The Markagunt Plateau is considered the top step in the Grand Staircase, the landscape that descends from the roof of southwestern Utah into the basement of the Grand Canyon.

Back in the 1970s, students exploring the region discovered what persisted as a geological mystery for decades: Older volcanic rocks covered more recently formed stones on the 11,000-foot plateau.

“What in the world might have happened to produce that? We’re talking about milelong blocks of rock that are just out of place,” said Bob Biek, a senior scientist with the Utah Geological Survey. “It really breaks a fundamental rule of geology that older rocks are typically overlain by younger rocks unless there’s something really weird going on, which, it turns out, there is.”

That weird something proved to be ancient “megaslides” that occurred 18 million to 25 million years ago and are far larger than any landslide known to have occurred in the Earth’s history, according to ongoing research sponsored by the geological survey and led by Biek. Thanks to years of geologic mapping in southwestern Utah, Biek and his team have identified three massive slides associated with volcanic highlands that have long since eroded away, replaced by today’s broad valleys and towering plateaus, like the Markagunt, Sevier Plateau and Black Mountains.

Biek named the three slides after these plateaus, collectively called the Marysvale gravity slide complex, named for the volcanic field that dominated this landscape millions of years ago. Debris from the slides covers 3,000 square miles in Piute, Iron, Garfield and Beaver counties and is a mile thick at its deepest on the southern end of the Tushar Mountains.

Christopher Cherrington | The Salt Lake Tribune
Christopher Cherrington | The Salt Lake Tribune

Put another way, the slides are almost as large as Yellowstone National Park and the debris could fill the Grand Canyon, according to an article Biek and his colleagues wrote in the latest issue of the Utah Geological Survey’s quarterly journal Survey Notes.

Collaborators include Peter Rowley, a retired U.S. Geological Survey geologist, and David Hacker, a geology professor at Kent State University in Ohio.

Landslides typically leave obvious signs on a landscape, but that was not the case with this series of ancient slides.

(Photo courtesy of the Utah Geological Survey) New research sponsored by Utah Geological Survey characterizes massive landslides that swept across southwest Utah 18 million to 25 million years ago. The high-speed flow of these slides created so much heat that it turned rock into a glasslike substance called pseudotachylyte, pictured here in the form of a thin layer left in the rock formation.

“It is old. It is deeply eroded and partly buried by younger deposits and cut by later faults,” Biek and his colleagues wrote. “The [Marysvale complex] is, by any geologic standard, insanely big — so large that we cannot see it from any single vantage point.”

Setting the stage for the slides was a volcanic uplift that had created vast highlands dotted with dozens of stratovolcanoes, like those found in the Cascade Mountains of Oregon and Washington, said Rowley, who was monitoring Mount St. Helens at the time of its 1980 eruption.

That eruption caused Mount St. Helens’ summit and north face to collapse in what was among the largest slides in recorded history. Multiply that event by a thousand and you’ll get an idea of the Markagunt slide’s magnitude.

It was the collapse of Mount St. Helens that helped Biek solve the mystery of the Markagunt Plateau’s volcanic rocks.

“It really opened the door to geologists to realize how unstable these volcanoes actually are. Before [Mount St. Helens’ eruption], they didn’t realize that volcanoes fell down,” Biek said. “There’s hundreds and hundreds of modern examples of volcanoes that you can see in the landscape today that have collapsed catastrophically. St. Helens is just one of the latest.”

Biek first identified the Markagunt slide, the largest of the three, in 2014, and subsequent searching identified the Sevier slide to the east in 2016 and the Black Mountains slide to the west last year.

"These Cascades-like volcanoes were built upon a weak foundation of clay-rich, greasy, unstable rock. Imagine piling volcanoes up. At some point they're going to become unstable and collapse," Biek said. "Likely it was an earthquake or an eruption that triggered each of these three slides."

He believes there could be other massive landslides hiding in plain sight elsewhere in the world.

"These things are so large that they replicate truly tectonic features that we see elsewhere in the landscape," Biek said. "One of the important things is to look at other volcanic fields elsewhere in the western U.S. and elsewhere in the world and see if they actually might host as-yet-unrecognized giant landslides."

Biek believes the Marysvale slides began with the Sevier slide 25 million years ago, followed 2 million years later by the Markagunt slide. The pattern culminated with the Black Mountains slide, following a western direction that tracked the movement of volcanic activity over time in this region where the Great Basin and the Colorado Plateau now meet.

Biek suspects each of the slides, which all moved from north to south, was a single catastrophic event that lasted only minutes. Moving debris reached freeway speeds and would have obliterated everything in their paths. Evidence for such cataclysms was found in the form of a strange rock, known as pseudotachylyte, which the geologists discovered in several places.

"That is melted rock. It is formed by the grinding action of rock against rock," Biek said. "The only way to actually generate enough heat to do that is if you move things very quickly."

What concealed these slides was their sheer size. The Markugunt slide was some 20 miles wide and extended 60 miles.

Rowley, who lives in New Harmony, had mapped the geology of this region for decades, filing about 200 reports during his career.

"I did a lot of the early mapping in the Tushars and I missed it all, the three slides," Rowley said, "I walked over them many times and never saw them. It was too big."

Anyone traveling across southwestern Utah will pass through landscapes shaped by these ancient slides.

"You won't know because they're so big and they don't look like a modern slide. But all the pieces are there," Biek said. "It's interesting just to know about the history of the landscape that you were living on."

Biek is developing a field guide to the slides and hopes to see interpretive signs and trails installed at sites where the public can see evidence of these cataclysmic events. Most of these locations are hard to reach, but one possible spot for interpretive facilities is a road cut near Panguitch Lake, where a millimeter-thick layer of pseudotachylyte is apparent.

“This is an exciting discovery,” Rowley said. “The public needs to know this stuff and in a language they understand.”