Exfoliation of rock deteriorates cli s through the formation and subsequent opening of fractures, which in turn can lead to potentially hazardous rockfalls. Although a number of mechanisms are known to trigger rockfalls, many rockfalls occur during periods when likely triggers such as precipitation, seismic activity and freezing conditions are absent. It has been suggested that these enigmatic rockfalls may occur due to solar heating of rock surfaces, which can cause outward expansion. Here we use data from 3.5 years of field monitoring of an exfoliating granite cli in Yosemite National Park in California, USA, to assess the magnitude and temporal pattern of thermally induced rock deformation. From a thermodynamic analysis, we find that daily, seasonal and annual temperature variations are su cient to drive cyclic and cumulative opening of fractures. Application of fracture theory suggests that these changes can lead to further fracture propagation and the consequent detachment of rock. 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 e cacy of other, more typical rockfall triggers. R ockfalls are common and hazardous in steep terrains around the world 1-4 , and are primary agents of landscape erosion in many environments 4-8 . In exfoliating landscapes (Fig. 1a), rockfalls frequently occur as detachments of the outer rock layers (exfoliation sheets) along surface-parallel fractures (joints). These detachments are typically thinner (measured normal to rock faces) than they are wide or long. The origin and formation of exfoliation sheets, particularly those formed in granitic landscapes, has been a subject of interest for more than a century 9-15 . The consensus that erosion-induced or palaeo-stresses are responsible for their formation has been challenged by recent work 16,17 proposing that a combination of regional compressive stresses and topographic curvature can generate exfoliation fractures. Regardless of their origin, understanding modern-day failure of rock masses along exfoliation fractures is important for studies of landscape erosion and rockfall hazards. Rockfalls can be triggered by a number of mechanisms, including precipitation, seismic shaking, and freeze-thaw conditions 6,18 . Yet many rockfalls lack recognized triggers and are seemingly spontaneous events, suggesting other factors at play. The role of thermal effects (temperature and insolation) on initiating rock deformation, where rock surfaces expand, contract, and eventually fail in response to cyclical temperature variations, was critically examined regarding exfoliation sheet formation, and subsequently dismissed 11,19,20 . However, these studies did not investigate what role thermal effects might have on the deformation of existing exfoliation sheets. Further, some studies 13 acknowledged that thermal effects could be important at depths of less than one metre-areas of obvious interest for rockfalls. Recent studies on building f...
