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

2. 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

3. 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.

4. Weathering examples Fe-oxide formation from oxidation

5. 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.

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

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

8. 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.

9. 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

10. 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.

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

12. 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.

13. 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)

14. 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.