WANDERING ROCKS OF DEATH VALLEY: Stones that slide across the ground under some kind of mysterious force
At Racetrack Playa in the Death Valley, California, pictured, heavy
rocks appear to move across the floor while no one is looking.
Scientists believe the movement may be caused by the ice stuck to the
bottom of the stones, which are then moved in the wind. However, no-one
has caught the stones in the act
At Racetrack Playa in the Death Valley National Park, California,
strange forces are at work. Forces capable of pushing heavy rocks across
the flat surface of a dried-out lake while no one is looking.
Scientists have scratched their heads over the trails left by these
sliding stones since early in the 20th Century. In the 1960s,
Californian geologists started a rock monitoring programme. They tracked
30 stones, weighing up to 25kg, 28 of which moved during a seven-year
period – some more than 200m. Analysis of the stones’ trails suggested
speeds of 1m per second. In most cases, the stones travelled in winter.
In the decades that followed, theories about ice and wind gained
support. Others involved algal slime and seismic vibrations.
So what’s happening? Are the stones sliding around in bad weather? ‘We
think so,’ says Dr Gunther Kletetschka, of the Academy of Science of the
Czech Republic and Charles University in Prague, who led a 2013 study
on the stones. His team’s research describes what happens when water on
the Playa surface freezes. According to their theory, ice stuck to the
stones remains frozen longer than the surrounding ice – because rock
conducts heat away faster. This reduces the force between the rocks and
the Playa surface, so they can be pushed by the wind.
No one has yet caught the stones in the act, and they’ve been strangely
still of late. Kletetschka thinks this is due to water levels. ‘The
reason for stones not moving recently is the presence of an artificial
trench, draining the excess water from the Playa,’ he says.