Transcript Lecture 13

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 the
forces 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 slope
in 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. Bedrock
2. Regolith
3. 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 Rates
1. Rock Composition and Structure
Jointing increases surface area exposed to weathering
Some rocks more soluble (eg. limestone) than
others (eg. granite)
2. Wetness and Precipitation
Promotes chemical and physical weathering
3. Temperature
Promotes chemical weathering
4. Freeze-thaw cycles
Volume increase of H2O upon freezing mechanically
splits rock, especially in humid continental, subarctic,
polar and alpine environments
Joints and fractures enhance rates of weathering
Smaller fractures throughout
(large)
Limestone bedrock, Kansas, USA
Photo: J.S. Aber, 1977
5.
Hydrology (Soil water and Groundwater)
Promotes chemical weathering within the parent
material
6.
Geographic Slope Orientation
Affects exposure to sun, wind and precipitation
Important worldwide, but especially at higher latitudes
7.
Vegetation
Acids from organic decay add to chemical
weathering; shields rock and soil; roots hold soil
together on steep slopes but split jointed bedrock
8.
Time
Effect of the above processes increases with time
Physical Weathering Processes
Rock is broken and disintegrated without chemical alteration
Surface 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 areas
and accumulates downslope, forming talus slopes
Talus slope
Glacier 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 due
to 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 domeshaped features on exposed landscapes
Exfoliation
Exfoliation
Dome
Half Dome,
Yosemite
National
Park, USA
Chemical Weathering Processes
Chemical weathering is the decomposition of rock minerals
Minerals can:
1.
Combine with oxygen or carbon dioxide in the air
2.
Dissolve or combine with water
Forms of Chemical Weathering:
1.
Hydrolysis
Minerals 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.
Oxidation
Oxygen oxidizes metallic elements to form oxides
(eg. iron oxide, Fe2O3)
More susceptible to further chemical weathering
3.
Carbonation and Solution
Water can dissolve 57 natural elements and many of
their compounds – “universal solvent”
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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 and
Stalagmite complex
Photo: Vladimir Maltsen
Mass Movement
Any unit movement of a body of material propelled and
controlled 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 downslope
Driving 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 or
ice 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 to
freezing and thawing, wetting and drying, temperature
changes and animal disturbance
Effects of Lahar
Form of earthflow
Soil Creep
Debris avalanche