1. Chapter 15: Preliminaries to
Erosion: Weathering and
Mass Weathering
McKnight’s Physical Geography:
A Landscape Appreciation,
Tenth Edition, Hess

2. Preliminaries to Erosion:
Weathering and Mass Weathering
• Denudation
• The Impact of Weathering and Mass Wasting
on the Landscape
• Weathering and Rock Openings
• Weathering Agents
• Mass Wasting
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3. Denudation
• Overall effect of
disintegration, wearing
away, and removal of
rock material
• Three types of activities:
– Weathering
– Mass wasting
– Erosion
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Figure 15-1

4. The Impact of Weathering and
Mass Wasting on the Landscape
• Fragmentation of bedrock
• Mass wasting can result
in open scar on
landscape; accumulation
of debris
• Grand Canyon is an
example
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Figure 15-2

5. Weathering and Rock Openings
• Weathering destroys
bedrock and fragments it
into smaller components
• Any exposed bedrock is
weathered
• Openings in bedrock
surface allow weathering
to transfer deeper
• Openings typically
microscopic
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Figure 15-3

6. Weathering and Rock Openings
• Five types of openings
– Microscopic: numerous, occur in
spaces between rock crystals
– Joints: cracks resulting from
stress that do not show
appreciable displacement
– Faults: breaks in bedrock with
displacement
– Lava vesicles: gas openings in
cooled lava
– Solution cavities: holes created
by percolating water
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Figure 15-4

7. Weathering and Rock Openings
• The importance of jointing
– Almost all lithospheric
bedrock is jointed
– Block separation evident
since weathering emphasizes
fracture
– Joint systems
– Large joints that extend
through large distances and
thicknesses are called master
joints
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Figure 15-6

8. Weathering Agents
• Most are atmospheric
• Oxygen, carbon dioxide, and water most important
• Temperature changes
• Water penetration in bedrock openings
• Biotic agents
– Burrowing and/or plant roots
– Chemical alteration
• Three principal categories
– Mechanical
– Chemical
– Biotic
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9. Weathering Agents
• Mechanical Weathering
– Physical disintegration of
rock without changes to its
chemical composition
• Frost wedging
– Freeze-thaw action of water
– Ice wedges downward in
openings
– Ice melts and water falls
farther into larger opening
– Process repeats
– Frost shattering
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Figure 15-7

10. Weathering Agents
• Salt wedging
– Salt left behind from
evaporated water collects
and pries apart rock
openings
• Temperature changes
– Diurnal and seasonal
temperature fluctuations
modify volumes slightly
– Fracturing of rock over long
time scales
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Figure 15-8

11. Weathering Agents
• Exfoliation
– Curved layers peel off of
bedrock
– Exfoliation dome
– Unloading through erosion
– Hydration
• Other mechanical
weathering processes
– Chemical and biotic impacts
on mechanical weathering
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Figure 15-14

12. Weathering Agents
• Chemical Weathering
– Decomposition of rock material
through chemical alteration of
minerals
– Greater surface area is
decomposed faster
– Moisture required for most
processes
• Oxidation
– Oxygen combined with metallic
elements in minerals to form new
products
– Iron oxide: rusting
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Figure 15-15

13. Weathering Agents
• Hydrolysis
– Union of water and another substance to produce a new
substance
– Igneous rock is particularly susceptible
• Carbonation
– Reaction between carbon dioxide and carbonate rocks
• Less common processes exist as well
• Chemically weathered rocks are less coherent and
have loose particles
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14. Weathering Agents
• Biological weathering
– Plants and animals alter rock
structure
– Impacts of lichens
– Burrowing animals
• Climate and weathering
– High temperatures and abundant
precipitation increase chemical
weathering
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Figure 15-17

15. Mass Wasting
• Process by which weathered
material is moved short
distances by gravity
• Factors influencing mass
wasting
– Angle of repose
– Impact of water on lubrication of
rock material
– Clay-water mixture very slick and
mobile substance
– Quick clays
– Subarctic mass wasting
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Figure 15-19

16. Mass Wasting
• Fall
– Rockfall
– Talus/Scree
– Uniform accumulation of rockfall
material: talus apron
– Material tends to collect in cone
shaped heaps: talus cones
– Talus cones grow up the
mountain
– Slow talus flow in glaciers: rock
glaciers
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Figure 15-20

17. Mass Wasting
• Slide
– Landslide, instantaneous mass
slope collapse with no fluid flow
lubrication
– Initiated from added weight from
rainfall or earthquakes
– Rock avalanches
– Lost material leaves land scar
– Damming of valley streams
– Rotation of sliding material:
slump
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Figure 15-23

18. Mass Wasting
• Flow
– Wasting initiated or enhanced by
addition of water
– Water primary force; clay can
enhance motion as well
– Earthflows: water saturated land
moved downhill
– Mudflows: originate in arid
basins; muddy downslope flows
that can accumulate large rock
material
– Debris flows
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Figure 15-25

19. Mass Wasting
• Creep
– Unobtrusive downslope flow
of soil and regolith
– Freeze/thaw and wet/dry
effects on creep
– Burrowing animals and
plant root effects
– Principle variables are
slope angle, vegetative
cover, and moisture supply
– Terracettes
– Solifluction: soil flowage
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Figure 15-30

20. Summary
• Denudation is the overall process of rock weathering,
disintegration, and mass wasting
• Weathering and mass wasting will drastically alter
landscapes
• Weathering breaks rocks into smaller fragments
• Any exposed rock is weathered
• There are five primary types of openings that are
involved in weathering
• There are numerous weathering agents, but they are
classified into three primary categories
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21. Summary
• Mass wasting is the process by which weathered
material is moved by gravity
• There are numerous factors which influence the
magnitude of mass wasting that takes place
• Falls involve the downward motion of rock material
with no added water
• Slides are instantaneous mass slope collapses, such
as landslides
• Flows involve the displacement of weathered material
by water—earthflows and mudflows
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22. Summary
• Creep is a slow-moving displacement of weathered
material
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