2. Outline
• Shaping the Earth surface
-uplift vs. erosion
-describing the surface: relief, contours
-factors controlling landscape development
• Hydrologic cycle
• Mass movements
-Basics
-Various types
-Submarine mass movements
• Weakening the surface
-Slope stability
-Failure surfaces and triggers
-Link to plate tectonics
-Hazard and prevention
Interlude F & Chapter
Interlude F & Chapter 16
16
3. Shaping the Earth Surface
• Result of competition between uplift and erosion.
• Adding or subtracting elevation.
• Driven by tectonics:
• Uplift- vertical surface motion upwards
• Subsidence-vertical motion downwards
• Driven by surface processes:
• Erosion- surface lowering by mass removal
• Deposition- surface rising by mass addition Interlude F & Chapter
16
7. Factors Controlling
Landscape Development
Agents of transport & erosion
• Water, ice wind
Elevation distribution
• Relief, slope angle
Climate
• Mean temp and precipitation amount
Life
• Ecology/ biota can weaken/stabilize the surface
Exposed material type (substrate)
• Erodibility
Time Interlude F & Chapter
16
8. Outline
• Shaping the Earth surface
-uplift vs. erosion
-describing the surface: relief, contours
-factors controlling landscape development
• Hydrologic cycle
• Mass movements
-Basics
-Various types
-Submarine mass movements
• Weakening the surface
-Slope stability
-Failure surfaces and triggers
-Link to plate tectonics
-Hazard and prevention
Interlude F & Chapter
Interlude F & Chapter 16
16
9. Hydrologic Cycle
Water plays major role in surface erosion & deposition.
Hydrologic cycle- describes never ending water flow
between various reservoirs
Interlude F & Chapter
16
10. Hydrologic Cycle
Average time spent
The reservoirs: in each reservoir:
Interlude F & Chapter
16
12. Outline
• Shaping the Earth surface
-uplift vs. erosion
-describing the surface: relief, contours
-factors controlling landscape development
• Hydrologic cycle
• Mass movements
-Basics
-Various types
-Submarine mass movements
• Weakening the surface
-Slope stability
-Failure surfaces and triggers
-Link to plate tectonics
-Hazard and prevention
Interlude F & Chapter
Interlude F & Chapter 16
16
13. Intro to Mass Movements
• Most humans consider Earth to
be
• Earth’s surface, however, is
mostly unstable
• Due to
weathering/erosion/gravity
Interlude F & Chapter
16
14. What is a Mass Movement?
• Downslope motion of earth materials by gravity
• Type of natural hazard
• Natural landscape process
These hazards can produce catastrophic losses
Interlude F & Chapter
16
15. Intro to Mass Movements
• Mass movements are important to rock cycle
• Initial step in sediment transportation
• Significant landscape change agent
• All slopes are unstable, they change continously
• Mass movements are often
Interlude F & Chapter
16
16. Types of Mass Wasting
Classified based on 4 factors:
1. Material type (rock, regolith (loose debris), snow, ice)
2. Rate of movement (fast, intermediate or slow)
3. Nature of moving mass (cloud, slurry, or distinct blocks)
slurry-liquid with suspended soilds
4. Surroundings (subaerial or submarine)
Interlude F & Chapter
16
17. Types of Mass Wasting
Creep, solifluction, rock glaciers (not shown) slumping
Flows (mud) avalanches, falls (rock)
Interlude F & Chapter
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18. Types of Mass Wasting
Creep –slow downhill movement of regoith
Due to expansion and contraction
via wetting and drying, freezing and thawing
Grain moves:
perpendicular to slope upon expansion
Verically by gravity upon contraction
Interlude F & Chapter
16
19. Types of Mass Wasting
Creep tilts trees, gravestones, and walls
Interlude F & Chapter
16
20. Types of Mass Wasting
Solifluction –slow downhill movement of tundra
melted permafrost slowly flows over deeper, frozen soil
generates hillsides with solifluction lobes
Rock glaciers – mix of rocks fragments and ice
rocks added faster than ice accumulation
Behave like glacial ice, flowing slowly downhill
Interlude F & Chapter
16
21. Types of Mass Wasting
Slumping – sliding of regolith as coherent blocks
Slip occurs along a spoon-shaped failure surface
Variety of sizes and rates of motion
Distinctive features
head scarp, bulging toe
Interlude F & Chapter
16
22. Types of Mass Wasting
Mud & debris flows – h20 rich mass movement
Mudflow- slurry of water and fine sediment
Debris flow- mudflow with large rocks
Interlude F & Chapter
16
23. Types of Mass Wasting
Lahar – a volanic mud or debris flow
volcanic ash from recent/ongoing eruption
water from heavy rain or melted glacial ice
Interlude F & Chapter
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24. Types of Mass Wasting
Landslides – movement down nonvertical slope
• Rock slide – consists of rock only
• Debris slide – mostly reoglith
Movement down failure surface is quick
Debris can move 300 km/hr on cushion of air
Interlude F & Chapter
16
25. Landslide Case Study
• Vaiont Dam disaster – illustrates need to evaluate geology when
siting structures
• Built in 1960 in a deep gorge in Italian Alps.
• Limestone over shale dipped toward valley center.
• On 10/9/63, 600 million tons of limestone fell into the reservoir.
• A wave crested the dam, destroyed villages, and killed 2,600.
Interlude F & Chapter
16
26. Types of Mass Wasting
Avalanches – turbulent clouds of debris and air
• Snow avalanche – over-steepened snow detaches
• Debris avalanche – rock and dust fragment
• Move up to 250 km/hr on a cushion of air
Interlude F & Chapter
16
27. Types of Mass Wasting
• Rock & debris falls – vertical freefall
• Bedrock/regolith that falls rapidly
• Block impact, fragment and keep moving
• Talus blocks pile up at slope base
Interlude F & Chapter
16
28. Types of Mass Wasting
Submarine mass movements.
• 3 types – based on degree of disintegration
1. Submarine slumps – coherent blocks break and sip
2. Submarine debris flows – moving material breaks apart
3. Turbidity currents – sediement moves as a turbulent cloud
Usually well-perserved
Interlude F & Chapter
16
29. Types of Mass Wasting
Submarine mass movements.
• Gigantic submarine slope failures are widespread
• Large than land-based failures
• Important process sculpting adjacent land
• Create catastrophic tsunamis
Interlude F & Chapter
16
30. Outline
• Shaping the Earth surface
-uplift vs. erosion
-describing the surface: relief, contours
-factors controlling landscape development
• Hydrologic cycle
• Mass movements
-Basics
-Various types
-Submarine mass movements
• Weakening the surface
-Slope stability
-Failure surfaces and triggers
-Link to plate tectonics
-Hazard and prevention
Interlude F & Chapter
Interlude F & Chapter 16
16
31. Weakening the Surface
• Mass movements require earth materials
• Fragmentation & weathering.
Interlude F & Chapter
16
32. Weakening the Surface
• Slope stability is a trade-off between 2 forces:
1. Downslope force – gravitational pull
2. Resisting force – material properties that resist motion
• Movement occurs when downslope forces win
Interlude F & Chapter
16
33. Slope Stability
1. Downslope forces (Fd) = gravity
Weight of earth materials
2. Resisting forces (Fr) = material strength
3. Cohesion friction
Interlude F & Chapter
16
34. Slope Stability
• Loose material assumes an “angle of repose”.
• Maximum stable angle
• Due to material properties
Particle size, shape, surface roughness
• Typical repose angles:
• Fine sand:35
• Coarse sand: 40
• Angular pebbles: 45
Interlude F & Chapter
16
35. Failure Surfaces
• Weak subsurface layers can initiate motion
• Failure surface- types include
• Saturated sand/clay layers
• Joints parallel to surface
• Weak sedimentary bedding (shale)
• Metamorphic foliation
Interlude F & Chapter
16
36. Failure Triggers
• Destabilizing event usually triggers slopes failure
• Triggers are natural & anthropogenic:
• Shocks or vibration
• Changes in slope characteristics
• Changes in slope strength
Interlude F & Chapter
16
37. Failure Triggers
• A triggering event is not necessary for movement
• Slope materials slowly weaken over time
• Gravity
• Result: movements often
Interlude F & Chapter
16
38. Failure Triggers
• Shocks & vibrations.
• Vibrations decrease material friction
• On unstable slope, downslope force takes over
• Many sources of vibration are common:
• Motion of heavy machinery, vehicles
Interlude F & Chapter
16
39. Failure Triggers
• Vibrations cause saturated sediments to liquefy
• Quick clay – pore water slurries clay flakes when shaken
• Saturated sand – fluidized by increase in pore pressure
Interlude F & Chapter
16
40. Failure Triggers
• Changes in characteristics can destabilize a slope
• Angle – steepening slope beyond angle of repose
• Loading –add weight to top of slope
• Water – as rain or via humans
Interlude F & Chapter
16
42. Link to Plate Tectonics
• Tectonic processes influence mass movements
• Create uplift – topo and relief leads to steep slopes
• Fragment crust – joints and faults ease disintegration
• Generate earthquakes – trigger
Interlude F & Chapter
16
43. Identifying Slope Hazards
• Geologic mapping can identify risk regions
• Past failures
• Currently unstable slopes:
• Cracked and bulging ground
• Measureable changes in surveyed land features
GPS can detect slow movements
Interlude F & Chapter
16
44. Prevention
• Action can reduce mass movement hazards
• Revegetation – adding plants has 2 positive effects
• Removes water
• Roots
Interlude F & Chapter
16
45. Prevention
• Action can reduce mass movements hazards
• Slowing or eliminating undercutting- increases stability
• Removing erosion agent at slope base
• Reducing effect of erosion agent
Interlude F & Chapter
16
46. Prevention
• Engineered structures.
• Rock staples – rods drilled into rock to hold steep face
Interlude F & Chapter
16
47. Prevention
• Engineered structures.
• Avalanche sheds – structures that shunt avalanche snow
• Controlled blasting – surgical removal of dangerous rock
Interlude F & Chapter
16
