8: Mass Wasting
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At the end of this chapter, students should be able to:
- Explain what mass wasting is and why it occurs on a slope
- Explain the basic triggers of mass-wasting events and how they occur
- Identify types of mass wasting
- Identify risk factors for mass-wasting events
- Evaluate landslides and their contributing factors
This chapter discusses the fundamental processes driving mass-wasting, types of mass wasting, examples, and lessons learned from famous mass-wasting events, how mass wasting can be predicted, and how people can be protected from this potential hazard. Mass wasting is the downhill movement of rock and soil material due to gravity. The term landslide is often used as a synonym for mass wasting, but mass wasting is a much broader term referring to all movement downslope. Geologically, a landslide is a general term for mass wasting that involves fast-moving geologic material. Loose material along with overlying soils are what typically moves during a mass-wasting event. Moving blocks of bedrock are called rock topples, rock slides, or rock falls, depending on the dominant motion of the blocks. Movements of dominantly liquid material are called flows. Movement by mass wasting can be slow or rapid. Rapid movement can be dangerous, such as during debris flows. Areas with steep topography and rapid rainfall, such as the California coast, Rocky Mountain Region, and Pacific Northwest, are particularly susceptible to hazardous mass-wasting events.
- 8.1: Slope Strength
- Mass wasting occurs when a slope fails. A slope fails when it is too steep and unstable for existing materials and conditions. Slope stability is ultimately determined by two principal factors: the slope angle and the strength of the underlying material. Force of gravity, which plays a part in mass wasting, is constant on the Earth’s surface for the most part, although small variations exist depending on the elevation and density of the underlying rock.
- 8.2: Mass-Wasting Triggers and Mitigation
- Mass-wasting events often have a trigger: something changes that cause a landslide to occur at a specific time. It could be rapid snowmelt, intense rainfall, earthquake shaking, volcanic eruption, storm waves, rapid-stream erosion, or human activities, such as grading a new road. Increased water content within the slope is the most common mass-wasting trigger. Water content can increase due to rapidly melting snow or ice or an intense rain event.
- 8.3: Landslide Classification and Identification
- Mass-wasting events are classified by type of movement and type of material, and there are several ways to classify these events. The figure and table show the terms used. In addition, mass-wasting types often share common morphological features observed on the surface, such as the head scarp—commonly seen as crescent shapes on a cliff face; hummocky or uneven surfaces; accumulations of talus—loose rocky material falling from above; and toe of the slope, which covers existing surface material.
- 8.4: Examples of Landslides
- This page contains various examples of landslides, including details such as causes, effects, and severity.
Mass wasting is a geologic term describing all downhill rock and soil movement due to gravity. Mass wasting occurs when a slope is too steep to remain stable with existing material and conditions. Loose rock and soil, called regolith, are what typically move during a mass-wasting event. Slope stability is determined by two factors: the angle of the slope and the shear strength of the accumulated materials. Mass-wasting events are triggered by changes that oversteepen slope angles and weaken slope stability, such as rapid snow melt, intense rainfall, earthquake shaking, volcanic eruption, storm waves, stream erosion, and human activities. Excessive precipitation is the most common trigger. Mass-wasting events are classified by their type of movement and material, and they share common morphological surface features. The most common types of mass-wasting events are rockfalls, slides, flows, and creep.
Mass-wasting movement ranges from slow to dangerously rapid. Areas with steep topography and rapid rainfall, such as the California coast, Rocky Mountain Region, and Pacific Northwest, are particularly susceptible to hazardous mass-wasting events. By examining examples and lessons learned from famous mass-wasting events, scientists have a better understanding of how mass-wasting occurs. This knowledge has brought them closer to predicting where and how these potentially hazardous events may occur and how people can be protected.
Thumbnail: Talus cones produced by mass wasting, north shore of Isfjord, Svalbard, Norway. Image used with permission (Public Domain; Wilson44691).