The domes and arches etched into Yosemite's famed granite cliffs may seem frozen in time, but in reality they're constantly moving.
The dramatic rock formations were formed as layers of rock peeled away from the mountainside, like an onion. The flakes remain attached at a few points but are completely hollow in the middle. In Yosemite, these precarious attachments — geologists call them "exfoliations" — fall at a rate of one a week, on average. Most often, they collapse because water repeatedly freezes and thaws in the cracks, destabilizing the cliffs. Sometimes they fall apart during an earthquake.
Other times though, rockfalls happen on sunny days. Now geologists from the U.S. Geological Survey and the National Park Service have found a potential cause for the seemingly spontaneous rockfalls: heat. As the temperature rises from morning to afternoon, the thin outer layer of rock moves ever so slightly away from the cliff, then returns as the evening cools.
A pair of geologists collected evidence for this idea in the park's Royal Arches, a cliff overlooking Yosemite Valley. For 3.5 years, Brian Collins of USGS's landslide hazards program and Greg Stock of the park service monitored a 19-meter-tall exfoliation that clings to a near-vertical cliff.
Their measurements revealed that a 20-metric ton wall of granite can move about one a centimeter a day.
"We look around the landscape and we see thousands and thousands of these flakes and we have to assume they're all moving," Collins said. "They're kind of breathing."
As the cliffs inhale and exhale, the tips of the cracks weaken. Over time, the cracks open wider and heat stress can prompt the rock to fall.
Los Angeles Times
