Wind and Deserts
Introduction
• Wind is an important agent of erosion and sediment
transport
– In areas with limited rainfall
– In areas with limited vegetation
– This is generally desert areas that are sparsely populated
Air Pressure on Earth
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Cold air at the poles creates the Polar high pressure zone
Warm air at the equator produces the Equatorial low
20-30° North or South of the equator is the Subtropical high pressure zone
Sub-polar low at the northern part of the temperate zone
Air Circulation on a non-rotating Earth
• Air flows
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From polar highs to sub-polar lows
From subtropical highs to equatorial lows
From subtropical highs to sub-polar lows
In a non-rotating Earth, air would flow directly north or south
Global Wind Patterns
• Coriolis effect
– Causes winds in the northern hemisphere to veer to the right
– Causes winds in the southern hemisphere to veer toward the left
• Produces
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Polar easterlies
Westerlies
Northeast trades
Southeast trades
Sediment Transport
– Saltation
• Near the Earth’s surface
• Accounts for ¾ of sand transport in areas covered by dunes
– Saltation forms sand ripples
• Small grains are moved more easily than large ones
• Large ones left behind
• Ripples disappear in high winds that can move all grains
– Larger grains move by surface creep
– Finer particles are carried aloft in turbulent eddies
Sand Ripples, Death Valley, CA
Transport of Dust
• Zone of quiet air 0.5 mm thick above the surface
– Silt and clays (dust) don’t protrude above this zone and hence are not
picked up
– Larger grains stick up into more turbulent zone and are moved
– Dust particles need to be dislodged to begin to move
Deposition of Dust
• Happens because
– Wind velocity decreases
– Particles collide with rough or moist surfaces that trap them
– Particle accumulate to form aggregates which settle out because
of their mass
– Particles are washed out of the air by rain
– Vegetation acts as a trap
– When a topographic obstacle gets in the way
Effect of Obstacles
Problems of Windblown Sediment
• Loss of soil material
• Blowing sand can damage crops
• Animals suffer
– Cattle asphyxiated by dense dust
– Hair and skin sandblasted from their hind quarters
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Windshields can be frosted
Engines are damaged by dust
Reduction of visibility
Medical problems for humans by inhalation of dust
Major Dust Storms
Wind Deflation
• Produces depressions called blowouts
• Can remove soil
• Produced depressions, now known as spungs in South
Jersey
– South Jersey was a polar desert during the Pleistocene
• Ground was frozen in a continuous or discontinuous permafrost
• Sand was blown away until the surface encountered the water table, or
permafrost
Wind Deflation
Desert Pavement
• Deflation leads to removal of fines
• Coarse particles left behind
• Forms a desert pavement
Abrasion
• Ventifacts
– Wind sculpted particles
– Smooth surface, but often with abrasion marks
– Angular junctions of faces
Eolian Deposits
• Dunes
– Ridge of deposited sand
– Probably develop around a surface irregularity
– Typically asymmetrical
• Gentle slope up wind
• Steep slope downwind
• Slip face at angle of repose
– Migrate downwind
Dune Types
Loess
• Wind-deposited dust
– Thicker than the little bit of wind-blown dust that is practically
everywhere
– Largely silt-sized, but with some fine sand and clay
– Resource because productive soils develop on it as parent
material
• Key characteristics
– Forms uniform blanket
– Contains fossils of land plants and air-breathing animals
Loess
• Sources
– Deserts
– Floodplains of glacial melt-water streams
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Middle part of North America
East-central Europe
Similar composition to other glacial materials
Thickest downwind from former braided melt-water streams
Deposits thin downwind
Loess in the Central US
Wind-Blown Dust on the Seafloor
Deserts
• Now defined as areas where annual rainfall is less than 250 mm
(10 inches)
• Or where potential evaporation rate exceeds precipitation rate
• 25% of the land area outside the polar regions
• Types
– Subtropical Deserts
• Two most extensive belts of deserts are associated with belts of dry, descending air
centered between 20° and 30° North or South of the equator
• Sahara, Kalahari, Rub-al-Khali, Australian
Deserts of the World
Deserts
• Continental Interiors
– Land areas far from coasts and sources of moisture
– Gobi, Takla Makan in Asia
• Rainshadow
– Mountains create a barrier to water-bearing winds
– Form on the lee side of mountains in the “rain shadow”
– Eastern sides of the Cascades and Sierra Nevada
Deserts
• Coastal Deserts
– Places where cold, upwelling seawater cools air blowing on
shore
– As the air encounters the warm coast it contains too little
moisture
– Coastal deserts of Peru and southwestern Africa
• Polar Deserts
– Precipitation is low due to sinking of cold, dry air
– Water is in the form of ice
Desert Climate
• Dry
• Temperature fluctuations
– Very hot during the day
• 139.5 °F is record high in the Libyan desert of North Africa
• Other, appropriately located deserts get hot during the day
– Cool off dramatically at night
• Little or no cloud cover allows radiational cooling
• Windy
– Heated air rises during the day drawing in more air to replace it
Weathering and Mass Wasting
• Dryness
– Lack of vegetation
– Relatively slow chemical weathering
– Importance of mechanical weathering
• Mechanical weathering produces distinctive landforms
– Flat-lying sedimentary rocks that are eroded produce
characteristic features
• Mesa—table top mountains
• Butte—narrow, flat-topped hill or spire
Weathering and Mass Wasting
• Desert Varnish
– Thin, dark, coating of manganese oxide
– Formed by prolonged exposure to air
– Manganese may come from desert dust or by microoganisms
that release it
Desert Varnish
Fluvial Processes
• Flash floods
– Rain comes in bunches
– Little vegetation to impede flow
– Arroyos
• Stream valleys
• Filled during flash floods
• Dry much of the time
Arroyo
Fluvial Processes
• Alluvial fans
– Delta on land
– Produced where streams flow out of the mountains onto flat
valley floors
• Bajada—Coalescing alluvial fans
• Playa—lakebed on the floor of a desert valley
Playa and Bajada
Fluvial Processes
• Pediment
– Broad, relatively flat surface eroded across bedrock, and thinly covered with
alluvium
Pediment in the Mojave
Fluvial Processes
• Inselbergs
– Erosion remnants
– Isolated knobs, rocks, or hills
– Form in areas of homogeneous, resistant rocks surrounded by
less resistant rock
Inselberg
Desertification
• Expansion of desert areas
– Can result from natural or man-triggered climate changes
– Symptoms
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Declining groundwater tables
Increasing saltiness of water and topsoil
Reduction in supplies of surface waters
Unnaturally high rates of soil erosion
Destruction of native vegetation
Desertification
Advancing Barchans in Egypt