A series of massive volcanic eruptions between eight and 12 million years ago in what is now Idaho may have been larger than colossal events known to have taken place in Yellowstone.

In all, there were 12 such eruptions centered on the region of the Snake River Valley, and while that is roughly half as many as had been previously believed, they were “significantly larger” than had been calculated.

“While it is well-known that Yellowstone has erupted catastrophically in recent times, perhaps less widely appreciated is that these were just the latest in a protracted history of numerous catastrophic super-eruptions that have burned a track along the Snake River eastwards from Oregon to Yellowstone from 16 million years ago to the present,” said Tom Knott of the University of Leicester Department of Geology’s Volcanology Group.

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Knott was the lead author of the new study, which was just published in the journal Geological Society of America Bulletin.

“The size and magnitude of this newly defined eruption is as large, if not larger, than better known eruptions at Yellowstone, and it is just the first in an emerging record of newly discovered super-eruptions during a period of intense magmatic activity between 8 and 12 million years ago,” he added.

To reach their conclusions, the researchers painstakingly pieced together numerous pieces of evidence, including examining widespread glassy deposits fused to the landscape, each of which preserves subtly distinctive magnetic, mineralogical, and chemical characteristics that allow them to be traced great distances. They also drilled deep into rock composed of ancient volcanic ash.

Yellowstone Supervolcano Much Bigger than Thought

They found that one of the super-eruptions from the Yellowstone hotspot-track, defined as the Castleford Crossing eruption, occurred about 8.1 million years ago; they estimated the eruption volume exceeded 1,900 cubic kilometers (456 cubic miles). To put that into some kind of perspective, that would be more than enough to completely fill Lake Ontario.

The strength of that eruption was calculated to be approximately 8.6 on the Volcanic Explosivity Index (VEI). That means it was hundreds of times more powerful than the Mt. Pinatubo eruption in the Philippines in 1991, which had a VEI of 6 and was itself 10 times more powerful than the 1980 Mt. St. Helens event.

The hotspot track gradually worked its way eastward to Yellowstone, where an eruption 640,000 years ago formed the Yellowstone Caldera. The most recent eruption at Yellowstone was 170,000 years ago, and the last lava flow was about 70,000 years ago.

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Despite occasional alarm about the devastating impacts of another supervolcanic eruption beneath the park, Bob Smith, a University of Utah scientist who has studied the geodynamics of Yellowstone for 60 years, told the Billings Gazette that the hotspot has now run into the thicker North American Plate, meaning it will take a lot more energy and heat to blow through than when it was located in the thin and fractured Snake River Plain.

“People are always joking that Billings is going to be wiped out,” Smith said, “but there may not be enough energy for the hotspot to burn through the lithosphere. We may argue that Yellowstone may not have a super eruption again.”