Namib Desert, Namibia DESERTS

Transcription

1 Namib Desert, Namibia DESERTS

2 ORIGIN OF DESERTS - Hadley cells Rising air Figure 15.1

3 Distribution of deserts on Earth Figure 15.15

4 Deserts in time: Modern aeolian regions Glacial-maximum (18ka) aeolian regions Glacial-maximum loess and aerosol dust tracks

5 Jurassic deserts

6 Fl = lift force Fd = drag force W = weight

7 Fd ~ τa where A is grainʼs area = d 2 π/4 for spherical particles; τ is surface shear stress τ = ρu 2 where ρ = air density; u = friction velocity of air hence Fd ~ ρu 2 d 2 π/4 Fl + - Fd = drag force W

8 Fl Fd W=weight

9 Fl=ΔpA The lift force is similar to the lift given to airplane wings as fastmoving air flows over them (the Bernoulli effect). W Fd

10 p=pivot Quartz sand grains

11 In summary: The beginning of sand movement is called entrainment, when wind "picks up" sand grains and puts them into motion. As a wind blows over a surface, the moving air exerts lift and drag forces on loose grains. The lift force is similar to the lift given to airplane wings by fast-moving air. Drag forces exert a forward push on grains sticking up from the surface. The lift and drag forces on a sand grain must overcome two types of resistance to movement. The resistance of gravity acting on a grain depends on particle size: larger grains with more mass are more resistant to movement (W). Frictional and cohesive forces between grains also produce resistance to movement.

12 The term "threshold of movement" refers to the point when wind forces overcome resistance to movement--and the grain leaves its position to bounce, roll or slide across the ground. There is a minimum force needed to move particles of a given size. The threshold velocity indicates the minimum force. The graph shows two threshold velocity lines for sand. The fluid threshold applies to wind moving over a surface when no particles are in motion. Once sand movement has begun, saltation the impacts of grains colliding with the surface puts other grains into motion. Less wind energy is needed to keep sand moving after some grains are in motion, and the impact threshold is approximately 80% of the fluid threshold. Note the high thershold velocities deposition for silt & clay due to high electrostatic cohesion of these particles, which, when entrained, tend to move by suspension. suspension

13 The rate of sand movement (amount( of sand transported per unit time, eg m 3 /yr) ) increases exponentially with wind speed. Several formulas have been proposed:

14 Deserts are characterized by two main types of morphologies: 1-Desert pavements, which are surfaces covered with closely packed, interlocking angular or rounded rock fragments of pebble and cobble size. 2- Dunes, which are made of well-sorted and rounded grains, usually quartz. Dunes Desert pavement Sonora, Arizona

15 Desert pavements form by the gradual removal of the sand, dust and other fine grained material by the wind (deflation) and intermittent rain leaving only the larger fragments behind. Desert pavement Thickening sand layer

16 A more sophisticated model calls into action the role of soil development and the shrink/swell properties of clays/salts underneath the pavement. Phase 1: sediment crust

17 Phase 2: vesicular crust * * tiny nonconnected circular pores formed by entrapped air within soil

18 Phase 3: soil peds development

19 Desert pavements are often associated with ventifacts

20 Dunes form as sand grains come to rest around obstacles deposition

21 The sediments that accumulate on the windward slope are called topset deposits. Sand grains on the windward slope move by saltation. When they reach the crest, they form an unstable and temporary surface called the brink. When enough sediments are captured on the brink they eventually tumble over the edge onto the slip-face. This motion provides the advancement of the dune as it migrates in the direction of the wind.

22 Desert dunes classification is based upon shape and include barchan dunes, parabolic dunes, transverse dunes, linear (longitudinal) dunes, seif dunes, star dunes. Barchan Unidirectional wind and moderate sand supply rate Parabolic Unidirectional wind and low sand supply rate Transverse Unidirectional wind and high sand supply rate Linear or longitudinal Converging wind directions and moderate sand supply rate

23 Star dunes Highly variable wind direction

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25 Barchan dunes Baja California Barchan dunes Mars Longitudinal dunes Star dunes Transverse dunes

26 Sul lato sopravvento delle dune si formano ripples asimmetrici con creste perpendicolari alla direzione del vento

27 Ripples are small-scale geomorphic features that tend to develop on sandy surfaces that are in a state of relative equilibrium or slow deposition. Surfaces experiencing either marked erosion or vigorous deposition generally do not display rippling. A=wind ripples B=water current ripples C=wave ripples

28 Iβ small for lee slope and high for stoss slope

29 Ripples formation. Saltating sand grains possess a "characteristic flight path". Since, in most natural sand surfaces, one grain size predominates with a normal distribution around the peak size, saltating sand grains are striking the surface at a relatively uniform angle (approximately ten degrees for the average grain). When surface unevenness occurs and a small hollow is created, less saltation impacts will occur on the upwind side of the hollow than on the downwind slope. As a result, surface creep along slope AB is considerably greater than creep along slope CA, as slope CA resides in a "saltation shadow." Consequently, sand is removed from point A and deposited at point B, creating a ripple. This, in turn, produces a second hollow downwind of the newly-created ripple and the process repeats itself with numerous parallel ridges forming at right angles to the wind direction.

30 The wind-generated cross-bedded stratification of fossil sand dunes can be used to infer past wind directions for paleoclimatic reconstructions Cross-bedding indicates high and unidirectional current velocity, often winds in terrestrial settings, forming sand dune lee-slopes.

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32 Weathering in Desert environments 1- Mostly mechanical weathering. 2- Little chemical weathering (e.g., oxidation of mafic minerals and formation of hematite). 3- Desert varnish -- veneer coating of manganese oxides

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34 Mechanical Weathering in Desert environments Thermal expansion/contraction. Repeated daily heating and cooling of rock heat causes expansion; cooling causes contraction. Salt wedging. Growth of minerals in cracks. Water evaporates, ions in solution combine to form minerals

35 Chemical Weathering Hydrolysis - any reaction in which water participates. a) Ion exchange H+ replaces other cations. b) Dissolution - mineral completely dissolves, leaving only ions in solution. c) Oxidation - reaction in which elements gain or lose electrons (example: rust).

36 oxidation of mafic minerals and formation of hematite

37 The formation of desert varnish is not well understood. Microscopic and microchemical observations show that a major part of varnish is clay, which could only arrive by wind. Clay, then, may act as a substrate to catch additional substances dissolved in acqueous aerosols that chemically precipitate when the rock reaches high temperatures in the desert sun. Another important characteristic of desert varnish is that it has an unusually high concentration of manganese minerals (Birnessite). This significant enrichment is thought to be caused by biochemical processes (many species of bacteria use manganese). Main composition: Clays Birnessite (Mn) Hematite (Fe)

38 Products of Weathering Mineral Residual Products Material in Solution Quartz quartz grains silica Feldspar clay minerals silica, K +, Na +, Ca 2+ Amphibole (hornblende) clay minerals, limonite, hematite silica, Mg 2+, Ca 2+ Olivine limonite, hematite silica, Mg 2+