Chapter 5

Weathering and Soil

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Introductory Geology

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Earth’s external processesWeathering – the physical breakdown (disintegration) and chemical alteration (decomposition) or rock at or near Earth’s surface

Mass wasting – the transfer of rock and soil downslope under the influence of gravity (Chapter 9)

Erosion – the physical removal of material by mobile agents such as water, wind, ice, or gravity

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Weathering

Types of weathering• Mechanical weathering • Chemical weathering• Other function together

Weathering Processes• Dissolution • Oxidation• Hydrolysis

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WeatheringMechanical & chemical

processes work together

Solid rock

Chemical weathering attacks susceptible minerals

Rock crumbles by mechanical weathering

Chemical/mechanical processes form clay and disperse minerals

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Mechanical Weathering

Increased surface area

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Types of Weathering

Mechanical weathering – breaking of rocks into smaller pieces

• Frost wedging – freezing/thawing of water in fractures, disintegration of rocks

• Unloading – exfoliation of igneous and metamorphic rocks at the Earth’s surface due to a reduction in confining pressure

• Thermal expansion – expansion and contraction due to heating and cooling

• Biological activity – disintegration resulting from plants and animals

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Frost Wedging

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Unloading - Exfoliation

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Exfoliation

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Thermal Expansion

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Biological

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Chemical Weathering

Breaks down rock components and internal structures of minerals

Most important agent involved in chemical weathering is water (responsible for transport of ions and molecules involved in chemical processes)

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Chemical Weathering ProcessesDissolution

– Aided by small amounts of acid in the water– Soluble ions are retained in the underground

water supply

Oxidation– Chemical reaction where compound or

radical loses electrons– Important in decomposing ferromagnesian

minerals

Hydrolysis– The reaction of any substance with water– Hydrogen ion attacks and replaces other

positive ions

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Simple Dissolution

Halite (salt) dissolving in water

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Dissolution by Acidic Water

CaCO3 + 2[H+(H2O)]

Marble and limestone (calcium carbonate) decompose due to acidic water

Calcium Carbonate + Acidic Water

Ca2+ + CO2 + 3(H2O)

Soluble Calcium + Carbon Dioxide + Water

Acid rain caused by air pollution

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Oxidation

4Fe3+ + 3O22-

2Fe2O3

Iron (Fe) minerals rust when exposed to water and oxygen

Oxidation: loss of electron from element

Iron oxidation produces

Hematite (Fe2O3): reddish brown rust

Limonite [FeO(OH)]: yellowish rust

Iron + Oxygen

Hematite (iron oxide)

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Hydrolysis

Al2Si2O5(OH)4 + 2K+ + 2HCO3 -) + 4SiO2

Silicate minerals decomposed by water due to hydrolysis

Produces clay, soluble salt, silica

Potassium Feldspar + Carbonic Acid + Water

Kaolinite + Potassium + Bicarbonate + Silica(clay) (salt)

2KAlSi3O8 + 2(H+ + HCO3- ) + H2O

in solution

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Products Weathering

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Chemical Weathering Alternations

Decomposition of unstable minerals

Generation or retention of materials that are stable

Physical changes such as the rounding of corners or edges

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Weathering FactorsRates of weathering

– Advanced mechanical weathering aids chemical weathering by increasing the surface area

Rock characteristics– Rocks w/ calcite (marble and limestone) readily

dissolve in weakly acidic solutions

– Silicate minerals weather in the same order as their order of crystallization

Climate– Temperature and moisture, most crucial factors

– Chemical weathering is most effective in areas of warm, moist climates

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Chemical WeatheringRate variation by rock type

Granite Marble

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Weathering rates

organic activity - organic acidsraises wtr rates by orders of magnitude

climate - temp and amount of water

water flow through rock

bedrock composition

topography: groundwater flow

time - to equilibrium?

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Rock Characteristics

Bowen’s Reaction Series

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Weathering Processes

Reaction rates and weathering susceptibilities approximate a “backwards backwards Bowen’s Reaction SeriesBowen’s Reaction Series”

first minerals (highest P/T) to crystallize are least stable (first to weather)

last minerals (lowest P/T) to crystallize are most stable (last to weather)

Residual material is altered to a stable composition and physical form

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Differential Weathering

Masses of rock do not weather uniformly due to regional and local factors

Results in many unusual and spectacular rock formations and landforms

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Differential Weathering

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Differential weathering

Controlled by jointing patterns

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Joint-controlled weathering in igneous rocks

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Soil

Soil is a combination of mineral and organic mater, water, and air

Results from weathering

Regolith: rock and mineral fragments produced by weathering (weathered debris)

Soil supports the growth of plants

Good soil: mix of decomposed rock and humus (decay plant and animal)

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Typical soil components

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Soil

Soil Formation

Soil Profile

Soil Types

Soil Erosion

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Soil Formation

Parent material– Residual soil – parent material is the underlying

bedrock

– Transported soil – forms in place on parent material that has been carried from elsewhere and deposited

Time– Important in all geologic processes– Amount of time for soil formation varies for

different soils depending on geologic and climatic conditions

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Soil Formation

• Parent material– Residual soil: parent material is the

underlying bedrock

– Transported soil: forms in place on parent material that has been carried from elsewhere and deposited

• Time– Important in all geologic processes

– time for soil formation varies depending on geologic and climatic conditions

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Soil Formation• Climate

– Most influential control of soil formation– Key factors: temperature and precipitation

• Plants and animals– Organisms influence the soil’s physical and

chemical properties

– Also furnish organic matter to the soil

• Slope– Steep slopes often have poorly developed soils

– Optimum terrain is flat-to-undulating upland surface

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Soil Formation

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Soil development variations due to topography

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Soil Profile

Soil forming processes operate from the surface downward

Vertical differences are called horizons – zones or layers of soil

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• O horizon – organic matter• A horizon – organic and mineral matter

– High biological activity

– O and A horizons together make up the topsoil

• E horizon – little organic matter– Zone of eluviation and leaching

– The O, A, E, and B horizons together are called the solum (“true soil”)

• B horizon – zone of accumulation• C horizon – partially altered parent material

Soil Profile

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Idealized soil profile

A

E

B

C

Parent

O

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Soil Profile showing horizons

AE

B

C

Parent

O

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Soil profile showing Solum

A

E

B

O

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Soil Development

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Soil TypesCharacteristics of each soil type primarily depend on prevailing climatic conditions

Climate conditions relate to vegetation

Three generic soil types• Pedalfer• Pedocal• Laterite

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Soil Types• Pedalfer

– Humid (>63 cm rainfall), temperate– Forest vegetation – Fe oxides and Al-rich clays in the B horizon

• Pedocal– Arid (<63 cm rainfall), temperate– Dry grasslands and brush vegetation – High accumulations of calcium carbonate

• Laterite– Tropical climates, hot and wet– Lush grasslands and trees vegetation– Intense chemical weathering

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Soil Types

Pedalfer Pedocol Laterite

Humid Arid Tropical

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Soil ErosionConstant recycling of Earth materials, part of rock cycle

Water and wind are powerful erosion forces that move soil components

Raindrop like tiny bomb to soil particles• sheet erosion: thin sheets of water

• rills: tiny channels

• gullies: deeper cuts

Sediment: soil that reaches a stream

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Soil Erosion• Natural rates of soil erosion depend on

– Soil characteristics

– Climate

– Slope

– Type of vegetation

• In many regions, rate of soil erosion is significantly > rate of soil formation

• Sedimentation and chemical pollution– Related to excessive soil erosion

– Occasionally soil particles are contaminated with pesticides, industrial pollutants etc.

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Soil ErosionGood soil development critical for agriculture and forestry

Agricultural soil conservation measuresPlanting trees as windbreaks

Plowing hill contours and terracing

Crop rotation

Forestry soil conservation measuresEliminate clear cutting (selective harvest)

Careful design of logging roads

Harvest away from drainage area