Lecture Outlines

Physical Geology, 15/e

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Plummer, Carlson & Hammersley

Weathering and Soil

Physical Geology 15/e, Chapter 5

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• weathering is the group of destructiveprocesses that change physical andchemical character of rocks at or nearEarth’s surface

• erosion is physical picking up of rockparticles by water, ice, or wind

• transportation is the movement oferoded particles by water, ice, or wind

Rocks exposed at Earth’s surface areconstantly changed by water, air,temperature variations and other factors

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Weathering, Erosion and Transportation

Mechanical Weathering – physicaldisintegration of rock without changes ofthe chemical composition

Chemical weathering – decompositionof rock from exposure to atmosphericgases (carbon dioxide, oxygen and watervapor) and water.

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How Weathering Changes Rocks

Destruction of building materials

Discoloration of surface outcrops

Production of soil

Impacts the atmosphere

• Removes carbon dioxide

Creates interesting and unusual rockshapes

• Spheroidal weathering

• Differentialweathering

Spheroidal weathering

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Effects of Weathering

Pressure release – removal of overlyingrock allows expansion and fracturing.Exfoliation domes

Frost action – mechanic effect offreezing (and expanding) water on rocks.Frost wedging and frost heaving

Other Processes

• Plant growth – growing roots widen

fractures

• Burrowing animals

• Thermal Variation – largetemperature changes fracture rocks byrepeated expansion and contraction

• Salt pressure

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Mechanical Weathering

Plant Roots

Frost Wedging

Thermal Variation

Role of Oxygen/Oxidation – chemically

active oxygen from atmosphere

• iron oxide stains are common result

Role of Acids – hydrogen cations replaceothers in minerals

• carbonic acid from atmospheric CO2 dissolvedin water

• sulfuric, hydrofluoric acids emitted byvolcanic eruptions

• some minerals, such as calcite, may be totallydissolved

• human activity, such as mining and burning offossil fuels, produces acids

Solution Weathering

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Chemical Weathering

Chemical Weathering of

Feldspars – most commonminerals in crust

• slightly acidic rain water attacksfeldspar

• clay minerals produced

Chemical Weathering of OtherMinerals

• ferromagnesian minerals• Weathering and Diamond

Concentration• more complex silicate bonds

lead to lower weatheringsusceptibility

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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

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Chemical Weathering

• 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

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Weathering, the Carbon Cycle and Global Climate

Soil - a layer of weathered, unconsolidated material ontop of bedrock

• common soil constituents:

o clay minerals

o quartz

o water

o organic matter

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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

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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 rockcomposition evolves with time and chemical weathering

• Slope – Soils are thin or nonexistent on steep slopes and are thickand well developed on flat or gently sloping uplands.

• Living Organisms – provide organic matter

• Climate – most influential factor affecting soil thickness andcharacter

• Time

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Soil

Soil Erosion

• The O and A horizons are the most valuable andalso the most vulnerable to erosion.

• How Soil Erodes

• Soil particles are small and are thereforeeasily eroded (carried away) by water andwind

• 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

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Soil

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

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Soil classification

End of Chapter 5