A basin of ancient water
Salinas Grandes is a high-altitude desert basin sitting at 3,450 meters (11,319 feet) above sea level. This vast salt flat, covering approximately 212 square kilometers (82 square miles), is the remnant of an ancient lake. Millions of years ago, the uplift of the Andes mountains created this endorheic basin—a closed drainage system with no outlet to the sea. Over geologic time, water from rain and snowmelt flowed down from the surrounding volcanic terrain, filling the basin. The intense solar radiation and dry air at this altitude caused the water to evaporate, leaving behind concentrated layers of minerals.
This process continues today. During the rainy season (December to March), a shallow layer of water can cover the salt flat, creating a massive natural mirror. As the water evaporates during the dry season (April to November), new layers of salt crystallize on the surface. The salt crust has an average thickness of about 30 centimeters (12 inches), consisting mainly of halite (sodium chloride). Beneath the solid crust lies a mineral-rich brine that contains significant deposits of lithium, potassium, and boron. This has made the region a focal point for lithium exploration, a process that requires vast quantities of water and is a concern for the 33 local Indigenous Kolla and Atacama communities who rely on the basin's delicate water balance.
Reading the layers
The salt flat is a mineral deposit; it is a high-resolution archive of past climate conditions. Scientists can drill cores deep into the salt crust to study the alternating layers of salt and mud. These layers, known as varves, correspond to annual cycles of evaporation and precipitation. A dry year with high evaporation produces a thicker layer of salt crystals. A wetter year with more rainfall and runoff from the mountains results in a thicker layer of mud and sediment washed into the basin.
By analyzing the thickness, composition, and chemistry of these millennia-old layers, paleoclimatologists reconstruct a detailed history of regional climate. They can track long-term variations in rainfall and identify periods of prolonged drought. These mineral deposits preserve a direct record of ancient lake levels and water chemistry, showing how the regional environment has responded to climatic shifts over long timescales. This data is important for understanding the natural climate variability of the South American Altiplano and for refining predictive models of future environmental conditions.