Yosemite National Park in Danger, Granite Cliffs at Risk of Falling Due to Heat

The domes and arches that make up Yosemite's granite walls were formed as layers of rock peeled from the side of mountain. While the flakes look like they are attached, they are hollow from inside. These attachments in Yosemite that the geologists call "exfoliations" fall at the rate of one each week. The reason attributed to their fall is due to water that freezes and thaws inside the cracks repetitively, making the cliffs weak from inside, thus destabilizing them. Sometimes an earthquake in the region too may cause a fall.

There are also times when the rock fall happens on days that are sunny, with no seismic activity. The geologists of U.S. Geological Survey and National Park Service reveal the possible reason for this spontaneous fall, heat.

During the day, the cliffs experience varied degrees of temperature making the outer layers of rock expand, making the layer of rock slide away from the cliff and resume its position as the temperature cools in the evening.

Two geologists, Greg Stock from the Yosemite National Park and Brian Collins from USGC's landslide hazard program, monitored the wall and collected data to study this phenomenon. By using tools to measure the shift, they were able to record movements as slight as 0.001 centimeters.

"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 slowly opens wider and the stress from the heat can prompt the rock to fall.