Posted : August 2014
Author : the admin
DEATH VALLEY NATIONAL PARK, Calif. (AP) - For years
scientists have theorized about how large rocks - some weighing hundreds of
pounds - zigzag across Racetrack Playa in Death Valley National Park, leaving
long trails etched in the earth. Now two researchers at the Scripps Institution
of Oceanography at the University of California, San
Diego, have photographed these "sailing
rocks" being blown by light winds across the former lake bed.
Cousins Richard Norris and James Norris said the movement is
made possible when ice sheets that form after rare overnight rains melt in the
rising sun, making the hard ground muddy and slick. On Dec. 20, 2013, the
cousins catalogued 60 rocks moving across the playa's pancake-flat surface. "Observed
rock movement occurred on sunny, clear days, following nights of sub-freezing
temperatures," they wrote in a report published Wednesday in the online
scientific journal PLOS ONE. The conclusion proves theories that have been
floated since geologists began studying the moving rocks in the 1940s.
The phenomenon doesn't happen often because it rarely rains
in the notoriously hot and dry desert valley. The rocks move about 15 feet per
minute, according to the report. Richard Norris, 55, a paleobiologist at
Scripps, and James, 59, a research engineer, launched their "Slithering
Stones Research Initiative" in 2011, the Los Angeles Times reported (http://lat.ms/VNVWou).
After getting permits from the National Park Service, they
installed a weather station in the area and placed 15 stones equipped with
global positioning devices on the playa. The "GPS stones," which were
engineered to record movement and velocity, were stationed at the southern end
of the playa where rocks begin their strange journeys after tumbling down a
cliff.
At the end of last year, Richard and James Norris returned
to inspect the instruments. "We found the playa covered with ice," Richard
told the Times. "We also noticed fresh rock trails near shards of thin ice
stacked up along the shoreline." The following afternoon, "we were
sitting on a mountainside and admiring the view when a light wind kicked up and
the ice started cracking," he said. "Suddenly, the whole process
unfolded before our eyes."
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