A team of scientists say they believe they have solved the mystery of how hundreds of rocks slide mysteriously across a dry lake bed in Death Valley, Calif.
The "sailing rocks" at the Racetrack Playa, a dry lake bed, have been under investigation by researchers since the 1940s. The rocks, some weighing as much as 320 kilograms, move across the lake bed over a period of years, leaving tracks.
A team lead by paleobiologist Richard Norris from University of California, San Diego, decided to monitor a set of test rocks that were deliberately placed on the lake bed in the winter of 2011. The rocks were watched remotely using a high-resolution weather station.
Because the stones can sit for more than a decade without moving, researchers didn’t think they would have results for some time. But around the end of 2013, the rocks began moving.
"Science sometimes has an element of luck," Norris said in a release. "We expected to wait five or 10 years without anything moving, but only two years into the project, we just happened to be there at the right time to see it happen in person."
In December 2013, researchers discovered that the playa was covered with a pond of water about seven centimetres deep. Rock movement occurred shortly after.
The team was able to chronicle a set of events that set the rocks in motion.
First the playa fills with water that must be deep enough to form floating ice but shallow enough to expose the rocks. When nighttime temperatures dip, the pond forms thin sheets of ice which can move freely but are thick enough to maintain strength.
Then, when the sun comes out, the ice melts and breaks up into large floating panels, and light winds drive these ice panels across the lake bed, pushing the rocks in front of them and leaving trails in the soft mud.
"On Dec. 21, 2013, ice breakup happened just around noon, with popping and cracking sounds coming from all over the frozen pond surface," said Norris in the study published on Aug. 27 in the journal PLoS ONE (Public Library of Science).
Rocks moved under light winds at about three to five metres per minute by ice that was only three to five millimetres thick. The rocks remained in motion for a few seconds to 16 minutes.
Previous theories have postulated dust devils or hurricane-force winds as the trigger for the sailing stones.
Researchers say they observed some rocks travelling more than 60 metres in one motion. Because they only used small test rocks, the scientists say they can't be 100 per cent sure about their theory since they haven't observed how the bigger, heavier rocks moved.
The team also concluded that climate change is having an effect on the sailing rocks.
"The last suspected movement was in 2006, and so rocks may move only about one millionth of the time," said study co-author Ralph Lorenz of Johns Hopkins University.
"There is also evidence that the frequency of rock movement, which seems to require cold nights to form ice, may have declined since the 1970s due to climate change."