A. Introduction

• the wind is an exogenic agent

• the wind erodes, transports, & deposits material

• defn - "eolian" - (caused by the wind) refers to the erosion, transportation, and deposition of materials

• eolian processes tend to be more efficient in air & semi-arid climates - beaches & deserts

• the wind creates landforms (eolian landscapes with eolian landforms)

• the ability of the wind to move materials is small compared to that of water & ice because of its small density - but it can perform considerable work over time

• e.g., a 30 mph wind can transport ~ 1/2 ton of sand per day over a 1-meter-wide section of sand dune

• the amount of transported sand increases with wind speed

• intermediate-sized grains are transported most readily - (small grains are mutually cohesive & present a smooth surface to the wind)

Processes:

"deflation" - a process of wind erosion that removes & lifts individual particles, literally blowing away the unconsolidated, dry, or noncohesive sediments

"abrasion" - the mechanical wearing & erosion (grinding) of bedrock with a "sandblasting" action - rate of abrasion is a function of: hardness of surface rocks; wind velocity & constancy

• deflation & abrasion create particular landforms

"blowout depressions" - small (1 m) to large (hundreds of m) depressions formed via deflation (chemical weathering removes cementing materials)

• abrasion is restricted to the area in close proximity to the ground (1-2 m)

• rocks that have experienced abrasion, appear pitted, grooved, or polished

• land forms are aerodynamically shaped

"yardangs" - (larger scale) streamlined rock structures, formed by abrasion on the windward side & deflation on the leeward side - appear elongated & aligned with the most effective wind direction (Sphinx)

C. Eolian Transportation

• wind exerts a drag or frictional pull on particles

• transportation distance of particles varies with grain size

• only the finest material can be transported significant distances - "duststorm" vs. "sandstorm" (~ 2 m)

• fine debris (e.g., volcanic debris) can be transported worldwide within days

• ~ 80% of wind transport accomplished by skipping & bouncing action along ground ("saltation") - grains > 0.2 mm - aerodynamic lift, elastic bounce, & impact

• ~ 20 % of wind transport accomplished by "surface creep" - slides & rolls particles too large for saltation

• conservation measures slow eolian erosion & transportation prosesses (e.g., fences & walkways, vegtation)

• the wind is a minor contributor as far as weathering is concerned - but it plays a significant role in relocating material

• most deserts are covered by desert pavement - not sand

• saltating sand grains run into small patches of sand, lose their KE, & accumulate

• "slipface" - forms as the dune grows larger than 12" in height - forms on the leeward side at an angle (30 - 340)

• dune also has a gently sloping windward side

• constant flow of new material makes the slipface unstable ("avalanche slope")

• the built-up material slides down the slope & the slipface then readjusts seeking a stable angle ("angle of repose") - in this way the dune migrates downwind

• "freedunes" - actively moving dunes; reflect the interactions between the wind & the moving sand

• "tied dunes" - dunes fixed in place because of their encounter with vegetation or rock outcrops

• only ~ 10% of land between 30 degrees N. & 30 degrees S. is covered w/active sand dunes - thus limited extent of desert area covered w/sand dunes

• "snow dunes"" - same dune forming principles apply - form as wind deposits snow in drifts

• "loess" - large quantities of fine-grained clays silts left as glacial (Pleistocene) outwash deposits; subsequently blown by the wind great distances & redeposited as a generally unstratified, homogeneous blanket of material covering existing landscapes

• no specific landforms were created; assumes the general topography of the preexisting landscape

• because of its coherent structure, it weathers & erodes into steep bluffs

• loess deposits are very fertile - well drained, deep, & good moisture retention

• when subject to overgrazing or to dryness, can be lifted by winds to form dust storms (e.g., Dust Bowl of American Plains in the 1930s)

• dry [arid (desert)] climates - ~ 26% of Earth's land surface [semiarid (steppe)climates - ~ 35%]

• spatial distribution of these dry lands results from: (1) subtropical highs from15 degrees & 35 degrees N & S; (2) rain shadows on lee sides of mountain ranges; & (3) distance from moisture laden air masses (e.g., central Asia)

• these dry lands possess unique landscapes resulting from: (1) intermittent precipitation events; (2) weathering processes; & (3) wind

• desert lands experience high potential evaporation, high inputs of insolation, strong radiational cooling at night, low precipitation amounts, intermittent running water, hard, poorly vegetated surfaces (desert pavement), high surface runoff during rainstorms - a harsh environment !!

Desert Landforms & Landscapes (cont.)

"alluvial fans" - a fan shaped fluvial landform at the mouth of a canyon - particularly noticeable in arid landscapes where streams are intermittent - is produced as the water slows down upon leaving the constricted channel of the canyon

desert landscapes:

• comprise the largest single climatic region on earth (both arid & semiarid climates considered)

• are regions of special plant & animal adaptations

• wind & water processes operate together in arid regions

• although water events are infrequent, running water is still the major erosional agent in deserts

• are comprised of unique rock structures

• stark erosional remnants stand above the surrounding terrain

• such remnants, are resistant horizontal rock strata that have eroded differentially - the less resistant sandstone strata have been removed

Basin & Range Province of the Western U.S. :

• "province" - a large region with similar geologic traits (308,800 mi2)

• comprised of arid & semiarid basins & mountain ranges found in the rain shadow of mountains to the west

• the relief is sharp

• has a dry climate

• few permanant streams

• interior drainage

• landscape comprised of parallel sequences of upward-faulted ridges (horsts) & downward-faulted valleys (grabens)

• eroded material from the ridges accumulates to a great depth in the basin

• graben, horst, playa lake, bolson, pediment, alluvial fan