Deserts and Wind Action What is a desert

Deserts and Wind Action • What is a desert? • Global desert locations/regions • Factors that produce desert environments • Arid Environments: water erosion/deposition & landforms wind erosion/deposition & landforms

Water & Arid Environments Erosion/Deposition/Landforms Sahara zinternal drainage - the streams drain toward landlocked basins instead of toward the sea zflash floods - because of the lack of vegetation, heavy rainfall runs rapidly over the surface and can create sudden local floods of high discharge

Ephemeral Streams Flash Flooding: Heavy storms No Vegetation Impermeable crust =Debris Flows

Wadis: Gorge like dry valley z dry washes: channels created by the rapid downcutting by floodwaters which tend to produce narrow canyons with vertical walls and flat, gravelstrewn floors Lack of regular precip. = No lateral erosion occurs

z playa lake: a shallow temporary lake (following a rainstorm) on a flat valley floor in a dry region z playa: a very flat surface underlain by hard, mud-cracked clay z bajada: a broad gently-sloping depositional surface formed by the joining of individual alluvial fans

Desert Playa – dry barren area in lowest part of a basin Bajada – broad, gentle surface at the base of the mountains; alluvial fans combined together Bajada Playa

Photo by Marli Miller.

Alluvial Fans

Bajada / Bahada

Step 1 Step 3 Step 2

Pediment Bajada

Desert Range Desert Ridges Desert Basin Bolson Wind Erosion Progressive Erosion Of an Arid Landscape

Wind Erosion, Transport, Deposition & Landforms

Wind Action zwind can be an important agent of erosion and deposition in any climate, as long as sediment particles are loose and dry zwind differs from running water in two ways: ybecause air is less dense than water, wind can erode only fine sediment - sand, silt and clay ywind is not confined to channels as running water is, so water can have a widespread effect over vast areas

z. Erg -- sand sea

Aeolian Transportation (Same types as found in rivers) Traction

Only small particles can remain suspended for long periods of time Most particles are moved by saltation Saltation refers to short jumps of grains dislodged from the surface and jumping a short distance. As the grains fall back to the surface they may dislodge other grains that then get carried by wind until they collide with ground to dislodge other particles.

Dust Storms: z When loose silt and clay are easily picked up from barren dry soil, such as in a cultivated field - silt and clay can remain suspended in turbulent air for a long time, so a strong wind may carry a dust cloud hundreds of meters upward and hundreds of kilometers horizontally

Sand storms:

Wind Erosion & Desert Landforms

zventifacts: rocks with jagged windabraded surfaces – abraded by windblown sediments

Ventifacts, Death Valley

Wind Erosion Deflation: Removal of sand & dust by the wind

Desert Pavement (Produced by deflation) Cobblestone sheet: sand, silt clay removed

Reg (deflation surface)

Hamadas – areas strewn with boulders and larger stones. Sand gets removed & eventually underlying rock exposed

Deflation z. Deflation Blowout/Hollow: the removal of clay, silt, and sand particles from the land surface by wind. Leaves a depression on the land surface caused by wind erosion z. Pillar: erosional remnant of the former land that may be left at the center of a blowout z. Oasis Formation: Deflation extends to G. W. T

Yardangs -- isolated outcropping of wind-eroded rock (deflation & abrasion) Allign with wind direction

Zeugen (water & wind feature) Water evaporation creates hard resistant upper crust Base & sides erode chemically and by wind – form block like ridges A. K. A Hoodoos Cap Rock Mushroom Rock Pedestal Toadstool

Hoodoo Development • Resistant Cap • Abrasive saltation (undercutting)

Inselbergs (wind abrasion, water & chemical) German for “Island Mountain” Steep sided hills that rise above a surrounding relatively flat plain More resistant to erosion than the rocks that once made up the surrounding plain Sheds water due to its steep slopes Surrounding less resistant rock collects this water and is subjected to more rapid rates of chemical weathering

Differential Erosion Features “Badlands” (Cap Rock, layers of alternating resistance, gullies) Water & Wind

Baked cap rock Less resistant More resistant Less resistant

z plateaus: a broad, flat-topped area elevated above the surrounding land bounded, at least in part, by cliffs (Excarpments) z mesa: a broad, flat-topped hill bounded by cliffs and capped with a resistant rock layer z butte: a narrow hill of resistant rock with a flat top and very steep sides plateau mesa butte

Cap Rock

Wind/water erosion continues Gap widens Gets isolated Pinacle Bute Mesa Plateau

Arches

Wind Deposition Aeolian – transported & dropped by wind Don’t Forget the Other Sediment Types in Desert Environments: Alluvial – flash flooding Evaporate – minerals (salt pan)

rocks from mineral evaporates Used as a palette by First Nations Manganese, iron oxides, hydroxides, and clay minerals form most varnishes and provide the shine.

Desert Deposition: Wind Related “Aeolian Deposits” z. SAND DUNES z. LOESS/DUST

Sediment of upland areas is carried off by wind & deposited in lower areas. A desert basin filling up this way is called a Bolson.

Sand Dunes z. Mounds of loose sand grains heaped up by the wind z. Need strong winds that generally blow in the same direction zsand grains well-sorted and wellrounded because wind is very selective as it moves sediment

zslip face: the steep downwind slope of a dune. Zone of stagnant air. zwind ripples: Occur as a result of larger grains accumulating as smaller grains are transported away. Wind ripples on sand surface - Monument Valley, Utah

Leeward Deposition

Types of Dunes z. Barchan: a crescent-shaped dune with a steep slip face on the inward or concave side which is formed where the sand supply is limited – regular wind

Barchan Dunes z. Crests curved and point to direction the wind blows (Horns) Eddy currents steepen the leeward slope Horns develop & move first

Transverse Dune: z. A relatively straight, elongate dune oriented perpendicular to the wind direction. Can form from merging Barchans. Transverse dunes

Parabolic z. Somewhat similar in shape to a barchan dune, except that it is deeply curved and is convex in the downwind direction Horns are anchored by vegetation = zones of stagnant air Could also be the result of a deflation hollow

Seif/Longitudinal/Linear zlongitudinal dune: one of the largest types of dunes which is a symmetrical ridge of sand that forms parallel to the prevailing wind direction Long straight dunes that form in areas with a limited sand supply and converging wind directions

Linear (Seif) Dunes Cont… z Elongate parallel to wind direction Short and Blair, 1986, Geomorphology from Space

z. Star Dunes - form where wind direction is highly variable Star dunes

Star Dunes cont…

Dune Migration: Nuisances $$$ Clear roads/parking lots$$$ Windbreaks; plant vegetation $$$

Wind Deposition: Loess jet stream: a narrow belt of strong winds with velocities sometimes greater than 200 mph

Wind Deposition: Loess z A deposit of fine wind-blown silt, clay and minerals that get weakly cemented by calcite have a high porosity (leeward side of deserts) Mississippi Valley Argentina Ukraine China

Where does Loess come from? Usually originates from dry lakebeds, eroded hillsides, floodplains, glacial outwash plains

Benefits of Loess Deposits

Problems with Loess High soil erosion rate Unsustainable farming practices, combined with huge population pressures, have led to severe environmental degradation. tons of sediment clog the Yellow River annually and pose a serious downstream flood risk. Earthquake = unstable

ARID LANDFORMS TOPOGRAPHIC MAPS

Alluvial Fans

Barchans

Playa

Deflation Hollows

Feature X in the photograph is being shaped by A. saltation. B. abrasion. C. deflation. D. corrosion.

Which process is responsible for shaping the boulder in the diagram? A. traction B. abrasion C. corrosion D. hydration