13.4: Wind Driven Sediment Transport

Sediment Transported by Wind

While water is still the dominant agent of erosion in most desert environments, wind is also a notable agent of weathering and erosion. Possessing lower energy than water, wind transport nevertheless moves sand, silt, and dust. When wind blows, it carries sediment, called the sediment "load" (Figure \(\PageIndex{1}\)). In much the same way that water carries sediment, wind does likewise, and the sediment is distributed between the bedload and suspended load. As with water, in wind these components depend on wind velocity.

Figure \(\PageIndex{1}\): Dust blows across a playa in the arid Mojave Desert. “01-2007-SearlesLake-duststorm” by Bobak Ha'Eri, is licensed under CC BY-SA 2.5.

Sand size material moves by a process called saltation in which sand grains are lifted into the moving air and carried a short distance where they drop and splash into the surface (Figure \(\PageIndex{2}\)). This movement knocks and dislodges other sand grains which are then carried a short distance and splash dislodging still others.

Figure \(\PageIndex{2}\): Three modes of wind-driven sediment movement. Wind moves sediment by suspension in the air, saltation (bouncing at the surface of the Earth) and creep (essentially incrementally being pushed from behind). “Diagram showing mechanics of saltation” by NASA, is in the public domain.

The zone of saltating sand is an effective agent of erosion through sand abrasion. Rocks and boulders lying on the surface may be blasted and polished by saltating sand (Figure \(\PageIndex{3}\)). When predominant wind directions shift, multiple sandblasted and polished faces may appear. Such wind abraded desert rocks are called ventifacts (Figure \(\PageIndex{4}\)).

Figure \(\PageIndex{3}\): Windblown quartz grains eroded from outcrops of the nearby Navajo Sandstone with a color that has been caused by iron staining. They are relatively even in size or well-sorted, and are rounded and frosted due to abrasion. "Hematite-stain-sand" by Mark A. Wilson, is in the public domain.

Figure \(\PageIndex{4}\): Wind-abraded ventifact. A black cobble on the desert floor is surrounded by smaller gravel and sand sized sedimentary fragments. The ventifact contains a distinctive faceted ridge formed by wind abrasion on the windward side of the cobble. The windward and leeward sides of this rock are smooth due to abrasion. “VentifactMojaveDesert031511” by Wilson4469, is licensed under CC0.

Query \(\PageIndex{1}\)

Deflation and Desert Pavement

Desert pavement is a layer of interlocking pebbles that protect smaller sediments below (Figure \(\PageIndex{5}\)). When a rock is removed a layer of sand can be found below. Wind easily removes the sand, leaving only the larger pebbles behind. Deflation, the lowering of the land surface by the removal of sand, silt and clay by wind erosion, has been long supposed responsible for the formation of desert pavement (Figure \(\PageIndex{6}\)).

Figure \(\PageIndex{5}\): Gravel and cobble sized sedimentary rock fragments are closely juxtaposed in an exposed in desert pavement. There is very little space between these particles; sand sized and smaller particles have been removed by wind transport “Desert pavement” by Michael C. Rygel, is licensed under CC BY-SA 3.0.

Figure \(\PageIndex{6}\):

The wind continually removes the smallest sized sediment until only the larger sizes are left, deflating the surface and creating desert pavement. ”Deflation” by Noonat, is licensed under CC BY-SA 3.0.

There is evidence that other factors may be at work, as well. One theory states that the rocks are lifted to the surface by shrinking and swelling of clay layers below, while other studies suggest that pebbles started at the surface and smaller sediments were deposited around them. Regardless of how it forms, once desert pavement is in place, it helps shield any underlying sediments from eroding away.

Hazards of Blowing Sediment

For every bucket of sand that migrates into the Kelso Dunes, an undetermined volume of dust is generated that disappears into the wind. Dust storms in the Mojave region can be quite intense, and a hazard when dust concentrations are high enough to cause white-out conditions. Windblown dust can be harmful to people breathing it. Dust may contain toxic compounds and it can carry pathogens such as the virus that causes Valley Fever; it also carries away valuable topsoil. On the other hand, dust that settles into stony soils of the desert provides improved retention of moisture and adds nutrients. Thus, dust can be both beneficial and destructive.

References

1. Harden, D. R. (2004). California Geology. Pearson Prentice Hall.
2. Johnson, C., Affolter, M. D., Inkenbrandt, P., & Mosher, C. (2023). An Introduction to Geology. Salt Lake Community College. https://slcc.pressbooks.pub/introgeology/
3. Our Dynamic Desert. (2009, December 18). Our Dynamic Desert. Retrieved August 31, 2023, from https://pubs.usgs.gov/of/2004/1007/intro.html