Hot weather stresses everyone and everything. A swampy afternoon taxes your body, strains your home and car, and can wreak havoc on infrastructure like power grids and roadways. It stands to reason that the hot sun can apply stress to the Earth, too, and a study published today in Nature Geoscience suggests that rapid temperature cycles can stress the faces of mountainsides to their breaking point, leading to potentially dangerous rockfalls.

The study, conducted in Yosemite National Park over a three-and-a-half year period by researchers at Yosemite and the U.S. Geological Survey's Landslide Hazards Program, sought to answer the question of why sudden rockfalls can occur for lack of an obvious cause, such as heavy rain or earthquakes.

They concluded that daily temperature cycles during certain periods of the year are enough to exacerbate "exfoliation" fractures in the surface layer of a granite cliff—possibly to the point of complete failure. We saw a dramatic example of one such rockfall in summer 2015, when the park’s iconic Half Dome lost 5 million pounds of granite. This video from Yosemite National Park gives an overview of rockfalls.  

Exfoliation is a much less gentle process in nature than it is in skin care. It’s a type of erosion where the surface of a rock cracks and peels away in layers due to weathering. The study found that a growing exfoliation layer on a mountainside in Yosemite National Park flexed in and out on hot afternoons and days that saw large temperature ranges; that is, days with warm afternoon high temperatures and cool nighttime low temperatures.

During the study, the scientists measured temperature, sunlight, and the size of the cracks in one of these layers to determine how much of a role daily temperatures played in weathering the mountainside. They found that the greatest outward deformation, or expansion, of the rock layers occurred during the mid-afternoon hours during peak heating, and the greatest inward deformation of the rocks, or contraction, took place during the coldest part of the morning. The deformation wasn’t tiny, either—they found that the layers can expand and contract up to one centimeter during the course of a day.

The vast majority of rockfalls around the world occur as a result of triggers like precipitation (the possible cause of the 2015 Half Dome rockfall) or seismicity, but the authors point out that the remaining 15 percent are either heat induced or happen without explanation. Many of these events occur during the warmest parts of the day during the warmest parts of the year, lending further credence to their findings. A deeper understanding of these sudden events can help scientists better understand this phenomenon, possibly helping keep people safe from sudden falling rocks in the future.