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DESERT PROCESSES ANDLANDFORMS
INTRODUCTION• Landscape evolution in arid and semi-arid regions
of the earth can take on a distinctivecharacteristic of their own.
• These areas are obviously marked by low annualprecipitation, distinctive floral/faunal habitation,and characteristic ephemeral (seasonal)erosion/deposition processes, which include bothwater and wind as principal driving agents.
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Characteristics of Deserts1. Dry Climate: Rate Evaporation > Rate of
Precipitationa) - Arid climate: <10 to 15 inches/year (Desert
Regime)b) Semi-arid: 10-25 inches/year (Steppe Regime)c) Nature of Rainfall/precipitation- overall year
round precipitation is low in desert areas, but isintense in instantaneous accumulation rates(heavy storm downfall common), flash floodsare common. As a result, rain/surface erosion isthe dominant erosion process in deserts (NOTWIND as one might expect)
2. Causal Factors of Dryness:
• Latitudinal Effects of Global SurfaceHeating/Global Weathering patterns– e.g. Sub-tropical latitude deserts such as Sahara,
Arabian, Australian.• Orographic/Rain Shadow Effects
– e.g. interior of Pacific Northwest, Mohave desertof 163 California.
• Lack of significant vegetative cover (associatedwith inhospitable climate lacking water).
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3. High Landscape erosion Potential: associatedwith lack of vegetative cover, intense rainfallevents, flash floods, weathering, all coupletogether to present erosion potential.
4. Weathering- Physical weatheringpredominates (chemical weathering is atminimum in absence of water) with suchprocesses as salt wedging, thermal exansion,frost wedging, and plant rooting.
5. Poor soil development- pedogenesis isgenerally limited in arid areas but distinctivenonetheless.
6. Impermeable surface layers possible- in theform of bedrock, hardpans and salt beds.
7. Sand- desert sand generated during thephysical weathering process, as opposed toclay dominated soils in humid/chemicallyweathered areas.
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8. Drainage Patterns- Dominated by ephemeralstreams, flowing either seasonally or duringstorm periods.
9. Closed or internal drainage basins arecommon in desert regions as opposed toopen drainage basins in humid areas.
GLOBAL DISTRIBUTION OF DESERTS
• Desert and Steppe conditions cover upwardsof 25-30% of total continental land areaforming the largest component of all climaticregimes.
• Most global deserts lies between 30˚Southand 30˚North of the Equator
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GLOBAL DISTRIBUTION OF DESERTS
A:Subtropical Deserts
• At latitudes near or above the Tropics ofCancer and Capricorn. Tropic of Cancer: includes Sahara Desert of N.
Africa, the Arabian Desert of the Middle East.Tropic of Capricorn: includes much of Australian
interior, and interior Pampas of Argentina.
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Subtropical Deserts
Mid-latitude Deserts
• the Gobi Desert of central Asia (Mongolian)and the Western interior of the United States.
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Mid-latitude Deserts
Desert Facts and Figures:
• Largest Desert: Sahara of Northern Africa (10% oftotal desert area)
• Sandiest Desert: the Arabian Desert of ArabianPeninsula
• Hottest Temperature Recorded: 136 degrees F atLibya, 165 1922, Sahara Desert .
• Greatest 1 day temperature fluctuation: 100 degreesF in Algeria, N. Africa (1927), Sahara Desert HighestAnnual Avg. Temperature: 94 degrees F in Ethiopia,Sahara Desert
• Least Annual Average Precipitation: 0.03 inches inArica, Chile (coastal Chile and Peru form some of thedriest desert areas on earth)
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FLUVIAL PROCESSES IN DESERT/ARIDREGIONS
• Running water is the most important agent oferosion and transportation in desertlandscape development
A: Erosional Characteristics:
i. Rain splash, sheetwash, rilling andflashflooding is common in surface drainageprocess owing to lack of vegetative cover.
ii. Although sporadic, intense rainfall episodesresult in great volumes of sediment beingmobilized in a relatively short period of time.
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iv. Stream gradients may be steep butephemeral nature of flow results inunpredictable imbalance between erosion,transporation and deposition.
v. Drainage basins tend to be closed (i.e. notthrough flowing) with internal drainage andbasin sedimentation
vi. Desert drainage systems tend to be bedloaddominated by coarse sand and gravel, finersilts, clays and sands are readily transportedand removed from the sediment populationby the sorting action of wind.
FLUVIAL EROSIONAL DESERTLANDSCAPE FEATURES
• Fluvial Erosion is the dominant landscapemodification process in deserts, the high ratesof mechanical/physical weathering by thefluvial process in desert landscape isresponsible for large amounts of landscapemodification.
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1. Butte/Mesa Desert Topography
• A result of differential erosion with resistantcap rocks holding up topographic features.
• Features are found in areas with flat lyingsedimentary rock layers with alternatingresistances to erosion.
• Sandstone or perhaps limestone often formresistant cap rocks.
A. Butte:- Round/oval
shaped, flattoppedtopographicfeature
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Buttesfrom Navajo Spirit Tour US.
• Mesa-elongated/tablelike, flat-toppedtopographicfeature
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Messa
• Pinnacles- Tower-like spires of rock, erosional remnants
formed by cap-rocks.
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• Wadi• Wadis (also known as arroyos), such as Wadi
Rum which is situated in Jordan, and WadiBani Khalid in Oman, both within the ArabianDesert, are dry river valleys which experienceinfrequent flows of water. They are deep,steep-sided ravines which vary in size frombeing just a few metres to several hundreds ofkilometres long
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The Wadis
Canyons• Are steep-
sided, deeperversions ofwadis. Theyare created bythe incision(vertical/downward erosion)by the actionof flowingwater indeserts.
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2. Inselbergs• Erosionally
resistant rockmass, thatstands in reliefas more easilyerodedmaterial isstriped/eroded from thesurroundinglandscape.
Inselberg
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4. Pediments• gently inclined, concave up, ramp that extends
outward from a mountain front, found alongthe lower slopes of mountains in desert regions
Pediments
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4. Badland Topography-• Intricately rilled and barren terrain in arid
regions.• Common in areas underlain by horizontal strata
of shale and clay formations that are poorlyconsolidated and subject to rilling and gullying.
• An extensive network of convoluted rills andgullies forming a "badland" topography
Badland Topography-
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Fluvial Deposition In Arid Landscapes• Eroded debris is commonly deposited in form
of talus slopes and alluvial deposits on valleyfloors.
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Depositional Sites
• Pediment Zone- "foot of the mountains“- The zone at base of desert mountain ranges
that forms the site of fluvial depositions frommountain canyons. (forms deposition site asthe break in slope of streams exiting steepmountain canyons, results in decreasedgradient, velocity and subsequent deposition)
• Intermontane Basins:• Part of the internal drainage network, low
areas between mountain ranges, often site ofcomplex interaction between lake basins,aeolian processes, and fluvial regimes
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Fluvial Depositional Features• Alluvial Fans• fan-shaped
deposit ofalluvial debrisas mountainstreamdrainagesempty ontothe piedmontarea.
• Bajada- Coalescing alluvial fans from adjacent mountain
canyons forming a "fan apron" along themountain front.
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• Playa lake• is an ephemeral lakebed, meaning that the lake is
filled with water only for transient time periods.
Fluvial erosion & Depositional featuresin Desert Areas
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AEOLIAN PROCESSES IN DESERTECOSYSTEMS: WIND PROCESSES
• Desert winds readily move sediment in sparsevegetative regimes, but the size fraction isgenerally limited to clays, silts, and fine sand.
• Wind may seem extensive in deserts, but playsrelatively minor role in major landscapemodification processes.
• Wind- related to horizontal air movementswith turbulence in air flow paths, and variablevelocities common.
• Aeolian" processes- are those related to wind(greek root of wind).
• Driving Force: Solar Insolation + AtmosphericPhenomena
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Wind Patterns and Moving Air Massesin Desert Areas
1. Driving Force:- Wind: generated by differences in air
pressure High-to-low pressure air flow = "wind“
- Solar Insolation Atmospheric Pressure Differential: derived from
unequal heating of earth's surface by solarradiation
2. Local Winds- E.g. land heating during day, air warms up and
into motion
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Aeolian Erosion Processes
1. Deflation- general movement of loose particles by shear
force in suspension or by rolling along groundsurface
a) blowouts/Deflation Basins- Areas which are selective deflated on finer
grain-size fractions of loose unconsolidatedground sediment
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b) oasis, swamp or pan Because the blowouts/hollows or cavities that
form trap surface water, or at least store morewater than their immediate surroundings, thispromotes some chemical weathering of theunderlying rock, which in turn causes furtherremoval of talus by wind action. The depth of the water table regulates the
depth to which a depression hollow can erode,because as soon as the water table isintersected, an oasis, swamp or pan is formed.
Makgadikhadi Pan in Botswana
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An Oasis in Peru
2. Abrasion- Sand and silt act as effective tools to abrade 5
(scour) the land surface while it is transportedby wind.
- i.e. sandblasting effect eroding and settingadditional sediment into motion.
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a) Rock pitting, etching, faceting, and polishing.
b) Ventifacts-polished andabraded stoneson desert floor.
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c) DesertPavement
A layer ofcoarse graveland bouldersleft strandedon desertfloor as windselectivelydeflates finergrain sizesout of area.
d) Yardang- Is one large desert landform that is sculpted by
the wind though deflation and abrasion.
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Yardangs in Egypt
e) ZeugensTabular masses of harder rocks separated by
trenches/furrows. Typically zeugen are mushroom-shaped rock that has been eroded by the abrasiveaction of windblown sand
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An Example Zeugen
Pedestal Rocks (Mushroom)• Many rock-outcrops in the deserts are easily
susceptible to wind deflation and abrasion.• Softer layers in these outcrops are worn out
easily, leaving remnants of more resistant rocksin the shape of mushroom with a slender stalkand capped by a broad and rounded pear shapeabove.
• Usually a product of chemical weathering, butwind abrasion also contributes in that thechemically weakened (weathered) softer layersare easily abraded by wind.
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Aeolian Transportation:
• Finest silt and clays are often carried furthestout of desert basin, while coarser sand isdeposited within 171 the desert floor.
• Generally sand is deposited at any pointwhere wind velocity decreases either naturallyor via shadow/pockets.
DEPOSITIONAL LANDFORMS
• Wind is a good sorting agent. Depending uponthe velocity of wind, different sizes of grains aremoved along the floors by rolling or saltation andcarried in suspension and in this process oftransportation itself, the materials get sorted.
• When the wind slows or begins to die down,depending upon sizes of grains and their criticalvelocities, the grains will begin to settle.
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• Therefore good sorting of grains can befound in depositional landforms made bywind.
• More formally, wind will deposit its talusload when its supply of kinetic energy is toolow to carry the talus mass.
• Since a desert has an abundant supply ofsand and with nearly constant winddirections prevailing, depositional featuresin arid regions can develop anywhere.
DEPOSITIONAL LANDFORMS
• The three commonly distinguished landformsformed by wind deposition of talus areDraas,Dunes andRipples.
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Draas and ergs
• Draas are huge sand accumulations andwhere these converge, a landform termed ergis formed – such as the star-shaped GreatContinental Erg in Algeria. There areapproximately twenty ergs in the Sahara,which collectively cover about 15% of theSahara’s surface area.
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Sand Dunes• Dry hot deserts are good places for sand dune
formation.• Obstacles to initiate dune formation are equally
important. Dune form, i.e. their shape and size, iscontrolled or determined by three factors,namely the strength and direction of wind, theamount of sand available and the amount (if any)of vegetation present.
• All dunes are mobile to some extent, and can beclassified into live dunes and fixed dunes on thebasis of their mobility.
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Live dunes or free dunes
• have no fixed position, but migrate downwindby erosion on the gently inclined windwardside and deposition on the leeward side (slipface) in the same way as described for fixeddunes.
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Free Dune
Fixed dunes
• Tend not to move and their shapes arerelatively stable and static and they are usuallysecured down by vegetation, rocks andopposing winds.
• They are formed when transported sandsettles in the lee of an obstacle such as a bushor a rock, causing the obstruction to grow insize, capturing more sand.
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Fixed Dunes at Bankass Mali
Longitudinal dunes• Also known as linear dunes, form when the
supply of sand is poor and the wind direction isconstant, or where sand is more abundant andcross winds converge - often along coasts wherethe winds from the sea and those from the landmeet and push the sand into long lines.
• They appear as long ridges of considerable lengthbut low in height.
• The wind channels between existing dunes andforms a vortex flow, which then shapes andmaintains the dune form.
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Seif dunes• Originate from barchanlike forms, but have only
one wing or point due to shifting wind conditionsdisturbing the one point.
• The remaining lone wings of seifs can grow verylong and high. Seifs are the dominant dune form inthe Sahara and some of them are up to 100 m longand have a local relief of up to 100 m as well.
• The seifs in the central Namib Desert south ofWalvis Bay reach heights between 50 m and 250m, reputed to be of the highest dunes in the world.
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Barchan Dunes• Solitary crescent shaped dunes with their tips
pointing downwind (i.e. horns pointdownwind). Steep leeward slipface on onconcave side of dune.
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Formation of Barchan Dunes
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Transverse dunes
• When the supply of sand is plentiful, regularshaped dunes like barchans can coalesce andlose their individual characteristics, formingcrescent-shaped (or barchanoid) ridges. If theridges become fairly straight, they are calledtransverse dunes
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Star dunes
• Mountainous piles of sand known as stardunes dominate the inland margin of the sandsea where high winds blow from all directions.They are reputed to be among the highestdunes in the world, as high as 220 m or evenhigher when they rest on a raised surface.
• Star dunes are named for their shape as seenfrom above, a lot of sharp ridges windingoutwards and downwards from a central cres
Star Dunes
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Parabolic dunes (blowout dunes)• Parabolic dunes form when sandy surfaces are
partially covered with vegetation.• They are also crescent shaped, but unlike the
barchan dunes, their points or wings (horns)point into the wind, while the arch is downwind.
• Parabolic dunes reach heights of up to 20 or 30m except at their crescent, where more sandpiles up as it is halted or slowed by surroundingvegetation.
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Loess
• This is the lightest material carried by thewinds which form a so-called blanket coveringthe existing land.
• This blanket is easily eroded and rainpenetrates through them rapidly. A largeportion of the world’s loess has its origin fromdeserts
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Loess is a predominantly silt-sized sedimentoriginating from broken-down rock fragments,which is formed by the accumulation of wind-blowndust. Loess is fairly even- sized sediment, paleyellow or buff in colour, typically non-stratified andoften calcerous – i.e. of calcium carbonate origin
Algeria, Tassili n Ajjer, Sahara, sandripples on a desert dune
