141_lecture35

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Science-Technology

Published on January 8, 2009

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Slide 2: Whitney Lynn Slide 3: DESERTS are regions that receive very little rainfall, generally <25 cm/yr (<10 in/yr) and are sparsely vegetated. DESERTS are characterized by dryness, NOT sandiness. DESERTS are not devoid of life. This is the desert. There’s nothing out here. Nothing. -N. Karavasiles Yuha Desert, CA Slide 5: Classification of DESERTS is based on aridity. Use ARIDITY INDEX. Aridity index (AI) is the ratio of annual evaporation to annual precipitation. AI = 1 indicates evaporation = precipitation Represents a humid climate AI = 4 indicates four times as much evaporation as precipitation Represents desert climate. AI = 1.5-4 represents semi-arid climates. Slide 6: TYPES OF DESERTS SUBTROPICAL DESERTS Produced by atmospheric convection cells in equatorial regions. Sahara Desert, Arabian Desert Slide 8: TYPES OF DESERTS RAIN SHADOW DESERTS Produced on the leeward side of mountain ranges. Cascades, Sierra Nevada, Rockies Slide 9: TYPES OF DESERTS CONTINENTAL INTERIOR DESERTS Exist because land mass is so large, the interior is far from ocean moisture sources. Gobi Desert Slide 10: TYPES OF DESERTS DESERTS NEAR COLD OCEAN CURRENTS Cold air from cold ocean currents move over land. Air warms. Evaporates water from the land surface. Atacama Desert in Chile Slide 11: TYPES OF DESERTS POLAR DESERTS Remain cold all year. Only cold, dry air circulates in these regions. Greenland, Antarctica Slide 12: WEATHERING IN DESERTS Mostly mechanical weathering. Exfoliation Frost wedging Salt crystallization Some chemical weathering Chemical weathering produces DESERT VARNISH. DESERT VARNISH is a coating of manganese and iron oxides on rocks in deserts. Extremely slow process Slide 13: WEATHERING IN DESERTS DESERT VARNISH Slide 14: WATER IN DESERTS WATER is the most dominant force of erosion in deserts! Most desert erosion happens rapidly in FLASH FLOOD events. These short lived desert streams rapidly erode ARROYOS. Slide 15: WATER IN DESERTS FLASH FLOOD Before After Slide 16: WATER IN DESERTS ARROYOS Slide 18: WATER IN DESERTS PEDIMENT PEDIMENTS are gently sloping (<5?) erosional surface produced by erosion at the base of mountains. Slide 19: WATER IN DESERTS INSELBERG INSELBERGS are final stage of the erosion of a mountain. Result may be reduced to a resistant, steep-sided bedrock knob. Uluru (Ayers Rock), Australia Slide 20: STREAM DEPOSITON IN DESERTS Generally produces ALLUVIAL FANS at the base of mountains. Slide 21: STREAM DEPOSITON IN DESERTS Water may also collect in desert basins as temporary lakes. The lake may then evaporate producing a PLAYA. PLAYAS are deposits of salts precipitated as the lake water evaporates. Deposits include BORAX (Sodium borate or Na2B4O7.20H2O). 20 Mule Team Boraxo and Death Valley Days Great Salt Lake/Dead Sea Slide 22: PLAYA Death Valley, CA Slide 23: THE WORK OF WIND IN DESERTS After water, wind is the second-most effective force in the erosion of desert features. Wind is produced by heat changes in deserts. Ventifact Slide 24: THE WORK OF WIND IN DESERTS Most changes in deserts are due to movement of particles. Occurs because there is very little vegetation to hold soil or sediment in place. Particles moved in deserts can then abrade other rocks causing erosion. EROSION BY WIND - DEFLATION Slide 25: DESERT PAVEMENT Slide 26: SALTATION Slide 27: DUNE FORMATION Slide 28: Navajo Sandstone, UT Slide 29: DUNE SHAPES Influenced by Sand type Aridity Prevailing wind direction Vegetation Slide 30: TRANSVERSE DUNES Are parallel ridges. Are found in arid and semi-arid regions where sand is plentiful. Form where wind direction is constant. Form where there is little vegetation. Form perpendicular to wind direction. Slide 31: TRANSVERSE DUNES Slide 32: LONGITUDINAL DUNES Are parallel ridges. Form where sand supply is moderate. Form where wind direction varies in a narrow range. Form with ridges aligned parallel to wind direction. Slide 33: LONGITUDINAL DUNES Slide 34: BARCHAN DUNES Are crescent-shape ridges that form perpendicular to wind direction. Form where sand accumulates around sparse vegetation. Form on hard ground with very little sand. Form perpendicular to wind direction. Horns at ends of crescent point downwind. Velocity slows at center and sand is deposited there. Slide 35: BARCHAN DUNES Slide 36: PARABOLIC DUNES Are horseshoe shaped with horns pointing upwind. Form at sandy lake and ocean shores. Develop initially as transverse dunes. Transverse dunes are then subjected to deflation. Slide 37: PARABOLIC DUNES Slide 38: STAR DUNES Are the most complex of dunes types. Form when wind blows in a variety of directions (3 or more predominate). Dunes grow to a high central point. May have 3-4 arms radiating from central point. Slide 39: STAR DUNES Slide 40: LOESS Is wind-blown silt-sized particles. Resembles fine-grained lake mud. Accumulates downwind of plentiful supplies of silt. Generally as glaciers melt, rock flour is released. Rock flour is silt-sized particles. These particles are transported away from the glacier by water and later deposited. Once deposited, it is moved around by wind. Has accumulated to great thickness in the American Midwest and in China. Slide 41: LOESS Slide 42: Dust Bowl, Kansas, 1935

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