Rockfalls in steep terrain could be triggered by warm weather.

Precipitation, earthquakes and freeze–thaw cycles are known to increase the risk of rockfalls, but some falls have no known cause. Brian Collins of the US Geological Survey in Menlo Park, California, and Greg Stock of the US National Park Service in El Portal, California, attached 'crackmeters' to a 500-metre cliff in California's Yosemite National Park.

They found that a crack behind a slab in the cliff widened every day as the rock expanded in the heat, and closed up at night. It was also wider during the summer than in the winter. Over 3.5 years, the crack steadily opened up, and the authors say that such rock slabs could eventually fall, even without stresses such as an earthquake or heavy rain.

The study suggests that warm summer afternoons are most likely to see such rockfalls, which matches records at Yosemite and other places around the world.

Nature Geosci. http://dx.doi.org/10.1038/ngeo2686 (2016)