Chapter 5 weathering and soil
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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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GEOL 101

Introductory Geology

Chapter 5Weathering and Soil

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

  • 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

    Increased surface area

    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

    Unloading - Exfoliation


    Thermal Expansion


    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

    • 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

    Halite (salt) dissolving in 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

    Products Weathering

    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

    • 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 WeatheringRate variation by rock type




    • 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

    Bowen’s Reaction Series

    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

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

    • Results in many unusual and spectacular rock formations and landforms

    Differential Weathering

    Controlled by jointing patterns

    Differential weathering

    Joint-controlled weathering in igneous rocks


    • 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


    • Soil Formation

    • Soil Profile

    • Soil Types

    • Soil Erosion

    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 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 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 Formation

    Soil development variations due to topography

    Soil Profile

    • Soil forming processes operate from the surface downward

    • Vertical differences are called horizons – zones or layers of 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







    Soil Profile showing horizons







    Soil profile showing Solum





    Soil Development

    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 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 Types







    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 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 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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