Chapter 15
Weathering and Mass Wasting
OVERVIEW
This is the first chapter of Part Four. It begins the study of the matter and energy flows
that shape the surface of the Earth. This chapter examines how rock material is broken
down by weathering and how weathered material moves downhill under the influence
gravity.

A variety of weathering processes cause rock to break down into smaller particles
and to decompose chemically at the Earth’s surface.

Physical weathering is the disintegration of rock into smaller fragments of the same
mineral composition by processes such as frost action, salt-crystal growth, and
unloading.

Chemical weathering is the decomposition of rock resulting from mineral alteration
processes such as hydrolysis, oxidation, and acid solution.

Over much of the land surface, the underlying bedrock is covered by a layer of
weathered material called regolith.

Regolith is the source of sediment carried by wind, water, and glacial ice, and the
parent material for soil development.

Frost action is one of the most important physical weathering processes in cold
climates. It occurs when water freezes in joints in the rock, and the expansion of the
water during repeated freezing forces the joints to enlarge.

Salt crystal weathering operates extensively in dry climates and is the result of the
growth of salt crystals in rock pores. Groundwater moves to the surface through
capillary action and evaporates, leaving the salts behind producing grain by grain
breakup of sandstone.

Unloading is a form of physical weathering that occurs when the removal of
overlying layers causes the rock to expand, cracking in layers parallel to the surface
that break away from the rock in sheets.

Physical weathering can also occur when rocks are subject to intense heating and
cooling and through the growth of plant roots that can wedge rocks apart.

The chemical weathering processes of hydrolosis and oxidation change rock
minerals into clay minerals and oxides.

Acid action is most commonly due to the work of weak solutions of carbonic acid
dissolving certain rocks, particularly limestone and marble.

Mass wasting is the spontaneous downhill movement of soil, regolith, and rock on
slopes under the influence of gravity.

Regolith and soil are more susceptible to mass wasting than bedrock.

Soil creep is the gradual downhill movement of particles as they are rearranged by
wetting and drying, freezing, and thawing and other processes.

Water-saturated regolith can move quickly down a slope in an Earthflow.

Mudflows and debris floods can develop when intense rains fall on exposed soil
surfaces.

A large mass of bedrock which breaks free from a slope can slide rapidly downhill as
a landslide. The rock mass usually disintegrates as it moves.

Earthflows, mudslides, and landslides can be induced both by natural processes and
by human activities. They can have a major impact on the environment and present a
serious hazard to humans. Human activities that extract mineral resources involve
scarification of the land surface.

The periglacial system refers to the distinctive landforms and geomorphological
processes of arctic and alpine tundra environments which have a strong annual
temperature cycle and extremely cold winters.

Much of the tundra environment has a layer of permafrost (perennially frozen
ground) beneath a surface active layer (seasonally thawed ground). Permafrost is
classified as continuous, discontinuous, sub-sea, and alpine. Continuous permafrost
may reach up to 450 meters in depth.

Ground ice in permafrost can occur as ice wedges and pingos.

Intense frost action can generate patterned ground features such as ice-wedge
polygons and stone polygons.

Summer thawing of the active layer can produce saturated soils that flow downhill to
form solifluction lobes and terraces.

Human activity which disturbs the surface cover of the permafrost may lead to
thermal erosion, in which the permafrost melts to greater depths leading to the
development of depressions and ground subsidence known as thermokarst.
KEY TERMS
weathering
mass wasting
bedrock
chemical
weathering
physical weathering
sediment
alluvium
soil creep
Earthflow
mudflow
landslide
scarification
periglacial
Permafrost
ground ice
ice wedge
patterned ground
solifluction
thermokarst
STUDY QUESTIONS
1. What is weathering? What is the distinction between physical and chemical
weathering?
2. What are regolith, bedrock, and sediment? How are they related to one another?
3. Sketch and label a cross-section of a hill slope to show the following features:
bedrock, outcrop, regolith, and soil.
4. What is frost action, and how does it break rock into smaller fragments? Give two
examples of landforms produced by frost action.
5. What is salt-crystal growth, and why, under natural conditions, is it most effective in
dry climates?
6. What is an exfoliation dome and how is it produced?
7. Limestone outcrops often show surface forms produced by chemical weathering.
What chemical weathering process produces these forms, and why is limestone
especially vulnerable to this process?
8. Define mass wasting, and discuss why regolith is more susceptible than bedrock to
this process.
9. Soil creep occurs when a soil particle repeatedly lifts perpendicular to a slope and
settles vertically under the influence of gravity. Sketch a diagram to show how this
can move a soil particle down a slope. What mechanisms might cause lifting of the
soil particle?
10. Compare Earthflows and mudflows in terms of their behavior, causes, and potential
hazard to humans.
11. What are landslides, and where do they usually occur?
12. What are some of the most common ways in which human activity causes mass
wasting?
13. What is the periglacial system?
14. Sketch a diagram to show how the change of ground temperature with depth varies
between summer and winter in permafrost. Label the active layer and permafrost
layer on your diagram.
15. Describe the formation of ice-wedge polygons.
16. How do solifluction lobes develop?
CHAPTER QUIZ
Multiple Choice Questions
1. Which of the following refers to bedrock exposed at the land surface?
a) regolith
b) alluvium
c) outcrop
d) sediment
2. Which of the following is a form of physical weathering?
a) salt-crystal growth
b) acid solution
c) both a and b
d) neither a nor b
3. Steplike slump features are commonly associated with:
a) soil creep
b) earthflows
c) mudflows
d) landslides
4. Talik is a(n):
a) pocket of unfrozen ground
b) type of patterned ground
c) form of ground ice
d) ice-cored mound
5. Most solifluction activity occurs:
a) below the permafrost table
b) within the active layer
c) when the soil freezes in winter
d) all of the above
True/False Questions
1.
2.
3.
4.
All permafrost contains ground ice. (T/F)
Mudflows usually move faster than Earthflows. (T/F)
Industrial activity can increase regional chemical weathering rates. (T/F)
A physical weathering process strongly associated with arid environments is salt
crystal weathering. (T/F)
5. Accumulations of sediment deposited by streams, waves, currents and wind are called
residual regolith. (T/F)
Short Answer Questions
1. What conditions are necessary for frost action to be an effective weathering process?
2. Why are landslides often able to move long distances at a high velocity?
3. What is discontinuous permafrost?
Short Essay Questions (1 - 2 paragraphs)
1. Discuss the roles of weathering and mass wasting in converting solid rock into river
sediment.
2. What are some of the more important ways in which human activity can enhance or
induce mass wasting?
Internet Resources
1. Digital materials and virtual weathering:
<http://www.sciam.com/2000/0200issue/0200dorsey.html>
2. Photographs of lateritic weathering: <http://mindepos.bg.tuberlin.de/eurolat/lateri/welcome.html#1>
3. Weathering images: <http://www.geo.duke.edu/geo41/wea1.htm>;
<http://www.geo.duke.edu/geo41/wea2.htm>
4. Mass wasting images: <http://www.geo.duke.edu/geo41/mmo.htm>;
<http://www.geo.duke.edu/geo41/mmo2.htm>
5. An overview of weathering with links to images:
<http://enterprise.cc.uakron.edu/geology/natscigeo/Lectures/weath/weath.htm>
6. Images of mass wasting features in North Dakota:
<http://www.ndsu.nodak.edu/nd_geology/nd_mwast/index_mw.htm>
7. The US Geological Survey National Landslide Hazards Program:
<http://landslides.usgs.gov/index.html>
8. Photographs of Italian mudslides and landslides:
<http://www.geocities.com/RainForest/8211/>
9. Mass movement causes and consequences: <http://www.aber.ac.uk/iges/ctig/hazards2000/massmovement/>
10. Images of mass wasting features: <http://www.science.ubc.ca/~geol351/imgset.html>