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Earth Science Unit 1.4:Weathering and Soils
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The Rock Cycle
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WeatheringThe breakdown of rock near the surface
due to surface processes, such as:
Chemical Alteration – Solution & leaching of water– Biological action – Hydration
Mechanical – Impact – Wedging: Frost, Plant Roots, Salt Crystal Growth,
Expansion of Hydrated Minerals
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Weathering Rates
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Differential Weathering and Erosion
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Differential Weathering and Erosion
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Mass-WastingMovement of Large Amounts of Material
Downhill under Gravity
• Creep
• Mudflows
• Slump
• Rockfalls
• Avalanches
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Surface Area and
Weathering
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Spheroidal Weathering
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Spheroidal Weathering and Exfoliation
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Spheroidal Weathering
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Joints and Spheroidal Weathering
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What Determines Soil Type
• Climate
• Vegetation
• Drainage
• Time
• Parent Material – Residual - Transported – Least Important Factor for Mature Soils
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Soil Formation ProcessesLeaching from Surface
– K, Mg, Na – Ca – Si – Al, Fe
Accumulation beneath Surface– Al, Fe in Humid Climates – Ca in Arid Climates
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Soil Horizons and Profiles
Soil Horizons
• Layers in Soil
• Not Deposited, but Zones of Chemical Action
Soil Profile
• Suite of Layers at a Given Locality
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Principal Soil Horizons • O - Organic (Humus) Often Absent• A – Leaching (Clay Removed)
• E - Bleached Zone - Only in Certain Soils• B – Accumulation
– Absent in Young Soils– Distinct in Old Soils– Al, Fe, Clay (Moist)– Si, Ca (Arid)
• C - Parent Material (bedrock)
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Soil ClassificationThis may be the most
difficult classification problem in science because of the many factors involved.
Varied Bases for Classification
• Parent Material • Special Constituent
Materials• Maturity • Structure • Climate & Vegetation
Multiple Objectives• Scientific
– Genesis & Evolution • Agricultural
– Fertility – Most Effective Use
• Engineering– Slope Stability – Expansion and
Shrinkage – Stability of
Excavations
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"The 7th Approximation"
• U.S. Soil Conservation Service
• 12 Soil Orders
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"The 7th Approximation"
Degree of Weathering and B Horizon Development
Little Slight Moderate Large Extreme
Entisols Aridisols
Inceptisols Alfisols
Spodosols Ultisols
Mollisols Oxisols
Soils Defined by Special Constituent Materials
Andisols Volcanic Ash
Histosols Peat, Organic Matter
Vertisols “Self-Mixing” Clay Soils
Gelisols Soils on Permafrost
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Soils of the U.S.
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Typical Soil Profile (Spodosol)
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Aridisol, Kuwait
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Ultisols:Alabama
Tennessee
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Oxisol, California (a Paleosol)
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Paleosol, Scotland