Question of the Day - 04 June 2021

Welcome to the vast ranks of people who've submitted a question to QoD. We're sure many of them remember "their first time" too.

And yes, we're happy to provide the answer to this question. Your husband isn't making it up -- though he is exaggerating a bit. If you were envisioning boulders racing around a track, that's not how it works. 

Rather, the "Racetrack" refers to the Racetrack Playa area within Death Valley, named for the boulders that, yes, do move "by themselves," leaving a trail in their wake.

Until very recently, it was a mystery that had bewildered observers since the early 1900s: How on Earth do boulders weighing as much as 700 pounds move, with no obvious or visible outside force, for distances of up to 1,500 feet across the flat dry lake bed? And when we say "flat," we mean it. There's only 1.5 inches difference in elevation between the north and south end of the 2.8-mile long playa.

While it's known colloquially as the Racetrack, that itself is a bit of hyperbole. The rock travel, in fact, is so slow that until recently, no one had ever actually seen it happen in the more than a century that has elapsed since the phenomenon was first observed. The speculation was that rock movement occurred when the lake bed was wet and winds were simultaneously blowing at speeds of 50 mph or more. Another theory was that a combination of winds and ice "floated" the boulders enough for them to move. But no one knew for sure.

The average rainfall in Death Valley is just two inches per year, while in some years there's no rain at all. The area is technically an endorheic lake, meaning it's a closed-drainage basin. Hence, when it does rain, the runoff from the surrounding mountains all gathers on the lake bed. It has nowhere to flow to, so it quickly evaporates in the desert heat, leaving a super-slippery mud slick in its wake. The mud then cracks when it dries and contracts, causing those familiar polygon-shaped "cornflakes" to form on the surface.

The "sailing stones" or "running rocks" are slabs of dolomite and syenite that tumble off the mountains and previous studies showed that they move only once every two or three years, with the resultant tracks enduring for three or four years. They tend to be concentrated at the lower end of the lake bed. Interestingly, those stones with rough bases move in straight lines, while the smooth-bottom ones tend to meander. Again, for decades, there was only speculation as to how and why any of this happens.

But in a study a few years ago, for the first time, researchers were actually present when the rocks moved. Their findings were published in late 2014. Using motion-activated GPS sensors and time-lapse photography, the team chronicled a total of 60 rocks and boulders that moved between December 2013 and January 2014 and they determined that ice and wind are factors, but not in the way previously thought.

What they discovered was that the rocks move when ice sheets that are a few millimeters thick form on shallow water. As this "windowpane" ice starts to melt and breaks into large sheets, the wind blows the ice sheets across the lake surface and, when they come into contact with a rock, they have enough power and momentum to push the rock along the playa surface.

So it’s not that the wind blows the rocks across slippery mud or that the rocks "float" thanks to ice and wind. Rather, the wind pushes the ice and the ice pushes the rocks. Mystery solved.