Weathering and Soils

The Rock Cycle

The Rock Cycle

WeatheringBreakdown of Rock near the Surface Due to

Surface Processes Chemical Alteration • Solution & Leaching • Biological Action • HydrationMechanical • 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-WastingMovement of Large Amounts of Material

Downhill under Gravity • 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

• 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 ProcessesLeaching 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

ActionSoil 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 FormationBalance 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 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

"The 7th Approximation"

• U.S. Soil Conservation Service• 12 Soil Orders

"The 7th Approximation"

Degree of Weathering and B Horizon DevelopmentLittle Slight Moderate Large ExtremeEntisols Aridisols

Inceptisols AlfisolsSpodosols UltisolsMollisols Oxisols

Soils Defined by Special Constituent MaterialsAndisols Volcanic AshHistosols Peat, Organic MatterVertisols “Self-Mixing” Clay SoilsGelisols Soils on Permafrost

Soils of the U.S.

Typical Soil Profile (Spodosol)

Aridisol, Kuwait

Ultisols:Alabama

Tennessee

Oxisol, California (a Paleosol)

Paleosol, Scotland