1. McKnight's Physical Geography 11e
Lectures
Chapter 15
Preliminaries to Erosion: Weathering and Mass Wasting
Andrew Mercer
Mississippi State University
© 2014 Pearson Education, Inc.

2. Learning Goals of This Chapter
Define denudation, weathering, mass wasting, and erosion.
Explain how the processes of denudation work together to change the landscape.
Explain the influence of rock openings on weathering processes.
Identify the five types of rock openings.
Define joint, joint system, and master joint, and explain their influence on topographic development and weathering.
Define agent as it relates to weathering and erosion.

3. Learning Goals of This Chapter
Differentiate between mechanical, chemical, and biotic weathering.
Explain frost wedging, and identify its weathering category.
Explain salt wedging, and identify its weathering category.
Explain temperature changes as a type of weathering, and identify its weathering category.
Explain exfoliation, identify its weathering category, and cite examples.
Explain oxidation, and identify its weathering category.

4. Learning Goals of This Chapter
Explain hydrolysis, and identify its weathering category.
Explain carbonation, and identify its weathering category.
Explain plant wedging, and identify its weathering category.
Besides plant wedging, explain two other types of biological weathering.
Explain differential weathering.
Explain the influence of climate on weathering.
Define angle of repose, and explain the four factors that influence mass wasting.

5. Learning Goals of This Chapter
Identify the four main types of mass wasting, and differentiate between their movements.
Differentiate between rock glacier and talus cone.
Differentiate between landslide and slump.
Differentiate between earthflow and mudflow.
Differentiate between creep and solifluction.

6. Preliminaries to Erosion: Weathering and Mass Wasting
Denudation
Weathering and Rock Openings
Weathering Agents
Mass Wasting

7. Denudation
Overall effect of distintegration, wearing away, and removal of rock material
Three types of activities:
Weathering
Mass wasting
Erosion

8. Denudation Fragmentation of bedrock
Mass wasting can result in open scar on landscape; accumulation of debris
Grand Canyon is an example

9. 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

10. 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

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

12. 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

13. Weathering Agents Mechanical Weathering Frost wedging
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

14. Weathering Agents Salt wedging Temperature changes
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

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

16. Weathering Agents Chemical Weathering Oxidation
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

17. Weathering Agents Hydrolysis Carbonation
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

18. Weathering Agents Biological weathering Climate and weathering
Plants and animals alter rock structure
Impacts of lichens
Burrowing animals
Climate and weathering
High temperatures and abundant precipitation increase chemical weathering

19. 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

20. Mass Wasting
Types of mass wasting

21. 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

22. 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

23. 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

24. 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
Soilfluction – soil flowage

25. 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.
Mass wasting is the process by which weathered material is moved by gravity.

26. Summary
There are numerous factors that 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.
Creep is a slow-moving displacement of weathered material.