Zion National Park: Massive Landslide Created Utah Landmark Study Says

Formed by a landslide? Zion National Park is renowned for its stunning vistas and dramatic rock formations. A new study suggests that the picturesque Utah park was created by an enormous landslide or “rock avalanche” some 4,800 years ago.

The study, performed by researchers at the University of Utah, appears as the cover story of the June issue of GSA Today, the journal of the Geological Society of America. The team used a computer simulation to model the progression of the landslide as it exposed various layers of deposits, dispersing the flow throughout the valley. When they compared the results to the present day, they found the simulation was very close to the geological reality of current day Zion National Park. The simulated results matched the known landslide deposits on the valley floor.

In effect, an entire mountainside collapsed, damming the Virgin River. The landslide created the iconic flat stone floor of what is now Zion National Park and also resulted in the creation of a lake that fed the area for seven centuries. Evidence of the lake, which would have covered about 2.4 square miles near the area where the present-day Zion Lodge is located, comes in the form of an existing clay bed and fossilized mollusks found throughout the valley.

Massive landslide

The landslide that essentially created the iconic features of Zion National Park was truly massive in nature, with a volume of 10.1 billion cubic feet, (or 286 million cubic meters). Jeff Moore was the senior author of the study and is an assistant professor of geology and geophysics at the University of Utah. He gives an idea of the scope of the rock avalanche in a media release.

“The ancient Zion landslide would cover New York City’s Central Park with 275 feet of debris. And you would need 90 times the volume of concrete in Hoover Dam to recreate the mountainside that failed.”

The computer simulation estimates that the landslide reached speeds of up to 180 miles per hour. Amazingly, researchers estimate that despite the huge amount of rock involved in the landslide, the rock avalanche was over by about the one minute mark, leaving the landscape of Zion National Park permanently changed.

The landslide involved the face of The Sentinel, one of the mountains on the west side of the Zion Canyon. The Sentinel face would have been about 900 miles high, made of Navajo Sandstone. It is currently measured at an elevation of 7,157 feet.

Zion National Park – The Mystery Solved

Theories over the formation of Zion National Park have evolved over the years. Many scientists originally theorized that Zion Canyon had been formed so flat due to the movement of debris from glaciers. The Sentinel landslide was first documented in a research paper in 1945.

The University of Utah researchers dated the landslide by examining the boulders that would have been thrown up by the event for signs of exposure to the sky. Measuring for the amount of a substance created by cosmic rays allowed them to determine roughly when the rock was first exposed to the atmosphere by the destructive force of the rock avalanche.

The team did not find any evidence of an earthquake or other event that may have caused the Sentinel landslide in Zion National Park, leaving its origins a question mark.

Zion National Park – Landslide Risk?

There is geological evidence of other prehistoric landslides in Zion National Park, including one in the Hop Valley area about 2,600 years ago. While the kind of massive rock avalanche that created Zion National Park’s unique topography are rare, there have been smaller landslides much more recently, including two that occurred in 1990 and 1995, disrupting local roads. Jeff Moore spoke to the Deseret News about the continuing geological activity.

“From a long-term perspective, this is a part of a cycle. 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.”

The pattern of failing canyon walls and landslides will continue into the future.

“All that sediment on the flat floor is also on its way out. When that goes, Zion Canyon will look a lot different and feel a lot different.”

Zion National Park, recently voted Utah’s “best” attraction in a local TV poll, draws record numbers of visitors and seems set to for a new attendance record for the third year in a row. Deseret News reports that nearly 1 million people visited the park during the first four months of this year alone. That has put the stunning natural reserve – and sometimes is visitors – in jeopardy. Search and rescue calls doubled in 2015 alone with no corresponding increase in operating budgets.

While the processes that move the earth are unchanging, danger to modern day tourists at Zion National Park is more likely to come from wandering off the path than any landslide activity.

[Image by George Frey/Getty Images]