Death Valley--
a land of austere beauty,
bone dry desert, extremes of both heat and cold.
An hours drive off paved roads
lies Racetrack Playa, known for the nearly flat
mud cracked surface of the dry lake, where rocks move
through unknown forces until now.
Most people suggest that movement is driven by hurricane-force winds blowing
over rain-slicked mud.
Others, suggest that floating ice sheets lift rocks,
describe remarkably similar trails. In 1976,
Bob Sharp and Dwight Carey tested the idea that large
ice sheets might move stones. One of the experiments that we did
was to create a corral about 15 feet in diameter
in which two rocks were placed to
see if ice had something to do with this movement.
What happened during that time period is one of the rocks moved out,
the other one stayed, so at least at that time we were convinced that
rice probably didn't play a role in the movement of every stone on the playa.
We tested these ideas by installing a weather station,
we also embedded GPS units in our own rocks,
lug them to the playa, and there they waited for two years.
Jim Norris explains:
my business partner and I designed a custom GPS
recorder, a little module about three inches in diameter, that we can put into
stones.
And we built fifteen of those. Those stones are deployed
out on the Playa as we speak.
And they were designed so that they would sit and wait
and as one of the other researchers on this project said.
"probably the most boring experiment ever." True, until this year.
In late November 2013,
rain and snow created a shallow pond on the southern third of the playa.
We observed rocks in motion December 4th,
20th, 21st, and January 9.
The days dawn clear and cold with a light breeze.
In the morning the pond was covered with floating ice,
sunlight formed melt pools between large areas of
still frozen pond. Ice in the center of the playa melted first.
Water was blown onto the shore causing the remaining floating ice to move.
Abruptly, just before noon, the ice broke up into large moving sheets--
popping and crackling all over the pond surface. In the deeper part in the pond,
moving ice began to splinter and break up leaving wakes of open water and ice
chips downstream of
each rock. Large ice panels, hundreds of feet across,
battered some rocks into motion. By good luck and with a long camera lens we
caught some rocks moving on January 9th.
In this image, the liquid water is ripple by the wind,
the floating ice is darker gray. There are a series of stationary rocks not
engaged by ice
and there is a moving stone. And it's off.
Here we see a time lapse set of images at about natural speed of the rock.
It moves in fits and starts pushed by the ice panel
and finally and briefly gets going.
Movement is so slow that it is easy to miss
particularly if there are no stationary rocks nearby
to act as reference points.
This rock was about the size of a cantaloupe
and was hard to find months later when the pond was gone because it moves
several more times.
In a sped up version, the rock quickly stops moving as disengage from the ice,
the ice panel drifts out of view.
In December and January, ice pushed up and over rocks but the trails were
initially invisible
beneath the muddy water. By early afternoon, the shallow pond had been blown
away
revealing more than 60 rock trails all remarkably similar.
Our GPS rocks also moved and some scribed trails over 700 feet long.
Rocks moved only 2 to 25 meters a minute,
a slow shuffling walk at best. As evening came on,
the wind slackened and the water flowed back to the south
refilling the pond around the rocks. By February,
the pond had dried out revealing the inscribed surface of the racetrack.
Rocks had moved at least four times in the two and a half months the pond existed.
Dick Norris notes: To me the most remarkable things about this
are that rocks move first of all very slowly, only at a very slow pace about
like that. And that this happens in the middle of the day
when the sun is shining and it's just simply beautiful out here.
We had thought before we began the study of the racetrack phenomenon
that this must happen during howling winds--times of really miserable weather
conditions, when you really
wouldn't want to be out here and that was why the phenomenon had never been
observed before
because the conditions were simply too nasty for anyone
to be willingly out here. Rocks are driven by surprisingly thin ice.
Indeed most of the ice is no thicker than a windowpane
and clear as glass. Rocks move through a combination of a shallow pond,
ice, light breezes, and sun to set them in motion.
Ponds, in particular, are rare in Death Valley
so rocks may not move for many years or even a decade or more.
For now, the sliding rocks continue to fascinate
and we finally know what to look for to see the move. And who knows
there may still be parts of the story yet to be discovered.