The Sliding Rocks of Racetrack Playa – A Geological Who Done It Mystery

I love interesting geological phenomena and the challenge of mysterious landforms and features.  Here are a couple of pictures that would make anyone scratch their heads and ask, how did that happen? In this case there is still some uncertainty but by proposing and testing hypotheses geologists have established some good ideas for how these rocks came to be where they are.

A sliding rock one one of the playas of NV and CA.   No one has seen one of these move but presumably they have moved in the past because they have left records of their movements in he trails they have left behind.   Did this happen over minutes, days, years, or even thousands of years?  Some recent experimentation at some of these sites may finally solve some of the mystery. Photo credit: NASA/GSFC/Cynthia Cheung.

Based just on a few pictures and observation many hypotheses have been proposed.  For example, it could be a prank like the crop circles in Britain or it could be animals have pushed the rocks around.  How about wind or maybe the rocks are lighter than they look and sort of floated along. Did the rocks move all at once or slowing over time?  Was the ground dry or wet when they moved?  Maybe they were moved by earthquakes like the rubbing boulders of the Atacama desert valley which I wrote about earlier.

These sliding rocks are in Nevada (click for larger image)

Did this rock slide across a dry or wet surface? Was it pushed by an animal or person? Notice the lack of any other tracks in the area. © iStockphoto / Mike Norton

These rocks are found on several playas in Nevada and California but similar rocks “racetracks” have been found in other places in the world.  These particular playas are some of the flattest natural surfaces on earth.  Racetrack playa only rises about one inch over 4/12 miles!!  (Reference and additional pictures). They were created in valleys with no natural outlet. Normally very dry, when there are thunderstorms water from the surrounding mountains comes down into the valley bringing new sediments which have filled the valleys to eventually produce very wide but very shallow lakes. Since all the water leaves via evaporation salts are precipitated out of the water leaving this whitish surface.   The satellite image above shows a mountainous surrounding death valley that has several of these playas.In  2010 a group of university students spent time collecting data at this site to try to solve the puzzle.   Remember that hypotheses are a form of guess based on present knowledge with the observations at hand.  Some hypotheses can be dismissed out of hand without much experimentation or additional observation.  For example, you can see the “tracks”  are obviously not found next to additional tracks of other animals or humans who might have been shoving the rocks to their current positions.  The rocks also apparently were not thrown to their positions by volcanoes or somehow rolled down the mountains to their current positions as there are tracks that are quite clearly the result of sliding.   Also, notice that in one picture the tracks are not straight but make some very obvious changes in direction.

Some of the moving rocks are large. This one is about 10 inches tall. Researchers in the late 1960s and early 1970s documented the movements of one very large rock that they named Karen. (The two men named all the rocks after women.) They estimated that Karen weighed 700 pounds. Credit: NASA/GSFC/Maggie McAdam

Recent observations and some tests have shown that the rocks have indeed moved over the last year having moved several inches in some cases.   Some of the larger rocks have moved further than the smaller rocks which might seem odd since they are the heaviest but consider that one of the forces at play in these locations is very strong winds, it could be that larger rocks have large surface area and sit up higher and so catch more resistance from the wind that can reach up to 150 mph at this location.    It seems that the combination of wet clay, wind and possibly ice combine to produce conditions that allow even a 700 pound rock to be pushed very slowly across this nearly perfectly flat surface.   For more details about the research that has been done and additional pictures please visit this report on the results of this 2010 research project.

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