科学美国人60秒:Heat ID'd as Subtle Cause of Rockfalls

The floor of Yosemite Valley is littered with piles of rocks that crumbled off the park’s iconic cliffs. These rockfalls happen all the time, because Yosemite’s granite walls are riddled with cracks produced by geologic stress. Scientists know the rockfalls can be triggered by things like earthquakes, rainfall, and freezing and thawing ice. But many falls occur without an obvious cause. Now researchers think that heat may be the culprit.

“We noticed that there had been a bunch of rockfalls that were happening in the summertime, on particularly hot days.” Brian Collins, a research civil engineer with the U.S. Geological Survey. “And we noticed when we looked at the timing that they were happening in the afternoon, when we thought the temperatures were at their hottest.”

Collins and Greg Stock, Yosemite’s park geologist, wanted to know if small rock movements, induced by changing temperatures, might weaken cracks and contribute to rockfalls. So the researchers—who are both climbers—found a suitable fracture near the base of a 500-meter-tall cliff and installed instruments called crackmeters, which monitored the width of the crack over time.

The devices revealed that the crack grew almost a centimeter wider during the warmest part of the day. It shrank again when temperatures cooled off, for instance, at night and during the winter. But overall, the scientists found that the average width of the fracture grew over the course of a summer and over the entire three and a half-year study period, bringing the crack closer to breaking.

“We think that what happened was that every cycle — as the rock goes back and forth and back and forth — you’re getting to a part that we call subcritical crack growth. And that means that the crack where the rock is attached to the cliff is actually fracturing at a microscopic level. And so if you do that over the course of a year, then eventually you’re going to do some permanent damage to those points of attachment.”

The study is in the journal Nature Geoscience. [Brian D. Collins and Greg M. Stock, Rockfall triggering by cyclic thermal stressing of exfoliation fractures]

The slab of rock the scientists studied hasn’t fallen yet, and Collins doesn’t know how many cycles it will take before this or any other fracture finally breaks. When it does, the trigger might be a particularly sweltering day, when the partially detached slab is farthest away from the cliff. Or it could be another process, helped along by the fact that temperature changes already weakened the crack.

Either way, the results will help researchers assess the rockfall hazard in steep, rugged terrain. Because now they know when the mercury goes up, rocks are more likely to come down.

—Julia Rosen