Ch. 14: Mass Movements Flashcards
What is a mass movement?
It is the downslope of a body of material made up a soil, sediment, or rock propelled by the force of gravity.
-They can occur on land, or beneath the ocean as submarine landslides
Identify the parts of a slope. Where on the slope do erosion,
transportation, and deposition processes dominate?
-Waxing - near the top of slope, and is a convex surface (dominated by erosion)
-Free face - below the waxing area, and is steep
- exists if more resistant rock is present; also dominated by erosion
- Debris slope - below the free face
- rock fragments (coarse sediments) that fell from above tend to accumulate here and can be deposited here or still be in transportation
- Warning slope - at the bottom of the slope, and is concave
- Fine sediment washed w/ water from above is deposited here
List and describe the five main factors that cause mass movements to initiate. In particular, define the angle of repose and explain how it is related to the forces that are at play in the
context of mass movements. What are those driving and resisting forces, and how do they interact when a mass movement occurs?
-Slope angle - How the slope is is angled could really affect the kinetic energy of the mass movement. it depends on how gentle or steep the slope is.
-The angle of repose is the steepest angle at which a sloping surface is stable. it represents a balance between the driving force (gravity) and resisting force (friction and cohesion). for many materials, the angle of repose commonly ranges between 33 to 37°, but it also depends on the material type, grain size, and cohesion.
- the larger the slope angle, the higher the chance of mass movement
-Forces - driving force = gravity; it works to bring material down the slope
- it’s affected by the weight, size, and shape of the material, and how far the surface angle is past the angle of repose (is the slope over steepened?); also affected by the amount of moisture available
- resisting force = the shear strength of slope material, which work against gravity
-cohesiveness (how well the particles hold well together)
- Internal friction
-If the weight and gravity from above overcome the friction and cohesion, then the slope fails.
- Water - acts as lubricants between rock/sediment layers
-heavy rainfall events can oversaturate a slope surface, reducing friction and cohesion; surfaces with high clay content are especially sensitive- when clay is saturated, it forms a viscous fluid that fails easily (even at low angles), especially if weight lays on top of it.
Earthquakes - shakes debris loose and fractures rock
Volcanic eruptions - eruption can melt snow and ice, which saturates sediment and rock, and creates lahar
- lahar - a hot and cold mixture of volcanic debris and water that flows quickly down the slopes of volcanoes
What are the four main classes of mass wasting? What properties are used to differentiate them? Briefly describe each and explain the differences between the sub-classes.
-Falls -involves the free-fall movement of material from steep slope or cliff face. It’s differentiated based on the nature of the falling material and the mechanism of detachment
- rockfall is a volume of rock that falls through the air and hits a surface
-talus slope - blend of cone-shaped piles of irregular broken rocks at the base of a steep incline due to rockfall
- Landslides - sudden fast movement of cohesive mass of regolith or bedrock that isn’t saturated w/ moisture
-properties used to differentiate landslides are the type of movement, nature of the sliding surface, and the material involved
-Translational slides - movement along a flat surface roughly
- Rotational slides (“slumps”) - occurs when surface material moves along a concave surface, and overlying material rotates; surface of the rupture is curved, and there’s little internal deformation
Flows - refers to the fast movement of unconsolidated material, such as soil, regolith, and rock fragments downslope; when the soil is saturated, flow occurs and it’s categorized based on grain size of material and water content
-Debris flow - mixture of loose soil, rock, organic matter, air and water that move downslope as a slurry
- Earthflow - occurs in fine-grained materials or clay-bearing rocks. It has a characteristic hourglass shape
- Mudflow - a type of earthflow that’s wet enough to flow faster
Creeps - persistent, gradual mass movement of surface soil in which individual soil particles are lifted and disturbed overtime
-they can occur by freeze/thaw cycles, wet/dry cycles, or by grazing livestock; common in permafrost
-Solifluction - a form of soil creep that occurs in permafrost in which the gradual downslope movement of wet material occurs because the underlying frozen soil acts a barrier to the percolation of water
How can humans cause disturbances that lead to mass movement? (scarification)
Humans can cause disturbances that lead to mass movements through multiple ways, such as:
-Highway roadcut - could change subsurface water flow patterns which could destabilize slopes due to water seeping into soil.
-Surface mining - could also alter natural drainage patterns and increase surface runoff, which could lead to the saturation of slopes
-Housing development - the heavy machinery used in the construction of urban developments usually involve the digging and grading of slopes. This can change the natural balance of forces on slopes
-Forest clear-cut - this exposes soil to erosion by wind and water; the loss of vegetation decreases the ability of soil to stick together
