Unit 8 Flashcards
Landslide
a sudden event where large quantities of rock and soil move down steep slopes
Mass Wasting
the downslope movement of rock, regolith, and soil under the direct influence of gravity
What process does Mass wasting normally follow?
Weathering
What is the controlling force of mass wasting?
Gravity
What does saturating sediment do?
Saturation reduces friction
Angle of repose
The steepest angle at which a material can remain stable
What are the controls and triggers of mass wasting
- Oversteepened Hills
- Removal of Vegetation
- Earthquakes
- Slope materials gradually weaken over time- eventually if the strength falls below what is necessary to maintain slope stability, a landslide will occur
How can plants protect against erosion?
Plants protect against erosions by binding soil and regolith together.
They also shield the soil surface from raindrop impacts
How is vegetation removed?
By humans or by forest fires
How can earthquakes trigger mass wasting?
Earthquakes and aftershocks can dislodge rocks and unconsolidated materials
Liquefaction
During periods of ground shaking, water-saturated surface materials behave as fluid-like masses that flow
How are mass wasting processes classified?
By the type of material:
– Debris, mud, and earth are used if soil and regolith move
– “Rock” is used if bedrock moves
Types of Motion in Mass wasting
- Fall
- Slide
- Flow
Fall (Type of Motion)
The free fall of detached pieces
Talus Slopes
Built by rock fall
Slide (Type of Motion)
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
Flow (Type of Motion)
occurs when material moves downslope as a viscous fluid
Most are saturated with water
What are the different rates of movement for classifying mass wasting processes?
- Fast
- slow
Fast (Rate of Movement)
A rock avalanche is the most rapid type of mass wasting – Rocks float on air as they move downslope
Slow (Rate of movement)
- Creep moves particles a few millimeters per year
- A wide range of rates exists between these two extremes
What are the classifications for mass wasting processes?
- Type of Motion
- Rate of movement
Rapid forms of mass wasting
- Slump
- Rockslide
- Debris Flow
- Lahar
- Earth Flow
Slump
- Rapid form of mass wasting
– 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
- Rapid form of mass wasting
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
– Sometimes triggered by melting snow or rain
• Most common during the spring
– Sometimes triggered by earthquakes
• New Madrid, Yellowstone, Gros Ventre
Debris Flow
rapid form of mass wasting that involves the flow of soil and regolith with water (mudflow if the material is fine grained)
– 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
Lahar
Rapid form of Mass Wasting
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
Earthflow
Rapid form of mass wasting
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!
Slow forms of mass wasting
- Creeps
- solifluction
- The Sensitive Permafrost Landscape
Creep
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
Solifluction
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 permafros
The Sensitive Permafrost Lanscape
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
