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This is Scientific American 60-second science. I'm Julia Rosen. Got a minute?

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 walls are riddled with cracks produced by geologic stress.

Scientists know that 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.

Well, we noticed that there had been a bunch of rock falls that were happening in the

summertime on particularly the 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 rock falls. So the researchers, who are both climbers,

found a suitable fracture near the base of a 500-meter tall cliff and installed instruments called

crack meters, 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

...