Monday March 10,
2008
6:00 - 7:30 pm
Pellissippi State Technical Community College
10915 Hardin
Valley Road, Knoxville
J.L. Goins Administration Building, Cafeteria Annex
MARCH PRESENTATION
RACETRACK
PLAYA, DEATH VALLEY, CALIFORNIA: SHEDDING
NEW LIGHT ON AN AGE-OLD PARADOX
Geoffrey
J. Gilleaudeau
Ph.D. Candidate
Department of Earth and Planetary Sciences
University of Tennessee-Knoxville
Abstract
Racetrack
Playa is a desiccated mud flat located in the northwestern
section of Death Valley National Park, California. The playa sits
at the bottom of a hydrologically closed basin that was once the
site of a Pleistocene to early Holocene perennial saline lake.
The basin has undergone considerable tectonic subsidence through
Tertiary time due to extension of the crust and the formation of
the Basin and Range province. The basin is bound by a large
normal fault (creating a half-graben), as well as the Last Chance
Range to the west and the Cottonwood Mountains to the east. The
modern surface of Racetrack Playa has extremely low relief, lies
well above the water table, and consists of fine sand, silt and
clay.
A curious aspect of the playa surface is the presence of large
boulders (up to 700 lbs.) that sit at the terminations of long
(>1 km) irregular tracks that have seemingly been carved in
the substrate. Individual tracks appear to cross each other, take
sharp (>90°) turns, and curve gradually into semi-circles
several meters in diameter. These tracks seem to indicate that
boulders have been mechanically transported, or slid, across
great distances of the playa surface. Despite decades of park
visitation, no individuals have ever reported witnessing the
boulders in motion. The mechanism of their transport thus remains
a mystery. Early 20th century explanations for the mysterious
boulder tracks ranged from the supernatural to the
extraterrestrial to the absurd. More plausible hypotheses have
included transport by ice, strong winds, and swelling clays.
Calculations indicate, however, that wind speeds >800 mph
would be necessary to move the largest boulders even across a wet
clay surface. In addition, ice events are extremely rare in Death
Valley, and micro-changes in topography caused by swelling clays
are not likely to account for >1 km of transport. After a
casual field trip to Racetrack Playa in the spring of 2007, I
propose a new mechanism to account for the unusual transport of
large boulders across the playa surface. This mechanism involves
extreme slickening of the surface by cyanobacterial mats after
precipitation events, and subsequent transport by strong winds
channeled through the valley. While only a hypothesis, extreme
reduction in the surface coefficient of friction by
cyanobacterial mats seems to be the only plausible explanation
for this bizarre phenomenon.
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