Weathering

1. Weathering http://soest.hawaii.edu/coasts/cgg_main.html

3. Chemical Weathering Mechanical Weathering H2O CO2 H2O+ CO2 = H2CO3 2KAlSi3O8+2H+2HCO3+H2O Al2Si2O5(OH)4+2K+2HCO3+4SiO2 feldspar plus rainwater react to form clay plus dissolved salts and silicic acid

4. (A) Weathering processes attack corners along three surfaces. Mechanical weathering breaks rock into small pieces thus increasing the surface area attacked by chemical weathering... this produces sediment. (B) Angular rock pieces are quickly rounded.

6. Chemical Weathering Dependent on properties of water – “polar molecule”

7. Chemical Weathering Hydrolysis = weathering of silicates by water Hydrolysis – polar nature of water molecule allows easy bonding with cations Water molecules swarm on cations and “dissolve” them…cations often replaced by H+

8. Chemical Weathering Hydrolysis Kaolinite – clay… common name?? Feldspar - most abundant mineral on Earth 2KAlSi3O8+2H+2HCO3+H2O Al2Si2O5(OH)4+2K+2HCO3+4SiO2 Feldspar Kaolinite

9. Blue – oxygenBrown – siliconPurple - cations • a. b.

10. Oxidation = O bonds with cation (K, Ca, Fe, Mg) by electron sharing (covalent) Chemical Weathering Fe2SiO4 + 2H2CO3 + 2H2O  2Fe2+ + 2OH - + H4SiO4 + 2HCO3 Olivine + carbonic acid + water iron and hydroxyl ions in solution + silicic acid in solution + bicarbonate ions in solution Oxidation would strip the oxygen from the atmosphere in a few million years. 2Fe2+ + 4HCO3 + ½ O2 +2H2O Fe2O3 + 4H2CO3 Hematite or Limonite

11. Dissolution = dissolving of solid compound Chemical Weathering Limestone is an important crustal rock – karst topography CaCO3+H2CO3 =Ca+2HCO3 Calcite dissolved by acid CO2+H2O=H2CO3 Carbonic acid

12. Chemical Weathering Karst topography Sinkhole – limestone cavern with roof collapsed Cenote – flooded limestone cavern

13. Biological Weathering • Simple breaking • Movement and mixing • Carbon dioxide produced by • respiration forms carbonic acid • Organisms influence moisture in soil

14. Chemical weathering dominates in warmer, wetter climates Mechanical weathering dominates in cool, dry climates

15. Vulnerability to weathering is reverse of crystallization history Bowens Reaction Series First to weather Olivine – Ca Feldspar Pyroxene Amphibole – Ca/Na Feldspar Biotite – Na Feldspar Orthoclase Muscovite Quartz Last to weather Chemical weathering produces soils composed of oxides – Bauxite (Al oxide), Laterite (Fe oxide)

16. Soil Profile O Horizon – decayed and loose organics (topsoil) A Horizon - inorganic mineral particles mixed with some organics B Horizon – clays with little organics C Horizon – transition between bedrock and soil

18. a. Inceptisol Steep slopes Histosol High organic poor drainage Mollisol Organic grassland Alfisol Leached fertile soil Ultisol Intensely weathered low fertility Andisol Formed on volcanic rock Aridisol Desert, clay, salt, gypsum Entisol Young soil lacking layers Gelisol Permafrost f.

20. Residual soils contain accumulations of stable elements and compounds – Al, Fe bauxite, gibbsite

21. Sedimentary Minerals Microcrystalline quartz (chert, agate, quartz) SiO2 Clay - kaolinite Calcite – CaCO3 Hematite/limonite Fe2O3 Granite – quartz, feldspar, biotite, amphibole Bauxite Al2O3 H2O Saprolite – quartz, clay, hematite, Al-oxide

23. What processes have changed this granite?

24. Mechanical Weathering Water expands when it freezes, this can crack open rocks, ice wedging

26. Mechanical Weathering When large areas of igneous rock are exposed at the surface they tend to peel like an onion. “exfoliation” Half-dome, Yosemite product of stresses perpendicular to the surface exfoliaton

27. Mechanical Weathering Spheroidal weathering, rock disintegrates at corners and edges

28. Mechanical Weathering Abrasion, sand blasting forms ventifactsand other weathering surfaces

29. Mechanical Weathering Talus slope

30. Mechanical Weathering Salt weathering

31. Mechanical Weathering How do natural bridges form?

32. Mechanical Weathering Step 1- large parallel joints are opened by ice wedging Step 2- continued ice wedging and exfoliation produce “fins”

33. Mechanical Weathering Step 3 – fins develop exfoliation “arches” that widen to windows with continued Ice wedging…a bridge continues to widen until becomes unstable and collapses.

34. Erosion

35. Erosion

36. Soil Profile – deforestation Tropical forest soils have thin O horizon with thick B horizon of bauxite • Typical deforestation pattern • Large international logging • companies pay poor governments • to allow clearing of heavy forest… • can be major source of $$ • Land is cleared by burning which • doesn’t usually harm valued hardwoods

37. Cleared lands are made available to poor farmers. Heavy rains wash away • humus layer and rich organics disappear. This exposes thick laterite soils (bauxite) • produced by chemical weathering. With forest gone the hot sun bakes the soil • which turns to brick within 3-5 years forcing farmer to move to newly burned area.

38. Managing the global problem of tropical deforestation is very complex. Many tropical economies are severely depressed and rely upon newly opened lands for farming and the sale of timber for income. However these sources are not sustainable and deforestation only continues the cycle of impoverishment and depression. When you are poor, protecting a forest does not mean as much as having a full meal.

39. Common reactions Hydrolysis 2KAlSi3O8 + 2H+ + 2HCO3- + H2O   Al2Si2O5(OH)4 + 2K+ + 2HCO3- + 4SiO2 orthoclase + H ions + bicarbonate ions + water  clay (kaolinite) + dissolved potassium + bicarbonate ions + dissolved silicate Oxidation (a) Fe2SiO4 + 2H2CO3 + 2H2O   2Fe2+ + 2OH - + H4SiO4 + 2HCO3 fayalite + carbonic acid + water  dissolved iron and hydroxyl ions + dissolved silicate + bicarbonate ions (b) 2Fe2+ + 4HCO3- + ½O2 + 2H2O    Fe2O3 + 4H2CO3 iron + bicarbonate in solution + gaseous oxygen + water  ferric oxide mineral (hematite) + carbonic acid Dissolution (a) CO2        +        H2O       H2CO3 carbon dioxide gas + water  carbonic acid (b) CaCO3 + H2CO3      Ca2+   +   2HCO3- calcite + carbonic acid  dissolved calcium + dissolved bicarbonate