Chapter 3

• Weathering


  The breakdown of rocks at the Earth's surface, by the action of rainwater, extremes of temperature, and biological activity
• Weathering does not involve the removal of rock material
• The products of weathering are a major source of sediments for erosion and deposition
• Residue of weathering


  Chemically altered and unaltered materials
• Types of weathering


  • Physical
  • Chemical
  • Biological
• Physical weathering


  • Caused by the effects of changing temperature on rocks, causing the rock to break apart
  • Sometimes assisted by water
• Types of physical weathering


  • Freeze-thaw
  • Exfoliation
• Freeze-thaw


  Water continually seeps into cracks, freezes and expands, eventually breaking the rock apart
• Exfoliation


  Cracks develop parallel to the land surface as a consequence of the reduction in pressure during uplift and erosion
• Chemical weathering


  Rain water reacting with the mineral grains in rocks to form new minerals (clays) and soluble salts
• Chemical weathering processes


  • Hydrolysis
  • Oxidation
  • Reduction
  • Hydration
  • Carbonation
  • Solution
• Hydrolysis


  Weathering reaction that occurs when the two surfaces of water and compound meet
• Oxidation


  Reaction that occurs between compounds and oxygen, resulting in the removal of one or more electrons from a compound
• Hydration


  Reverse of oxidation, caused by the addition of one or more electrons producing a more stable compound
• Carbonation


  Reaction of carbonate and bicarbonate ions with minerals, usually taking place as a result of other chemical processes
• Biological weathering


  Disintegration of rock and mineral due to the chemical and/or physical agents of an organism
• Biological weathering


  • Trees putting down roots through joints or cracks in the rock
  • Piddock shells boring into rocks for protection
  • Bacteria, algae and lichens producing chemicals to break down the rock
• Soil forming factors


  • Parent Material
  • Climate
  • Organisms/Biological factors
  • Topography
  • Time

  
• Jenny's state factor equation for soil genesis


  S = f (C, O, R, P, T . . .)
• Components of Jenny's state factor equation

  • C = Climate
  • O = Organisms
  • R = Relief (topography)
  • P = Parent Material
  • T = Time
• Pedogenic processes are the physical, chemical or biological processes taking place in the soil as a result of the soil forming factors
• Soil forming processes


  • Additions
  • Losses
  • Transformations
  • Translocations

  
• Additions


  Materials added to the soil, such as decomposing vegetation and organisms (organic matter), or new mineral materials deposited by wind or water
• Losses


  Through the movement of wind or water, or uptake by plants, soil particles (sand, silt, clay, and organic matter) or chemical compounds can be eroded, leached, or harvested from the soil
• Transformations


  The chemical weathering of sand and formation of clay minerals, transformation of coarse organic matter into decay resistant organic compounds (humus)
• Translocations


  Movement of soil constituents (organic or mineral) within the profile and/or between horizons

  
• Mechanisms of soil formation

  • Accumulation of materials
  • Leaching and losses
  • Transformation
  • Translocation
  • Decomposition
  • Humification
  • Eluviation and illuviation

  
• Accumulation of materials


  Materials are added to the soil such as organic matter and decomposing materials or new mineral materials deposited by the forces of ice, water or wind and they accumulate over time
• Leaching and losses


  Removal of soluble components of the soil column as water washes down through the soil, carrying away bases and causing acidification
• Accumulation of organic materials

  1. Water-logging prevents oxidation or breakdown by soil organisms

  2. Held up by root systems in well-drained soils

  3. Depositions by wind, water or ice
• Nitrogen fixation


  Some plants with the help of bacteria fix atmospheric nitrogen and ammonia compounds into the soil as nitrates
• Leaching and losses


  1. Removal of soluble components of the soil column

  2. Washing away of bases like calcium, acidification through hydrogen ions

  3. Movement of water, wind, ice or uptake by plants removes accumulated materials like clay, organic matter, silt or other compounds
• Transformation and illuviation


  1. Transformation - chemical weathering of silt, sand, formation of clay minerals, change of organic materials

  2. Illuviation - clay and other accumulated materials washed from upper horizons and deposited in lower horizons

  3. Plants and animals physically and chemically break down materials, improving drainage and nutrient composition
• Illuviation


  Soil particles held in suspension, such as clay, are accumulated (eg. deposited)
• Eluviation


  Soil particles held in suspension, such as clay, are removed (eg. washed away)
• Podsolization


  Strongly acid soil solutions cause breakdown of clay minerals, silica, aluminum and iron form complexes with organic substances, removed from surface zone and accumulated in sub-surface layers
• Laterization


  High temperatures and heavy precipitation result in rapid weathering of rocks and minerals, eluviation and leaching occur
• Calcification


  Evapotranspiration exceeds precipitation, causing upward movement of dissolved alkaline salts from groundwater, then downward movement of salts resulting in deposition in B horizon, can form caliche
• Gleying


  In waterlogged, anaerobic conditions, iron compounds are reduced and either removed from soil or segregated as mottles/concretions
• Typical weathering products of common minerals

  • Quartz as sand grains
  • Clay minerals plus potassium, sodium, calcium
  • Chlorite plus iron and magnesium
  • Serpentine plus iron and magnesium
  • Calcium and carbonate
  • Iron oxide minerals plus iron and sulphuric acid