What Moves Gravel-Size Gypsum Crystals Around the Desert?

The Salar de Gorbea, some 13,000 feet above sea level, in the Atacama Desert of northern Chile, is like another planet. Active volcanoes dominate the muted, but colorful, vegetation-free landscape. In a few places, groundwater collects in salty, acidic pools. It evaporates in the sun, leaving behind gypsum crystals as big as your feet that protrude from the ground like daggers.

But they don’t stay put. Somehow they are scattered all over the place. And about three miles away it’s even weirder: It looks as if someone intentionally swept them into 15-foot-high piles, with some crystals merged together like giant gobs of rock candy.

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The gypsum crystals can be as big as a person’s foot and protrude from the ground like daggers.

Credit
Kathleen Benison

How they got there was a mystery, until someone stumbled upon a whirlwind so powerful, it defied textbooks. In a paper published in Geology in March, Kathleen Benison, a geologist at West Virginia University, documented how what she calls a gravel devil may be responsible for the large crystals’ movement around the desert.

“I remember holding one of the crystals and noticing how they were all broken,” Dr. Benison said. “I looked up, and there was one of these gravel devils.”

She watched for five minutes as a huge white cloud that appeared to materialize in a valley between two volcanoes moved across the landscape and over the pools before it vanished, right above the gypsum dunes. This happened every afternoon during her three-day visit in March 2007, but it is unclear how regularly they occur.

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The gypsum forms into gravel dunes.

Credit
Kathleen Benison

Like windblown sand grains, the crystal surfaces were scratched, suggesting that the wind had carried them. But typically anything bigger than a grain of sand can be moved only by gravity or surface water. Desert whirlwinds aren’t supposed to be strong enough to carry anything as large as these gravel-size gypsum crystals.

But whirlwinds occasionally defy thermodynamic speed limits, said Nicholas Heavens, a planetary scientist at Hampton University who was not involved in the study but wrote a commentary about it. In Arizona, dust devils have been seen and proved capable of carrying small rodents, and in 2013, a ghostly wind ripped the side mirror off a police car in Hartford.

Dr. Heavens has no doubt that the gravel devil exists. It’s just extreme: To lift the crystals in the air and transport them, the speed at the center of a gravel devil must be around 150 miles per hour, he said. That’s at least the strength of an F0 tornado, and more like an F1.

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After the gravel devils move the gypsum, some of it clumps together when…

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