weathering

Weathering, Soil, and Mass Wasting: The Earth Striving to Reach Physical and Chemical Equilibrium

A) Introduction: Le Chatelier's Law

  • Quote: "—nothing stays out of equilibrium for long."

  1. When minerals and rocks are formed miles below the Earth's surface, they are in chemical and physical equilibrium.

  2. When these "deep-formed" rocks and minerals are brought to the surface of the Earth, they are now in a new physical and chemical environment—they are out of equilibrium.

    • To reach equilibrium, they chemically and physically adjust—this process is called weathering.

B) Weathering: Mechanical (Physical) or Chemical Adjustment

  1. Mechanical Weathering: Physically breaking big pieces into small pieces while retaining the original rock's characteristics.

    • Frost Wedging: Alternate freezing and thawing of water in cracks of rocks. Water expands about 9% when it freezes.

    • Unloading ("sheeting"): Reduction of weight from mountains by erosion makes rocks rebound (bounce back), causing sheets of rock to break loose—like layers of an onion.

    • Joints: Cracks form as magma cools and crystallizes. The mass shrinks—like steam condensing into water.

    • Biological Activity: Plant roots wedge into cracks, breaking apart rocks.

  2. Chemical Weathering:

    • Occurs when chemical disequilibrium is resolved by adding or removing elements to create chemically stable substances.

    • Carbonic Acid Formation:

      CO2+H2O→H2CO3CO_2 + H_2O \rightarrow H_2CO_3CO2​+H2​O→H2​CO3​

    • Granite Weathering Reaction:

      2KAISi3O8+2H2CO3+H2O→AlSi2O5(OH)4+2KHCO3+4SiO22KAISi_3O_8 + 2H_2CO_3 + H_2O \rightarrow AlSi_2O_5(OH)_4 + 2KHCO_3 + 4SiO_22KAISi3​O8​+2H2​CO3​+H2​O→AlSi2​O5​(OH)4​+2KHCO3​+4SiO2​

      • Potassium feldspar changes into clay, releasing potassium salt (fertilizer) and silica.

    • Spheroidal Weathering: Clay minerals absorb water, increasing the crystal size. This creates outward pressure, peeling rock layers like onion skins.

  3. Factors Affecting Weathering Rates:

    • Surface Area: Larger exposed surface increases weathering.

    • Mineral Makeup:

      • Silicate minerals weather in the sequence outlined in Bowen's Reaction Series:
        Olivine → Pyroxene → Amphibole → Biotite → K-Feldspar → Muscovite → Quartz.

    • Climate:

      • Abundant rainfall produces more carbonic acid.

      • Warm climates support vegetation, producing humic acid.

C) Soil

  1. Definition:

    • Regolith: Rock and mineral fragments produced by weathering.

    • Soil = Regolith + Water + Air + Organic Matter.

  2. Soil Texture and Structure:

    • Texture: Determined by particle size (sand → silt → clay).

    • Structure Types: Platy, prismatic, blocky, and spheroidal.

  3. Controls of Soil Formation:

    • Parent Material: Residual (on bedrock) or transported (by streams).

      • Influences rate of weathering and soil fertility.

    • Time: Longer formation periods create thicker soils.

    • Climate: Temperature and precipitation determine whether mechanical or chemical weathering predominates.

    • Plants and Animals: Organic matter primarily comes from plants, forming humic acid.

    • Slope:

      • Steep slopes → poorly developed soil.

      • Flat to hilly uplands → well-drained, fertile soil.

  4. Soil Profile:

    • Horizons are layers of soil within a profile:

      • O Horizon: Organic material (Topsoil).

      • A Horizon: Mineral matter with high biological activity (Topsoil).

      • E Horizon: Light-colored; "eluviation" (leaching of nutrients).

      • B Horizon: Subsoil—clay and minerals accumulate.

      • C Horizon: Altered parent rock.

  5. Soil Types:

    • Pedalfers: Found in moderate rainfall areas, rich in aluminum and iron oxides.

    • Pedocals: Found in dry regions, rich in calcium carbonate.

    • Laterites: Tropical regions; silica removed, leaving oxides.

  6. Soil Erosion:

    • Caused by raindrops, sheet erosion, rills, gullies, and stream channels.

  7. Rates of Erosion:

    • Influenced by climate, slope, and vegetation.

  8. Weathering and Ore Deposits:

    • Bauxite Formation: Aluminum enrichment via deep leaching.

    • Acid Stripping: Converts pyrite (FeS2) into sulfuric acid, which dissolves metals, enriching ore.

D) Mass Wasting (Gravity-Driven Movement)

  1. Factors Triggering Mass Wasting:

    • Water's Role: Reduces cohesion between soil particles; adds buoyancy.

    • Over-Steepened Slopes:

      • Angle of repose: Maximum slope angle (~33° for sand).

      • Physical undercutting by streams, waves, or poor construction practices.

    • Seismic Activity: Earthquakes and vibration destabilize slopes.

    • Vegetation Removal: Reduces root anchoring and slope stability.

  2. Types of Mass Wasting:

    • Falls: Free-falling rock or debris.

    • Slides: Coherent masses move downslope.

    • Flows: Material moves as a viscous fluid.

  3. Specific Mass Movements:

    • Slump: Curved surface movement; material piles at the slope's base.

    • Rockslide: Fractures parallel to the slope slip; rain and snow reduce friction.

    • Debris Flows:

      • In semi-arid regions, saturated soil and regolith are washed downslope.

      • Lahars: Volcanic debris flows.

    • Earthflow: Saturated soil and regolith flow in humid regions.

    • Creep: Slow downhill soil movement.

    • Solifluction: Soil movement over permafrost.