Scattered across some of the barren vastness of Death Valley is a strange phenomenon. Rocks can be found with perfect trails behind them, indicating that they’ve recently moved. But how does this happen when the primary environment is dry, still sand flats? And just how common is this phenomenon?
Let’s take a closer look at the sailing stones and just what causes this interesting geological oddity!
What Are Sailing Stones?
In the relatively early 1900s, it was discovered that some stones appear to be moving of their own volition. Specifically, these were found in a dry lake bed called the Racetrack Playa in Death Valley.
These stones are comprised of a few types. Syenite is the most common on the Western end of the Racetrack, but most of the rocks consist of dolomite. In particular, black dolomite is the most common material among sailing stones.
These stones have been observed for decades, but recent advances have removed the mystery. For those in a hurry, the mechanism which moves the sailing stones is called “ice shove” which I’ll cover in more detail below.
While the stones often form direct lines behind them, some of the various tracks behind these stones can take sharp corners or even curves depending on the exact circumstances during which they were last moved.
These stones range from very small up to over 700lbs. While wind might have explained the occasional small rock, it certainly doesn’t account for the reports of massive boulders that are also moving along the playa surfaces in Death Valley.
Geology of Death Valley
Death Valley is a strange spot. It’s consistently one of the hottest places in the world, but it’s actually the remnants of an ancient sea. The valley itself is a “grabden” which means that it’s a depression between two rises in the earth’s crust, specifically the mountains which border the valley to the East and the West.
Death Valley is very low, possibly the lowest spot in North America. It sits roughly 282 feet beneath sea level,
Death Valley’s unique geographic position is what makes it one of the hottest, driest places on the planet. It’s actually in the “rain shadow” of four different mountain ranges.
Rain shadows are a phenomenon where a mountain range has two sides, the windward and leeward sides. The windward side has a tendency to catch the vast majority of rain that comes in that direction, while the “shadow” on the other side generally remains quite dry. It’s a common desert phenomenon, but Death Valley is surrounded in such a way that it receives very little rain at all.
Due to its unique composition and location, Death Valley’s geology has been extensively studied. It appears there’s not much information to glean from some of the stones, the heavily metamorphosed, grey quartzite found there is heavily metamorphosed and not easy to discern the origin of, for instance.
Limestone emerges in this area. Limestone is generally indicative of ancient marine bodies of water. There are also extensive salt and boron evaporite deposits, the result of relatively quick crystallization of the aforementioned minerals. The region is famous for large amounts of boron minerals.
In short, Death Valley is a bit of an outlier as far as its environment goes. It’s very low, incredibly dry, and has a variety of different formations spread throughout the region.
Discovery and Study of the Sailing Stones
In 1915, people began to notice the sailing stones. The strange tracks left behind by the stones were initially noticed by a prospector, which brought geological interest and formal study. Many hypotheses were put forward for the sailing stones.
While it would seem easy enough to observe this phenomenon, it actually proved quite difficult. While the stones only seem to move every few years, this movement also takes place with incredible speed. It was estimated that the entire movement might only take ten seconds in some cases.
At one point, several of the sailing stones were even surrounded with rebar to see if it would disrupt their movements. Alas, the stones proved too tricky, and one even completely escaped the rebar that was supposed to contain it and help scientists understand how the stones would move.
Several of the stones have ended up being permanently monitored. They have alphabetical labels and a few have even had GPS trackers added to them.
These were all decent theories. In particular, the playa surfaces where sailing stones are found often have a very small boundary layer, sometimes as low as 2” above the solid surface. This means even relatively low stones are subjected to the full force of winds whipping through the valley.
How The Sailing Stones Mystery Was Solved
Wind, erosion, and the small amounts of water which actually fall in the area were all proposed mechanisms for the strange movements of the sailing stones but it took until 2014 for the stones to have a real answer. Other, less realistic, proposals included things like magnetic activity.
In 2014, however, someone got the brilliant idea to subject a couple of stones to time-lapse photography. This showed conclusively that small ice rafts form around the rocks, which actually causes a bit of buoyancy. This lowers the friction required for the stones to move.
When the temporary pond begins to break up in the morning sun, the stones are then shoved along by comparatively low mass sheets of ice due to the reduced friction created. This doesn’t appear to require high wind speeds, relatively gentle motion will move the stones along well enough due to the conditions.
Other Locations With Sailing Stones
The relative rarity and unique geological conditions of the Racetrack Playa in Death Valley are what make up this phenomenon. Most places simply don’t have the weather cycle and unique surface left behind there, so it’s not a common phenomenon.
That said, there also appear to be some sailing stones in other locations. The most frequently cited appears to be the Little Bonnie Claire Playa, which is nearby in Nevada.