DESERTS

Deserts

• What is a desert?
• < 25 cm/yr rainfall
• Evaporation > precipitation
• Too dry to support much vegetation
• Deserts
• Cover ~25% of Earth's surface area outside of polar regions
• Have varied landscapes - mountains, plains, plateaus; with sand or without

Distribution of Deserts

• Low-latitude deserts
• Deserts tend to occur at 30° latitude - Low-latitude desert
• Related to atmospheric circulation

Effect of Latitude

• Warm, moist air rises at equator; cool, dry air sinks at poles
• The rotation of Earth causes Coriolis Effect and breakdown of single cell into three cells in each hemisphere
• Coriolis effect causes deflection to right in northern hemisphere, to left in sourthern hemisphere

Wind Patterns

Distribution of Deserts
Mid-latitude Deserts

• Effect of topography
• Rain shadow
• Warm, moist air rises on windward side of mountains, cools, and rains out
• On lee side, dry cool air sinks
• Mid-latitude (rain shadow) deserts include Great Basin Desert, eastern Washington state, Death Valley, Gobi desert
• Effect of large bodies of water
• Large bodies of water may add moisture to a region that might otherwise be dry
• Ex: Gulf of Mexico and south Texas

Desert Erosion

• Wind erosion is more significant in arid regions than humid regions, but most desert landscapes are formed by running water
• Chemical weathering occurs, but it is much slower than in humid climates

Desert Landforms

• Desert Landforms
• Playa lake - intermittent lake of internal drainage basin (evaporites form)
• Playa - dry lake bed
• alluvial fan
• Bajada - coalesced alluvial fans
• Inselbergs - bedrock knobs projecting above sediment-filled basin
• Arroyo - steepwalled stream channel; often dry

Desert Landforms (cont.)

• Plateau - uplifted plain (not restricted to deserts
• Mesa - table-shaped mountain; smaller than plateau, larger than butte
• Butte - flat-topped mountain

Processes of Wind Erosion

• Processes of Wind Erosion
• Deflation - removal of loose and dry sediment by wind.
• Blowouts - shallow depressions caused by deflation in unprotected areas  (Fig. 13.11)
• Desert pavement - pavement of cobbles and pebbles which remain after deflation removes fine material (Fig. 13.12)
• Abrasion - removal of material by impacting particles
• Ventifacts - cobbles and pebbles polished and smoothed by wind

Dunes

• Dunes - mounds or ridges of sand transported by wind.
• Types of Dunes:
• Barchan - crescent shaped; solitary; tips point downwind; constant wind direction; limited sand supply
• Transverse - dunes at right angle to wind; abundant sand
• Parabolic - crescent shaped, but end point upwind
• Longitudinal - dunes parallel to wind direction; limited sand supply

Loess

• Loess
• Windblown silt deposit; unstratified; tan
• Vertical cliffs; forms excellent soil

• Occurs in U.S. midwest (windblown glacial sediments), China, and other areas

• Desertification - expansion of desert into non-desert areas
• What was causing the expansion of the Sahara?