ScienceShot: A Mineral Mystery in the Driest Place on Earth
During his travels in South America in 1835, Charles Darwin came to the point where “the desert of Atacama commences, where nothing can exist.” Considered the driest place on Earth, the Atacama Desert receives less than 1 to 2 mm of rainfall each year. The only thing growing, it seems, is one of the world’s strangest mineral deposits (pictured). They appear as endless salt-encrusted desert basins, and their chemical composition makes them so unusual that some geologists say these minerals couldn’t exist in nature if the deposits weren’t right before their eyes. For instance, nitrate salts, like saltpeter, could be made by bacteria, except none have been detected in the desert. Other salts made from ions like dichromate are found nowhere other than the Atacama Desert. How these mineral deposits were made has puzzled geologists since Darwin, but a new study says that the minerals came from three separate sources. One source was sea spray from the Pacific Ocean, 50 kilometers to the west, bringing chlorine and sulfur, which landed in the desert and dried into salt crystals. Other minerals formed from thin air, as nitrogen in the atmosphere reacts with salts and dust, depositing nitrate minerals on the ground over time. And the last source, researchers report in an upcoming issue of Geology, was rain falling into the Andes Mountains 100 kilometers east of the Atacama, leaching material like dichromate ions into ground water flowing toward the desert; over millions of years, the mountains grew taller and the change in elevation forced more ground water toward the surface. At the same time, the parched desert climate was becoming increasingly drier. This aridity caused dissolved minerals in the ground water to precipitate into the vast mineral deposits seen today, creating a landscape unlike anywhere else in the world.
*Correction, 26 February, 10:30 a.m.: Previously, this article stated that the Pacific Ocean lies to the east of the Atacama, and the Andes Mountains to the west of the Atacama Desert. Rather, the Pacific Ocean is to the west, and the Andes Mountains are to the east of the desert.