Weathering Figures

Weathering and Sedimentary Rocks

• Weathering (Ch. 6)
• Chemical
• Mechanical (Physical)

• Soils (Ch. 6)
• Sedimentary Rocks (Ch. 7)

• Weathering - Process of breaking down and alteration of materials at the earth’s surface
• Types of Weathering
• Mechanical (or Physical) Weathering - materials broken down into smaller fragments. Ex:  qtz in granite -> qtz sand
• Chemical Weathering - Chemical alteration or dissolution. Ex: feldspar -> clay

• Erosion
• Weathering plus transport
• Transporting Agents
• Water - bedload, suspended load, dissolved load
• Wind
• Glaciers

• Why this distribution of sedimenary minerals?

• Mineral/Rock type
• Climate
• chemical weathering more rapid in warm, humid climate
• water important for weathering reactions - ion highway
• Soil cover
• soil: surface accumulation; rock and mineral matter, organic matter (humus), water, and air
• stores water, biologically active, acidic

• Water a key player -
• serves as chemical highway
• ions in and out; O₂ in
• universal solvent
• more effective if acidic
• What is the pH of water exposed to the air?
• Carbonic acid: CO₂ + H₂O = H₂CO₃

• Three important chemical weathering processes
• Dissolution
• Oxidation
• Hydrolysis

• Dissolution - rocks dissolve.  Examples:
• Halite dissolution   NaCl -> Na⁺ + Cl^-
• quartz -> quartz grains and dissolved SiO₂ (SiO₂ + 2H₂O -> H₄SiO₄ , silicic acid)
• Mg-pyx dissolution   MgSiO₃ + 2H₂CO₃ + H₂O  ->  Mg²⁺ + 2 HCO₃^- + H₄SiO₄ (silicic acid)
• Calcite dissolution   CaCO₃ + H₂CO₃ -> Ca²⁺ + 2HCO₃^-

• Oxidation - electrons transferred from one element to another. Examples
• “Rusting”  4Fe° + 3O₂ -> 2Fe₂O₃
• Fe° donates electron, O₂ accepts electron
• Also get Fe²⁺ to Fe³⁺
• Weathering of Fe-pyroxene  4FeSiO₃ + O₂ + 8H₂O -> 2Fe₂O₃ + 4H₄SiO₄
• Oxidation of organic matter   CH₂O + O₂ -> CO₂ + H₂O -> H₂CO₃

• Hydrolysis - substitution of H⁺ for a cation in a silicate mineral
• Key process in silicate weathering
• Ex: weathering of a feldspar
• Solids:  KAlSi₃O₈ + H₂O -> Al₂Si₂O₅(OH)₄   (K-spar + water -> kaolinite)
• Solids + dissolved constituents:  KAlSi₃O₈ + 2H⁺ + H₂O -> Al₂Si₂O₅(OH)₄ + K⁺ + SiO₂
• Balance equation:  2KAlSi₃O₈ (K-spar) + 2H⁺ + H₂O -> Al₂Si₂O₅(OH)₄ (kaolinite) + 2K⁺ + 4SiO₂

• Processes of Mechanical Weathering - serves to increase surface area which enhances chemical weathering
• Frost wedging
• develops talus slopes - large piles of angular rocks beneath cliffs
• Fracturing
• Joints - regular pattern of fractures in rock; due to crustal stresses; process called jointing
• Sheeting - type of joint believed to be caused by release of pressure (unloading)
• Exfoliation Dome
• Sheeting - type of joint believed to be caused by release of pressure (unloading)

Exfoliation Dome
Fig. 6-15.  Exfoliation
dome in granite in NH
• Thermal expansion and contraction - Hot, dry regions (*T > 30ºC);  No experimental confirmation, but...
• Root wedging (organic activity)
• Abrasion - collision, grinding of particles
• running water and waves
• wind
• glaciers

• Spheroidal weathering - weathering processes which tend to give rock spheroidal shape.
• Salt cracking - growing of salt crystals in cracks causes expansion of crack