3.3. Slope Processes Flashcards
Mass movement
describe all the down slope movement of weathered material (regolith) due to the force of gravity.
Regoliith
Weathered material
Shear Stress
a force that causes layers or parts to slide upon each other in opposite directions
Shear Strength
a material’s ability to resist forces that can cause the internal structure of the material to slide
How Shear Stress and Strength relate to mass movement
Mass movements occur when the force of gravity (meaning the shear stress) exceeds the shear strength (meaning friction) of the slope materials.
Classification of mass movements
1) Type of material soil / regolith or rock / bedrock 2) Type of motion Fall - free-fall of material Slide - material slides as one mass along flat surface Flow - material flows as a fluid 3) Speed of movement Fastest - rock avalanche: floats (flows) on trapped air Slowest - creep: inches per year (or less)
Slowest to fastest mass movements (increasing in speed going downwards)
- Soil creep
- Solifluction
- Debris flow
- Mud flow
- Rock slides
- Slumps
- Rock fall
- Rock avalanche
Flows
involve the internal movement of soil and rocks down slope.
Characteristics of flows
- Flows occur under wet conditions than slides.
- Flows range in speed from very slow to fast.
- They occur on relatively gentle slopes. Slope gradients are commonly from 10% to as steep as 30%.
- Flows involve a lot of mixing of soil and rock particles within the moving mass.
- Flows usually consist of unconsolidated material.
Soil Creep
very slow downhill movement of material.
Cause of Soil Creep
caused by heaving - alternate expansion/contraction of material from freezing/thawing or wetting/drying
Heave
- Heave occurs when soil expands and contracts due to freezing and thawing, or wetting and drying.
- When soils become saturated or frozen, they expand, pushing rocks and boulders upwards perpendicular to the ground surface.
- When the soil dries or thaws, the boulders move down vertically resulting in a net down slope movement.
- forms terracettes
Solifluction
slightly quicker type of flow than creep, although it is still a slow flow process
Characteristics and causes of Solifluction
- It happens at a rate of 5cm-1metres per year
- This type of slow flow occurs in wetter soil than creep and usually occurs in areas of permafrost. Permafrost refers to the layer of groundwater that fills in the pore spaces of soil and rock that is permanently frozen.
- In times of warm weather, the ground will begin to thaw from the surface downward. All of the freshly melted water cannot absorb into or move through the permafrost layer. This causes the upper layer of soil and regolith to become saturated and flow down the slightest of slopes as it slips over the frozen ground underneath.
- “Solifluction Lobes” are a sign of this happening
Debris Flows
made up of loose rocks, mud and water. Debris flows also transport large objects such as trees.