Chapter 7: Erosional Forces

Section 4: Wind

I. Wind Erosion

A. Air 1. Differs from other erosional forces because it

cannot pick up heavy sediments.

2. Wind carries and deposits sediments over large areas.

a. For example, wind is capable of picking up and carrying dust particles from fields or volcanic ash high into the atmosphere and depositing them thousands of kilometers away.

I. Wind Erosion

B. Deflation and Abrasion1. Wind erodes Earth’s surface by deflation and

abrasion.

2. When wind erodes by deflation, it blows across loose sediment, removing small particles such as silt and sand.

a. The heavier, coarser material is left behind.

I. Wind Erosion

B. Deflation and Abrasion3. When wind blown sediment

strikes rock, the surface of the rock gets scraped and worn away by a process called Abrasion.

a. Abrasion is similar to sand blasting.

b. The rocks become pitted and worn down gradually over time due to abrasion.

I. Wind Erosion

B. Deflation and Abrasion

4. Deflation and Abrasion happen to all land surfaces but occur mostly in deserts, beaches, and plowed fields.

a. These areas have fewer vegetation to hold the soil in place; so when wind blows over them, they can be eroded rapidly.

b. Grasslands or pasture lands have many plants to hold the soil in place, therefore there is little soil erosion caused by the wind.

I. Wind Erosion

C. Sandstorms

1. Even when the wind blows strongly, it seldom carries sand grains higher than 0.5 m from the ground.

a. However sand storms do occur when the wind blows forcefully in the sandy parts of the deserts; sand grains bounce along and hit other sands grains, causing more and more grains to rise into the air.

2. Most sandstorms occur in deserts, but they can occur in other arid regions.

I. Wind ErosionD. Dust Storms

1. When soil is moist, it stays packed on the ground, but when it dries out, it can be eroded by wind.

2. Because soil is composed largely of silt and clay sized particles they are less dense than the same sized sand particles, so wind can move them high into the air.

3. When silt and clay particles stick together; a faster wind is needed to lift these fine particles of soil than is needed to lift grains of sand.

a. However after they are airborne, the wind can carry them long distances because they are less dense.

I. Wind Erosion

D. Dust Storms4. Where the land is dry, dust storms can cover hundreds of

kilometers.

5. Dust Storms blow topsoil from open fields, overgrazed areas, and places where vegetation has disappeared.

a. Silt and dust from Kansas fell in New England and in the North Atlantic Ocean all the way from Kansas during the 1930’s Dust Bowl.

b. Dust blown from the Sahara has been traced as far away as the West Indies-a distance of at least 6,000 km.

I. Wind Erosion

1. Which factors increase the amount of erosion?

I. Wind Erosion

2. Which factors decrease the amount of erosion.

I. Wind Erosion

3. Estimate the missing Erosion rating.

II. Reducing Wind Erosion

A.Vegetation1. One of the best ways to slow or stop wind

erosion is to plant vegetation.

2. These practices helps conserve soil and protect valuable farmland.

a. Strip Cropping

b. Cover Cropping

c. No-Till Farming

II. Reducing Wind Erosion

B. Windbreaks1. Farmers plant trees to act as windbreaks that

prevent soil erosion.

a. As the wind hits the trees, its energy of motion is reduced, therefore it is no longer able to lift particles.

2. One study showed a thin belt of cottonwood trees reduced the effect of a 25 km/h wind to about 66 percent of its normal speed.

II. Reducing Wind Erosion

B.Windbreaks3. Tree belts also trap snow and hold it on land.

a. This increases the moisture level of the soil, which helps prevent further erosion.

II. Reducing Wind Erosion

C. Roots1. Plants with fibrous root systems, such as

grasses, work best at stopping wind erosion.

2. Grass roots are shallow and slender with many fibers.

a. They twist and turn between particles in the soil and hold it in place.

II. Reducing Wind Erosion

C.Roots3. Planting vegetation is

a good way to reduce the effects of deflation and abrasion.

4. If the wind is strong and the soil is dry, nothing can stop erosion completely.

III. Deposition by WindA. Loess

1. Wind deposits of fine grained sediments.

a. Examples found near the Mississippi and Missouri Rivers

b. Strong winds that blew across glacial outwash areas carried the sediments and deposited them.

III. Deposition by Wind

A. Loess2. Loess is as fine as talcum powder.

3. Many farmlands of the Midwestern United States have fertile soils that developed from loess deposits.

III. Deposition by Wind

B. Dunes1. Created when wind blows sediments against

an obstacle such as a rock or clump of vegetation.

a. Wind drops sediments because is loses its energy of motion.

2. The sediment itself then becomes an obstacle, trapping even more material.

III. Deposition by Wind

B. Dunes

3. If the wind blows long enough, the mound will become a dune.

4. A DUNE is a mound of sediments drifted by the wind.

III. Deposition by Wind

B. Dunes4. Dunes are common in desert regions.

5. Also along the shores of oceans, seas, or lakes.

a. Sand or other sediment will continue to build up and form a dune until the sediment runs out or the obstruction is removed.

b. Some desert sand dunes can grow to 300 feet high (100 meters), but most are much shorter.

III. Deposition by Wind

C. Moving Dunes1. A sand dune has two sides.

a. Side facing the wind has a gentler slope.

b. The side away from the wind is steeper.

c. The shape of the dune tells you the direction.

2. Unless sand dunes are planted with grasses, most dunes move, or migrate away from the direction of the wind. (See figure 22 next slide)

III. Deposition by WindC. Moving Dunes

3. Some dunes are known as traveling dunes because they move rapidly across desert areas.

III. Deposition by Wind

III. Deposition by Wind

III. Deposition by WindD. Dune Shape

1. The shape of the dune depends on the amount of sand or other sediment available, the wind speed and direction, and the amount of vegetation present.

III. Deposition by WindD. Dune Shape

2. Barchan Dune

a. The open side of a Barchan faces the direction that the wind is blowing.

b. This type of dune forms on hard surfaces where the sand supply is limited

c. Crescent shaped.

III. Deposition by WindD. Dune Shape

3. Transverse Dune.a. Forms when sand is

abundant.

b. Named because the long directions of these dunes are perpendicular to general wind direction.

III. Deposition by WindD. Dune Shape

4. Star Dunea. Found in areas where

wind direction changes.