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
Wind has sand blasted these boulders on surface of Mars..
p.540a
Headstone dated
1864 in Michigan
showing effects of
wind - smoothed
p.540b
Ventifact 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 yardang 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. Deflation: 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 in Death Valley, CA
Fig. 18-5, p.542
Deflation and origin 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. Dune 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
a. Profile view of sand dunes. B. dune migrates when sand moves up the windward
side and slides down leeward slope. Produces cross beds….
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: form long, parallel ridges of sand aligned roughly parallel to prevailin
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 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 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- 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 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
Fig. 18-13b, p.546
2. 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 river
-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
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
Fig. 18-16, p.548
Deserts
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
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. 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.
Fig. 18-19, p.550
Shiny black coating on this rock is rock varnish. Varnish composed of iron and
manganese oxides.
Fig. 18-20, p.554
What types of landforms are found in deserts?
1. 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 p
-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
Fig. 18-21a, p.555
Racetrack playa in Death Valley, CA
Fig. 18-21b, p.555
Aerial view of alluvial fan in Death Valley, CA
Fig. 18-22, p.556
Coalescing Alluvial fans forming a bajada in Death Valley
Fig. 18-23, p.556
Pediments are erosional bedrock surfaces formed by erosion along mountain front.
Fig. 18-24, p.557
Panoramic view of Monument Valley, mesas and buttes….
Fig. 18-25a, p.557
Fig. 18-13a, p.546
p.551
Fig. 18-11a, p.545
Fig. 18-10a, p.544
Fig. 18-12a, p.545
Fig. 18-10b, p.544
Fig. 18-8b, p.544
Fig. 18-3a, p.541
Fig. 18-3b, p.541
Fig. 18-25b, p.557
p.539
Fig. 18-CO, p.534
Fig. 18-8a, p.544
Fig. 18-11b, p.545