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Chapter 13
Weathering, Karst
Landscapes, and Mass
Movement
Geosystems 5e
An Introduction to Physical Geography
Robert W. Christopherson
Charlie Thomsen
Exogenic/Endogenic Systems
Chapter 13 begins the treatment of the exogenic system
(internal processes that produce flows of heat and material
from deep below the crust, powered by radioactive decay).
This is the solid realm of Earth. . Chapters 11 and 12
covered the endogenic system. As the landscape is
formed, a variety of exogenic processes simultaneously
operate to wear it down. The endogenic system (external
processes that set air, water (streams and waves), and ice
into motion, powered by solar energy). This is the fluid
realm of Earth's environment; it builds and creates initial
landscapes, while the exogenic system works towards low
relief, little change, and the stability of sequential
landscapes. The distinction is shown in Figure 11-5:
Exogenic/Endogenic Systems
What is weathering?
Have you noticed highways in mountainous in cold
climates that appear rough and broken? Roads that
experience freezing weather seem to pop-up in
chunks each winter? Or, maybe you saw older
marble structures, such as tombstones, etched and
dissolved by rainwater? Similar physical and
chemical weathering processes are important to the
overall reduction of the landscape and the release of
essential minerals from bedrock. A simple
examination of soil gives evidence of weathered
mineral grains from many diverse sources.
Key Learning Concepts for this Topic:
Define the science of geomorphology.
Illustrate the forces at work on materials residing on a slope.
Define weathering and explain the importance of parent rock and
joints and fractures in rock.
Describe frost action, crystallization, hydration, pressure-release
jointing, and the role of freezing water as physical weathering
processes.
Describe the susceptibility of different minerals to the chemical
weathering processes called hydrolysis, oxidation, carbonation, and
solution.
Review the processes and features associated with karst topography.
Portray the various types of mass movements and identify examples
of each in relation to moisture content and speed of movement
1. What is geomorphology and what is its
relationship with physical geography?
Geomorphology is a science that analyzes and describes the
origin, evolution, form, and spatial distribution of landforms. It is
an important aspect of the study of physical geography and the
understanding of the spatial-physical aspects of landforms.
An interesting example of geomorphology is the Grand Canyon of
Arizona: the erosional processes that reduce a landscape like the
Grand Canyon are balanced against the resistance of the materials that
make up the landscape. Weathering and erosional forces naturally
oscillate, especially in the desert, with high rainfall variability coming
in episodic thunderstorms. Each rainfall event at the Grand Canyon
operates on available slopes and cliffs. The river receives materials
and discharges its flow of water and sediment load. The variation in
rock resistance is responsible for the cliffs and slopes in the rock:
more resistant rock cliffs, less resistant rock slopes.
2. Define landmass denudation. What processes are
included in the concept?
Denudation is a general term referring to all
processes that cause reduction or rearrangement of
landforms. The principal denudation processes
affecting surface materials include weathering, mass
movement, erosion, transportation, and deposition.
3. What is the interplay between the resistance of rock
structures and weathering variabilities?
Weathering is greatly influenced by the character of the bedrock: hard
or soft, soluble or insoluble, broken or unbroken. The differing
resistance of rock, coupled with these variations in the intensity of
weathering, result in differential weathering.
Interactions between the structural elements of the land and
denudation processes are complex, and represent a constant struggle
between internal and external processes. An important question to ask
is whether or not this dynamic interplay is progressive, evolving and
building landforms in an orderly manner through stages? Do
landscapes initially form and subsequently age in graceful stages until
they are flat? Or does the interplay of forces fluctuate back and forth
across a never achieved steady-state equilibrium? The debates in
geomorphology are fueled by the fact that landscapes evolve on a
much longer time span than does a human life, or a research grant, or
thesis project, or the edition of a textbook. Modern geomorphology
has moved away from simple descriptive classifications.
4. A brief overview of the geomorphic cycle model; What was
W. M. Davis’ principal contribution to the models of
landmass denudation?
Davis theorized that a landscape goes through an initial uplift that is
accompanied by little erosion or removal of materials and then enters
a prolonged period of stability -an idea he later modified. The raised
elevation of the landscape is such that streams begin flowing more
rapidly downhill, cutting both headward (upstream) and downward.
According to this cyclic model, the landscape eventually evolves into
an old erosional surface. But Davis's theory, although it helped launch
the science of geomorphology and was innovative at the time, was too
simple and did not account for the processes being observed as
systems theory entered geomorphology. Although not generally
accepted today, his thinking about the evolution of landscapes is still
influential.
5. What are the principal considerations in the dynamic
equilibrium model?
The balancing act between tectonic uplift and reduction by
weathering and erosion, between the resistance of rocks and
the ceaseless attack of weathering and erosion, is
summarized in the dynamic equilibrium model. The
dynamic equilibrium model is a balancing act between
tectonic uplift and erosion, between the resistance of crust
materials and the work of denudation processes. Landscapes
evidence ongoing adaptation to rock structure, climate, local
relief, and elevation. A dynamic equilibrium demonstrates a
trend over time. Endogenic events (such as earthquakes and
volcanic eruptions), or exogenic events (such as heavy
rainfall or forest fire), may provide new sets of relationships
for the landscape.
6. Describe conditions on a hillslope that is right at the
geomorphic threshold.
As changing conditions provide new sets of relationships for the
landscape, the system eventually arrives at a geomorphic threshold, or
that point at which the system breaks through to a new set of
equilibrium relationships and rapidly realigns landscape materials
accordingly. Slopes, as parts of landscapes, are open systems and
seek an angle of equilibrium among the forces described here.
Conflicting forces work together on slopes to establish an optimum
compromise incline that balances these forces. A geomorphic
threshold (change point) is reached when any of the conditions in the
balance is altered. Many factors could alter the hillside's equilibrium,
such as an earthquake, or the building of a house or dam (adding
mass). All the forces on the slope then compensate by adjusting to a
new dynamic equilibrium. (See next slide).
Figure 13.2: A slope in disequilibrium
The disturbed hillslope in
Figure 13.2 is in the midst
of compensating
adjustment. The failure of
saturated slopes caused a
landslide into the river and
set a disequilibrium
condition. As a
consequence, the new dam
of material threw the
stream into disequilibrium
between its flow and
sediment load.
7. What factors might push a slope beyond its
geomorphic threshold ?
Slopes seek an angle of equilibrium among the
forces described in the text (Figure 13-3). In other
words, slopes exhibit maximum inclines that balance
conflicting forces. A geomorphic threshold (change
point) is reached if one of the conditions in the
balance alters. When this occurs all the forces on the
slope compensate by adjusting to a new dynamic
equilibrium. A slope is an open system responding to
variable inputs and producing variable outputs.
Angle of equilibrium among the
geomorphic forces (Figure 13:3)
8. What are the general components of an ideal slope?
Figure 13-3b illustrates basic slope components that vary with
conditions of rock structure and climate. Slopes generally feature an
upper waxing slope near the top. The convex surface curves
downward and grades into the free face below. The presence of a free
face indicates an outcrop of resistant rock that forms a steep scarp or
cliff.
Downslope form the free face is a debris slope, which receives rock
fragments and materials from above. The debris slope grades into a
waning slope, concave surface along the base of the slope. This
surface of erosional materials gently slopes at a continuously
decreasing angle to the valley floor.
A slope is an open system seeking an angle of equilibrium.
Conflicting forces work simultaneously on slopes to establish an
optimum compromise incline that balances these forces.
Slope Mechanics (Figure 13.3B)
The relationship between rates of weathering and breakup of slope materials, coupled
with the rates of mass movement and material erosion, shapes slopes. A slope is
stable if its strength exceeds these denudation processes and unstable if materials are
weaker than these processes.
10. Describe weathering processes operating on an open expanse of
bedrock. How does regolith develop? How is sediment derived?
Rocks at or near Earth's surface are exposed to both physical and chemical weathering
processes. Weathering encompasses a group of processes by which surface and
subsurface rock disintegrates into mineral particles or dissolves into minerals in
solution. Weathering does not transport the weathered materials; it simply generates
these raw materials for transport by the agents of wind, water, and gravity. In most
areas, the upper surface of bedrock is partially weathered to broken-up rock called
regolith. In some areas, regolith may be missing or undeveloped, thus exposing an
outcrop of unweathered bedrock. Loose surface material comes from further
weathering of regolith and from transported and deposited regolith. This
unconsolidated sediment and weathered rock forms the parent material from which soil
evolves.
When rock is broken and disintegrated without any chemical alteration, the process is
called physical weathering or mechanical weathering. By breaking up rock, physical
weathering greatly increases the surface area on which chemical weathering may
operate. Chemical weathering refers to actual decomposition and decay of the
constituent minerals in rock due to chemical alteration of those minerals. A familiar
example of chemical weathering is the eating away of cathedral facades and etchings on
tombstones caused by increasingly acid precipitation.
11. Describe the relationship between mesoscale climatic
conditions and rates of weathering activities.
Important in determining weathering rates are
climatic elements, including the amount of
precipitation, overall temperature patterns, and any
freeze-thaw cycles. Physical weathering dominates
in drier, cooler climates, whereas chemical
weathering dominates in wetter, warmer climates.
Extreme dryness reduces most weathering to a
minimum, as is experienced in desert climates
(BW). In the hot, wet-tropical and equatorial
rainforest climates (Af), most rocks weather rapidly,
and the weathering tends to be deep below the
12. What is the relation among parent rock,
parent material, regolith and soil?
Bedrock is the parent
rock from which
weathered regolith
(partially weathered
rock overlaying bedrock,
whether residual or
transported) and soils
develop. While a soil is
relatively youthful, its
parent rock is traceable
through similarities in
composition. This
unconsolidated
fragmental material,
known as sediment,
combines with weathered
rock to form the parent
material from which soil
evolves. See Figure 13-5.
13. What is physical weathering? Give an example.
Physical, or mechanical, weathering is the term used when
rock is broken and disintegrated without any chemical
alteration. By breaking up rock, physical weathering
greatly increases the surface area on which chemical
weathering can take place. An example of physical
weathering is frost action. When water freezes, its volume
expands. This creates a powerful mechanical force, which
can exceed the tensional strength of rock. Repeated
freezing and thawing of water break rocks apart. The work
of ice begins in small openings, such as existing joints and
fractures, gradually expanding until rocks are split apart.
14. Why is freezing water such an effective physical
weathering agent?
Water expands by as much as 9% of its volume as it
freezes. This expansion creates a powerful
mechanical force that can exceed the tensional
strength of rock. In a weathering action called frostwedging, ice crystals grow in preexisting cracks in
rock and push the sides apart along joints or
fractures. The work of ice probably begins in small
openings, gradually expanding until rocks are
cleaved. Softer supporting rock underneath the slabs
already has weathered physically–an example of
differential weathering.
15. What is chemical weathering? Contrast this
set of processes to physical weathering.
Chemical weathering is the actual decomposition of minerals in rock.
Chemical weathering involves reactions between air and water and
minerals in rock. Minerals may combine with water in chemical
reactions (such as carbonation), or carbon dioxide and oxygen from
the atmosphere (such as oxidation). Although physical weathering
may create greater surface area for further weathering to take place,
chemical weathering can dissolve minerals throughout the rock.
An example that demonstrates the difference between physical and
chemical weathering is the absorption of water in rocks. In cold
climates dominated by physical weathering, the process of hydration
takes place. In hydration water present in the rock expands with
freezing and cracks the rock into smaller pieces. In humid climates,
where chemical weathering occurs, water percolates into the rock, a
process called hydrolysis, and breaks down the silicate minerals in
rock. Hydrolysis dissolves silicate materials leaving behind resistant
minerals, such as quartz.
16. What is meant by the term spheriodal
weathering? How is spheroidal weathering formed?
Spheroidal weathering is an example of the way
chemical weathering attacks rock. The sharp edges
and corners of rock are rounded as the alteration of
minerals progresses through the rock. Joints in the
rock offer more surfaces of opportunity for
weathering. Water penetrates joints and fractures
and dissolves the rock's weaker minerals or
cementing materials. The resulting rounded edges
are the basis for the name spheroidal weathering.
17. What is hydrolysis? How does it affect rocks?
When minerals chemically combine with water, the
process is called hydrolysis. Hydrolysis is a
decomposition process that breaks down silicate
minerals in rocks. Water is not simply absorbed in
hydrolysis but actively participates in chemical
reactions to produce different compounds and
minerals.
18. Iron minerals in rock are susceptible to which form of
chemical weathering? What characteristic color is
associated with this type of weathering?
Iron minerals in rock are susceptible to oxidation.
Oxidation is an example of chemical weathering that
occurs when oxygen combines with certain metallic
minerals to form oxides. The rusting of iron in
rocks or soils produces a reddish-brown stain of iron
oxide. – (Example of what planet?????)
19. With what kind of minerals do carbon compounds react,
and under what circumstances does this reaction occur?
What is this weathering process called?
Carbon compounds react with carbonic acid, created
when water vapor dissolves carbon dioxide.
Carbonic acid is strong enough to react with many
minerals, especially limestone, in a process known
as carbonation. When rainwater attacks formations
of limestone, the constituent minerals are dissolved
and wash away with the mildly acidic rainwater.
20. Describe the development of limestone topography. What
is the name applied to such landscapes? From what was this
name derived?
Limestone is so abundant on Earth that many landscapes are
composed of it. These areas are quite susceptible to
chemical weathering. Such weathering creates a specific
landscape of pitted, bumpy surface topography, poor surface
drainage, and well-developed solution channels
underground. Remarkable labyrinths of underworld caverns
also may develop. These are the hallmarks of karst
topography, originally named for the Krs Plateau in the
former Yugoslavia, where these processes were first studied.
Approximately 15% of Earth's land area has some
developed karst, with outstanding examples found in
southern China, Japan, Puerto Rico, Cuba, the Yucatán of
Mexico, Kentucky, Indiana, New Mexico, and Florida.
Limestone and Karst regions: (Figure 13.14)
21. What are some of the unique erosional and depositional
features you find in a limestone cavern?
Conditions Necessary for Karst:
Limestone must contain 80% or more of
CaCO3.
Joints, cracks, pipes, and other openings
must exist for water to flow.
Must be an aerated zone between the
ground surface and the water table.
Some vegetation must occur to add organic
acids to downward percolating water.
Karst and Florida
22. Define the role of slopes in mass movements, using the terms angle
of repose, driving force, resisting force, and geomorphic threshold.
All mass movements occur on slopes. The steepness of a slope
determines where loose material comes to rest, depending on the size
and texture of the grains; this is called the angle of repose. This angle
represents a balance of driving and resisting forces.
The driving force in mass movements is gravity, working in
conjunction with the weight, size, and shape of the grains or surface
material, the degree to which the slope is oversteepened, and the
amount and form of moisture available–whether frozen or fluid. The
greater the slope angle, the more susceptible the surface material is to
mass movement. The resisting force is the shearing strength of slope
material, that is, its cohesiveness and internal friction working against
mass movement. To reduce shearing strength is to increase shearing
stress, which eventually reaches the point at which gravity overcomes
friction.
Geomorphic threshold is the threshold up to which landforms change
before lurching to a new set of relationships, with rapid realignments
of landscape materials and slopes.
23. What are the classes of mass movement (wasting)? [mass
wasting movie at the end of chapter]. What is rockfall?
Four basic classifications of mass movement are
used: fall, slide, flow, and creep. Each involves the
pull of gravity working on a mass until the critical
shearing strength is reduced to the point that the
mass falls, slides, flows, or creeps downward. A
rockfall is simply a quantity of rock that falls
through the air and hits a surface. During a rockfall,
individual pieces fall independently, and
characteristically form a pile of irregular broken
rocks called a talus cone at the base of a steep cliff.
24. Name and describe the type of mudflow
associated with a volcanic eruption.
The hot eruption of Nevado del Ruiz in central
Colombia, South America, melted about 10% of the
ice on the mountain's snowy peak, liquefying mud
and volcanic ash, and sending a hot mudflow
downslope. Such a flow is called a lahar, an
Indonesian word referring to flows of volcanic
origin. The Mount Saint Helens eruption also
produced a lahar in the Toutle River valley.
25. What is scarification, and why is it considered a type of mass
movement? Give several examples of scarification. Why are humans
a significant geomorphic agent?
Large open-pit strip mines–such as the Bingham Copper
Mine west of Salt Lake City, the Berkeley Pit in Butte,
Montana, and the extensive strip mining for coal in the East
and West, are examples of human-induced mass
movements, generally called scarification. At the Bingham
Copper Mine, a mountain literally was removed (see next
slide). The disposal of tailings and waste material is a
significant problem with such large excavations because the
tailing piles prove unstable and susceptible to further
weathering, mass wasting, or wind dispersal.
Fig. 13.28: Scarification by Strip Mining.
Movie: Mass Wasting
Mass wasting: the downslope movement of
earth under the influence of gravity. Various
factors in mass wasting are discussed,
including the rock’s effective strength and
pore spaces, as well as different types of
mass wasting such as creep, slump, and
landslides. Images of an actual landslide
illustrate the phenomenon.
End of Chapter 13
Geosystems 5e
An Introduction to Physical Geography
Robert W. Christopherson
Charlie Thomsen