解开死谷石头自行滑动之谜

STEVE INSKEEP, HOST:

Now let's talk about the athletics of rocks. A century ago, miners working in California's Death Valley reported seeing big rocks on the desert floor with long trails behind them, as if they'd been pushed by some unseen force. Every once in a while, the rocks have moved again, but no one has ever seen it happen. Now after 60-plus years of trying, scientists think they have solved the mystery of the slithering rocks of Death Valley.

NPR's Christopher Joyce reports.

CHRISTOPHER JOYCE, BYLINE: For decades scientists have been trekking out to a dry lake bed called The Racetrack in Death Valley to see these rocks for themselves. Many are boulders weighing 500 pounds and leading up to many of them are those erratic trails in the sand.

RICHARD NORRIS: They do things like, they take high angle turns, sometimes they reverse course.

JOYCE: Geologist Richard Norris at Scripps Institution of Oceanography was very puzzled when he first saw them.

NORRIS: They are just going, you know, every which way out there, but in a very regular kind of fashion, like, they're moving in fleets.

JOYCE: Fleets of slithering boulders. Norris joined a community of scientists who had been obsessed with trying to find out what was going on here. There were several theories. One long-standing one was wind storms. Once, in 1953, a researcher landed a plane on the lake bed and tried moving the rocks with the wind off his propellers. The rocks didn't slide, though a few did roll. Some people thought maybe ice was forming underneath the rocks on cold nights and they were ice skating, but no one actually saw that happening.

So Norris got together with his cousin, who's an engineer. They said, OK, what if we put GPS trackers on those rocks and video cameras around them to catch the rocks in the act?

JOYCE: And no one ever said to you, Richard, you're nuts?

NORRIS: Well, kind of. (Laughter) I think that a lot of people thought, you know, why are you putting GPS trackers on rocks?

JOYCE: They set everything up and just waited. Then early one morning in December, rain had fallen the night before. A thin sheet of ice had formed over the lake bed or, playa, as it's called. There was a light breeze and then the sun came up and the answer to the mystery unfolded.

NORRIS: The ice began to melt out in the center of the playa and the ice began to pop and crackle all across the playa surfaces. The ice began to move.

JOYCE: Thin sheets of ice, dozens of them, 40 or 50 feet across. The wind was pushing each sheet over a film of water and watery sand, shoving them up against the rocks.

NORRIS: It's basically being like, a tugboat or a bulldozer - it's pushing the rocks very slowly along.

JOYCE: But by the end of the day, some of the rocks had moved hundreds of feet. The ice and water had evaporated and in the now dry sand, furrowed trails marked each rock's journey.

Norris thinks the conditions for this to happen occur very rarely; once in decades perhaps and they had been very lucky to catch it. One of the first people they told about the discovery was Paula Messina at San Jose State University. She had written her PhD thesis on the sliding rocks and like many scientists, she believed that freak wind storms were moving those rocks.

PAULA MESSINA: And even though they were showing me data and photographs, I still couldn't believe it. I went down to The Racetrack and had to observe it for myself because I thought, no, it can't possibly be.

JOYCE: But now she says, she and scientists who believed the wind hypothesis are now laughing about how wrong they were and Norris is laughing with them.

NORRIS: It's so much fun. Pretty much everybody was out there because it was a neat problem and it was fun to do. And I think there is no purer form of science than that.

JOYCE: Norris's team has published the findings in the journal PLOS ONE. Christopher Joyce, NPR News.

INSKEEP: Like a rolling stone. This is NPR News.