|Rocks at Racetrack Playa seem to move on their own, leaving mysterious trails behind.|
These are the “sliding rocks” or “sailing stones” of Death Valley. First documented by miners back in the 1900s, these rocks range from pebbles to 600-pound (272-kilogram) boulders and seem to move of their own accord. The only evidence of their activity is a series of long, perplexing trails left in the valley’s dried mud.
Scientists have been trying to solve the mystery of the sliding rocks since 1948, and have since proposed all manner of explanation: dust devils, flooding, ice sheets, hurricane-force winds, and algal films.
Geologists previously speculated that some combination of wind, rain and ice would have a role. But few expected that the answer would involve ice as thin as windowpanes, pushed by light breezes rather than strong gales.
But thanks to new photographic and meteorological evidence presented by Richard Norris of the Scripps Institution of Oceanography and James Norris of Interwoof, we may finally have a conclusive answer as to what makes the stones stir. The researchers’ findings were published in the journal PLOS ONE.
A Goldilocks Phenomenon
According to Norris, a geological oceanographer and paleontologist, Death Valley’s rocks move under a delicate mix of water, ice, sun, and wind. Norris and his cousin were able to document the rock movements by inserting GPS tags into chunks of limestone and syncing their movements with readings taken from a custom-built weather station.
Unlike one of the previous explanations, which had thick ice capturing the rocks and carrying them along like a miniature glacier, Norris said their evidence shows that thin ice floes break up and pile against the rocks. This creates enough friction to cause the rocks to skim across the muddy surface of a temporary pond. If you were there to see it, the rocks would look like ice-breaker ships plowing through sea ice—though in this instance, it’s the ice that’s moving the ships.
But the conditions have to be just right, what Norris called a sort of Goldilocks phenomenon. If the ice is too thick, or the day is too sunny, or the wind isn’t steady enough, then nothing happens.
Oh, and you have to have standing water—a rare phenomenon in itself for an area that receives less than 2 inches of rainfall annually.
“The process of ice breaking up and shoving rocks around happens every year if you go up into Saskatchewan or Ontario, but you don’t normally associate it with a hot, dry place like Death Valley,” said Norris. “And yet here’s the same kind of process unfolding occasionally—very occasionally—in this place that we associate with a very different kind of climate.”
Scientists have long known that whatever it is that causes the stones to move, it doesn’t happen very often. In fact, the mudflat where you’ll find the rocks and their trails, called Racetrack Playa, can go a decade or more without showing any new signs of movement.
A balance of very 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
The researchers’ findings were published in the journal PLOS ONE.