NS

Class 14 - Weathering

  • Geomorphology - (morph means shape or form) is a sub-discipline of geography concerned with the origin of and processes by which landforms change

  • Landforms - the product of endogenic and exogenic processes.

  • Endogenic processes - build ‘initial’ landscapes (e.g., rock formation, mountain building).

  • Exogenic processes - destroy or modify landscapes

  • Denudation - refers to all exogenic processes working to reduce elevations or relief (i.e., flatten the surface):

    1. weathering

    2. mass movement

    3. erosion

    4. transportation

    5. deposition

The Dynamic Equilibrium Framework:

  • Phase 1 - little change in elevation, endogenic and exogenic factors are roughly balanced
  • Phase 2 - rapid elevation increase where endogenic > exogenic briefly. roughly equal thereafter.
  • Phase 3 - Highly variable but lower elevations, exogenic > endogenic factors
  • Phase 4 - Stabilizing mean elevation, overall balance between endogenic and exogenic factors

Weathering:

  • Weathering - refers to processes that physically or chemically alter rock at or near the surface.
  • Physical weathering – processes that modify the physical state of rock only, no chemical change
    • frost action (freeze thaw)
    • root wedging (tree roots)
    • salt-crystal wedging
    • impact from precipitation, abrasion, and rock falls
    • pressure-release jointing/exfoliation
  • Chemical weathering - processes that alter rock chemistry, changing it from one mineral state to another, weakening the overall structure of solid rock
    • Dissolution of carbonate minerals by acidic precipitation (dissolution of limestone or the carbonate cement within sedimentary rock)
    • Hydration of minerals
    • Hydrolysis; deconstructs weak silicate minerals to form clay
    • Oxidation, rusting of iron-bearing minerals in rock
  • Karst topography - associated with limestone dissolution. Limestone is a sedimentary rock that forms in the ocean from shelled marine organisms (i.e., in the ocean).
  • Factors influencing weathering processes:
    • Rock type
    • Rock chemistry
    • Rock composition
    • Rock structure
    • Climate
    • Sub-surface water
    • Slope orientation
    • Vegetation

Erosion:

  • Erosion - refers to the translocation of weathered materials by wind, water or ice, driven by gravity and solar energy.

  • Erosion Processes - when driving forces exceed the resisting forces

  • Driving forces - climate, gravity, momentum, slope, tectonics

  • Resisting forces - friction, inertia, cohesion of particles, vegetation

  • Slopes:

    • Stable if its strength exceeds the weathering and erosion processes (e.g. gentle slope, clay soil)
    • Unstable if slope materials are weaker than the weathering and erosion processes (e.g. steep slope, sand soil)

Mass Movement:

  • Type of erosion where massive quantities of earth materials flow downslope under the force of gravity, and in some cases aided by other factors that reduce resisting forces such as cohesion and friction.

  • Classes of mass movement, sorted by rate of movement and wetness

  • Soil Creep: Slow, Dry Mass movement

    • important in cold regions

  • Rock Fall: Dry, Fast Mass movement

    • topples are a type of rock fall, fractured rock that is forward leaning

  • Rotational slump/slide: mostly dry and not too fast/slow

    • Mass has slumped down and sticks out

  • Translational slide: Slow and middle of slow/fast

  • Mudflow: wet and fast

    • higher water content, reduces friction between sediments and regolith (includes boulders and small clay particles)

  • Human-induced scarification: leads to slides

    • moves more earth today than all natural processes combined