Mystery of how rocks move across Death Valley lake bed solved
The cracking sounds were ferocious. An ankle-deep, frozen lake in Death Valley National Park was breaking apart under sunny skies.
As cousins Richard Norris and James Norris watched, a light wind began moving huge flows of ice across the surface of the water and into rocks weighing up to 200 pounds. Propelled by the ice masses, the rocks began to slide across the slick, muddy bottom of the normally dry lake bed, known as “the Racetrack Playa.”
“My god, Jim, it’s happening,” Richard yelled.
James Norris grabbed a camera.
Their photos last Dec. 21 provided the final evidence in solving a mystery of the Racetrack Playa that has long puzzled visitors and scientists: What mechanism moves rocks across flat dirt in the heart of the hottest, driest place on earth?
Rocks of various heft – some weighing 600 pounds or more – leave trails that wiggle like snakes or form complete loops or even rectangles. The trails are cut sharply into the earth but no other tracks are visible.
Theories over the decades have included sporadic hurricane-force winds when the surface is covered with rain water, or rocks carried across the mud by small rafts of ice, or UFOs.
But until the Norrises had an incredible stroke of luck that day last December, no one had scientifically verified the phenomenon. The findings were formally presented today in the online scientific journal PLOS ONE.
“I’m amazed by the irony of it all,” James Norris said, nodding toward the glistening playa earlier this month. “In a place where rainfall averages two inches a year, rocks are being shoved around by mechanisms typically seen in arctic climes.”
“And the movement is incredibly slow,” he added. “These rocks clock in at about 15 feet per minute.”
Geologists have been studying the moving rocks since 1948, when the first scientific study suggested they were driven by dust devils. One reason the mystery endured is that the movements are episodic, often with no motion for periods of decades until a precise series of natural events occurs.
The first requirement is rain in a parched climate. Next, temperatures must fall low enough to freeze the water before it evaporates. Then the sun has to come out and thaw the ice. Finally, wind has to blow strongly enough to break the ice into floes and move it across shallow water underneath. Even a light wind will do.
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