Weathering of Rocks

Weathering of Rocks

Introduction to Weathering

  • Weathering of rocks is a constant, slow process.
  • Importance of Weathering:
    • Shapes the landscape around us.
    • Creates soil necessary for plant and crop growth.
    • Slowly destroys buildings and monuments.

Types of Weathering

  • Two basic types are:
    • Physical (Mechanical) Weathering
    • Chemical Weathering
Physical (Mechanical) Weathering
  • Physical weathering breaks rocks into smaller pieces.
Processes Involved in Mechanical Weathering

  1. Frost Wedging

    • Occurs in areas with cycles of freezing and thawing of water.
    • Water expands upon freezing, which can crack rocks.
    • Water infiltrates cracks, freezes, and gradually enlarges them through repeated cycles leading to large fractures in rocks.
    • Example: A water bottle freezing causing cracking.

    Diagram of Frost Wedging Process

    • Impact on Daily Life:
      • Potholes in roads due to frozen water in cracks.
      • Salt used on roads can lead to salt crystal growth in cracks, contributing to pothole formation.

  2. Salt Crystal Growth

    • Similar to frost wedging but involves the growth of salt crystals.
    • Occurs in arid regions or near coastlines where salt spray evaporates.
    • Salt crystals expand and wedge rocks apart.
    • Example: Small salt crystals growing, causing cracks in rocks.

  3. Sheeting (Exfoliation Weathering)

    • Influences intrusive igneous rocks formed under high pressure.
    • As these rocks are exposed at the Earth's surface, pressure is released, allowing rocks to expand.
    • Leads to separation in outer parts of the rock, mimicking a peeling effect.
    • Commonly seen in granite forming large, rounded boulders.

    Example of Exfoliation Dome

  4. Biological Activity

    • A catch-all category for various forms of mechanical weathering.
    • Classic example: Roots of trees growing into rocks, prying them apart.
    • Also includes action from burrowing organisms or human activities like mining.
Chemical Weathering
  • Chemical weathering changes the minerals in rocks, leading to decomposition into new substances.
  • Water is the major agent in chemical weathering.
  • Types of Chemical Weathering:
    • Dissolution
    • Oxidation
    • Hydrolysis
Processes of Chemical Weathering

  1. Dissolution

    • Water dissolves certain minerals due to its polar nature.
    • Example: Halite and calcite can be dissolved when water is slightly acidic, such as when combined with carbon dioxide, creating weak carbonic acid.
    • This process can lead to karst landscapes characterized by limestone dissolution and the formation of caves.
    • Example: Karst landscapes in Southeast Asia due to limestone dissolution.

  2. Oxidation

    • Involves the rusting of iron-containing minerals.
    • Oxidation decomposes iron-bearing minerals like olivine and biotite into iron oxides (e.g., hematite, limonite).
    • Visual example: Weathered basalt cobble with oxidation leading to orange/red coloration.
    • Environmental Concern: Oxidation of pyrite, leading to sulfuric acid formation that can impair aquatic ecosystems and water quality.

  3. Hydrolysis

    • The process by which water breaks down silicate minerals.
    • Water is slightly acidic, which facilitates hydrolysis.
    • During hydrolysis, hydrogen replaces potassium in minerals like feldspar, forming clay (kaolinite).
    • Result: When granite is weathered, potassium feldspar converts to clay, while quartz remains intact, resulting in sandy products from weathered granite.

    Diagram of Hydrolysis Process

Conclusion

  • Interdependence of Weathering Types:
    • Mechanical weathering can increase surface area available for chemical weathering.
    • Chemical weathering can soften rocks, making them more susceptible to mechanical weathering.
    • Both weathering processes occur simultaneously in nature, influencing the rock's breakdown and landscape changes.