Death Valley's sailing stones are pushed by windblown sheets of ice
On the Racetrack Playa in Death Valley, rocks slide across a dry lakebed and leave long trails with nobody touching them. Scripps researchers finally filmed the cause: thin panes of winter ice, broken up by morning sun and nudged by light wind, shove the stones along. It happens only every two or three years.
A dry lake, or playa, is a basin that once held standing water and dried out because evaporation outpaced inflow. As the water leaves, dissolved salts such as sodium carbonate and borax stay behind, building a flat clay floor crusted with minerals. An alkaline crust makes an alkali flat; a salty one, a salt flat or salt pan. In the Great Basin of the western United States, the dry end of an enclosed basin is called hardpan.
Names change with the map. Playa is Spanish for beach and is used in Mexico, the American West and for ancient lake sediments in the Sahara. South Americans say salar or salina, South Africans say pan, Australians distinguish salt pans from clay pans, Arabic speakers use sabkha or shott, Central Asians call cracked-mud flats takyr, and Iranians call salt flats kavir. The densest cluster, almost 22,000 of them, is scattered across West Texas and the eastern edge of New Mexico on the southern High Plains. Most are small, but Bolivia's Salar de Uyuni, the largest salt flat on Earth, covers 10,582 square kilometres.
Many fill with a skin of water in wet seasons. A thin layer blown back and forth by wind can polish the bed extremely hard and smooth, while deeper water leaves cracked mud and tent-shaped desiccation ridges, and very little water lets dunes form. The Racetrack effect needs a precise recipe: water deep enough to freeze into floating ice but shallow enough to leave rocks exposed, ice strong yet mobile, then sun and breeze. The tracks usually last three or four years.
The lakebeds themselves are bare, but rings of saltbrush and shadscale around them feed livestock and wild grazers through winter. Seasonally flooded playas in southwest Idaho, Nevada and Utah shelter species found nowhere else. In 2006 scientists described a new giant fairy shrimp, a sizeable predator that had stayed hidden in murky, clay-laden water; its tiny cysts can lie dormant in the dry bed for years until conditions suit hatching.
Source: Dry lake
