Chapter 3- Weathering Processes Two major categories Mechanical (or Physical) processes

1. Chapter 3- Weathering Processes • Two major categories • Mechanical (or Physical) processes • physical breakdown of larger particles into smaller particles • Chemical processes • Chemical reactions that facilitate soil profile development • Removal of chemical elements • Addition of altered chemical elements • Addition of new chemical elements

2. Mechanical processes aid Chemical processes • Increases surface area to volume ratio • Chemical processes dominate soil development • Mechanical processes • breakage can occur along bedding planes • Along stress induced fractures or joints • Micro or macro scale • Along crystal boundaries

3. Pressure release • Caused by unroofing of deeply buried rock- typically igneous, but can be any kind of rock

4. Sheet joints due to pressure release create an exfoliation dome known as Enchanted Rock in Enchanted Rock State Park, Texas

5. Freeze Thaw • Water, when frozen, expands and can fracture rock if the cracks have the appropriate geometry and the climate is suitable. • Salt Crystal Growth • Similar to freeze thaw fracturing, crystal growth occurs as chemicals dissolved in the water precipitate out as water evaporates- creating salt crystals that pry the rock apart • e.g., K-horizons with high percentage of salt in sandy soils push sand grains apart so they are held in salt suspension • Can also induce chemical reactions on mineral grains- chemical weathering

6. This boulder in Death Valley, California has disintegrated in situ due to saline waters entering fractures, and prying apart the cracks Best developed early in pedogenesis

7. Thermal Fracturing • Implication of freeze thaw expansion and contraction due to changes in temperature and thermal properties of the parent material • Also relates to fire related actions as well • Spalling occurs when fire superheats the surface • Also vaporizes water which exerts force • Can be influenced by micro-environmental conditions

8. Influence of fire and environmental conditions on weathering of boulders

9. Sand in the bottom of a 5 m deep weathering pit at Cookie jar Butte in Utah

10. 13 m deep weathering pit in Utah Likely formed from a variety of mechanical and chemical processes

11. Sand accumulated at the bottom of a 18 m deep weathering pit in Utah.

12. Chemical weathering is dominant factor in pedogenesis Requires mechanical weathering to facilitate continued chemical reactions Surface area to volume ratio…

13. Congruent vs Incongruent weathering • Congruent- no by products from the chemical reaction in an water-based solution • i.e., the stuff goes into solution and completely dissolves • e.g., salt (NaCl) or limestone (Calcite {CaCO3} • NaCl + H2O ------------->Na+ & Cl- • Crystalline & liquid aqueous aqueous • CaCO3 + CO2 + H20 ----->Ca2+ + 2HCO3- • Crystalline & gas & liquid aqueous aqueous • Solubility of calcite is dependent upon CO2 and H+ concentration

14. Soil environment is loaded with acids and gases that can supply the either of the two things that drive the dissolution of calcite • organics create organic acids • CO2 is abundant in soils and is greater than in the actual atmosphere • The reactions are self sustaining up to a point • saturation of solution with respect to CaCO3

16. Soils over limestone are influenced by insoluble materials in the rock eolian influx biological components Complete dissolution of pure limestone produce very little in the way of soil. Incomplete dissolution creates a special kind of material called terra rosa red earth

17. Terre Rosa fills sinkholes in karst terrane

18. processes of chemical weathering Oxidation/Reduction • - a function of available free oxygen and the Eh (redox potential) - Oxygen strips electrons away from other atoms the process is reversible • - often related to water levels in streams and groundwater systems

19. Incongruent chemical weathering creates weathering by-products or materials Especially commonplace when considering silicates Why? - silica is only slightly soluble in most aqueous solutions with lower pH values (0.0007 g/liter or 7 ppm in most solutions less than pH of 9) That means that some other elements will dissolve but in so doing cause secondary chemical reactions that create new minerals, especially those that can react with the silicate structures Examples clay minerals; oxidation processes

20. Oxidation interaction between substances and oxygen (and • in some cases water) 4FeO + O 2Fe O E.G., • 2 2 3 The iron goes from the ferrous reduced state (Fe2+) to the ferric ( Fe3+) state MgFeSiO + 2H O Mg(OH) + H SiO + FeO 4 2 2 2 3 silicic magnesium olivine acid hydroxide 2FeS + 7H O + 15O 2Fe(OH) + 4H SO 2 2 2 4 pyrite sulfuric acid

21. Clay minerals occur as layers or sheets arranged in a variety of • ways - layers consist of sheets of aluminum and silica that alternate between discrete layers > 1:1 - clay minerals include Kaolinite, Allophane, and Halloysite > 2:1 - clay minerals include Illite, Smectite, Montmorillonite, and Glauconite usually derived from the decomposition of silicate • minerals like feldspars, micas, and hornblende Can have unique properties that are important for • good construction > shrink-swell clays like Smectite

22. Examples of incongruent chemical weathering

23. Solubility of materials determines which things remain to be able to create alternative minerals in the soil, most notably clay minerals Polynov’s Ion mobility series Cl- > SO42- > Na2+ > Ca2+ > Mg2+ > K2+ > Si2+ > Fe3+,2+ > Al2+ Phase I--><--II--><---III---><--IV--><---V----

24. Other factors Mobility-how susceptible atoms are to movement • due to normal chemical processes Leaching • - removal of elements from upper portions of the weathering profile to locations lower in the weathering profile Chelation- mobilization of relatively immobile • ions by encapsulating them in a complex molecular structure - common way to move things like Aluminum, Iron and Calcium - usually involves organic compounds that fix the metal into a mobile compound

25. Chelation structures

26. Abrasion pH’s for common minerals As cations are released in the grinding process, the solution, which begins as a neutral pH becomes increasingly more basic However, in most environments, this tendency is offset by production of more H+ cations

27. H+ ions attack weaker spots in the crystalline structure of aluminosilicate minerals

28. Etching related to chemical weathering acting and • pieces breaking off along cleavage surfaces - keeps rates from achieving some steady state

29. processes of chemical weathering solution • - removal of atoms from mineral structures - causes minerals to fall apart related to pH of the solvent • - typically the water

30. chemical processes Ion exchange • - usually a function of a polarized molecular arrangement > causes a propensity to try and balance the charges by replacing ions with other ions through a process called adsorption > esp. Cations - often measured as CEC (Cation Exchange Capacity) > a measure of the adsorption ability of a surface - particularly common in clay minerals

31. Measurement of weathering • Total chemical analysis is one way • given as oxides- oxygen is a common balancing ion