Soil Weathering and Horizons: Physical and Chemical Processes - Prof. Timothy R. Ellsworth, Study notes of Earth Sciences

Download Soil Weathering and Horizons: Physical and Chemical Processes - Prof. Timothy R. Ellsworth and more Study notes Earth Sciences in PDF only on Docsity! Lec 6 Soil Weathering and Horizons Ch 2 1 Weathering Ch. 2 - Refers to the chemical and physical alteration of parent materials: - Weathering overview: Physical disintegration – Big things become smaller Chemical decomposition – Minerals change - loss/gain - Minerals formed under different conditions than found in soils: No oxygen Higher temperatures and pressures No water New conditions create a new equilibrium; low pressure, lots of H2O, O2 Soils are continually leached with water, removing weathering products allowing weathering processes to continue. - Old soils have lost large amounts of silicon, calcium, potassium, and other minerals. - Why? Because they were released by weathering and leached - Physical weathering alters the size and shape of rocks and minerals. - Chemical weathering alters the composition of soil minerals. Weathering Lec 6 Soil Weathering and Horizons Ch 2 2 WEATHERING CAN BE CLASSIFIED AS PHYSICAL OR CHEMICAL WEATHERING -Physical weathering alters the size and shape of rocks and minerals; bedrock> boulders> cobbles> gravel> sand> silt - Chemical weathering alters the composition of soil minerals. New conditions create a new equilibrium: Primary minerals become secondary minerals - Physical Weathering - (Disintegration) Physical weathering results in a reduction in particle size and hence, an increase in the surface area of a mass of material. Forces Freezing and thawing ice pressures Wetting and drying shrink-swell Heating and cooling exfoliation, differential expansion Abrasion, Impact water, ice, wind, colluvium Plant roots forcing cracking Lec 6 Soil Weathering and Horizons Ch 2 5 Chemical Weathering - Chemical weathering results in unstable primary minerals being transformed into more stable secondary minerals. Chemical weathering processes: Hydration CaSO4 + 2H2O = CaSO4·2H2O Anhydrite Gypsum 5Fe2O3 + 9H20 = Fe10O15 ·9 H20 Hematite Ferrihydrite Hydration is the chemical bonding of water to cations and anions. The hydrated cations and anions are larger in size than the unhydrated ions and hence, this results in disruption of the mineral's structure. Chemical Weathering Hydrolysis KAlSi3O8 + H2O > HAlSi3O8 + K+ + OH- orthoclase feldspar > clay HAlSi3O8 + 11H2O > Al2O3 + 6H4SiO4 monosilicic acid - Hydrolysis is the reaction of water with a mineral resulting in the splitting of water into H+ and OH- ions. -Hydrolysis is considered to be the most important chemical weathering process. - What happens to the reaction products? Lec 6 Soil Weathering and Horizons Ch 2 6 Chemical Weathering Carbonation plus Acid Reactions CO2 + H2O > H2CO3 > H + + HCO3 - H2CO3 + CaCO3 > Ca+ + 2HCO3- - Carbonic acid is a weak acid, nitric acid (HNO3) is also found in soils as are organic acids. - Sulfuric acid (H2SO4) is common in reduced soils that have undergone oxidation: FeS2 + 7/2O2 + H2O > 2H2SO4 + Fe+2. Chemical Weathering Oxidation and Reduction Many rocks and minerals contain species that are subject to oxidation or reduction: Fe3+ + e- > Fe2+ reduction S2- + 2O2 > SO42- + 8e- oxidation Reduced species (Fe2+, S2-) are subject to abiotic or biologically mediated oxidation in well aerated soils. Oxidized species (Fe3+, SO42-) can serve as terminal electron acceptors in poorly aerated soils and be reduced. Oxidation-reduction changes the size, but most importantly the valence of the species, disrupting mineral lattices and resulting in their decomposition. Lec 6 Soil Weathering and Horizons Ch 2 7 Chemical Weathering Complexation - Higher plants and microorganisms produce a variety of organic compounds (e.g. oxalic acid C2O4H2) that can complex metal ions in the soil Fe3+ + complexing agent = complexed-Fe - Complex metal ions are more mobile and subject to greater amounts of leaching in the soil profile. - Active process in Spodosol formation Chemical Weathering Dissolution CaSO4 2H2O + 2H2O > Ca2+ + SO42- + 4H2O Water is an excellent solvent for polar species. Water hydrated the cations and anions, shielding the + and - charges, eventually resulting in the total dissolution of the mineral. Lec 6 Soil Weathering and Horizons Ch 2 10 Soil horizons Specific horizons are distinguished from other horizons by differences in: color organic matter content texture structure Soil horizons are zones in which one or more of the following dominates: - Organic matter accumulation - Loss (eluviation) due to movement of water through the profile - Gain (illuviation) due to deposition of material that has been eluviated from other horizons or - Zones that are transformed in place. Lec 6 Soil Weathering and Horizons Ch 2 11 MASTER SOIL HORIZONS O - organic A - mineral E - mineral B - mineral C - mineral R - mineral Note: Master horizons can be subdivided: A1, A2, Bt1, Bt2 Parent material changes, “Lithologic Discontinuity” e.g., loess over till: Ap, A, AB, Bt1, 2Bt2, 2BC, 2C Additional Master Soil Horizons L Limnic horizon, used with Histosols, material deposited under water, plant tissues, diatoms, fish droppings, etc. includes diatomaceous earth (di), marl (ma), coprogenous earth (co). W Water, frozen or unfrozen Lec 6 Soil Weathering and Horizons Ch 2 12 S O L U M R E G O L I T H O A E Bt C R O Horizons Organic horizons of mineral soils. Formed above or in upper portion of mineral soil. Lec 6 Soil Weathering and Horizons Ch 2 15 A E Bs C Spodosol Horizons B Horizons - Mineral horizon that form below an A, E, or O horizon. Characterized by either an illuvial (gain) or residual concentration of silicate clays, iron and/or aluminum oxides, humus, carbonate, gypsum, silica, or some combination of these. - Most common B horizon in Midwest is an illuvial concentration of silicate clays (Bt). - Usually contains more clay than overlying horizons, often has blocky and/or prismatic structure, with ped surfaces coated with clay (skins). Lec 6 Soil Weathering and Horizons Ch 2 16 C Horizons - Horizons or layers excluding hard rock that are little affected by pedogenic processes. - May or may not be parent material, geologic, not pedologic structure A C R Horizons Hard consolidated bedrock. May contain fractures, but these are small enough that few roots can penetrate. R A Bw C Lec 6 Soil Weathering and Horizons Ch 2 17 A and B Horizons R Horizon TRANSITION HORIZONS Horizons dominated by the properties of one master horizon, but with some properties of another master horizon. AB BA Dominate horizon listed first EB E/B B/C B&E