Giant dust devils with crystal blades

This is very cool news, although you would be wise to steer very clear of one of these awesome spectacles – flying blades of gypsum in tornadic hot winds.

Gypsum crystals coat the ground of the Atacama desert in Chile. In this picture from Kathleen Benison, you can see how large they are.

Geologist Kathleen Benison was working in the Atacama desert in Chile where the harsh environment creates large bladed-habit gypsum crystals due to evaporation. The crystals can also be found broken, heaped into piles or scattered around. How did this transport happen? The wind would have to be very strong, tornado force, to move the large crystals. And giant whirling dust devils are exactly what she saw.

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.

Her paper is published in the latest issue of Geology.

Whirlwinds, dry convective helical vortices, move large gypsum crystals in the Andes Mountains of northern Chile. The crystals are entrained from a saline pan surface, where they grew in shallow surface brines. They are transported as much as 5 km and deposited in large dune-like mounds. The dune gravel is cemented relatively quickly by gypsum cement precipitating from near-surface saline groundwater, resulting in gypsum breccia. This marks the first occurrence of gravel-sized grains moved efficiently in air by suspension, provides a new possible interpretation for some ancient breccias and conglomerates, and improves understanding of limits of extremity of Earth surface environments.

Dust devil in the Arizona desert. Photo: NASA/U. of Michigan

Another researcher, Nicholas Heavens, comments on the discovery of these “dry convective helical vortices” (DCHVs) .

Benison has identified an unusual site with potentially frequent, intense DCHVs in a setting channelized by the local topography. This area could be studied by in situ measurements or radar to understand why DCHVs become so intense here and also to study the dynamics of tornadic-strength vortices in a potentially advantageous field environment.

Dust devils are ground-based rotating columns of air resulting from strongly heated surfaces. They are frequently seen in the desert on a hot day. When they are strong enough, they can entrain dust, hay, or smoke and fire into their whirling dance. Heavens points out that they are not as strong as tornadoes (below 29 m/sec: the minimum velocity on the Enhanced Fujita (EF) Scale used to rate tornadoes). He also notes that DCHVs can be strong enough to transport small animals:

DCHVs can mobilize and entrain small mammals such as kangaroo rats and rabbits. By dropping kangaroo rats and jackrabbits from a tower, Ives (1947) found that their terminal velocity was 11–13 m/sec, which should be the necessary updraft velocity for a DCHV to keep them aloft.

Such calculations lend credence to the explanation for waterspouts or DCHVs as the transport mechanism of anomalous falls such as those of fish or frogs, famous Fortean strange rains. But these events are still mysterious since no one has documented the falls as a direct result of whirling winds.

Benison was lucky enough to observe the huge cloud and note they were not that anomalous. But she did not directly observe the crystals being transported. However, as noted, it seems potentially possible that the winds are strong enough to accomplish this. Heavens calculates “any vortex transporting the gypsum particles would contain horizontal velocities of ≈40–120 m/sec, a range that spans the entire EF scale.”

Extremely scary.

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