Deserts
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DesertsDeserts
Definition of DesertDefinition of Desert• A desert is an area with less than 25 cm (10 inches) of annual precipitation
• aridity index = potential evaporation/precipitation
greater than 4.0
• Deserts may be cold, temperate or hot. All major continents have one type of desert or the other.
Wind-Swept Wind-Swept Sand DunesSand Dunes
Desert Life Forms (Roadrunner)Desert Life Forms (Roadrunner)
Desert Life Forms (Lizard)Desert Life Forms (Lizard)
Types of DesertsTypes of Deserts
• Desert Types- Subtropical Desert – 30o Latitude- Deserts on Leeward side of major Mountain
ranges- Interior Deserts- center of continents far from
ocean- Coastal desert- prevailing onshore wind cooled
by cold ocean current- Polar deserts- extremely cold and dry
World's DesertsWorld's Deserts
Atmospheric ConvectionAtmospheric Convection
A Rain-Shadow DesertA Rain-Shadow Desert
Ocean-Current DesertOcean-Current Desert
Desert Landscape (Features)Desert Landscape (Features) Weathering and desert streams create Desert features• Weathering in Desert is mostly mechanical
- Little chemical weathering produces manganese and iron-oxide stains, called desert vanish
• Stream Erosion- Arroyo- channel with water during periods of high
discharge but dry most part of the year- Pediments- large-scale gently inclined surfaces- Inselberg- steep-sided knob of durable rock- Playa- dry lake bed
Chemical Weathering in DesertsChemical Weathering in Deserts
Desert Landforms Produced By WaterDesert Landforms Produced By Water
UluruUluru (Ayers Rock) (Ayers Rock) InselbergInselberg
PlayasPlayas
Playas (cont'd)Playas (cont'd)
Playas (cont'd) Playas (cont'd) ––A Playa in Death Valley, CaliforniaA Playa in Death Valley, California
Swimmers in hypersaline Dead SeaSwimmers in hypersaline Dead Sea
Evaporite deposits indicate a dry climate in the geologic record
Water-Carved CanyonWater-Carved Canyon
Work of WindsWork of Winds
• Erosion by Wind- Deflation- wind removes finer particles from
the surface- Desert pavement- layer of pebbles left behind
after deflation- Abrasion- sand blasting- Ventifacts- wind-shaped stones with sharp-
edge faces- Yardangs- streamlined desert ridges
Blowout Caused by DeflationBlowout Caused by Deflation
Desert PavementsDesert Pavements
Desert Pavements (cont'd)Desert Pavements (cont'd)
Desert Pavements (cont'd)Desert Pavements (cont'd)
Desert Pavements (cont'd) Desert Pavements (cont'd) – –
Source: Martin Miller
Desert Pavements (cont'd) Desert Pavements (cont'd) – –
Remnants of Remnants of Wind AbrasionWind Abrasionin addition to in addition to occasional flash occasional flash flood erosionflood erosion(surface grains (surface grains are frosted)are frosted)
Origin of VentifactsOrigin of Ventifacts
Origin of Ventifacts (cont’d)Origin of Ventifacts (cont’d)Wind
Yardangs , White Desert, EgyptYardangs , White Desert, Egypt
Transport By WindTransport By Wind• No dissolved load
• Suspended Load- most consist of dust (silt, clay, pollen, bacteria, salt crystals, etc.)
• Bed Load- sediments moved along or near the ground
– Rolling or saltation- bed loads lifted off the ground momentarily due to force of collision with other grains
Transport of Wind-Borne SedimentTransport of Wind-Borne SedimentSuspended Load Transports Sahara sediment to Caribbean and Amazon Rain Forest
Deposition of DunesDeposition of Dunes
• Reduced wind velocity results in sediments deposition
• Dunes are hills of loose wind-born sand - Size, shape, and orientation of dune are determined
by available sand, vegetation, and wind
Beach Sand DunesBeach Sand Dunes
Deposition of Wind’s Bed Load Deposition of Wind’s Bed Load
Rain – Shadow Desert in Lee Of Mountains
Large Scale Dunes Large Scale Dunes (Gobi Desert)(Gobi Desert)
Dune MigrationDune Migration
Just like ripples in a stream
Dune Migration (cont'd)Dune Migration (cont'd)
Deposition and Dune TypesDeposition and Dune Types
• Dune Types- Transverse- ridges that are perpendicular to
prevailing wind direction- Longitudinal- ridges that are parallel to
prevailing wind direction- Barchans- crescent-shaped with horns pointing
downwind- Horseshoe (Parabolic)- crescent-shaped with
horns pointing upwind- Star- winds from three or more directions
Transverse Dunes Transverse Dunes
Transverse Dunes Transverse Dunes
Longitudinal DunesLongitudinal Dunes
Longitudinal DunesLongitudinal Dunes
Barchan DunesBarchan Dunes
Barchan Dunes in Baja CaliforniaBarchan Dunes in Baja California
Parabolic DunesParabolic Dunes
Star DunesStar Dunes
Star DunesStar Dunes
Lithified Sand Dunes (Jurassic Lithified Sand Dunes (Jurassic Navajo Sandstone)Navajo Sandstone)
LoessLoess
• Loess are deposits of glacial outwash silt
Loess from the Columbia River BasinLoess from the Columbia River Basin
DesertificationDesertification• Desertification- invasion of desert conditions into
formerly non-desert areas• Drought and overpopulation are main causes• Signs
- Lowering of water table- Marked reduction of water supply- Increased salinity in water and soil- Progressive destruction of native vegetation- Accelerated soil erosion
Map of the SahelMap of the Sahel
End of Lecture 16End of Lecture 16
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