SAILING STONES
Sailing stones, sliding rocks, rolling stones, and moving
rocks are a geological phenomenon where rocks move and inscribe long tracks
along a smooth valley floor without human or animal intervention.
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.
Trails 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.
The Racetrack’s stones speckle the
playa floor, predominately in the southern portion. Historical accounts
identify some stones hundreds of feet from shore, yet most of the stones are
found relatively close to their respective originating outcrops. Three
lithologic types are identified: (1) syenite, found most abundant on the west
side of the playa; (2)dolomite, subrounded blue-gray stones with white bands;
and (3) black dolomite, the most common type, found almost always in angular
joint blocks or slivers. This dolomite composes nearly all stones found in the
southern half of the playa, and originates at a steep promontory,
850 ft-high (260 m), paralleling the east shore at the south end of
the playa. Intrusive igneous rock originates from adjacent slopes (most of
those being tan-colored feldspar-rich syenite). Tracks are often tens to
hundreds of feet long, about 3 to 12 inches (8 to 30 cm) wide, and
typically much less than an inch (2.54 cm) deep. Most moving stones range
from about six to 18 inches in diameter.
A balance of very specific
conditions is 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
- strong sustained wind to keep stones going
And in some hypotheses:
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 revar 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.
A panorama of the Milky Way with the
tracks of sailing stones below. Notice the stone on the right side.
Ten of the initial twenty-five
stones moved in the first winter with Mary Ann (stone A) covering the longest
distance at 212 feet (65 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 (6.4 cm)
in diameter, Nancy (stone H) was the smallest monitored stone. It also moved
the longest cumulative distance, 860 feet (260 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
inches (74 by 48 by 51 cm) block of dolomite and weighs an estimated 700
pounds (318 kg). Karen did not move during the monitoring period. The
stone may have created its 570 feet (170 m) 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 (800 m)
from the playa. Karen was rediscovered by San Jose geologist Paula Messina in
1996.
Continued research in the 1990s
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.
Another sailing stone in Racetrack Playa.
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).
Wind and ice both are the favored hypothesis for these sliding rocks. Noted
in "Surface Processes and Landforms", Don J. Easterbrook mentions
that because of the lack of parallel paths between some rock paths, this could
be caused by degenerating ice floes resulting in alternate routes. Even though
the ice breaks up into smaller blocks, it is still necessary for the rocks to
slide.
Twenty-first
century developments
Further understanding of the geologic processes at work in Racetrack Playa
goes hand in hand with technological development. In 2009, development of
inexpensive time-lapse digital cameras allowed the capturing of transient
meteorological phenomena including dust devils and playa flooding. These
cameras were aimed at capturing various stages of the previously mentioned
phenomena, though discussion of the sliding stones ensued. The developers of
photographic technology describe the difficulty of capturing the Racetrack’s
stealthy rocks, as movements only occur about once every three years and, they
believed, lasted approximately ten seconds. Their next identified advancement
was wind-triggered imagery, vastly reducing the ten million seconds of non-transit
time they had to sift through.
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.
Explanation
A rock with a GPS unit inside a cavity bored into its top.
Based on a study published in the on-line journal PLOS-ONE in August 2014,
news articles reported the mystery solved when researchers observed rock
movements using GPS and time-lapse photography. The research team, led by
cousins James and Richard Norris witnessed and documented 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.
These observations contradicted earlier hypotheses of winds or thick ice
floating rocks off the surface. Instead, rocks move when large ice sheets a few
millimeters thick floating in an ephemeral winter pond start to break up during
sunny days. These thin floating ice panels, frozen during cold winter nights,
are driven by light winds and shove rocks at up to five meters per minute (~0.3
km/h). Some GPS measured moves lasted up to 16 minutes, and a number of stones
moved more than 5 times during the existence of the playa pond in the winter of
2013-14
Experiments
Aquarium experiment
Ralph Lorenz, a NASA scientist, investigated the phenomenon in 2006. To
illustrate the "ice raft" theory, Lorenz developed an experiment
using a kitchen-table model using a Tupperware container to show how heavy
rocks might glide across the surface of the lake bed. A bed of sand is added to
the bottom of the Tupperware, a rock placed on the sand, and water added until
there is only a small edge of the rock sticking out. After putting the
container in the freezer until the water is frozen, then removing the container
and letting the ice begin to melt, Lorenz could end up with a small raft of
floating ice with a rock embedded in it. All he had to do was gently blow on
the floating ice sheet to get the rock to drag across the sand. Gunther
Kletetschka with his NASA's academy students showed the process of lifting a
small stone in a thin aquarium and captured the lifting process on camera.
