Finally understanding the origin of geometric shapes in salt deserts

In Bolivia, at an altitude of 3,600 meters above sea level, the Salar de Uyuni salt desert offers intrepid tourists an almost unreal landscape: a white “slab” of salt polygons as far as the eye can see. There are many other regions in the world where such geometries appear spontaneously. However, the mechanism by which they form has remained unknown. But Cedric Bohm, at the University of Leeds, UK, and his colleagues think they’ve found a reason why these polygons appear in nature: It would be the interaction of the downward flow of salt-laden water and the rise of evaporation.

Salt deserts consist of dry lakes, located in valleys in semi-arid regions, where water evaporation exceeds precipitation. The soil is enriched with water via Precipitation falls on the surrounding mountains, but not directly from the surface. When the lake empties, salt crusts appear that form polygons.

In the past, two attempts have already been proposed to explain this phenomenon. In one, the drying surface cracks, allowing salt to rise to the surface and produce crusts in these gaps. In the other case, salt builds up on the surface, causing the veneer to have an impossible horizontal expansion, causing wrinkles. “These two hypotheses fail in the same place: the size of the polygons formed must be proportional to the thickness of the crust, which is not the case,” notes Cedric Bohm. On the contrary, the geometries are pretty much the same size, with a diameter of between one and two meters, regardless of the desert area and how much salt is present.

“Before our study, the problem had been considered from the point of view of the lake crust, the researcher continues, so we chose to see what was happening instead, from the point of view of soil fluid dynamics. The team modeled the porous medium beneath the surface of the desert, which is composed of minerals (various salts) and water, to replicate the convection of low and high salinity water. Thus, evaporation carries minerals to the surface, causing salt crust to grow. Conversely, salt-laden liquid tends to sink over the edges of the convection cells.

But why are the shapes so regular? The convective cells of the lake initially have a circular outline. As it grows, it interacts with neighboring cells, creating a straight border of drafts between them and giving rise to a honeycomb structure on the surface.

“We hope that subcrustal fluid dynamics is the dominant phenomenon to explain the formation of salt polygons,” asserts Cedric Beom. Field readings reinforced the study’s conclusions: in modeling as in reality, the most salt-rich subsurface regions are located below the salt hills; The predictions for the size of the polygons are consistent with the shapes observed in the field.

The significance of the scientists’ work goes beyond simple curiosity: It’s also a matter of responding to health problems. After the river was diverted, Lake Owens in California dried up. However, the arsenic in the ground has risen to the surface in salt crusts and can now be carried by winds towards the settlements. A better understanding of this dynamic makes it possible to implement treatment strategies.

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