[meteorite-list] Mystery of Moving Rocks in Death Valley Solved!

[Ron Baalke]

http://www.grindtv.com/outdoor/nature/post/racetrack-playa-mystery-death-valley-solved/

Racetrack Playa mystery in Death Valley solved

For decades, scientists have been trying to figure out how rocks moved 
across a dry lake bed and left trails behind, but now they know blown 
ice sheets cause it

by David Strege
GrindTV
August 27, 2014 

[Image]
Racetrack Playa researcher Richard Norris standing by a trail likely formed 
more than a decade before this December 16, 2012 photo. Trails can last 
for years or decades between events. Photo from Richard Norris courtesy 
of Scripps Oceanography

The phenomenon of the "sailing stones" on Racetrack Playa in Death Valley 
National Park has baffled scientists for decades.

By some mysterious force of nature, rocks move along the flat-as-a-pancake 
playa and leave long trails behind. What causes the stones to move?

[Image]
Parallel trails carved into the wet, mud-cracked surface of Racetrack 
Playa in Death Valley. Photo by Jim Norris courtesy of Scripps Oceanography

One popular theory was that strong winter winds upward to 90 mph combined 
with just enough rain to make the clay slippery caused the stones to "sail."

Another is that ice sheets pick up the rocks, or ice forms around the 
rock enabling it to move with the wind, leaving a series of rock trails.

But now, the mystery is solved.

Scientists can say conclusively that these synchronized trails left by 
rocks, some up to 700 pounds, are caused by thin sheets of ice pushing 
the rocks across the desert floor under certain conditions, a theory that 
had been previously dismissed in 1976 after a test.

The conclusion was reached by a team led by paleobiologist Richard Norris 
of the Scripps Institution of Oceanography, UC San Diego, with the results 
published Wednesday in the journal PLOS ONE.

[Video]
Scripps Oceanography details the phenomenon in this six-minute video (it 
also illustrated the event on a whiteboard):

As part of the Slithering Stones Research Initiative, researchers custom 
built motion-activated GPS units and fitted them into 15 rocks and placed 
them on the playa in the winter of 2011, with permission from the National 
Park Service. They expected it would take five to 10 years before something 
happened.

[Image]
A GPS tracking unit was fitted into 15 rocks that were placed on the Racetrack 
Playa. Photo by Richard Norris courtesy of Scripps Oceanography

Ralph Lorenz, one of the paper's authors from Applied Physics Laboratory 
at John Hopkins University, called it "the most boring experiment ever."

But in December 2013, something happened.

"Science sometimes has an element of luck," Norris said. "Only two years 
into the project, we just happened to be there at the right time to see 
it happen in person."

Three inches of water covered the playa and shortly after their arrival, 
rocks began moving. The study showed that sailing rocks require a rare 
combination of these events:

1. The playa fills with water deep enough to form floating ice during 
cold winter nights but shallow enough to expose the rocks. 

2. As overnight temperatures drop, the pond freezes to form thin sheets 
of "windowpane' ice. 

3. When the sun comes out, the ice begins melting and breaking up into 
large floating panels. These ice panels, driven by light winds, push the 
rocks ahead of them, leaving trails in the soft mud below the surface. 
When the playa dries out months later, the trails become clear.


[Image]
Example of "windowpane" ice collected on the Racetrack Playa. It was much 
thinner than expected. Photo by Richard Norris courtesy of Scripps Oceanography

"On Dec. 21, 2013, ice breakup happened just around noon, with popping 
and cracking sounds coming from all over the frozen pond surface," said 
Richard Norris. "I said to Jim [Norris, a cousin], 'This is it!'"

Indeed it was.

Forget hurricane-force winds, the rocks were moved by quarter-inch thick 
ice panels by light winds of 10 mph. The rocks moved only a few inches 
per second or a speed deemed imperceptible at a distance without a  stationary 
reference point.

"It's possible that tourists have actually seen this happening without 
realizing it," said Jim Norris of the engineering firm Interwoof in Santa 
Barbara. "It is really tough to gauge that a rock is in motion if all 
the rocks around it are also moving."

Lorenz said the last suspected movement previously was in 2006, so rocks 
may move only about 1 millionth of the time, and there is evidence to 
suggest that the frequency of rock movement has declined since the 1970s 
because of climate change.

[Image]
Racetrack Playa is partly flooded shortly after the December 21, 2013 
move event in which hundreds of rocks scribbled trails in the mud under 
the floating ice. Photo by Richard Norris courtesy of Scripps Oceanography

Asked if the mystery of sliding rocks has finally been solved, Richard 
Norris replied, "We documented five movement events in the 2 1/2 months 
the pond existed and some involved hundreds of rocks. So we have seen 
that even in Death Valley, famous for its heat, floating ice is a powerful 
force in rock motion. But we have not seen the really big boys move out 
there. Does that work the same way?"

No word whether the Slithering Boulder Research Initiative is now forming.