Monday, September 19, 2011

The mysterious "sailing stones" of Racetrack Playa

note: Someone complained about the image originally published with this blog post (based on lack of digital rights to that image).  The previous image came from Wikimedia commons and I do not believe its use infringed with the rights as stated on its page on Wikimedia commons when I found it there at that time, in September of 2011.  However, that image has been removed and replaced with the above image, which also comes from Wikimedia commons and which carries a prominent statement that the author of the above image has released it into the public domain.  You can visit that Wikimedia commons page yourself by following this link:

The text of the original post follows:

The Mojave Desert is a stark and beautiful geographical feature which holds a special place in my heart for two reasons.

First, growing up in California, I had the wonderful privilege as a boy of going on long car trips with my family across the great southwest to places such as Carlsbad Caverns, the Grand Canyon, Bryce Canyon, Zion Canyon, and Sedona, Arizona: all destinations which required passage through the vast Mojave. I have many fond memories, not only of these well-known sites but also of smaller out-of-the-way stops along the road, where signs or park rangers explained details of the incredible natural history of the area, and talked about the wildlife and the human history of those who have made that area their home.

Second, as a young Army officer I was deployed several times to the National Training Center in Fort Irwin, California, smack in the middle of the Mojave Desert and not far from Death Valley. During the long days and nights in the desert I had the opportunity to drink in its unworldly beauty and stunning panoramas.

I well remember driving for hours at night in a column of Bradley Fighting Vehicles across the desert landscapes, the entire world bathed in the green light of my night vision goggles, across enormous valleys that seemed like they would never end, protruding from my Bradley turret and trying to stay awake and keep track of where we were at the same time, or coming down steep mountain passes in the late afternoon into the great Central Corridor, and feeling you could see for hundreds of miles, as if you were on a giant desert planet far from earth which dwarfed all human beings and their pathetic noisy machines of war.

There are several gigantic dry lakebeds there, such as Bicycle Lake or Silver Lake, entirely flat and bone dry, thousands of feet in length and long enough to accommodate multiple landing strips for huge Army cargo aircraft such as C-130s.

In other parts of the Mojave Desert, these dry lakes are called "playas," from the Spanish term for a beach (perhaps given that name with grim desert humor in the days of the Old West). Travelers driving to Las Vegas from Southern California along Interstate-15 pass by a couple large playas within sight of the interstate.

Further north, in Death Valley itself (still part of the Mojave) is the famous Racetrack Playa, so called because it contains mysterious "sailing stones" which leave racetrack-like trails in the mud of the dry lake. These wandering boulders, some of which weigh over 700 pounds, have mystified scientists and lay visitors alike, because to date no one has actually seen them move, much less photographed their lonely voyages, but move they do, from one location to another, leaving behind deep broad grooves in the earth to show their progress.

Here is a link to an online paper from the USGS written by two geologists who analyzed the trails of the Racetrack Playa rocks using GPS data. Here's another link to a discussion of the phenomenon which examines some of the theories that have been put forward over the years and proposes one which seems to be a likely candidate for the truth.

As both articles point out, the surface of the playa is almost perfectly flat, so flat in fact that just a couple inches of rain will cover the entire lakebed, which appears to rule out gravity as a driving force for these mysterious rock movements.

Some hypotheses put forward in the past suggested that strong winds might be able to move the rocks when the soil floor of the playa becomes wet and slick, but as the second article points out, the force of winds required to move rounded boulders weighing several hundred pounds through mud would be beyond hurricane speed. I can personally attest to the fact that winds in the Mojave can be quite extreme, and I have seen them roar through with enough force to blow the heavy rear hatch of a Bradley Fighting Vehicle back and forth, but even so I have never seen them blowing boulders around.

The theory put forward in the second article mentioned above, however, seems quite plausible, and is echoed in various other publications on the web, which is that thin layers of water which pool in the playa bottom after a rare desert rainfall or after runoff from the snow in the surrounding mountains can and often does freeze at night. These very thin ice sheets can be moved by much less severe winds, such as winds in the neighborhood of 10 to 60 mph. If so, boulders and stones in the ice sheet might well be carried along as the ice sheet is slid across the lakebottom surface by the wind, scouring marks in the soil below as they go.

The second article linked above indicates that several boulder tracks are often parallel and identical in their twists and turns, providing evidence that appears to support the allegation that they moved within a single ice sheet. However, the tracks recorded by GPS in the first article linked above, as well as the photograph entitled "Figure 1: Two diverging sliding cobbles" on the first page, appears to argue against the mechanism of a single ice sheet moving the two boulders shown. Other theorists have suggested that "collars" of ice could form around individual rocks, enabling the wind to blow them more easily but still individually.

An interesting video showing the strong winds blowing thin layers of liquid water along the playa bed at Racetrack can be seen here.

The Mojave Desert in general and the dry-lakebed playas in particular appear to provide strong geographic support for the hydroplate theory of West Point and MIT graduate Dr. Walt Brown. We have discussed before the fact that the location of an extensive zone of barren volcanic geography at the front edge of a sliding "hydroplate" (in this case, the plate carrying North America) is consistent with Dr. Brown's theory that the great plates slid rapidly during the events surrounding a catastrophic rupture of the earth's surface and flood, building up tremendous friction and melting rock into magma especially along their forward edges. For a previous discussion of that concept, see "Ancient volcanic activity in the Mojave Desert."

The playas themselves are examples of geographic features in which runoff is limited but evaporation is high, creating conditions in which the floodwaters were trapped and slowly evaporated away, leaving high salt concentrations behind. As Dr. Brown explains on this page in his book, after the flood event:
Drainage of the waters that covered the earth left every continental basin filled to the brim with water. Some of these postflood lakes lost more water by evaporation and seepage than they gained by rainfall and drainage from higher elevations. Consequently, they shrank over the centuries. A well-known example was former Lake Bonneville, part of which is now the Great Salt Lake.
The playas of the Mojave are smaller examples of the same phenomenon.

The "sailing stones" of Racetrack Playa are a fascinating natural mystery, and one that may be solved soon. Then again, perhaps these silent travelers will continue to keep their secrets for many more thousands of years.