Lecture Outlines Physical Geology, 15/e Plummer, Carlson & Hammersley
Weathering and Soil Physical Geology 15/e, Chapter 5
Weathering, Erosion and Transportation Rocks exposed at Earth s surface are constantly changed by water, air, temperature variations and other factors weathering is the group of destructive processes that change physical and chemical character of rocks at or near Earth s surface erosion is physical picking up of rock particles by water, ice, or wind transportation is the movement of eroded particles by water, ice, or wind
How Weathering Changes Rocks Mechanical Weathering physical disintegration of rock without changes of the chemical composition Chemical weathering decomposition of rock from exposure to atmospheric gases (carbon dioxide, oxygen and water vapor) and water.
Effects of Weathering Destruction of building materials Discoloration of surface outcrops Production of soil Impacts the atmosphere Removes carbon dioxide Creates interesting and unusual rock shapes Spheroidal weathering Differentialweathering Spheroidal weathering
Mechanical Weathering Pressure release removal of overlying rock allows expansion and fracturing. Exfoliation domes Frost action mechanic effect of freezing (and expanding) water on rocks. Frost wedging and frost heaving Plant Roots Other Processes Plant growth growing roots widen Frost Wedging fractures Burrowing animals Thermal Variation large temperature changes fracture rocks by repeated expansion and contraction Thermal Variation Salt pressure
Chemical Weathering Role of Oxygen/Oxidation chemically active oxygen from atmosphere iron oxide stains are common result Role of Acids hydrogen cations replace others in minerals carbonic acid from atmospheric CO2 dissolved in water sulfuric, hydrofluoric acids emitted by volcanic eruptions some minerals, such as calcite, may be totally dissolved human activity, such as mining and burning of fossil fuels, produces acids Solution Weathering
Chemical Weathering Chemical Weathering of Feldspars most common minerals in crust slightly acidic rain water attacks feldspar clay minerals produced Chemical Weathering of Other Minerals ferromagnesian minerals Weathering and Diamond Concentration more complex silicate bonds lead to lower weathering susceptibility
Chemical Weathering Weathering Products Quartz Clay Minerals Iron Oxides Factors Affecting Weathering Availability of water Climate chemical wreathing is faster in warm humid climates. freeze thaw cycles enhance frost wedging Rock composition Slope
Weathering, the Carbon Cycle and Global Climate Present in the atmosphere as carbon dioxide gas Combines with water to form carbonic acid Weathers rocks and leads to limestone formation in bodies of water Returned to mantle by tectonic plate movement Released back to atmosphere by volcanic eruptions
Soil Soil - a layer of weathered, unconsolidated material on top of bedrock common soil constituents: o clay minerals o quartz o water o organic matter
Soil Soil horizons O horizon - uppermost layer; organic material A horizon dark-colored, rich in organic matter and high in biological activity E horizon - zone of leaching; fine-grained components removed by percolating water B horizon - zone of accumulation; clays and iron oxides leached down from above; formation of hard pan in wet climates C horizon - partially weathered bedrock
Soil Factors Affecting Soil Formation Parent material Residual soil - weathering of underlying rock Transported soil - brought in from elsewhere wind-transported soil is called loess Soil composition determined by parent rock composition evolves with time and chemical weathering Slope Soils are thin or nonexistent on steep slopes and are thick and well developed on flat or gently sloping uplands. Living Organisms provide organic matter Climate most influential factor affecting soil thickness and character Time
Soil Soil Erosion The O and A horizons are the most valuable and also the most vulnerable to erosion. How Soil Erodes Soil particles are small and are therefore easily eroded (carried away) by water and wind water erosion is the most significant type; wind erosion is generally less significant Rates of Erosion soil characteristics, climate, slope, vegetation Consequences of Erosion Removal of an essential resource Sedimentation of water bodies
Soil classification World Soil Orders Alfisols: gray to brown surface horizon, common in humid forests Andisols: soils formed in volcanic ash Aridisols: soils formed in dry climates (low organic matter) Entisols: young soils that have no horizons Gelisols: weakly weathered soils with permafrost within 2 meters of the surface Histosols: wet, organic soils with little mineral material Inceptisols: very young soils with weakly developed soil horizons Mollisols: nearly black surface horizon rich in organic matter Oxisols: heavily weathered soils (also called laterites) Spodosols: acid soils low in plant nutrient ions Ultisols: strongly weathered soils low in plant nutrient ions and clays Vertisols: clay soils that swell when wet and shrink when dry
End of Chapter 5