In California, in a valley between the Cottonwood and Last Chance mountain ranges in Death Valley National Park, there is the Racetrack Playa, a dry lake where the rocks move by themselves, the sailing stones.
The dry lake, called Racetrack Playa, is located in the heart of Death Valley at 1100 meters above sea level.
The Racetrack Playa is one of the driest and hottest places in North America, its 4.5 km long surface is occupied by hundreds of rocks of different weights and sizes called “sailing stones” because of their movement on the sand.
This strange phenoment has led geologists and experts to wonder for decades about its causes and now the mystery seems to be solved.
How do these stones move and leave traces of their passage?
The first attempt to scientifically explain this process dates back to 1948.
There was talk of sand vortexes and the strong wind that slowly pushed these blocks of rock of various sizes into the muddy substrate.
The problem was that the wind alone could not move rocks weighing more than 70 kilos.
A study carried out in 1955 attributed the responsibility for slipping to a thin layer of ice.
In May 1968, a seven-year programme of monitoring the movement of 30 moving stones was started, in which each monitored stone was identified by a name.
At the end of the programme, it was observed that 28 of the 30 stones monitored had continued on the route that had already begun before the monitoring: the longest cumulative distance, 262 m, had been covered by the smallest of the group.
At the end of August 2014 a more detailed scientific study with video documents finally solved the enigma.
Boulder monitoring in the Racetrack Playa
According to a group of geologists from the Scripps Institution of Oceanography in California, the boulders are moved by layers of ice below the surface, which are formed when it rains and the lake bed is filled with rainwater.
The water that covers the Racetrack playa lake in the cold season freezes at night: during the daytime melting, the thin but strong sheets of ice that float are able, under the pressure of a light breeze, to move the rocks that rest on the soft mud.
The group of geologists equipped 15 boulders with motion-activated GPS units, constantly monitored by a weather station and time-lapse cameras.
In December 2013, while the Playa was covered by about 7 centimetres of water and the surface layer was frozen due to the warming of the sun, the ice began to crack.
Shortly afterwards, the rocks began to move.
Here is the video with the movement of the rocks:
Trajectories and speeds are determined by the wind: usually the rocks move slowly, making up to 5 metres per minute, pushed by winds blowing up to 16 kilometres per hour.
The rocks followed by the researchers remained in motion from a few seconds to 16 minutes; some moved as much as 224 metres, and in one case 60 metres at the same time.
In order for the rocks to move, certain concomitant circumstances must occur.
The Playa must be covered with a layer of water high enough to freeze in winter but low enough to leave the rocks uncovered.
When the temperature drops at night and the water surface freezes, the ice must be 3-6 millimetres thick: thin enough to break easily, thick enough to push a rock.
In the heat of the sun, the ice breaks into large floating panels, which, dragged by the wind, move over the little water and mud that remains, pushing the rocks along the Playa.
The boulders, in contact with the earth, scratch the surface of the ground leaving behind them the famous trails.
You can read the whole story here.