Study: Hot Weather Triggers Yosemite Rockslides

YOSEMITE NATIONAL PARK (CBS SF) – There may finally be an answer to why there are dangerous rockslides in Yosemite National Park during the hottest days of the year.

According to the U.S. Geological Survey, a three-year study has found that rockslides are more likely "to happen in the hottest part of the day, during the hottest part of the year."

"Summertime rockfalls have been something of a mystery," said Greg Stock, Yosemite park geologist and coauthor of the study. "With this work we now have a plausible explanation for why they happen."

Scientists say that Yosemite rockfalls are part of the natural process of erosion and quite common. But they are a hazard and understanding their cause will help make the popular tourist attraction safer for park visitors.

In an effort to find the cause, U.S. Geological Survey and NPS geologists placed sensitive deformation and temperature gauges in a crack behind a large, partially detached slab of granite clinging to a Yosemite Valley cliff.

The scientists found that daily heating and cooling of the rock surface caused the crack to open and close by nearly half an inch.

The resulting stress can cause such cracks to grow, destabilizing the rock slabs to the point where they fall, in a process called exfoliation.

"Our research provides clear evidence that thermal effects can move large slabs of rock and that these movements, over time, can lead to rock falls," said Brian Collins, USGS geotechnical engineer and coauthor of the study.