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Abstract:
Patterns, such as the hexagonal salt ridges that emerge in salt deserts around the world, are a common phenomenon in geophysical settings. They are similar to other natural phenomena such as fairy circles, columnar joints and patterned ground, whose origin can be explained by an instability in a dynamical system. So far, the origin of polygonal salt patterns in salt deserts is unknown, even though they are a common landform and their understanding makes an important contribution to climate modeling and the understanding of the emission of atmospheric dust from their surface. In this work I provide a model that explains the emergence of the salt ridge hexagons. I present evidence, which confirms the predictions made by my model. To explain the driving mechanism of salt polygons, I couple the surface expression of ridges on the salt crust to the dynamics in the underground and model the process as the buoyancy driven convection of the saline water in the porous medium below the salt crust. I investigate the driving mechanism by means of mathematical modeling, numerical simulation, analogue experiments and a field study. I find the conditions for instability of the stratified saline water and present direct hydrological evidence for the existence of plumes of high salinity which are co-located with the patterns visible at the surface. With my model I am able to explain the length - and timescale of pattern emergence as well as the robustness of pattern wavelengths against changing natural circumstances. The results from my investigation of buoyancy driven convection in porous media can be connected to other research of similar systems, such as CO2 sequestration. Last but not least, with my work I hope to make a contribution to the better understanding of these otherworldly landscapes that draw the eye and fascinate the observer, in the hope that a better understanding of nature will lead to increased efforts to preserve it.
