In the Death Valley National Park, in California’s Mojave Desert, one of the strangest geological phenomena known can be observed; the so-called sliding stones or traveling stones. It is a phenomenon through which, without the influence of any living being, the stones of the landscape literally slide along the ground, leaving grooves in the earth.
This phenomenon, first described in 1948, is observed in rocks of multiple sizes. Although the trail they leave is clearly visible, it is not always easy to measure the speed of movement, which is extraordinarily slow. What has been observed is that the movements are often relatively parallel and the changes of direction are usually synchronized.
Punctual movements, not constant
One of the first findings found, when analyzing the phenomenon, is that the movements of the rocks are not constant. Thanks to the analysis with photographs, it was possible to verify that the stones of the Valley of Death can remain still for years, and even decades. However, and thanks to GPS observations, we know that they can occasionally reach speeds of up to 6 meters per minute —0.36 km/h—.
The first speculations about the process involved supernatural agents, but evidently, scientists assured that there must be a much more mundane explanation.
Different researchers proposed different hypotheses. The first studies attributed the phenomenon to whirlpools of sand. Attempts were made to test this hypothesis using aircraft engines to emulate the winds, and it was found that it was possible to move small rocks as long as the wind speed was greater than 20 meters per second and the surface was wet.
Another hypothesis suggested that the presence of algae mats could reduce friction and facilitate the movement of rocks in the presence of strong winds. However, for much of the year the ground is dry, and it only floods during the winter, being frozen for much of the time. It seems unlikely that algae were relevant. Also, some rocks are extraordinarily heavy, and it would take very strong winds to move them over wet or algae-covered soil. Some calculations even established the need for winds of up to 80 meters per second.
But the hypothesis that gained the most strength, given the very particular climatic conditions, was that of ice sheets. According to the researchers who defended it, during the winter thick ice sheets would form on the ground, which would significantly reduce friction, allowing them to move with the wind much more easily.
solving the mystery
All these hypotheses had a problem. Although they had been tested under experimental conditions, none could be verified in vivo, since the movement of the rocks could only be inferred from their tracks and their change of position, but had never been observed.
In a study carried out by a group of researchers led by Professor Richard D. Norris, from the Scripps Institution of Oceanography in La Jolla, California, and published in 2014, rocks in motion were observed for the first time.
It turned out that none of the hypotheses raised were confirmed. They did not move through humidity, or by mats of algae, or by thick sheets of ice.
The phenomenon of rock movements, in fact, happens not when the great ice sheets form during the winter nights, but when they begin to melt in the morning. The surface of the ground, still flooded with water, retains large sheets of ice just a few millimeters thick, which are beginning to break. The light winds then push the stones which, frozen, move thanks to the almost zero friction between the layers of ice embedded in the water. The movements, according to the study published in PLoS ONE , can last up to 16 minutes.
The reason why movements occur infrequently, and for many years they do not occur, has to do, therefore, with the rarity of the conditions that must be met. The driest winters would not provide enough water for the phenomenon to happen; warm winter nights do not allow enough ice to form.
References:
Norris, R. D. et al. 2014. Sliding Rocks on Racetrack Playa, Death Valley National Park: First Observation of Rocks in Motion. PLoS ONE, 9(8), e105948. DOI: 10.1371/journal.pone.0105948
