Yosemite National Park

A view of a rockfall from the east side along El Portal Road (Highway 140) within Yosemite National Park in California.

Summer after summer, scientists with the U.S. Geological Survey (USGS) could not figure out why rockfalls kept occurring in Yosemite National Park. The slides are common after earthquakes, heavy rain or snow, but none of those played a factor in these particular avalanches.

“These were happening in the summer time, in the afternoon on beautiful, blue sky, sunny days,” USGS geotechnical engineer Brian Collins told USA Today. “It was a mystery.”

A recent study conducted by the USGS has found the culprit: heat.

Yosemite National Park geologist Greg Stock and USGS civil engineer Brian Collins download data from instruments that measure how much rocks move from daily temperature changes.

(U.S. Geological Survey)

According to a release, researchers with USGS and the National Park Service (NPS) placed deformation and temperature gauges in a crack behind a large, partially detached slab of granite clinging to a Yosemite Valley cliff. They found that the daily heating and cooling of the rock’s surface was making the crack open and close by almost half an inch.

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The stress from this constant back-and-forth is what causes the cracks to grow, destabilizing them to the point where they may fall, a process called exfoliation.

“We were expecting it to move somewhat,” said Collins. “The big eye opener was these rocks are opening much, much more than we thought every day.”

In 2015, there were 66 documented rockfalls, rockslides and debris flows, which displaced roughly 25,000 tons of rock, according to the National Park Service.

Though the falling rocks do pose a threat to tourists and mountain climbers, Collins said there’s no reason for them to cancel their summer plans to visit Yosemite Park.

“These types of rockfalls aren’t all that common,” Collins told USA Today. “People visiting the park and other environments like this should keep in mind that rockfalls are a natural process and just be aware.”

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An unknown man in a head lamp on the edge of Glacier Point in Yosemite stares at the incredible view. (Christian Adam/Caters)