landform development dynamic equilibrium model uplift creates potential energy of position...
TRANSCRIPT
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Landform Development
Dynamic Equilibrium Model
Uplift creates potential energy of position (disequilibrium)
Sun provides heat energy
Hydrologic cycle provides kinetic energy
Atmosphere and crustal reactions provide chemical energy
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Landforms constantly adjusted toward equilibrium
1. Equilibrium Stability
2. Destabilizing Event (‘geomorphic threshold’ met)
(eg. lava flow, tectonics, heavy rainfall, forest fire,
deforestation, climate change)
3. Period of Readjustment
4. New Condition of Equilibrium Stability
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Hillslopes
Material loosened by weathering may be eroded and transported but the agents of erosion must overcome theforces of friction before downslope movement occurs
Slopes are often convexo-concave
Convex at the top (waxing slope and free face)
Concave at the bottom (debris slope and waning slopein the depositional zone)
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Weathering Processes
Weathering processes disintegrate rock into mineral particles or dissolve them into water
Two forms:
1. Physical weathering
2. Chemical weathering
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Parent Material 1. Bedrock2. Regolith3. Sediments
Soil Thickness
1. Rate of organic and mineral soil production
2. Rate of weathering and erosion
3. Rate of organic soil decomposition
4. Time
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Factors Affecting Weathering Rates1. Rock Composition and Structure
Jointing increases surface area exposed to weatheringSome rocks more soluble (eg. limestone) thanothers (eg. granite)
2. Wetness and Precipitation Promotes chemical and physical weathering
3. Temperature Promotes chemical weathering
4. Freeze-thaw cyclesVolume increase of H2O upon freezing mechanicallysplits rock, especially in humid continental, subarctic,polar and alpine environments
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Joints and fractures enhance rates of weathering
(large)
Smaller fractures throughout
Limestone bedrock, Kansas, USAPhoto: J.S. Aber, 1977
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5. Hydrology (Soil water and Groundwater)Promotes chemical weathering within the parentmaterial
6. Geographic Slope OrientationAffects exposure to sun, wind and precipitationImportant worldwide, but especially at higher latitudes
7. VegetationAcids from organic decay add to chemical weathering; shields rock and soil; roots hold soil together on steep slopes but split jointed bedrock
8. TimeEffect of the above processes increases with time
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Physical Weathering Processes
Rock is broken and disintegrated without chemical alterationSurface area susceptible to chemical weathering increases
Freeze-thaw weathering
•H2O increases in volume by 9% upon freezing•Repeated freezing and thawing breaks rocks apart•Humid continental, subarctic, polar and alpine environments
Frost wedging pushes portions of rock apart. The loosened, angular rock falls from cliffs in steep areasand accumulates downslope, forming talus slopes
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Talus slopeGlacier National Park,USA – formed due to freeze-thaw weathering)
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Crystallization
Dry weather: moisture drawn upward to rock surfaces Dissolved minerals crystallize. Crystals spread mineral grains apart (especially sandstone) Opened spaces are then open to water and/or wind erosion.
Hydration
Minerals absorb water and expand Stresses rock – grains forced apart Granular disintegration enhances chemical weathering dueto large increase in exposed surface area
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Pressure-release jointing
Overburden removed through weathering
Pressure released - heave for millions of years
Layers of rock peel off in curved slabs “pressure-release jointing”
Exfoliation (sheeting) leaves massive, arch and dome-shaped features on exposed landscapes
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Exfoliation
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Exfoliation Dome
Half Dome,YosemiteNationalPark, USA
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Chemical Weathering Processes
Chemical weathering is the decomposition of rock minerals
Minerals can: 1. Combine with oxygen or carbon dioxide in the air2. Dissolve or combine with water
Forms of Chemical Weathering:
1. HydrolysisMinerals chemically combine with water in a reaction to the mild acids in precipitation water(eg. feldspar converted to clays and silica)Disintegration etches, erodes and softens rock
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2. OxidationOxygen oxidizes metallic elements to form oxides(eg. iron oxide, Fe2O3)More susceptible to further chemical weathering
3. Carbonation and SolutionWater can dissolve 57 natural elements and many oftheir compounds – “universal solvent”
• Carbonic acid (H2CO3) in precipitation • Reacts with rock minerals containing Ca,Mg, K and Na• Minerals dissolved into H2O (eg. CaCO3)• Washed away in rainwater
Cause of karst topography and landscapes such as sinkholes, tower karst and stalagtites/stalagmites.
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Florida Sinkhole
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Stalactite andStalagmite complexPhoto: Vladimir Maltsen
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Mass Movement
Any unit movement of a body of material propelled andcontrolled by gravity. Slopes and gravitational stresses are always involved
Physical and chemical weathering weaken rock near the surface, making it susceptible to mass movement
Angle of repose: Slope achieved at equilibrium as grains flow downslopeDriving force: Gravitational forces. The greater the slope angle, the greater the likelihood of mass movement.Resisting force: Cohesiveness and internal friction
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Types of Mass Movements
1. Rockfall- rock falls through air and hits a surface- pile of irregular, broken rocks results
2. Debris avalanche (faster than landslide since water orice fluidize the debris)- rock, debris and soil
3. Landslides (translational or rotational)- sudden movement of cohesive mass of bedrock/regolith
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4. Flows (formed due to increased moisture content)
5. Creep (persistent, gradual mass movement)-very slow movement of individual soil particles due tofreezing and thawing, wetting and drying, temperaturechanges and animal disturbance
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Effects of Lahar Form of earthflow
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Soil Creep