Wind, water and ice are shown once again to be key geological drivers
FRISCO — Scientists have not only solved the mystery of the moving rocks at Death Valley’s Racetrack Playa — they documented the movement on video and even took measurements by attaching GPS units to some “non-native” rocks as part of a research project in the Southern California desert.
Some of the rocks weigh up to 100 pounds and leave behind distinct tracks as they scoot across the dry lake bed. Scientists have been studying the area for decades, but nobody has seen the process in action until now, according to a press release from the Scripps Institution of Oceanography (UC San Diego).
The researchers published their findings last week in the journal PLOS ONE, concluding that the rocks are pushed by wind after thin ice forms on the surface of the playa. There is some evidence that the movement of the rocks has diminished since the 1970s, which may be linked with a warming climate.
When some of the researchers arrived to monitor the site in December 2013, they found the playa covered in shallow water, and soon, the rocks started to move.
“Science sometimes has an element of luck,” researcher Richard Norris said. “We expected to wait five or ten 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.”
Their observations show that moving the rocks requires a rare combination of events. First, the playa fills with water, which must be deep enough to form floating ice during cold winter nights but shallow enough to expose the rocks. As nighttime temperatures plummet, the pond freezes to form thin sheets of “windowpane” ice, which must be thin enough to move freely but thick enough to maintain strength. On sunny days, the ice begins to melt and break up into large floating panels, which light winds drive across the playa, pushing rocks in front of them and leaving trails in the soft mud below the surface.
“On Dec. 21, 2013, ice breakup happened just around noon, with popping and cracking sounds coming from all over the frozen pond surface,” said Richard Norris. “I said to Jim, ‘This is it!’”
Previous theories explaining the movement of the rocks included hurricane-force winds, dust devils and even slippery films of algae. But the detailed study shows the rocks are pushed by winds as light as 10 mph, skating along on ice less than a quarter inch thick. The rocks moved at a speed of about six to 20 feet per minute — almost imperceptibly to observers at a distance. Some rocks moved for just a few seconds while others moved for as long as 16 minutes.
“The last suspected movement was in 2006, and so rocks may move only about one millionth of the time,” said Lorenz. “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.”
“We documented five movement events in the two and a half months the pond existed and some involved hundreds of rocks”, said Richard Norris, “So we have seen that even in Death Valley, famous for its heat, floating ice is a powerful force in rock motion. But we have not seen the really big boys move out there …. Does that work the same way?