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Julie Friedman
U.S. Geological Survey field assistant Julie Friedman stands next to a 10-foot-tall rock fragment formed by a hydrothermal explosion in Yellowstone’s Hot Spring Basin. The fragments are called breccia and are a conglomerate of sharply angled rocks embedded in finer material. (Courtesy photo)

Wild things happen in Yellowstone National Park all the time — geysers shoot water hundreds of feet into the air and mud pots gurgle with steaming-hot sludge.

Despite such wonders, it’s a bit hard to imagine a tsunami-like wave rushing across Yellowstone Lake. Yet that’s one theory of how the largest hydrothermal explosion crater complex documented in the park, likely the largest in the world, was created at Mary Bay. The complex at the northeastern end of Yellowstone Lake is 1.7 miles long and 1.4 miles wide.

The tsunami theory is discussed in a recently published paper on Yellowstone National Park’s hydrothermal processes and large explosions, a special comprehensive paper for The Geological Society of America.

“This is by far the most exhaustive study,” said Ken Pierce, a U.S. Geological Survey researcher at the Northern Rocky Mountain Science Center in Bozeman, who co-authored the paper with fellow USGS scientists Lisa Morgan and Pat Shanks III.

Big-bang theory

While the park’s supervolcano and seismic activity may draw researchers’ attention and the public’s interest, the hydrothermal processes at work in Yellowstone are no less fascinating, especially the Mary Bay explosion.

According to the researchers, the tsunami-like wave could have been triggered by the shifting of a fault underneath the lake, much the same way that a tsunami in the Indian Ocean was caused by a 2004 underwater earthquake.

Based on their investigations, Morgan, Pierce and Shanks write that the wave could have splashed the lake’s water more than a mile inland. As water suddenly receded in the northeast corner of the lake, the Mary Bay hydrothermal area that had been kept stifled by the pressure of the lake water could have flashed to steam so quickly that it created an explosion capable of hurtling debris at about 2,200 feet per second up to 2.5 miles away.

Such explosions are not volcanic and don’t directly involve magma.

“We can’t say that the tsunami was the trigger, or if it was the seismic activity,” Morgan said. “Most people don’t associate tsunamis with lakes. It was a seismic event that triggered a large wave.”

Ice left, steam came

No matter the cause, there’s no doubt the Mary Bay explosion occurred. A complex of craters has been mapped on the bottom of the bay.

Prior to this research, it was believed that all of the large hydrothermal explosions in Yellowstone were triggered by the retreat of the glaciers — once more than 3,000 feet thick — about 14,000 to 16,000 years ago. Such an ice cap would have kept a lid on hydrothermal features. Another possibility is that when ice dams broke and waters flooded out, the features were suddenly exposed. As the pressure declined, explosions would have rocked Yellowstone’s ancient caldera.

But Morgan said their research also found evidence of more recent explosions, averaging about one every 700 years. 

“We found out that they did not all happen at one time and that they were occurring pretty regularly,” Morgan said.

There have been at least 20 big hydrothermal explosions that made craters larger than 300 feet in diameter since glaciers retreated in Yellowstone. Most of the sites are now covered by water, such as Indian Pond, Turbid Lake and Twin Buttes. Such documented sites have crater depths that range from 9 feet to several hundred feet deep.

Triggering the boom

Morgan, Pierce and Shanks predict that such explosions could happen again — with little or no warning — given the right circumstances. Triggers could include earthquakes, landslides or even the lowering of the water table in drought. Given those considerations, the researchers theorize that an explosion large enough to create a crater 300 feet wide might be expected every 200 years.

“These events are much more frequent than any volcanic event in Yellowstone,” Morgan said. “So they’re one of the more frequent hazards.”

She said more work needs to be done to find out how to monitor and predict the hydrothermal explosions to protect park visitors and workers.

“I think it’s important for the park to have a full understanding of the breadth of the potential hazards in Yellowstone,” Morgan said.

Contact Brett French at or at 657-1387.