Sailing stones
Encyclopedia
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Racetrack Playa
The Racetrack Playa, or The Racetrack, is a scenic dry lake feature with "sailing stones" that leave linear "racetrack" imprints. It is located above the northwestern side of Death Valley, in Death Valley National Park, Inyo County, California, U.S.....
, Death Valley
Death Valley
Death Valley is a desert valley located in Eastern California. Situated within the Mojave Desert, it features the lowest, driest, and hottest locations in North America. Badwater, a basin located in Death Valley, is the specific location of the lowest elevation in North America at 282 feet below...
, where the number and length of travel grooves are notable. The force behind their movement is not confirmed and is the subject of research.
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 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 the direction it came from. Trail length also varies – two similarly sized and shaped rocks may travel uniformly, then one could move ahead or stop in its track.
Description
Dolomite
Dolomite is a carbonate mineral composed of calcium magnesium carbonate CaMg2. The term is also used to describe the sedimentary carbonate rock dolostone....
on the south end of the playa, but some are intrusive igneous rock
Igneous rock
Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava...
from adjacent slopes (most of those being tan-colored feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
-rich syenite
Syenite
Syenite is a coarse-grained intrusive igneous rock of the same general composition as granite but with the quartz either absent or present in relatively small amounts Syenite is a coarse-grained intrusive igneous rock of the same general composition as granite but with the quartz either absent or...
). Tracks are often tens to hundreds of feet long, about 3 to 12 in (7.6 to 30.5 cm) wide, and typically much less than an inch (2.54 cm) deep.
A balance of specific conditions are thought to be needed for stones to move:
- a saturated yet non-flooded surface,
- a thin layer of clay,
- very strong gusts as initiating force, and
- strong sustained wind to keep stones going
Research history
National Park Service
The National Park Service is the U.S. federal agency that manages all national parks, many national monuments, and other conservation and historical properties with various title designations...
later wrote more detailed descriptions and Life
Life (magazine)
Life generally refers to three American magazines:*A humor and general interest magazine published from 1883 to 1936. Time founder Henry Luce bought the magazine in 1936 solely so that he could acquire the rights to its name....
magazine featured a set of photographs from the Racetrack. 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 wind
Wind
Wind is the flow of gases on a large scale. On Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space...
s 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
Ice
Ice is water frozen into the solid state. Usually ice is the phase known as ice Ih, which is the most abundant of the varying solid phases on the Earth's surface. It can appear transparent or opaque bluish-white color, depending on the presence of impurities or air inclusions...
sheets around the stones either help to catch the wind or move in ice floes.
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 (0.45359237 kg) track-making stone with seven rebar
Rebar
A rebar , also known as reinforcing steel, reinforcement steel, rerod, or a deformed bar, is a common steel bar, and is commonly used as a tensioning device in reinforced concrete and reinforced masonry structures holding the concrete in compression...
segments placed 25 to 30 in (63.5 to 76.2 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.
Karen (stone J) is a 29 by block of dolomite and weighs an estimated 700 lbs. Karen did not move during the monitoring period. The stone may have created its 570 foot long 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 a truck and winch would have caused. A possible sighting of Karen was made in 1994 a half mile 0.5 miles (804.7 m) from the playa. Karen was rediscovered by San Jose geologist Paula Messina in 1996.
Professor John Reid led six research students from Hampshire College
Hampshire College
Hampshire College is a private liberal arts college in Amherst, Massachusetts. It was founded in 1965 as an experiment in alternative education, in association with four other colleges in the Pioneer Valley: Amherst College, Smith College, Mount Holyoke College, and the University of Massachusetts...
and the University of Massachusetts
University of Massachusetts
This article relates to the statewide university system. For the flagship campus often referred to as "UMass", see 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 1992-1993. 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 0.5 miles (804.7 m) wide. Physical evidence included swaths of lineated areas that could only have been created by moving thin sheets of ice. So wind alone and wind in conjunction with ice floes 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 layer
Boundary layer
In physics and fluid mechanics, a boundary layer is that layer of fluid in the immediate vicinity of a bounding surface where effects of viscosity of the fluid are considered in detail. In the Earth's atmosphere, the planetary boundary layer is the air layer near the ground affected by diurnal...
s (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). This means that stones just a few inches high feel the full force of ambient winds and their gusts, which can reach 90 miles per hour (144.8 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 favoured 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.
A study published in 2011 postulated that small rafts of ice form around the rocks and when the local water level rises, 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 by 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.
External links
- The Sliding Rocks of Racetrack Playa by Paula Messina
- The Mystery of the Rocks on the Racetrack at Death Valley by Lena Fletcher and Anne Nester
- http://www.physicsforums.com/showthread.php?p=437132
- Moving Rocks of Death Valley's Racetrack Playa by Brian Dunning
- Why Are Death Valley's Rocks Moving Themselves? by Philip Schewe (FoxNews)