By Lisa M. Krieger, Bay Area News Group

When rocks get hot, they do what people do: shed a few layers.

And the hottest hours of the day — during the hottest months of the year — are prime time for unexplained rockfalls and cracking, according to a study of the granite domes and cliffs in the Sierra Nevada, which finds that hikers, tourists and wildlife are at less risk of being flattened during cooler times.

“Cliffs move in and out, and detach,” said Menlo Park-based U.S. Geological Survey scientist Brian Collins, who uses instrumentation to measure restless rocks at Twain Harte reservoir and Yosemite National Park.

“People look at landscape as static, that it will be there forever,” said Collins whose study, in essence, is explaining what turns mountains into molehills. “But it’s changing all the time.”

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