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The Work of Wind and Deserts
GEOL: CHAPTER 15 The Work of Wind and Deserts
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The Mesquite Flat sand dunes in Death Valley, California, are a mix of dominantly transverse-type dunes with some crescent-type dunes and star-type dunes. The Mesquite Flat sand dunes in Death Valley, California, are a mix of dominantly transverse-type dunes with some crescent-type dunes and star-type dunes.
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Learning Outcomes LO1: Discuss the role wind plays in transporting sediment LO2: Explain the two processes of wind erosion LO3: Identify the types of wind deposits LO4: Describe the air-pressure belts and global wind patterns
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Learning Outcomes, cont.
LO5: Describe the distribution of deserts LO6: Identify the various characteristics of deserts LO7: Identify the different types of desert landforms
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More than 6,000 years ago, the Sahara was a fertile savannah supporting a diverse fauna and flora, including humans. Then the climate changed, and the area became a desert. How did this happen? Will this region change back again in the future? These are some of the questions geoscientists hope to answer by studying deserts. By this point in the semester, you should have some theories of your own! More than 6,000 years ago, the Sahara was a fertile savannah supporting a diverse fauna and flora, including humans. Then the climate changed, and the area became a desert. How did this happen? Will this region change back again in the future? These are some of the questions geoscientists hope to answer by studying deserts.
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Desertification The expansion of deserts into formerly productive lands Human agriculture has altered natural vegetation patterns Sahel hard hit: south of Sahara Desert Famines Malnutrition Poverty
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Wind Sediment Transport
Wind is less dense than water, so can only transport smaller sediments Bed load: moves by saltation or by rolling or sliding Saltation: wind lifts sand grains that dislodge other grains upon hitting the surface
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This should look familiar. Think of stream transport.
Most sand is moved near the ground surface by saltation. Sand grains are picked up by the wind and carried a short distance before falling back to the ground, where they usually hit other grains, causing them to bounce and move in the direction of the wind. This should look familiar. Think of stream transport. Figure 15.1 Saltation Most sand is moved near the ground surface by saltation. Sand grains are picked up by the wind and carried a short distance before falling back to the ground, where they usually hit other grains, causing them to bounce and move in the direction of the wind.
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Erosion Natural forces of wind and water have eroded the rocks in Monument Valley, Arizona, to create “mitten” buttes such as this one. Figure 15.2 Erosion Natural forces of wind and water have eroded the rocks in Monument Valley, Arizona, to create “mitten” buttes such as this one.
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Wind Sediment Transport, cont.
Suspended load: Silt- and clay-sized particles Will usually stay at surface unless physically disturbed Once in the air, smaller particles can travel thousands of miles
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Wind Erosion: Abrasion
Caused by saltating sand grains Analogous to sandblasting Rarely more than a meter above ground Typically modifies existing features Ventifacts: surfaces altered by windborne particles
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a b Ventifacts a. A ventifact forms when windborne particles (1) abrade the surface of a rock, (2) forming a flat surface. If the rock is moved, (3) additional flat surfaces are formed. b. Numerous ventifacts are visible in this photo, which also shows desert pavement in Death Valley, California. Desert pavement prevents further erosion and transport of a desert's surface materials by Figure 15.3 Ventifacts
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Wind Erosion: Deflation
Removal of loose surface sediment by wind Deflation hollows/blowouts from differential erosion Desert pavement: close-fitting pebbles and cobbles caused by wind eroding away smaller particles
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Figure 15.4 Deflation Hollow A deflation hollow, the low area, between two sand dunes in Death Valley, California. Deflation hollows result when loose surface sediment is differentially removed by wind. A deflation hollow, the low area, between two sand dunes in Death Valley, California. Deflation hollows result when loose surface sediment is differentially removed by wind.
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Figure 15.5 Desert Pavement
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Figure 15.5 Desert Pavement
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Dune Formation Mound or ridge of wind-deposited sand
Form around an obstruction that stops sand grains Shallow windward slope Steep leeward slope: angle of repose Dune can migrate in direction of strong prevailing winds
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Figure 15. 6 Sand Dunes Large sand dunes in Death Valley, California
Figure 15.6 Sand Dunes Large sand dunes in Death Valley, California. The prevailing wind direction is from left to right, as indicated by the sand dunes in which the gentle windward side is on the left and the steeper leeward slope is on the right. Sand Dunes Large sand dunes in Death Valley, California. The prevailing wind direction is from left to right, as indicated by the sand dunes in which the gentle windward side is on the left and the steeper leeward slope is on the right.
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Dune Migration a. Profile of a sand dune.
Wind Direction of dune migration b. Dunes migrate when sand moves up the windward side and slides down the leeward slope. Such movement of the sand grains produces a series of crossbeds that slope in the direction of wind movement. Sand moves by saltation Windward side Leeward slope Wind a. Profile of a sand dune. Figure 15.7 Dune Migration Stepped Art Fig. 15-7, p. 308
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Cross-Bedding Ancient cross-bedding in sandstone beds in Zion National Park, Utah, helps geologists determine the prevailing direction of the wind that formed these ancient sand dunes. Figure 15.8 Cross-Bedding Ancient cross-bedding in sandstone beds in Zion National Park, Utah, helps geologists determine the prevailing direction of the wind that formed these ancient sand dunes.
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Dune Types Barchan Longitudinal Transverse Parabolic Determined by:
Sand supply Wind direction and velocity Vegetation
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Barchan Dunes Crescent shaped, tips point downwind
Form in areas with a flat, dry surface Little vegetation Nearly constant wind direction Mobile Up to 30 meters high
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Barchan Dunes Figure 15.9 Barchan Dunes
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Longitudinal Dunes Long parallel ridges of sand that are parallel to prevailing winds Limited sand supply Winds converge from slightly different directions 3-100 meters in height Up to 100 km long
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a b Longitudinal Dunes a. Longitudinal dunes form long, parallel ridges of sand aligned roughly parallel to the prevailing wind direction. They typically form where sand supplies are limited. b. Longitudinal dunes, 15 m high, in the Gibson Desert, west central Australia. The bright blue areas between the dunes are shallow pools of rainwater, and the darkest patches are areas where the Aborigines have set fires to encourage the growth of spring grasses. Figure Longitudinal Dunes
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Transverse Dunes Long ridges perpendicular to prevailing wind direction Abundant sand with little or no vegetation “Sand seas” Up to 200 meters high Up to 3 km long
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Figure Transverse Dunes Transverse dunes form long ridges of sand that are perpendicular to the prevailing wind direction in areas of little or no vegetation and abundant sand. Transverse dunes form long ridges of sand that are perpendicular to the prevailing wind direction in areas of little or no vegetation and abundant sand.
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Parabolic Dunes Common in coastal areas Abundant sand
Strong onshore winds Form where vegetation cover is broken, from a deflation hollow or blowout Tips point upwind
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Figure 15.12 Parabolic Dunes
a. Parabolic dunes typically form in coastal areas that have a partial cover of vegetation, a strong onshore wind, and abundant sand. b. A parabolic dune developed along the Lake Michigan shoreline west of St. Ignace, Michigan.
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Loess Wind-blown silt and clay deposits
Quartz grains, feldspar, micas, calcite Three sources: Deserts Pleistocene glacial outwash deposits River floodplains in semiarid regions
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Loess, cont. Easily eroded Steep cliffs and rapid stream erosion
10% Earth surface 30% U.S. surface Fertile soils
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Global Air-Pressure Belts
Air pressure: density of air exerted on surroundings (weight) Heated air = lower surface air pressure; much solar heating Cooler air = higher surface air pressure; less solar heating
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Global Air-Pressure Belts, cont.
Equatorial zone receives most solar energy Surface air rises, cools, releases moisture Rising air is drier as it moves poleward At degrees latitude, it sinks, compresses, and warms to form a high-pressure area conducive to deserts
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Global Wind Patterns Winds: air flows from high-pressure areas to low-pressure areas Coriolis effect: apparent deflection of moving objects because of Earth’s rotation Winds deflected to the right in the Northern Hemisphere To the left in the Southern Hemisphere
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The General Circulation Pattern of Earth’s Atmosphere
Figure The General Circulation Pattern of Earth’s Atmosphere Air flows from high-pressure zones to low-pressure zones, and the resulting winds are deflected to the right of their direction of movement (clockwise) in the Northern Hemisphere and to the left of their direction of movement (counterclockwise) in the Southern Hemisphere. This deflection of air between latitudinal zones resulting from Earth’s rotation is known as the Coriolis effect.
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Distribution of Deserts
30% of land surface Low and middle latitudes Potential evaporation greater than yearly precipitation Semiarid: more precipitation than arid
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Distribution of Deserts, cont.
Arid = desert Deserts Less than 25 cm precipitation per year High evaporation rates Poorly developed soils Mostly devoid of vegetation
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The Distribution of Earth’s Arid and Semiarid Regions
Semiarid regions receive more precipitation than arid regions, yet they are still moderately dry. Arid regions, generally described as deserts, are dry and receive less than 25 cm of rain per year. The majority of the world’s deserts are located in the dry climates of the low and middle latitudes. Figure The Distribution of Earth’s Arid and Semiarid Regions Semiarid regions receive more precipitation than arid regions, yet they are still moderately dry. Arid regions, generally described as deserts, are dry and receive less than 25 cm of rain per year. The majority of the world’s deserts are located in the dry climates of the low and middle latitudes.
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Climate Characteristics of Deserts
Hot summer days: 32ºC – 50ºC Cooler on winter days: 10ºC - 18ºC Precipitation variable and unpredictable Plants are small, widely spaced, grow slowly Stems and leaves minimize water loss
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Desert Vegetation Desert vegetation is typically sparse, widely spaced, and characterized by slow growth rates. The vegetation shown here is in Death Valley, California. Figure Desert Vegetation Desert vegetation is typically sparse, widely spaced, and characterized by slow growth rates. The vegetation shown here is in Death Valley, California.
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Weathering and Soils Mechanical weathering dominates
Temperature fluctuations Frost wedging Some chemical weathering, though minor Desert soils are thin, patchy and subject to erosion
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Desert Streams Most precipitation comes from brief and heavy cloudbursts Rapid runoff quickly fills channels, with rapid sediment transport and much erosion Most streams flow intermittently and don’t reach the sea (internal drainage) Some permanent streams: Colorado River
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Playa Lakes Excess water from rainstorms accumulates in low-lying areas Temporary: hours to months Shallow and saline water Evaporates to leave a playa (salt pan)
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Figure 15.16 Playas and Playa Lakes
A playa lake formed after a rainstorm near Badwater, Death Valley National Park. Playa lakes last from a few hours to several months.
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Figure 15.16 Playas and Playa Lakes
Salt deposits and salt ridges cover the floor of this playa in the Mojave Desert. Salt crystals and mud cracks are characteristics features of playas.
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Alluvial Fans and Pediments
Sediment-laden streams leave mountains and create fan-shaped sediment deposit on flat desert areas Common in the Basin and Range province in western North America Pediment: an erosion surface of low relief gently sloping away from a mountain range
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Remember the Alluvial fans from Chapter 12?
Figure Alluvial Fan A ground view of an alluvial fan, Death Valley, California. Alluvial fans form when sediment-laden streams flowing out from a mountain deposit their load on the desert floor, forming a gently sloping, fan-shaped, sedimentary deposit.
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Mesas and Buttes Mesa: broad and flat-topped; erosional remnant bounded by steep slopes Buttes: mesa eroded into a pillar-like structure Both have an erosion-resistant cap rock underlain by more easily eroded rock
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Buttes Right Mitten Butte and Merrick Butte in Monument Valley Navajo Tribal Park on the border of Arizona and Utah. Figure Buttes Right Mitten Butte and Merrick Butte in Monument Valley Navajo Tribal Park on the border of Arizona and Utah.
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Virtual Field Trip The effects of wind erosion
The features of wind deposition A desert landform
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