Weathering of Rocks and Minerals

WEATHERING OF ROCKS AND MINERALS

  • Definition: Process of disintegration and decomposition of rocks and minerals due to physical and chemical agents, resulting in the formation of regolith (unconsolidated residues).
  • Parent Material: Unconsolidated, chemically weathered mineral material from which soil develops.

A. PHYSICAL WEATHERING

  • Definition: Mechanical disintegration of rocks into smaller fragments without creating new substances.

1. Physical Condition of Rocks

  • Permeability: Coarse textured sandstones weather more quickly than fine-textured basalts.
  • Unconsolidated volcanic ash also weathers faster compared to gravels.

2. Action of Temperature

  • Rocks expand when heated in the daytime and contract at night.
  • This cycle of expansion and contraction leads to stress and fragmentation.
  • Dark colored rocks experience greater temperature variations than light colored rocks, enhancing their weathering potential.

3. Action of Water

  • Fragmentation and Transport: Water can break down rocks, transport soil materials, and influence erosion through its velocity which increases its transporting power exponentially (to the sixth power of its velocity).
  • Freezing: Water in cracks expands when it freezes, exerting pressure on the rock which leads to its breaking apart.
  • Wet-Dry Cycle: Some minerals (e.g., montmorillonite) swell when wet and shrink when dry, causing cracking and physical disintegration of rocks.

4. Action of Wind

  • Wind contributes to erosion, particularly by carrying fine materials that abrade rock surfaces, leading to their breakdown (e.g., in desert regions).

5. Atmospheric Electrical Phenomenon

  • Lightning: Lightning can fracture rocks and widen cracks due to rapid heating of surrounding materials.

B. CHEMICAL WEATHERING

  • Definition: Decomposition of rocks and minerals by chemical reactions, most critical in soil formation.
  • Chemical weathering favors increased surface area for chemical transformations.

1. Carbonation

  • Reaction of carbon dioxide with water forms carbonic acid which dissolves minerals (e.g., limestone).
    • Example:
      ext{CaCO}3 + ext{H}2 ext{CO}3 ightarrow ext{Ca(HCO}3)_2
      (Calcium carbonate to calcium bicarbonate)

2. Hydrolysis

  • Involves splitting of water into H⁺ and OH⁻ ions leading to the formation of hydroxides and new soluble substances.
  • Example:
    KAlSi3O8 + 2H2O ightarrow K^+ + Al(OH)3 + 3SiO_2
    (Orthoclase to potassium and Al(OH)₃)

3. Oxidation

  • Addition of oxygen to minerals, occurring more vigorously in moist conditions, leading to the formation of oxides and hydrated forms (e.g., iron oxidation).
    • Example:
      4FeO + O2 ightarrow 2Fe2O_3
      (Formation of hematite)

4. Reduction

  • Removal of oxygen from minerals, occurs in oxygen-depleted conditions, leading to changes within mineral structures.
  • Example:
    2Fe2O3 + 6H2O ightarrow 4FeO + 6H2O
    (Transforming hematite to ferrous oxide)

C. BIOLOGICAL WEATHERING

  • Role of Organisms:
    • Humans: Activities such as construction increase rock surfaces for weathering.
    • Animals (e.g., ants, moles): Create passages in soils, contribute to chemical weathering through excretion and bioturbation.
    • Plants: Roots penetrate rock fissures, expanding and breaking rocks. Tree roots can dissolve minerals via organic acids, enhancing decomposition.
    • Microorganisms: Help decompose minerals and organic matter releasing nutrients into the soil.
  • Example: Earthworms passing soil through their bodies enhance both physical and chemical changes.

D. IMPORTANCE OF WEATHERING

  • Essential for soil formation and nutrient availability for vegetation.
  • Influences landscape evolution and ecosystem dynamics.