Erosion & Mass Movement

• erosion is the process by which the products of weathering are transported

• agents of erosion are gravity, wind, glaciers, water, ocean waves, currents streams, groundwater

What is erosion?

What is Soil Erosion?1. occurs world wide and is

normally a slow process2. ordinarily, new residual soil

forms about as fast as the existing soil erodes

3. However, unwise use of the land and unusual climatic conditions can upset this natural balance

How is erosion accelerated?

1. unwise farming and ranching methods increase soil erosion

• one example is clearing of trees,small plants, and animal overgrazing

• another example is furrows plowed in land

When soil is washed away by rainfall, furrows become larger forming gullies (gravity and water at work)

•Sheet erosion is another type of soil erosion that strips away parallel layers of topsoil

•may occur when continuous rainfall evenly washes away the topsoil

•can also be caused by wind during unusually dry periods

Severe soil erosion from wind in the 1930's was a result of drought coupled with unsound farming practices.

The photo on the right though, was taken at 10:00 AM, March 13, 1991 in downtown Topeka, Kansas, looking into the sun. There was so much soil in the air it darkened the sky. So the problem is not just something of the past.

constant erosion reduces the fertility of the soil by removing the A horizon, which is rich in humus (unable to grow crops)

What is soil conservation?

• Methods of protecting the soil by reducing soil erosion and loss of soil nutrients.

1. contour plowing: soil is plowed in circular bands that follow the shape of the land (prevents water from flowing directly down slope)

2. strip cropping: crops are planted in alternate bands

•Contour cropping of corn, alfalfa and wheat.

Contour farming (planting crops along the slope of the land) and contour strip cropping (alternating

different crops in the same field) reduces soil erosion. Traps soil on the field.

3. terracing: construction of step-like ridges that follow the contours of a sloped field

During heavy rain storms terraces catch water and lead it slowly off the field. Slowing down the speed

of flowing water reduces soil erosion.

4. crop rotation: farmers plant one type of crop one year and a different type of crop the next year

                                                

    <> With the development site cleared of all vegetation, area is now prone to soil losses in excess of 70 tons

per acre.

Gravity and Erosion

• Gravity, through its downward pull, causes rock fragments to move down inclines

• mass movement is the movement of fragments down a slope (rapid or slow)

rockfall: the fall of rock from a steep cliff

•is the most rapid

•can be from tiny fragments to large boulders

Figure 21 Rockslide The scar on the side of this mountain in northwestern Wyoming was made by an enormous rockslide that happened more than 75years ago.The debris in the slide formed a dam 70 m high across the Gros Ventre River.

landslide: sudden movement of masses of loose rock and soil down the slope of a hill

Slumgullion landslide

Hinsdale County, Colorado700 years old and still moving

mudflow: the rapid movement of a large mass of mud

slump: downhill movement of a large block of soil under the influence of gravity

A slump is a type of slope failure which involves the rotational movement of soil or rock

creep: extremely slow downhill movement of weathered rock material

erosion of mountains: weathering and erosion wear down rugged peaks to rounded peaks and gentle slopes

Rocky Mountains Appalachian Mountains

Avalanche

• The quick and sudden shift of snow and ice down the slope of a mountain.

• Causes: (Gravity is the ultimate force that pulls the snow downhill)– Warmer temperatures in spring– Massive accumulation of snow and ice– Plate tectonics (earthquakes)– Human activities (snowmobiling, snow skiing,

explosions, etc)– Glacier movement

Avalanche Problems

• Loss of life, property, roads, railways and etc.

• Avalanches differ from other mass movements because no soil is moved it is the movement of snow and ice down the slope on top of the soil.

Avalanches