ch15

Chapter 15: Preliminaries to Erosion: Weathering and

Mass Weathering

McKnight’s Physical Geography: A Landscape Appreciation,

Tenth Edition, Hess

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

2© 2011 Pearson Education, Inc.

Denudation

• Overall effect of disintegration, wearing away, and removal of rock material

• Three types of activities:– Weathering– Mass wasting– Erosion


Figure 15-1

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


Figure 15-2

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


Figure 15-3


• 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


Figure 15-4


• 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


Figure 15-6

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


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


Figure 15-7

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


Figure 15-8

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


Figure 15-14

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


Figure 15-15

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


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


Figure 15-17

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


Figure 15-19

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


Figure 15-20

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


Figure 15-23

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


Figure 15-25

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


Figure 15-30

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


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


Summary

• Creep is a slow-moving displacement of weathered material
