Chapter 21: Dry Regions: The Geology of Deserts

Deserts are defined not solely by low annual precipitation below 25 centimeters, but by the combination of high evaporation rates, minimal vegetation cover, and pronounced diurnal temperature fluctuations. The chapter categorizes deserts into five primary types based on their geographic and climatic origins: subtropical deserts formed by atmospheric circulation patterns, rain-shadow deserts created by orographic effects, coastal deserts influenced by cold ocean currents, continental-interior deserts isolated from moisture sources, and polar deserts characterized by perpetual aridity despite frozen precipitation. In arid environments, physical weathering mechanisms such as salt crystallization and thermal expansion dominate over chemical weathering, fundamentally altering how landscapes evolve. Flash floods and ephemeral streams generate characteristic erosional features including talus aprons and alluvial fans, while wind erosion through saltation and deflation produces lag deposits and creates striking rock formations such as ventifacts and yardangs. The chapter explores sand dune morphology and migration patterns across multiple dune types, from crescent-shaped barchan dunes to longitudinal seif dunes, and describes prominent landforms including mesas, buttes, and hoodoos shaped by differential erosion. Desert surfaces develop distinctive features such as desert varnish, a dark patina of manganese and iron oxides, and desert pavement composed of closely-packed coarse sediment. The chapter also addresses biological adaptation in desert ecosystems, the expanding global phenomenon of desertification driven by climate change and human land use, and the far-reaching environmental consequences of dust storms that transport sediment and contaminants across intercontinental distances.