Death Valley in California is intensely dry, with a climate veering from searing heat to bitter cold. It is also the site of a long-running mystery. A dried-up lake known as Racetrack Playa in the mountains above Death Valley is littered with hundreds of rocks that appear to move, leaving a trail in their wake in the dry mud.
Various explanations had been offered for the moving rocks, based on magnetic fields, hurricane force winds and much else. The problem, though, is that no one had seen the rocks actually move. But now, using time-lapse video, scientists found that a rare combination of weather drives the rocks along on their strange voyage.
Even though Death Valley is one of the driest places on Earth, around once a decade it can rain in winter, turning the dried-up playa into a shallow lake. And in the intensely cold winter nights, thin sheets of ice then float on the water’s surface. The following morning, as the sunshine warms the lake, the ice starts to melt and breaks up into sheets of ice in light winds. These broken ice sheets can then shove the rocks along like small bulldozers, and as the rocks move they leave tracks in the mud on the lakebed.
Researchers managed to video the rocks moving last winter. Following a series of storms, the playa filled with water 7cm (3in) deep and overnight the water froze. When the sun rose the next morning, the ice cracked with loud popping noises, and the rocks began to shift. Hundreds of rocks went into motion, including even boulders weighing 300kg (660lb). Video footage of the rocks can be seen at http://bit.ly/1qWRlZy.
The rocks began moving in December 2013, and by January this year some had slid up to 224m (735ft), moving along at speeds of no more than 0.3km/h (0.2mph), too slow to spot by eye. And they followed the direction of the gentle winds blowing at the time — when the winds changed direction, so did the rocks. The study was published in the journal PLOS ONE.