Goh Cheng Leong Chapter 7: Arid or Desert Landforms YouTube Lecture Handouts

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Watch video lecture on YouTube: Aeolian Landforms (By Wind) - 13 Erosional & 3 Depositional Arid or Desert Landforms Aeolian Landforms (By Wind) - 13 Erosional & 3 Depositional Arid or Desert Landforms
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  • 1/5th world is desert (rocky, stony or sandy)

  • Deserts which are absolutely barren are true deserts

  • Confined between north & south of equator

  • Lie on Western part of continents where Trade winds are offshore – bathed by cold currents with desiccating effect (moisture is not condensed into precipitation) – examples – Sahara, Arabian, Thar, Iranian, Kalahari, Namib, Atacama, Great Australian Desert

  • Continental Interior – Mid latitudes - Gobi and Turkestan – have extreme temperatures

Types of Desert

  • Hamada (Rocky Desert): Stretch of bare rocks, sand and dust – bare and sterile – Sahara Desert

  • Reg (Stony Desert): Extensive sheets of angular pebbles which the winds are not able to blow off – more accessible with camels. Called serir in Libya & Egypt.

  • Erg (Sandy Desert): ripples of sand dunes – Calanscio Sand Sea in Libya. Called koum in Turkestan.

  • Badlands: Arid areas in South Dakota, hills are eroded by rainstorms into gullies and ravines – Painted Desert of Arizona (SE of Grand Canyon of Colorado River)

  • Mountain Desert: Outlines of peaks and ranges. Steep slopes are cut by wadis & action of frost has carved irregular edges – Tibesti Mountain & Ahaggar Mountains in Sahara Desert

Mechanism of Arid Erosion

  • Insufficient rainfall (less than 5 inch) – irregular

  • High temperature

  • Rapid rate of evaporation

  • Sub-aerial denudation by weathering, wind action and water

  • Intense heating in day and cooling in night – creates stress in weakened rocks

  • Peeling off the outer thin layers – onion peeling or exfoliation

  • Water gets into cracks and freezes – it expands by 10% volume – create fragments that accumulate as screes – rocks become teeth of wind erosion

  • Winds are more efficient in arid areas

  • Deflation – lifting and blowing away of loose material from ground. Fine dust will be removed from origin and deposited far away – lowering of land surface – creates deflation hollows (Qattara Depression in Sahara – 450 ft BSL)

  • Abrasion – Rock surface is scratched and polished; more effective at rock base where wind can carry high amount of material (Telegraph poles in desert are covered by metal for foot or two above ground)

  • Attrition – particles roll against one another and rounded into millet seed sand

Erosional Landforms in Desert

Rock Pedestals or Mushroom Rocks: wears away softer material and grooves and hollows are cut. Rock pillar will be further eroded near the base where friction is greatest – undercutting produces mushroom rock or gour in Sahara

Image of erosional landform aeolian

Image of Erosional Landform Aeolian

Image of erosional landform aeolian

Zeugen: tabular – soft rock below hard rock – creates ridge and furrow landscape. Joints open by mechanical weathering – stand at 10 to 100 feet above sunken furrows. Abrasion lowers zeugen and widens furrows

Image of Zeugen

Image of Zeugen

Image of Zeugen

Yardangs: Vertical bands in direction of prevailing winds. It excavates softer rocks into narrow corridors – found in Atacama in Chile & interior deserts of Central Asia

Image of Yardangs

Image of Yardangs

Image of Yardangs

Mesas and Buttes: Mesa (Spanish word meaning table) is flat mass with resistant top and steep sides. Hard mass resists denudation by wind and water – in Arizona or fault blocks (Table Mountain of Cape Town, South Africa). Mesas may be reduced to buttes by continued erosion. These are separated by canyons.

Image of mesa and butte aeolian

Image of Mesa and Butte Aeolian

Image of mesa and butte aeolian

Inselberg (Island Mountain): Isolated residual hill rising abruptly from level ground – steep slopes and rounded tops – north Nigeria, Western Australia & Kalahari Desert

Image of Iinselberg wind

Image of Iinselberg Wind

Image of Iinselberg wind

Ventifacts or Dreikanter (3 surfaces): pebbles faceted by sand blasting – polished and resemble Brazil nuts. If wind direction changes, another facet is developed with sharp edges. Form smooth mosaic region covered by rock fragments.

Image of Dreikanter

Image of Dreikanter

Image of Dreikanter

Deflation Hollows: Formation of small depression and minor faulting can initiate depression; eddying action of oncoming winds will wear off weaker rocks until water table is reached. This will form oasis or swamps. For example, Faiyum Depression in Egypt is 130 ft BSL. Formation of Great Dust Bowl in USA.

Image result for erosional landform aeolian

Deflanation Hollow

Image result for erosional landform aeolian

Depositional Landforms in Desert

  • Dust grains can travel as far as 2,300 miles before getting deposited

  • Dust from Sahara fall as blood rains in Italy or Switzerland glaciers

  • Dust settles in Hwang Ho Basin from Gobi Desert to several hundred feet depth

  • Coarse and heavy grains remain static and form dunes based on size of particle, wind velocity, location and nature of surface where they are transported, presence or absence of water and vegetation

  • Dunes: Hills of sands by accumulation and wind movement – it can be active or live versus inactive or fixed rooted with vegetation – common in ergs – can be of many shapes

  • Barchans (crescent or moon shaped) – individual or group – common in Turkestan or Sahara. Accumulation at obstacle like grass patch or rock heap – occur transversely – horns thin out and become lower in direction of wind due to frictional retardation around edges

  • Windward side – convex and gentle sloping – sand is driven up on this side

  • Leeward side – concave and steep – sand slips down the leeward side

  • Advancement can be from 25 to 50 feet a year

  • Long rooted sand holding trees and grass are planted to check advancement of dunes and prevent fertile land from devastation. Many can coalesce into single line of irregular ridge

Image of Barchans, Crescent and moon shaped

Image of Barchans, Crescent and Moon Shaped

Image of Barchans, Crescent and moon shaped

Seifs (Longitudinal dunes): seif means sword – long narrow ridges parallel to the direction of prevailing winds. Crestline rise and fall in regular succession like teeth of monstrous saw. Sahara desert, south of Qattara Depression; Thar and West Australian Desert

Loess: Fine dust blown beyond the desert limits is deposited on neighboring lands – fine loam, rich in lime, coherent and porous; water sinks and surface gets dry; streams have deep cut through loess and badland topography may develop – NW China, Hwang Ho basin (250,000 square miles with depth of 200 to 500 feet) and called as Hwangtu (yellow earth) – originally comes from village in Alsace, France. Called as limon in Germany, France and Belgium (wind borne). In Mid West USA – derived from ice sheets and called as adobe.

Landforms by Action of Water in Deserts

  • Rainfall – 5 to 10 inches with thunderstorms, torrential downpours

  • Light vegetation can lead to flash floods

  • Loose gravels are swept down the hill – cut gullies and ravines forming badland topography

  • Liquid material in flash floods makes flow liquid mud

  • Mass of debris at foot of hill – alluvial cone or fan or dry delta – evaporation by hot sun and downward percolation into ground as mounds of debris

  • Gullies along with corrasion are deepened by torrents during cloudburst – wadis (dry most of the time)

  • Desert streams are fed by melting snow of distant mountains outside desert as exotic streams (called chebka in Algeria)

  • Outflowing rivers are short and intermittent

  • Drainage is internal – water sometimes does not disappear and temporary lake is formed – high percent of salts

  • Lakes and alluvial plains are formed – playas, Salinas and salars in USA & Mexico; shots in N. Africa

  • Floor of depression has bajada and pediment

  • Bajada – depositional feature of alluvial material by intermittent streams

  • Pediment – erosional plain at base of mountain scraps

Image of Piedmont Slope

Image of Piedmont Slope

Image of Piedmont Slope