The sailing stones (sliding rocks, moving rocks) are a geological phenomenon where rocks move in long tracks along a smooth valley floor without human or animal intervention. They have been recorded and studied in a number of playas around Racetrack Playa, Death Valley, where the number and length of travel grooves are notable. The force behind their movement is not understood and is subject to research.
Racetrack stones only move 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 wander. Stones sometimes turn over, exposing another edge to the ground and leaving a different track in the stone's wake.
Sliding rock trails fluctuate in direction and length. Some rocks which start next to each other start out travelling parallel, but one may abruptly change direction to the left, right, or even back the direction it came from. Length also varies because two similarly size and shaped rocks could travel uniform, then one could burst ahead or stop dead in its track.
Speed is an unknown variable. Since these stones are rarely transported and nobody has witnessed the movement, the speeds the rocks travel at are not known.
Most of the so-called gliding stones originate from an 850 foot (260 m) high hillside made of dark dolomite on the south end of the playa, but some are intrusive igneous rock from adjacent slopes (most of those being tan-colored feldspar-rich syenite). Tracks are often tens to hundreds of feet (low to high tens of meters) long, a few to 12 inches (8 to 30 cm) wide, and typically much less than an inch (2.5 cm) deep.
A balance of specific conditions are thought to be needed for stones to move:
- A saturated yet non-flooded surface,
- Thin layer of clay,
- Very strong gusts as initiating force,
- Strong sustained wind to keep stones going.
Geologists Jim McAllister and Allen Agnew mapped the bedrock of the area in 1948 and made note of the tracks. Naturalists from the National Park Service later wrote more detailed descriptions and Life magazine featured a set of photographs from The Racetrack. Speculation about how the stones may 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. They maintain that ice sheets around the stones either help to catch the wind or move in ice flows.
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 flow hypothesis by corralling selected stones. A corral 5.5 feet (1.7 m) in diameter was made around a 3 inch (7.5 cm) wide, 1 pound (0.5 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 flows, 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.
Ten of the initial twenty-five stones moved in the first winter with Mary Ann (stone A) covering the longest distance at 212 feet (64.5 m). Two of the next six monitored winters also saw multiple stones move. No stones were confirmed to have moved in the summer and some winters none or only a few stones moved. In the end all but two of the thirty monitored stones moved during the seven year study. At 2.5 inches (6.5 cm) in diameter Nancy (stone H) was the smallest monitored stone. It also moved the longest cumulative distance, 860 feet (262 m), and the greatest single winter movement, 659 feet (201 m). The largest stone to move was 80 pounds (36 kg).
Karen (stone J) is a 29 by 19 by 20 inch (74 by 48 by 51 cm) block of dolomite and weighs an estimated 700 pounds (about 320 kg). Perhaps not surprisingly Karen didn't move during the monitoring period. The stone may have created its 570 straight and old track from momentum gained from its initial fall onto the wet playa. However, Karen disappeared sometime before May 1994, possibly during the unusually wet winter of 1992 to 1993. Removal by artificial means is considered unlikely due to the lack of associated damage to the playa that the needed truck and winch would have done. A possible sighting of Karen was made in 1994 a half mile (800 m) from the playa.
Professor John Reid led six research students from Hampshire College and the University of Massachusetts 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 1992-1993. At least some stones were proved beyond a reasonable doubt to have been moved in ice flows 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. So wind alone as well as in conjunction with ice flows are thought to be motive forces.
Physicists studying the phenomenon in 1995 found that winds blowing on playa surfaces can be compressed and intensified. 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 cm). This means that stones just a few inches high feel the full force of ambient winds and their gusts, which can reach 90 mph (145 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).
Wind and ice both are the favored hypothesis for these mysterious sliding rocks. Noted in Don J. Easterbrook's "Surface Processes and Landforms", he mentioned that because of the lack of parallel paths between some rock paths, this could be caused by the breaking up of ice resulting in alternate routes. Even though the ice breaks up into smaller blocks, it is still necessary for the rocks to slide.
Friday, September 25, 2009
All I Ask Is A Tall Rock And A Star To Sail Her By
Posted by M. Christian