Mass Wasting

Mass Wasting(erosion caused by gravity)

Chapter 6, Lesson 3

Mass Wasting

Have you ever seen or heard a news report about a large number of boulders that fell down the side of a mountain and ended up on a road?

The downhill movement of a large mass of rocks or soil because of the pull of gravity is called mass wasting. A mass wasting event has two important parts:

• A large amount of material moves together as a mass.

• Gravity is the main cause of the movement.

In a mass wasting event, the whole mass moves together, all at one time. The mass does not move as separate pieces over a long period of time. Gravity is the transporting agent, not water, ice, or air. 

Why does it happen?

Mass wasting commonly occurs when soil on a hillside becomes soaked with rainwater. The water-soaked soil becomes so heavy that it breaks loose and slides down the hillside. 

For mass wasting, GRAVITY is the cause of the erosion, and deposition occurs when the material reaches the bottom of the hill

Multiple Choice

Which agent of erosion causes Mass wasting to happen?

• Wind

• Water

• Gravity

• Glaciers

How does vegetation help?

Recall that vegetation on a steep slope reduces the amount of erosion during a heavy rainfall. The thick vegetation on slopes also tends to prevent mass wasting.

Why does this happen?

1) The root systems of plants hold sediment in place like an anchor. 2) Vegetation like thick grass also cushions falling rain and allows water to soak slowly into the soil. 

Multiple Choice

If you want to hold soil in place, and prevent mass wasting from happening...you should...

• remove some vegetation

• plant some vegetation

Foldable Time

1)Take a blank piece of notebook paper

2) Fold hot dog style

3) Cutting through the FRONT FLAP ONLY, make three cuts so there ends up being FOUR flaps

4)On the front of each flap you will be writing the four types of mass wasting

5) Inside each flap you will be writing an explanation of each type AND draw a picture of each.

The first type of mass wasting is a rockfall

-A rockfall is a type of fast-moving landslide that happens when rock or earth falls,or rolls from a cliff or down a very steep slope. 

-Rockfalls can be very dangerous depending on where they occur, the size of the rocks involved, and how fast the rocks fall or bounce downslope. When people, buildings, vehicles, or highways are in their path, these rockfall events can cause property loss, personal injury.

The second type of mass wasting caused by gravity is a mudslide

- A mudslide can be caused by heavy rains loosening the sediment on a slope, causing mud to flow downhill quickly

They consist mainly of mud and water plus fragments of rock and other debris, so they often behave like floods. They can move houses off their foundations or bury a place within minutes.

Slump

The third type of mass movement caused by erosional gravity is Slump

Slump is a type of mass wasting that takes place when material moves fast, but in a large mass.

Think of it as like the side of a hill, fall down as one large chunk

Creep

Creep is the fourth type of mass wasting that is caused by gravity

When material moves so slowly downhill that it is hardly noticeable it is called creep. Creep may cause trees and other objects to lean over.

S-L-O-W, Invisible MOVEMENT of SOIL DOWNHILL

Amount of Erosion caused by mass wasting

The amount of erosion that occurs during mass wasting depends on the following factors:

• the type of rock involved,

• the amount of water in the soil,

• how strongly the rock and soil are held together, and

• how steep the slope is.

Mass wasting is more destructive on steep slopes than on gentle slopes.