Weathering and Soils
The Rock Cycle
The Rock Cycle
Weathering
Breakdown of Rock near the Surface Due to
Surface Processes
Chemical Alteration
• Solution & Leaching
• Biological Action
• Hydration
Mechanical
• Impact
• Wedging: Frost, Plant Roots, Salt Crystal
Growth, Expansion of Hydrated Minerals
Never Safe From Weathering
Weathering Rates
Differential Weathering and Erosion
Differential
Weathering
and Erosion
Mass-Wasting
Movement of Large Amounts of Material
Downhill under Gravity
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Creep
Mudflows
Slump
Rockfalls
Avalanches
Surface Area
and
Weathering
Surface-Volume Effects
Spheroidal Weathering
Spheroidal Weathering and Exfoliation
Spheroidal Weathering
Joints and Spheroidal Weathering
What Determines Soil Type
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Climate
Vegetation
Drainage
Time
Parent Material
– Residual - Transported
– Least Important Factor for Mature Soils
Soil Formation
Young Soils
• Strongest Influence Is Parent Material
Mature Soils
• Strongest Influences: Climate, Vegetation,
Drainage
Soil Formation Processes
Leaching from Surface
• K, Mg, Na
• Ca
• Si
• Al, Fe
Accumulation beneath Surface
• Al, Fe in Humid Climates
• Ca in Arid Climates
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
Principal Soil Horizons
• O - Organic (Humus) Often Absent
• A – Leaching
– K, Mg, Na, Clay Removed
• E - Bleached Zone - Present Only in Certain Soils
• B – Accumulation
– Absent in Young Soils
– Distinct in Old Soils
– Al, Fe, Clay (Moist)
– Si, Ca (Arid)
• C - Parent Material
Limits of Soil Formation
Balance Between:
• Downward Lowering of Surface
• Downward Migration of Horizons
If erosion rapid or soil evolution slow, soils
may never mature beyond a certain point.
Extremely ancient soils may have lost
everything movable
Soil Classification
This 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
"The 7th Approximation"
• U.S. Soil Conservation Service
• 12 Soil Orders
"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
Soils of the U.S.
Typical
Soil Profile
(Spodosol)
Aridisol, Kuwait
Ultisols:
Alabama
Tennessee
Oxisol, California (a Paleosol)
Paleosol, Scotland