Photo: Jeff Moore If you've ever gazed into Zion canyon in southwestern Utah's Zion National Park, it's hard not to be awed by the site of the Sentinel, the majestic 7,157-foot-high mountain that stands guard over the landscape.
But flash back to 4,800 years ago -- for perspective, about 250 years before Egypt's Great Pyramid of Giza was built -- and the scene looked a bit different. The Sentinel was considerably bigger, and the floor of Zion Canyon wasn't quite so flat. That's when the mountainside collapsed, and released a massive avalanche of rocks that careened down the slope at up to 200 miles an hour. The event dumped enough debris to bury New York City's Central Park, all 843 acres of it, under a layer 275 feet deep.
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A new article in June issue of Geological Society of America's journal GSA Today provides the first definitive estimate of the landslide's date, as well as estimates of its size and dynamics.
The study's lead author, University of Utah assistant professor of geology and geophysics, used computer simulations and studies of deposits from the landslide to calculate that the event deposited 10.1 billion cubic feet of debris. He estimated that to recreate the mountainside's bulk, it would take 90 times the amount of concrete used to build Hoover Dam.
"This catastrophic landslide of massive proportions had two effects," he explained in a press release. "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."
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Moore and colleagues sampled 12 boulders from the park, analyzing them for their quantities of beryllium-10, a substance created by exposure to cosmic radiation striking the Earth. The longer a boulder is exposed, the greater the concentration of beryllium-10.
While that data helped to fix the date, another question still remains: What caused the massive landslide?
"We found no evidence indicating there was an ancient earthquake at the time, but there's not a detailed record of paleo-earthquakes in the Zion area," Moore said. "Rock avalanches frequently occur with an earthquake trigger but just as often occur with no apparent trigger at all."
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