Geology And Geophysics

Racetrack Playa Death Valley Moving Rocks Sliding Stones



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"Racetrack Playa Death Valley Moving Rocks Sliding Stones"
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They are known as the sliding rocks or the moving rocks or the sailing stones of the Racetrack Playa. No one has ever seen them move but move they do, leaving clear tracks in the dried mud that can be hundreds of feet long. No evidence of interference from animals or people is ever seen near rocks that have moved. Sometimes, a monitored rock is even found flipped over without any evidence of tampering.

The Playa itself is a dry lakebed located in the Panamint Range, between the Cottonwood Mts. to the east and the Last Chance Range to the west. It is 2.5 miles from north to south and 1.25 miles east to west at an elevation of 3,711 feet.

An early theory proposed that the rocks were simply moving downhill but measurements show the playa is essentially flat, therefore refuting this theory. In fact, where there is a measurable slope, rocks actually move slightly uphill.

Although no definitive theory has been formed, there is some consensus as to the major contributing factors. Most investigators agree, because of the "wakes" left by the rocks, movement occurs when the surface of the playa is wet. Rainfall causes the playa to become muddy and this mud forms a slippery, low-friction surface.

Dr. Paula Messina has introduced another theory of why the surface becomes so slick when it is wet and that is the activity of bacteria that usually lie dormant until there is moisture present. These bacteria form a film on the surface, sometimes compared to Teflon, adding to the Slip N' Slide effect.

Wind gusts are considered necessary to start the rocks moving, with strong, sustained wind needed to keep a stone sailing. The general trend of the tracks is north-northeast, consistent with prevailing winds. Low lying areas in the surrounding mountains form natural wind tunnels allowing strong winds sweep across the playa. One wind tunnel is located in the southeastern portion of the playa, which coincidentally is 5cm lower, collecting rain water and also home to natural springs causing this area to turn muddy more often and for longer.

The central portion of the playa is an area where two wind tunnels converge. While the trails in the southeastern portion are straighter, the trails in the central portion are the most convoluted, showing the effects of the competing winds.

While most investigators concede the wind can account for the movement of smaller rocks, tracks have been found behind rocks weighing upwards of 700 pounds and these are considered, by some, too heavy to be moved by even the strongest wind.

These rocks are thought to move with the help of ice rafts forming around the rocks, carrying them along when the ice sheets move. Bob Sharp and Dwight Carey tested this theory in 1972. They labeled stones and corralled them to see if ice was responsible for moving the stones. The theory was that while individual rocks could move between the rebar stakes rocks imbedded in a large ice raft would run into the stakes, altering the direction and nature of the tracks. This study was inconclusive. Some rocks moved beyond the rebar corrals while others in the same corral didn't move at all. This led to further speculation that small rings of ice instead of large rafts might be forming around individual rocks helping them skate along.

One theory never presented until now that could contribute to the solution of what moves the racing rocks was formed by the author and dubbed The Oatmeal Bowl Theory.

When I was a child my mother often served oatmeal for breakfast. One morning, staring at my bowl of boring, hot mush, I saw the bowl move. At first I thought it was my imagination but then it moved again! I pointed it out to my mom and my sisters. They thought I was doing it for a joke but after a few minutes it was obvious the bowl was moving on its own. In fact, watching on other mornings, the bowl would actually hop up a little as it moved a few inches across the table. My mother challenged me to explain it. I told her I thought the hot cereal was heating the air trapped under the bowl until it expanded so much that it could actually move a heavy ceramic bowl filled with food. It was the only explanation. The reason it happened to my bowl of cereal was because my sisters would start stirring in sugar and milk right away, cooling the oatmeal. Since I didn't like oatmeal I delayed, allowing time for the air underneath the bowl to heat sufficiently to cause movement.

When I first heard about the Racetrack Playa I immediately thought of my childhood breakfast experience as an explanation, or at least a partial explanation, of the reason why the rocks move. Irregularities in the rock surface could trap air and moisture underneath. As the rock heated, so would the trapped air or water. If it was heated enough the air could move the rock when it expanded or the moisture, as it evaporated and expanded could also move the rock. Just as my oatmeal bowl would hop a bit, a rock could move forward or even flip over, especially if the edge was caught by the wind at the critical moment.

While I am not in a position to adequately test this theory, it would be interesting to see if those involved in tracking the movement of these rocks could do so. By itself it is not a complete, alternative explanation. Rather, it is possibly another piece of the puzzle as to why the rocks move.

http://www.pbs.org/wnet/nature/deathvalley/rocks.html
http://geology.com/articles/racetrack-playa-sliding-rocks.shtml
http://ngm.nationalgeographic.com/2007/11/death-valley/cahill-text

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