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Death Valley National Park > Racetrack Playa

Mystery Solved!
Solution: The rocks slide across the playa on a sheet of ice with the help of aerodynamic forces, which are produced by natural wind tunnels.

Sliding Rocks on Racetrack Playa

by Dennis Brooks

Terrain

Racetrack Playa is a lake-bed, which is dry most of the time and is nearly perfectly flat. The Playa is located between the Cottonwood Mountains to the East and the Last Chance Range to the West. Its elevation is 3608 feet above sea level. The floor is about 2.8 miles long (north to south) and about 1.3 miles wide (east to west). It is nearly level with the North end being only 1.5 inches higher than the South end.

At the northern end of the lake-bed is a rock formation known as the Grandstand. Rocks from the Grandstand and other rock formations break off and fall onto the lake-bed. These Rocks come into the Playa from all around the lake giving it plenty of rocks of all shapes and sizes. Some of the rocks are boulders that weigh up to 700 pounds. The rocks are scattered all across the lake-bed.

The surface of the floor is covered with cracked mud, which consists of silt and clay. As the mud dries, it shrinks and breaks into interlocking polygon-shaped tiles. During the rainy season, enough water comes in to fill the cracks and cover the tiles. From a distance, it looks like a huge lake.

The water leaves deposits of mud and silt behind, which settles as a thin layer over the floor and in between the polygon-shaped tiles. When heavy rains come during the winter, the tiles are temporarily erased because the mud fills in the cracks. In the spring, dry conditions cause the mud to crack again and form new tiles where the old ones had been.

The Conditions

When it rains in the area, the mountains surrounding Racetrack Playa produce runoffs that cover the Playa floor. During the hot summer days, the rain water transforms the surface of the Playa into a very soft and slippery mud. However, the water evaporates quickly because of the heat. When the water evaporates, the mud dries, shrinks, and cracks into the polygon-shaped tiles. When it rains at night, the shallow lake turns into a sheet of ice because of the extreme temperatures at such a high elevation. The thickness of the ice depends on the depth of the water at the time it freezes. When the sun comes up the next day, first the ice turns into water and the drying cycle starts over.

The prevailing winds in the area blow across Racetrack Playa from southwest to northeast. During winter storms, the winds sweep down from the surrounding mountains across the Playa at speeds up to 90 miles per hour. This sets up a natural wind tunnel across the Playa and simulates the conditions created by researchers who test-fly aircraft indoors.

Moving Rocks

The scattered rocks and boulders move across the floor from time to time. They slide across the Playa, cutting trails in the sediment as they go. Rocks, stones, and boulders of all sizes and shapes are found all over the floor of the playa. Many of them have long trails behind them. This gives the impression that the rocks are racing across the floor when no one is watching. That is how the Playa got its official name, The Racetrack. However most people simply refer to it as Racetrack Playa.

Trails

Thanks to the observations and reports visitors bring back from their trips to the area, we can get a detailed analysis of the rocks' movement. Visitors bring back photos taken from every angle of the rocks and their trails. They also add their comments, which help us see what takes place on the Racetrack. All of this help us develop a collage of the rocks' movement. We find these images and comments on the internet and use them in our research.

The rocks sitting at the ends of the trails tell a story of their playful activities. The trails range in length from a few inches to over 2,500 feet. Rocks with rough bottoms leave straight tracks with multiple groves while those with smooth bottoms may wander about over the surface as they travel. Some of the larger boulders slide along leaving a very shallow trail or no trail at all.

Sometimes a rock will turn over while traveling. Then it continues to move making a new trail with its other side.

Some visitors report that they have seen some rocks that appeared to have dropped out of the sky. These rocks leave only a crater and no trail at all. The shape of trails behind the rocks show that the rocks move when the floor of the Racetrack is covered with a very soft mud.

Scientists have observed that when the rocks move, there is always wind and water and that most of the rock trails are inline with the direction of the prevailing winds. The longest and straightest trails are more numerous in the southeastern part of The Racetrack. This is where the wind is channeled through the mountains to form a natural wind tunnel. Two other wind tunnels cross the Racetrack near its center and create turbulence in the area. The area where the two wind tunnels converge is where the rock trails are the most convoluted.

The rocks seem to defy nature. They slide in straight lines, go in circles, and zig zag across the floor. Even though the lake bed is considered to be flat, the elevation at one end is slightly higher than the other. Since the rocks are moving in all directions, sometimes they travel slightly uphill.

Explanation

It is the wind that creates the primary forces that move the rocks. The sheet of ice that forms over The Racetrack at night provides the surface conditions that allow the rocks to move so smoothly across the floor of the lake bed.

When the winds comes down through the mountain pass at 90 miles per hour, two thing happens. First The Racetrack is transformed into something that closely resembles a tarmac rather than a dirt racetrack. When the wind sweeps over the sheet of ice, aerodynamic conditions above the surface of the ice are created. The conditions are similar to the conditions an airplane experiences as it travels through the air.

These conditions can be compared to an airplane's flight because of one rule. An airplane will fly whether it is moving through the air or whether it remains stationary and the air is moving over its wings. Thus, the wind tunnel. The wind tunnel creates a low pressure area on the already slick ice. This makes the surface of the ice almost frictionless. Without much friction to stop their movement, the rocks slide along with the force of wind blowing them from one location to another.

Sure, the aerodynamic forces will fly the rock over the surface of the ice after they start moving, but what gets them airborne from the beginning? It takes a considerable amount of force to dislodge a large rock from its resting place and set it in motion. So, how can a 700 pound rock go from a body at rest to a body in motion? Also, some of the rocks are blocked by a mud barrier produced by their last stop. This makes it even more difficult to move. The force must not only get the rock moving, but it must also have enough power to break through the mud barrier from a resting position.

The second thing that happens in the wind tunnel is that the rocks experience the same aerodynamic forces that the ice sheet experiences. However, the low pressure area created over the rocks gives them lift. If the rocks have a smooth surface, the wind will lift them up onto the ice and send them sailing down The Racetrack. If the conditions are just right, a smooth rock may go airborne and fly across The Racetrack making a crater when it lands.

That explains how small, smooth rocks get their start, but what about the big jagged rocks? How do they get started? The big rocks get their start from a force called parasitic drag. Where aerodynamic lift allows aircrafts to fly straight, parasitic drag causes them to crash. Fortunately, on The Racetrack, these two opposing forces work together to give the big rocks their start.

When the wind sweeps across a smooth rock, the wind-stream separates briefly starting at the tail of the rock; then it comes back together again at the face of the rock. The separation of the wind-stream causes the low pressure on top of the rock and results in its lift. With large, flat-faced rocks, the wind-stream separates, but it does not come back together except downstream. This condition creates parasitic drag.

Also, instead of the low pressure forming on the top of the rock, it forms at the face of the rock. This parasitic drag is not just a little low pressure; it is a virtual vacuum. When the wind reaches a high enough speed, the big rock is sucked into this vacuum to start its journey. After it gets started moving, it continues to move with the wind. Sometimes it shaves ice along the way and pushes it ahead as it goes. The shaved ice may build up and cause enough resistance to stop the rock's movement. At other times, the big rocks fly across the floor with nothing in their way.

Answers

When the wind dislodges rocks, some of the rocks break through a mud barrier and the barrier separates into four or five smaller clumps. When the barrier breaks, the clumps of mud are already in motion. The wind continues to sweep the clumps along with the bigger rock. Sometimes their trails diverge slightly, and their tracks are shorter and not as deep as some of the other rocks. Visitors have observed that the mud tracks are generally parallel and have even spacing.

Also, smaller rocks generally make deeper tracks because they can move along with only a thin sheet of ice between them and the soft mud. As they move, the ice breaks up over the soft mud and leaves a deeper, wider trail. Since heavier rocks generally need to move on thicker ice, by the time they start moving, the ground below the ice-sheet is also frozen solid giving them a much harder surface on which to travel.

According to the National Park Service, there are 8 other playas near Death Valley. This phenomenon occurs at every one of them because they all have clay-bottom lake-beds, enough moisture to make the surfaces slick, loose rocks, and strong winds to move the rocks around.

Solution: The rocks slide across the playa on a sheet of ice with the help of aerodynamic forces, which are produced by natural wind tunnels.

About the Author

Dennis Brooks retired from the US Army in 1990 and attended Drury University (Springfield, Missouri) where he received his bachelors of science degree. After graduation, he started doing independent research on various subjects including AIDS, dyslexia, and dieting. He worked as a teacher in public schools, and as a private tutor, teaching reading. He is presently retired and spends most of his time writing and surfing the Internet looking for mysteries to solve. He also studies physical science and astronomy. He hopes to someday write a book on unsolved mysteries.

Contact the author:dennisbroo@gmail.com

References:

The Mysterious Sailing Stones of Death Valley
http://www.dunego.net/amazing/37-nature/79-the-mysterious-sailing-stones-of-death-valley.html

The Sliding Rocks of Racetrack Playa
http://geology.com/articles/racetrack-playa-sliding-rocks.shtml

Racetrack Playa
http://digital-desert.com/death-valley-geology/racetrack/

The Mystery of the Racing Rocks, How do these rocks move?
http://www.pbs.org/wnet/nature/deathvalley/rocks.html

Y7 Solve the mystery of the sliding rocks of Race Track Playa
http://gorgeousgeography.wordpress.com/2008/06/16/y7-solve-the-mystery-of-the-sliding-rocks-of-race-track-playa/

The Sliding Rocks of the Racetrack Playa Mystery
http://www.instructables.com/community/The-Sliding-Rocks-of-the-Racetrack-Playa-Mystery/

Physics Forums
http://www.physicsforums.com/showthread.php?t=60676&page=2

The Mystery of the Rocks on the Racetrack at Death Valley
http://sophia.smith.edu/~lfletche/deathvalley.html

Racetrack Playa
http://en.wikipedia.org/wiki/Racetrack_Playa





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