Ch. 15 - Mass Movement-The Work of Gravity

Class: GEOL-101

Notes:

Rescuers climb over huge boulders after the Jiweishan landslide in Central China destroyed an iron ore plant and several homes, killing 74 people.

15.1 Importance of Mass Movement

Describe how mass movement processes can cause natural disasters and discuss the role that mass movement plays in the development of landscapes.

Slopes

Presentation content:

• Slopes are the most common elements in our physical landscape
  • Slopes may appear to be stable, but all are under the influence of gravity
  • At one extreme movement may be gradual and practically imperceptible
  • At the other, it may consist of a roaring debris flow or thundering rock avalanche

Landslides as Geologic Hazards

Presentation content:

• A landslide is a sudden event where large quantities of rock and soil move down steep slopes
  • When people and communities are in the way, a natural disaster may result
  • The term landslide has no specific definition in geology
• Mass wasting refers to the downslope movement of rock, regolith, and soil under the direct influence of gravity

The Role of Mass Wasting

Presentation content:

• The Role of Mass Wasting in Landform Development
  • Mass wasting is the geologic process that often follows weathering
    • Sediment is ultimately transported to the sea
• Combined effects of mass wasting and running water produce stream valleys
  • The most common and conspicuous of Earth’s landforms

Notes from the lecture:

• Creates more surface area -> materials start to erode more easily
• Grand cannyon: 5M years ago a downcut formed it and it has been eroted since then

Slopes Change Through Time

Presentation content:

• Most rapid and spectacular mass-wasting events occur in areas of rugged, geologically young mountains
• Mass wasting and erosional processes slowly change these rugged mountains to more subdued terrain
• If dynamic internal processes did not continually produce regions having higher elevations, the system that moves debris to lower elevations would eventually cease
• Gravity is the controlling force of mass wasting, but several other factors play important roles.

Notes from the lecture:

• If you remove a lot of vegetation, that instantaneously changes the stability, material gets washed away if it rains, not absorbed by vegetation anymore.
• Similar to earthquakes, earthquakes do not kill people, the buildings are the ones that kills people.
• 0-1 cm rainfall 2004 -> 9cm rainfall 2005

15.2 Controls and Triggers of Mass Movement

Summarize the factors that control and trigger mass movement processes.

The Role of Water

Presentation content:

• When sediment pores fill with water, cohesion among particles are destroyed
• Water can lubricate materials
• Water adds weight to a mass of material
• Example: Colorado Front Range

Notes from the lecture:

• There is a certain amount of saturation that materials can handle before allowing to flow.

Overstepped Slopes

Presentation content:

• Many situations where oversteepening takes place
  • Examples: stream valleys and human activities
• Unconsolidated granular particles assume a stable slope at the angle of repose
  • The steepest angle at which a material can remain stable (at rest)
  • Different for various materials
• Oversteepened slopes are unstable and can trigger mass wasting

Notes from the lecture:

• Precipitation is playing a big role here similar to slope

• If it is not unconsolidated material...

Removal of Vegetation

Presentation content:

• Plants protect against erosions by binding soil and regolith together
  • Plants also shield the soil surface from raindrop impacts
• Vegetation is removed by forest fire or by humans (timber, farming, development)
  • Wildfires are inevitable in the western United States
  • Fast-moving destructive debris flows triggered by intense rainfalls are some of the most dangerous post-fire hazards

Notes from the lecture:

• Could be detrimental, specially in an overstepped slope
• You are increasing the rate/probability of erosion occurring.
• Every year after we have these large forest fires we see a destabilization of slope
  • The next big thing is when raining season begins.

Earthquakes as Triggers

Presentation content:

• Earthquakes and aftershocks can dislodge rocks and unconsolidated materials
• Examples from California and China
  • 1994 Northridge earthquake in California triggered 11,000 landslides
  • 2008 earthquake in China caused landslides which created temporary dams and “earthquake-created” lakes
• Liquefaction
  • During periods of ground shaking, water-saturated surface materials behave as fluid-like masses that flow

Notes from the lecture:

• Very specifically targeted to those tectonically active regions (plate boundaries)
• Liquefaction
  • Grains of sand vibrate and pressurize that water and destabilizes that material
  • Also facilitate things like free flow.
• Note:
  • Large ground material falling into the ocean can create a tsunami

Landslides Without Triggers?

Presentation content:

• Many rapid mass wasting events occur without a discernible trigger
• Slope materials gradually weaken over time—eventually if the strength falls below what is necessary to maintain slope stability, a landslide will occur
  • Timing of these events is random
  • Accurate prediction is impossible

Notes from the lecture:

• Landslide potential across the US
  • Eastern US: high potential (Appalachians)
  • Western US: very localized potential (only high on certain regions)
• Why less across the West?
  • Less rainfall
  • As you move across the mountains on the West coast you move into the mountain shadow, a very stable zone with not a lot of precipitation.
• Why do we classify this regions?
  • Insurance / protecting human life?

15.3 Classification of Mass Movement Processes

List and explain the criteria that are commonly used to classify mass movement processes.

Two things to consider

Presentation content:

• Two things to consider for classifying mass-wasting processes:
  • Type of Material
    • Debris, mud, and earth are used if soil and regolith move
    • “Rock” is used if bedrock moves
  • Type of Motion
    • Fall
      • The free fall of detached pieces is called a fall
      • Talus slopes are built by rock falls

Notes from the lecture:

• Break it down by the type of material and by different types of motion.
• Telus slopes is the product of large waterfalls, slope where you have large amounts of rock fall.

Motion

Presentation content:

• Type of Motion
  • Slide
    • A slide occurs when there is a distinct zone of weakness separating the slide material from the underlying material
      • Rotational slide—surface of rupture is concave up
      • Translational slide—material moves along a flat surface such as joints, faults, or bedding planes.
  • Flow
    • Flow occurs when material moves downslope as a viscous fluid
      • Most are saturated with water
• Rate of Movement
  • Fast
    • A rock avalanche is the most rapid type of mass wasting
      • Rocks float on air as they move downslope
  • Slow
    • Creep moves particles a few millimeters per year
    • A wide range of rates exists between these two extremes

Notes from the lecture:

• Translation slide -> flat surface
• Rotational slide -> curved surface
• Rock fall is basically moving at the rate where gravity carries it down
• If you have a large amount of rock material falling, it allows for these rocks to travel larger distance because they start functioning as a "pillow" for the next falling rock
• As water freezes it contracts, when it warms it expands, this is a destabilizer.
• Examples:
  • Blackhawk Rock Avalanche
    • How did you get all that material from the mountain 8 km away
    • There is probably a pillow of water material in the floor
  • Landslides in the Valles Marineris canyons of Mars
    • Massive 50km canyons
    • There must have been water on mars at some point for rocks to flow and travel that far.

15.4 Common Forms of Mass Movement

Distinguish among slump, rockslide, debris slide, debris avalanche, debris flow, mudflow, lahar, earthflow.

Rapid Forms of Mass Wasting

Presentation content:

• Slump
  • A slump is the movement of a mass of rock or unconsolidated material as a unit along a curved surface (rotational slide)
    • Can involve a single mass or multiple blocks
  • Occurs along oversteepened slopes

• Rockslide
  • A rockslide occurs when blocks of bedrock slide down a slope
    • A debris slide occurs when unconsolidated material slides down a slope
  • Generally very fast and destructive
    • Generally fast process because of gravity
  • Sometimes triggered by melting snow or rain
    • Most common during the spring
    • Water facilitates the triggering of this. -> mechanical weathering that can act as a trigger.
  • Sometimes triggered by earthquakes
    • New Madrid, Yellowstone, Gros Ventre (1925)

• Debris Flow

  • A debris flow is a rapid form of mass wasting that involves the flow of soil and regolith with water (mudflow if the material is fine grained)
    • Not solid rock
  • Tend to occur more frequently in semi-arid mountainous regions
    • Sudden rainfall or snowmelt washes large quantities of sediment into rivers
      • Lack of vegetation to anchor soil
  • Often confined to channels and canyons
    • Common in semi-erid regions

• Lahar

  • Debris flows composed mostly of volcanic materials
    • Example: Mount St. Helens
  • Historically some of the most deadly volcanic hazards
    • Can occur during a volcanic eruption or when a volcano is quiet
    • Take place when highly unstable layers of ash and debris become saturated with water
    • Generally follow stream channels
  • Notes:
    • Large amount of ice melt incorporating volcano material
    • Allows for the flow of slushy volcanic material
    • Washes out the side of a volcano
      • As a result the volcano has a lateral blow-out
      • Really dangerous because it is basically acting as a blasting directed to places where there are houses and habitants.

• Earthflow

  • Earthflows form on hillsides in humid regions during heavy precipitation or snowmelt
    • Water saturates the soil and regolith
  • Commonly involve materials rich in clay and silt
    • Very viscous, move at slower rates than more fluid debris flows
  • Range in size from a few meters to more than a kilometer long and several hundred meters wide!
  • Notes:
    • Ground became saturated
    • You can have a situation where you have a slump and an earthflow

15.5 Very Slow Mass Movements

Review the general characteristics of slow mass movement processes and describe the unique issues associated with a permafrost environment.

Creep

Presentation content:

• Creep is the gradual movement of soil and regolith downhill
  • Imperceptibly slow!
  • Aided by the alternate expansion and contraction of the surface material
  • Caused by freezing and thawing or wetting and drying
• Causes fences and utility walls to tilt

Notes from the lecture:

• Very very slow process
• A pebble moves just a few meters for a couples of years

Solifluction

Presentation content:

• Solifluction is the downslope movement of water logged soils
  • Literally: “soil flow”
  • Promoted by a deeper dense clay hardpan or impermeable bedrock layer
  • Common in regions underlain by permafrost
    • Occurs in the active layer, the zone above the permafrost

Notes from the lecture:

• Only happens in regions that experience permafrost
• Think of it as peanut butter staying in the plate
• Also one of these very very slow processes.
• Moving very slowly

The Sensitive Permafrost Landscape

Presentation content:

• Permafrost is permanently frozen ground
  • Summers are too short and cool to melt ice below the shallow surface
  • Deeper ground remains below 0ºC (32ºF) throughout the year
• Extensive around the Arctic Ocean
  • Land use is regulated to prevent the permafrost from melting

Notes from the lecture:

• You have to worry about subsidence
• The Foundation warms underneath -> starts process of liquifaction
• Starts melting slowly, as a result you start getting subsidence of the ground.
• Issue that we see more in northern latitudes
  • Oil gas pipes on Alaska, what would happen with them? -> this is a concern especially for infrastructure.
• Melting of permafrost releases methane.
  • Look up videos of people drilling the permafrost and turn up a lighter, it turns on fire.

15.6 Detecting, Monitoring, and Mitigating Landslides

Summarize the indicators of an active landslide and list some ways that engineers can mitigate various kinds of mass movement.

Detecting

Presentation content:

• Detecting a developing case of mass movement can allow for evacuations that save lives, permit real-time monitoring of the situation, and trigger efforts to diminish the threat (mitigation).
• Method used to identify active landslides:
  • Field mapping of ground deformation. This involves locating cracks or bulges in the ground surface, cracked concrete foundations, tilted trees, and other disturbances.
  • Remote mapping of ground deformation by aerial photography, lidar, and satellite imaging also helps identify active landslide sites

Mitigating Active Landslides

Presentation content:

• When active landslides threaten infrastructure such as roads or buildings, geologists and engineers employ a variety of stabilizing methods.
• These include techniques to
  1. “de-water” the failing mass (for example, rerouting stream channels or installing horizontal drain pipes);
  2. Buttress the slope with stronger material (broken rock or reinforced concrete);
  3. Bind the sliding surfaces with rock bolts for fractures or steel (or timber) piles for unconsolidated masses.

End of Chapter 15 - Concept Checks

15.1 The Importance of Mass Movement

1. Define mass movement. How does it differ from erosional agents such as streams, glaciers, and wind?
2. In what sort of landscape are rapid mass movement processes most likely to occur? Describe how these geologic hazards might become geologic risks.
3. Sketch or describe how mass movement combines with stream erosion to expand valleys.

15.2 Controls and Triggers of Mass Movement

1. How does water affect mass movement processes?
2. Describe the significance of the angle of repose.
3. How might a wildfire influence mass movement?
4. Describe the relationship between earthquakes and landslides.

15.3 Classification of Mass Movement Processes

1. List and sketch three ways material can move during mass movement events.
2. In what way has scientific thinking changed about how rock avalanches move at such great speeds?

15.4 Common Forms of Mass Movement

1. Without looking back at the figures in this section, sketch and label a simple cross section (side view) of a slump.
2. What factors led to the massive rockslide at Gros Ventre, Wyoming?
3. How is a lahar different from a debris flow that might occur in southern California?
4. Contrast earthflows and debris flows.

15.5 Very Slow Mass Movements

1. Describe the basic mechanisms that contribute to creep. How might you recognize that creep is occurring?
2. During what season does solifluction in the Arctic occur? Explain why it occurs only during that season.
3. What is permafrost? How might disturbing permafrost lead to unstable ground that may slide, flow, or subside?

15.6 Detecting, Monitoring, and Mitigating Landslides

1. List two observations which indicate that a slope may be failing.
2. Describe some monitoring methods used when an active landslide is detected.
3. Name two techniques that are used to strengthen failing slopes.