WEATHERINGWEATHERINGCHAPTER 6CHAPTER 6

The Rock CycleThe Rock Cycle

Sedimentary Sedimentary Rocks are Rocks are formed from formed from the lithification the lithification of sediments.of sediments.

The Sediments that Make up The Sediments that Make up Sedimentary Rocks are Sedimentary Rocks are

Produced by:Produced by:Mechanical & Chemical Weathering

Weathering is Weathering is the response the response of Earth’s of Earth’s materials to a materials to a changing changing environment.environment.

WeatheringWeathering

Chemical Chemical alteration alteration (decomposition) of (decomposition) of rock at or near the rock at or near the Earth's surface. Earth's surface.

Physical Physical breakdown breakdown (disintegration)(disintegration)

Products of Products of WeatheringWeathering

Lithic (Rock) FragmentsLithic (Rock) Fragments(granite, basalt, schist, etc.)(granite, basalt, schist, etc.)

Dissolved IonsDissolved Ions(Calcium, Potassium, Sodium, etc.)(Calcium, Potassium, Sodium, etc.)

Residual Minerals:Residual Minerals: Stable MineralsStable Minerals

Quartz, Orthoclase, MuscoviteQuartz, Orthoclase, Muscovite Rust MineralsRust Minerals

Hematite, GoertiteHematite, Goertite Clay Minerals Clay Minerals

Bentonite, MontmorilloniteBentonite, Montmorillonite

ErosionErosion

IncorporationIncorporation and and transportationtransportation (physical (physical removal) of material by mobile agents under removal) of material by mobile agents under the influence of gravity.the influence of gravity. Mobile agents include:Mobile agents include:

water water windwind iceice

Mass WastingMass Wasting Rapid transfer of rock material downslope Rapid transfer of rock material downslope

under the influence of gravity.under the influence of gravity.

Mechanical WeatheringMechanical Weathering

Mechanical Weathering Mechanical Weathering

Mechanical WeatheringMechanical Weathering – physical – physical forces that break rock into smaller forces that break rock into smaller pieces without altering the rock’s pieces without altering the rock’s mineral composition.mineral composition.

Produces lithic fragments.Produces lithic fragments. Aids further breakdown by Aids further breakdown by chemical chemical

weatheringweathering..

Mechanical Weathering Mechanical Weathering Types of Mechanical Weathering:Types of Mechanical Weathering:

Frost WedgingFrost Wedging Unloading Unloading Thermal ExpansionThermal Expansion Biological ActivityBiological Activity

Root WedgingRoot Wedging BurrowingBurrowing Decaying OrganismsDecaying Organisms Human ActivitiesHuman Activities

Frost Wedging Frost Wedging Freeze-Thaw Freeze-Thaw

Cycles:Cycles: Liquid water expands Liquid water expands

approximately 9% approximately 9% upon freezing.upon freezing.

Freezing water exerts Freezing water exerts outward pressure on outward pressure on the walls of a confined the walls of a confined space.space.

Expands and enlarges Expands and enlarges fractures in rock.fractures in rock.

Example – potholes in Example – potholes in roadways.roadways.

Frost WedgingFrost Wedging

Frost Wedging Frost Wedging

Insert Animation # 65: Insert Animation # 65: Physical WeatheringPhysical Weathering

Frost Wedging Frost Wedging Freeze-Thaw Cycles:Freeze-Thaw Cycles:

Aided by regular patterns of Aided by regular patterns of jointsjoints ( (fracturesfractures in rock along in rock along which no movement occurs).which no movement occurs).

Joints are produced duringJoints are produced during: : magma contraction magma contraction during during

cooling/crystallization,cooling/crystallization, unloadingunloading, and , and tectonic eventstectonic events (tensional or (tensional or

compressional). compressional). Joints also Joints also allow water to allow water to

penetrate to depthpenetrate to depth and allow and allow the process of weathering to the process of weathering to begin before the rock is begin before the rock is exposed.exposed.

Unloading Unloading Large masses of Large masses of

igneous bodies igneous bodies (particularly granite) (particularly granite) exposed by erosion.exposed by erosion.

Great reduction in Great reduction in pressure from the pressure from the removal of overlying removal of overlying rocksrocks ( (unloadingunloading). ).

Causes outer layers to Causes outer layers to expand and separateexpand and separate from the remaining from the remaining igneous body.igneous body.

Forming Forming expansion expansion fractures fractures (tensional (tensional jointsjoints).).

Expansion fractures generate onion-like Expansion fractures generate onion-like layers of concentric slabs (layers of concentric slabs (sheetingsheeting) ) that separate and that separate and spallspall off forming off forming exfoliation domesexfoliation domes..

Sheeting Sheeting

Exfoliation Exfoliation DomesDomes

Thermal Expansion Thermal Expansion Repeated heating and cooling.Repeated heating and cooling. Causes Causes expansionexpansion and and contraction contraction of of

minerals with different expansion rates.minerals with different expansion rates. Exerts stress on the rocks outer shell.Exerts stress on the rocks outer shell.

Not a significant form of Not a significant form of mechanical weathering.mechanical weathering.

Example: Glass breaks if Example: Glass breaks if cooled to quickly after cooled to quickly after being hot and vise versa.being hot and vise versa.

Biological Activity Biological Activity

Weathering accomplished by the Weathering accomplished by the activities of organisms including: activities of organisms including: plantsplants, , burrowing animalsburrowing animals, , decaying organismsdecaying organisms, and , and humanshumans..

Biological Biological ActivityActivity

Plant roots in search Plant roots in search of nutrients grow of nutrients grow into fractures and into fractures and wedge them apart wedge them apart ((root wedgingroot wedging).).

Burrowing animalsBurrowing animals bring fresh material bring fresh material to the surface where it will be more to the surface where it will be more quickly weathered.quickly weathered.

Biological Activity Biological Activity

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Decaying organismsDecaying organisms produce acids (humic produce acids (humic acids) that enhance chemical weathering.acids) that enhance chemical weathering.

Biological Activity Biological Activity

The "black" water that drains from the sandstone rocks of the Cordillera del Cóndor is actually the color of strong tea -- from the tannins and humic acids in the water leached from the surrounding vegetation.

Decaying Leaves

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HumanHuman construction, construction, mining, agricultural, mining, agricultural, deforestation deforestation activities.activities.

Biological Biological Activity Activity

Chemical WeatheringChemical Weathering

Chemical Weathering Chemical Weathering Chemical transformation (decomposition) Chemical transformation (decomposition)

of unstable of unstable minerals at or near the Earth’s minerals at or near the Earth’s surface.surface. Residual MaterialsResidual Materials::

Formation of one or more new Formation of one or more new residual mineralsresidual minerals that that are are stablestable in surface environments. in surface environments.

Stable MineralsStable Minerals – – Quartz, Orthoclase, MuscoviteQuartz, Orthoclase, Muscovite Rust Minerals Rust Minerals –– Hematite, GoertiteHematite, Goertite Clay Minerals Clay Minerals –– Bentonite, MontmorilloniteBentonite, Montmorillonite

Ions dissolved in solutionIons dissolved in solution released to the environment. released to the environment. Calcium, Potassium, Sodium, etc.Calcium, Potassium, Sodium, etc.

Chemical Weathering Chemical Weathering Most important agent is Most important agent is waterwater

Responsible for Responsible for transport of ionstransport of ions and and molecules involved in molecules involved in chemical chemical processesprocesses..

Carries naturally occurring Carries naturally occurring acidsacids..

Chemical Weathering Chemical Weathering

Types of Chemical Weathering:Types of Chemical Weathering: DissolutionDissolution Oxidation Oxidation HydrolysisHydrolysis

Dissolution Dissolution Minerals that are dissolved in water.Minerals that are dissolved in water.

Aided by small amounts of Aided by small amounts of acidacid in water. in water. Various organic acids are Various organic acids are

released into the soil by released into the soil by decaying organisms – decaying organisms – Humic Humic AcidsAcids..

Sulfuric AcidSulfuric Acid produced by produced by weathering of pyrite.weathering of pyrite.

Carbonic AcidCarbonic Acid is produced when is produced when COCO22 in the atmosphere is in the atmosphere is

dissolved into precipitation (may dissolved into precipitation (may be enhanced by CObe enhanced by CO22 in soil). in soil).

CaCOCaCO33 + (H + (H++ + HCO + HCO33--) ) Ca Ca2+2+ + 2HCO + 2HCO33

--

calcite + carbonic acid calcite + carbonic acid calcium ion + bicarbonate ion calcium ion + bicarbonate ion

OxidationOxidation Any reaction when electrons are Any reaction when electrons are

lost from one element.lost from one element.

Important in the Important in the decomposing decomposing ferromagnesian silicate mineralsferromagnesian silicate minerals – olivine, pyroxene, and – olivine, pyroxene, and hornblende.hornblende.

Oxidation can only occur after Oxidation can only occur after the iron is freed from the silicate the iron is freed from the silicate structure by structure by hydrolysishydrolysis..

4Fe4Fe+2 (Ferrous Iron)+2 (Ferrous Iron) + 3O + 3O22 2Fe 2Fe+3 (Ferric Iron)+3 (Ferric Iron)22OO3 3

iron + oxygen iron + oxygen iron oxide ( iron oxide (hematite and limonitehematite and limonite))

Oxidation of pyrite yields (HOxidation of pyrite yields (H22SOSO44) sulfuric acid ) sulfuric acid

and iron oxide [FeO(OH)] (limonite) – and iron oxide [FeO(OH)] (limonite) – acid mine drainageacid mine drainage

Hydrolysis Hydrolysis The reaction of any substance with The reaction of any substance with

water.water. Water molecules dissociateWater molecules dissociate to form hydrogen (H to form hydrogen (H--) )

and hydroxyl (OHand hydroxyl (OH--) ions.) ions. Hydrogen ion attacks and replaces other ions.Hydrogen ion attacks and replaces other ions. SilicatesSilicates are decomposed primarily by this process. are decomposed primarily by this process.

2KAlSi2KAlSi33OO88 + 2(H + 2(H++ + HCO + HCO33--) + H) + H22O O Al Al22SiSi22OO55(OH)(OH)44 +2K +2K++ + 2HCO + 2HCO33

-- + 4SiO + 4SiO22

Potassium Feldspar + carbonic acid + water Potassium Feldspar + carbonic acid + water kaolinite (residual kaolinite (residual clay) + [potassium ion + bicarbonate ion + silica (in solution)]clay) + [potassium ion + bicarbonate ion + silica (in solution)]

A conceptual diagram showing how the weathering of A conceptual diagram showing how the weathering of granitic rock yields quartz grains for quartz sandstone, clay granitic rock yields quartz grains for quartz sandstone, clay for shale, and calcium for limestone.for shale, and calcium for limestone.

RoundingRounding of corners or edges of corners or edges ((spheroidal weatheringspheroidal weathering).).

Successive shells loosening as the weathering process Successive shells loosening as the weathering process continues to penetrate deeper into the rock.continues to penetrate deeper into the rock.

Physical Alterations Caused Physical Alterations Caused by Chemical Weatheringby Chemical Weathering

Factors Influencing Rates of Factors Influencing Rates of WeatheringWeathering

(or rate of response of Earth’s (or rate of response of Earth’s materials to a changing environment)materials to a changing environment)

Surface AreaSurface Area Rock Characteristics Rock Characteristics

Mineral composition and stabilityMineral composition and stability Mineral solubility Mineral solubility Fractures, bedding planes, voids Fractures, bedding planes, voids

ClimateClimate TopographyTopography Differential WeatheringDifferential Weathering

Increase in Surface AreaIncrease in Surface Area

Rock CharacteristicsRock Characteristics Mineral composition and stability Mineral composition and stability

Rates of chemical weathering of silicate minerals from least Rates of chemical weathering of silicate minerals from least resistant to most resistant resistant to most resistant follow Bowen's Reaction Seriesfollow Bowen's Reaction Series..

Minerals that formed at high temperature decompose fastest.Minerals that formed at high temperature decompose fastest. Silicate minerals decompose in the Silicate minerals decompose in the same order as their order same order as their order

of crystallization due to the number of Si-O bonds within the of crystallization due to the number of Si-O bonds within the silicate structuresilicate structure..

Rates of Chemical Weathering (Solubility) Rates of Chemical Weathering (Solubility) on Granite (left) vs. Limestone (right)on Granite (left) vs. Limestone (right)

Rocks containing calcite (marble and limestone) readily Rocks containing calcite (marble and limestone) readily

dissolve in weakly acidic solutions = high solubilitydissolve in weakly acidic solutions = high solubility

MineralMineralStabilityStability

ClimateClimate Warm temperatures and abundant moistureWarm temperatures and abundant moisture

promote mechanical and chemical weathering. promote mechanical and chemical weathering. Climates with Climates with frequent freeze-thaw cyclesfrequent freeze-thaw cycles

promote mechanical weathering.promote mechanical weathering. Temperature and moisture dictate Temperature and moisture dictate vegetation vegetation

type and amounttype and amount. . Amount of decaying organic matter producing Amount of decaying organic matter producing

carboniccarbonic and and humic acidshumic acids.. Human activitiesHuman activities can affect the composition of can affect the composition of

the atmosphere impacting rates of chemical the atmosphere impacting rates of chemical weathering (increased carbon dioxide and weathering (increased carbon dioxide and sulfur and nitrogen oxides – sulfur and nitrogen oxides – acid rainacid rain).).

TopographyTopography

Weathering and Weathering and erosion increases as erosion increases as elevation increases.elevation increases.

Differential WeatheringDifferential Weathering Rocks do not weather uniformly due to Rocks do not weather uniformly due to

regional and local factors.regional and local factors. Results in many unusual and spectacular Results in many unusual and spectacular

rock formations and landforms.rock formations and landforms.

Differential Differential Weathering – Weathering –

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