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Roger Weller, geology instructor regional geology planetary gems
by Hillary Ditzhazy
The Sliding Rocks of Death Valley
The rocks in Death Valley California are an
unexplained phenomenon that geologists have been studying for years. They are
located on a dried surface in Death Valley, California and move seldom under
certain weather conditions. While no one has actually seen these rocks move,
they leave behind shallow furrows, trails, which suggest the rocks slide around.
So what makes these rocks move? Geologists have studied this phenomenon for
years, and, while there are still mysteries, there have been some theories that
give reason to their movement.
The dried surface they are located on is called the racetrack
playa, which is a
surface that is made up of very fine clay and silt sediments.
The racetrack playa is a flat dry lake, elevation 3708ft, nestled in between
mountains and ranges. When there is heavy rain fall, the water from the top of
the hills slides down into the playa to make a temporary muddy lake. As the
water evaporates, a layer of soft mud is left, and, when dried, it shrinks and
cracks into palm sized polygon shapes.
Credit Photo: http://www.nature-blog.com/2007/10/death-valley-moving-rocks.html
So what makes these rocks move? When the mud is wet, it becomes very slippery, but that’s not enough to move the rocks. Since the playa valley is completely level, the gravity would pull the rocks down and not push horizontally. Geologists tried to connect the matter of size and shapes to the movements. They found a larger more angular rock will travel as far as 200 ft in one advance and furrows will be in a straight and parallel line to other rocks beside it of the same stature. The smaller, more lightweight and rounded rocks can move over 600 feet in a single advance with more curve like snake patterns.
Credit Photo: distoedakilo.blogspot.com/2007_08_01_archive.html )
In the 1950’s scientists began studies of the
valley and soon came up with the theory, a combination of high speed wind and
the slick mud would allow these rocks to glide. Dr Robert P. Sharp, of
California Institute, studied this theory for seven years by tagging rocks and
tracking weather conditions. For the most part his research added up to conclude
it was just a matter of wind and slick silt mudd. While this theory seemed to
satisfy the mystery, a few rocks tended to not follow through?
In 1955 George M. Stanley proposed a theory that
rocks move with the assistance of ice sheets forming after the playa surface is
flooded. Due to the playa’s low elevation, the temperature can drop down to
freezing between winter and early spring. However, this theory was not tested
until 1976 when Dr. Robert Sharp and Dwight Carey tested this theory and found
it false. They claimed "’that wind moves the stones when conditions are just
right, that this normally happens at least every one to three years on Racetrack
Playa, and that ice sheets are not necessary.’" (Sharp and Carey)
It was not until
1995, when John B. Reid, Jr., and a group of geology majors began research, and
testing this theory. He and his coworkers wetted the valley themselves and put
their own dolomite, a rough material, on the surface, it did not move easily.
Thus, he concluded wet mud alone could not provide enough friction on the
surface to move a 700lb rock. However if this was possible, the assistance of
ice and wind together explains the larger rocks movements. While this theory is
concluded Sharp, Cary and other geologist still insist that wind can move the
rocks around with or without the assistance of ice.
Some believe this to be a hoax and think that people are actually moving the
rocks while no one is watching. If anything it would be a unique way of bowling.
Yet this was ruled out when the question came about of how could a 100lb human
carry or move a 700 lb rock? On top of that how can these lighter than human
rocks leave distinct furrows and what moves them leave nothing? Still no theory
completely solves this puzzling mystery there are still unexplored elements.
Some think that magnetism might also be taking an effect to help move the rocks,
and even lightning, seeing that the major movement of the rocks is committed
under thunder storms.
Sharp, R.P., and Carey, D.L., 1976, Sliding Stones, Racetrack Playa, California:
Geological Society of America Bulletin, v. 87, p. 1704-1717