Large stones have been sliding across Death Valley, leaving behind tracks that have left scientists clueless to what causes them to move. The stones appear to magically move on their own, but the explanation to why they move has finally been proven with science.
Scientists finally solved the "sailing stones" mystery by using GPS tags to observe the movements of the rocks. They found that the stones, which range from pebbles to 600-pound boulders, stroll along Death Valley under a mix of water, ice, sun and wind.
According to researchers Richard Norris of the Scripps Institution of Oceanography and his cousin James Norris of Interwoof, ice, sun and wind are the culprits of the sliding stone mystery. The stones slide when thin ice melts and breaks up under the sun and piles against the stones, causing enough friction that makes the stones move with the wind.
Published in the journal PLOS One, the researchers found the stones can only slide under the perfect conditions. The ice can't be too thick, the sun not too bright and the wind must be steady for the stones to make their journey across the muddy surface of the temporary pond of Death Valley.
Norris and his cousin placed stones that had GPS tags inserted into the limestone on the Racetrack Playa (a playa is a dry lake bed). of Death Valley last year to find solve the mystery. The GPS was synced to a custom-built weather station to track the movement.
"We recorded the first direct scientific observation of rock movements using GPS-instrumented rocks and photography, in conjunction with a weather station and time-lapse cameras," the authors write.
What followed was a lot of waiting and watching, but they were able to get photographic and meteorological evidence to crack the case. The cousins witnessed 60 rocks moving across the Valley on Dec. 20 of last year.
"There was this crackling sound or popping sound all over the playa," says Norris. "One moment it was quiet, and the next moment it was popping everywhere as the ice began to break up, and I said to my cousin, 'This is it! We're actually seeing this whole thing happen!' "
The sliding stones were first documented in the 1900s by miners. The mystery has gone unsolved since 1948, when scientists began to uncover why the stones moved -- the only clue being the trails left in the dried mud. Previous theories included dust devils, ice sheets, hurricane-force winds and flooding.
"In contrast with previous hypotheses of powerful winds or thick ice floating rocks off the playa surface, the process of rock movement that we have observed occurs when the thin, three to six [millimeter], "windowpane" ice sheet covering the playa pool begins to melt in late morning sun and breaks up under light winds of (about four to five meters per second)," they write.
For the stones to slide, there also has to be standing water, which is rare in the area that gets only two inches of rainfall each year.
"The process of ice breaking up and shoving rocks around happens every year if you go up into Saskatchewan or Ontario, but you don't normally associate it with a hot, dry place like Death Valley," says Norris. "And yet here's the same kind of process unfolding occasionally, very occasionally, in this place that we associate with a very different kind of climate."
Decades can go by before new movements are seen in Racetrack Playa.
