For a century, the moving rocks in the Death Valley, California have intrigued visitors and scientists alike and gave birth to all sort of theories more or less strange. Although they weight about 300 kg, these stones can be found on the floor of the playa with long trails behind them. Somehow the stones slide across the playa, cutting a furrow in the sediment as they move. Trails differ in both direction and length. Even two similarly sized and shaped rocks may travel uniformly.
Scientists have investigated the origins of stone movement since the early 1900s, but as of August 2014, no one has seen the phenomenon in action. That is because the stones can sit for a decade or more without moving. Last year, however, the researchers from Scripps Institution of Oceanography, UC San Diego have finally caught the stones in action.
For their experiment the researchers used a weather station capable of measuring gusts to one-second intervals and fitting 15 stones with custom-built, motion-activated GPS units. The results showed that the cause of the moving stones is a rare combination of events produced by rain, wind, ice and sun.
First, the playa fills with water deep enough to form floating ice during cold winter nights but shallow enough to expose the stones. During night, when the temperature drops, the stones cover with ice thin enough to move freely but thick enough to maintain strength. On sunny days, the ice covering the pool 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.
In the study, the rocks moved under light winds of about 3-5 m per second and were driven by ice less than 3-5 mm. As a result they speed was only a few inches per second (2-6 meters per minute), which is almost imperceptible at a distance and without stationary reference points.
‘It’s possible that tourists have actually seen this happening without realizing it,’ said Jim Norris of the engineering firm Interwoof in Santa Barbara.
Watch paleooceanographer Richard Norris describing the phenomenon.
More info at scripps.ucsd.edu
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