Lecture 11 // Mass Wasting Flashcards
Mass Wasting
The downslope movement of Earth material (bedrock, regolith, sediments and soil).
Contributing Factors 1: Gravity
Has the greatest effect on steep slopes that are close to the angle of repose.
- Angle of repose = the steepest angle possible for a certain type of material (eg. loose sand ~25° and bedrock upwards of 40°).
Contributing Factors 2: Water Content
Adds weight to slopes.
- Freeze-thaw action can widen fractures over time.
- Grains touching each other have internal friction. Water gets between grains and lubricates grain to grain contacts, lessening cohesion.
Contributing Factors 3: Vegetation
The roots of vegetation acts to bind the soil, increasing cohesion.
- Clear cuts are much more prone to landslides than vegetated slopes.
Contributing Factors 4: Oversteepening
Increasing the slope angle, either naturally (eg. wave erosion of sea cliffs) or human-caused (eg. cutting into slopes to build roads).
Contributing Factors 5: Triggers
- ‘Pushes the unstable slope over the edge’.
- Examples: severe rainstorm, multiple freeze thaw cycles in spring, earthquake.
Types of Mass Wasting
- Slow
- Fast
The types of mass wasting are defined by the type of movement involved, the speed at which it is moving, and in some cases the water content..
Slow Types: Creep
The very slow downslope movement of Earth materials, simply due to gravity pulling on grains.
Slow Types: Solifluction
The downslope movement of thawed, watersaturated soil above a permafrost layer.
Slow Types: Slump
A rotational slide occurs when regolith moves downslope along one or more curved failure surface(s) at the headwall, and accumulates at the toe.
Fast Types: Mudflows, Debris Flows, Earth Flows
Flows are given different names depending on the water content, with mudflows having the greatest water content and earth flows the least. Mudflows originating from volcanic eruptions are called lahars.
Mudflow on the island of Leyte in the Philippines buried a school full of 1,126 children and their teachers under 40 meters of water saturated mud and debris.
Fast Types: Rock Fall
- Talus accumulates at the base of the slope.
Fast Types: Rock Slide
The slope fails along a plane of weakness that parallels the slope. This might be along joints, or it might be along bedding planes between different rock types. Shale layers are a common slip surface because they can hold great amounts of water that then act as a lubricant.
Fast Types: Extraordinary Large Mass Wasting Events
- The collapse of portions of volcanic islands.
- Submarine landslides – the largest mass wasting events on Earth are when portions of the Hawaiian islands slid to the bottom of the ocean.
The Sea-to-Sky Highway
Hwy. 99
Notorious for rock fall, rock slides and debris flows, especially the section between Horseshoe Bay & Squamish.
Why? •Fractured and jointed bedrock •Oversteepening •Freeze-thaw cycles common •Intense rainstorms (triggers)
