chapter 12 deserts and wind

8/2/2019 Chapter 12 Deserts and Wind

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Essent ials of GeologyEssent ials of Geology

David SalleeDavid Sallee

Chapter 12Chapter 12The Work of WindThe Work of Wind

and Desert sand Desert s

IntroductionIntroduction

Wind is anWind i s an

effective agenteffective agent

of erosion andof erosion and

transportationtransportation

in desertin desert

regionsregions

Wind alsoWind also

moves loosemoves loose

sediment in asediment in a

variety of othervariety of other

environmentsenvironments

How Does Wind TransportHow Does Wind Transport

Sediment?Sediment? Bed loadBed load

Sediments tooSediments toolarge or heavy tolarge or heavy tobe carried inbe carr ied insuspension bysuspension bywater or windwater or wind

Suspended l oadSuspended l oad SiltSilt -- and clayand clay--

sized part iclessized part iclesconstit ute most ofconstit ute most ofthis load, heldthis load, heldaloft for manyaloft for many

milesmiles

How Does Wind Erode?How Does Wind Erode?

Wind is anWind i s an

efficientefficient

sorter ofsorter of

sedimentsediment

WindWind

erosionerosion

producesproduces

manymany

unusualunusual

featuresfeatures

Wind Erosion: AbrasionWind Erosion: Abrasion

The impact of grainsThe impact of grains

during saltation isduring saltat ion is

much likemuch like

sandblasting, butsandblasting, but

limit ed to less thanlimit ed to less than

1m above the ground1m above the groundsurfacesurface

Etching, polishing,Etching, polishing,

and pitti ng are someand pitti ng are some

of the effectsof the effects

Ventifacts are stonesVentifact s are stones

wit h shape modifiedwit h shape modified

by abrasionby abrasion


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Wind Erosion: Deflat ionWind Erosion: Deflati on

The removal ofThe removal ofloose surfaceloose surfacesediment by w indsediment by w ind

erosion createserosion createsblowoutsblowouts

Desert pavement isDesert pavement isleft behind afterleft behind afterw ind has removedw ind has removedthe finerthe finer --grainedgrainedmaterial and itmaterial and itprotects theprotects theunderlyingunderlyingmaterial frommaterial fromdeflationdeflation

Types ofTypes of

DesertsDeserts

HamadaHamada

ErgErg

RegReg

What Are the DifferentWhat Are the Different

Types of Wind Deposits?Types of Wind Deposits? Dunes occur inDunes occur in

several distinctiveseveral distinctive

types, consist oftypes, consist of

sand, and aresand, and are

deposited near t heirdeposit ed near t heir

sourcesource

Loess consists ofLoess consists of

w indblown silt andw indblown silt and

clay deposit s oftenclay deposit s often

found far from theirfound far from theirsourcesource

Loess is the siltLoess is the silt -- sized ti l l , which can besized ti l l , which can be

picked up and blown very far distancespicked up and blown very far distances

by wind. Loess makes up much of t heby wind. Loess makes up much of t he

rich soils of the Midwest U.S. grain belt.rich soils of the Midw est U.S. grain belt.

The Form at ion andThe Form at ion and

Migration of DunesMigration of Dunes Dunes are moun dDunes are moun d-- oror

ridgeridge-- shaped deposit sshaped deposit s

of sandof sand

form w hen an objectform w hen an object

on the surface slowson the surface slows

the wind so thatthe wind so that

deposition takes placedeposition tak es place

usually areusually are

asymmetri cal in crossasymmetri cal in cross--

section, w ith a steepersection, w ith a steeper

downwind slopedownwind slope

migrate downwind asmigrate downwind as

sand accumulates onsand accumulates onthat side of the dunethat side of the dune

Dune TypesDune Types BarchanBarchandunesdunes crescentcrescent --

shapedshaped

tips pointtips pointdownwinddownwind

form on f lat,form on f lat,dry surfacesdry surfaceswi th l i t t lewi th l i t t levegetation,vegetation,limit ed sandlimit ed sandsupplysupply

constant w indconstant w inddirectiondirection

the mostthe mostmobile dunemobile dune --up to 10m/ yrup to 10m/ yr

Dune TypesDune Types

Longitudinal or seifLongitudinal or seif

dunesdunes

long, parallel ri dges oflong, parallel ri dges of

sandsand

aligned generally parallelaligned generally parallel

to w ind direction, butto w ind direction, but

result when windsresult w hen winds

converge from slightlyconverge from slightly

different directionsdifferent directions

can be up to 100m high,can be up to 100m high,

common in Austr alia,common in Australia,

Saudi Arabia, and EgyptSaudi Arabia, and Egypt


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Dune TypesDune Types TransverseTransversedunesdunes form longform long

ridgesridges

perpendicularperpendicularto theto theprevailingprevailingwind directionwind direction

abundant sandabundant sandsupplysupply

crests up tocrests up to200m high200m high

barchan dunesbarchan dunesor barchanoidor barchanoidforms mayforms mayform alongform alongedgesedges

DuneDune

TypesTypes Parabolic dunesParabolic dunes

common incommon in

coastal areascoastal areas abundant sandabundant sand

supplysupply

strong onshorestrong onshore

windwind

tips pointtips point

upwind,upwind,

anchored byanchored by

vegetationvegetation

center i s oftencenter i s often

blown outblown out

Dune TypesDune Types Star dunesStar dunes common in Saudicommon in Saudi

ArabiaArabia

pyramidal hills ofpyramidal hills of

sandsand

several ri dgesseveral ri dges

radiate from theradiate from the

crestcrest

form where windform where wind

direction isdirection is

variablevariable

stable dunes,stable dunes,

many aremany are

landmarks forlandmarks for

desert tr avelersdesert travelers

LoessLoess WindblownWindblown

silt andsilt and

clayclay

deposits,deposits,

far fromfar from

theirtheir

sourcesource

Soils onSoils on

loessloess

depositsdeposits

are someare some

of t he mostof the most

fertile infertile inthe worldthe world

Dist ribut ion of AirDist ribut ion of Air --PressurePressure

Belt s and Global Wind Patt ernsBelt s and Global Wind Patt erns Warm ai r ri ses, coolWarm air rises, cool

air sinksair sinks low pr essure systemslow pr essure systems

are dominant at loware dominant at lowlatitudes, high pressurelatitudes, high pressure

at polesat poles descending atmospheredescending atmosphere

cells in betw een (atcells in between (atabout 3 0 N and Sabout 30 N and Slat itude) result inlat i tude) result inanother high pressureanother high pressuresystemsystem

the Coriolis effectthe Coriolis effectdescribes the deflectiondescribes the deflectionof winds in theof winds in thehemispheres due to thehemispheres due to therotat ion of Earthrotat ion of Earth

Where Do Desert s Occur?Where Do Desert s Occur? Where evaporationWhere evaporation

exceedsexceedsprecipitat ionprecipitation major i ty of themajor i ty of the

worlds desertsworlds desertsfound in the low t ofound in the low t o

middle latitu des,middle latitudes,associated w ithassociated w ithhighhigh--pressurepressurebeltsbelts

some found in thesome found in thedeep interiors ofdeep interiors ofcontinentscontinents

others areothers arerainshadowrainshadowdesertsdeserts


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What Are theWhat Are the

Character ist ics of Desert s?Character ist ics of Desert s? Temperature,Temperature,

Precipitation, andPrecipitation, and

VegetationVegetation

temperaturestemperatures

vary great lyvary great ly --

extreme highsextreme highs

and lowsand lows

receive less thanreceive less than

25cm of25cm of

precipitat ion/ yearprecipitat ion/ year

plants areplants are

diverse, yetdiverse, yet

wid ely spaced,wi dely spaced,

small, and slowsmall, and slow --

growinggrowing

What Are t heWhat Are t he

Character isti cs of Desert s?Character isti cs of Desert s? Weathering andWeathering and

SoilsSoils

mechanicalmechanicalweathering isweathering isdominant due todominant due totemperaturetemperaturefluctuations andfluctuations andfrost wedgingfrost wedging

rock varnish coatsrock varnish coatsmany rocks wit hmany rocks wit hiron andiron andmanganese oxidesmanganese oxides

soils are th in andsoils are th in andpatchypatchy

What Are t heWhat Are t he

Characterist ics of Desert s?Characterist ics of Desert s?

Mass Wasting, Streams, and Groundw aterMass Wasting, Streams, and Groundw ater

runnin g wat er accounts for most of t he erosionrunnin g wat er accounts for most of t he erosion

in desertsin deserts

rainfall comes in bri ef, heavy cloudbursts th atrainfall comes in bri ef, heavy cloudbursts th at

moves large amounts of sediment quicklymoves large amounts of sediment quickly

downstreamdownstream

intern al drainage describes a desert str eam thatintern al drainage describes a desert str eam that

never reaches the sea, depositing it s load wit hinnever reaches the sea, depositing it s load wit hin

its bedits bed

w ater table is beneath t he str eam channelsw ater table is beneath t he stream channels

What Types of Landform sWhat Types of Landform s

are Found in Desert s?are Found in Desert s?

Alluvial FansAlluvial Fans form whenform when

sedimentsediment -- ladenladenstreams flow outstreams flow outfrom m ountainfrom mountainfronts into thefronts into theflat desert floorflat desert floor

poorly sorted,poorly sorted,fans out f romfans out f romthe mouth of thethe mouth of thestreamstream

may coalesce t omay coalesce toform a bajadaform a bajada

Landform s in Desert sLandform s in Desert s

PlayaPlaya

results from t heresults from t he

evaporation of aevaporation of a

playa lake, leavingplaya lake, leaving

a salt pana salt pan

characterized bycharacterized by

mud cracks andmud cracks and

salt crystalssalt crystals

salts are oftensalts are often

concentratedconcentrated

enough to beenough to be

minedmined

commerciallycommercially

Landforms inLandforms in

DesertsDeserts

PedimentsPediments erosionalerosional

bedrockbedrock

surfacessurfaces I nselbergI nselberg

resistant,resistant,projecting highprojecting highin desertsin deserts

Mesas andMesas andButtesButtes steep sided, flatsteep sided, flat

toppedtopped

chapter 12 deserts and wind

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