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Salt Polygons and Porous Media Convection

Jana Lasser, Joanna M. Nield, Marcel Ernst, Volker Karius, Giles F. S. Wiggs, Matthew R. Threadgold, Cédric Beaume, and Lucas Goehring
Phys. Rev. X 13, 011025 – Published 24 February 2023
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Abstract

From fairy circles to patterned ground and columnar joints, natural patterns spontaneously appear in many complex geophysical settings. Here, we investigate the origins of polygonally patterned crusts of salt playa and salt pans. These beautifully regular features, approximately a meter in diameter, are found worldwide and are fundamentally important to the transport of salt and dust in arid regions. We show that they are consistent with the surface expression of buoyancy-driven convection in the porous soil beneath a salt crust. By combining quantitative results from direct field observations, analog experiments, and numerical simulations, we further determine the conditions under which salt polygons should form, as well as how their characteristic size emerges.

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  • Received 1 June 2022
  • Revised 5 December 2022
  • Accepted 25 January 2023

DOI:https://doi.org/10.1103/PhysRevX.13.011025

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Open access publication funded by the Max Planck Society.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Fluid DynamicsNonlinear DynamicsInterdisciplinary Physics

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Why Death Valley Is Full of Polygons

Published 24 February 2023

The geometric patterns on dry, salty lake beds are generated by convection of high- and low-salinity water underground, according to simulations and observations.

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Authors & Affiliations

Jana Lasser1,2,*, Joanna M. Nield3, Marcel Ernst2, Volker Karius4, Giles F. S. Wiggs5, Matthew R. Threadgold6, Cédric Beaume6, and Lucas Goehring7,†

  • 1Graz University of Technology, Institute for Interactive Systems and Data Science, Inffeldgasse 16c, 8010 Graz, Austria
  • 2Max Planck Institute for Dynamics and Self-Organization Am Fassberg 17, 37077 Göttingen, Germany
  • 3Geography and Environmental Science, University of Southampton Highfield, Southampton SO17 1BJ, United Kingdom
  • 4Geowissenschaftliches Zentrum, Georg-August-University, Goldschmidtstrasse 3, 37077 Göttingen, Germany
  • 5School of Geography and the Environment, University of Oxford, South Parks Road, Oxford OX1 3QY, United Kingdom
  • 6School of Mathematics, University of Leeds, Leeds LS2 9JT, United Kingdom
  • 7School of Science and Technology, Nottingham Trent University, Nottingham NG11 8NS, United Kingdom

  • *jana.lasser@tugraz.at
  • lucas.goehring@ntu.ac.uk

Popular Summary

Salt deserts are some of the most extreme and beautiful landscapes in the world: flat silver-white plains covered with bizarre and seemingly unnatural shapes. The most prominent feature of these fantastic landscapes is a characteristic tiling of polygons, formed by ridges in the salt-encrusted surface, and always a few meters across. To date, no mechanism has been able to explain the origins of these patterns, with their distinctive size and shape. Here, we outline and explore a new model of their formation, driven by convection happening in the wet salty soil beneath the crust.

In a salt desert, surface evaporation leaves the near-surface groundwater enriched in salt and heavier than the fluid beneath it. This can lead to precipitation of salt at the surface as well as convection in the soil, with narrow, regularly spaced downwelling flows of high salinity. Using extensive observations from sites like Death Valley in California and lab experiments, alongside detailed numerical simulations, we show that convection can explain the emergent shapes as well as the robust length scale of the patterns. We find that convection results in a higher salt flux into the surface above the downwellings, where salinity gradients in the groundwater are weaker. This preferential precipitation of salt templates the growth of ridges at the locations of the downwellings.

Our study reveals the origin of an otherworldly landscape that fascinates hundreds of thousands of tourists every year.

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Vol. 13, Iss. 1 — January - March 2023

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It is not necessary to obtain permission to reuse this article or its components as it is available under the terms of the Creative Commons Attribution 4.0 International license. This license permits unrestricted use, distribution, and reproduction in any medium, provided attribution to the author(s) and the published article's title, journal citation, and DOI are maintained. Please note that some figures may have been included with permission from other third parties. It is your responsibility to obtain the proper permission from the rights holder directly for these figures.

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