SALT LAKE CITY — How long does it take for a mountain to fall?
A new study says in just under 90 seconds, a Utah mountainside collapsed at what is now Zion National Park, moving as fast as 180 mph with rocky debris that would cover New York City's Central Park with 275 feet of material.
The earthy makeover created the flat floor of Utah's most visited national park by damming the Virgin River to create a lake that lasted 700 years.
"It was certainly a violent and short-lived event. The whole slide was over in 60 to 90 seconds," said Jeff Moore, the study’s senior author and an assistant professor of geology and geophysics at the University of Utah, pointing out one would need 90 times the volume of concrete in the Hoover Dam to re-create the gargantuan landslide.
Moore spent five years studying alpine landslides and deposits in the Swiss Alps while working at the Swiss Federal Institute of Technology, laying the foundational work to map the Sentinel rock collapse.
The study, which was conducted with Jessica Castleton, who is now at the Utah Geological Survey, was published Thursday and featured on the cover of the June issue of the Geological Society of America’s journal GSA Today.
"The Swiss Alps have a lot of similar deposits and were mapped a long time ago," Moore said. "A lot of them dammed rivers and blocked valleys."
The Sentinel landslide was first described in a scientific paper in 1945, but this new examination provides more details on when it happened — 4,800 years ago — and provides insight into its characteristics.
Computer simulations show the huge slide rushed southeast across Zion Canyon in about 20 seconds, averaging 112 mph and reaching a top speed of between 180 mph and 200 mph.
The original deposit was 2 miles long and a mile wide, with its lower end right at the mouth of the canyon.
"This catastrophic landslide of massive proportions had two effects," Moore said. "One was constructive — creating paradise through cataclysm. More than 3.6 million people last year enjoyed the flat and tranquil valley floor of Zion Canyon, which owes its existence to this landslide. The other aspect is the extreme hazard that a similar event would pose if it happened today."
In fact, the landslide's deposit still produces smaller slides, including one in 1995 that damaged a road in the canyon, Moore said.
"From a long-term perspective, this is a part of a cycle," he said. "The rivers dig deeper into the bedrock, the canyon walls grow higher. Ultimately they cannot sustain these heights and they respond by failing in small and big landslides alike."
Moore and other researchers obtained permission from the National Park Service to take samples from 12 boulders on the landslide's surface, analyzing their beryllium-10 content.
The amount of the radioactive isotope — formed by exposure to cosmic rays — helps scientists pinpoint when a boulder's surface was first exposed after landslide activity.
Moore said previous studies used indirect methods of dating the landslide, with one that estimated it was 7,900 years old.
Only 55 percent of the landslide's deposit — or the flat valley floor of the canyon — remains.
"All that sediment on the flat floor is also on its way out," he said. "When that goes, Zion Canyon will look a lot different and feel a lot different."
Other researchers involved in the study include Jordan Aaron, who wrote the computer code to simulate the landslide's distance and speed and is a geological engineering doctoral student at the University of British Columbia, and Marcus Christi and Susan Ivy-Ochs, rock dating experts at ETH Zurich, the Swiss Federal Institute of Technology.