Weathering, Erosion, and Mass Wasting

• Weathering: Converts rock into sediment, producing soils, clays, and dissolved substances; a key process in the rock cycle.

• Erosion: Natural processes (e.g., wind, rivers) that remove sediments from one location and transport them elsewhere.

• Mass Wasting: The downslope movement of earth materials under the influence of gravity.

Lecture Outline

1. Weathering, erosion, mass wasting, and the rock cycle

2. Controls on weathering

3. Chemical weathering

4. Physical weathering

5. Soils: the residue of weathering

6. Mass wasting

7. Classification of mass movements

8. Understanding the origins of mass movements

Interaction between Weathering, Erosion, Mass Wasting, and the Rock Cycle

• Weathering is essential as it breaks down rocks into smaller particles.

• Erosion and mass wasting contribute to the transport of these particles, changing landscapes over time.

Controls on Weathering

• Factors influencing the rate of weathering include:

  • Properties of Parent Rock:

    • Different minerals weather at varying rates. Minerals have distinct solubilities in water (e.g., quartz is low)

    • The structure (e.g., fractures vs. massive) affects weathering susceptibility.

  • Climate:

    • Rainfall and Temperature: Both significantly impact weathering rates.

  • Soil:

    • Presence of soil and vegetation can promote or inhibit weathering processes.

  • Time:

    • The longer the exposure, the more weathering occurs.

Factors Controlling Rates of Weathering (Table 16.1)

Chemical Weathering

• Involves the alteration of minerals through reactions with air and water.

• Key Processes:

  • Hydrolysis: Water reacts with minerals, such as feldspar breaking down into clay.

  • Reaction with Carbon Dioxide: Carbonic acid is formed, promoting further reactions in moist soils.

Example: Disintegration of Granite (Page 7)

1. Granite is composite of multiple minerals that decay at different rates.

2. Cracks form along crystal boundaries & weaker minerals disintegrate first (feldspar, biotite, and magnetite).

3. This process ultimately leads to disintegration of granite into smaller fragments.

Role of Surface Area (Page 8)

• Small rocks, having larger surface area, weather more rapidly than larger counterparts.

Carbon Dioxide's Impact on Weathering (Page 9)

• Increased levels of atmospheric CO₂ decrease weathering rates; cooler temperatures lead to lower weathering activity.

• Conversely, weathering itself reduces atmospheric CO₂, which influences global temperatures.

• Chemical reactions include:

  • CO2 + H2O
    ightarrow H2CO3 (Formation of carbonic acid)

Chemical Weathering of Silicates (Page 10)

• Weathering of feldspar results in the formation of kaolinite clay and silica, contributing to the soil composition.

  • KAlSi3O8 + H2CO3
    ightarrow Al2Si2O(OH)4 + K^+ + HCO3^-

Stability of Common Minerals (Table 16.2)

• Outlines relative stabilities from most stable (e.g., quartz, iron oxides) to least stable (e.g., halite, calcite).

Physical Weathering

• Mechanisms Influencing Rock Breakage:

  • Natural zones of weakness, biologic activity, frost wedging, exfoliation, and weathering duration.

• Factors contributing to physical weathering include rock type, climate, temperature, and topography.

Frost Wedging (Page 19)

• Process: Water freezes in rock cracks, expands, and pushes the rock apart.

• Results in the breakup of rocks into smaller pieces.

Exfoliation (Page 20)

• Rocks expand and contract due to temperature changes, leading to flaking or peeling of the outer layers, particularly relevant in granite formations.

Soil: The Residue of Weathering

• Soils can be viewed as geological systems with inputs, outputs, and transformation processes.

• Processes of Soil Formation Include:

  • Transformations: Loss and alteration of materials.

  • Translocations: Movement of materials within the soil profile.

Basic Soil-Forming Processes (Page 25)

• Losses include: Water erosion, leaching, and removal of organic material.

• Additions include: Organic material, airborne dust, and chemicals from bedrock.

Recognized Soil Types (Table 16.3)

• Differentiates soils based on formation conditions and characteristics:

  • Alfisols: Humid climates, clay accumulation.

  • Aridisols: Dry climates, low organic matter.

  • Andisols: Formed from volcanic ash.

  • Gelisols: Presence of permafrost.

  • Histosols: Rich in organic material.

  • Other mentioned types: Inceptisols, Mollisols, Oxisols, Spodosols, Ultisols, and Vertisols.

Mass Wasting

• Defined as the movement of rock and soil down slopes due to gravitational forces.

• Occurs when gravitational forces exceed materials' internal strength.

• Key Factors Influencing Mass Wasting:

  • Nature of slope materials (angle of repose)

  • Water content

  • Steepness and overall stability of the slope

Influential Factors on Mass Movements (Table 16.4)

Classification of Mass Movements

• Based on three characteristics:

  • Nature of Material: Rock vs. unconsolidated materials

  • Velocity of Movement: Slow, moderate, fast

  • Nature of Movement: Flow, slide, or fall

Types of Movements (Page 48)

• Slow:

  • Creep, slump.

• Moderate:

  • Earthflow, debris flow.

• Fast:

  • Mudflow, landslide, rockfall, rock avalanche.

Understanding the Origins of Mass Movements

• Examples include the Gros Ventre Disaster (1925) and the Vaiont Dam Disaster (1963).

  • The Gros Ventre went through a series of geological events leading to a mass slide including unsupported sandstone layers, saturation from rainfall, and floods from debris dams.

  • The Vaiont Dam saw a rockslide that led to catastrophic flooding and loss of life, illustrating the importance of understanding geological stability before human intervention.

Thought Questions (Page 76)

• Evaluate the geologic conditions necessary to assess mass movement potential in certain terrains.

• Analyze conditions that may amplify or reduce landslide potential in various climates.