Chapter 19 Deserts Wind and Deserts • A desert is any region that receives less than 25cm (10in) of rain per year. • Evaporation equals or exceeds rainfall. • Boulder gets about 62 cm (24 in) of rain per year. • Deserts tend to have sparse vegetation due to low moisture, and high relief due to short duration catastrophic rain with no vegetation to hold soils. Desert Terms • Desert • Desert pavement Desert Erosion • Descending air • Deflation • Leeward • Yardang • Plateau • Ventifact • Water is the primary agent of erosion. • Mesa • Dune • Wind is a secondary agent of erosion. • Butte – Barchan • Zone of erosion – Transverse dune – Longitudinal dune • Zone of deposition – Blowout • Alluvial fan • Loess • Playa Geography of Deserts Rising and Descending Air • The geographic location of deserts is • Rising air masses expand, cool, controlled by descending air masses. and precipitate moisture. • This occurs on the leeward side (rain • Descending air masses shadow) of a mountain range. compress, warm, dry, and • This also occurs at latitudes 30 evaporate moisture. degrees N and 30 degrees S as a result of global air circulation. 1 Rising and Descending Air Rising and Descending Air 40 Thought questions Geography of Deserts • Why are ski areas in Colorado on the west side of the divide? • What is the prevailing wind direction? • Is the air rising or falling on the western slope? • Why is it so windy in Boulder? • Does precipitation add or subtract heat from the air? Desert Landforms Desert Landforms: • Desert regions with sedimentary Plateau, Mesa, Butte bedrock develop characteristic landforms. – A plateau is a broad, level, elevated region commonly bounded partly by cliffs. – A mesa is smaller, flat-topped hill bounded by cliffs. – A butte is a narrow hill of resistant rock bounded by cliffs. 2 Desert Landforms: Desert Landforms: Plateau, Mesa, Butte Plateau, Mesa, Butte Desert Landforms: Desert Landforms: Plateau, Mesa, Butte Plateau, Mesa, Butte Desert Landforms Zones of Erosion and Deposition • Desert regions with igneous or metamorphic bedrock also develop characteristic landforms with – Zones of erosion which have little or no soil or vegetation, high relief and near total outcrop of bedrock. – Zones of deposition which have low relief, some soil development, and sparse vegetation. • Alluvial Fans are fan-shaped deposits of material in zone of deposition. • Playa Lakes are ephemeral (temporary) lakes. 3 Zones of Erosion and Deposition Zones of Erosion and Deposition Alluvial Fan Alluvial Fans Alluvial Fan with Debris Flows Alluvial Fan 4 Playa Lake Dunes and Deflation • Wind moves sand and silt in zone of deposition. • Silt and Clay are removed. • Sand is moved to leeward end of valley to form a dune field. • Pebbles and cobbles are left behind to form an desert pavement. • Process is called deflation. Dune field, Great Sand Dunes Dunes: Barchan Transverse Blowout Longitudinal (Linear) Barchan Dunes: Blowout Dunes: Crescent-shaped with limbs (Parabolic Dunes) downwind; Crescent-shaped with limbs Formed when sand moves upwind; formed when winds across outcrop / pavement breaks through vegetation 5 Transverse Dunes: Longitudinal Dunes: Perpendicular to wind direction; Parallel to wind direction; formed formed in regions of abundant sand in regions of limited sand. (beaches). Longitudinal Dunes: Desert Pavement Desert Desert Varnish Varnish 6 Yardangs, Iran Desert Result from wind erosion of Varnish partially lithified dunes. Yardangs, Iran Result from wind erosion of Ventifact partially lithified dunes. is a wind- feceted cobble or pebble Loess • Silt and clay-sized particles are Loess deposited by wind in wetter regions. • Loess is a well-sorted deposit of wind-deposited silt and clay sized particles. • Loess is a common peri-glacial deposit. 7 Desert Terms Clicker Question • Desert • Desert pavement • Descending air • Deflation • Temperate deserts primarily occur: • Leeward • Yardang • Plateau • Ventifact – A. At 30º N and 30º S latitude and in the • Mesa • Dune rain shadows of mountain ranges. • Butte – Barchan – B. At the equator • Zone of erosion – Transverse dune – C. Below mountain ranges – Longitudinal dune • Zone of deposition – D. In Africa – Blowout • Alluvial fan – E. In the southern hemisphere. • Loess • Playa Clicker Question Clicker Question • Temperate deserts primarily occur: • A crescent-shaped sand dune with its limbs downwind is a – A. At 30º N and 30º S latitude and in the rain shadows of mountain – A. Blowout dune ranges. – B. Barchan – B. At the equator – C. Transverse dune – C. Below mountain ranges – D. Longitudinal dune – D. In Africa – E. Waddi – E. In the southern hemisphere. Clicker Question Clicker Question • A crescent-shaped sand dune with its • A crescent-shaped sand dune with its limbs downwind is a limbs upwind is a – A. Blowout dune – A. Blowout dune – B. Barchan – B. Barchan – C. Transverse dune – C. Transverse dune – D. Longitudinal dune – D. Longitudinal dune – E. Waddi – E. Waddi 8 Clicker Question • A crescent-shaped sand dune with its limbs upwind is a – A. Blowout dune – B. Barchan – C. Transverse dune – D. Longitudinal dune – E. Waddi 9.