Rockfall at Yosemite's El Capitan '10 times bigger' than slide that killed tourist a day earlier
A massive hunk of rock fell off Yosemite National Park’s El Capitan on Thursday, a day after a rockfall in the same area of the granite monolith killed a British tourist and seriously injured his wife, park officials said.
One person was hurt in Thursday’s slide, though it was unclear if the individual was a climber or a motorist. Tourists were asked to use Southside Drive to exit Yosemite Valley, as the rockfall left Northside Drive closed.
On Wednesday, a sheet of granite the height of a 13-story building — about 130 feet long, 65 feet wide and in some sections 10 feet thick — separated from the rock face and dropped to the base of El Capitan, officials said.
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 with no sign of rain or seismic activity. 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.
As the cliffs inhale and exhale, so to speak, the tips of the cracks weaken. Over time, the cracks slowly open wider and the stress from the heat can prompt the rocks to fall.