Weathering Processes: Physical, Chemical & Biological

• Weathering is the disintegration and decomposition of rocks and minerals by physical, chemical, and biological agents, leading to the formation of regolith.
• Regolith refers to unconsolidated residues of weathered rock on or above solid rocks.
• Weathering is the process where the Earth's crust or lithosphere is broken down via atmospheric, hydrospheric, and biospheric activities, transforming solid rocks into parent material or regolith.
• Parent material is the regolith or its upper portion, defined as the unconsolidated mineral material from which soil develops, weathered chemically to varying degrees.
• Physical or mechanical weathering involves disintegration.
• Chemical weathering involves the decomposition process.
• Biological weathering works in conjunction with physical and chemical processes.

Agents of Weathering

• Weathering is classified into three types based on agents involved.
• Physical/Mechanical Weathering: Involves the physical condition of rock, temperature changes, water action, wind action, and atmospheric electrical phenomena.
• Chemical Weathering: Includes hydration, hydrolysis, solution, carbonation, oxidation, and reduction.
• Biological Weathering: Encompasses actions by humans, animals, plants, their roots, and microorganisms.

Physical Weathering

• Rocks are broken down into smaller pieces without any new substances being formed.
• Permeability is the single most important factor with coarse textured sandstone weathering more readily than fine textured basalt.
• Unconsolidated volcanic ash weathers faster than coarse deposits like gravels.
• Temperature variations cause rocks to expand when heated by the sun and contract when cooled at night.
• Alternate expansion and contraction weakens the rock surface, causing it to crumble due to poor heat conduction.
• Minerals within rocks expand and contract at different rates which causes stress, resulting in fragmentation and exfoliation (peeling off of surface layers).
• Water acts as a disintegrating, transporting, and depositing agent and flowing water is a cutting and carrying force, forming gullies and ravines.
• Transporting power of stream varies as the sixth power of its velocity meaning greater speed equals more transporting power and carrying capacity.
• 15 cm/sec water speed carries fine sand.
• 30 cm carries gravel.
• 1.2 m carries stones (1kg).
• 9.0 m carries boulders (several tons).
• Disintegration is more intense near a river's source.

Additional Physical Weathering Processes

• Action of Freezing: In cold regions, water freezes in cracks, expanding by about one-tenth breaks rocks apart.
• Alternate Wetting and Drying: Substances like smectite and montmorillonite expand when wet and shrink when dry, leading to cracks and eventual breakage.
• Action of Glaciers: Glaciers in cold regions accumulate snow that changes to ice sheets.
• Glaciers exert enormous pressure, carrying loose materials, depositing them in warmer areas upon melting.
• Action of Wind: Wind erodes and transports material, causing abrasion that breaks down rocks.
• Atmospheric Electrical Phenomenon: Rainy season lightning breaks or widens cracks in rocks.

Chemical Weathering

• This is the decomposition of rocks and minerals by various chemical processes and is vital for soil formation.
• Chemical weathering occurs on the surface of rocks and minerals, altering them and forming secondary products through chemical transformation.
• Chemical weathering is more efficient as the surface area of the rock increases.
• Quartz is more resistant to chemical attack than minerals like olivine or pyroxene.

Average Mineralogical Composition

• Granite contains 52.4% Feldspar, 31.3% Quartz.
• Basalt contains 46.2% Feldspar, 44.5% Pyrox-amphi.
• Shale contains Feldspar 30.0%, Quartz 2.3%.
• S. Stone contains 11.5% Feldspar, and 66.8% Quartz.
• L. Stone contains 76% Carbonates.

Chemical Processes

• Chemical combination of water molecules with a substance or mineral causing a change in structure.
• Minerals in rocks hydrate when exposed to humid conditions, causing swelling, increased volume, loss of luster, and softening.
• Example Hydration reactions include:
  • 2Fe₂O₃ + 3H₂O → 2Fe₂O₃.3H₂O (Haematite to Limonite)
  • Al₂O₃ + 3HOH → Al₂O₃.3H₂O (Bauxite to Hyd. aluminium Oxide)
  • CaSO₄ + 2H₂O → CaSO₄.2H₂O (Anhydrite to Gypsum)
  • 3(MgO.FeO.SiO₂) + 2H₂O → 3MgO.2SiO₂.2H₂O + SiO₂ + 3H₂O (Olivine to Serpentine)
• Hydrolysis: Most important chemical weathering process caused by the dissociation of H₂O into H⁺ and OH⁻ ions
• These ions combine with minerals, leading to exchange, decomposition of crystalline structure, and formation of new compounds.
• KAISI₃O₈ + H₂O → HAISI₃O₈ + KOH (Orthoclase to Acid silt clay).
• HAlSi₃O₈ + 8 HOH → Al₂O₃.3H₂O + 6 H₂SiO₃ (recombination)
• Clay, bases, and silicic acid from the reactions are available to plants.
• Water containing CO₂ (absorbed from atmosphere), reacts with minerals to produce insoluble clay minerals, positively charged metal ions (Ca,Mg,Na,K) and negatively charged ions (OH, HCO₃)

Solution and Carbonation

• Some rock substances are directly soluble in water, which removes them, leading to holes, rills, and eventual decomposition.
• Action increases with acidified water from organic and inorganic acids.
• Halites, like NaCl dissolve in water: NaCl + H₂O → NaCl, H₂O (dissolved ions with water).
• Carbonation: CO₂ dissolves in water to form carbonic acid: 2H₂O + CO₂ → CO₃.

Impact of Carbonic Acid

• Carbonic acid attacks and dissolves many rocks and minerals, especially limestone, leading to disintegration by removing cement holding sand particles.
• Calcite + H₂CO₃ → Ca(HCO₃)₂ : CaCO₃+H₂CO₃(Calcite) to Ca(HCO₃)₂(Ca bi carbonate)
• Oxidation: When oxygen is added or combined with minerals, usually from O₂ in soil water and the atmosphere absorption.
• Increases with moisture and forms hydrated oxides such as:
  • 4FeO + O₂→ 2Fe₂O₃ (Ferrous oxide to Ferric oxide).
  • 4Fe₃O₄ + O₂→ 6Fe₂O₃ (Magnetite to Haematite).
  • 2Fe₂O₃ + 3H₂O → 2Fe₂O₃.3H₂O(Haematite to Limonite).
• Reduction: Removal of oxygen or reverse of oxidation.
• Changes soil color to grey, blue, or green as ferric iron converts to ferrous iron compounds in waterlogged conditions.
• 2Fe₂O₃ - O₂ → FeO (Haematite to Ferrous oxide reduced).
• Chemical weathering of igneous and metamorphic rocks involves primary mineral destruction and secondary mineral production.
• In sedimentary rocks, it destroys weak bonding agents.

Biological Weathering

• Biological agents decompose and disintegrate rocks/minerals.
• Man and Animals: Actions include cutting rocks for construction, accelerating rock decomposition.
• Making holes aids weathering.
• Ants/termites build galleries, excrete acids, and speed disintegration.
• Rabbits destroy soft rocks, and moles contribute decaying matter.
• Earthworms cause physical and chemical changes.
• Higher Plants and Roots: Root growth exerts force that breaks rocks apart.
• Grass roots prevent erosion and conserve moisture.
• Some roots form drainage channels.
• Roots produce acids and CO₂, which impacts weathering.
• Micro-organisms: Mosses, bacteria, fungi, and actinomycetes play a role in mineral decomposition and soil formation and liberate nutrients for plants.

Explore physical weathering's disintegration by agents like temperature, and chemical weathering processes such as hydration and hydrolysis. Understand biological weathering and the formation of regolith from parent material. Learn weathering's impact on permeable vs. non-permeable rock.