The moving rocks of Death Valley, California have long perplexed geologists and fascinated general public for decades. Some pretty outlandish theories have been proposed over the years to attempt to explain this very odd phenomenon. Finally, after years of research and some high tech kit, researchers at the Scripps Institution of Oceanography have finally discovered the real reason behind the creeping stones.
Let's take a look.
One of the mysterious creeping rocks [Image Source: Tahoenathan/Wikimedia Commons]
Since the early 1900's, visitors to Death Valley have mused over the moving rocks and how they seemed to slide of their own accord. The area in question is a completely dry lake, or playa, etched with tracks in the wake of many now sedentary rocks. As you can imagine, this has proved a little hard to decipher, rocks simply don't move on their own on a flat surface. To complicate matters, the area is pretty inhospitable and it was near impossible to witness them in action, until now.
The tracks seemed to suggest that the rocks moved, though very slowly and strangely erratic. In either case, they couldn't be observed unless you wanted to sit out in the desert for years on end. A team of researchers decided to solve this problem once and for all. They employed time lapse cameras and GPS tracking to catch these pesky rocks in action. Thanks to the devotion of the team we can now, finally, dispell any mystery around the sailing stones of Death Valley.
The phenomenon was first noticed in 1915 when the area was explored during natural resource exploration studies. The moving stones quickly caught the geologists attention and were published in various geological publications, quickly sparking the imagination of everyone who read it.
Where are they in Death Valley?
The home of the moving rocks in question is called the Racetrack Playa in Death Valley National Park, California. Playas, in case you weren't aware, are desiccated lakes that once contained water which has since completely evaporated leaving a completely dry lake bed. This occurs when the evaporation rate exceeds the replenishment lake supply of water. This means that the lake will always lose its battle against evaporation and always stay dry for most of its existence.
[Image Source: Scripps Oceanography/YouTube]
The Racetrack Playa is one example amongst many playas around the world. It is located 1,132 meters above sea level and is 4.5 kilometers long and 2.1 kilometers wide. The lake is not always bone dry. During heavy rains or melting snow, it does get filled up, albeit temporarily. Once the rains subside, exposure to hot sun inevitably evaporates all of the shallow waters leaving a cracked, hexagonal, mosaiced dry floor.
Playas are not unique to this part of the world. But what makes this one special, is the presence of the once mysterious roaming stones. The playa floor has a large collection of dolomite (a type of limestone) and syenite (similar to granite) that can weigh anywhere from a few hundred grams to hundreds of kilograms. Some of these stones appeared to be able to move without human or animal intervention leaving tell-tail "racetracks" or trails in their wake. These tracks are often around 100 meters long and are usually less than 2.5 centimeters deep. The moving stones in question, tend to between 15 to 46 centimeters in diameter.
How could these rocks move?
James Crook, a prospector, first visited the site in 1915 and again in 1948. Geologists Jum McAllister and Allen Agnew also mapped the area. It was quickly noted that this phenomenon was not isolated to the Racetrack Playa. Little Bonnie Claire Playa of Nye County, Nevada also appeared to have the same kind of moving stones. In 1952, a National Park Ranger made some measurements and several geologists proposed various hypotheses about the phenomenon.
Theories included a form of localized magnetic effect. This was quickly dismissed as the rocks in questions aren't particular ferrous and do not show a kind of gradual assembly towards a source. Others had suggested that strong winds could move the stones once the lake bed was wet, acting as a lubricant. The most likely one proposed was that a mixture of wind, temperature, and water were the culprits.
It was this later hypothesis that the research team would put to the test.
Sliding stone with GPS [Image Source: Wikimedia Commons]
Tracking the stones
Although it is impossible to watch them move, they could be tracked. Movement appeared to only occur every three years or so. The research team attempted to build a corral 1.7 meters in diameter around a select few of the rocks. This was to see if it was something to do with ice floes. If true, the corral should impede or prevent movement.
Much to their surprise, some stones were able to "escape" the corral and not be impeded by the rebar segments. Interesting, and it seemed to suggest that the ice moving the stones must be smaller than the gaps between the rebars. Their research continued into the 1990's. By this time, wind and ice interaction had become the favored explanation.
In 2009, the development of time-lapse cameras allowed the researchers to capture the playa flooding events. Later advancements in technology allowed the team to add wind triggered imagery to their arsenal. This allowed them to greatly reduce the 2,700 hours or so of non-transit time. GPS technology was later added in 2013 to allow the researchers to monitor more than 6o rocks. This was coupled with time lapse photography to see what was going on.
Their observations seemed to contradict the previous theory of strong wind and/or thick ice interaction "floating" or "carrying" the rocks over the lake bed.
So what's the secret behind the moving rocks?
With the aid of time-lapse photography, GPS tracking, and a long wait the team has finally managed to catch the moving rocks red handed. Their footage showed that as the lake fills with a shallow layer of water it freezes around the rocks when the temperature is cold enough. During the day, the sun's heat partially melts this ice layer which then breaks up into sheets. These sheets, driven by the wind, literally bulldoze the rocks around the valley floor. Once all the ice melts and the water evaporates, the stones are left once more to sit and wait. The only evidence, until now, of the entire event being the mysterious tracks in their wake.
What is also amazing, is that the rocks move without the need for very strong winds. The ice barges are also surprisingly thin, at times only the thickness of a plane of glass. These sheets, along with the rocks, are moved by the wind, sometimes up to five meters per minute. That's an incredible 0.3 km/h. Some of the rocks have even been shown to move up to 224 meters during observation periods. Amazing for a stone.