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Chapter 21
DesertsDeserts
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
Chapter 21
Chapter 21
Deserts• Deserts cover• Primary control on deserts? • Characterized by…
• Extreme• Specialized• Unique
Chapter 21
What Is a Desert?• Land so arid, that…
• Evaporation prevents permanent surface water• Vegetation covers <15% of surface• Annual rainfall <10’ (25 cm)
• Deserts exist in both hot and cold climates
Chapter 21
Hot vs. Cold Deserts
• Cold deserts:• High latitudes • High elevations• Near cold oceans currents
• Hot deserts:• Low latitudes• Low elevations• Far from oceans
Chapter 21
Desert Features• Desert surfaces often include…
• Exposed bedrock• Accumulated clasts• Unweathered sediment • Precipitated salt• Windblown sand
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
Chapter 21
Chapter 21
Types of Deserts• 5 Types (distinctive landscapes & biota):
1. Subtropical (sahara, arabian)
2. Rain shadow (eg eastern OR)
3. Coastal (atacama)
4. Continental interiors (Gobi- southern mongolia)
5. Polar (antarctica)
Chapter 21
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
Chapter 21
• At 20-30o N & S latitude across time (as plates move) • African deserts bracket equator
• North: sahara and arabian• Rainforest straddling equator • South: namib and Kalahari
Subtropical Deserts
Chapter 21
Rainshadow Deserts
• Wet ocean winds are driven over mountains• 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 (east side of Sierras)
Chapter 21
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
Chapter 21
Interior Deserts
• Air looses moisture crossing continents • Land far from ocean moisture can be arid
• Example: gobi desert in Mongolia
Chapter 21
Polar Deserts
• Above 66º N & S latitude there is a little moisture vapor• Air circulation carries • It is so cold, air can’t
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
Chapter 21
Chapter 21
Desert Processes• Deserts are characterized by:
• Erosion by water and wind• Weathering.• Soil formation.• Deposition.
• These processes result in unique landscapes
Chapter 21
Weathering
• Physical weathering dominates • Rare chemical weathering leaches ions• Evaporation -> salts both break and cement grains
Chapter 21
Desert Soils
• Desert soils are thin, with poorly defined horizons • Usually colored like the bedrock • Trace elements/oxidation cause wide color variations/red
Chapter 21
Desert Varnish
• Exposed surfaces develop desert varnish.• Dark coating of iron and manganese oxides • Forms very slowly by bacterial activity and dust and water• Native Americans left petroglyphs in desert varnish
Chapter 21
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
Chapter 21
• 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
Chapter 21
• Coarse clasts can’t be lifted by wind• Finer sediment removed; forms lag deposits (deflation)• Creates desert pavement
-> Surface layer of stones• Becomes resistant to erosion
Wind Erosion and Transport
Chapter 21
• 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
Chapter 21
• Deflation – lowering land surface via erosion• Concentrated wind erosion creates blowouts
• Desert processes clearly operating on mars
Wind Erosion and Transport
Chapter 21
• 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
Chapter 21
• 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
Chapter 21
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
Chapter 21
• Wind carries 2 types of sediment load 1. Surface load – coarser 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
Chapter 21
Chapter 21
• Deserts exhibit diverse landscapes • Reflect interplay of erosional and depositional
Desert Landscapes
Chapter 21
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.
Chapter 21
• Cliffs & Mesas – • Resistant – • Weak –
Desert Landscapes
Chapter 21
• 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
Chapter 21
• Pediments • Ramp-like bedrock surfaces sloping up to a mountain front
• Bajada • Sloping surface of coalesced alluvial fan at mountain flank
Desert Landscapes
Chapter 21
• Dunes – windblown sand accumulations • Sand carried by wind accumulates around obstacle • Over time, a dune grows, begins to moved downwind • Dunes generate enormous cross-beds
Desert Landscapes
Chapter 21
• 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
Chapter 21
• Dunes • Sand oceans (ergs) develop in some deserts
• Arabian peninsula, Namibia.
• Past ergs – large regions of cross-bedded sandstones.
Desert Landscapes
Chapter 21
• Desert organisms are adapted to harsh conditions • Dessication.• Temperature extremes.
Desert Life
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
Chapter 21
Chapter 21
Desertification• Desertification – aridification of nondesert areas • Human activity expands desert areas
• Examples: aral sea and mid US
Chapter 21
Desertification• Desertification caused by…
• Overpopulation, overgrazing • Careless agriculture• Diversion of water supplies• climate change
• Leads to hardship (i.e. famine).