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Largest known landslide occurred 21 million years ago

Largest known landslide occurred 21 million years ago

An enormous landslide more than 21 million years ago in what is now Utah could be the largest known in all of Earth's land areas, geologists are reporting.

The so-called Markagunt gravity slide covered an area greater than the state of Rhode Island within minutes-moving fast enough to melt rock into glass due to the immense friction, the researchers said. Any animals in the way would have been quickly mowed down.

Geologists previously knew about parts of the landslide, but in new work, geologist David Hacker of Kent State University in Ohio hiked through wilderness to find features indicating the slide was much bigger than previously realized. The findings are published in the November issue of the journal Geology.

The landslide took place in an area between what is now Bryce Canyon National Park and the town of Beaver, Utah, Hacker and colleagues said, and covered about 1,300 square miles (3,400 square km). That would make it one of the two largest known continental landslides (larger slides exist on the ocean floors).

Its rival in size, the “Heart Mountain slide,” which took place around 50 million years ago in northwest Wyoming, was discovered in the 1940s. The Markagunt slide could prove to be much larger, once it is better mapped, Hacker and colleagues said.

They suggest it occurred when not just one mountainside gave way, but a whole portion of a volcanic mountain range whose base had been pushed up higher and higher by molten rock, or magma, gathering beneath.

“Large-scale catastrophic collapses of volcanic fields such as these are rare but represent the largest known landslides on the surface of the Earth,” the authors wrote. The landslide was over 55 miles (90 km) long, Hacker added, though today, “looking at it, you wouldn't even recognize it as a landslide.”

Understanding the mega-landslide could help geologists better understand these extreme events, he said. The Markagunt and the Heart Mountain slides document for the first time how large portions of ancient volcanic fields have collapsed, Hacker explained, representing “a new class of hazards in volcanic fields.”

Such events could theoretically happen in modern volcanic fields, or groups of volcanic mountains, such as the Cascade Mountains, which include Mt. St. Helens in Washington, he added. But many conditions must come together to produce a landslide. “We study events from the geologic past to better understand what could happen in the future,” said Hacker, who plans to continue mapping and analyzing the slide.


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