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Scientists uncover massive 21-million-year-old landslide in southern Utah

Trees lay on a hillside in the aftermath of a mudslide and related flooding on March 25, 2014 in Oso, Washington. The prehistoric landslide discovered in Utah was likely 20 times larger.

David Ryder, Getty Images

Hiking through the wilderness areas of the Dixie National Forest in Utah, David Hacker and his fellow geologists were in the midst of mapping the rugged region when they stumbled upon something historic.

They knew the area had been hit by a series of prehistoric landslides. But they discovered that many of those smaller ones were actually one, massive event that happened 21 million years ago and covered an area the size of Rhode Island.

The landslide - dubbed the Markagunt gravity slide - stretched over about 1,700 miles, from what is now Bryce Canyon National Park to the town of Beaver, Utah. That makes it the biggest known landslide to have occurred above sea level. (Larger ones have taken place under the oceans.) The previous record holder was a landslide nearly 50 million years ago at Heart Mountain in northwestern Wyoming that stretched across 1,300 miles.

"It would have been like that landslide in Washington (in March) but at least 20 times bigger," said Hacker, an associate professor of geology at Kent State University who wrote on the findings in the November issue of the journal Geology with co-authors Robert F. Biek of the Utah Geological Survey and Peter D. Rowley of Geologic Mapping Inc. of New Harmony, Utah.

"This was a huge area," he said. "The part that is coming out at you over the land surface would have been a giant sheet of rock moving very quickly. It would have been a wall of rock several hundred feet tall."

Hacker said the presence of pseudotachylytes, rocks that were melted into glass by immense friction, indicated that the massive slide happened in a flash - the movement of earth taking place within mere minutes. Any animals in its path would have perished.

An expert on catastrophic geological events, Hacker said the slide originated when a volcanic field consisting of many strato-volcanoes, a type similar to Mount St. Helens in the Cascade Mountains, collapsed. Other contributing factors might have been that the volcanic rocks were resting on slippery clay. What actually sparked the slide remains unknown.

"That is what we are trying to figure out," he said. "We don't yet know the trigger."

Hacker and his team had gone to map the area as part of the Utah Geological Survey Mapping program. Using satellite images, GPS and stereo air photos that allow you to see in 3D, the team came upon something called a ramp fault which indicates an area where the landslide broke to the surface. The direction of the fault suggested that the slide covered a much bigger area than earlier believed.

"That was our Eureka moment," he said.

Told of the new finding, Columbia University's Mark H. Anders didn't appear ready to concede that the Utah slide was bigger than Heart Mountain. Anders has worked extensively on the slide in Heart Mountain and his latest research dated it to over 48 million years and established it was fast moving - rather than something that took place over years or decades.

"I would still put my money on Heart Mountain being larger," he said. "The safe way to say this is they are comparable in volume and they both are related in one way shape or form to volcanic activity. "

George H. Davis, a structural geology professor at the University of Arizona who has worked in Colorado Plateau of southern Utah for decades but did not take part in the study, countered that the findings made sense given the fragility of volcanoes.

"Volcanoes are meant to collapse," Davis said. "That is to say, they look so strong, they are standing so high. But internally, they are weak because of the all the acid that rots the center of these volcanoes. The huge load of this volcano can't be supported by the weak interior. That is what, in part, causes the flanks of a volcano to collapse and move away from the center in the form of giant landslides."

Davis also said the landslide discovery highlights the surprises that can come from a mundane task like mapping.

"These geologist hadn't set out to see if we could find one landslide bigger, bigger than Heart Mountain. That wasn't even on their minds," he said. "What we are seeing here is the power of geological mapping ... It's the kind of the thing you can't conjure up or imagine if you were back in the office or in the lab."

Davis and Hacker also said understanding past monster disasters like the Utah slide can help scientists and disaster management specialists better prepare for the next big one and think beyond individual volcanoes to consider the impact of volcano fields. Such concerns have grown in recent years as deadly landslides have become more commonplace in populated areas - the Washington disaster in March killed 43 people and one the Philippines in 2006 left more than 1,000 dead.

"We have never witnessed a Yellowstone super volcano like eruption but we know what the devastation is like in the past from our mapping," he said. "We can anticipate how a big of a magnitude of devastation will occur. If we didn't have these mapped out, we could still be living around these when they go off. Now we know how far to evacuate people when they go off."

  • Michael Casey

    Michael Casey covers the environment, science and technology for CBSNews.com