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Geol 104: Deserts Deserts • Any region that receives <25 cm (10 in) of rain annually and can support little or no vegetation (due to low soil moisture). • Cover ~25% of Earth’s land surface outside of polar regions. • Deserification is increasing amount of desert land annually. Conditions Leading to Deserts • Latitude and prevailing wind direction. • Topography and prevailing wind direction. • Orientation of coast and prevailing wind direction. • Distance from coast and prevailing wind direction. • Cold ocean currents and prevailing wind direction. Wind • Wind is the flow of air from areas of higher pressure to lower pressure. • The ultimate driving force for wind is solar energy. • Wind is controlled by: – Pressure gradient – Coriolis effect – Friction 1 Geol 104: Deserts Pressure Gradient Effect • The greater the difference in air pressures the greater the wind speed. • Isobars - lines on weather map connecting areas of equal air pressure. • Pressure gradient - amount of pressure change over a given distance. The closer the isobars the greater the pressure gradient and the stronger resulting winds. Coriolis Effect • All free-moving fluids are deflected on the globe due to the rotation of the Earth. • Wind deflection due to the Coriolis effect: – is always directed at right angles to the direction of air flow, – affects wind direction but not wind speed, – is affected by the wind speed (ie, stronger the wind, greater the deflection), and – is strongest at the poles, weakening to zero at the equator. Friction Effect • Friction slows surface winds which weakens the Coriolis effect causing the winds to cross isobars. • Roughness of terrain dictates angle of air flow across isobars (little effect over water but large over rough terrain). 2 Geol 104: Deserts Global Air Circulation • Combination of Coriolis effect and the differential heating of equator vs poles and land vs water. • General pattern consists of (fig. 13.3): A. Equatorial Lows • Near the equator, hot, moist air rises resulting in abundant precipitation and tropical rain forests. B. Subtropical Highs • As upper-level flow from equatorial lows sinks back down the associated heating produces hot arid regions (subtropical deserts) centered at about 30°N and S latitudes. Global Air Circulation (cont) C. Subpolar Lows • Form at convergence of Subtropical High and Polar High. Warm, moist mid-latitude air rises and meets with cold, dry polar air in zone of atmospheric instability at about 60°N and S latitudes. D. Polar Highs • Extremely cold, dry polar air subsides spreading toward the equator due to greater density of cold air. Prevailing Winds A. Easterly Trade Winds – Easterly flowing winds between the Subtropical highs and Equatorial lows. B. Prevailing Westerlies – Westerly flowing winds between the Subpolar lows and Subtropical highs. C. Polar Easterlies – Easterly flowing winds between the polar highs and Subpolar lows. 3 Geol 104: Deserts Global Precipitation Patterns • Major precipitation patterns are influenced by: 1. Global Circulation Patterns (latitude) • High precipitation at zones of convergence, low at zones of divergence. 2. Coastline vs Prevailing Wind Direction • Wind moving off water yields high precipitation. 3. Topography and Extent of Landmass • Rainshadow effect - low precipitation on leeward side of mountains (fig. 13.4). • Precipitation greatly decreases with distance from shore (source of moisture). Global Distribution of Deserts • Regions of low precipitation or high evaporation. • See fig. 13.3 • Note relationship between location of deserts and global air circulation and precipitation patterns. Wind Action in Deserts • Winds are very strong in desert areas because: 1. Low humidity 2. Great temperature range 3. Little or no vegetation 4 Geol 104: Deserts Wind Erosion • Deflation - removal of loose materials by wind; often results in lowering of land surface. • Desert Pavement - pebbles and cobbles left behind as wind erodes smaller particles. Wind Transportation • Dust Storms • Most sand transport is near the surface by saltation. • Ventifact - cobble of boulder with flattened. Polished faces due to wind-blown sand. Wind Deposition • Dunes – Mound or ridge of well sorted, rounded windblown sand. – Types of Dunes • • • • Barchan Transverse Parabolic Longitudinal • Loess – Homogeneous, unlayered deposit of windblown silt and clay, usually of glacial origin. 5 Geol 104: Deserts Desert Landforms • Desert Streams - seasonal flow. – Wash - stream bed that is dry most of the time. • Desert Lakes - seasonal due to seasonal stream flow. – Playa - lake bed that is dry most of the year. • Debris flows - desert soils can become saturated and flow. – Flash floods - slow infiltration of rain leads to rapid increase in discharge and short lag time. Desert Landscapes • Plateau – Large elevated area of fairly flat land, capped by resistant rock. • Mesa – Broad flat-topped hill with steep cliff faces capped by resistant rock, smaller than plateau. • Butte – Narrow flat-topped hill with very steep cliffs capped by resistant rock, smaller than mesa. • See fig. 13.7 Desertification • Process whereby productive potential of arid or semiarid land falls by 10% or more (10-25% - moderate, 25-50% - severe, and >50% - very severe). • Estimates are: – 3.1 million mi2 (area 12 times size of Texas) have become desertified in past 50 years. – Current trend could threaten livelihood of 1.2 billion people.Each year, 23,000 mi2 undergo low to moderate, and 81,000 mi2 (area the size of Kansas) undergo severe desertification. 6 Geol 104: Deserts Human factors that leave topsoil vulnerable to desertification: • Overgrazing on fragile arid or semi-arid range land. • Deforestation without reforestation. • Surface mining without reclamation. • Irrigation in areas of high evaporation leading to salt buildup. • Soil compaction by machinery, livestock, etc. Global implications • What are the global implications of desertification? • How is desertification (and remediation of desertification) linked to social and biophysical processes? • What are the local implications of desertification (how might it affect you)? 7