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The strangely smooth shape of boulders in Chile's Atacama Desert, one of the world's driest locations, could be due to the rocks rubbing against each other during earthquakes.
Jay Quade at the University of Arizona in Tucson and his colleagues analysed the patterns of erosion shown by the boulders. The researchers determined that the rocks' smooth sides, and depressions in the sediment around them, could be best explained by rubbing and rocking motions experienced during earthquakes. In February 2010, two members of the team were present when an earthquake with a magnitude of 5.2 struck about 100 kilometres from their location, enabling them to observe the rocks rubbing against each other for about a minute.
Earthquakes of a similar or larger magnitude occur roughly once every four months, and the authors calculate that the boulders could have experienced 40,000–70,000 hours of rubbing over the past 1.3 million years.
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One million years of rubbing rocks. Nature 489, 339 (2012). https://doi.org/10.1038/489339d
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DOI: https://doi.org/10.1038/489339d
