Earth Science Unit 1.4:Weathering and Soils

The Rock Cycle

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

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

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 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 Action

Soil Profile• Suite of Layers at a Given Locality

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)

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