Part 2 Deserts and Winds

Sculptors of the Earth : 

Sculptors of the Earth Deserts and Winds

Deserts : 

Deserts Humid regions can be characterized by rounded hills and curving slopes Deserts, though, have angular rocks, sheer canyon walls, and surfaces covered in pebbles or sand The same geological processes are at work in both regions, though

Deserts : 

Deserts The desert landscape is influenced by mechanical weathering Debris consists of rock whose mineral content has not changed

Deserts : 

Deserts Chemical weathering is not completely absent Over long time spans, clays and thin soils do form

Deserts : 

Deserts The role of water In deserts, most streams are ephemeral – they only carry water when it rains Some might flow a few days or even just a few hours each year Some may not even carry water every year In the western states, these are called “washes” or “arroyos”

Deserts : 

Deserts Ephemeral Streams Known for dangerous flash floods after heavy rains So much water falls, that the soil cannot absorb it The lack of vegetation allows water to quickly run off the land The floods usually end as quickly as they start Erosion Because there are fewer plants in the desert to anchor soil, erosion during a rain even is impressive

Deserts : 

Deserts Basin and Range Because arid regions lack permanent streams, they have interior drainage Many streams are intermittent Water does not run out of the area toward the oceans In the USA, the dry Basin and Range is an excellent example of this behavior

Deserts : 

Deserts The Basin and Range includes Southern Oregon Nevada Western Utah Southeastern California Southern Arizona Southern New Mexico

Deserts : 

Deserts Basin and Range Contains more than 200 relatively small mountain ranges that rise 3000 – 4900 feet ab above the basins Rains produce runoff that is heavily loaded with sediment When the fast moving runoff from the canyons meets the more gently sloping grounds of the basins the streams of runoff quickly lose velocity and most of its load of sediment is dropped off This creates an alluvial fan

Deserts : 

Deserts Alluvial Fan A fan shaped deposit of sediment formed when a stream’s slope is abruptly reduced

Deserts : 

Deserts On rare occasions of abundant rainfall, or snowmelt from the mountains, streams may flow to the center of the basin and convert the basin floor to a shallow lake This is known as a playa lake A flat area on the floor of an undrained desert basin that fills and becomes a lake after a heavy rain or snowmelt They only last a few days or a weeks before evaporation removes the water

Deserts : 

Deserts Some permanent streams do manage to cross desert areas Colorado River Nile River Starts in the lakes and mountains of central Africa and covers almost 1900 miles without a single tributary adding to its flow Before this could happen, though, it had to have enough water at the beginning to survive the soil infiltration and evaporation along the way

Deserts : 

Deserts Running water in the desert, though infrequent, is important Most desert erosion results from running water Water does most of the erosional work in deserts

Winds : 

Winds Compared with running water, wind does not do nearly as much erosional work on the land, even in deserts Strong desert winds pick up, transport, and deposit great quantities of fine sediment Think of the dust bowl! The Dust Bowl was a time in the USA when much of the topsoil was removed in the Great Plains and surrounding areas because of poor land management practices, such as repeatedly plowing up ground

Winds : 

Winds The Dust Bowl

Winds : 

Winds Wind erodes the desert in two ways Deflation Abrasion

Winds : 

Winds Deflation The lifting and removal of loose particles such as clay and silt Coarser sand particles roll or skip along the surface in a process called “saltation” These large particles make up the bed load In portions of the Dust Bowl, deflation lowered the land by a meter or more in only a few years Deflation results in shallow depressions called “blowouts” Range in size from small dimples less than a meter deep and three meters wide to depressions more than 45 meters deep and several kilometers wide

Winds : 

Winds

Winds : 

Winds Deflation Desert Pavement In portions of many deserts, the surface is characterized by a layer of coarse pebbles and cobbles that are too large to be moved by the wind Deflations creates a stony surface layer called “desert pavement” when it removes all the sand and silt and leaves only the coarse particles The remaining surface is protected from further deflation, unless vehicles or animals break it up

Winds : 

Winds How desert pavement is made

Winds : 

Winds Abrasion Happens when windblown sand cuts and polishes exposed rock surfaces Blowing sand can grind away at boulders and smaller rocks, sometimes sandblasting them into odd shapes Since blowing sand rarely travels more than a meter above the surface, the wind’s sandblasting effects are limited vertically

Winds : 

Winds Wind deposits The wind can create landforms when it deposits its sediments, especially in deserts and along coasts Two major wind deposit landforms are Loess Sand dunes

Winds : 

Winds Loess Windblown silt that blankets the landscape Dust storms over thousands of years picked up this material, transported it, and deposited it The thickest and most extensive deposits occur in western and northern China In the USA it can be found in large quantities in SD, NB, IA, MO, IL, and the Columbia Plateau in the Pacific Northwest

Winds : 

Winds Sand Dunes Like water, wind deposit its load when its velocity falls Sand begins to accumulate wherever an obstruction crosses its path and slows its movement Unlike deposits of loess, which form blanket-like layers over broad areas, winds commonly deposit sand in mounds or ridges called dunes Dunes begin near obstructions, but as they grow the begin to act as an obstruction themselves enabling themselves to grow even larger

Winds : 

Winds Sand Dunes Are often steeper on the sheltered side and have a more gently sloping inclined side facing the wind The wind loses velocity as it reaches the crest of the dune and dumps its load on the sheltered side As the dune becomes steeper, the sand eventually slides down the slope So, in this way, dunes tend to migrate in the direction the wind blows

Winds : 

Winds Sand Dunes As sand is deposited on the sheltered side of the dune, it forms layers inclined in the direction the wind is blowing The sloping layers are called cross beds When dunes are eventually buried under other layers of sediment and become sedimentary rock, the cross beds remain as a record of their origins Excellent examples of this can be seen in Zion National Park

Winds : 

Winds Sand Dunes What forms sand dunes assume depends on the wind direction and speed, how much sand is available, and the amount of vegetation There are six major types of dunes Barchan Transverse Barchanoid Longitudinal Parabolic Star

Winds : 

Winds Barchan Dunes Solitary dunes shaped like crescents Form on flat, hard ground where supplies of sand and vegetation are limited Move slowly and reach heights of only about 100 feet

Winds : 

Winds Transverse Dunes Called transverse because ridges are perpendicular to the direction of the wind Form in series of long ridges if prevailing winds are steady, sand is plentiful, and vegetation is sparse Typical in coastal areas and the “sand sea” in parts of the Sahara and Arabian deserts Can reach heights of 650 feet and extend for miles

Winds : 

Winds Barchanoid Dunes A cross between Barchan and Transverse Scalloped rows of sand that form at right angles to the wind Resemble rows of barchans that have been positioned side by side Can see at White Sands National Monument in NM

Winds : 

Winds Longitudinal Dunes Long ridges of sand that form parallel to the prevailing wind Occur where sand supplies are moderate and the prevailing wind direction varies slightly Can reach 320 feet high and extend for miles Are found in portions of North Africa, Arabia, and central Australia

Winds : 

Winds Parabolic Dunes Look like backward Barchans Their tips point into the wind Form where vegetation covers the sand Often form along the coast where strong offshore winds and abundant sand are available

Winds : 

Winds Star Dunes Isolated hills of sand mostly found in parts of the Sahara and Arabian deserts Their bases resemble stars Usually have three or four sharp ridges that meet in the middle Develop in areas of variable wind direction Can reach heights of 300 feet