WIND and DESERTS
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WIND and DESERTS Wind is a turbulent fluid and therefore transports sediment in much same manner as rivers do: 1. Bed load : sediments too large or heavy to be carried in suspension are are transported by either saltation or rolling/sliding

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WIND and DESERTS

Wind is a turbulent fluid and therefore transports sediment in much same

manner as rivers do:

1. Bed load: sediments too large or heavy to be carried in suspension are are transported by

either saltation or rolling/sliding

saltation: intermittent bouncing along surface; wind starts sand grains rolling, picks them up and carries

them some distance before they fall back to surface, striking another grain,

causing that grain to bounce....

-usually restricted to within 1m of surface; high winds can result in sand blast effect...

2. Suspended load: silt and clay sized particles carried by wind, create clouds of dust or even

dust storms

How does wind Erode?

wind action produces distinctive erosional features, and very well sorted deposits

1. Abrasion: impact of saltating grains on an object...equivalent to sand blasting

-usually does not create major erosional features, but modifies existing features-etching, pitting,

smoothing or polishing

Products of this type of erosion:

a. ventifacts: stones whose surfaces have been polished, pitted, grooved, or faceted by

wind erosion. if wind blows from several directions, or if stone is moved, it will have several facets or faces...

-most common in deserts, also on beaches in humid regions

b. yardangs: elongated, streamlined ridges that look like overturned ships hull. typically found grouped in

clusters aligned parallel to direction of the wind.


Most sand is moved near the ground by saltation. Sand grains are carried by

wind a short distance, where they fall back to ground, strike other sand grains

and cause those grains to bounce and be carried by wind.

Fig. 18-1, p.537



Headstone dated grains are carried by

1864 in Michigan

showing effects of

wind - smoothed

p.540b


Ventifact grains are carried by forms when: a. wind borne particles abrade surface of a rock. B. large

ventifacts lying on desert pavement in Death Valley, CA

Fig. 18-3, p.541


Profile view of streamlined grains are carried byyardang in playa deposits of Egypt

Fig. 18-4, p.541


Wind abrasion caused these structures in Libya desert-exposed limestone

Fig. 18-2, p.538


2. desert-exposed limestoneDeflation: removal of loose surface sediment by the wind.

a. deflation hollows or blowouts: shallow depressions resulting from differential erosion of

surface materials common on Southern Great Plains of US

b. desert pavement: wind removes fine materials, leaving coarser cobbles, etc behind.

intermittent rainfall moves grains so that they form a carpet over surface, protecting it from

further erosion

Different types of Wind deposits:

-most common type is that of the Dune-mounds or ridges of wind deposited sand. Dunes

form when wind blows over or around an obstruction. sand is deposited. feature grows as

more sand is deposited-a kind of self generating wind break

-most dunes are asymmetric profile: gentle windward slope, steeper leeward slope that is inclined

in the direction of the prevailing wind

-sand grains move up the windward slope by saltation, and accumulate on leeward side forming

an angle between 30 to 34 degrees-which is the angle of repose of dry sand.


Deflation hollow desert-exposed limestone in Death Valley, CA

Fig. 18-5, p.542


Deflation and desert-exposed limestoneorigin of desert pavement. A. find grained material is winnowed by wind,

Leaving concentration of larger particles, forming desert pavement (B). Desert

pavement in Mojave Desert (c

Fig. 18-6, p.542


1. desert-exposed limestoneDune Types:

a. barchan dunes: crescent shaped dunes whose tips point downwind form in areas that are flat,

dry surface with little vegetation, constant wind direction, limited supply of sand; small-max

height of 30m, most mobile of sand dunes-moving 10m per year

b. longitudinal dunes (seif dunes): long parallel ridges of sand aligned parallel to direction of

prevailing winds. form where sand supply is somewhat limited

size variable: 3m to 100m high, up to 100km in length. very well developed in Australia,

Saudi Arabia, Egypt, Iran

c. transverse dunes: long ridges formed perpendicular to prevailing wind direction; areas of

abundant sand and very little vegetation.

-when viewed from air look like waves on ocean- often called 'sand seas'

-crest can be as high as 200m, width up to 3km

d. parabolic dunes: crescent shape like barchan dune, except tips point up wind common along

coastal regions with abundant sand, strong onshore winds, partial cover of vegetation

forms when partial vegetation occurs, deflation forms depression around vegetated area-sand

builds up to form dune

e. star dune: found in Saudi Arabia, has star shape. amongst tallest dunes in world-over 100m high


Large sand dunes in Death Valley, with ripples. Prevailing wind from left to right

Fig. 18-7, p.543


Fig. 18-8, p.544


Cross bedding in Zion Nat’l Park: can be used to determine wind direction in past…

Fig. 18-9, p.544


Barchan dunes: form in areas with limited amount of sand, nearly constant wind

direction, and dry surface with little vegetation.

Fig. 18-10, p.544


Longitudinal dunes nearly constant wind : form long, parallel ridges of sand aligned roughly parallel to prevailing

wind direction. Typically form where sand supply is limited. B. longitudinal dunes, 15m

high in Australia. Light blue areas are ponds of standing water…

Fig. 18-11, p.545


Transverse dunes nearly constant wind form long ridges of sand that are perpendicular to prevailing wind

direction, in areas of little or no vegetation and abundant sand. Great Sand Dunes

Nat’l Park, Colo

Fig. 18-12, p.545


Transverse dunes nearly constant wind form long ridges of sand that are perpendicular to wind direction, in

areas of little vegetation and abundant sand. Prevailing wind lower left to upper right

Fig. 18-12b, p.545


Star dunes- nearly constant wind pyramidal hills of sand

that develop where wind direction

is variable.

b. Ground level view of star dune

in Namibia

Fig. 18-14, p.547


Parabolic dunes nearly constant wind commonly form in coastal areas with partial cover of vegetation, strong

onshore wind and abundant sand.

Fig. 18-13, p.546


Parabolic dune developed along W Mich shoreline nearly constant wind

Fig. 18-13b, p.546


2. nearly constant wind Loess: wind blown silt and clay deposits composed of angular quartz grains, feldspar, micas

derived from 3 main sources: deserts, Pleistocene glacial outwash deposits, and floodplains of rivers

-must be stabilized by moisture and vegetation in order to accumulate.

-easily eroded because of its unconsolidated nature-forms steep cliffs

-occur on every continent-loess derived soils are very fertile (Great Plains of N Amer,

Ukraine, etc)

Wind is the Primary force causing Deserts-what are wind patterns controlled by?

-by global pattern of air pressure belts and wind

-air pressure controls pressure belts which impact wind directions: warm air is low pressure,

cold air is high pressure....so air flows from high pressure zones to low pressure zones,

setting up wind patterns.

-because the Earth is rotating at same time that this pressure differential occurs,

the resultant winds are deflected by the rotation

-rotational effects...this is called Coriolis Effect: winds in northern hemisphere are deflected to

the right of their direction of motion, whereas those in the southern hemisphere are deflected to

the left of the direction...


Terrace wheat fields nearly constant wind

in loess soil in China.

Many farmers live in

caves carved out of

the unconsolidated

loess.

Fig. 18-15, p.547


General circulation of Earth’s atmosphere nearly constant wind

Fig. 18-16, p.548


Deserts nearly constant wind

a. where do deserts occur?

-deserts occur in arid environments: high evaporation rates, receive less than 25cm of water

per year, poorly developed soils and have little vegetation

-majority of deserts occur in dry climates in low to mid latitudes

-in more northern latitudes, some deserts occur behind high mountain ranges- these are called

rainshadow deserts because rainfall occurs on seaward side of mountains (e.g. Sierra

Nevada in W US...)

b. characteristics of deserts

climate conditions are different but geologic processes are the same....

1. Temperature, Precipitation, and Vegetation

-deserts typically are hot, dry and have very little vegetation. temperatures are very hot during

day, can get to freezing at nite because of absence of ground cover to retain heat and moisture

-vegetation is typically of wide varieties adapted to hot, dry climates

2. Weathering and soils

-mechanical weathering is dominant in desert regions- frost wedging

-any chemical weathering takes place during winter months when precipitation occurs

rock varnish- thin dark coating on surface of rocks, composed of iron and manganese

oxides. origin from wind blown dust or microorganisms that live on rock surface


Distribution of arid and semi-arid regions on Earth nearly constant wind

Fig. 18-17, p.548


Rainshadow deserts: mid to high latitudes. Form on leeward side of high mountain

ranges. Marine air forced upwards by mountain range, forms clouds that produce

rain. Warm and dry air descends on leeward side..

Fig. 18-18, p.549


3 side of high mountain. Mass wasting, streams, groundwater

-running water causes most of the erosional effects we see in deserts-even though it is present

infrequently

-rainfall occurs in short, localized events: usually more rain falls than can be absorbed so

erosion occurs

-most desert stream are intermittent-only contain water when it rains. supply to water table is

also intermittent, so water table usually deeper than in humid regions

4. Wind

wind produces some dramatic erosional features. wind is very good transportation agent

for sand and finer sized grains


Desert vegetation typically sparse, grow slowly. side of high mountain

Fig. 18-19, p.550


Shiny black coating on this rock is side of high mountainrock varnish. Varnish composed of iron and manganese oxides.

Fig. 18-20, p.554


  • What types of landforms side of high mountain are found in deserts?

  • playa lakes: after intense rainstorm, water may not be able to be absorbed by ground

  • so it collects in depressions most are shallow, last for several hours up to several

  • months; water often very saline

  • 2. playa- once water evaporites from playa lake, the dry lakebed is called a playa or salt pan.

  • -characterized by mud cracks and precipitated salt crystals

  • 3. alluvial fans form where streams debouch from canyons onto desert floor

  • -these are usually very poorly sorted deposited, fan-shaped in form; multiple fans

  • coalesce to form a bajada

  • 4. pediment: erosional bedrock surfaces of low relief that slope gently away from

  • mountain bases. most are covered with thin layer of debris, alluvial fans or bajadas

  • -related to erosion and retreat of mountain front over long period of time

  • 5. inselberg: isolated steep sided erosional remnants on pediments. usually due to rocks

  • resistant to weathering

  • 6. mesa: broad, flat topped erosional remnant bounded on all sides by steep slopes.

  • contain easily eroded sedimentary material that is capped by horizontal resistant rocks

  • (sandstone, limestone, basalt).

  • 7. butte: isolated pillar-like structures that originally were a mesa, but continued erosion

  • formed this feature...form where resistant rocks at top are breached allowing more

  • easily eroded rocks beneath to be removed


Playa lake formed after rainstorm in Mojave Desert, CA side of high mountain

Fig. 18-21a, p.555


Racetrack side of high mountainplaya in Death Valley, CA

Fig. 18-21b, p.555


Aerial view of side of high mountainalluvial fan in Death Valley, CA

Fig. 18-22, p.556


Coalescing Alluvial fans side of high mountain forming a bajada in Death Valley

Fig. 18-23, p.556


Pediments side of high mountain are erosional bedrock surfaces formed by erosion along mountain front.

Fig. 18-24, p.557


Panoramic view of Monument Valley, side of high mountainmesas and buttes….

Fig. 18-25a, p.557


Fig. 18-13a, p.546 side of high mountain


p.551 side of high mountain


Fig. 18-11a, p.545 side of high mountain


Fig. 18-10a, p.544 side of high mountain


Fig. 18-12a, p.545 side of high mountain


Fig. 18-10b, p.544 side of high mountain


Fig. 18-8b, p.544 side of high mountain


Fig. 18-3a, p.541 side of high mountain


Fig. 18-3b, p.541 side of high mountain


Fig. 18-25b, p.557 side of high mountain


p.539 side of high mountain


Fig. 18-CO, p.534 side of high mountain


Fig. 18-8a, p.544 side of high mountain


Fig. 18-11b, p.545 side of high mountain


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