SEISMICITY AND THE STRANGE RUBBING BOULDERS OF THE ATACAMA DESERT, NORTHERN CHILE

We encountered large fields of 0.5 to 8 metric ton boulders in the Atacama Desert of northern Chile worn to smoothness around their mid-sections. These smoothed surfaces exhibit varying degrees of desert varnish, from very dark to fresh and white where currently in contact with adjacent boulders. We suggest that the boulder smoothing is the cumulative result of millions of years of rubbing between boulders during earthquakes. ¹⁰Be cosmogenic ages of boulder tops from these fields average about 1-2 x 10⁶ years, unsurprisingly old given the hyperaridity of the Atacama. Quite by chance, during our second field visit to one major boulder site we experienced an earthquake which rocked but did not tip the boulders, causing them to rub against each other for about a minute. The earthquake measured 5.3 and was centered about seventy miles east of the site. In the seismically active Atacama earthquakes of this energy occur about once every four months, suggesting that the average boulder experiences ~50-100 kilohours of abrasion. This remarkable 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 dry planets like Mars.