YSM Issue 87.4

FEATURE
geology
d e at h
va l l e y’S
BY GENEVIEVE SERTIC
PHOTO BY LIDIYA KUKOVA
For nearly a century, scientists have struggled with the mystery of
the sailing stones of Death Valley. These massive rocks, weighing up
to 320 kilograms, scraped out tracks as long as 224 meters in parallel
formation, giving the valley its second name, “Racetrack Playa.”
What scientists did not understand was how these immense stones
managed to move, or “sail.” No forces powerful enough seemed to
exist in the environment.
Finally, at the end of August, a team of researchers from the
Scripps Institute of Oceanography published their surprising
findings from three years of observations in Death Valley, during
which they quite literally saw the process in motion. This discovery
finally emerged after years of debate within the scientific community
over conflicting theories about the sailing stones, and the answer is
of interest to scientists and tourists alike.
It is not pure force, but rather the right combination of conditions
that thrusts these massive stones along the lakebed. The Scripps
team found that the rocks only move when a thin layer of ice forms
overnight from rainwater runoff from the surrounding mountains.
The next day, when the sun shines down on Death Valley, the thin ice
sheet breaks up into panels, which flow steadily in the direction of
even a light wind. The panels push the massive rocks along with them
at about two to five meters per minute.
The rainwater runoff must be seven millimeters to form
“windowpane” ice three to six millimeters thick—thin enough to be
broken into panels, but still strong enough push the sailing stones
forward. The exact movement of the
sailing stones depends on the magnitude
and direction of the wind. Light, steady
breezes of four to five meters per second
help the rocks move along their path.
Some previous theories had predicted
ice and wind to play a role in the sailing
stones, but not in the same way that
the researchers discovered. Powerful
wind, thick ice sheets, and algal films
that reduce the friction between the
rocks and the lakebed were all previous
conjectures. However, the researchers
found that the wind that sweeps through
the dry lake does very little to move the
s a i l i n g sto n e s
rocks. The ice sheets that form are not thick enough to move the
rocks directly. And only winds of up to 80 meters per second—about
as fast as a NASCAR race car—could move the stones even with the
help of an algal film. Only thin, floating ice panels pushed with a
gentle breeze are able to move the stones.
Led by paleobiologist Richard Norris, the Scripps team started
their work on Racetrack Playa in 2011. To measure the movement of
the rocks, they monitored the stones and environmental conditions
with time-lapse cameras, GPS systems, and a weather station that
measured the velocity of gusts every second. Because the National
Park Service did not allow the researchers to use the native rocks in
the playa for their experiment, the team attached the GPS systems to
15 rocks similar to those in Racetrack Playa and placed them in the
dry lake. Dr. Norris and the other researchers were not expecting to
actually see any motion because the rocks seldom move—at most
once every decade. It was by pure chance that they were present
when the phenomenon occurred on December 21 last year. The
researchers heard the ice begin to crack around noon and saw the
spectacle firsthand.
The discovery has explained other phenomena surrounding the
sailing stones of Death Valley as well. In some areas, the ice panels
themselves scrape through the sand and leave tracks in their wake,
which explains why there are some trails with no stone marking the
end. Some pairs of rocks also lose synchronization with each other
along their trails, which is likely a result of splitting ice sheets that
maneuver around one stone but not the
other.
The sailing stones were a mystery
to the public as well as to scientists.
Visitors to Death Valley now have an
explanation for the tourist attraction,
and scientists now have a case study of
a surprising force of nature: thin panels
of ice floating on water that together
force massive rocks hundreds of meters
IMAGE COURTESY OF INQUISITR WEBSITE
The sailing stones of Death Valley have perplexed
scientists for decades. Now, researchers think they
have found an explanation for how a light breeze is
enough to move these massive rocks.
forward. The rocks look the same as
before—sitting motionlessly at the end of
the tracks streaking the playa—but now
we understand the story that these sailing
stones tell.
26 Yale Scientific Magazine October 2014 www.yalescientific.org