It’s a strange phenomenon that has long perplexed scientists. How are these blocks of rock, sometimes weighing several dozen or even hundreds of kilos, able to move on their own? Observation leaves no room for doubt.
In several desert plains, notably in the Death Valley Park in California, one can indeed find large stones which seem to have left a long trail behind them, suggesting that the stone slid on the ground for sometimes several hundred meters. The trace is not necessarily rectilinear and reveals sudden changes in direction, for no apparent reason.
However, the ground here is completely flat. No slope to suggest that the stone could have rolled under the action of the gravitygravity. Studied since the early 1900s, these moving stones, also called “sailing stones”, “sliding stones” or “walking stones” remained a mystery until 2014, when the mechanism of their movementmovement was finally revealed.
Stones surfing on ice sheets
If meteorological conditions, such as wind and ice, had been suspected since the 1970s, it was only thanks to the precise study of the movement of stones, with measurements GPSGPS and pictures in time-lapsetime-lapsecoupled with a measurement of the conditions weather reportweather reportthat the secret of the moving stones will be understood.
The mysterious “sailing stones” of Death Valley in motion. © LiveScience, YouTube
An answer to the riddle of the shifting stones
It thus appeared that everything happens during the winter, while an ephemeral lake forms in the plain. As temperatures drop, this thin layer of water actually freezes, especially during the night. A crustcrust Ice then forms just at the surface of the ground, including under the blocks of rock that have fallen from the surrounding reliefs. But during the day, under the action of soleilsoleilthis ice will partially melt, forming large frozen plates only a few millimeters thick, floating on a layer of water.
Under the action of the wind, these thin panels of ice can move. If a stone is on such a panel, it is therefore carried away in the movement, at a vitessevitesse which can reach five meters per minute, for slides that can sometimes last more than a quarter of an hour!
Less moving stones due to global warming
This astonishing phenomenon, however, remains relatively rare due to the fact that it is based on a combination of very specific weather conditions.
It could even disappear completely due to climate change which is currently impacting these environments. Researchers have indeed noted a decline in the movement of shifting stones in recent decades.
