1. Mass Movements

2. Mass Movements What is mass movement?
It is the transfer of rock and soil downslope due to gravity.
The force of gravity causes material to move downslope.

3. Mass Movements
Some types of mass movement are so slow that you cannot see them. Others, are very sudden.

4. Mass Movements: Triggers
What triggers mass movements?
Saturation of surface materials with water;
Oversteepening of slopes;
Removal of vegetation; and
Earthquakes.

5. Water
Heavy rains and rapid melting of snow can trigger mass movement by saturating surface materials with water
When the pores in sediment become filled with water, the particles slide past each other more easily.
Moist sand can hold its shape but, when saturated, it will ooze downhill.

6. Oversteepened slopes
Loose particles can maintain a stable slope up to a certain angle. If the steepness of a slope exceeds the stable angle, mass movements become more likely.

7. Causes of oversteep slopes
When a stream undercuts a valley wall
Waves pounding against the base of a cliff
Excavating during construction

8. Removal of vegetation
Plants make slopes more stable. Their roots bind the soil together. When plants are removed by forest fires or human activity the likelihood of mass movement increases.

9. Earthquakes (trigger)
Earthquakes are one of the most dramatic triggers of mass movements. An earthquake and its aftershocks can dislodge enormous amounts of rock and other material.

10. Types of Mass Movements
Geologists classify mass movements based on:
The kind of material that moves
How it moves
The speed of the movement

11. Types of Mass Movements
We will discuss the following types of mass movements:
Rockfalls;
Slides;
Slumps;
Flows; and
Creep.

12. What type of weathering might cause a rock to break free and fall?
Rockfalls
A rockfall occurs when rocks or rock fragments fall freely through the air.
This type of mass movement is common on slopes that are too steep for loose material to remain on the surface.
What type of weathering might cause a rock to break free and fall?

14. Slide
In a slide, a block of material moves suddenly along a flat, inclined surface.
Rockslides contain segments of bedrock.
One of the fastest type of mass movement. (can move up to 124 mph!)
What can trigger a slide?

15. Slumps
A slump is a downward movement of a block of material along a curved surface.
Does not travel very fast or far
Slumps leave a crescent-shape cliff just above the slump.
What might trigger a slump?

16. Flows
Flows are mass movements of material containing a large amount of water, which move downslope as a thick fluid.

17. Mudflows Move quickly Mixture may have the consistency of wet concrete
Will take most everything in its path down with it

18. Earthflows Move slowly Occur most often on hillsides in wet climates
Water saturates the soil, the material breaks away and leaves a tongue-shaped mass

20. Creep Creep is the slowest type of mass movement
Travels only a few centimeters per year
Freezing and thawing of the ground results in gradual downhill movement of the soil.
Structures that were once vertical will tilt downhill

21. Creep
Creep evidence apparent at the base of the tree trunks.