Death Valley's 'sailing stones' moved unseen for decades, until cameras caught them.
On a dry lakebed in Death Valley, rocks, some as heavy as a person, leave long trails behind them, yet for decades nobody saw one move. Explanations ranged from hurricane-force winds to the supernatural. In 2014 researchers published the answer: sheets of ice just millimetres thick, pushed by gentle winds.
Racetrack Playa is a flat, usually bone-dry lakebed in Death Valley National Park, California. Scattered across it are stones of dolomite and syenite, most 15 to 46 centimetres across, each at the end of a track that can stretch about 100 metres. Some tracks run straight while others wander, and stones that start side by side can suddenly part ways. The first documented account came from a prospector in 1915, and geologists described the tracks in print in 1948.
Explaining them proved surprisingly hard. Early researchers suggested strong gusts over wet mud; others argued some stones were too heavy for any wind. From 1968 Bob Sharp and Dwight Carey named and tracked 30 stones for seven years. Nearly all moved, and only in winter; a stone called Nancy, just 6.4 centimetres wide, travelled 260 metres in total. They tested the ice idea by fencing a stone inside a ring of metal stakes, but it slipped out between them, suggesting any ice involved was small. Later studies found signs of large ice floes, and physicists showed that winds at the surface of a flat playa can strike even small stones at full force.
The mystery was finally settled with GPS-tagged stones and time-lapse cameras. On 20 December 2013, researchers watched more than 60 rocks move at once, some travelling as far as 224 metres over that winter. The cause was neither gales nor thick ice. After rain, a shallow pond forms on the playa and freezes on cold nights into ice panels a few millimetres thick. On sunny mornings the ice breaks up, and light winds push the floating panels, which shove the rocks along at up to about 5 metres a minute. Some moves lasted up to 16 minutes.
Because it needs a rare sequence of flooding, freezing and thawing, movement happens only every few years. One statistical study suggests it has become less frequent since the late twentieth century, possibly as winters grow warmer and drier. The playa has also suffered at human hands: stones have been stolen, and in 2016 someone carved initials into a rock and left miles of tyre tracks across its surface.
Source: Wikipedia — Sailing stones · Text summarised from Wikipedia (CC BY-SA 4.0)
