Lecture 5: Weathering and Soil Formation GY111 Physical Geology Lecture 5: Weathering and Soil Formation Lectures on Weathering and Soil Formation for Physical Geology.

Weathering Chemical Weathering Physical Weathering Hydration: chemical reaction that consumes H2O [ 2KAlSi3O8 + 2H2CO3 + H2O = Al2Si2O5(OH)4 + 4SiO2 + 2K+ +2HCO3- ] Oxidation: chemical reaction that consumes oxygen [ Fe2SiO4 + ½O2 = Fe2O3 + SiO2 ] Dissolution: dissolving of minerals into solution [ NaCl + H2O = Na+ OH- + H+ + Cl- ] Physical Weathering Ice wedging Thermal stress Spheroidal weathering Biologic activity Soil creep Solifluction Exfoliation Weathering: 1. Chemical Weathering: Hydration: chemical reaction that consumes H2O [ 2KAlSi3O8 + 2H2CO3 + H2O = Al2Si2O5(OH)4 + 4SiO2 + 2K+ +2HCO3- ] Oxidation: chemical reaction that consumes oxygen [ Fe2SiO4 + ½O2 = Fe2O3 + SiO2 ] Dissolution: dissolving of minerals into solution [ NaCl + H2O = Na+ OH- + H+ + Cl- ] 2. Physical Weathering: Ice wedging Thermal stress Spheroidal weathering Biologic activity Soil creep Solifluction Exfoliation

Physical Weathering Examples Fe-oxide formation from oxidation. Spheroidal weathering along joint fractures. Biologic activity such as tree root wedging. Frost wedging during freeze-thaw cycles. Most effective in mid-latitude climates. Exfoliation forms in joint fractures that form parallel to Earth’s surface when rocks are exhumed by weathering.

Weathering examples Fe-oxide formation from oxidation

Weathering Examples cont. Joint patterns allowing physical and chemical weathering 2. Spheroidal weathering along joint fractures. Note that weathering is more effective at corners producing a spherical shape from the original cube.

Weathering Examples cont. Biologic activity 3. Biologic activity such as tree root wedging.

Weathering Examples cont. Frost wedging 4. Frost wedging during freeze-thaw cycles. Most effective in mid-latitude climates.

Weathering Examples cont. Exfoliation dome formation (Stone Mt. GA) 5. Exfoliation forms in joint fractures that form parallel to Earth’s surface when rocks are exhumed by weathering.

Weathering Factors Climate Bedrock type (mineralogy) Rainfall Average temperature In some climates chemical weathering dominates, in others physical weathering dominates Bedrock type (mineralogy) Bowen’s Reaction series Topography (Soil formation) Steep: little or no soil Flat: abundant soil Duration of weathering process Weathering Factors: 1. Climate Rainfall. Average temperature. In some climates chemical weathering dominates, in others physical weathering dominates. 2. Bedrock type (mineralogy) Follows Bowen’s Reaction series: high T minerals weather fastest. 3. Topography (Soil formation) Steep: little or no soil Flat: abundant soil 4. Duration of weathering process

Soil Profile A horizon: >50% organic humus mixed with sand, silt and clay B horizon: sand size particles surrounded by a matrix of soluble residue and clay minerals C horizon: bedrock is weathered but still recognized Soil Profile: 1. A horizon: >50% organic humus mixed with sand, silt and clay. 2. B horizon: sand size particles surrounded by a matrix of soluble residue and clay minerals. 3. C horizon: bedrock is weathered but still recognized.

Soil Profile Schematic A, B and C horizons 1. Soil profile schematic displaying the A, B, and C horizons.

Soil Types Pedalfer: originate in temperate humid climate zones. Well developed A, B and C horizons. Pedocal: originate in arid and semi-arid temperate climates. Contain abundant CaCO3 in B horizon; All horizons are poorly developed. Laterites: originate in humid tropical climates. Contain mainly Al2O3. Horizons are poorly developed. Soil Types: 1. Pedalfer: originate in temperate humid climate zones. Well developed A, B and C horizons. 2. Pedocal: originate in arid and semi-arid temperate climates. Contain abundant CaCO3 in B horizon; All horizons are poorly developed. 3. Laterites: originate in humid tropical climates. Contain mainly Al2O3. Horizons are poorly developed.

Soil Porosity & Permeability Porosity: percentage of void space in material Permeability: ability of material to transmit fluid Aquifer: a material with good permeability Sandstone Limestone Any type of highly fractured rock (fault zone) Soil Porosity & Permeability: 1. Porosity: percentage of void space in material. 2. Permeability: ability of material to transmit fluid. 3. Aquifer: a material with good permeability: Sandstone Limestone Any type of highly fractured rock (fault zone)

Exam Summary Know examples of chemical and physical weathering. Know how to describe the soil horizons. Know the factors that determine weathering rates. Know the soil types and geographic examples. Know the definitions of porosity and permeability.