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

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.

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.

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.

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.

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

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

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

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

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

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

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 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

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

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

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

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

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

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

Mass Wasting Types of mass wasting

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

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

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

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

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.

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.