Earth Science 7.3 Landscapes Shaped by Wind

Key Concepts: Students will identify today: How deflation causes erosion in the desert How abrasion shapes desert landscapes What types of landforms are deposited by wind How sand dunes differ from one another

Wind Erosion Compared with water, wind does not produce as much erosion in the desert landscape. Wind can still be an important force however and contributes to shaping the landscape. Humid areas can resist erosion by wind because plants hold the soil together and moisture binds the soil particles together. In deserts, the soil is dry and dusty and there are few plants with roots to hold the soil together.

Wind Erosion Strong desert winds also pick up, move and deposit sediment in great quantities. These are known as dust storms. In the Great Depression of the 30s, farmers in the central US experienced the affects of wind erosion when a severe drought struck.

Wind Erosion With all the vegetation removed to grow crops; the area became a Dust Bowl when the strong winds picked up the soil and blew it in huge dust storms. The heavy winds picked up all the topsoil and blew it away; leaving behind barren soil which was unable to grow anything.

Wind Erosion Deflation Abrasion Winds erode in the desert through two ways Deflation Abrasion

Wind Erosion Deflation is the lifting and removal of loose particles of clay and silt by strong winds. Courser sand particles roll or skip along the surface in a process called saltation. These larger sand particles make up the “bed load” much as they do in a stream, rolling along the bottom. In portions of the Dust Bowl, deflation lowered the land by a few meters in only a few years.

Deflation Deflation also results in shallow depressions called blowouts. In the Great Plains, you can find thousands of blowouts Blowouts range in size from simple depressions less than a meter deep to large blowouts several kilometers across

Desert Pavement In sections of the desert, the surface can be found to have a course layer of pebbles; stones too large to be moved by even the heaviest of winds. This stony surface is created by deflation and is called desert pavement. Deflation removes all the sand and particles small enough to be carried away. What remains is this layer of desert pavement.

Abrasion Wind can erode by abrasion as well. Abrasion is when wind-blown sand cuts and polishes exposed rock surfaces. Blowing sand can grind away at boulders and smaller rocks, sometimes sandblasting them into odd shapes. Sand rarely travels more than a meter above the surface so much of the affect is limited to cutting close to the ground.

Wind Deposits: Loess Loess is windblown silt that blankets the landscape Dust storms pick up this material, transport it and deposit it. The thickest and most extensive deposits of loess are in China. The silt in them comes from nearby deserts. This fine buff-colored sediment gives the Yellow River it’s name.

Wind Deposits: Sand Dunes Like running water, wind releases it’s load of sediment when it’s velocity falls and the energy available for transport diminishes. Sand begins to accumulate whenever an obstruction crosses it’s path and the wind diminishes.

Wind Deposits: Sand Dunes Unlike deposits of loess, which forms blanket-like layers over broad areas, winds commonly deposit sand in mounds or ridges called dunes. Dunes can occur whenever an obstruction, such as a rock or small plant, causes the wind’s velocity to slow and particles drop to the ground. Once the sand starts to mound it acts as it’s own windbreak and more and more sand collects.

Wind Deposits: Sand Dunes Dunes are often steeper on the sheltered side and more gently inclined on the side facing the wind. Wind blows sand up the gentler windward side. Once the sand blows over the crest of the dune, the wind slows and the sand drops down. The sheltered side of the dune gradually becomes steeper and the sand eventually slides down the side of the slope.

Wind Deposits: Sand Dunes As sand is deposited on the sheltered side of the dune, it forms layers inclined in the direction the wind blows. These sloping layers are called cross-beds. When the dunes are eventually buried under sediment and become sedimentary rock, the cross beds remain as a record of their origin.

Wind Deposits: Types of Sand Dunes Sand Dunes are not random. They occur in a variety of consistent shapes. There are six major types of sand dunes. Barchan Dunes Transverse Dunes Barchanoid Dunes Longitudinal Dunes Parabolic Dunes Star Dunes

Wind Deposits: Types of Sand Dunes Barchan Dunes: Solitary sand dunes shaped like crescents are called barchan dunes. These form on flat hard ground where supplies of sand and vegetation are limited. Barchan dunes move slowly and only reach heights of about 30 meters. If the wind direction remains consistent, barchan dunes remain symmetrical. If the wind direction varies, one tip of the dune can grow larger than the other.

Wind Deposits: Types of Sand Dunes Transverse dunes: If prevailing winds are steady, sand is plentiful, and vegetation is sparse, dunes form in a series of long ridges. These are called transverse dunes because the ridges are perpendicular to the direction of the wind. Transverse dunes are typical in many coastal areas. Transverse dunes in the Sahara and Arabian deserts create dunes which reach up to 200 meters in height.

Wind Deposits: Types of Sand Dunes Barchanoid Dunes: A common dune form that is between a barchan and transverse dune is the barchanoid dune. These scalloped rows of sand form at right angles to the wind. The rows resemble a series of barchans that have been placed side-by-side.

Wind Deposits: Types of Sand Dunes Longitudinal Dunes: Longitudinal dunes are long ridges of sand that form parallel to the prevailing wind. These dunes occur where sand supplies are moderate and the prevailing wind direction varies slightly. In sections of North Africa, Arabia, and central Australia, longitudinal dunes can reach nearly 100 meters high and extend for more than 100 kilometers.

Wind Deposits: Types of Sand Dunes Parabolic Dunes: Parabolic dunes look like backwards barchans. Their tips point into the wind instead of away from it. They form where some vegetation covers the sand. Parabolic dunes often form along the coast where strong onshore winds and abundant sand are available.

Wind Deposits: Types of Sand Dunes Star Dunes: Star dunes are isolated hills of sand mostly found in parts of the Sahara and Arabian deserts. Their bases resemble stars and they usually have three or four sharp ridges that meet in the middle. Star dunes develop in areas of variable wind direction, and they sometimes reach heights of 90 meters.

Key Concepts: Today we learned: Wind erosion is more effective in deserts than in humid areas. There are two types of wind erosion: deflation and abrasion The wind can create landforms when it deposits it’s sediments. Layers of loess and sand dunes are landscape features created by wind deposits. A sand dunes form depends on it’s wind direction and speed, the amount of sand available, and the amount of vegetation.

Ticket to Go: Write down 4 facts that you learned today that you did not know already in connection to wind erosion and sand dunes.