Yosemite's granite

The floor of [Yosemite:arithematic] 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:reason] ."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:noted] when we [looked:booked] 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:change] temperatures, might weaken cracks and contribute to rockfalls. So the researchers - who are both climbers - found a suitable [fracture:building] near the base of a 500-meter-tall cliff and installed instruments called crackmeters, which monitored the width of the crack over time.