Chapter 5.3

1. Chapter 5.3 Mass Movements

2. Triggers of Mass Movements • What force causes material to move down slope? • The transfer of rock and soil down slope due to gravity is called mass movement

3. Mass Movement • This home in Pacific Palisades, California, was destroyed by a landslide triggered by the January 1994 Northridge earthquake

4. Q & A • Are snow avalanches a type of mass movement? Yes. These thundering down slope movements of snow and ice can also transport large quantities of rock, soil, and trees. 10,000 snow avalanches occur each year in the mountainous western United States.

5. Avalanches 101

6. Triggers of Mass Movement • Water • Heavy rains and rapid melting of snow can trigger mass movement by saturating surface materials with water

7. Triggers of Mass Movement • Oversteepened Slopes • An oversteepened slope can result when a stream undercuts a valley wall or waves pound against the base of a cliff

8. Triggers of Mass Movements • Plants make slopes more stable because their root systems bind soil and regolith together • When plants are removed by human activities the likelihood of mass movement increases

9. Triggers of Mass Movement • Earthquakes are one of the most dramatic triggers of mass movements • An earthquake and its aftershocks can dislodge enormous amounts of rock and unconsolidated material

10. Types of Mass Movements • Geologists classify mass movements based on the kind of material that moves, how it moves, and the speed of movement • Five kinds • Rockfalls • Slides • Slumps • Flows • Creep

11. Rockfalls • A rockfall occurs when rocks or rock fragments fall freely through the air • Common on slopes that are too steep for loose material to remain on the surface

12. Rockfall

13. Slides • In a slide, a block of material moves suddenly along a flat, inclined surface • Slides that include segments of bedrock are called rockslides • Often occur in high mountain areas • Reaching speeds of over 124 mph

14. Landslide – Kelso, Japan

15. Landslide - Italy

16. Out Running a Landslide

17. Slumps • A slump is the downward movement of a block of material along a curved surface • As the block moves, its upper surface sometimes tilts backward and leaves a crescent-shaped cliff just above the slump

18. Flows • Flows are mass movements of material containing a large amount of water that move downslope as a thick fluid • Mudflows move quickly and carry a mixture of soil, rock, and water that has a consistency of wet concrete • Earthflows move relatively slowly and carryclay-rich sediment (They move about a millimeter per day to several meters per day)

19. Afghanistan - Mudflow

20. Creep • Creep the slowest, downhill movement of soil and regolith (only travels a few millimeters or centimeters per year) • Alternates freezing and thawing • Freezing expands the water in the soil, lifting soil particles at right angles to the slope • Thawing causes contraction, which allows the particles to fall back to a slightly lower level

21. Time Lapse Glacier

22. Creep

23. Norwegian Helicopter Causing Rockfall

24. Review • What is mass movement? • How does water trigger mass movements? • How does a rockfall differ from a rockslide? The transfer of rock and soil downslope due to gravity Water fills the pores in sediment, allowing the particles to slide past one another more easily In a rockfall, rocks or rock fragments fall freely through the air. In a rockslide, a block that includes segments of bedrock moves suddenly along a flat, inclined surface

25. Review • What is the slowest type of mass movement? • When highway engineers build a road in a mountainous area, they insert drainage pipes into the slopes alongside the road. Explain why. Creep The pipes allow water to drain out of the soil and regolith in the slopes, reducing the chance that a mass movement will carry slope material onto the road.