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Eolian Desert Systems.

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Presentation on theme: "Eolian Desert Systems."— Presentation transcript:

1 Eolian Desert Systems

2 Eolian (wind-driven) Distribution Desert belts Rainshadows Far inland
10-30° N and S of equator (Hadley circulation cells) trades; downwelling cool, dry air more land, more absoprtion (less if reflective) and shift in circulation Rainshadows downwind side of mountain in US, winds blow W to E lose moisture over mountains Far inland center of continent away from moisture source




6 Deserts 20-25% of E’s surface low rainfall (<25 cm/ yr on avg.)
NOT ALL HOT! (freeze at night!) NOT ALL DRY (ephemeral) usually sparsely vegetated any rain usually generates flash floods Drains to center of basin to form pans and lakes

7 Transport Types dependent on wind velocity Traction Saltation
coarse grains Saltation grain collisions aid transport through dispersion Suspension fines (<0.05 mm) are winnowed out and put into suspension at very high winds, coarser grains are blown

8 Deposits Dust deposits (silt) Sand deposits Lag deposits loess
travels great distances Glacial eolian Indiana; Ohio, China, Mississippi Dust storms Sand deposits Lag deposits


10 Deposits Sand deposits well sorted since fines were winnowed away

11 Lag deposits leftovers gravel sized deflation pavement

12 Subenvironments alluvial fans ephemeral lakes (playa)
water may collect in center of basin dune fields/ sand seas sand- poor areas exist between dunes

13 Dunes Ripples, dunes Draas Controls Cross-beds HUGE!  up to 5.5 km;
h >0.4 km Controls as wind V increases,  lengthens (smear out) as grain size increases, height goes up (piles up)

14 Ripples Ripples amplitude = .01 – 100 cm, up to 20 cm apart size fnc. (wind V, particle size, ripple type) ** most formed by bombardment of saltating grains and associated creep impact ripples


16 Dune Morphology Well-sorted, Q-rich Heavy minerals
smaller grain size Shell fragments possible Gross morphology similar to water ripples Generally only preserve lower portion of foreset Contains smaller scale bedforms (ripples) Hard to extract wind directions since winds often change direction

17 Dunes Dune height and spacing are regular Described by power function
Dh=cDsn Dh dune H Ds spacing N function of sand supply, wind speed C constant

18 Dune types Classifed by patterns formed by slip faces (lee)
Different patterns due to different # wind directions



21 No slip faces Eolian sand sheet Ergs Sand seas (Namibia)
smaller Sand seas (Namibia) larger flat undulating sand bodies low < x strata (0-20°) Ephemeral stream deposits possible

22 Form by unidirectional winds
barchan/barchanoid ridge/transverse continuum function of sed supply horns point downwind ridge – commected crescents  ( parabolic/ blow-out dune function of vegetation horns point upwind lateral edges anchored by vegetation so, middle migrates  ) Differentiated by regional patterns



25 2 wind directions Linear~ symmetrical ridge
uniform sand accumulation but wider and steeper upwind, tapers downwind Navajo (NW Arizona) linear dunes with 100 m sand free between often vegetated elongate, sharp-edged ridge of opposing crescents depositional and erosional Sinai Desert up to 300 km! Synonyms Sief Longitudinal Reversing ~ assymetrical ridges slip faces time 1; time 2



28 This is a C-band, VV polarization radar image of the Namib desert in southern Namibia, near the coast of South West Africa. The image is centered at about 25 degrees South latitude, 15.5 degrees East longitude. This image was one of the first acquired by the Spaceborne Imaging Radar-C/X- Band Synthetic Aperture Radar (SIR-C/X-SAR) when it was taken on orbit 4 from the shuttle Endeavour on April 9, The area shown is approximately 78 kilometers by 20 kilometers. The dominant features in the image are complex sand dune patterns formed by the prevailing winds in this part of the Namib desert. The Namib desert is an extremely dry area formed largely because of the influence of the cold Benguela ocean current that flows northward along the coast of Namibia. The bright areas at the bottom of the image are exposed outcrops of Precambrian rocks. This extremely barren area is a region rich in diamonds that through the centuries have washed down from the mountains.

29 3 wind directions Star HUGE! > 400 m high arms radiate from center
need high speed, variable winds AND lots of available sand




33 Interdunes Deflation low sand accumulation Lags Deposition Dry or Wet


35 Interdune Areas Dry WET Evaporites
Ripples formed from sands avalanching off dunes Poor sorting gently dipping layers with poor structure Bioturbated WET water accumulates and traps silts/ clays look for evidence of terrestrial life Tracks Diatoms gastropods, etc. Evaporites when lake dries up, can get evaporates Gypsum mudcracks, other evidence of dessication





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