5.1 Physical and chemical weathering processes

Rocks break down through physical and chemical weathering processes. Physical weathering splits rocks without changing their chemistry, while chemical weathering alters their composition. Water plays a crucial role in both types, causing expansion, dissolution, and chemical reactions.

Temperature fluctuations stress rocks, leading to cracking and breakage. Chemical reactions like hydrolysis, oxidation, and carbonation transform minerals. These processes shape landscapes, create soils, and contribute to the rock cycle, constantly reshaping Earth's surface.

Weathering Processes

Physical vs chemical weathering

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• Physical weathering mechanically breaks down rocks into smaller fragments without altering their chemical composition (frost wedging, exfoliation, abrasion)
  • Frost wedging occurs when water seeps into cracks, freezes, and expands, widening the cracks (ice wedging)
  • Exfoliation happens when outer rock layers peel off due to pressure release (onion-skin weathering)
  • Abrasion is the wearing down of surfaces through friction and impact from rocks and sediments (sandblasting)
• Chemical weathering alters a rock's mineral composition through chemical reactions (hydrolysis, oxidation, carbonation)
  • Hydrolysis involves water reacting with minerals to break chemical bonds (feldspar to clay)
  • Oxidation occurs when oxygen reacts with minerals to form new compounds (rusting of iron)
  • Carbonation happens when carbonic acid reacts with minerals (limestone dissolution)

Water's role in weathering

• In physical weathering, water contributes through:
  • Frost wedging, where water expands by ~9% when freezing, exerting pressure on rock (ice segregation)
  • Hydration, where absorbed water causes some minerals to swell, creating pressure (salt crystallization)
• In chemical weathering, water:
  • Acts as a reactant in hydrolysis and carbonation reactions (feldspar hydrolysis, limestone dissolution)
  • Serves as a solvent, allowing ions to move and react more easily (mineral dissolution)
  • Is slightly acidic as rainwater due to dissolved atmospheric CO2, enhancing chemical reactions (acid rain)

Temperature effects on weathering

• Thermal expansion and contraction cause rocks to expand when heated and contract when cooled
  • Repeated temperature changes stress rocks, leading to cracking and breakage (thermal shock)
• Thermal fatigue occurs when minerals in rocks have different thermal expansion rates
  • Differential expansion and contraction of minerals create stresses, weakening and breaking rocks apart (granular disintegration)
• Insolation weathering involves rapid day-night temperature changes, especially in deserts
  • Rock surfaces expand and contract, leading to cracking and flaking (exfoliation)

Chemical reactions in weathering

• Hydrolysis is the reaction between water and minerals, replacing cations with hydrogen ions (H+)
  • Example: Feldspar + Water -> Clay minerals + Dissolved cations (K+, Na+, Ca2+)
• Oxidation is the reaction between oxygen and minerals, often involving iron (Fe)
  • Example: Pyrite (FeS2) + Oxygen + Water -> Iron oxide (rust) + Sulfuric acid
• Carbonation is the reaction between carbonic acid (H2CO3) and minerals, particularly calcium-bearing ones
  1. Atmospheric CO2 dissolves in water: CO2 + H2O -> H2CO3
  2. Carbonic acid reacts with minerals: Limestone (CaCO3) + H2CO3 -> Calcium bicarbonate (Ca(HCO3)2)