deserts - unimi.itusers.unimi.it/paleomag/ppt/e-environs.deserts.pdf · deserts are characterized...

Namib Namib Desert, NamibiaDesert, Namibia

DESERTSDESERTS

Figure 15.1

Rising air

ORIGIN OF DESERTS - Hadley cells

Figure 15.15

Distribution of deserts on EarthDistribution of deserts on Earth

Deserts in time:

Modern aeolianregions

Glacial-maximum(18ka) aeolianregions

Glacial-maximumloess and aerosol dusttracks

Jurassic desertsJurassic deserts

W = weight

Fl = lift force

Fd = drag force

+ -Fd = drag force

Fd ~ τA where A is grainʼs area = d2π/4 for spherical particles; τ is surface shear stress

τ = ρu2 where ρ = air density; u = friction velocity of air

hence

Fd ~ ρu2 d2π/4 Fl

W

W=weight

Fd

Fl

Fl=ΔpA

Fd

W

The lift force is similarto the lift given toairplane wings as fast-moving air flows overthem (the Bernoullieffect).

p=pivot

Quartz sand grains

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 dragforces on loose grains. The lift force is similar to the lift given toairplane wings by fast-moving air. Drag forces exert a forward pushon grains sticking up from the surface.

The lift and drag forces on a sand grain must overcome two types ofresistance to movement. The resistance of gravity acting on a graindepends on particle size: larger grains with more mass are moreresistant to movement (W). Frictional and cohesive forces betweengrains also produce resistance to movement.

The term "threshold of movement" refers to the point when wind forces overcomeresistance to movement--and the grain leaves its position to bounce, roll or slideacross the ground. There is a minimum force needed to move particles of a givensize. The threshold velocity indicates the minimum force.

The graph shows two threshold velocity lines for sand. The fluid threshold appliesto wind moving over a surfacewhen no particles are in motion.Once sand movement has begun,the impacts of grains collidingwith the surface puts other grainsinto motion. Less wind energy isneeded to keep sand moving aftersome grains are in motion, and theimpact threshold is approximately80% of the fluid threshold.Note the high thershold velocitiesfor silt & clay due to high electrostaticcohesion of these particles, which,when entrained, tend to moveby suspension.

deposition

saltation

susp

ensio

n

The The rate rate of sand movement (of sand movement (amountamountof sand transported per unit time, of sand transported per unit time, egegmm33/yr/yr) increases ) increases exponentiallyexponentially with withwind speed.wind speed.

Several formulas have beenSeveral formulas have beenproposed:proposed:

Deserts are characterized by two main types ofmorphologies:1-Desert pavements, which are surfaces covered with closelypacked, interlocking angular or rounded rock fragments ofpebble and cobble size.

2- Dunes, which are made of well-sorted and rounded grains,usually quartz.

Desert pavementDesert pavementSonora, ArizonaSonora, Arizona

Dunes Dunes

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 fragmentsbehind.

Desert pavementThickening sand layer

A more sophisticated model calls into action the role of soil developmentand the shrink/swell properties of clays/salts underneath the pavement.

Phase 1:sedimentcrust

Phase 2:vesicularcrust

* tiny non-connectedcircular poresformed byentrapped airwithin soil

*

Phase 3:soil pedsdevelopment

Desert pavements are often associated with Desert pavements are often associated with ventifactsventifacts

Dunes form as sand grains come to rest aroundobstacles

deposition

The sediments that accumulate on the windward slope are calledtopset deposits. Sand grains on the windward slope move bysaltation. When they reach the crest, they form an unstable andtemporary surface called the brink. When enough sediments arecaptured on the brink they eventually tumble over the edge ontothe slip-face.

This motion provides the advancement of the dune as it migratesin the direction of the wind.

BarchanUnidirectional wind and moderate sand supply rate

ParabolicUnidirectional wind and low sand supply rate

TransverseUnidirectional wind and high sand supply rate

Linear or longitudinalConverging wind directions and moderate sand supply rate

Desert dunes classification is based upon shape and includebarchan dunes, parabolic dunes, transverse dunes, linear(longitudinal) dunes, seif dunes, star dunes.

Star dunesStar dunesHighly variable wind directionHighly variable wind direction

BarchanBarchan dunes dunes Baja CaliforniaBaja California

BarchanBarchan dunes dunes MarsMars

Longitudinal dunesLongitudinal dunes Star dunesStar dunes

Transverse dunesTransverse dunes

Sul latoSul latosopravventosopravventodelle delle dune dune si siformanoformanoripplesripplesasimmetriciasimmetriciconcon creste cresteperpendicolariperpendicolarialla direzionealla direzionedel del ventovento

Ripples are small-scale geomorphic features thattend to develop on sandy surfaces that are in a stateof relative equilibrium or slow deposition. Surfacesexperiencing either marked erosion or vigorousdeposition generally do not display rippling.

A=wind ripples

B=water currentripples

C=wave ripples

Iβ small for leeslope and highfor stoss slope

Ripples formation. Saltating sand grains possess a "characteristic flightpath". Since, in most natural sand surfaces, one grain size predominateswith a normal distribution around the peak size, saltating sand grains arestriking the surface at a relatively uniform angle (approximately ten degreesfor the average grain).

When surface unevenness occurs and a small hollow is created, lesssaltation impacts will occur on the upwind side of the hollow than on thedownwind slope. As a result, surface creep along slope AB is considerablygreater than creep along slope CA, as slope CA resides in a "saltationshadow." Consequently, sand is removed from point A and deposited atpoint B, creating a ripple. This, in turn, produces a second hollow downwindof the newly-created ripple and the process repeats itself with numerousparallel ridges forming at right angles to the wind direction.

• Cross-bedding indicates high andunidirectional current velocity, oftenwinds in terrestrial settings, formingsand dune lee-slopes.

The wind-generated cross-bedded stratification of fossilThe wind-generated cross-bedded stratification of fossilsand dunes can be used to infer past wind directions forsand dunes can be used to infer past wind directions forpaleoclimatic paleoclimatic reconstructionsreconstructions

Weathering in Desert environments1- 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

Mechanical Weathering in Desert environments

Thermal expansion/contraction. Repeated dailyheating and cooling of rock heat causesexpansion; cooling causes contraction.

Salt wedging. Growth of minerals in cracks.Water evaporates, ions in solution combineto form minerals

Chemical WeatheringChemical Weathering

Hydrolysis - any reaction in which waterparticipates.

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).

oxidation of maficminerals andformation ofhematite

The formation of desert varnish is not well understood. Microscopicand microchemical observations show that a major part of varnish isclay, which could only arrive by wind. Clay, then, may act as asubstrate to catch additional substances dissolved in acqueousaerosols that chemically precipitate when the rock reaches hightemperatures in the desert sun. Another important characteristic ofdesert varnish is that it has an unusually high concentration ofmanganese minerals (Birnessite). This significant enrichment isthought to be caused by biochemical processes (many species ofbacteria use manganese).

Main composition:ClaysBirnessite (Mn)Hematite (Fe)

Mineral Residual Products Material in Solution

Quartz quartz grains silica

Feldspar clay minerals silica, K+, Na+, Ca2+

Amphibole (hornblende)clay minerals, limonite,

hematite silica, Mg2+, Ca2+

Olivine limonite, hematite silica, Mg2+

Products of WeatheringProducts of Weathering