Smooth Sailing Stones 

Imagine a stationary boulder of rock on a flat land sliding along a smooth valley floor independent of any human or animal involvement. This bizarre concept of self-moving boulders poses the question, “How could this phenomenon possibly occur?”. This extravagant geologic phenomenon is referred to as sailing stones. For a century, these eerie rocks and their long trails have puzzled scientists and tourists worldwide. These boulders of rock, composed of syenite, dolomite, and black dolomite, appear to move on their own, leaving a graceful and long trail behind them as evidence of movement (Stanley, 1955; Figure 1). However, understanding this process has been anything but smooth sailing.

 It has been observed that tracks of sailing stones are situated in Little Bonnie Claire Playa in Nevada but most prevalently, in Racetrack Playa, Death Valley in California. Averaging approximately 0.5 meters in diameter, these boulders leave a trail measuring tens to hundreds of meters behind them. In order for the stones to move smoothly in a path, there are specific conditions of the surrounding area that must be considered such as, presence of ice, a thin layer of clay, strong wind forces, and a saturated yet non-flooded surface (Lorenz et al., 2011). A theory, referred to as the “Ice Raft” Theory, was proposed discussing that small “rafts” of ice form around the surface of the rocks therefore reducing the forces of friction enabling these rocks to slide off of the surface of a valley floor when large gusts of wind blow (Kirk, 1952). The wind is considered as an initiating force that plays a factor in helping the rocks to be displaced.

In support of the “Ice Raft” Theory, it was found that the valley in which these rocks are moving along was once covered by an intermittent lake where water from spring systems are drained towards the underground. This suggests the idea that it is the water that buoyantly lifts the frozen bedrock along with the strong-blowing winds that cause the movement of the rocks to occur (Lorenz et al., 2011).

     This sliding rock phenomenon is currently an ongoing field of research and scientists have documented rock movement through multiple observations of move events. However, what they saw was completely different than the Ice Raft Theory. Instead of frozen water pushing the boulders to move, it was the thawing and breaking up of large ice sheets during warmer and sunny days (Norris et al., 2014). Rocks moving through these thin floating ice panels were observed by attaching a GPS unit on the rock itself (shown in Figure 2), and it was seen that they are driven by light winds that then thrust rocks at up to four to five meters per minute.

     Evidently, this geologic phenomenon is very remarkable and it is impressive that natural forces are responsible for these sailing stones. One day they may be at one spot and the next day they would have moved meters away from their original location. This continuing field of study provides researchers and scientists further explanation as to how these rocks “sail”.

References:

• Kirk, L. (1952). Trails and Rocks Observed on a Playa in Death Valley National Monument, California.SEPM Journal of Sedimentary Research, Vol. 22(22 (3), pp.173-181.

• Lorenz, R., Jackson, B., Barnes, J., Spitale, J. and Keller, J. (2011). Ice rafts not sails: Floating the rocks at Racetrack Playa. American Journal of Physics, 79(1), p.37.

• Norris, R., Norris, J., Lorenz, R., Ray, J. and Jackson, B. (2014). Sliding Rocks on Racetrack Playa, Death Valley National Park: First Observation of Rocks in Motion. PLoS ONE, 9(8), p.e105948.

• Stanley, G. (1955). Origin of Playa Stone Tracks, Racetrack Playa, Inyo County, California. Geological Society of America Bulletin, 66(11), pp.1329-1350.

• Unexplained Mysteries, (2014). Death Valley’s sailing stones mystery solved – Unexplained Mysteries. [online] Available at: http://www.unexplained-mysteries.com/news/271569/death-valleys-sailing-stones-mystery-solved [Accessed 15 Mar. 2015].