When Racetrack Playa, near Death Valley is dry, an assortment of rocks can be seen with movement tracks inscribed in the playa floor. The stones have become known by several names, including ‘sailing’ stones, racing stones, roving rocks and their scientific title – peripatetic rocks (Dell’Amore, 2010).
The tracks have been observed and studied since the early 1900s, but no one has seen the rocks moving. Racetrack stones move only once every two or three years and most tracks last for around four years (Reid et al, 1995). These stones are thought to have been moved without animal or human influence.
Racetrack Playa is located in Death Valley National Park, Inyo County, California, USA. The playa is at an elevation of 1,130m above sea level and is 4.5km long (north-south) by 2.1km wide. The playa is flat, with the northern end only 4cm higher than the southern. During periods of sustained rain, water washes down from the surrounding mountains and drains into the playa, temporarily forming a shallow, endorheic lake (Kletetschka et al, 2013).
Multiple theories surround the movement of the rocks, with bizarre theories such as vibrations and algal undercoating’s being less credible. Two of the more scientifically supported hypotheses are described below.
WATER & WIND:
The role of wind in the movement of rocks on the playa has been studied in great detail by many scientists. But to suggest that wind alone transports the rocks across Racetrack Playa is to greatly over-estimate the wind speeds in the local area. Bacon et al (1996) reject the hypothesis on the fact that the local wind speeds are too weak and the playa surface is too rough (after Sharp, 1960).
However, numerous papers support the hypothesis that the sailing stones are moved by strong winter winds after periods of heavy rain (Sharpe and Carey, 1976). Wetting of the playa surface is required to the extent that a thin, slimy, water-saturated mud layer overlies a still firm base. Strong winds then push the rocks over the slippery surface, creating the tracks in the mud. The prevailing southwest winds blow to the northeast and most of the rock trails are parallel to this direction, leading to popular support of this hypothesis.
ICE & WIND:
Although Racetrack Playa is found within Death Valley, an arid region which experiences temperature highs of up to 40°C during the day, ice may still form on the playa. The seasonal flooding of the playa creates a shallow lake which may be cooled during the night and could experience freezing due to the shadows of nearby high mountains. Stanley (1955) noted similarities in the signatures of neighbouring tracks and with survey data concluded that rocks were moved within an ice sheet floating atop a shallow layer of water.
The theory is as follows: as water accumulates, strong winds blow thin sheets of water quickly over the flat surface of the playa. Sheets of ice form on the surface as temperatures fall below freezing and rocks are locked into the ice. Wind then drives the floating ice along with the rocks over the surface of the playa. More recently Kletetschka et al, (2013) proposed that after a period of precipitation followed by below freezing temperatures, ice collars form around the rocks. As these collars grow they exert a buoyancy force on the rocks. Eventually the ice becomes thick enough to lift the ice and the attached rocks.
The formation of ‘ice rafts’ reduces friction between the rocks and the sediment below – allowing modest winds to transport the rocks. These rafts scrape the playa surface thus inscribing the trails which are visible when the playa is dry.
There appears to be no general consensus on how the roving rocks of racetrack playa actually move. There are teams of people who are attempting to catch the stones moving on camera in order to better ascertain the forces at play.
See the rocks at Racetrack Playa (remaining stationary) in this time-lapse video:
Bacon, D., Cahill, T., and Tombrello, A. (1996) ‘Sailing Stones on Racetrack Playa’, Journal of Geology, 104: 1, pp.121 – 125.
Dell’Amore, C. (2010) ‘Pictures: What drives Death Valley’s Roving Rocks?’ Available at: http://news.nationalgeographic.com/news/2010/09/photogalleries/100908-racetrack-rock-mystery-moving-pictures/.
Kletetschka, G. Hooke, R.L. Ryan, A. Fercana, G. McKinney, E. (2013). Sliding stones of Racetrack Playa, Death Valley, USA: The roles of rock thermal conductivity and fluctuating water levels. Geomorphology Volume 195: pp. 110–117
Reid, B.J. Edward, P.B. Copenagle, L. Kidder, J and Pack, S.M. (1995). Sliding rocks at the Racetrack, Death Valley: What makes them move? Geology. Volume 23: pp. 819–822
Sharpe, P.R. and Carey L.D. (1976). Sliding stones, Racetrack Playa, California. Geological Society of America Bulletin. Volume 87: pp. 1704-1717
Stanley, G.M. (1955). Origin of playa stone tracks, Racetrack Playa, Inyo County, California. Bulletin of the Geological Society of America Volume 66 (11): pp. 1329–1350
Written by Daniel Chaplin and Ciaran Nash, Undergraduate Students, Geography, Trinity College Dublin.