GEOG 1113: Physical Geography Arid Processes and Landscapes March 31, 2015

Arid Landscapes 3 factors influence arid climates: –Subtropical high pressure –Rainshadow –Distance from large bodies of water

Arid Landscapes: Desert Geomorphology The Southwestern United States Region lies in the rainshadow of the Sierra Nevadas Landforms as influenced by rock structure –Basin and Range province of mountains and valleys –Non-deformed horizontal rocks layers have been shaped by stream erosion, differential weathering, and mass wasting

Desert Landforms and Geomorphology Note: Tops of most landforms once part of same surface, since partially eroded away

Playa

Arroyo

Eolian Erosion and Transport Geomorphic processes involving wind Most active in desert regions because: –Strong winds are common –A large supply of sand and silt that is available and can be blown –Vegetation cover is minimal and wind is free to erode sediment

Fluid Behavior of Wind Wind acts like a fluid, like water, but less dense Faster wind can move larger particles Threshold Velocity for wind to carry different sized particles

Particle Transport Silts and Clays carried in suspension Sand bounces along – saltation, or Sand rolls slowly along – creep

Eolian Erosional Landforms 2 types of wind erosion: –Deflation – wind blows loose soil away: leaves coarser pebbles & cobbles, called Desert Pavement (bottom left) when deflation causes basin to form, called Deflation Hollow (bottom right)

Eolian Erosional Landforms Deflation/Desert Pavement

Eolian Erosional Landforms Abrasion – wind blows sand along a surface to polish & abrade it VentifactsYardangs

Eolian Deposition and Landforms: Air Flow and the Formation of Sand Dunes Formation of dunes is dependent on the way wind and topography collectively influence patterns of sand erosion and deposition –Bare, sandy patches exposed to wind are locations for dune to form –Sand must accumulate and then is shaped by air flow

Eolian Deposition and Landforms: Air Flow and the Formation of Sand Dunes Fully active dunes have distinct components: –Backslope is the windward surface where erosion and saltation dominate –Crest is the highest point of the dune, marking the boundary between erosion and deposition –Slip face is the portion of the dune where deposition occurs © 2007, John Wiley and Sons, Inc.

Eolian Deposition and Landforms: Classification of Sand Dunes Overall form of dunes can vary based on a variety of factors: –Sand supply –Amount of vegetation present –Wind speed and direction

Loess Fine-grained, wind-blown silt – high in calcium – usually from alluvial deposits or glacial till Can be transported farther than sand

Loess Deposits around the World Loess Deposits

Loess Terraces

Regions Prone to Desertification

Desertification in African Sahel

The Sahel

Human Interactions with Eolian Processes: Desertification in the Great Plains The Dust Bowl of the 1930s Western Great Plains is a semi-arid landscape in the rainshadow of the Rocky mountains Population began increasing in the 1870s with the Homestead Act –Much of native grasslands were destroyed with increased farming

Human Interactions with Eolian Processes: Desertification in the Great Plains In the 1930s a period of intense drought occurred –Decimated the landscape and the regional farming economy Grasslands were plowed so soils were exposed and easily deflated by the very high winds –Black blizzards