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The Strange Rubbing Boulders of the Atacama

ScienceDaily (Oct. 11, 2011) — A geologist's sharp eyes and upset stomach has led to the discovery, and almost too-close encounter, with an otherworldlygeological process operating in a remote corner of northern Chile's Atacama Desert.

The sour stomach belonged to University of Arizona geologist Jay Quade. It forced him and his colleagues PeterReiners and Kendra Murray to stop their truck at a lifeless expanse of boulders which they had passed before without noticing anything unusual.
"I had just crawled underneath the truck to get out ofthe sun," Quade said. The others had hiked off to look around, as geologists tend to do. That's when Quade noticed something very unusual about the half-ton to 8-ton boulders near the truck: theyappeared to be rubbed very smooth about their midsections. What could cause this in a place where Earth's most common agent of erosion -- water -- is as almost nonexistent?
About the only thing that cameto mind was earthquakes, said Quade. Over the approximately two million years that these rocks have been sitting on their sandy plain perhaps they were jostled by seismic waves. They caused themgradually grind against each other and smooth their sides. It made sense, but Quade never thought he'd be able to prove it.
Then, on another trip to the Atacama, Quade was standing on one of theseboulders, pondering their histories when a 5.3 magnitude earthquake struck. The whole landscape started moving and the sound of the grinding of rocks was loud and clear.
"It was this tremendous sound, likethe chattering of thousands of little hammers," Quade said. He'd probably have made a lot more observations about the minute-long event, except he was a bit preoccupied by the boulder he was standingon, which he had to ride like a surfboard."The one I was on rolled like a top and bounced off another boulder. I was afraid I would fall off and get crushed."
He managed to stay atop his boulder,...
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