Our Hazardous Environment GEOG 1110 Dr. Thieme

1. Lecture 11: Mass Wasting Our Hazardous EnvironmentGEOG 1110Dr. Thieme

2. Mass Wasting • downslope movement of rock or unconsolidated material • driven by gravity

3. Steep Slope Free Face of El Capitan formed on resistant granite bedrock in Yosemite Valley, California

4. Gradual Slope Convex, Straight, and Concave Slope Segments formed on relatively weak metamorphic rock on Santa Cruz Island, California

5. Angle of Repose • the steepest angle that can be assumed by loose fragments on a slope without downslope movement, usually ~ 30-37 degrees • Slope stability can be estimated from: • Shear stressexerted tangent to the slope, i.e. the slope’s steepness • Shear strength of particles on the slope: • composition and thickness of soil and rock layers • water content

6. Roadside cut too steeply in Gwinnett County, triggering rock fall

7. Fall - a fast, dry process • Slide - fast but often triggered by rains • Flow – fast and wet • Creep – slow and dry

11. FALL– commonly occurs in mountainous areas as a result of frost wedging Talus apron at the base of a mountain in the Sierra Nevada

12. SLIDE • movement of blocks of rock, usually in settings where strata are inclined, or where joints and fractures exist parallel to slope • translational if the slip surface is straight • rotational if it is curved

13. A rock slidein Oregon, a convergent margin setting

14. SLUMP • downward sliding of a mass of rock or unconsolidated material moving as a unit along a curved surface

16. FLOW • fast to moderate movement of sediment containing a large amount of water • volcanic eruptions trigger lahars • heavy rains trigger earthflows moving at moderate speeds in humid settings • mudflowsare triggered by catastrophic rains, particularly in arid and semiarid settings • debris flows occur when large objects are entrained by flowing mud

17. Alahar triggered by rainfall on the Nima II River in Guatemala. The lahar developed on the slopes of Santiaguito volcano

18. Earth Flow • form on hillsides in humid regions • Water saturates the soil • commonly involve materials rich in clay and silt

19. Tongue-shaped earthflow along a newly constructed highway, following heavy rain on clay-rich material

21. Mudflow following a torrential rain in the semiarid Panamint valley, Nevada

22. A series of debris flows in 1999 destroyed much of the city of Caraballeda in Venezuela

23. A debris flow in Virginia triggered by Hurricane Camille in 1969. Debris flow recurrence interval for the southern Appalachian Mountains on the order of once every 3 years.

24. Creep • Gradual movement of soil and regolith downhill • Aided by the alternate expansion and contraction of the surface material

27. Solifluction • common in regions underlain by permafrost • can occur on gentle slopes • promoted by a dense clay hardpan or impermeable bedrock layer

28. Solifluction lobes are particularly common in settings which have permafrost, where the “active layer” thaws in summer

30. Slope Stability Resisting Force (Shear Strength) F = ___________________________ Driving Force (Shear Stress) Failure occurs when F = 1 or less

31. Factors affecting F • type of earth materials • slope angle and topography • climate • vegetation • water • time

32. Material Type • Slumps are most common in weak rock (mudstone, shale) and in unconsolidated material • Planes of weakness easily form in colluvium and other unconsolidated material beneath soil but above bedrock • Differences in resistance in layered rock cause layers to slab off and fall

34. Slope and Topography • Slope angle θ is responsible for driving force • Areas of high relief are more prone to mass wasting • Different types of mass wasting occur on steep as opposed to gentle slopes

35. Climate • Arid climates have rock falls, debris flows, and shallow soil slips due to: • sparse vegetation • thin soils • more exposure of bare rock • Humid climates have deep and complex landslides, earthflows, and soil creep

36. Vegetation • cushions the impact of falling rain and allows it to infiltrate into the soil, resisting slope failure • plant roots add strength and cohesion to slope materials, resisting slope failure • adds weight to a slope and may increase the probability of a landslide

37. Water • many landslides develop during rainstorms when slopes become saturated • slumps and other more gradual mass movements follow the deep infiltration of water into the slope • erosion of the base or toe of a slope decreases its stability

38. Time • seasonal changes in soil moisture and freezing and thawing of soil water drive some mass wasting (slump, flow, creep, solifluction) • strength of driving or resisting forces may increase or decrease over longer time periods due to chemical weathering of earth materials

39. Figure 6.18

40. Natural Service Functions • Landslides disturb natural ecosystems, which may provide an impetus for genetic diversification • Landslides dam streams, creating lakes and wetland habitats for aquatic organisms • Landslides can expose economically valuable minerals such as gold and diamonds