Seismicity and the strange rubbing boulders of the Atacama Desert, northern Chile

We found clusters of 0.5-8 t boulders worn to smoothness around their midsections in the Atacama Desert of northern Chile. We suggest that the boulder smoothing is the cumulative result of at least 1 m.y. of rubbing between boulders during earthquakes. Be-10 exposure ages of boulder tops from these fields average similar to 1.3 m.y., unsurprisingly old given the hyperaridity of the Atacama. During a visit to one major boulder site, we experienced an earthquake that rocked but did not tip the boulders, causing them to rub against each other for about a minute. This M-w 5.2 earthquake was centered similar to 100 km northeast of the site. In the seismically active Atacama, earthquakes of this energy or greater occur about once every four months, suggesting that the average boulder has undergone similar to 40,000-70,000 h of abrasion over the past 1.3 m.y. This unusual evidence underscores the largely unrecognized role that seismicity probably plays in hillslope sediment transport in the nearly rainless Atacama Desert, and perhaps on other seismically active but now dry worlds like Mars.