From Death Valley to Chile to Iran, salt polygons of the same size are forming at sites around the world—and underground fluid flows may hold the key to solving the age-old mystery of why.
Geometric shapes such as pentagons and hexagons form spontaneously in a wide range of geological conditions. Dried mud, ice, and rock often crack into polygons, but these patterns tend to vary wildly in size.
So why are all the plays so consistently similar? The answer lies underground proposed by physicist Yana Lasser and her colleagues on February 24 in Physical Review X.Using complex mathematical models, computer simulations and experiments conducted at Owens Lake in California, the team linked what they saw on the surface with what was happening below.
“Fluid flows and convection underground can definitely explain why the patterns form,” says Lasser of the Graz University of Technology in Austria.
This three-dimensional approach was key to explaining the universality of salt polygons.
Salt flats are formed in places where there is little precipitation and significant evaporation occurs. Groundwater that seeps to the surface evaporates, leaving behind a crust of salts and other minerals that were dissolved in the water. Most impressively, this process results in low ridges of concentrated salt that divide the playa into polygons: mostly hexagons with a few pentagons and other geometric shapes.
The type of salt varies from one playa to another. Common salt, or sodium chloride, dominates some layers, but others contain more sulfite salts. And the salt crusts themselves have a thickness of several millimeters to several meters. This variation appears to be the reason for the failure of previous attempts to describe play patterns.
Regardless of whether the crust is a meter or a millimeter thick, salt pans have polygons measuring 1 to 2 meters. Previous models were based on fracturing, dilation, and other phenomena that described how cracks in mud and rock create polygons with dimensions that vary with crustal thickness.
As groundwater evaporates from the surface, it concentrates salt in the remaining groundwater. This salty water, now denser and heavier, sinks, forcing other less dense water to rise. Laser and his colleagues showed that over time, a circulation known as convection tends to push downward plumes of saltier water into the network of vertical sheets. The surface above these sheets accumulates more salt, so thick salt ridges grow there. Thinner salt crusts form between where less salty water rises, spontaneously forming the characteristic polygons common to beaches around the world.
The equations the researchers used describe the relative salinity of the groundwater, the pressure in the fluid and the speed at which the water circulates. A computer simulation that embraced all the complexity of a three-dimensional problem started without a salt crust or polygons and created something that closely resembled real games.
“This fluid-dynamic model makes a lot more sense than a model that ignores what’s going on below the surface,” says physicist Julian Cartwright of Spain’s National Research Council, who is based in Granada and was not involved in the study.
Tests at Owens Lake helped the team test and refine the model. “Physics is a lot more than just sitting in front of a computer,” Lasser says, “and I wanted to do something that involved experiments.”
The lake dried up in the 1920s when water was diverted to Los Angeles. The deposited minerals in the remaining salt flats contain high natural concentrations of arsenic, which is carried along with wind-blown dust, creating a serious health hazard. Among other remediation measures, brine was pumped into the bottom of the lake to try to create a more stable salt crust. This human intervention allowed the researchers to test their ideas in a controlled way.
“The whole area is destroyed,” says Lasser, “but it was a perfect research environment for us.”
