A Catastrophic Ancient Landslide Shaped Zion National Park


The relatively flat valley floor in Zion Canyon is the result of a prehistoric landslide. Michele Falzone/Getty Images
The relatively flat valley floor in Zion Canyon is the result of a prehistoric landslide. Michele Falzone/Getty Images

Geological transformation can take millions upon billions of years. But sometimes it happens in the blink of an eye.

According to new research, a massive mountainside in what eventually became Utah collapsed 4,800 years ago — give or take 400 years on each side. The ensuing rock avalanche lasted only less than a minute, but dammed a river, created a lake and reshaped the canyon floor and helped shape today's Zion National Park.

The study, conducted by geologists from the University of Utah and published in the Geological Society of America's journal GSA Today, found that the rocks in the avalanche moved at speeds of up to 180 mph (290 kph). The landslide took place on the western side of Zion Canyon and involved the iconic rock formation known as The Sentinel — a peak that's large and imposing today, but that was significantly more so before it gave way.

The scientists measured known landslide deposits across the valley floor, and used computer simulations to recreate the geologic event. According to the simulation, the initial landslide lasted 20 seconds, then for another half minute the debris spread around the canyon, sloshing against rock walls like waves, and eventually settled. The amount of rocks in the huge landslide measured a volume of 10.1 billion cubic feet (286 million cubic meters). It's hard to picture just how much earth that is, but the scientists say that if you scooped it all up and poured it into New York City's Central Park, you'd fill the entire park with a giant rockpile towering 30 stories above the street. 

"This catastrophic landslide of massive proportions had two effects," Jeff Moore, an assistant professor of geology and geophysics at the University of Utah and the study's lead author, said in a press release announcing the discovery. "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."

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. The orange line above the slide shows where part of the Sentinel collapsed to generate the...
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. The orange line above the slide shows where part of the Sentinel collapsed to generate the...
Jeff Moore, University of Utah

Because of the landslide, the flow of the Virgin River became dammed, the study also found, and over five to 10 years following the event, a lake formed that lasted for 700 years. That lake eventually breached the walls of the dam, and the river's flow eroded a significant amount of the landslide material as the lake drained and shrank. Sediment from the lake and river form the floor of today's canyon, and have helped create a fertile, habitable landscape in the middle of sometimes barren terrain.

The Sentinel landslide area was first scientifically described in 1945, but this is the first in-depth recreation and comprehensive analysis of the event that took place in one of the American West's most iconic national parks. Moore expects that the Virgin's flow will eventually erode the entirety of today's canyon floor in another several thousand years, once more creating a steep, rocky canyon. The landslide was "a minute with up to 10,000 years of consequences," he said.

A hiker overlooks the Zion Canyon valley floor.
A hiker overlooks the Zion Canyon valley floor.
James O'Neil/Getty Images