SOILS 0.5

• Weathering is the process where rocks and minerals are physically and chemically disintegrated and decomposed to produce the earth’s crust including soil
• Two basic processes involved in weathering:

  A. Physical weathering:

  • Rocks and minerals disintegrate into smaller rock fragments
  • Disintegration results in the decrease in size of rocks and minerals without appreciably affecting their chemical composition
  • Involves mechanical disintegration due to external agents like water, ice, wind, and plants
  • Also includes thermal weathering where disintegration results from movement or stress on the rock surface, causing the surface layer to peel away referred to as exfoliation
• Two basic processes involved in weathering:

  B. Chemical weathering:

  • A process where decomposition results in changes in chemical composition, release of soluble materials, and formation of new minerals
  • Involves hydrolysis, hydration, dissolution, carbonation, oxidation-reduction, and complexation reactions
• Factors affecting weathering of rocks and minerals:

  1. Climatic conditions:

  • Control the kind and rate of weathering that takes place given sufficient time
  • Low rainfall leads to mechanical weathering dominating, resulting in a decrease in particle size with little change in composition
  • High rainfall leads to chemical and mechanical changes, resulting in the formation of new minerals and soluble products
  • Weathering rates are generally more rapid in humid tropical regions

  2. Physical characteristics:

  • Particle size: large crystals encourage disintegration, while finer-grained materials are more resistant to mechanical breakdown
  • Hardness and Cementation: rock cemented firmly resist mechanical breakdown and present a small amount of total surface area for chemical activity
• Chemical Weathering Processes:

  • Hydrolysis: decomposition involving the addition of water (e.g., feldspar reaction)
  • Hydration: association of water molecules with other molecules (e.g., hematite reaction)
  • Dissolution: dissolving, separating, or breaking into elements by the solvent action of water (e.g., gypsum reaction)
  • Carbonation: weathering accelerated by the presence of the hydrogen ion in percolating water like carbonic acid
• Chemical Weathering Processes:

  • Oxidation-reduction: particularly manifested in rocks carrying iron, an element easily oxidized
  • Complexation reaction: soil biological processes produce organic acids forming organic complexes with Al3+ ions and providing H+ ions to solubilize Al and Si
• Factors Affecting Weathering of Rocks Minerals:

  1. Climatic conditions control the kind and rate of weathering:

  • Low rainfall leads to mechanical weathering dominating with little change in composition
  • High rainfall results in chemical and mechanical changes, leading to the rule of new minerals and soluble products
  • Weathering rates are generally more rapid in humid tropical regions
• Factors Affecting Weathering of Rocks Minerals:

  2. Physical characteristics:

  • Particle size: large crystals encourage disintegration, while finer-grained materials are more resistant
  • Hardness and Cementation: rock cemented firmly resist mechanical breakdown, porous rocks are readily broken down into small particles
• Factors Affecting Weathering of Rocks Minerals:

  3. Chemical and structural characteristics determine the ease of decomposition:

  • Minerals like gypsum, slightly soluble in water, are quickly removed with adequate rainfall
  • Packing of ions in the crystal unit influences weathering rates, with olivine and biotite being easily weathered due to less tightly packed crystal units compared to muscovite
• Weathering is the process where rocks and minerals are physically and chemically disintegrated and decomposed to produce the earth’s crust including soil
• Two basic processes involved in weathering:

  A. Physical weathering:

  • Rocks and minerals disintegrate into smaller rock fragments
  • Disintegration results in the decrease in size of rocks and minerals without appreciably affecting their chemical composition
  • Involves mechanical disintegration due to external agents like water, ice, wind, and plants
  • Also includes thermal weathering where disintegration results from movement or stress on the rock surface, causing the surface layer to peel away referred to as exfoliation
• Two basic processes involved in weathering:

  B. Chemical weathering:

  • A process where decomposition results in changes in chemical composition, release of soluble materials, and formation of new minerals
  • Involves hydrolysis, hydration, dissolution, carbonation, oxidation-reduction, and complexation reactions
• Factors affecting weathering of rocks and minerals:

  1. Climatic conditions control the kind and rate of weathering that takes place given sufficient time

  2. Physical characteristics like particle size, hardness, cementation, and chemical and structural characteristics determine the ease of decomposition