20,000 Kilometers Over Death Valley: The GPS View of the

Sliding Rocks

Paula Messina

Geology Department

San José State University

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California, Nevada

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CA NV

Death Valley National Park

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Racetrack Playa: Physical Setting• +1131 meters

• ~4 km. (north-south); ~2 km. (east-west)

• Surface: sand, silt, clay; desiccation polygons

February, 2001: Seven Inches of Snow

Grandstand

Racetrack Road

Ditch

Ponding WaterSeptember, 1997

Sliding Rocks

Why Do Rocks Slide?

• Ice rafting?

• Wind alone?

What is the Complete Network?

• Does non-parallelism among trails imply wind alone?

• How can all the trails be mapped?

Does a Rock’s Character Contribute to its Activity?

• Do rounder rocks inscribe more-sinuous trails?

• Do larger (more massive) rocks produce shorter trails?

Global Positioning System to the Rescue

• 24 satellites• 20,000 km. (12,000

mile) orbits• 24 hour global

coverage• Sub-meter accuracy

• Full operational capability: April 1995

How Does GPS Work?

Mapping Mission

• July 16 - 23, 1996

• 162 rocks/trails

• Two field mappers

• Human “digitizers” walked over 100 km.

Cartographic Entities

Points Lines

Areas

• In-field Data Download

• Post-processed differential correction

• Export to ArcView GIS Shape File format

• Export to MS Excel for Quantitative Analyses

Final Map

• Predominant motion: SW-NE (slightly uphill)

• Trail length f (rock size)• Trail straightness f (rock shape)• Parallelism is

the exception; not

the rule.

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Inferred Wind Rose

Plots Showed No Statistically-Significant Correlations

• Trail length appeared spatially controlled– (see following slides)

What Factors Control A Rock’s Ultimate Distance Traveled?

Image and trail map for Diane,the rock that inscribed the longest trail

What Factors Affect Trail Straightness?

• Straightness=Total Trail Length

StartEnd Distance

• Trail straightness appeared spatially controlled

What Factors Control A Rock’s Ultimate Path?

Crooked trails Straight trails

What Factors Control A Rock’s Ultimate Path?

• GIS-Generated Aspect Map

Intervisibility: Ray Tracing• Cross-section:

• Map View:

Intervisibility Maps for Representative Rocks

Trail Character Correlates to Surrounding Terrain

The Racetrack: A Mosaic of Microclimates

• Wind tunnels

• Turbulence

The Final Word: How Can Rocks Converge?

More Information on the Sliding Rocks:

Paula Messina’s Racetrack Web Site

Smithsonian magazine: July, 1999

California Geology: January, 2001

“Savage Planet” Episode IV: “Extremes”

Discovery Channel Online’s Dancing Rocks Web Site