What Moves Gravel-Size Gypsum Crystals Around the Desert?

The movement of gravel-sized gypsum crystals around the desert is primarily caused by exceptionally strong whirlwinds, known as “gravel devils,” which are capable of lifting and transporting these larger grains.¹

This mechanism was documented in the high-altitude Atacama Desert of northern Chile, specifically at the Salar de Gorbea.²

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🌪️ The Role of Gravel Devils

Traditional geology posits that wind can only effectively move grains smaller than 2 mm in diameter (sand, silt, and clay-sized particles).³ The discovery of gravel-sized gypsum crystals being moved by wind rewrote this understanding, identifying the unique power of the “gravel devil.”⁴

• Mechanism: Gravel devils are essentially massive, powerful dust devils (dry convective helical vortices).⁵ They form in clear, dry conditions, typically during the afternoon.⁶
• Scale and Power: Scientists observed whirlwinds up to 500 meters in diameter lasting around five minutes.⁷ The strength of the updraft and rotational motion within these vortices is sufficient to entrain (lift) the relatively lightweight, gravel-sized gypsum crystals from the surface of saline pans.⁸
• Transport: Once airborne, the crystals are carried in suspension and can be transported as far as 5 kilometers before being deposited in large, dune-like mounds.⁹

💎 Why Gypsum is Susceptible

The mineral gypsum (¹⁰$CaSO_4 \cdot 2H_2O$), a calcium sulfate dihydrate, is crucial to this phenomenon because of its properties:¹¹

• Formation: It forms in arid environments when water carrying dissolved gypsum evaporates from shallow, internal drainage basins, leaving behind crystalline deposits.¹²
• Structure: When formed in water-saturated environments like the Salar de Gorbea, the resulting selenite variety of gypsum often grows as large, thin, bladed crystals that, despite their size (up to 27 cm long observed in the study), have a relatively low density compared to traditional rock gravel, making them easier for powerful winds to lift.

While typical wind action (a process called saltation) is responsible for moving sand-sized gypsum grains and forming the famous dunes at places like White Sands National Park, New Mexico, the gravel-sized movement is distinctly attributed to the much more intense force of the “gravel devils.”

For a visual on the incredible phenomenon of rock movement in dry lake beds, check out this video: The Geology Behind the “Sailing Stones” of Death Valley. This video, while focused on sailing stones moved by ice and wind, offers context on the unusual movement of geological materials in desert playas.