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 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.

Fig. 18-1, p.537 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.

Wind has sand blasted these boulders on surface of Mars.. p.540a

p.540b Headstone dated 1864 in Michigan showing effects of wind - smoothed

Fig. 18-3, p.541 Ventifact forms when: a. wind borne particles abrade surface of a rock. B. large ventifacts lying on desert pavement in Death Valley, CA

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

Fig. 18-6, 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

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

Fig. 18-8, p.544 a.Profile view of sand dunes. B. dune migrates when sand moves up the windward side and slides down leeward slope. Produces cross beds….

Cross bedding in Zion Nat’l Park: can be used to determine wind direction in past… Fig. 18-9, p.544

Fig , p.544 Barchan dunes: form in areas with limited amount of sand, nearly constant wind direction, and dry surface with little vegetation.

Fig , p.545 Longitudinal dunes: 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 , 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

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 b, p.545

Fig , p.547 Star dunes- pyramidal hills of sand that develop where wind direction is variable. b. Ground level view of star dune in Namibia

Fig , p.546 Parabolic dunes commonly form in coastal areas with partial cover of vegetation, strong onshore wind and abundant sand.

Fig b, p.546 Parabolic dune developed along W Mich shoreline

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 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...

Fig , p.547 Terrace wheat fields in loess soil in China. Many farmers live in caves carved out of the unconsolidated loess.

General circulation of Earth’s atmosphere Fig , 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 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

Fig , p.548 Distribution of arid and semi-arid regions on Earth

Fig , p.549 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..

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 , p.550

Shiny black coating on this rock is rock varnish. Varnish composed of iron and manganese oxides. Fig , 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 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

Fig a, p.555 Playa lake formed after rainstorm in Mojave Desert, CA

Fig b, p.555 Racetrack playa in Death Valley, CA

Fig , p.556 Aerial view of alluvial fan in Death Valley, CA

Fig , p.556 Coalescing Alluvial fans forming a bajada in Death Valley

Fig , p.557 Pediments are erosional bedrock surfaces formed by erosion along mountain front.

Fig a, p.557 Panoramic view of Monument Valley, mesas and buttes….

Fig a, p.546

p.551

Fig a, p.545

Fig a, p.544

Fig a, p.545

Fig b, p.544

Fig. 18-8b, p.544

Fig. 18-3a, p.541

Fig. 18-3b, p.541

Fig b, p.557

p.539

Fig. 18-CO, p.534

Fig. 18-8a, p.544

Fig b, p.545