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Importance of Deserts

Importance of Deserts• Sensitive to Climate Changes

> expansion & reduction can be documented> helps document past global climate changes> helps us to understand and prepare for futurechanges in weather and climate

• Useful in understanding Other planets

• Mars> an Arid planet.

• Sensitive to Climate Changes> expansion & reduction can be documented> helps document past global climate changes> helps us to understand and prepare for futurechanges in weather and climate

• Useful in understanding Other planets

• Mars> an Arid planet.

Arid Regions- a.k.a. DesertsArid Regions- a.k.a. Deserts• Arid Regions are exactly that...ARID.

> they lack moisture> can be either WARM or COLD

• Arid Regions are exactly that...ARID.> they lack moisture> can be either WARM or COLD

Characteristics of DesertsCharacteristics of Deserts

Besides being dry> few plants> consist of nearly continuous rockand/or sand exposures little soil development

• Weathering, erosion, & depositionstill operate.> in a balance with the local climate

Besides being dry> few plants> consist of nearly continuous rockand/or sand exposures little soil development

• Weathering, erosion, & depositionstill operate, but more slowly.> In a balance with the local climate

The Distribution of DesertsThe Distribution of Deserts• Deserts form in two regions

> areas of persistent high pressure- latitude effects related to Hadley Cellcirculation

> Areas on the lee side of Topographicbarriers- rainshadow effect

> other things influence too- cold water ocean currents- extremely cold temperatures

• Deserts form in two regions> areas of persistent high pressure

- latitude effects related to Hadley Cellcirculation

> Areas on the lee side of TopographicBarriers (lee= downwind side)- rainshadow effect

> other things influence too- cold water ocean currents- extremely cold temperatures

Area of persistent arid conditions (sub-tropical desert)

Distribution of Arid and Semiarid RegionsDistribution of Arid and Semiarid Regions

• Subtropical deserts occur at 20-30° N-S latitude.Gobi

Arabian

Sahara

Australian

MojaveSonoranChihuahuan

Atacama Kalahar

iNamib

Thar

Patagonian

Antarctica

Rainshadow DesertsRainshadow Deserts• Air that must cross coastal mountains loses much of its moisture, thuswhen it arrives over the inland areas it is drier.

Distribution of Arid and Semiarid RegionsDistribution of Arid and Semiarid Regions

• Rainshadow deserts occur in places down wind ofmajor mountain chains like the Himalayas, SierraNevadas, and Andes.

Weathering and SoilsWeathering and Soils

• Mechanical weathering dominates desert regions.• Chemical weathering is not effective

> Rock varnish is a thin coating of iron and/or manganese oxides onmany surface rocks in deserts.

Agents of ChangeAgents of Change• Water

> scarce> when it rains, it really rains hard!> erodes and deposits distinct landforms

- no plants to slow erosion down

• Wind> common> lack of plants facilitates effectiveness as anagent of change.

> erodes and deposits distinct landforms

• Water> scarce> when it rains, it really rains hard!> erodes and deposits distinct landforms

- no plants to slow erosion down

• Wind> common> lack of plants facilitates effectiveness as anagent of change.

> erodes and deposits distinct landforms

Exotic & Local Streams Exotic & Local Streams

•.

• Arroyos usually exhibit braided channels> coarse-grained sediments> prone to severe flash floods!

• Exotic Streams > Perennial rivers that begin outside the desert

- Nile River, Egypt.- Colorado River, Southwest US.

> Can be either Meandering or Braided

• Local braided streams in deserts flow

infrequently- known as Arroyos infrequently- known as Arroyos .

• Arroyos usually exhibit braided channels> coarse-grained sediments> prone to severe flash floods!

• Exotic Streams > Perennial rivers that begin outside the desert

- Nile River, Egypt.- Colorado River, Southwest US.

> Can be either Meandering or Braided

Arroyo- a dry, often braided stream channel in the desertArroyo- a dry, often braided stream channel in the desert

Nile River: An exotic stream

Pediments and Sediment-Filled ValleysPediments and Sediment-Filled Valleys

• Pediments are eroded bedrock, often covered by alluvial fans orbajadas.

• Eventually these bajadas extend far into the valleys between mountainranges.

Bajada -coalescedalluvialfans

-dry lakeinselbergs

Playa lakesPlaya lakes

Alluvial Fan

InselbergsInselbergs• are isolated mountains composed of more resistant rock types.• Location: Ayers Rock in central Australia.

Plateaus, Mesas, & ButtesPlateaus, Mesas, & Buttes

• Plateau> A large flat uplifted area, usually only partlydissected by stream activity

• Mesa > Broad flat topped erosional remnant of partof a plateau, with nearly vertical slopes.

• Butte> isolated pillar-like structure that streamshave eroded from a Mesa.

• Plateau> A large flat uplifted area, usually only partlydissected by stream activity

• Mesa > Broad flat topped erosional remnant of partof a plateau, with nearly vertical slopes.

• Butte> isolated pillar-like structure that streamshave eroded from a Mesa.

New Mexico

MesaMesa

ButtesButtes

Wind is a common agent of erosion and deposition in deserts

The dust storm shown below transports sand, silt and clay across the Sahara desert in North Tunisia

Wind ErosionWind ErosionMain agents are abrasion anddeflation.

• Deflation > the removal of loose surface sedimentby wind.

• Abrasion> the impact of saltating grains on thesurface.

> i.e. "sand-blasting"

Main agents are abrasion anddeflation.

• Deflation > the removal of loose surface sedimentby wind.

• Abrasion> the impact of saltating grains on thesurface.

> i.e. "sand-blasting"

• Deflation is the removal of loose surface sediment bywind.

• Deflation produces deflation hollows (blowouts) > and reg surfaces or desert pavement.

Desert pavement with Inselberg

Ventifacts Ventifacts Erosional Features

• Ventifacts > stones with wind-sculpted & polishedsurfaces.

> sand-blastingactually does thework.

Erosional Features

• Ventifacts > stones with wind-sculpted & polishedsurfaces.

> sand-blastingactually does thework.

18-618-6

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Wind DepositsWind Deposits• Eolian materials

> Sediments transported and deposited

by wind.

• Dunes are landforms> typically sand-sized particles.> rarely silt and or clay-sized.

• Loess (eolian silt)> wind-blown silt and clay associatedwith glaciations and arid regions.

• Eolian materials> Sediments transported and deposited

by wind.

• Dunes are landforms> typically sand-sized particles.> rarely silt and or clay-sized.

• Loess (eolian silt)> wind-blown silt and clay associatedwith glaciations and arid regions.

QTVR of dune crest

Barchan DunesBarchan Dunes

Barchan

Transverse DunesTransverse Dunes

Transverse

Parabolic DunesParabolic Dunes

Parabolic(Blowout)

Longitudinal DunesLongitudinal Dunes

Longitudinal

Star DunesStar DunesStar