Geology 200

Study Guide

Unit 6:
Mass Wasting

Overview

In the first five units of Geology 200 we examined the nature of materials that constitute the lithosphere and how these materials are formed. In this unit we begin to explore processes that occur at the Earth’s surface by looking at the products of the interactions between the lithosphere, the hydrosphere, the atmosphere, and gravity at or close to the surface of the Earth. The collective term used to describe these processes is mass wasting, which refers to the movement of rock or soil close to the surface of the Earth in a down slope direction. Mass wasting plays an important role in the development of a range of landforms, and usually occurs after the process of weathering. In essence, such landforms result from the removal of the products of weathering by mass wasting.

The unit opens with an introduction to the process of mass wasting. After this introduction, it moves on to the factors that control and those that trigger mass wasting. Common types of mass wasting include the processes of creep, flow, rockfall, and underwater slides. The unit concludes with a discussion of methods that can be used to help prevent landslides.

Objectives

After completing this unit, you should be able to

  1. define mass wasting and landslide.
  2. classify mass wasting on the basis of rate of movement of the Earth material, type of material, and nature of movement involved.
  3. outline the factors that control mass wasting, including triggering mechanisms.
  4. describe the most common types of mass wasting including creeps, flows, rockfalls, and rockslides.
  5. differentiate between types of flow including earthflows, solifluction, debris flows, mudflows, and debris avalanches.
  6. explain the character of underwater landslides and how they form.
  7. outline methods that can be used to prevent mass wasting of soils.
  8. describe methods that can be used to prevent rockfalls and rockslides on highways.

Section 1: Introduction to Mass Wasting

Slopes are ubiquitous landforms on the Earth’s surface that can range from gentle to extremely steep landforms. On one end of the spectrum, such slopes can be covered by loose soil and support significant vegetation. Other slopes comprise barren rock and rubble. The force of gravity makes earth materials move downslope. In some settings, the movement can be so gradual that it proceeds unnoticed. In other settings, however, movement may consist of rapid debris flows or rock avalanches. Mass wasting describes the downslope movement of soil, regolith, and rock. In a more constrained sense, mass wasting refers to the downslope movement of Earth materials under the influence of gravity, essentially as a coherent mass. We will view mass wasting from this perspective in this course. Mass wasting can cause so much damage as to constitute natural hazards.

The first reading in this section presents an outline of basic processes involved in mass wasting as well as some pertinent terminology. It also describes the associated damage and economic cost to humans. The second reading describes how mass wasting is classified based on rate of movement of Earth material, type of material, and the nature of movement.

Reading Assignment

Plummer, C. C., Carlson, D. H., & Hammersley, L. Physical Geology (15th ed.).

  • “Mass Wasting” (including the introduction to Chapter 9: Mass Wasting) (pp. 204-208).
  • “Classification of Mass Wasting” (pp. 210-212).

Study Questions

  1. What is mass wasting?
  2. Explain the meaning of the term landslide.
  3. What three variables can be used to classify mass wasting?
  4. Describe the term engineering soil.
  5. Explain the differences between debris, earth, and mud.
  6. How does a flow differ from a slide?
  7. How does a translational slide differ from a rotational slide?

PDF icon Answer Key

Section 2: Controlling Factors in Mass Wasting

In this section we examine factors that control mass wasting. The textbook briefly outlines the roles played by local relief, thickness of soil cover, orientations of planes of weakness, climatic factors, and vegetation. It then examines the influence of gravity on shear resistance on slopes, followed by the dual role of water in stabilizing and weakening slopes. The final part of the section outlines mechanisms that can trigger mass wasting on slopes.

Reading Assignment

Plummer, C. C., Carlson, D. H., & Hammersley, L. Physical Geology (15th ed.).

  • “Controlling Factors in Mass Wasting” (pp. 208-210).

Study Questions

  1. Explain how climate can influence the process of mass wasting.
  2. What is the shear strength of a soil, and what factors influence it?
  3. Explain how pore pressure and surface tension of water can influence the process of mass wasting, depending on the amount of water within a soil.
  4. Outline factors that can trigger a landslide.

PDF icon Answer Key

Section 3: Common Types of Mass Wasting

As noted in Section 1, mass wasting can be classified based on the rate of movement, the type of material, and the character of the movement exhibited. In this section we examine the types of mass wasting that can occur based on these three variables. We start with creep, which describes the slow downslope movement of soil on slopes. This is followed by a look at flows in which there is motion within a moving body of loose material. Flows include earthflows, debris flows, mudflows, and debris avalanches. The last class of mass wasting we will examine here includes rockfalls/rockslides, and rock avalanches. The section concludes with a look at underwater landslides.

Reading Assignment

Plummer, C. C., Carlson, D. H., & Hammersley, L. Physical Geology (15th ed.).

  • “Common Types of Mass Wasting” (pp. 212-224).
  • “Underwater Landslides” (pp. 224-226).

Study Questions

  1. Explain the process of creep, and outline the two important factors that influence creep.
  2. What conditions favour the development of earthflows?
  3. What is solifluction? How does this process relate to permafrost?
  4. Under what conditions are mudflows most likely to occur?
  5. What is a debris avalanche?
  6. How does a rockslide differ from a rock avalanche?
  7. In what settings do underwater landslides occur in oceans?

PDF icon Answer Key

Section 4: Preventing Landslides

The final section of this unit examines ways of preventing landslides. The first part of the section explores methods that can be used to avert mass wasting of soil. Because human modification of natural slopes during construction can lead to slope instability, we will discuss various forms of preventive measures that can be taken to avoid slope failure during construction. The second part of the section looks at ways to forestall rockfalls and rockslides on highway cuttings through mountainous country and at techniques used to stabilize such slopes.

Reading Assignment

Plummer, C. C., Carlson, D. H., & Hammersley, L. Physical Geology (15th ed.).

  • “Preventing Landslides” (pp. 226-228).

Study Questions

  1. Describe how construction can make a slope more susceptible to landslides.
  2. Outline preventive measures that can be taken during construction to prevent landslides.
  3. What caused the 2014 Oso landslide in Washington State?
  4. Give examples of planes of weakness on a slope. Explain how the orientation of planes of weakness can influence the stability of a slope.

PDF icon Answer Key

Unit 6 Self Test

You have now finished Unit 6, so please complete and submit the associated laboratory exercise and assignment. Instructions can be found in the Assignment Drop Boxes section of the course homepage.

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