Death Valley National Park in California's Mojave Desert is known for many things - Its extreme temperatures, varied altitude that ranges from 282ft. below sea level to 11,000ft. above, beautiful rugged terrain and most of all, 'rolling' or 'dancing' stones. The phenomenon whereby large boulders, some weighing as much as 500 lbs, move several hundred yards in a straight line or zigzag patterns, has confounded experts for years.
While the most logical explanation would be a gradual incline, the rolling rocks are all located close to Racetrack Playa, a dry lake bed that lies in the flattest section of the park. What's even more intriguing is that nobody has ever witnessed the movements. The only evidence that the rocks are moving is from the extensive tracks they leave behind in the dry sand.
Over the years, there have been numerous theories. Some thought the rocks were being propelled by hurricane-force winds, while others believed that the movement was the result of the thin ice that forms over the lake bed, acting as a sledge to help move the rocks forward. There were also the skeptics who believed that it was Park Rangers that physically moved them around and the wishful thinkers who attributed it to Unidentified Flying Objects (UFO's). But nobody had ever been able to prove how the rocks moved, until recently.
Determined to solve the mystery once and for all, a team of researchers from the Scripps Institution of Oceanography at UC San Diego turned to modern technology for assistance. In the winter of 2011, they situated 15 rocks fitted with GPS devices as well as a sophisticated weather station that tracked wind speeds at one second intervals, at the Racetrack Playa. Since the rocks move extremely rarely, the researchers thought they would have to wait a long time to get answers.
Turns out this was not the case at all. When lead researchers Jim Norris and Richard Norris visited the Racetrack Playa in early December, 2013, they noticed that it was filled with 3 inches of water. Then came the big day! At around noon on December 21st, 2013, the two heard popping and cracking sounds emanating from the water which had froze overnight and was now melting from the sun's heat. The scientists knew they had found their answer.
As scientists from Hampshire College in Massachusetts, had suggested a few years ago, during winter, the below freezing nightly temperatures of the area turn the surface water in the lake bed to ice. When the frozen layer starts to melt in the morning, it breaks up into chunks of floating ice that act like sledges. This, accompanied by the right amount of wind, helps the rocks roll across the sand. Since the ice sheets are extremely thin, they often break up while the rocks are rolling, explaining the willy-nilly patterns created by some.
The scientists who published their research in the online journal PLOS ONE on August 25th, 2014, say that similar to snow rollers, for the rocks to move, the conditions have to be perfect. First and foremost, the water in the Playa has to be substantial so that it can form floating ice when the temperatures drop during the winter, but not so deep that it submerges the rocks. Also, the ice has to be the right consistency - too thin and it will melt when the sun emerges, too thick and it will be unable to move around nimbly. Finally, there has to be a gentle breeze to get everything in motion.
What was surprising was how little it took to move the rocks - The sheets of ice needed to be just about 0.25 inches thick and the breeze, a mere 10mph. Of course, the rock movement is extremely slow and small - just a few inches and only for a few seconds - hardly enough for anyone to be able to observe any motion. In fact, the only reason the scientists were able to pinpoint when it happened, was thanks to the GPS records.
Unfortunately, though the scientists were able to observe five such movements on hundreds of small rocks during the two and half winter months when the Playa was filled with water, they were never able to catch any of the big boulders in action. Whether those move in the same way, is a mystery that still needs to solved!
Resources: wikipedia.org, cnet.com, fastcompany.com PLOS One.org