Mysterious Nature: The Sailing Stones

A panorama of the Milky Way with the tracks of sailing stones below (Dan Duriscoe)

Sailing stones, sliding rocks, and moving rocks all refer to a geological phenomenon where rocks move and inscribe long tracks along a smooth valley floor without human or animal intervention.

The stones move only every two or three years and most tracks develop over three or four years. Stones with rough bottoms leave straight striated tracks while those with smooth bottoms tend to wander. Stones sometimes turn over, exposing another edge to the ground and leaving a different track in the stone's wake.

Trails differ in both direction and length. Rocks that start next to each other may travel parallel for a time, before one abruptly changes direction to the left, right, or even back to the direction from which it came. Trail length also varies – two similarly sized and shaped rocks may travel uniformly, then one could move ahead or stop in its track.

Tracks of sliding rocks have been observed and studied in various locations, including Little Bonnie Claire Playa in Nevada, and most notably Racetrack Playa, Death Valley National Park, California, where the number and length of tracks are notable. At Racetrack Playa, these tracks have been studied since the early 1900s, yet the origins of stone movement were not confirmed and remained the subject of research for which several hypotheses existed. However, as of August 2014, timelapse video footage of rocks moving has been published, showing the rocks moving at low wind speeds within the flow of thin, melting sheets of ice. The scientists have thus identified the cause of the moving stones to be ice shove.  

Research History

A sailing stone in Racetrack Playa
Early Investigation

The first documented account of the sliding rock phenomenon dates to 1915, when a prospector named Joseph Crook from Fallon, Nevada visited the Racetrack Playa site. In the following years, the Racetrack sparked interest from geologists Jim McAllister and Allen Agnew, who mapped the bedrock of the area in 1948 and published the earliest report about the sliding rocks in a Geologic Society of America Bulletin. Their publication gave a brief description of the playa furrows and scrapers, stating that no exact measurements had been taken and suggesting that furrows were the remnants of scrapers propelled by strong gusts of wind – such as the variable winds that produce dust-devils – over a muddy playa floor. Controversy over the origin of the furrows prompted the search for the occurrence of similar phenomena at other locations. Such a location was found at Little Bonnie Claire Playa in Nye County, Nevada, and the phenomenon was studied there as well.

Naturalists from the National Park Service later wrote more detailed descriptions and Life magazine featured a set of photographs from the Racetrack. In 1952, a National Park Service Ranger named Louis G. Kirk recorded detailed observations of furrow length, width, and general course. He sought simply to investigate and record evidence of the moving rock phenomenon, not to hypothesize or create an extensive scientific report. Speculation about how the stones move started at this time. Various and sometimes idiosyncratic possible explanations have been put forward over the years that have ranged from the supernatural to the very complex. Most hypotheses favored by interested geologists posit that strong winds when the mud is wet are at least in part responsible. Some stones weigh as much as a human, which some researchers, such as geologist George M. Stanley, who published a paper on the topic in 1955, feel is too heavy for the area's wind to move. After extensive track mapping and research on rotation of the tracks in relation to ice floe rotation, Stanley maintained that ice sheets around the stones either help to catch the wind or that ice floes initiate rock movement.

Progress In The 1970s

Bob Sharp and Dwight Carey started a Racetrack stone movement monitoring program in May 1972. Eventually thirty stones with fresh tracks were labeled and stakes were used to mark their locations. Each stone was given a name and changes in the stones' position were recorded over a seven-year period. Sharp and Carey also tested the ice floe hypothesis by corralling selected stones. A corral 5.5 feet (1.7 m) in diameter was made around a 3 inches (7.6 cm) wide, 1 pound (0.45 kg) track-making stone with seven rebar segments placed 25 to 30 inches (64 to 76 cm) apart. If a sheet of ice around the stones either increased wind-catching surface area or helped move the stones by dragging them along in ice floes, then the rebar should at least slow down and deflect the movement. Neither appeared to occur; the stone barely missed a rebar as it moved 28 feet (8.5 m) to the northwest out of the corral in the first winter. Two heavier stones were placed in the corral at the same time; one moved five years later in the same direction as the first but its companion did not move during the study period. This indicated that if ice played a part in stone movement, then ice collars around stones must be small.

Racetrack Playa (Pirate Scott)
Continued Research

Professor John Reid led six research students from Hampshire College and the University of Massachusetts Amherst in a follow-up study in 1995. They found highly congruent trails from stones that moved in the late 1980s and during the winter of 1992–93. At least some stones were proved beyond a reasonable doubt to have been moved in ice floes that may be up to half a mile (800 m) wide. Physical evidence included swaths of lineated areas that could only have been created by moving thin sheets of ice. Consequently, both wind alone and wind in conjunction with ice floes are thought to be motive forces.

Physicists Bacon et al. studying the phenomenon in 1996, informed by studies in Owens Dry Lake Playa, discovered that winds blowing on playa surfaces can be compressed and intensified because of a playa's smooth, flat surfaces. They also found that boundary layers (the region just above ground where winds are slower due to ground drag) on these surfaces can be as low as 2 inches (5.1 cm). As a result, stones just a few inches high feel the full force of ambient winds and their gusts, which can reach 90 miles per hour (140 km/h) in winter storms. Such gusts are thought to be the initiating force while momentum and sustained winds keep the stones moving, possibly as fast as a moderate run (only half the force required to start a stone sailing is needed to keep it in motion).

In a study published in 2011 it was postulated that small rafts of ice form around the rocks and the rocks are buoyantly floated off the soft bed thus reducing the reaction and friction forces at the bed. Since this effect depends on reducing friction, and not on increasing the wind drag, these ice cakes need not have a particularly large surface area if the ice is adequately thick, as the minimal friction allows the rocks to be moved by arbitrarily light winds.

Reinforcing the "ice raft" theory, a research study published in 2013 pointed out narrowing trails, occurrence of intermittent spring systems, and absence of rocks at the end of the trails. The study identified the Racetrack mountain area that drains water towards the Racetrack playa while ice covered the intermittent lake. This suggests that it is this water that buoyantly lifts the icebergs with embedded rocks until friction with the playa bed is reduced sufficiently for wind forces to move them and cause the observed tracks. The study also provides mapping and analysis of the effect of artificial ditch preventing the visitors from driving on the playa and they claim that it may interfere with the sliding rock phenomenon.

Tracks are sometimes non-linear (Jon Sullivan)

Based on a study released in August 2014, news articles reported the mystery solved when researchers observed rock movements using GPS and time-lapse photography. They witnessed and documented a rock movement on December 20, 2013 that involved more than 60 rocks, with some rocks moving up to 224 meters between December 2013 and January 2014 in multiple move events. This study contradicted earlier hypotheses of winds or thick ice floating rocks off the surface. Instead, rocks move when large ice sheets just a few millimeters thick start to melt during periods of light wind. These thin floating ice panels push rocks up to five meters per minute. 


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