The Mysterious Rubbing Rocks of the Atacama Desert

South America's Atacama Desert gets less than a millimeter of rain a year, and some areas have probably never received rainfall. Yet there are rocks in the desert that seem to have been eroded by water. What's going on here?

You can see some examples of the rocks in the pictures above and below. These giant boulders have unnaturally smooth surfaces all along their midsections. You don't get smoothness like that without a water source around to erode and wear down the rough edges of the rocks. And considering these boulders weight anything from .5 to 8 tons, it's not as though they could have easily "migrated" from some other watery locale.

That mystery has fascinated University of Arizona geologist Jay Quade for some time, and now he says he's found the solution. Quade had first noticed the rocks while on a geological expedition in northern Chile. Feeling nauseous, he crawled underneath his jeep to get out of the blazing hot sun, which is when he noticed the unnatural smoothness of the rocks. Erosion was the only obvious explanation, but that made no sense in a desert that's been almost completely devoid of rain for the last two million years.

The Mysterious Rubbing Rocks of the Atacama Desert

The other, rather novel possibility Quade came up with was earthquakes. He figured that if a sufficiently powerful earthquake hit the area, the boulders - which were all packed closely together - would rub up against each other. Over a couple million years, enough serious earthquakes could occur to completely grind down the midsections of the rocks, creating the bizarre smoothness we see today. It's a neat theory, but not easily proven.

At least, that's what Quade thought until a return visit to the Atacama Desert, when he got proof in the most spectacular fashion imaginable. While he was standing on a boulder and surveying the landscape, a 5.3 magnitude earthquake struck. As Quade tried desperately to keep his balance on the now bouncing boulder, he could see - and especially hear - his hypothesis being proven, as all the boulders began gnashing and grinding together.

He recalls the scene:

"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. I was just astonished when this earthquake came along and showed us how it worked."

Quade believes the boulders were once found on top of the hills surrounding their present location on the salt flats. Earthquakes dislodged the rocks and sent them tumbling down the various hills, concentrating an unusually high quantity of boulders in this one small area. With the rocks packed so tight together, they started rubbing "shoulders" - Quade compares it to the experience on a crowded train platform - and over a couple million years these rocks have probably spent about 50,000 to 100,000 hours being rumbled by earthquakes and grinding together.

One interesting side result of this research is that it offers a mechanism for landscapes to change and evolve even in areas without the eroding effect of water. As Quade explains, this work could help us better understand the geological changes on other worlds:

"It also answers a mystery that had been eating at me for years: How do the boulders get transported off the hills when there is so little rain. How do you erode a landscape that is rainless? 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."

Via the Geological Society of America. Images by Jay Quade.