SFI 100 Lec 5 Weathering

• Weathering


  breakdown and alteration of rocks and minerals near Earth‘s surfacve into products that are more in equilibrium with the conditions found in the environment

  the breakdown of rocks to form sediments
• Importance of weathering

  1. Prerequisite to soil formation and leads to formation of three soil separates: sand, silt, clay
  2. Releases nutrients needed by plants
  3. Contribute to physical properties of soil
• Types of weathering

  1. Physical
  2. Chemical
  3. Biological
• Physical Weathering


  rock breakdown as a result of energy developed by physical forces

  also called mechanical weathering or disintegration
• Mechanisms of physical weathering

  1. Frost wedging
  2. Growth of crystals
  3. Thermal expansion and contraction
  4. Spheroidal weathering
  5. Pressure release
  6. Abrasion
• Chemical weathering


  the alteration of the chemical and mineralogical composition of weathered material

  products may be another material or a solution

  also called decomposition
• Mechanisms of chemical weathering

  1. Hydrolysis - the attack of small, highly charged ion on the crystal structure of a mineral
  2. Oxidation - element loses an electron; results to increase in positive valency and imbalance in structure
  3. Reduction - Opposite of oxidation; acceptance of electrons, decrease in positive valency; creates imbalance; occurs when material is saturated with water
• Mechanisms of chemical weathering (part 2)

  1. Carbonation - reaction of carbonate and bicarbonate ions; CO2 from atmosphere reacts with water to form carbonic acid
  2. Hydration and Dehydration - entry and exit of water; hematite+h2o to geothite, gypsum+h2o to anhydrite
  3. Solution - dissolving simple salts by water; product is liquid form
• Mechanisms of biological weathering

  1. Root action
  2. Lichens formation
  3. Chelation - production of chelates by organisms that can decompose by removing metallic ions; means “to grab” or “to bind”
• Goldich stability series


  determines the rate of weathering

  lists least stable(high temp minerals) and most stable(low temp minerals)
• Products of weathering

  • Quartz into sand grains
  • Feldspars into silicate clays (K, Ca, Na)
  • Muscovite into silicate clays (K)
  • Ferromagnesian minerals into secondary minerals (Fe, Mg, etc.)
• Biological weathering


  The disintegration and/or decomposition of rocks and minerals due to the physical and/or chemical agents of organisms