deserts
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Deserts and Wind Chapter 14 Geology Today Barbara W. Murck Brian J. Skinner Ancient sand dunes, Utah N. Lindsley-Griffin, 1999 Deserts Source: U.S.G.S. What is a desert? An area where annual precipitation is less than 250 mm (10 in) N. Lindsley-Griffin, 1999 Desert Myths: N. Lindsley-Griffin, 1999 “Deserts are barren” - Wrong, most have at least some vegetation Organ Pipe National Monument, AZ Desert Myths: N. Lindsley-Griffin, 1999 “It never rains” - Wrong, rains are infrequent, but over a long enough period of time, the effects are large. San Rafael Swell, UT Desert Myths: “Water has little effect on deserts” Wrong, flash floods have tremendous power to erode and transport material Tarbuck-Lutgens, 1998; N. Lindsley-Griffin, 1999 Desert Myths: N. Lindsley-Griffin, 1999 “All erosion is by wind” - Wrong, most desert features formed by running water during wetter Ice Age climates Source: U.S.G.S. Desert Myths: N. Lindsley-Griffin, 1999 “Natural arches form by wind erosion” Wrong, mechanical weathering and mass wasting (rockfalls) are more important. Arches National Park, UT Desert Types Subtropical deserts are controlled by global climate zones where dry air descends to surface. (Fig. 14.2, p. 399) N. Lindsley-Griffin, 1999 Desert Types N. Lindsley-Griffin, 1999 Most of the western United States is subtropical arid and semiarid climate. Source: U.S.G.S. Desert Types Rainshadow Deserts form where mountains block moist air. Rising winds cool, release moisture (Seattle, Oregon, Northern California) Dry winds Descending winds are dry, warm up as they compress (Mojave Desert, Death Valley) Desert Wet winds Lee Slope Windward slope N. Lindsley-Griffin, 1999 Desert Types Coastal deserts are caused by global oceanic and atmospheric circulation Houghton-Mifflin, 1998; N. Lindsley-Griffin, 1999 Desert Types Coastal deserts form where cold coastal currents flow parallel to shore Cold coastal current chills onshore winds so they cannot carry moisture Source: U.S.G.S. N. Lindsley-Griffin, 1999 Polar Deserts - moisture frozen into ice sheets, not available for plant growth Air is too cold to carry moisture. Little snow falls, but what does fall does not melt. Desert Weathering Mechanical weathering is more important than chemical weathering (frost wedging, root wedging) Capital Reef National Park, UT N. Lindsley-Griffin, 1999 Desert Weathering N. Lindsley-Griffin, 1999 Salt crystallization is an important weathering process because salt expands as it crystallizes. Mudcracks and evaporites Source: U.S.G.S. Wind Erosion Eolian processes are particularly effective in arid and semi-arid regions Dust Storm Tarbuck & Lutgens, N. Lindsley-Griffin, 1999 Wind Erosion Sediment moves by suspension, surface creep, saltation Fig. 14.4, p. 402 N. Lindsley-Griffin, 1999 Wind Erosion Desert pavement forms by deflation smaller particles blow away, leaving surface covered with tightly packed large particles. Fig. 14.6, p. 404 Tarbuck & Lutgens, J.R. Griffin, N. Lindsley-Griffin, 1999 Wind Erosion N. Lindsley-Griffin, 1999 Desert Pavement: pebble- to cobble-sized rock fragments covering desert surface after lighter fragments have been Fig. 14.6, p. 404 removed by wind. Wind Erosion Wind-blown sand hammers at exposed rock faces to produce smooth flat surfaces Fig. 14.5, p. 403 Source: U.S.G.S. N. Lindsley-Griffin, 1999 Wind Erosion Ventifacts have at least one smooth abraded surface facing upwind. Ventifact Rock faceted and polished by windblown sand Tarbuck & Lutgens, J.R. Griffin, N. Lindsley-Griffin, 1999 Blowout - a small saucer-shaped depression formed by wind erosion in dunes. Common in Nebraska sand hills where protective vegetation has been disturbed. Blowout Wind Deposition N. Lindsley-Griffin, 1999 Sand dunes - Mounds of wind-blown sand Loess - Thick deposits of cohesive, unstratified wind-blown dust Sand Dunes Barchan dunes Dunes form wherever a supply of sand is available, strong wind blows constantly, and a barrier causes wind to lose velocity so sand can be deposited Coral Pink Sand Dunes State Park, AZ Tifernine dune field, Africa (space shuttle) N. Lindsley-Griffin, 1999 Wind Deposition Houghton Mifflin 1998; N. Lindsley-Griffin, 1999 Sand dunes dip gently on the upwind or windward side, are steep on downwind slip face. Wind Deposition Source: U.S.G.S. Sand ripples in Monument Valley N. Lindsley-Griffin, 1999 Sand Dunes Sand moves up windward slope by creep and saltation. It drops out as the wind loses velocity in the lee of the dune. Barchan Dunes N. Lindsley-Griffin, 1999 Sand Dunes Sand dune slip face: sand oversteepens at top, mass-wastes down the steep slip face of the dune Tarbuck & Lutgens, J.R. Griffin, N. Lindsley-Griffin, 1999 Sand Dunes Transverse dunes Parabolic dunes Houghton Mifflin, 1998; N. Lindsley-Griffin, 1999 The type of sand dune that forms depends on amount of sediment supply and wind direction Longitudinal dunes Star dunes Barchans Sand Dunes N. Lindsley-Griffin, 1999 Barchan dunes, crescent-shaped with horns pointing downwind, require moderate sediment supply and wind strength. (Fig. 14.9A, p. 407) Danakil Depression, Ethiopia Sand Dunes Transverse dunes, continuous asymmetrical ridges perpendicular to the strongest wind, require abundant sediment. (Fig. 14.9B, p. 407) N. Lindsley-Griffin, 1999 Takla Makan Desert, China Sand Dunes 300 m high star dunes in Libya; radar satellite image N. Lindsley-Griffin, 1999 Star dunes, isolated and stationary, form where winds blow from all directions. (Fig. 14.9C, p. 407) Sand Dunes Common in Africa and Australia (Fig. 14.9E, p. 407) N. Lindsley-Griffin, 1999 Longitudinal dunes, long narrow ridges parallel to wind, form where little sediment is available and winds are strong. Sand Dunes Ancient sand dune deposits can be recognized by the steep sets of cross beds, separated by gently dipping sets. Steep sets = slip face Gentle sets = windward face N. Lindsley-Griffin, 1999 Eolian Sediment Fine-grained (sand, silt, clay) Well-sorted Distinctive cross bedding style: low angle layers separating steep layers Sand grains rounded with frosted surfaces Source: U.S.G.S. Checkerboard Mesa - pattern caused by intersection of joints and eolian cross beds Source: U.S.G.S. N. Lindsley-Griffin, 1999 Alluvial Fans N. Lindsley-Griffin, 1999 Streams deposit sediment at slope change at mouth of canyon Fault-block Mountains Sediment-filled Playa Basins of internal drainage are common in the Basin and Range region of the U.S. Sand Hills Nebraska’s Sand Hills - a giant sand dune field formed near the end of the Ice Ages Sand dunes held in place by vegetation - if grass is destroyed, blowouts will form Dark green areas on map are up to 75% eolian sand NE Conservation & Survey, J.R. Griffin , 1999 Yellow area - Sand hills Wind Deposits Deposits of eolian sand and loess, Midwestern United States AGI/NAGI, J.R. Griffin, 1999 Desertification and Land Degradation Desertification - invasion of desert conditions into nondesert areas: Drought or climate change in areas that are already semiarid. Land degradation desertification caused by human activities: Too many people for resource base, poor agricultural practices, overgrazing, deforestation, misuse of surface and groundwater, civil war. N. Lindsley-Griffin, 1999 Desertification in the central Plains, 1930s The Aral Sea Aral Sea is drying up as water from tributary rivers is diverted to irrigate new cropland. Climate: winters colder, summers hotter, dust storms common. Economy: fishing industry gone, salinization is ruining soil. This desert was once the Aral Sea Houghton Mifflin 1998; N. Lindsley-Griffin, 1999
