Powerful whirlwinds explain an odd feature of the Atacama desert

THE Salar de Gorbea, at the southern end of the Atacama desert, in Chile, is one of the most hostile places on Earth. It receives virtually no rainfall and the little water it does host is contained in ponds both acidic and salty. It therefore has no vegetation. It is, though, the site of some of the most extraordinary dunes on Earth.

Most dunes are made of sand: grains of silica that are 2mm across, or less. There are exceptions. The White Sands National Monument in New Mexico, for example, is so called because the ingredients of its dunes are sand-grain-sized crystals of gypsum. But this exception proves the rule, because the point about a dune is that it is created by the wind, and when it comes to minerals, the wind can generally pick up and move around only sand-sized objects. The dunes of Salar de Gorbea, however, are an exception that proves no rule at all. They, too, are white, because they are also made of gypsum. But the gypsum in question includes crystals more than 20cm long. How such dunes could form by wind action has long been a mystery. Kathleen Benison, of West Virginia University, thinks, however, that she has solved it.

Gypsum is a form of calcium sulphate created by the evaporation of water laden with that substance. Dr Benison knew that gypsum crystals of the size found in Salar de Gorbea’s dunes form in ponds 5km from those dunes. She thus suspected that these ponds are the source of the dunes’ crystals. This suspicion was reinforced, she explains in a paper just published in Geology, when she compared the internal bands marking stages of the growth of crystals from the dunes with those of crystals from the ponds. They appeared identical. That suggested crystals are somehow being transported from the ponds to the dunes.

She was able to rule out one mechanism for such transport—that the crystals had been moved by long-vanished streams or rivers—for several reasons. First, the Atacama is believed to have been too dry for streams to form for millions of years. Second, gypsum dissolves in water (this is, indeed, the reason dunes made of it are rare, for most deserts have at least some rainfall). And third, the faces of crystals from the dunes were scored in ways which indicated that they had been bashed around by strong winds.

The only inland winds obviously powerful enough to have done this are in the funnels of tornadoes. The Atacama desert does not, though, experience such storms. It does experience lesser whirlwinds, called dust devils. But the textbooks say that dust devils are not powerful enough to lift and carry objects the size of the crystals found in the dunes.

Textbooks, however, are not always correct, so Dr Benison decided to check for herself. She went to Salar de Gorbea and monitored the dust devils there. She found that devils do regularly form in valleys along the edge of the region. Some then pass over the ponds where the gypsum crystals are growing, pluck crystals out of those ponds, carry them the 5km to the dunes, and then dissipate, dropping their loads on the accumulating heaps.

What she does not yet know is how they do it, for the textbooks are, in one sense, correct. The most powerful recorded dust devils have wind speeds of 70kph. This is indeed insufficient to carry mineral particles bigger than 2mm across. For the devils of Salar de Gorbea to be transporting large gypsum crystals they must be far more powerful than that. Dr Benison seems therefore to have substituted one mystery for another. The devils clearly are responsible for Salar de Gorbea’s dunes. What is responsible for these devils’ great powers remains to be found out.