Massive rockfalls send giant slabs of granite rock tumbling to the ground in California's Yosemite National Park about 60 to 70 times per year. What's surprising is that 15 percent of these events happen on sunny days with no sign of rain or seismic activity. This, researchers say, has remained a long-standing geological puzzle - until now.
Yosemite's iconic granite cliffs formed as layers of rock peeled away from the mountainside. These precarious attachments, known to geologists as "exfoliations," fall at a rate of one a week, on average.
Generally they are triggered by an earthquake, heavy precipitation or freeze-thaw cycles that cause water trapped in a fissure to freeze, expand, and gradually wedge open a crack. From time to time, however, slabs of rocks spontaneously fall down.
To learn more about what causes Yosemite's mysterious rockfalls, Brian Collins, a geological engineer from the U.S. Geological Survey (USGS) and a mountain climber, worked with Yosemite National geologist Greg Stock to monitor the granite cliffs. They found that repeated day-to-night changes in temperature causes the rocks to eventually separate from the cliffs.
The study "is a very important piece of work that brings a new kind of life to rocky landscapes," said Jeffrey Moore, a geologist at the University of Utah in Salt Lake City. "A 1-centimeter deflection over the course of a day is not trivial."
For the study, researchers installed strain gauges and crackmeters to measure overall changes in length at three spots along a 19-meter-long and 4-meter-wide slab, which has its top and bottom edges barely attached to a south facing cliff in the park.
From May 2010 through October 2013, Collins and Stock measured the deformations present in the granite slab every five minutes. Looking a little like a scissor jack, the researchers' crackmeter recorded any movement in the crack, which changed from hour to hour and day to day, sometimes by as much as 0.4 inch in a day. Weather conditions including sunlight intensity, humidity, and air temperature were also taken into consideration.
Researchers found that as the temperature rises from morning to afternoon, the thin outer layer of rock heats up and moves ever so slightly away from the cliff, then contracts as the evening cools.
"Every day we found this movement," Stock said. "On top of that, there's a seasonal signal."
In other words, the slab would progressively expand outward in the summer and inward in the winter, slowly opening the crack from year to year. This constant back and forth movement inevitably destabilizes the slab, causing it to eventually reach its breaking point.
"We think that this process...is probably happening almost everywhere in Yosemite Valley and this process can account for these otherwise mysterious rockfalls that are occurring on these warm, clear days when you wouldn't expect a rockfall to happen," Stock added.
Their study was recently published in the journal Nature Geoscience.
