Chapter_10_Weathering Soil Erosion

Chapter 10: Weathering, Soil, and Erosion

Overview

• Focus on the processes that decompose rocks, form soil, and lead to erosion of the landscape.

Weathering and Erosion

• Weathering: the process that decomposes rocks into sediments.

• Types of Weathering:

  • Physical (Mechanical) Weathering:

    • Breaks rocks into smaller pieces without altering chemical composition.

    • Causes include:

      • Temperature changes.

      • Freeze-thaw cycles.

      • Salt crystallization.

      • Organic activity (roots).

      • Abrasion from moving particulates.

    • Results in smaller fragments like gravel and sand.

  • Chemical Weathering:

    • Alters the rock's chemical composition.

    • Influences include:

      • Water (especially acidic), oxygen, carbon dioxide, and acids.

    • Can lead to the formation of new minerals and the dissolution of existing ones.

    • Both processes can overlap and affect each other in nature.

Physical Weathering Processes

• Mechanisms of Physical Weathering:

  • Pressure Release Fracturing: rocks expand when uncovered from pressure.

  • Frost Wedging: water freezes in cracks, expanding and breaking rocks.

  • Abrasion: moving particles wear away the surface of rocks.

  • Organic Activity: biological processes contributing to soil structure and rock breakdown.

  • Thermal Expansion/Contraction: thermal cycles can create fractures in rocks.

Chemical Weathering Processes

• Dissolution:

  • Minerals dissolve in water, especially in acidic conditions (e.g., limestone dissolving in acidic rain).

• Hydrolysis:

  • Reaction with water alters minerals, forming new minerals (e.g., orthoclase feldspar altering into clay).

• Oxidation:

  • Reactions between minerals and oxygen can create rust-like compounds, leading to weathering in iron-rich rocks.

Soil Formation and Composition

• Components of Soil:

  • Mineral grains (clay, silt, sand).

  • Organic matter (litter and humus).

  • Water and gas.

  • Soil layers (horizons):

    • O Horizon: Mostly organic material.

    • A Horizon: Mixture of organic and mineral components.

    • B Horizon: Subsoil with fewer organics.

    • C Horizon: Partially weathered rock.

Soil Classification

• Classification Levels:

  • Soil Order: Highest classification based on physical and chemical characteristics.

  • Soil Series: Lowest classification based on geographic origin.

Influencing Factors on Soil Development

• Factors influencing soil characteristics:

  • Climate (temperature and precipitation).

  • Organism activity (plants, animals, and human impact).

  • Parent material (original rocks).

  • Time (duration allows for development).

Mass Wasting and Landslides

• Mass Wasting: downslope movement of material due to gravity.

• Landslide Triggers:

  • Heavy rainfall increases weight and reduces friction.

  • Earthquakes enable sudden movement.

  • Human-induced changes destabilize slopes.

Mass Wasting Types

• Types of Rapid Mass Wasting:

  • Flow: Loose material moves downslope as a slurry.

  • Creep: Slow movement facilitated by freeze-thaw cycles.

  • Earthflow and Mudflow: Viscous flows of material.

  • Slide/Slump: Large blocks sliding downhill over fractures.

  • Rockslide: Rocks break and move rapidly.

  • Fall: Material free-falls down cliffs.

Summary of Key Concepts

• Understanding weathering, soil composition, and mass wasting is essential in Earth Science as these processes influence landscapes, ecosystems, and human activity.