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