13.3: Desert Weathering and Erosion

Figure \(\PageIndex{1}\): Weathering and erosion of Canyonlands National Park have created a unique landscape, including arches, cliffs, and spires.

Weathering takes place in desert climates by the same means as other climates, only at a slower rate. While the higher temperatures of some deserts would spur faster weathering, water is the main agent of weathering, so the lack of it slows weathering. Precipitation occurs in deserts, only less than in other climatic regions. When precipitation does occur, often in the form of flash floods, a large amount of mechanical weathering can happen quite quickly.

Figure \(\PageIndex{1}\): Newspaper rock, near Canyonlands National Park, has many petroglyphs carved into the desert varnish.

One unique weathering product of deserts is desert varnish. Also known as desert patina or rock rust, they are thin dark brown layers of clays and iron and manganese oxides that form on very stable surfaces within arid environments. The exact cause of the material is still unknown, though cosmogenic and biologic mechanisms have been proposed.

Figure \(\PageIndex{1}\): A dust storm (haboob) hits the Mongolian Gobi.

While water is still the dominant agent of erosion in most desert environments, wind is a notable agent of weathering and erosion in many deserts. This includes suspended sediment traveling in dust storms that frequent deserts. Deposits of windblown dust are called loess. Loess deposits cover wide areas of the midwestern United States, much of it from dust that melted out of the ice sheets during the last ice age [7]. Lower energy than water, wind transport nevertheless moves sand, silt, and dust [8]. As noted in the chapter on water, the load carried by a fluid (like air) is distributed among bedload and suspended load. As with water, in wind, these components depend on wind velocity.

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 dislodging other sand grains which are then carried a short distance and splash dislodging still others [8].

Figure \(\PageIndex{1}\): Enlarged image of frosted and rounded windblown sand grains

Since saltating sand grains are constantly impacting other sand grains, windblown sand grains are commonly pretty well rounded with frosted surfaces. In dust storms, the fine-grained suspended load is effectively removed from the sand and the surface as silt is carried away, leaving the sand behind. Wind is thus an effective sorting agent separating sand and dust-sized particles [9]. When wind velocity is high enough, materials may slide or roll along the surface.

One extreme version of sediment movement was shrouded in mystery for years: sliding stones. These are large moving boulders along flat surfaces in deserts, leaving trails. This includes the famous example of the Racetrack Playa in Death Valley National Park, California. For years, scientists and enthusiasts attempted to explain their movement, with little definitive results [10; 11]. In recent years, several experimental and observational studies have confirmed that thin layers of ice allow the stones to move with high winds providing propulsive energy [12; 13]. These studies include measurements of actual movement, as well as re-creations of the conditions, with resulting movement in the lab.

References

7. Muhs, D. R. & Bettis, E. A. Quaternary loess-paleosol sequences as examples of climate-driven sedimentary extremes. Special Papers-Geological Society of America 53–74 (2003).

8. Bagnold, R. A. The physics of blown sand and desert dunes. Methum, London, UK 265 (1941).

9. Shao, Y. Physics and Modelling of Wind Erosion. (Springer Science & Business Media, 2008).

10. Stanley, G. M. Origin of playa stone tracks, Racetrack Playa, Inyo County, California. Geol. Soc. Am. Bull. 66, 1329–1350 (1955).

11. Clements, T. Wind-blown rocks and trails on Little Bonnie Claire Playa, Nye County, Nevada. J. Sediment. Res. 22, (1952).

12. Norris, R. D., Norris, J. M., Lorenz, R. D., Ray, J. & Jackson, B. Sliding rocks on Racetrack Playa, Death Valley National Park: first observation of rocks in motion. PLoS One 9, e105948 (2014).

13. Kletetschka, G. et al. Sliding stones of Racetrack Playa, Death Valley, USA: The roles of rock thermal conductivity and fluctuating water levels. Geomorphology 195, 110–117 (2013).