27. Wind work

Dan BarkerApril 2009

North shore, Long Island Sound

Wind is one of several geological agents that can move mass over a distance by eroding, transporting, and depositing solid particles, although the particles are generally smaller than those moved by ice, gravity, or water.

When wind blows constantly in one direction for long time spans,it can effect a net loss in surface material, particularly on islands.Brava, Cape Verdes

Wind carrying sharp-edged pyroclastic glass particles isespecially effective at cutting off vegetation at ground level,thereby removing protective cover from soil. Central High Desert, Iceland.

Particles carried by wind impact rock surfaces, etching hardercomponents into relief. Aeolian Buttes, CA.

In San Gorgonio Pass, CA, wind from a constant direction hasmobilized sand to produce these grooves in rock.

Sand blasting by glass shards has differentially etched variationsin this exposure of the Green Tuff, Island of Pantelleria.

Lag pavement by wind erosion, Reykjanes Peninsula, Iceland

Wind can transport material on a smallscale, as in this dust devil in the Danikil Depression,Ethiopia

or on a massive scale, such as this dust storm in the Atlanticwest of Mauritania. Saharan dust reaches Austin, Texas at leastonce a year, on average.

The late Robert P. Sharp ofCalTech investigated the vertical profile of windtransportation with thisset-up at Palm Springs, CA.The accumulation in eachcollector was periodicallyremoved and weighed. Thesecond from the bottomcontained the most (asbouncing sand grains). Thetop collector contained a little dust, plus an angryand hungry lizard. Most ofthe sand was transportedless than a foot above theground.

Dune sands derived from eroded coral reefs, west Maui.

Once sand has accumulated in a restricted patch, it forms asoft pad that traps bouncing sand grains, so that more and more sand accumulates in the same place. Kelso Dunes, CA. The next two images show the true scale of this dune.

On the crest of that dune, looking downward. Humans for scale.

On the crest of the same dune, looking upward from the same pointas the previous image.

Wind ripples on a lee side, Kelso Dunes

Wind can remove small particles, leaving a lag concentrate of coarser ones, Note sunrise fog in background. Namibia.

Dunes accumulate in leeward spots. Dicker Willem, Namibia.

Red dunes, Namibia

Dense (dark) minerals concentrate on crests of dunes. Namibia.

Seasonally varying winds produce star dunes, as here inthe Mojave desert, CA.

Finally, the most notorious locality for wind effects isRacetrack Playa, north of Death Valley, CA. Hereblocks of dolomite from the surrounding mountainsare stranded on the mud surface of a playa or intermittent lake. The blocks connect to trails ofdepressed and compacted mud that trace complicatedpaths across the playa. Even light materials such aswild burro droppings and sunglasses leave trails. Somearticles in the popular press invoke "local anomalies in the gravitational field" or "extraterrestrial forces" toexplain the horizontal migration of heavy blocks acrossthe horizontal mud surface of the playa.

Racetrack Playa, California

Dolomite block on Racetrack Playa, CA.

Stone and track, Racetrack Playa, CA

Block and compressed track, Racetrack Playa, CA.

A track with no engraver; ice? Or removed by vandals?

Two tracks in oppositedirections, with blocksat both ends.

The prevailing hypothesis is that dolomite blocks andother objects were frozenin a thin layer of ice overa wet mud surface. Windsmoved plates of ice in different directions, and theobjects gouged the tracks.The other possibility, ofincreasing probability. Isvandalism. No one hasever seen a block move.