Exfoliation of rock deteriorates cliffs 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 cliff 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 sufficient 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 efficacy of other, more typical rockfall triggers.
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This work was supported by the USGS Landslide Hazards Program and the US National Park Service, Yosemite National Park. We thank B. Murchey (USGS) for supporting the efforts to initiate this work. F. Sandrone, L. Gastaldo, B. Matasci and M. Jaboyedoff (École Polytechnique Fédérale de Lausanne, Laboratory for Rock Mechanics and the Université de Lausanne, Institute of Earth Sciences Terre, Lausanne, Switzerland) performed laboratory testing of rock samples for rock mechanics characterization. We appreciate discussions with R. S. Anderson, M. E. Reid, S. J. Martel and J. R. Moore, who provided helpful suggestions and encouragement throughout the project. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the US Government.
The authors declare no competing financial interests.
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Collins, B., Stock, G. Rockfall triggering by cyclic thermal stressing of exfoliation fractures. Nature Geosci 9, 395–400 (2016). https://doi.org/10.1038/ngeo2686
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