Sailing stones in Death Valley are seen showing signs of traveling along the ground. Tracks in the dirt clearly show the rocks made a journey across the surface, but no one knew how this could happen, until now.
The Death Valley stones, made from black dolomite, have never been seen moving. Long trails have been seen for a century, however, causing geologists and armchair scientists to ask how the rocks could move along the flat Racetrack Playa.
Theories to explain the sailing stones have ranged from the magnetic field of the Earth to gale-force wind. Some people even believed the large stones may be slipping on algae, caught between the rock and ground.
A biologist, an engineer and a planetary scientist worked together utilizing a weather station, to take the first-ever video of the rocks in motion. The team also employed high-precision GPS measurements as part of the investigation.
Racetrack Playa is subject to occasional floods each winter from melted snow and rain. Freezing nighttime temperatures can create an icy surface on the ground. If this ice sheet is floating on a thin layer of liquid water, conditions are created for daytime winds to push the stones.
Ice forms in thin sheets resembling broken glass creates a muddy surface, the study revealed. The rocks are then propelled across the surface of the ground by light winds. The effect is similar to a car hydroplaning on a wet road.
"It's a wonderful Goldilocks phenomenon. Ponds like this are vanishingly rare in Death Valley, and it may be a decade between heavy enough rain or snowfall events to make a substantial pond," Richard Norris, a paleobiologist at the Scripps Institution of Oceanography and lead author of the study, said.
Rocks move in colder climates, through a similar process to that which occurs in Death Valley. When ice-covered lakes begin to thaw in spring, ice floes can drive large stones across muddy surfaces. This process produces tracks similar to those seen near the sailing stones.
"So we have seen that even in Death Valley, famous for its heat, floating ice is a powerful force in rock motion," Norris said.
Racetrack Playa is a dry lakebed, three miles in length, littered with a few hundred rocks. Some of the rocks measure just a few inches in diameter, while the largest weigh in at up to 700 pounds. Despite their size, even the largest boulders show signs of having traveled across the lake bed.
Sailing stone tracks are all different lengths. Some are smooth and graceful, while others show evidence of making several turns. A few tracks do not connect to any rocks at all, a mystery unanswered in this latest study.
Study of the sailing stones in Death Valley was detailed in the online journal Plos One.
