This is how honeycomb patterns form in salt pans

So-called salt polygons can be observed in salt deserts all over the world. The sides of these one to two meter high polygons – usually hexagons – are formed by narrow salt ridges a few centimeters high. Until now it was not clear how such patterns came about. For example, it was assumed that cracks form in this form due to drying out, or that the salt crust grows continuously and forms such patterns. But that wouldn’t explain why the polygonal shapes around the world are so similar, despite local differences in geology, salt chemistry and environmental conditions.

Together with research colleagues from Germany and England, Jana Lasser from the Technical University of Graz has developed a mathematical model of the processes leading to the formation of the honeycomb structure. They also observed the movement of salt water in sandy soils in laboratory experiments and conducted field studies in Death Valley and near Owens Lake, California.

Flow as a driving factor
Accordingly, convective currents of high- and low-salinity water in the subsurface are responsible, the researchers report in the journal “Physical Review X.” The driving mechanism for pattern formation is the convective circulation of water of varying salinity in the soil beneath the salt crust. This explains not only the shape, but also the more or less equal size of the combs and the speed at which they grow.

As the water evaporates in the heat, it becomes saltier just below the surface, making it heavier than the less salty water below. At some point, the saltier water begins to sink in narrow channels and the fresher water rises – similar to the convection currents of hot and cold water in a pan. Lasser explained that these convection coils reach a depth of one to two meters.

Water saltier along the ridges
When several such streams form, they are compressed and arranged in a honeycomb pattern. The less salty water flows up the center of the honeycomb and the saltier water flows down the ridges. “Evaporation continuously draws the fresher water to the center of the structure. It is initially not yet saturated and can therefore still dissolve salt. It then flows out and at some point the salt is so highly concentrated that it partially crystallizes and forms the ridges,” says Lasser.

The complexity researcher refers to some pattern-forming phenomena in nature, “which are based on very similar dynamical systems”. As examples she cites the fairytale circles in dry grassland landscapes in Namibia, for example, where evaporation, precipitation and vegetation would influence each other, or hexagonal basalt columns such as the ‘Giant’s Causeway’ in Northern Ireland, which are created by cooling lava.