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).