Download - 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