CHAP 12: Mass Wasting *
Which of the following responses are examples of how water can promote mass wasting?
- Waves and streams can undercut banks and cliffs, causing them to fail. - When water is added to weathered particles, the mass of the sediment is increased, which can cause slope instability. - Water can saturate sediment, reducing the integrity of the slope and allowing it to move.
What are the controls and triggers of mass wasting?
1) Gravity is the controlling force 2) Important triggers include: • Saturation of the material with water - diminishes particle cohesion. Water adds weight. • Oversteepening of slopes - stable slope angle (angle of repose) is different for various materials. Oversteepened slopes are unstable. • Removal of anchoring vegetation • Ground vibrations from earthquakes - may cause expensive property damage. Can cause liquefaction - water saturated surface materials behave as fluid-like masses that flow.
What criteria are used to categorize the types of mass wasting?
1) Type of material involved: - Debris - Mud - Earth - Rock 2) Type of motion - Fall (free-falling pieces) - Slide (material moves along a surface as a coherent mass) - Flow (material moves as a chaotic mixture) 3) The velocity of the movement - Fast - Slow
What is a reason the Gros Ventre rockslide took place?
Because the river eroded material at the valley base, there was no support of the material upslope.
Describe how a soil particle moves downhill during the freeze-thaw process.
During the freeze, a soil particle moves at a right angle away from the surface, and thawing causes the particle to fall back to a slightly lower level.
What is Solifluction?
Promoted by a dense clay hardpan or impermeable bedrock layer. Common in regions underlain by permafrost. Can occur on gentle slopes.
What are the general characteristics of slump, rockslides, debris flow, earthflow and creep?
SLUMPS are movement of a mass of rock or unconsolidated material as a unit along a curved surface. Occurs along oversteepened slopes. ROCK SLIDES are blocks of bedrock slide down a slope. Generally very fast and destructive! DEBRIS FLOW consists of soil and regolith with a large amount of water. Often confined to channels. Debris flows composed mostly of volcanic materials on the flanks of volcanoes are called lahars. EARTH FLOW form on hillsides in humid regions. Water saturates the soil. Commonly involve materials rich in clay and silt. CREEP is a gradual movement of soil and regolith downhill. Aided by the alternate expansion and contraction of the surface material.
__________ is an example of mass wasting commonly seen in the active layer above permafrost.
Solifluction
Which of the following correctly states the role of rivers and mass wasting in the formation of the Grand Canyon?
The Colorado River erodes downward, then mass-wasting processes move material from high to low areas, then the Colorado River transports material downstream
Define mass wasting.
The downslope movement of sediment and rock under the direct influence of gravity.
If permafrost is melted, which of the following is a consequence?
The ground subsides because the volume of water is less than that of ice.
Which of the following statements is true regarding the aftermath of the Gros Ventre rockslide?
The rockslide debris dammed up the river and formed a lake.
When dry sediment accumulates, the angle of repose represents what property of the deposit?
The steepest angle at which those sediments can accumulate and remain stable.
Why would trees be J-shaped?
The trunks become tilted in the downhill direction due to creep, but continue to grow toward the sun.
The Gros Ventre slide is evident today by __________.
a scar, runout area, and landslide debris filling the valley
A __________ is a common result of creep.
broken retaining wall
Over time, mass wasting causes __________.
canyons to widen
A __________ acted as a sliding surface across which material moved downhill during the Gros Ventre Slide.
clay layer
When considering mass wasting, which of the following factors is most influential in the downslope movement of weathered particles?
gravity - Once weathering creates particles, the most influential process in moving them downslope is the constant force of gravity.
During the eruption of Mount St. Helens on May 18, 1980, snow and glacial ice on the mountain melted and rushed down the Toutle River. What is the name given to this specific type of mass wasting?
lahar - When snow and glacial ice melt during a volcanic eruption and flow down off a mountain, this very specific type of debris flow is called a lahar.
Creep is __________.
mass wasting by the gradual downhill movement of sediment, influenced by freeze-thaw or wet-dry cycles
Permafrost is __________.
permanently frozen ground of tundra and subarctic climates that has remained 0°C for two years or more
What is the process of mass wasting?
refers to the downslope movement of rock, regolith, and soil under the direct influence of gravity. Its a geologic process that often follows weathering. Combined effects if mass wasting and running water produce stream valleys.
What type of mass wasting tends to be the fastest moving and most destructive?
rock slides
Heavy rainfall caused the clay to lubricate and the sandstone to become __________.
saturated and heavy
What rock type is the easiest for a river to cut through?
shale
When sediment accumulates, the angle of repose can be increased by changing which of the following conditions?
slight addition of water to the sediment
For mass wasting to occur, there must be a ___________?
slope angle
A __________ is formed when vertical incision by a stream is the primary process, with relatively minimal mass wasting.
slot canyon
Which of the following is associated with creep?
slow and subtle movement of material
At a local landfill, the liner beneath the trash was being repaired. In the process, trash from the base of the slope was removed and piled on top of the landfill. After an unusually heavy rainfall, the slope failed very quickly along a curved surface. Which of the following types of mass wasting best describes this slope failure?
slump
Which example of mass wasting is characterized by a crescent-shaped scarp or cliff at its head?
slump
Which of the following types of mass wasting is extremely slow and occurs when saturated soil flows downslope?
solifluction - Saturated soils slowly move downslope in a process called solifluction.
What is used to keep the Trans-Alaska pipeline from melting permafrost?
stilts
What is name of the near-surface layer that thaws in the summer?
the active layer
