18.3: Weathering and Erosion Processes in Deserts

Weathering and Erosion Processes in Deserts

During our sedimentary section we discussed weathering and erosion in detail. Remember that water acts as a major agent of both physical and chemical weathering, and in desert climates water is scarce, which overall results in a much slower rate of weathering. When large amounts of precipitation fall over a short period of time, however, like during the rainy season, flash floods Links to an external site. and mud and/or debris flows Links to an external site. are likely to occur and are responsible for large amounts of rapid weathering and erosion (Figure 18.7).

As we learned about surface runoff in Chapter 9, the maximum particle size water can carry is tied to velocity, so high velocity flash floods play a major role in desert deposition. They are also a serious concern for desert travelers who must pay attention to regional weather. Surface runoff in deserts struggle to infiltrate the ground because the flow compacts the surface, plants are less common to slow flows, and soils in deserts can become hydrophobic Links to an external site.. Water typically runs off as sheetwash to stream channels called arroyos Links to an external site. or a dry wash that may be dry for part or most of the year. Dry ephemeral channels fill quickly, creating a mass of water and debris that charges down the channel, sometimes overflowing the banks of the arroyo.  People entering or camping in such channels have been swept away by these sudden flash floods.

Wind is another notable agent of weathering and erosion in deserts. Lower energy than water, wind transport can typically move sand, silt, and clay-sized sediment, but, just like water, maximum load size is dependent on velocity. Typically, silt and clay-sized material, like in dust storms or haboobs Links to an external site., can be carried miles up into the atmosphere and can travel across the globe. Dust from the Sahara Desert can reach the US and is found in sediment cores from the Atlantic Ocean. Dust from the Gobi Desert in China can reach the western United States and can be found in sediment cores in the Pacific Ocean.

Transported sand grains typically remain close to the surface and are moved by a process called saltation Links to an external site.. Sand grains are lifted into the moving air, carried a short distance, then drop and impact the surface, dislodging other sand grains and continuing the cycle. These wind-blown saltating sand grains are consequently well-rounded and have frosted surfaces (Figure 18.8). In areas where sand accumulates to form sand dunes, clumps of vegetation often anchor sediment on the desert surface. However, saltation from winds may still be sufficient to move or remove materials not anchored by vegetation causing bowl-shaped depressions in the sand called blowouts Links to an external site..

The saltating sand sandblasts and carves features into the bedrock, including distinctly shaped rocks and boulders called yardangs Links to an external site. and faceted rocks called ventifacts Links to an external site.. The wind is also responsible for the formation of desert pavement Links to an external site., a surface covered with closely packed, interlocking angular or rounded rock fragments of pebble and cobble size (Figure 18.9). Commonly many of these desert surfaces will also have desert varnish Links to an external site. (desert patina or rock rust), a thin dark brown layer of clay, iron and manganese oxides that form on stable surfaces within desert environments.