The sailing stones of Death Valley
Sailing stones or Traveling Stones are part of a geological phenomenon in which rocks move and inscribe long tracks along a smooth valley floor without animal intervention. The movement of the rocks occurs when large, thin sheets of ice floating on winter pond break up in the sun. Trails of sliding rocks have been observed and studied in various locations, including Little Bonnie Claire Playa, in Nevada, and most famously at Racetrack Playa, Death Valley National Park, California, where the number and length of tracks are notable.
The Racetrack’s stones speckle the playa floor, predominantly in the southern portion. Historical accounts identify some stones around 100 m from shore, yet most of the stones are found relatively close to their respective originating outcrops.
Tracks are often up to 100 m long, about 8 to 30 cm wide, and typically much less than 2.5 cm deep. Most moving stones range from about 15 to 46 cm in diameter. Stones with rough bottoms leave straight striated tracks, while those with smooth bottoms tend to wander. Stones sometimes turn over, exposing another edge to the ground and leaving a different track in the stone’s wake.
Trails differ in both direction and length. Rocks that start next to each other may travel parallel for a time, before one abruptly changes direction to the left, right, or even back to the direction from which it came. Trail length also varies – two similarly sized and shaped rocks may travel uniformly, then one could move ahead or stop in its track.
A balance of specific conditions is thought to be needed for stones to move:
- A flooded surface
- A thin layer of clay
- Ice floes
- Warming temperatures causing ice breakup
News articles reported the mystery solved when researchers observed rock movements using GPS and time-lapse photography. The largest rock movement the research team witnessed and documented was on December 20, 2013 and involved more than 60 rocks, with some rocks moving up to 224 meters between December 2013 and January 2014 in multiple movement events. These observations contradicted earlier hypotheses of strong winds or thick ice floating rocks off the surface. Instead, rocks move when large ice sheets a few millimeters thick start to break up during sunny mornings. These thin floating ice panels, frozen during cold winter nights, are driven by light winds and shove rocks at a velocity of up to 5 m per minute. Some GPS-measured moves lasted up to 16 minutes, and a number of stones moved more than five times during the existence of the playa pond in the winter of 2013–14.
sliding: che scivolano
Playa: (dallo spagnolo) spiagga, pianura sabbiosa
to speckle: punteggiare
shore: riva, costa
outcrops: sperone di roccia
to range: spaziare, avere una gamma di
rough bottoms: fondi ruvidi
to wander: girovagare
to turn over: capovolgersi
abruptly: all’improvviso, inaspettatamente
ice floes: banchi di ghiaccio
time-lapse photography: fotografia in time-lapse
to shove: spingere, dare spintoni
The Travelling Stones of Death Valley