The Strange Rubbing Boulders of the Atacama Desert

Posted on October 11, 2011

Atamaca Desert Boulders

Geologist Jay Quade discovered an unusual geological process operating in a remote corner of northern Chile's Atacama Desert. Quade noticed that half-ton to 8-ton boulders in the desert appeared to be rubbed very smooth about their midsections. He wondered what could cause this in a place where Earth's most common agent of erosion -- water -- is almost nonexistent.

Quade's theory was that friction from earthquakes caused the erosion. Over the approximately two million years that these rocks have been sitting on their sandy plain perhaps they were jostled by seismic waves. The seismic waves caused the boulders to gradually grind against each other and smooth their sides.

Quade never thought he'd be able to prove his theory until he made another trip to the Atacama desert. Quade was standing on one of the giant boulders, 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.

Quade says, "It was this tremendous sound, like the chattering of thousands of little hammers. The one I was on rolled like a top and bounced off another boulder. I was afraid I would fall off and get crushed."

The boulders apparently tumbled down onto the flat desert from the hills above -- probably dislodged by earthquakes. Analyses of the boulder top surfaces suggest that they have been there one to two million years. That age, combined with the fact that seismic activity in the area generates a quake like that Quade witnessed on the average of once every four months, suggests that the average boulder has experienced a total of 50,000 to 100,000 hours of bumping and grinding.

Quade says, "It raises the question in my mind of other planets like Mars. I would predict that these kinds of crowds of boulders might be found on Mars as well, if people look for them."

Quade presented his interesting findings today at the annual meeting of the Geological Society of America in Minneapolis.

Photo: Jay Quade

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