"The scientists found that daily heating and cooling of the rock surface caused the crack to open and close by nearly half an inch."
"Our data indicate that the warmest times of the day and year are particularly conducive to triggering rockfalls, and that cyclic thermal forcing may enhance the efficacy of other, more typical rockfall triggers."
"Our results have important implications for the triggering of rockfalls in exfoliating landscapes. First, our mea- surements indicate that seemingly static bedrock landscapes are, in fact, quite dynamic; that a 20 tonne sheet of rock can deform in and out of a near-vertical cliff face by up to 1 cm on a daily basis demonstrates the inherent instability of sheeted cliffs. Second, the observed cumulative outward deflection highlights a potential posi- tive feedback loop in promoting detachment of exfoliation sheets. Namely, as crack opening occurs, sheet curvature increases, and likewise tensile stresses (equation (6)). These changes will, in turn, promote still higher values of stress intensity and lead to propagation of fracture tips. Further opening may occur as loose blocks become wedged at the bottom of fractures, preventing full return of sheets to their original position43. Both increasing temperature and temper- ature fluctuations may also promote fracture. Thus, we expect that rates of deformation should increase for already partially detached exfoliation sheets, albeit nonlinearly. Finally, our results offer a potential explanation for rockfalls that have no recognized trigger despite sometimes detailed observation at the time of failure. These include records of spontaneous summertime rockfalls in Japan27, France28 , Brazil31 , Switzerland44 and Yosemite45 . In Yosemite45 , a disproportionate number (15%) of rockfalls with either an identified thermal stress trigger or an unrecognized trigger occur during the hottest summer months (July through September) and at the hottest times of the day (12:00 through 18:00 PST) compared to what would be expected under a random distribution (6%). We suggest that cyclic thermal stresses might be the trigger for these rockfalls and potentially many others around the world, highlighting the role of temperature in eroding steep landscapes."