Earth Science 7.3 Landscapes Shaped by Wind

1. Earth Science 7.3 Landscapes Shaped by Wind

2. 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

3. 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.

4. 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.

5. 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.

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

7. 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.

8. 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

9. 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 calleddesert pavement. • Deflation removes all the sand and particles small enough to be carried away. What remains is this layer of desert pavement.

10. 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.

11. 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.

12. 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.

13. 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.

14. 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.

15. 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.

16. 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

17. 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.

18. 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.

19. 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.

20. 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.

21. Wind Deposits: Types of Sand Dunes • Parabolic Dunes: Parabolic duneslook 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.

22. 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.

23. 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.

24. 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.