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

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

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)

Weathering Processes

Weathering processes disintegrate rock into mineral particles or dissolve them into water

Two forms:

1. Physical weathering

2. Chemical weathering

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

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

Joints and fractures enhance rates of weathering

(large)

Smaller fractures throughout

Limestone bedrock, Kansas, USAPhoto: J.S. Aber, 1977

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

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

Talus slopeGlacier National Park,USA – formed due to freeze-thaw weathering)

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

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

Exfoliation

Exfoliation Dome

Half Dome,YosemiteNationalPark, USA

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

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.

Florida Sinkhole

Stalactite andStalagmite complexPhoto: Vladimir Maltsen

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

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

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

Effects of Lahar Form of earthflow

Soil Creep

Debris avalanche