Chapter 15 Chapter 15

Weathering, Mass Wasting Weathering, Mass Wasting & Erosion& Erosion

Preliminaries to Erosion: Preliminaries to Erosion: Weathering and Mass WastingWeathering and Mass Wasting

WeatheringWeathering

Mass WastingMass Wasting

Breaking down of rock into Breaking down of rock into smaller components by smaller components by atmospheric and biotic agentsatmospheric and biotic agents

Involves the downslope Involves the downslope movement of broken rock movement of broken rock due to gravity due to gravity

ErosionErosionMore extensive and More extensive and distant removal of distant removal of fragmented rock material fragmented rock material

Denudation

Impacts of Weathering & Mass WastingImpacts of Weathering & Mass Wasting

WeatheringWeathering = fragmentation of = fragmentation of rock in place – no obvious movementrock in place – no obvious movement

Mass WastingMass Wasting = involves down = involves down slope movement – empty space slope movement – empty space above and debris strewn belowabove and debris strewn below

ErosionErosion = wearing away of the land = wearing away of the land by the action of water, ice or wind by the action of water, ice or wind

DenudationDenudation

Weathering followed Weathering followed by mass wasting by mass wasting followed by erosionfollowed by erosion

Results in a Results in a lowering of the lowering of the landscape over landscape over millions of yearsmillions of years

Colorado River is major cause of erosion. Weather and mass wasting have also played a part in the

carving of Grand Canyon

WeatheringWeathering

JointingJointing Weathering AgentsWeathering Agents

– Mechanical WeatheringMechanical Weathering– Chemical WeatheringChemical Weathering– Biological WeatheringBiological Weathering

Climate and WeatheringClimate and Weathering

WeatheringWeathering 11stst step in external process of rock step in external process of rock

disintegrationdisintegration Think of weathering as “aging of rock Think of weathering as “aging of rock

surfaces”surfaces” Where lithosphere and atmosphere Where lithosphere and atmosphere

meet (land/air interface)meet (land/air interface) Mechanical or chemicalMechanical or chemical Exposed bedrock is prone to Exposed bedrock is prone to

weathering – it’s more “fragile”weathering – it’s more “fragile” Underlying rock is “stronger”Underlying rock is “stronger”

Faults and JointsFaults and Joints faults = displacement of land joints = no displacement of land

displacement

no displacement

FaultsFaults

The cracking and displacement The cracking and displacement of adjacent blocks of rock as a of adjacent blocks of rock as a result of the severe stresses result of the severe stresses set up by tectonic activity set up by tectonic activity

JointingJointing VerticalVertical

HorizontalHorizontal

Cracks resulting from Cracks resulting from stress – very commonstress – very common

Show no displacementShow no displacement

Weathering AgentsWeathering Agents MechanicalMechanical ChemicalChemical BiologicalBiological

Most are atmospheric – easy for gas Most are atmospheric – easy for gas to penetrate bedrockto penetrate bedrock

Water can also penetrateWater can also penetrate Temperature changeTemperature change Biotic influenceBiotic influence

Mechanical WeatheringMechanical WeatheringDisintegration of rock materialDisintegration of rock material

No change in chemical compositionNo change in chemical composition

Mostly occurs at or near the surfaceMostly occurs at or near the surface Frost WedgingFrost Wedging = freeze/thaw action of water = freeze/thaw action of water Salt WedgingSalt Wedging = salt crystallizes = salt crystallizes Temperature changeTemperature change = heat expansion & cold = heat expansion & cold

contractioncontraction ExfoliationExfoliation = layers peel off = layers peel off

Frost WedgingFrost Wedging Water permeates cracks/fissures in Water permeates cracks/fissures in

rocks, then freezes, then meltsrocks, then freezes, then melts

ExfoliationExfoliation Curved layers Curved layers

peel off bedrock peel off bedrock Occurs at Occurs at

different scalesdifferent scales

Exfoliation - hill Exfoliation - single boulder

Impact of Mechanical WeatheringImpact of Mechanical Weathering

As rock is fragmented, the amount of rock As rock is fragmented, the amount of rock surface increasessurface increases

More surface area exposed = more More surface area exposed = more fragmentationfragmentation

Chemical WeatheringChemical Weathering

Oxidation Oxidation HydrolosisHydrolosis CarbonationCarbonation

Decomposition of rocks by alteration of Decomposition of rocks by alteration of mineralsminerals

Chemical weathering requires moistureChemical weathering requires moisture More in humid regions than aridMore in humid regions than arid Requires oxygen, water & carbon dioxideRequires oxygen, water & carbon dioxide 3 processes are:3 processes are:

OxidationOxidation

Oxidation is the Oxidation is the combination of a combination of a substance with substance with oxygenoxygen

Combines with Combines with metallic elements to metallic elements to form new form new compoundscompounds

Usually softer and Usually softer and more easily erodedmore easily eroded

Red Rocks - OxidationRed Rocks - Oxidation

Iron oxides are Iron oxides are produced produced

Iron oxides are red, Iron oxides are red, orange, or brown in orange, or brown in color color

"Georgia Red Clay" "Georgia Red Clay" derives its color from derives its color from the oxidation of iron the oxidation of iron bearing minerals bearing minerals

Oxygen combines with iron-bearing silicate Oxygen combines with iron-bearing silicate minerals causing "rusting" minerals causing "rusting"

HydrolysisHydrolysis

Decomposition of a chemical Decomposition of a chemical compound by reaction with water compound by reaction with water

Igneous rock is very susceptibleIgneous rock is very susceptible Increases volume of rock and brings Increases volume of rock and brings

on disintegrationon disintegration

CarbonationCarbonation

Reaction between carbon dioxide in Reaction between carbon dioxide in water & carbonate rockswater & carbonate rocks

Produces calcium bicarbonate (very Produces calcium bicarbonate (very soluble)soluble)

Easily removed by runoff or Easily removed by runoff or percolationpercolation

Biological WeatheringBiological Weathering Plants & AnimalsPlants & Animals Roots, lichensRoots, lichens Burrowing animalsBurrowing animals

Climate and WeatheringClimate and Weathering

Weathering is enhanced by climateWeathering is enhanced by climate More chemical weathering with high More chemical weathering with high

temperatures and lots of raintemperatures and lots of rain

Mass WastingMass Wasting

FallFall SlideSlide

– LandslideLandslide– SlumpSlump

FlowFlow– EarthflowEarthflow– MudflowMudflow– Debris FlowDebris Flow

CreepCreep

Mass WastingMass Wasting

22ndnd step in external process of rock step in external process of rock disintegrationdisintegration

Movement of rock and soil downslope Movement of rock and soil downslope under the influence of gravity under the influence of gravity

Denudation: weathering followed by Denudation: weathering followed by mass wasting followed by erosionmass wasting followed by erosion

FallFall: Falling of pieces of rock downslope: Falling of pieces of rock downslope SlideSlide: Landslide – instantaneous collapse : Landslide – instantaneous collapse

of a slopeof a slope FlowFlow: Gravity impelling force with water : Gravity impelling force with water

to bring section of a slope down i.e. to bring section of a slope down i.e. mudflowmudflow

CreepCreep: Slowest and least perceptible : Slowest and least perceptible gradual downhill movementgradual downhill movement

Talus SlopeTalus Slope

Pieces of rock become wedged in slope, then are loosened and fall to bottom

Visual Evidence of CreepVisual Evidence of Creep

Freeze Freeze ––Thaw ProcessesThaw Processes

Freezing lifts particle perpendicular, then melting deposits it further downslope