Chapter 21 Deserts
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Chapter 21
Deserts
Chapter 21
Outline• Deserts: extents and basics.
• Desert types-Subtropical, rain shadows, cont. interiors, coastal, polar.
• Desert Processes-Weathering, soils, desert varnish-Erosion by water and wind-Deposition and transport
• Desert Landscapes-Cliffs & mesas-Pediments and bajadas-Dunes, their types and controls
• Desertification
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Deserts• Deserts cover ~25% of all land surface• Primary control on deserts? Plate tectonics• Characterized by…
• Extreme dryness• Specialized ecosystems, low human populations• Unique geologic and surface processes
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What Is a Desert?• Land so arid, that…
• Evaporation prevents permanent surface water• Vegetation covers <15% of surface• Annual rainfall <10” (25cm)
• Deserts exist in both hot and cold climates
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Hot vs. Cold Deserts• Cold deserts:
• High latitudes• High elevations• Near cold ocean currents
• Hot deserts:• Low latitudes • Low elevations• Far from oceans
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Desert Features• Desert surfaces often include…
• Exposed bedrock• Accumulated clasts• Unweathered sediment• Precipitated salt• Windblown sand
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Outline• Deserts: extents and basics.
• Desert types-Subtropical, rain shadows, cont. interiors, coastal, polar.
• Desert Processes-Weathering, soils, desert varnish-Erosion by water and wind-Deposition and transport
• Desert Landscapes-Cliffs & mesas-Pediments and bajadas-Dunes, their types and controls
• Desertification
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Types of Deserts• 5 Types (distinctive landscapes & biota):
1. Subtropical (Sahara, Arabian, Kalahari)
2. Rain shadow (e.g. Eastern OR?WA, Nevada)
3. Coastal (Atacama)
4. Continental interiors (Gobi- southern Mongolia)
5. Polar (Antarctica)
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Subtropical Deserts
• Due to patterns of atmospheric circulation. • Equator – 0o latitude.
• Solar energy evaporates water, which rises as hot, moist air• Rising air cools and expands, forming abundant rain• This air, stripped of moisture, flows N and S
• Subtropics – 20o-30o N & S.• Sinking dry air wicks water• Landscape below dries• Earth’s largest deserts
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• At 20-30o N & S latitude across time • African deserts bracket
• North: Sahara and Arabian• Rainforest straddling equator• South: Namib and Kalahari
Subtropical Deserts
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Rainshadow Deserts
• Wet ocean winds are driven over• Windward air is forced to rise and cool
• Moisture condenses and rains• Leeward air, w/o moisture, sinks
• Sinking air warms and sucks water out of land• Example: Nevada 9east side of Sierras)
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Coastal Deserts• Cool air over cold ocean water holds little moisture
• Air absorbs moisture when it interacts with land• Atacama desert (Peru) -> driest place on Earth
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Interior Deserts
• Air looses moisture crossing continents• Land far from ocean (water source) can be arid
• Example: Gobi desert in Mongolia
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Polar Deserts• Above 66º N & S latitude there is little moisture vapor
• Air circulation carries dry air to polar regions• It is so cold, air can’t hold water
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Outline• Deserts: extents and basics.
• Desert types-Subtropical, rain shadows, cont. interiors, coastal, polar.
• Desert Processes-Weathering, soils, desert varnish-Erosion by water and wind-Deposition and transport
• Desert Landscapes-Cliffs & mesas-Pediments and bajadas-Dunes, their types and controls
• Desertification
Chapter 21
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Desert Processes• Deserts are characterized by:
• Erosion by water and wind• Weathering.• Soil formation.• Deposition.
• These processes result in unique landscapes
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Weathering
• Physical weathering dominates• Rare chemical weathering leaches ions• Evaporation -> salts both break and cement grains
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Desert Soils
• Desert soils are thin, with poorly defined horizons• Usually colored like the bedrock• Trace elements/oxidation cause wide color variations/red
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Desert Varnish
• Exposed surfaces develop desert varnish.• Dark coating of iron and manganese oxides• Forms very slowly by bacterial activity + dust + water• Native Americans left petroglyphs in desert varnish
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Water Erosion
• Though rare, water shapes desert landscapes.• Lack of veg/roots magnifies sediment erosion and transport• Flashy rainfall generates dangerous floods
• Rapid flow of thick, muddy, and viscous water sculpts land• Water can quickly infiltrate dry stream beds
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• Ground is scoured by wind• Sand & silt-sized sediment is lifted and moved:
• Surface load – grains moved along land surface• Saltation – sand skipped, bounced by grain impact• Suspended load – sediment carried in air
• High winds can carry dust across entire oceans
Wind Erosion and Transport
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• Coarse clasts can’t be lifted by wind• Finer sediment removed; forms lag deposits (deflation)• Creates desert pavement
-> Surface layer of stones• Becomes resistance to erosion…
Wind Erosion and Transport
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• However, deflation is now known to be followed by…• Soil accumulation below pavement (inflation!)• Pavement is lifted• -> Surface stones crack and fit together better
• Often coated with desert varnish
Wind Erosion and Transport
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• Deflation – lowering land surface via erosion…• Concentrated wind erosion creates blowouts…
• Desert processes clearly operating on Mars
Wind Erosion and Transport
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• Grains in wind often ‘sandblast” exposed surfaces• Produces unusual products:
• Ventifacts -> stones with wind-faceted upper surfaces• Yardangs -> wind-sculpted bedrock
Wind Erosion and Transport
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• Desert sediments accumulate in many settings• Erosional debris accumulates near source areas
• Talus aprons, at angle of repose, collect cliff debris• Rock blocks have shapes determined by jointing
Desert Deposition
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Desert Deposition
• Playas – Desert (salt) lakes without outlet streams• Internal drainage collects water from flash floods• Dissolved solids crystallize out as water evaporates
• Halite, gypsum.
Salar de Uyuni, Bolivia
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• Wind carries 2 types of sediment load1. Surface load – courser sand-sized particles
2. Suspended load – finer-grained silt-sized “dust”• Sand forms dunes in deserts; dust forms loess next to (downwind
of) deserts
Desert Processes
Chapter 21
Outline• Deserts: extents and basics.
• Desert types-Subtropical, rain shadows, cont. interiors, coastal, polar.
• Desert Processes-Weathering, soils, desert varnish-Erosion by water and wind-Deposition and transport
• Desert Landscapes-Cliffs & mesas-Pediments and bajadas-Dunes, their types and controls
• Desertification
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• Deserts exhibit diverse landscapes• Reflect interplay of erosional and depositional processes over
long time spans
Desert Landscapes
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Desert Landscapes• Cliffs & Mesas – bedrock landforms
• Bedrock exposed along cliffs- breaks away along joints• Cliff retreat in flat rocks creates plateaus…• Then mesas, buttes and, finally, chimneys.
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• Cliffs & Mesas – weathering dictates appearance• Resistant – form steep cliffs as thick as rock layer• Weak – weather slopes
Desert Landscapes
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• Cliffs & Mesas – Structure governs landforms• Non-horizontal bedding produces linear ridges
• Cuesta – steep joint-controlled cliff; less-steep dip slope• Inselberg – eroded remnant of almost-complete cliff retreat
Desert Landscapes
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• Pediments • Ramp-like bedrock surfaces sloping up to a mountain front
• Bajada • Sloping surface of coalesced alluvial fans at mountain flank
Desert Landscapes
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• Dunes – windblown sand accumulations• Sand carried by wind accumulates around obstacle• Over time, a dune grows, begins to move downwind• Dunes generate enormous cross-beds
Desert Landscapes
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• Dune types • Form depends on variation in…
• Wind – velocity and consistency of direction• Sediment supply – is sand scarce or abundant
Desert Landscapes
DuneDune WindWind SandSand
BarchanBarchanSteadySteady
1 direction1 directionScarceScarce
StarStarSteadySteady
Changing directionsChanging directionsScarceScarce
TransverseTransverseSteady, moderateSteady, moderate
1 direction1 directionPlentifulPlentiful
ParabolicParabolicSteady, strongSteady, strong
1 direction1 directionPlentifulPlentiful
LongitudinalLongitudinalSteady, strongSteady, strong
1 direction1 directionAbundantAbundant
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• Dunes• Sand oceans (ergs) develop in some deserts
• Arabian peninsula, Namibia.• Past ergs – large regions of cross-bedded sandstones.
Desert Landscapes
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• Desert organisms are adapted to harsh conditions• Dessication.• Temperature extremes.
Desert Life
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Outline• Deserts: extents and basics.
• Desert types-Subtropical, rain shadows, cont. interiors, coastal, polar.
• Desert Processes-Weathering, soils, desert varnish-Erosion by water and wind-Deposition and transport
• Desert Landscapes-Cliffs & mesas-Pediments and bajadas-Dunes, their types and controls
• Desertification
Chapter 21
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Desertification• Desertification – aridification of nondesert areas• Human activity expands desert areas
• Examples: Aral Sea and mid US (i.e. Oklahoma 1930s)- dust bowl
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Desertification• Desertification caused by…
• Overpopulation, overgrazing• Careless agriculture• Diversion of water supplies• Climate change
• Leads to hardship (i.e. famine).
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