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DESERT PROCESSES AND LANDFORMS
INTRODUCTION • Landscape evolution in arid and semi-arid regions of the earth can take on a distinctive characteristic of their own. • These areas are obviously marked by low annual precipitation, distinctive floral/faunal habitation, and characteristic ephemeral (seasonal) erosion/deposition processes, which include both water and wind as principal driving agents.
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Characteristics of Deserts 1. Dry Climate: Rate Evaporation > Rate of Precipitation a) - 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 is intense in instantaneous accumulation rates (heavy storm downfall common), flash floods are common. As a result, rain/surface erosion is the dominant erosion process in deserts (NOT WIND as one might expect)
2. Causal Factors of Dryness:
• Latitudinal Effects of Global Surface Heating/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 desert of 163 California. • Lack of significant vegetative cover (associated with inhospitable climate lacking water).
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3. High Landscape erosion Potential: associated with lack of vegetative cover, intense rainfall events, flash floods, weathering, all couple together to present erosion potential. 4. Weathering- Physical weathering predominates (chemical weathering is at minimum in absence of water) with such processes as salt wedging, thermal exansion, frost wedging, and plant rooting.
5. Poor soil development- pedogenesis is generally limited in arid areas but distinctive nonetheless. 6. Impermeable surface layers possible- in the form of bedrock, hardpans and salt beds. 7. Sand- desert sand generated during the physical weathering process, as opposed to clay dominated soils in humid/chemically weathered areas.
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8. Drainage Patterns- Dominated by ephemeral streams, flowing either seasonally or during storm periods. 9. Closed or internal drainage basins are common in desert regions as opposed to open drainage basins in humid areas.
GLOBAL DISTRIBUTION OF DESERTS
• Desert and Steppe conditions cover upwards of 25-30% of total continental land area forming the largest component of all climatic regimes. • Most global deserts lies between 30˚South and 30˚North of the Equator
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GLOBAL DISTRIBUTION OF DESERTS
A:Subtropical Deserts
• At latitudes near or above the Tropics of Cancer 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% of total desert area) • Sandiest Desert: the Arabian Desert of Arabian Peninsula • Hottest Temperature Recorded: 136 degrees F at Libya, 165 1922, Sahara Desert . • Greatest 1 day temperature fluctuation: 100 degrees F in Algeria, N. Africa (1927), Sahara Desert Highest Annual Avg. Temperature: 94 degrees F in Ethiopia, Sahara Desert • Least Annual Average Precipitation: 0.03 inches in Arica, Chile (coastal Chile and Peru form some of the driest desert areas on earth)
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FLUVIAL PROCESSES IN DESERT/ARID REGIONS • Running water is the most important agent of erosion and transportation in desert landscape development
A: Erosional Characteristics: i. Rain splash, sheetwash, rilling and flashflooding is common in surface drainage process owing to lack of vegetative cover. ii. Although sporadic, intense rainfall episodes result in great volumes of sediment being mobilized in a relatively short period of time.
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iv. Stream gradients may be steep but ephemeral nature of flow results in unpredictable imbalance between erosion, transporation and deposition. v. Drainage basins tend to be closed (i.e. not through flowing) with internal drainage and basin sedimentation vi. Desert drainage systems tend to be bedload dominated by coarse sand and gravel, finer silts, clays and sands are readily transported and removed from the sediment population by the sorting action of wind.
FLUVIAL EROSIONAL DESERT LANDSCAPE FEATURES • Fluvial Erosion is the dominant landscape modification process in deserts, the high rates of mechanical/physical weathering by the fluvial process in desert landscape is responsible for large amounts of landscape modification.
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1. Butte/Mesa Desert Topography
• A result of differential erosion with resistant cap rocks holding up topographic features. • Features are found in areas with flat lying sedimentary rock layers with alternating resistances to erosion. • Sandstone or perhaps limestone often form resistant cap rocks.
A. Butte: - Round/oval shaped, flat topped topographic feature
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Buttes from Navajo Spirit Tour US.
• Mesa- elongated/table like, flat-topped topographic feature
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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 Wadi Bani Khalid in Oman, both within the Arabian Desert, are dry river valleys which experience infrequent flows of water. They are deep, steep-sided ravines which vary in size from being just a few metres to several hundreds of kilometres long
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The Wadis
Canyons • Are steep- sided, deeper versions of wadis. They are created by the incision (vertical/down ward erosion) by the action of flowing water in deserts.
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2. Inselbergs • Erosionally resistant rock mass, that stands in relief as more easily eroded material is striped/erode d from the surrounding landscape.
Inselberg
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4. Pediments • gently inclined, concave up, ramp that extends outward from a mountain front, found along the 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 poorly consolidated and subject to rilling and gullying. • An extensive network of convoluted rills and gullies 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 valley floors.
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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 from mountain canyons. (forms deposition site as the break in slope of streams exiting steep mountain canyons, results in decreased gradient, velocity and subsequent deposition)
• Intermontane Basins: • Part of the internal drainage network, low areas between mountain ranges, often site of complex interaction between lake basins, aeolian processes, and fluvial regimes
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Fluvial Depositional Features • Alluvial Fans • fan-shaped deposit of alluvial debris as mountain stream drainages empty onto the piedmont area.
• Bajada - Coalescing alluvial fans from adjacent mountain canyons forming a "fan apron" along the mountain 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 features in Desert Areas
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AEOLIAN PROCESSES IN DESERT ECOSYSTEMS: WIND PROCESSES • Desert winds readily move sediment in sparse vegetative regimes, but the size fraction is generally limited to clays, silts, and fine sand. • Wind may seem extensive in deserts, but plays relatively minor role in major landscape modification processes. • Wind- related to horizontal air movements with turbulence in air flow paths, and variable velocities common.
• Aeolian" processes- are those related to wind (greek root of wind). • Driving Force: Solar Insolation + Atmospheric Phenomena
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Wind Patterns and Moving Air Masses in 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 solar radiation
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 ground surface
a) blowouts/Deflation Basins - Areas which are selective deflated on finer grain-size fractions of loose unconsolidated ground 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 more water than their immediate surroundings, this promotes some chemical weathering of the underlying rock, which in turn causes further removal 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 is intersected, 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 transported by wind. - i.e. sandblasting effect eroding and setting additional sediment into motion.
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a) Rock pitting, etching, faceting, and polishing.
b) Ventifacts- polished and abraded stones on desert floor.
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c) Desert Pavement A layer of coarse gravel and boulders left stranded on desert floor as wind selectively deflates finer grain sizes out 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) Zeugens Tabular masses of harder rocks separated by trenches/furrows. Typically zeugen are mushroom- shaped rock that has been eroded by the abrasive action 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 rocks in the shape of mushroom with a slender stalk and capped by a broad and rounded pear shape above. • Usually a product of chemical weathering, but wind abrasion also contributes in that the chemically weakened (weathered) softer layers are easily abraded by wind.
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Aeolian Transportation:
• Finest silt and clays are often carried furthest out of desert basin, while coarser sand is deposited within 171 the desert floor. • Generally sand is deposited at any point where wind velocity decreases either naturally or via shadow/pockets.
DEPOSITIONAL LANDFORMS
• Wind is a good sorting agent. Depending upon the velocity of wind, different sizes of grains are moved along the floors by rolling or saltation and carried in suspension and in this process of transportation itself, the materials get sorted. • When the wind slows or begins to die down, depending upon sizes of grains and their critical velocities, the grains will begin to settle.
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• Therefore good sorting of grains can be found in depositional landforms made by wind. • More formally, wind will deposit its talus load when its supply of kinetic energy is too low to carry the talus mass. • Since a desert has an abundant supply of sand and with nearly constant wind directions prevailing, depositional features in arid regions can develop anywhere.
DEPOSITIONAL LANDFORMS
• The three commonly distinguished landforms formed by wind deposition of talus are Draas, Dunes and Ripples.
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Draas and ergs
• Draas are huge sand accumulations and where these converge, a landform termed erg is formed – such as the star-shaped Great Continental Erg in Algeria. There are approximately twenty ergs in the Sahara, which collectively cover about 15% of the Sahara’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, is controlled or determined by three factors, namely the strength and direction of wind, the amount of sand available and the amount (if any) of vegetation present. • All dunes are mobile to some extent, and can be classified into live dunes and fixed dunes on the basis of their mobility.
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Live dunes or free dunes
• have no fixed position, but migrate downwind by erosion on the gently inclined windward side and deposition on the leeward side (slip face) in the same way as described for fixed dunes.
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Free Dune
Fixed dunes
• Tend not to move and their shapes are relatively stable and static and they are usually secured down by vegetation, rocks and opposing winds. • They are formed when transported sand settles in the lee of an obstacle such as a bush or a rock, causing the obstruction to grow in size, 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 is constant, or where sand is more abundant and cross winds converge - often along coasts where the winds from the sea and those from the land meet and push the sand into long lines. • They appear as long ridges of considerable length but low in height. • The wind channels between existing dunes and forms a vortex flow, which then shapes and maintains the dune form.
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Seif dunes • Originate from barchanlike forms, but have only one wing or point due to shifting wind conditions disturbing the one point. • The remaining lone wings of seifs can grow very long and high. Seifs are the dominant dune form in the Sahara and some of them are up to 100 m long and have a local relief of up to 100 m as well. • The seifs in the central Namib Desert south of Walvis Bay reach heights between 50 m and 250 m, 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 point downwind). Steep leeward slipface on on concave side of dune.
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Formation of Barchan Dunes
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Transverse dunes
• When the supply of sand is plentiful, regular shaped dunes like barchans can coalesce and lose their individual characteristics, forming crescent-shaped (or barchanoid) ridges. If the ridges become fairly straight, they are called transverse dunes
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Star dunes
• Mountainous piles of sand known as star dunes dominate the inland margin of the sand sea where high winds blow from all directions. They are reputed to be among the highest dunes in the world, as high as 220 m or even higher when they rest on a raised surface. • Star dunes are named for their shape as seen from above, a lot of sharp ridges winding outwards 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 30 m except at their crescent, where more sand piles up as it is halted or slowed by surrounding vegetation.
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Loess
• This is the lightest material carried by the winds which form a so-called blanket covering the existing land. • This blanket is easily eroded and rain penetrates through them rapidly. A large portion of the world’s loess has its origin from deserts
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Loess is a predominantly silt-sized sediment originating from broken-down rock fragments, which is formed by the accumulation of wind-blown dust. Loess is fairly even- sized sediment, pale yellow or buff in colour, typically non-stratified and often calcerous – i.e. of calcium carbonate origin
Algeria, Tassili n Ajjer, Sahara, sand ripples on a desert dune
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