Why Is the Ground White and Flat for Miles? Where Water Goes to Evaporate

A drainage basin with no outlet to the sea, which geographers call endorheic, collects whatever water runs into it, and in an arid climate that water leaves only by evaporating. Everything dissolved in it stays, so salts accumulate over long periods, concentrating until they precipitate out as a crust. The surface is extremely flat because water spreads to a level before evaporating, so every wet season redistributes material and levels the deposit, and the largest examples vary in elevation by less than a metre across areas of thousands of square kilometres. That flatness makes them useful as reference surfaces, and one is used to calibrate the altitude measurements of satellites.

The chemistry depends on what the inflowing water dissolved on its way:

• •Sodium chloride, ordinary salt, which is the commonest and gives the white colour
• •Sulphates including gypsum, which form their own distinctive deposits
• •Carbonates, which precipitate early as water concentrates
• •Borates, mined commercially from several such basins
• •Lithium, concentrated in the brine beneath rather than in the crust
• •The order of precipitation follows solubility, with the least soluble coming out first

Several of these basins in South America hold brine with high lithium concentrations, and a very large share of world production comes from them. The extraction method is straightforward and slow, pumping brine from beneath the crust into shallow ponds where evaporation concentrates it over many months, with impurities precipitating at successive stages until a lithium-rich solution remains for chemical processing. The process is cheap compared with hard rock mining and uses enormous quantities of water in regions where water is the scarcest resource, which is the substance of the objections raised by local communities and by hydrologists. Demand for battery materials has driven rapid expansion, and the tension between that demand and local water is one of the sharper resource conflicts of the present.

Many of these surfaces are patterned with polygons, typically hexagonal, a few metres across and outlined by low ridges, and the pattern is regular enough that people assume it was made. The explanation involves convection in the brine beneath the crust. Water evaporating at the surface leaves the remaining brine denser, and dense fluid above less dense fluid is unstable, so the brine overturns in circulating cells much as a heated liquid does. Those cells organise into a honeycomb arrangement, which is the same pattern convection produces in a heated layer of fluid generally, and salt precipitates preferentially where the flows meet, building ridges along the cell boundaries. The pattern is therefore a record of the circulation below, and its scale reflects the thickness of the brine layer.

These surfaces look dead and are not entirely. Salt-tolerant microorganisms inhabit the brine, in some cases colouring it pink through pigments they produce, and specialised invertebrates persist in the wetter margins. Flamingos feed on those invertebrates and breed on several such basins in large numbers. Ephemeral flooding after rain produces brief blooms of life and, where the water is shallow and still, a mirror effect that is the subject of a great many photographs. The deposits also record the past, since the layers beneath the surface preserve a history of wetter and drier periods, and many of these basins held substantial lakes during the last glaciation whose shorelines are visible as terraces on the surrounding hills.

A basin with no outlet loses water only by evaporation, so dissolved salts accumulate and precipitate, and repeated flooding levels the deposit into the flattest natural surfaces on earth. Salts come out in order of solubility, and lithium stays in the brine beneath. Extraction concentrates that brine in evaporation ponds and consumes water in regions where water is scarcest.