Researchers Discover How Massive Boulders Travel Across Deserts Without Help

In late 2013, two cousins can be seen roaming across Racetrack Playa in California’s own Death Valley. They were out looking for rocks—but not just any rocks. These cousins—Richard and Jim—are scientists, and they are out hunting highly mobile rock specimens—highly mobile for stones, anyway.

There weren’t any falling rocks around, thankfully. They were there trying to figure out why some of the rocks in the national park shifted and moved on their own—often in erratic, unpredictable directions. This phenomenon is known as “sliding rocks.”

The first question to come to mind might be: how are these mostly flat, smooth objects able to move around the desert floor? To answer this, it’s worth also asking if there have been any other examples of sliding rocks in the world. It turns out there are. 

Was the wind blowing these lumps of stone? No, they would be too heavy for that. Could they have been full of giant insects carrying them along? No, they checked for that too. Magic, maybe? It was quite a confounding question to answer. But that’s the way it goes in scientific research. 

Lightning used to convince our ancestors that, somehow, the gods were angry and that they had to be appeased. In a recent rock-related story, an island suddenly appeared one day around 600 miles away from Tokyo. Was this some kind of magic? 

While the earth beneath our feet may seem solid and static, there is plenty of activity going about underfoot all the time. If an island can suddenly erupt out of the ocean—just like Niijima did in 2013—then why couldn’t little rocks travel around the desert on their own steam?

Here’s another example of stones having a life of their own. This rock in Romania, called a “trovant,” gets larger and larger year after year, all by itself. A pebble trovant, starting at just half an inch, can grow up to 30 feet tall!

Imagine starting with a little pebble garden and ending up being overcrowded with these enormous boulders. So how did these little fellows grow into such towering golems? Once again, scientists put in the work just to find out the answer.

Let’s get back to the shifting rocks in the desert and back to the Noris cousins. They managed to determine that the rocks had indeed been moving—it wasn’t an optical illusion or a trick in the sand. They were infrequently changing their locations.

One strange thing they noticed was that not all the rocks were moving in uniform directions. And not all of the rocks moved at equal distances, either. Some stones would only venture a couple of feet, while others would blaze a trail in the hundreds.

Racetrack Playa is a remote and hard-to-reach location, but that has not stopped curious folks from making the trek to see the mysterious moving rocks. Despite being in a remote location, the site is not entirely inaccessible.

The Norris cousins had a long-standing interest in the moving stones at Racetrack Playa. In 2011, they decided to take matters into their own hands. Jim, an engineer, and Richard, a biologist, came up with the idea of introducing their own non-native rocks to the playa and tracking their movement using GPS sensors.

All that was left for them to do was to wait. They had to be patient and wait for the right conditions. But, it wasn’t until two years later, in 2013, when they returned to the site to check on the experiment, that their patience was rewarded. 

The Norris cousins, along with their research partner Ralph Lorenz, were able to observe the moving stones phenomenon for the first time ever recorded. With the help of their GPS and weather-sensing equipment, they were able to understand the mechanics of how the rocks moved and what was causing them to do so. 

The Norris cousins and Ralph Lorenz were prepared enough to capture the movement of the rocks on film. The footage they captured shows a large rock seemingly gliding around the playa, albeit at a plodding pace.

So what was the cause of the mysterious rock movement? The answer was ice. But not the large chunks of ice that one may typically think of—the movement was caused by sheets of extremely thin frozen water on the lakebed that were pushed around by the wind.

The Norrises cousin’s experiment measured the distance traveled by individual rocks and found that some of them had moved more than 200 feet. This distance traveled was made possible by the ice sheets on the lakebed, but what causes the formation of ice on a dry lakebed?

For the extremely thin ice to form, the right amount of rain must fall and accumulate on the surface. The lakebed must then freeze, but the ice must be of a specific thickness. As the sun rises and warms the valley, the ice has to crack and break into large sheets instead of melting away.

The researchers published their findings in a scientific journal, PLOS ONE, solving the mystery of the moving stones in Death Valley (and maybe, for some, taking away a little magic and mystery from the world). But Racetrack Playa still holds secrets yet to be uncovered. 

The mystery behind how the larger boulders move remains unsolved. Jim and Richard didn’t see the bigger boulders move for themselves, indicating that sheets of thin ice may not be sufficient to explain their movement. It seems that nature has one more magic trick up her sleeve to confound us humans—at least for now.