THE BOUNCING BOULDERS OF THE ATACAMA

Abstract

In the heart of Chile’s Atacama Desert thousands of boulders, some exceeding 5 m in diameter, lie at the bases of steep escarpments. The rapid and forceful descents of the boulders are commonly recorded by the trails of small impact craters the boulders left as they bounced down the escarpments. These craters are exquisitely preserved thanks to the hyper-arid climate of the Atacama, the driest place on Earth. Once at rest, most boulders experience no further downslope motion but instead degrade in place. These unique boulder fields preserve thousands of fallen boulders in various states of degradation, from solid intact boulders to conical mounds of debris. This thesis focuses on one of the several previously undescribed fallen boulder fields in the Coastal Cordillera between 19° and 23°S latitude. We characterize this site using field surveys, boulder morphology classifications, and a drone-based structure from motion (SfM) digital elevation model (DEM) in order to exploit the unparalleled preservation of the evidence of past rock falls. The rapid descent of individual boulders suggests triggering by either earthquakes or infrequent rainfall events; we have experienced the former in the field. The spacing of bounce marks left by the boulders as they fell down the scarp are irregular, and their paths are commonly oblique to the maximum slope line, highlighting the stochastic nature of the impact process, and confirming the importance of probabilistic three-dimensional rock fall models. We describe the state of degradation of the boulders using a three-fold classification for relative age of the boulders based on the relative size of their debris skirts, and find that larger numbers of morphologically older boulders are found further out from the scarp. We also identify a morphometric approach using slope and slope roughness to identify boulder source regions. The uniquely preserved fields of fallen boulders in the Atacama provide an excellent case study to test theories of the processes important to determine rock fall hazards.