Editor's note: The following story was constructed from a news release from University of Utah Senior Science Writer Lee Siegel and interviews with University of Utah Geology Professor Jeff Moore and Zion National Park geologist Dave Sharrow conducted by Kurt Repanshek. A story about Moore's research was the cover story of Geological Society of America’s journal GSA Today released today.
Imagine a 2.5-mile-long lake in Zion Canyon, one nearly 400 feet deep that shimmered in the canyon for roughly 700 years. It's hard to grasp. But even harder to envision is a mountainside peeling off the face of what is known today as the Sentinel in Zion National Park and filling the valley floor in seconds with 286 million cubic meters of rock and dirt.
It happened, in less than a minute, the rock slide whooshing down at a peak speed of perhaps 200 mph, crushing and burying everything, and anyone, in its path. Jeff Moore, a University of Utah assistant professor of geology and geophysics, calculated that catastrophic slide and detailed it in the cover story of Geological Society of America’s journal GSA Today.
Rock slides and avalanches happen throughout the National Park System, though they're most often associated with places such as Yosemite National Park, Glacier Bay National Park and Preserve, even Great Smoky Mountains National Park. In 2012 a massive landslide in Glacier Bay on Lituya Mountain not only ran more than 5 miles along the John Hopkins Glacier, but it created a 3.7 magnitude earthquake. Another in Glacier Bay two years later moved an estimated 68 million metric tons of earth more than 4.5 miles on the flanks of Mount La Perouse.
This Google Earth aerial image of Zion Canyon shows the location of a huge rock-avalanche landslide that collapsed off a peak named the Sentinel 4,800 years ago, dammed the Virgin River and created a lake that lasted 700 years until it filled with sediment, producing the flat, cottonwood-dotted canyon floor enjoyed today by millions of visitors to Zion National Park in Utah. The orange line above the slide shows where part of the Sentinel collapsed to generate the slide. A University of Utah study provides the first direct date for the slide and details of its size and speed/Jeff Moore
In Zion in southern Utah, though, big slides are not common. The last avalanche of note, in 1995, damaged the park road between the visitor center and Zion Lodge, and actually spalled off the prehistoric "Sentinel" slide, according to Dr. Moore. That slide, which occurred about 2 a.m. on April 12 and involved about 100,000 cubic yards of rock and soil, demonstrated, on a much smaller scale, of course, how the the Sentinel slide impacted the canyon.
This map shows the location of the huge Sentinel landslide in Utah’s Zion Canyon and the lake that formed behind it when the slide dammed the ancient Virgin River. A new University of Utah study indicates the landslide happened 4,800 years ago and that the lake lasted until it filled with sediment about 4,100 years ago. That is why the canyon floor is flat unlike steep, narrow canyons nearby/Jeff Moore, University of Utah
News reports from the time mention a blockage of the Virgin River that created a 30-foot deep lake that backed up for a quarter mile. More than 400 people were trapped inside the canyon as the slide and flooding took out roughly 300 feet of road.
"Who ever thought anything like that would happen?" Dave Karaskweski, the head of park maintenance, said as he gazed in awe at the huge pile of red sandstone that had tumbled into the gorge. "When it comes to Nature, nothing amazes me anymore. We're all like little ants in a sandpile."
While the prehistoric Sentinel slide's debris was described in a 1945 scientific paper, Dr. Moore is the first to date the slide at 4,800 years ago (give or take 400 years) and describe its magnitude. He figured its age by measuring the amount of beryllium-10 contained in the surface of boulders from the avalanche. Beryllium-10 is created by particles from incoming cosmic rays bombarding rock surfaces. The longer a boulder is exposed, the greater the amount of beryllium-10, allowing scientists to determine when the boulder’s surface first was exposed by the landslide.
With permission from the National Park Service, Dr. Moore and colleagues sampled 12 boulders from the landslide’s surface, crushed the notebook-sized rock samples and analyzed their beryllium-10 content.
"The Sentinel rock avalanche was a major factor shaping Zion Canyon. With the exception of the original carving of the canyon, that landslide has the greatest influence on the appearance of the canyon today than any other geologic event," said Dave Sharrow, the park's geologist. "This research has provided some astounding measures of the size and speed of the event."
According to the park's geologist, this new research answers at least one key question that has lingered through the decades: Was the valley floor filled by a rock slide coming off the west wall of the canyon, or by the collapse of a sandstone "fin" standing in the middle of the canyon.
"Nobody had been able to demonstrate what origin worked. What has happened now with the modeling techniques Dr. Moore (and his team) were able to conduct, they could place material in various places and various topographies and allow them to collapse and see what happens," said Mr. Sharrow. "What they were able to do was place a mass equivalent to what was in the canyon when the slide first occurred, place that amount on the west wall, let it fail, and see where it ends up.”
Scientists don’t know what triggered the Sentinel landside. “We found no evidence indicating there was an ancient earthquake at the time, but there’s not a detailed record of paleoearthquakes 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.”
The collapsed peak included sandstones from the Navajo and Kayenta formations, and the latter includes some weak shale layers that might have aided the slide.
This view looking up Zion Canyon shows the remains of the giant Sentinel landslide, surrounded by the red line, that a new University of Utah study concludes happened 4,800 years ago, shaping the flat canyon floor of Zion Canyon Park in Utah/Jeff Moore, University of Utah
What Moore does know is how massive the Sentinel slide was.
"The ancient Zion landslide would cover New York City’s Central Park with 275 feet of debris,” the geologist said. “And you would need 90 times the volume of concrete in Hoover Dam to recreate the mountainside that failed.”
Using computer simulations, the geologist determined that the rock slide filled a section of Zion Canyon in less than half-a-minute with a pile of debris rising 650 feet in some places, with an average depth of 310 feet.
“The Virgin River was dammed immediately," Moore said in an interview last week, adding that it took 5-10 years to create a 380-foot deep lake. “The big blockage was there, then came the lake, then the lake filled with sediment. Today the river is working through the landslide dam and the sediment.”
Observant park visitors can see the telltale remains of the slide.
"A lot of the slide blockage is still there. You can see it really great (on the left side of the road) as you drive up the road from Canyon Junction," the geologist explained. "The road is kind of steep as you go up through the debris, and then it kind of flattens out as you get onto the debris.”
That debris is the reason why the floor of Zion Canyon is flat, said Moore, without steep, craggy walls that come down in a well-defined V-shape.
“This catastrophic landslide of massive proportions had two effects,” the geologist 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.”
A nice output of Dr. Moore's research were animations created with his computer modeling that show the slide filling the canyon floor.
"Those are things that will help us show this to visitors and to people who want to learn about it,” said Mr. Sharrow. "You can show them a pile of rock on the ground all day long and it's hard for a person to grasp the concept of that much rock moving in a big event. So what probably will be the most benefit to us as a park will be the visualization tools that he’s provided.”
Should you keep an eye on the cliff faces towering over you on your next visit to Zion Canyon? Is there ongoing danger of another great slide?
“It’s hard to say. These kind of extreme magnitude hazard events are very rare," Dr. Moore replied when posed the question. "It would be incredibly difficult for any geologist to come out and say with any kind of certainty to say, 'Ah-hah, this is where the next one will come.' I think people should just enjoy this. It’s such a cataclysmic event, but the effects, for us, are quite positive.”
Added geologist Sharrow: "We’ve had one great big slide in the last 4,800 years, so today our odds are pretty good that nothing big is going to fall. These are rare events.”
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