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More on Racetrack Playa

Racetrack Playa
Notice the central furrows and raised, levee-like ridges created by these two sliding rocks. Photo by Paula Messina

What the trails tell

The shallow furrows and rounded, levee-like ridges that form the "trails" of the sliding rocks are clues that suggest the stones move only when the playa surface is soft and wet. Anyone who has ever slipped in a mud puddle knows that water-drenched mud makes an incredibly slick, low-friction surface. Once an object is put into motion on a very low-friction surface, it may move quite a distance before it stops.

Some researchers thought that gravity was the culprit and that the rocks might be sliding downhill on a very, very shallow slope. However, this hypothesis was discarded when it was shown that the northern end of the playa is several centimeters higher than the southern end, so many rocks actually move uphill!

Without any witnesses to the sliding rock phenomenon, it's been difficult to prove exactly what makes Racetrack rocks move. Researchers have also been hampered because traces left behind by sliding rocks are short-lived. Small rock trails may be washed away by a single rain storm. Even trails gouged into the playa by the largest boulders last no more than seven years.

A rock class=
Each of the Racetrack Playa rocks has been named. This is Sylvia. Photo by Paula Messina

A high-tech solution

Researchers noticed that although some trails change direction, most trend in a generally southwest to northeast direction. This is consistent with the direction of the prevailing winds. Could wind really provide the force that sets the largest Racetrack Playa boulders in motion?

One recent study used a high-tech approach in an attempt to solve the mystery of the sliding rocks. Detailed measurements using Global Positioning System (GPS) instruments were made of over 160 sliding rocks and their trails.

Racetrack Playa
This map shows Linda's convoluted trail.

After analyzing their rock trail map, researchers found that the longest, straightest trails are concentrated in the southeastern part of Racetrack Playa. In this area, wind is channelled through a low point in the mountains, forming a natural wind tunnel. In the central part of the playa two natural wind tunnels focus their energy from different directions. It's in this area that rock trails are the most convoluted.

So the evidence suggests that strong gusts of wind and swirling dust devils, in combination with a slick playa surface may set even the heaviest the rocks in motion. Off they go, scooting along downwind until friction slows them down and they come to rest. There the stones wait for the next time when slippery mud and wind spur them into action again.

Racetrack Playa in time
geologic time scale
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