MASS WASTING: THE WORK OF 15
GRAVITY
INTRODUCTION
Mass Wasting: The Work of Gravity
CHAPTER OUTLINE
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landslide
2.
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3.
4.
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5.
LEARNING OBJECTIVES/FOCUS ON CONCEPTS
15.2 Summarize .
15.4 Distinguish
15.5 Review describe
TEACHING STRATEGIES
Clicker Questions:
Muddiest Point:
A. Slope Angles and Mass Wasting
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B. Classification of Mass-Wasting Process
TEACHER RESOURCES
Web Resources:
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Specific Locations and Mass-Wasting Hazards:
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Imagery, Animations, and Videos:
Images
Animations
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Video
ANSWERS TO QUESTIONS IN THE CHAPTER:
CONCEPT CHECKS
15.1 THE IMPORTANCE OF MASS WASTING
1. Landslides are sudden events where large quantities of rock and soil plunge down steep slopes.
2. The controlling force of mass wasting is gravity. Mass wasting, by definition, needs no
3. Rapid mass-wasting events are most likely to occur in areas of rugged, geologically young
mountains because these environments are likely to have steep, unstable slopes.
4. Streams deepen valleys through erosion and down-cutting; widening and enlarging of a valley
15.2 CONTROLS AND TRIGGERS OF MASS WASTING
1. Water can enhance mass-wasting processes and greatly increases the chances that mass-
2. The angle of repose is the steepest angle that unconsolidated materials may take before sliding
down-slope. Coarser materials have greater angles of repose; finer materials have lower
3. Wildfires influence mass wasting in several ways. One, wildfires eliminate plants that act to
anchor soils, thus promoting mass-wasting events. Two, fire can dry and loosen upper layers of
4. An earthquake and its aftershocks can dislodge enormous volumes of rock and unconsolidated
15.3 CLASSIFICATION OF MASS-WASTING PROCESSES
1. Mass-wasting movement is generally described as a fall, slide, or flow. Falls involve the free fall
of detached individual pieces of any size. Slides are mass movements in which there is a
2. The high velocities of rock avalanches result when air becomes trapped and compressed
15.4 RAPID FORMS OF MASS WASTING
1. Student drawing should be similar to Figure 15.13 and show the curved surface of rupture of a
slump as well as scarps created from the moved block of material.
2. Rockslides occur when blocks of bedrock break loose and slide down a slope along a relatively
3. The massive rockslide at Gros Ventre, Wyoming, was triggered by heavy rains and melting
snow. Here, a great mass of sandstone, shale, and soil moved along a saturated clay bed,
4. A lahar is a type of debris flow common on steep slopes of some volcanoes. Lahars are caused
when unstable layers of ash and debris become saturated with water and flow down steep
5. Earthflows are different from debris flows in that they are often more viscous, and thus move
15.5 SLOW MOVEMENTS
1. Creep occurs when soil and regolith move gradually downhill. Alternating expansion and
contraction of surface materials contributes to creep, such as freeze-thaw cycles, or wetting
2. Solifluction occurs during the summer season in the Arctic. During this brief warm season, the
active layer above permafrost thaws to a depth of about 1 meter. Water can percolate through
3. Permafrost is the permanently frozen layer of soil and sediment found in Arctic regions where
summers are too short and cool to melt deeper frozen ground. Permafrost is defined as ground
EYE ON EARTH
EOE #1 BUCKSKIN GLACIER, DENALI NATIONAL PARK
1. The material deposited by the landslide is likely poorly sorted. Landslides produce
2. We would expect these grains to be angular because they are very near their source and
have had little time for weathering and rounding.
3. From this image, it is obvious that mass-wasting processes have an important role in
EOE #2 SANTA CLARITA, CALIFORNIA WILDFIRE
2. The blaze may have ignited naturally through a lightning strike, or from human causes
such as an unattended fire or discarded cigarette.
3. Debris flows can be triggered by intense rainfall following a wildfire event; fire removes
vegetation from the slope, making it more susceptible to mass-wasting triggers. Fire can
EOE #3 SIERRA NEVADA, CALIFORNIA
2. Talus slopes form as mechanical weathering processes produce angular rock fragments
that fall to the base of a slope.
4. Chapter 6 on weathering and soil also discusses the formation of talus slopes as a
EOE #4 DURANGO, CALIFORNIA
1. This mass-wasting event was triggered by heavy rains, and appears to be a mixture of
2. If the material completely filled the stream, we would expect water to dam behind the
GIVE IT SOME THOUGHT
1. Answers will vary by geographic location. Refer to the figures in the GeoGraphics Landslide
Risk section to determine the landslide risk in your area. For example, in the Midwest, slumping
2. Mass wasting is not an agent of-erosion because it does not require a medium of transport, such
as water, ice, or wind.
3. In this image, we see that mass wasting is moving materials from the steep rocky cliffs to the
4. Earthquakes are an internal earth process that can cause or contribute to a mass-wasting event.
Earthquakes can loosen materials on slopes, making them more susceptible to downslope
5. It is unlikely that mass-wasting events occur on the Moon because the moon lacks important
6. Talus slopes generally build over time as weathering processes break down rocks. This rock
avalanche was likely a single event that occurred rapidly. We can see the scar on the slope from
7. The landslide risk map shows the potential for mass-wasting events (landslide in common terms)
for different areas of the US. Criteria to construct this map likely included the nature of surface
8. The 1959 earthquake caused a massive section of rock to move downslope in a slide, damming
9. Answer will vary by student, but generally might include:
Deforestation involves the removal of tree (biosphere) from a forested area. Rainfall
(atmosphere) in the area may promote mass wasting through slumps and slides.
Spring thaw/melting snow (hydrosphere) can saturate soils and sediments (geosphere).
10. The land beneath this rail line likely experienced solifluction over time. In summer months, the
active layer of permafrost likely thawed to some depth. As water infiltrated this thawed layer, it