In a remote corner of Death Valley National park, cradled in between the Cottonwood and Last Chance mountain ranges, Racetrack Playa presents an intriguing natural history mystery. Here, slabs of dolomite and syenite, ranging in size from a couple of pounds up to 1,000 pounds, leave visible tracks as they slide across the playa surface, without any sign of human or animal intervention. For decades visitors and scientists alike have puzzled over what natural forces cause the stones to move across the playa.
Theories have ranged from the plausible (Death Valley National Park’s website has for years suggested that rain will wet the playa providing a slippery surface across which very strong winds may skid the stones across the slick mud) to the supernatural (strange magnetic forces or psychic energy as the culprit behind the sailing stones mysterious behavior). Unfortunately, as the stones had never been seen moving, the mystery had remained just that. Until now.
In a paper published with the scientific Journal Plos One, researchers Richard D. Norris and James M. Norris have provided photographic and scientific evidence as to how the stones manage to sail in their erratic patterns across the smooth playa surface.
In 2011, Richard and James, in conjunction with the Scripps Institution of Oceanography, launched the “Slithering Stones Research Initiative.” For the next two years they and a team of friends and relatives, working with permission from the National Park, installed a weather station and placed fifteen stones on the southern end of the playa equipped with GPS devices to record movement and velocity. Most of the natural sailing stones begin their strange journeys across the playa at this point, tumbling onto the hard, flat surface from a nearby cliff.
In December of 2013, Richard and James stumbled into a rare confluence of events that allowed them to not only witness the stones moving, but to photograph it.
How does it work? First you need rain to turn the playa back into a shallow lake, followed by a cold that will freeze a layer of ice over the playa before it evaporates. Then you need a sunny day and a light wind. The wind and sun work together to crack the ice into sheets hundreds of feet wide and as thin as 1/4-inch, which then blow against the rocks. The ice then acts as sails, sliding the rocks along the slippery mud in a direction determined by the wind. This process is very slow, the stones rarely moving faster than a few inches per-second, which makes the motion easy to over-look without a fixed point to compare them to.
It’s an amazingly fickle process: if the winds are too strong, the ice will break against the rocks rather than pushing them, if the sun is too warm the ice will melt before it can move the stones, and if the wind is too light the ice won’t move as needed. The researchers observed rocks being moved simultaneously by large ice sheets and the rocks moving multiple times and in directions dictated by prevailing wind, before stopping, which accounts for the stones erratic tracks. The researchers also saw ice sheets creating rock-less trails that the Park Services had previously suspected were the result of tourists stealing rocks.
Researchers admit that although they have seen how the smaller stones move, there may yet be a different series of events which is needed to move the larger rocks, so the mystery isn’t completely solved.
For a more detailed report, check out their full paper on Plos One.