Racetrack Playa valley. Photo by Jon Sullivan. Click on the image to see the original image at Wikimedia Commons.
Racetrack Playa is a plain without vegetation of a dry located above the northwestern side of Death Valley, in Death Valley National Park, Inyo County, CA, USA (click here to see in Google Maps). Although “playa” is the Spanish word for beach, it is also used in English to refer to a dry lake. Racetrack Playa occupies an area of 4.5 km (north-south) by 2 km (east-west) which is 1,130 m above sea level between the Cottonwood Mountains and Last Chance Range. The surface is extremely flat and dry for most of the year, when the surface is covered with small hexagonal mud curls, although floods partially during the rainy season forming a shallow lake that evaporates quickly. In winter it forms a relatively thick layer of ice.
The sailing stones
Despite its geomorphological and environmental interest, Racetrack Playa is known around the world for the phenomenon of sliding rocks or sailing stones, because on the surface of the basin appear scattered stones leaving a trail behind him, so it seems that something or someone had dragged over the surface of the ground without anyone’s seen them move ever. The phenomenon is so striking that it has been “investigated” by pseudoscientists who have attributed the movement of the stones to energy phenomena, gravity field anomalies, extraterrestrial activity and other funny hypotheses.
Sailing stone in Racetrack Playa. Photo by Laurence G. Charlot. Click on the image to see the original picture at Wikimedia Commons.
Looking for answers
The first scientific approaches to the study of this geomorphological process suggested the hypothesis that wind was the main cause of stone movements. Louis G. Kirk, a National Park Service Ranger speculated that local strong winds caused the movement of stones over the muddy surface after heavy rainfall. An experiment was conducted by Jim McAllister and Allen Agnew (USGS) in 1948, who had the idea that the movement of the stones was due to strong winds blowing over the flooded surface. The two researchers flooded a small part of the plain and a used an aeroengine to create a strong air flow to move the stones, but failed to replicate the natural result. Moreover, local winds can reach 150 kilometers per hour, but not enough to move some of the stones, which may weigh hundreds of kilograms in some cases.
Stones with divergent trajectories. Photo by Daniel Mayer. Click on the image to see the original picture at Wikimedia Commons.
During the following decades, the researchers could not explain the nature of this phenomenon, although it was suggested a possible link with the ice layer formed on the lake at certain times of the year. John Reid (Hampshire College) and his team also reported that wind alone is not enough to move stones, and hypothesized that the ice layer was pushed by the wind during the winter, dragging the stones. But Paula Messina (San José State University) analyzed the trajectory of different stones using GPS. She noted that some trails were linear, suggesting the influence of wind, but other are curve or irregular. She estimated that wind velocity needed for stones to move this way was of several hundred kilometers per hour. You can visit Paula’s website for more complete information.
Undecided sailing stone. Photo by Jon Sullivan. Click on the image to see the original image at Wikimedia Commons.
Ice and wind
More recently, Ralph Lorenz (Johns Hopkins University) and his team replicated the phenomenon in a very simple way. He realized that in some cases, rocks contrails direction abruptly changed when crossed with each other, as if the rocks had hit and taken different directions. The only way this happens is that there is a mass of ice around each rock upon impact with another, without stopping deviate due to the low coefficient of friction of ice. He tested his theory in his own house with stones, the freezer and a couple of tupperwares… and stones moved!
Details of Ralph Lorenz’s home experiment (Lorenz et al., 2008). Click to see larger image.
Lorenz’s team suggested that the movement of stones in Racetrack Playa is due to the effect of weak winds on buoyant stones that are included in “ice cakes”, as also occurs in arctic tidal beaches. Ice cakes allow the stones to move over the flooded bed.
Stones arrive at the playa from the slopes around or by other processes. During the rain, water has no outlet possible, so that it accumulates and the area is flooded. If the temperature is low enough, a layer of ice is formed on the surface of liquid water. The stones partially embedded in the floating ice rise slightly above the bottom with the increasing level of water. Both the friction between the ice and water and between the stones and the bed are very small, so that blowing wind with some intensity pushes the ice (and the rocks embedded). If the stones and mud at the bottom have a light touch, the dragged stones leave a trail that remains once the ice has melted and the water has evaporated.
Why moving stones have not been observed? According to Lorenz, “movement happens for only tens of seconds, at intervals spaced typically by several years”, and “this would demand exceptional patience as well as luck” (see comments here). So, the rocks are probably traveling on the coldest and windiest days that occur over a period of several years. The most likely time would be in the very early dawn. Do you dare?
Know more
Bacon, D., Cahill, T. and Tombrello, T.A. 1996. Sailing Stones on Racetrack Playa. Journal of Geology 104: pp.121-125.
Lorenz RD, Jackson BK, Barnes JW, Spitale J, Keller, JM. 2011. Ice rafts not sails: Floating the rocks at Racetrack Playa. American Journal of Physics 79: 37.
Kirk LG. 1952. Trails and rocks observed on a playa in Death Valley National Monument, California. Journal of Sedimentary Petrology 22: 173-181.
Messina P, Stoffer P. 1999. Differential GPS/GIS analysis of the sliding rock phenomenon of Racetrack Playa, Death Valley National Park. In: Slate JL (Ed.), Proceedings of Conference on Status of Geologic Research and Mapping, Death Valley National Park. US Geological Survey Open File Report 99-153: 107-109.
Reid JB, Bucklin EP, Copenagle L, Kidder J, PackSM, Polissar PJ, Williams ML. 1995. Sliding rocks at the Racetrack, Death Valley: What makes them move? Geology 23: 819-822.
Sharp WE. 1960. The movement of playa scrapers by wind. Journal of Geology 68: 567-572.
Sharp RP, Carey DL. 1976. Sliding Stones, Racetrack Playa, California. Geological Society of America Bulletin 87: 1704-1717.
Sharp RP, Carey DL, Reid JB Jr, Polissar PJ, Williams ML. 1996. Sliding rocks at the Racetrack, Death Valley: What makes them move: comment and reply. Geology 24: 766-767.
This post was also published simultaneously in G-Soil.
