Mike K
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 How Do Rocks Race? | |
While visiting Death Valley\'s Racetrack, I met Gunther Kletetschka, a research professor, studying how the rocks at the racetrack move and create the playa mud trails we are so fond of.
This question as to the mechanism of the rock trails is one that has captured people\'s imagination for some time. No one has ever actually seen the rocks move, as it is likely to be under the most uncomfortable conditions. Proposals I had heard in the past involved the wet playa being frozen, or mostly frozen, and very strong winds driving the rocks across the playa, leaving tracks.
Prof. Kletetschka\'s hypothesis is that the playa floods with an inch or two of water in a rare Winter storm, then freezes from above as the air temperature plunges at night. A sheet of ice forms around the rocks and encases their base forming an ice collar. More water flows into the playa via known springs or additional runoff, and the ice sheet floats the rocks a fraction of an inch above the playa clay mud. A strong wind, measured up to 100 mph, drives the partial ice sheet across the playa taking the rocks with them. The wind is blowing the ice sheets with the embedded rocks, not the rocks slipping on the ice as had previously been proposed.
It has been shown that with extremely high winds most rocks will tumble across the frozen playa, not slide. At times, these ice sheets could be fairly large in area and have substantial mass, thus having the force to move larger rocks and create larger trails.
This theory of floating ice sheets taking the rocks, also explains why small and large rocks can move as one, with parallel tracks. If the rocks were simply blown over the frozen playa, the small rocks would move much faster or tumble producing different shaped tracks.
If more water floods the playa the rocks will be lifted free of the playa clay mud and the ice movement will result in the rock being carried away without leaving a trail. This also explains why more than 1/2 of all the Racetrack rock trails have no rock at either terminus. I originally thought that vandals carried the smaller rocks home, but this proposal is far more sensible.
Of course the wind can change direction and velocity, making for curves in the tracks, or different storm events can lift and move some rocks and not other nearby rocks. This results in tracks going in very different directions, but these tracks could be created in different events and possibly years apart.
The idea that the floating ice sheets carry the weight of the rocks is much more plausible as the largest rocks are very heavy, estimated to be several hundred to 700 pounds as they are easily several cubic feet of solid rock.
Of course Prof. Kletetschka has a good deal of data to back up his hypothesis; I just summarize his proposal here as a context for sharing some of my racetrack photos.
http://geology.com/nasa/racetrack-playa/
When taking these photos I wondered why the polygon pattern is so uniform in size, approximately 3\" in diameter. Kletetschka explained that the playa is a mixture of clay and silt, and when the clay dries it shrinks creating the cracks in the surface. The surface of the playa is very uniform in composition and thus the cracks, patterns in the playa, are also very uniform in frequency.
Sorry for running over the image count: I was hoping this would be considered \"an exception\" as it seemed to be the best forum for this subject.
Mike K
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