This is scientific American 62nd 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's granite 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 rockfalls that were happening in the summertime,

on particularly the hot days.

Brian Collins, a research civil engineer with the US Geological Survey.

And we noticed when we looked at the timing that they were 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 Gregstock, 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

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.

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