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

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 28 Aug 2014 12:24:20 -0700 (PDT)
Message-ID: <201408281924.s7SJOKaT005988_at_zagami.jpl.nasa.gov>

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.
Received on Thu 28 Aug 2014 03:24:20 PM PDT