GEOG101: Physical Geography

Lecture 16: Monday, March 23, 2009

Topics
1. Landmass Denudation
2. Weathering Processes
3. Karst Topography and Landscapes
4. Mass Movement Processes

Landmass Denudation
• Geomorphology: The science of landforms, including their origin, evolution, form, and spatial distribution
• Denudation: Processes that wear away or rearrange landforms
• Major Denudation Processes: weathering, mass movement, erosion, transportation, and deposition
• Differential weathering: Different weathering rates of surface rocks due to the differing resistance of rocks and the variation of processes at work on the rock
• Dynamic Equilibrium View of Landforms: (1) equilibrium stability (fluctuating around some average) --> (2) a destabilizing event --> (3) a period of adjustment --> (4) development of a new and different condition of equilibrium stability;
• Controlling factors for slope stability:
      Slope angle (gravity)
      Forces of friction
      Inertia (the tendency of objects at rest to remain at rest)
      Cohesion of particles to each other
• Regolith, Soil, and Parent Materials
      Regolith: broken-up rocks from bedrock
      Bedrock: the parent rock from which the regolith and soils develop

Weathering Processes
• Factors Influencing Weathering Processes
      Characteristics of the bed rocks: hard or soft, soluable or insoluble, broken or unbroken
      Joints: fractures or separations in rock ? increase the surface area of rock exposed to physical and chemical weathering
      Climatic elements: precipitation, temperature, and freeze-thaw cycles
            Dry, cold climate --> physical weathering dominated
            Wet and warm climate --> chemical weathering
      Geographic orientation: face orientation controls the slope's exposure to Sun, wind, and precipitation
      Vegetation
      Ground water and water movement

• Physical Weathering (Mechanic Weathering)
      Frost Action (freeze-thaw action)
            When water freezes, its volume expands ~9%. Expansion creates mechanic forces.
      Crystallization
            Dissolved minerals in the water evaporates and grow crystals (e.g., salt), breaking the rocks
            Mostly in Arid or semiarid environments
      Pressure-Release Jointing
            Erosion and transportation reduce the weight of in some areas --> isostatic uplift;
            Creating fractures and joints amenable for furthering weathering
            Sheeting -- Exfoliation domes

• Chemical Weathering :
      Hydration and Hydrolysis:
            Hydration--combination with water, involves little chemical change;

e.g., CaSO4 (gypsum) --> CaSO4.2H2O;
Hydration -> expansion and dehydration -> condensation;
            Hydrolysis--Minerals chemically react with water:
Feldspar (K, Al, Si, O) + carbonic acid + water --> residual clays + dissolved minerals + silica

      Oxidation: Metalic elements combine with oxygen to form oxides;
e.g., Iron (Fe) + Oxygen (O2) -> iron oxide (hematite; Fe2O3)
       Carbonation and Solution: Minerals dissolve into a solution;
Water dissolves CO2 --> forming carbonic acid (H2CO3);
Calcium carbonate + carbonic acid and water -> calcium biocarbonate (Ca2+ + HCO3-)
• Spheroidal Weathering: a descriptive term for a type of chemical weathering:
      Sharp edges and corners of rocks are rounded because these locations offer more surface for more weathering

Karst Topography and Landscapes
• Formation of Karst
      The Limestone formation contains > 80% calcium carbonates
      Complex patterns of joints for conducting water through subsurface drainage channels
      An air-containing zone between ground surface and water table
      Vegetation cover to supply organic acids
• Lands Covered with Sinkholes:
      Sinkholes: Circular depressions created by dissolution of carbonate
      Collapsed sinkholes and karst valley
• Caves and Caverns
      Subsurface dissolution
      Stalactite and stalagmites

Mass Movement Processes
• Mass Movement (mass wasting) Mechanics
      All mass movements occur on slopes due to gravitational stress
      Angle of repose: The angle of slope at a balance of the driving forces (gravity) and resisting force (friction and shearing)
• Classes of Mass Movements
      Falls and Avalanches (rockfall, talus slope, talus cones; debris avalanches): induced by heavy rains, volcanic eruption, earthquakes
      Landslides: A large amount of material fails simultaneously
      Flows: mud and debris saturated with water move a large amount of material, especially from barren mountain slopes during heavy rains
      Soil creep: A persistent, gradual mass movement of surface soil
• Human-Induced Mass Movements (Scarification)
      Reasons: highway roadcut, surface mining, building of shopping mall, high buildings, housing development



Please report any problems of this website to Dr. Jiang