1. 5. Weathering processes Local geology and climate Local geology and climate Mineral-water reaction Mineral-water reaction Mechanical weathering Mechanical weathering Slope stability and unconsolidated debris Slope stability and unconsolidated debris

2. Rocks and minerals

3. Climate and weathering

4. Weathering processes Weathering encompasses the breakdown of rock into smaller components by a combination of physical, biological, and chemical processes Weathering encompasses the breakdown of rock into smaller components by a combination of physical, biological, and chemical processes

5. Processes of decomposition Chemical processes lead to release of compounds in solution AND creation of new mineral products Chemical processes lead to release of compounds in solution AND creation of new mineral products Chemical weathering reflects tendency for new minerals to be formed which are stable under conditions prevailing at the earth’s surface Chemical weathering reflects tendency for new minerals to be formed which are stable under conditions prevailing at the earth’s surface

6. Magnitude of chemical weathering depends on: Magnitude of chemical weathering depends on:. Composition and structure of minerals. How they break apart. How mobile the constituents are Principal reacting agents are: Principal reacting agents are:. Water. Carbon dioxide. Oxygen

7. Most common processes: Most common processes:. Oxidation. Hydrolysis. Carbonation

8. Mineral – water reactions 1) Mineral + water  solution. Reaction in which the same phases (mineral and water) are present before, during and after the reaction. No alteration products – many minerals dissolve in rain water (e.g. calcite, dolomite). No alteration products – many minerals dissolve in rain water (e.g. calcite, dolomite)

9. Mineral – water reactions 2) Mineral A + water = Mineral B + solution. A new phase (mineral B appears during the reaction. Water is here a reactant, not merely a solvent 3) Mineral (1) + solution (A) = Mineral (2) + solution (B). Reaction involving ion exchange. Change in the composition of both phases

10. Processes - oxidation Oxidation: oxygen dissolved in soil and subsurface water can oxidize minerals Oxidation: oxygen dissolved in soil and subsurface water can oxidize minerals Oxidation occurs when oxygen is added in a chemical reaction Oxidation occurs when oxygen is added in a chemical reaction Oxidation is “normal” sate of most silicate minerals at the surface Oxidation is “normal” sate of most silicate minerals at the surface With oxidation, minerals converted to oxides With oxidation, minerals converted to oxides Metallic elements are: calcium, magnesium, iron. Metallic elements are: calcium, magnesium, iron.

11. Processes - hydrolysis Water reacts directly with minerals through hydrolysis (to produce a new compound nearly always softer and weaker than the original) Water reacts directly with minerals through hydrolysis (to produce a new compound nearly always softer and weaker than the original) Igneous rocks particularly susceptible to hydrolysis because their silicate minerals combine readily with water. Igneous rocks particularly susceptible to hydrolysis because their silicate minerals combine readily with water. Hydrolysis invariably increases the volume of the mineral (which contributes to mechanical weathering) in addition to the chemical change Hydrolysis invariably increases the volume of the mineral (which contributes to mechanical weathering) in addition to the chemical change

12. Processes - carbonation Process by which carbon dioxide in water reacts with carbonate rocks (such as limestone and dolomite) to produce a solution – calcium bicarbonate Process by which carbon dioxide in water reacts with carbonate rocks (such as limestone and dolomite) to produce a solution – calcium bicarbonate Calcium bicarbonate can readily be removed by runoff or percolation. Can also be deposited in crystalline form if water is evaporated. Calcium bicarbonate can readily be removed by runoff or percolation. Can also be deposited in crystalline form if water is evaporated.

13. Factors influencing chemical weathering Mobility: relative mobility of dissolved constituents. Role played by amount of water “leaching” weathering zone Mobility: relative mobility of dissolved constituents. Role played by amount of water “leaching” weathering zone Leaching: downward movement of water through the weathering zone which leads to the removal of soluble products Leaching: downward movement of water through the weathering zone which leads to the removal of soluble products Significance of leaching: provides mechanisms by which constituents are transferred to lower levels. Significance of leaching: provides mechanisms by which constituents are transferred to lower levels.

14. Mechanical weathering processes Force needed to accomplish disintegration provided by: Force needed to accomplish disintegration provided by:. Expansion of rocks and minerals. Growth of foreign substances in spaces within the parent material Growth: Growth:. freeze-thaw action. Salt wedging. Biotic activity Expansion: Expansion:. Temperature changes. Unloading. Hydration and swelling

15. Salt wedging Caused by crystallization of salts from evaporating water (dry-arid climates) Caused by crystallization of salts from evaporating water (dry-arid climates) Water often drawn upward in rock openings by capillary action Water often drawn upward in rock openings by capillary action. This water carried dissolved mineral salts. Evaporation  salts left behind as tiny crystals With time, crystals grow and expand. With time, crystals grow and expand.

16. Expansion of parent minerals or rocks T° changes. Change sin the volume of mineral particles. Weakens the coherence of mineral grains T° changes. Change sin the volume of mineral particles. Weakens the coherence of mineral grains Unloading. Expansion of large segments of rock masses which occurs when pressure is released by erosion. Unloading. Expansion of large segments of rock masses which occurs when pressure is released by erosion. Hydration and swelling: Expansion which occurs when minerals are formed or when they are altered by the addition of water to their structure Hydration and swelling: Expansion which occurs when minerals are formed or when they are altered by the addition of water to their structure

17. Slope stability and unconsolidated debris Whether debris created by weathering will resist erosion depends on balance between internal resistance of materials and magnitude of external forces acting on them Whether debris created by weathering will resist erosion depends on balance between internal resistance of materials and magnitude of external forces acting on them Slope stability: Slope stability:. Internal resistance: shear strength. External forces: shear stress