Eric H Christiansen Brigham Young University

16. Eolian Systems 1. Wind erosion 2. Wind transportation of sediment 3. Sand transport and deposition 4. Dune Types 5. Dust transport and deposition

Global Eolian System Driven by wind Solar energy produces differential heating Location of continents and oceans impacts heating Coriolis effect deflects moving air Mountains deflect wind

Global Eolian System Climate zones effect wind transport, deposition and erosion Wind is most effective in desert or semi-arid environments Low rainfall Dry soil Little vegetation Wind may effect sediment in other climate zones Coastal beach environments Areas impacted by human development Salt accumulation in soil from irrigation Construction sites

Deserts and loess: Wind prevails

Wind Erosion Two forms of wind erosion Deflation Removal of material by wind transport Abrasion Mechanical weathering of rock by wind blown sand

Deflation The blowing away of loose sand, silt, and clay sized particles Larger particles remain Only takes place in desert areas, or in the absence of vegetation Roots tend to hold soil in place

Deflation Desert pavement is formed when sand and silt are blown away Deflation may excavate depressions in excess of 50 m Only limit is the depth of groundwater

Lag surfaces: More common than dunes

Wind Erosion: Abrasion Natural sandblasting Kinetic energy of wind is transferred to wind-driven particles Particles impact rock surfaces & knock off small pieces Creation of a ventifact

Abrasion: Ventifacts Abraded and polished rock surface produced by wind erosion At least one polished surface facing the wind Prevailing wind direction may be determined Uncommon compared to stream erosion

Abrasion Yardangs - elongated, streamlined ridges Form parallel to the wind direction Most easily formed in weakly lithified sediments

Abrasion: Yardangs

Eolian Sediment Transport Wind is not as effective at moving particles as is water Viscosity of air is 17.6 * mPa-s Viscosity of water is 1 mPa-s Wind must overcome resisting forces Weight of grains Cohesion between grains Dust Bowl Oklahoma 1930’s Dust Bowl Kansas

Sediment Transport Sediment grains may move by: Depends on: Particle Size, Wind Velocity Saltation, creep and suspension Wind blowing past a grain: Lift – formation of a zone of low pressure around a grain Drag – impact of air molecules on the grain Critical wind velocity needed to move a specific grain

Saltation Causes most sediment movement in sand dunes Movement of sand grains during saltation is similar to ping pong balls One grain is lifted into the air, subsequently falls to the surface Falling grain collides with another, causing it to bounce into the air

Saltation Loose sand surfaces are most impacted One grain may impact several Chain reaction of collisions Entire sand surface may be moving Sand grains may be lifted from a centimeter to a meter off the surface

Creep Surface creep is the rolling of sand grains across the surface by wind Some grains are too large to be lifted Wind and saltating grains impart energy to roll grains Do not lose contact with the surface Particles up to ~ 2mm

Suspension Silt and clay particles (<0.06 mm) may be carried in suspension Carried high in the atmosphere Up to 2500 m Transported long distances Dust from the Sahara may reach the Caribbean

Eolian Deposits - Sand Wind blown sand accumulates in Sand sheets Ripples Dunes Sand Seas Variation in forms caused by Sand supply Wind speed and direction

Sand Sheets Occur in areas of lower wind velocity and larger grain size Grains are too large to move by saltation Miniature desert pavement Moving portion of sheet is a few cm thick, total is a few meters thick Small areas on margins of sand seas Some much larger Eolian Deposits - Sand

Sand Sheets

Ripples Ripples form perpendicular to wind from saltating grains Ripples occur at regularly spaced intervals Ripples may form on dune surfaces

Dunes A hill or ridge of sand deposited by wind Form from small irregularities in the land surface Air flow is disrupted A pocket of low wind velocity is formed Sediment accumulates in the pocket Growing mound further disrupts air flow Sand transported up the windward face by saltation and surface creep, accumulates at crest Crest becomes unstable, sand avalanches down the slip face

Dunes Dune geometry Asymmetrical shape Shallow upwind slope, O Steep downwind slope, O, angle of repose

Dunes Deposition of sediment on the slip face produces cross bedding Crossing bedding within the dune is the remnants of older buried slip faces Asymmetry of dune gives the direction of the prevailing wind

Dunes Barchan Dunes Isolated, crescent shaped dunes with horns pointing downwind Develop in areas of limited available sand with moderate winds from one direction Transverse Dunes Similar environment to Barchan dunes Greater availability of sand

Dunes Linear Dunes Develop in areas of limited available sand with strong, bi-directional winds Form evenly spaced ridges Star Dunes Isolated sand hill with pyramidal form Formed in areas with no single prevailing wind direction

Dunes Parabolic Dunes A “U” or “V” shaped dune with long trailing arms Common in coastal areas with abundant sand, and prevailing ocean winds Migrate landward Typically vegetated

Eolian Transport of Dust South Mountain, Phoenix, Arizona: Clayton Esterson

Eolian Deposits - Dust Dust particles may remain suspended until: Wind velocities drop, and particles can’t remain suspended Dust particles collide in the air, and stick together forming larger particles Rain washes dust from atmosphere

Eolian Deposits - Dust Loess – thick deposits of wind blown dust Dominated by silt size grains Transported long distances Not always associated with desert areas Generally absent in humid tropical regions Common in glaciated areas

16. Eolian Systems 4. Various types of dunes form, depending on wind velocity, sand supply, and constancy of wind direction. 5. Wind-blown dust (loess) forms blanket deposits, which can mask the older landscape beneath them. The source of loess is desert dust or the fine rock debris deposited by glaciers. Some deep oceanic sediment is wind-blown dust. 6. Desertification, the loss of farmable land on the margins of deserts, can be caused by human activity or by slight climatic fluctuations 1. Wind is not an effective agent of ersion, but it can produce deflation basins and yardangs as well as small pits and grooves on rocks. 2. The major result of wind activity is the transportation of loose sand and dust. Wind transports sand by saltation and surface creep. Dust is transported in suspension. 3. Sand dunes migrate as sand grains are blown up and over the windward side of the dune and accumulate on the lee slope. The internal structure of a dune consists of strata inclined in a downwind direction..