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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