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GEOL 101 Introductory Geology. Chapter 5 Weathering and Soil. Earth’s external processes. Weathering – the physical breakdown (disintegration) and chemical alteration (decomposition) or rock at or near Earth’s surface

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Chapter 5 Weathering and Soil

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Chapter 5 weathering and soil

GEOL 101

Introductory Geology

Chapter 5Weathering and Soil

Earth s external processes

Earth’s external processes

  • Weathering – 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



  • Types of weathering

    • Mechanical weathering

    • Chemical weathering

    • Other function together

  • Weathering Processes

    • Dissolution

    • Oxidation

    • Hydrolysis

  • Weathering1

    Mechanical & chemical processes work together


    Solid rock

    Chemical weathering attacks susceptible minerals

    Rock crumbles by mechanical weathering

    Chemical/mechanical processes form clay and disperse minerals

    Mechanical weathering

    Mechanical Weathering

    Increased surface area

    Types of weathering

    Types ofWeathering

    • 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

    Frost wedging

    Frost Wedging

    Unloading exfoliation

    Unloading - Exfoliation



    Thermal expansion

    Thermal Expansion



    Chemical weathering

    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)

    Chemical weathering processes

    Chemical Weathering Processes

    • Dissolution

      • 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

  • Simple dissolution

    Simple Dissolution

    Halite (salt) dissolving in water

    Dissolution by acidic water

    Dissolution by Acidic Water

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

    Calcium Carbonate + Acidic Water

    CaCO3 + 2[H+(H2O)]

    Ca2+ + CO2 + 3(H2O)

    Soluble Calcium + Carbon Dioxide + Water

    • Acid rain caused by air pollution


    • 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

    4Fe3+ + 3O22-2Fe2O3

    Hematite (iron oxide)



    • Silicate minerals decomposed by water due to hydrolysis

    • Produces clay, soluble salt, silica

    Potassium Feldspar + Carbonic Acid + Water

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

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

    Kaolinite + Potassium + Bicarbonate + Silica


    in solution

    Chapter 5 weathering and soil

    Products Weathering

    Chemical weathering alternations

    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

    Weathering factors

    Weathering Factors

    • Rates 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

  • Chemical weathering rate variation by rock type

    Chemical WeatheringRate variation by rock type



    Weathering rates


    • organic activity - organic acids

      • raises wtr rates by orders of magnitude

    • climate - temp and amount of water

    • water flow through rock

    • bedrock composition

    • topography: groundwater flow

    • time - to equilibrium?

    Rock characteristics

    Rock Characteristics

    Bowen’s Reaction Series

    Weathering processes

    Weathering Processes

    • Reaction rates and weathering susceptibilities approximate a “backwards Bowen’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

    Differential weathering

    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

    Differential weathering1

    Differential Weathering

    Differential weathering2

    Controlled by jointing patterns

    Differential weathering

    Joint controlled weathering in igneous rocks

    Joint-controlled weathering in igneous rocks

    Chapter 5 weathering and 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)

    Typical soil components

    Typical soil components

    Chapter 5 weathering and soil


    • Soil Formation

    • Soil Profile

    • Soil Types

    • Soil Erosion

    Soil formation

    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

  • Soil formation1

    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

    Soil formation2

    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

    Soil formation3

    Soil Formation

    Soil development variations due to topography

    Soil development variations due to topography

    Soil profile

    Soil Profile

    • Soil forming processes operate from the surface downward

    • Vertical differences are called horizons – zones or layers of soil

    Chapter 5 weathering and soil

    Soil Profile

    • 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

    Idealized soil profile

    Idealized soil profile







    Soil profile showing horizons

    Soil Profile showing horizons







    Soil profile showing solum

    Soil profile showing Solum





    Soil development

    Soil Development

    Soil types

    Soil Types

    • Characteristics of each soil type primarily depend on prevailing climatic conditions

    • Climate conditions relate to vegetation

    • Three generic soil types

      • Pedalfer

      • Pedocal

      • Laterite

    Soil types1

    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

    Soil types2

    Soil Types







    Soil erosion

    Soil Erosion

    • Constant 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

  • Soil erosion1

    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.

    Soil erosion2

    Soil Erosion

    • Good soil development critical for agriculture and forestry

    • Agricultural soil conservation measures

      • Planting trees as windbreaks

      • Plowing hill contours and terracing

      • Crop rotation

    • Forestry soil conservation measures

      • Eliminate clear cutting (selective harvest)

      • Careful design of logging roads

      • Harvest away from drainage area

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