Potentially hazardous rockfalls can be triggered by normal daily changes in temperature, according to a study published online in Nature Geoscience this week. The study offers an explanation for seemingly spontaneous summertime rockfalls in steep mountain landscapes.
Rockfalls are often triggered by earthquakes, heavy rain or freezing conditions, but sometimes occur on warm sunny days, when none of these triggers are present. About 15% of rockfalls from the famous granite cliffs in California’s Yosemite Valley occur during the hottest summer months and at the hottest times of the day, suggesting a role for temperature.
Brian Collins and Greg Stock used so-called crackmeters to monitor over three years of changes in the width of a crack partway up a 500-metre-tall cliff in Yosemite National Park. The crack separates a partially detached slab of rock from the rest of the cliff and the authors found it opened and closed each day, corresponding to daily changes in temperature. As the seasons turned, they observed a cumulative opening of the crack. Their analysis shows that daily and seasonal temperature cycles will cause progressive growth of the crack until the slab of rock breaks free in a rockfall. They conclude that the warmest times of the day and year are particularly conducive to triggering rockfalls, which is consistent with the rockfall record for Yosemite and with observations in similar landscapes elsewhere.
In an accompanying News & Views article, Valentin Gischig writes: “...as the climate warms in the coming decades, thermally induced rockfalls may become even more important to hazard assessment and cliff erosion.”