As far as we know, the Wah Wah Springs eruption is the largest known explosive volcanic eruption. – Eric Christiansen

PROVO — Geologists at BYU have discovered what may have been the world's largest supervolcano, which collapsed and exploded in western Utah.

Geologists say 30 million years ago, more than 5,500 cubic kilometers of magma erupted near Wah Wah Springs.

"As far as we know, the Wah Wah Springs eruption is the largest known explosive volcanic eruption," lead author and BYU geology professor Eric Christiansen said.

What happened then is hardly visible to the naked eye now, but underneath and in surrounding formations the evidence was there.

The eruption and collapse of this supervolcano released a lot of magma.

“The sky would have been darkened for days, perhaps weeks, because the ash, so much ash in the atmosphere would have completely blocked out the sun, just penetrating darkness,” study author and BYU geology professor emeritus Myron Best said. “Then eventually, when all of the ash cleared out of the atmosphere, you would see this vast landscape of steaming hot volcanic ash.”

Best said there was unimaginable devastation over a flat landscape covering about 12,000 square miles in western Utah and eastern Nevada. It extended beyond what is now Panguitch, north almost to Delta, and south almost to St. George.

It’s taken about 30 summers and 600 students in their summer field course to map out the geology and find the remnants of this supervolanic eruption.

The mapping of five mountain ranges led to the discovery of a caldera, or a huge hole in the landscape, lying across the Utah-Nevada state line. It measures about 30 miles across and 3 miles deep. Deposits left behind in southern Utah from this single eruption are 13,000 feet thick.

They used radiometric dating, X-ray fluorescence spectrometry, and chemical analysis of the minerals to verify that the volcanic ash was all from the same ancient super eruption.

As part of this monumental discovery, a repository at BYU includes drawers from floor to ceiling containing rock samples, thousands of them, collected over 30 years.

Airborne ash was caught up in the jet stream and carried as far as Nebraska.

“The gassy magma is erupting explosively, and the pyroclastic flow spreads across the landscape,” Best said.

“The accompanying collapse of this large lid to the magma chamber would have induced significant numbers of earthquakes,” Christiansen said. “At the same time, there would have been a roar and a hiss and a rumble.”

“For comparison, the eruption of Krakatoa (in Indonesia in 1883), could be heard thousands of miles away, and that was small in comparison to this supervolcanic eruption,” Christiansen said.

Unlike Yellowstone, which is still an active caldera, Utah and Nevada’s shared supervolcano will likely never erupt again.

“The geologic conditions that led to the eruption of these supervolcanoes have changed,” Christiansen said. “The whole plate tectonic environment is different now, so the chance of a super eruption in western Utah are very small.”