading sequence of hazards," he said. "None of it involves a volcanic eruption."
There's a "bold" and somewhat controversial theory, says Poland, that a magnitude 6 earthquake ruptured the bottom of Yellowstone Lake causing a tsunami that displaced enough water to rapidly reduce the level of Yellowstone Lake by 45 feet. The reduced "confining pressure" resulting from the displaced water over Mary Bay may have been insufficient to contain the hydrothermal pressure previously held captive beneath the lake bed.
Titanic hydrothermal explosions like those that formed Turbid Lake, Elliott's Crater and Mary Bay also appear to correspond with the retreat of massive ice sheets, which likely also contributed to a loss of confining pressure.
It's still a topic of study and discussion among geologists today, but the Mary Bay explosion, like the recent explosion at Biscuit Basin, is a stark reminder that Yellowstone's landscape is incredibly dynamic. Changes occur all the time — sometimes in spectacular and terrifying fashion.
The recent explosion, Poland said, "Was the right combination of events — highlighting a hazard that we've known about and have been talking about for a while, but hasn't gained much traction.
"Fortunately, (the Biscuit Basin explosion) was very well seen, without having an impact, other than closing a boardwalk. It would have been devastating if people had been injured."
The USGS plans to continue installing monitoring stations in partnership with the National Park Service, in hopes of learning more about Yellowstone's ever-changing hydrothermal landscape, Poland added. And, no doubt, hydrothermal explosions will be part of the ongoing conversation about potential hazards.
Upcoming visit?
If you intend to visit Yellowstone, or other national parks and wild spaces, it's important to remember they are not theme parks. There are sometimes staggering forces at work, which merit both respect and reflection.
For a virtual tour of various hydrothermal craters within Yellowstone National Park, including Mary Bay, and an account of what it was like to actually be in the park during the Biscuit Basin explosion, make sure to watch the video shared above.
