Chapter 19: Deserts and Wind

Deserts and arid regions occupy nearly one-third of Earth's terrestrial surface and are fundamentally defined by water scarcity in which evaporation persistently exceeds precipitation, rather than simply by low rainfall amounts. These environments are classified into two primary categories: true deserts with extremely limited moisture and steppes with semiarid conditions. The formation of major desert systems depends on global atmospheric circulation patterns, such as subtropical high pressure zones that suppress rainfall in equatorial and tropical latitudes, as seen in the Sahara and Arabian deserts. Coastal deserts intensify under the influence of cold ocean currents that reduce atmospheric moisture, while interior deserts form in continental rain shadows where mountains block moisture-bearing winds. Despite their apparent aridity, deserts remain dynamically shaped by running water as the dominant erosional agent, with brief but intense flash floods in ephemeral drainage channels producing significant landscape modification. Desert weathering typically generates mechanical fragments rather than chemical alteration due to moisture limitations. The resulting landscape evolution progresses through recognizable stages: initial mountain block erosion creates alluvial fans that merge into bajadas, basin centers accumulate temporary playa lakes that eventually become salt flats, and prolonged erosion leaves isolated bedrock remnants called inselbergs. Wind acts as a secondary but distinctive erosional and depositional force through deflation that lowers surfaces and removes fine sediment, abrasion that polishes rock surfaces into ventifacts and elongated yardangs, and protective desert pavement formation where coarser fragments concentrate as finer materials are transported away. Sediment transport occurs through saltation of sand grains skipping near the surface and suspension of dust traveling substantial distances. Wind-deposited features include six morphologically distinct dune types determined by wind regime consistency, available sediment supply, and vegetation stabilization, ranging from crescent-shaped barchans to complex star dunes. Loess deposits, composed of windblown silt from desert regions and glacial sources, accumulate in significant thicknesses across continental interiors and form exceptionally fertile agricultural soils. Understanding deserts requires recognition that human activities including overgrazing and deforestation accelerate land degradation processes, transforming marginal semiarid zones into expanding deserts through desertification.