Surprising cause of Yosemite rockfalls discovered
More than four million people are expected to visit Yosemite National Park this year, and along with the park's natural beauty, they could encounter danger from its majestic granite cliffs, which experience dozens of rockfalls each season. Last year, the National Park Service says at least 25,000 tons of rock came crashing down.
Rockfalls can be caused by rain or snow, seismic activity, or the effects of freezing. But others had no obvious cause.
"Summertime rockfalls have been something of a mystery," Greg Stock, Yosemite park geologist and coauthor of the study, said in a press release.
Now research out of the U.S. Geological Survey (USGS) reveals that summer heat can trigger rockfalls at Yosemite. The USGS examined the parks's granite cliffs over the course of three years, discovering that rockfalls were more likely to happen at the hottest time of the day, during the hottest parts of the year.
"With this work we now have a plausible explanation for why they happen," Stock said.
To study the rockfalls, the USGS and the National Park Service set up sensitive "deformation and temperature gauges" in a crack located behind a partially detached slab of granite on a Yosemite Valley cliffside. The researchers discovered that the rock surface's daily cooling and heating resulted in the crack opening and closing by almost a half an inch. Stress from these temperature shifts can destabilize the rock slabs in a process known as exfoliation, leading it to separate and crash to the ground below.
"Our research provides clear evidence that thermal effects can move large slabs of rock and that these movements, over time, can lead to rockfalls," said Brian Collins, USGS geotechnical engineer and study co-author.
The report was published in the journal Nature Geoscience.
Rockfalls are a hazard to visitors at the park, and the study can enable geologists who work in and study the area to develop a better understanding of how to manage the potentially deadly phenomenon.
The National Park Service reports that in 2015 alone, there were 66 documented rockfalls and probably a number of others that nobody witnessed. On its website, the park service urges visitors to call 911 within the park "if you witness a rockfall of any size, encounter fresh rock debris, or hear cracking or popping sounds emanating from the cliffs."
