Moving stones

Sometimes stones change their direction of motion

Most of the sliding stones fall to the bottom of a dried-up lake from a dolomite hill 260 m high, located on the southern limb of Raistraek Playa. The mass of stones reaches several hundred kilograms. The tracks that follow them have a length of several tens of meters, a width of 8 to 30 cm and a depth of less than 2.5 cm.

Stones move only once in two or three years, with most of the tracks lasting 3-4 years. Stones with a ribbed bottom surface leave more direct traces, and stones lying on the flat side wander from side to side. Sometimes stones turn over, which affects the size of their footprint.

Until the beginning of the 20th century, the phenomenon was explained by supernatural forces, then during the formation of electromagnetism an assumption arose about the effect of magnetic fields, which, in general, did not explain anything.

In 1948, geologists Jim McAllister and Allen Agnew mapped the location of the stones and marked their tracks. A little later, employees of the US National Park Service drew up a detailed description of the place and Life magazine published photos from Raistrake Playa, after which attempts to explain the movement of stones began. Most hypotheses agreed that the wind, with the wet bottom of the lake, at least partially explains the phenomenon. In 1955, geologist George Stanley from the University of Michigan published an article stating that the stones were too heavy and the local wind was not able to move them. He and his co-author proposed a theory according to which, during the seasonal flooding of a dried lake on the water, an ice crust is formed that promotes the movement of stones.

Sharp and Carey Studies

In May 1972, Robert Sharp ( California Institute of Technology ) and Dwight Carey ( University of California, Los Angeles ) launched a stone tracking program. Thirty stones with relatively fresh tracks were marked, and their initial position indicated by pegs. For 7 years of research, scientists have developed a theory according to which water that accumulates during the rainy season in the southern part of the lake is carried by the wind along the bottom of a dried lake and moistens its surface. As a result, hard clay soil becomes very soft and the coefficient of friction decreases sharply, which allows the wind to budge even one of the largest stones (it was called Karen) weighing about 350 kg.

Ice hypotheses have also been tested. Water moved by the wind can be covered with an ice crust at night, into which stones located on the water path freeze. The ice around the stone could increase the cross section for interaction with the wind and help move the stones along the streams of water. As an experiment, around a stone with a width of 7.5 cm and a mass of 0.5 kg, a corral with a diameter of 1.7 m was created with a distance between the fence supports of 64 to 76 cm. If a layer of ice formed around the stones, then it could catch on the support during movement fence and slow down the movement or change the trajectory that would be reflected in the trail of stone. However, no such effects were observed - in the first winter, the stone passed near the fence support, moving to the northwest beyond the fenced area 8.5 meters. The next time, two heavier stones were placed inside the pen - one of them moved in the same five years later direction as the first, but the second for the period of research did not budge. This suggests that the ice crust affects the movement of stones only if it is small.

Ten of the marked stones moved into the first winter of exploration, and stone A (which was called Mary Ann) crawled 64.5 m. It was noted that many stones also moved in the next two winter periods, and in the summer and in other winters they stood still. After 7 years, only two of the 30 observed stones did not change their location. The smallest of the stones (Nancy) was 6.5 cm in diameter, and it moved to a maximum total distance of 262 m, and then to 201 m in just one winter. The most massive stone, whose movement was recorded, weighed 36 kg.

Further research

In 1993, Paula Messina, California State University of San Jose, defended a thesis on moving stones, which showed that, in general, stones did not move in parallel. According to the researcher, this confirms that the ice does not contribute to movement. After studying the coordinate changes of 162 stones (which were carried out using GPS ), it was determined that neither the size nor their shape affect the movement of the boulders. It turned out that the nature of the movement is to a large extent determined by the position of the boulder on Raistrake Playa. According to the created model, the wind above the lake behaves in a very complex way, in the center of the lake even forming a whirlwind .

In 1995, a group led by Professor John Reid noted that the traces of the winter of 1992–1993 were very similar to those of the late 1980s. It was shown that at least some stones moved with streams of ice-covered water, and the width of the ice crust was about 800 m, as evidenced by characteristic traces scratched by a thin layer of ice. It was also determined that the boundary layer , in which the wind slows down due to contact with the ground, on such surfaces can be only 5 cm, which means the possibility of winds (whose speed in winter reaches 145 km / h) even on very low stones.

In 2014, a work was published in PLOS , the authors of which describe the mechanism of movement of stones. Scientists placed several of their stones weighing 5-15 kg at the bottom of the lake, providing them with navigation sensors and surrounding cameras. The reason for the movement was large (tens of meters), but thin (3–6 mm) sections of ice formed after freezing in the previous frosty nights ^[1] . This floating ice, carried away by the wind and the ice current, transported stones at a speed of 2–5 m / min.

• Messina, P., 1998, The Sliding Rocks of Racetrack Playa, Death Valley National Park, California : Physical and Spatial Influences on Surface Processes // Department of Earth and Environmental Sciences, City University of New York, New York, 1998.

• The Sliding Rocks of Racetrack Playa