Modul 15 - Weathering

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Module 15Module 15

WeatheringWeathering


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The Dynamic EarthThe Dynamic Earth

WEATHERINGWEATHERING

Earth is very dynamicEarth is very dynamic

Temperature (T) and pressure (P) increase withTemperature (T) and pressure (P) increase with

increasing depth below Earthincreasing depth below Earths surfaces surface Tectonic activityTectonic activity upliftsuplifts rocks formed at higher Trocks formed at higher T

and P deep below Earthand P deep below Earths surface to regions ofs surface to regions oflower T and P closer to the surfacelower T and P closer to the surface

At the lower T and P at or near EarthAt the lower T and P at or near Earths surface, thes surface, theminerals composing the uplifted rocks:minerals composing the uplifted rocks:

Are unstableAre unstable

Are constantly exposed to agents ofAre constantly exposed to agents ofweathering, such as Oweathering, such as O22, acidic H, acidic H22O, rain, wind,O, rain, wind,ice, etc.ice, etc.

Are thus relentlessly destroyed by weathering,Are thus relentlessly destroyed by weathering,

erosion, and mass wastingerosion, and mass wasting


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Formation of Metamorphic RocksFormation of Metamorphic Rocks

Uplift and exposure

Weathering

All rocks exposedAll rocks exposed

at earthat earths surfaces surface

undergoundergo

Rocks CycleRocks Cycle


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Weathering, Erosion, and TransportationWeathering, Erosion, and Transportation


WeatheringWeathering

The group of processes that change rock at orThe group of processes that change rock at or

near Earthnear Earths surfaces surface

ErosionErosion

The removal of rock particles from their sourceThe removal of rock particles from their source

by flowing water, wind, or glacial iceby flowing water, wind, or glacial ice

TransportationTransportation

The movement of eroded particles by flowingThe movement of eroded particles by flowing

water, wind, or glacial icewater, wind, or glacial ice


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Types of WeatheringTypes of Weathering

Chemical weatheringChemical weathering

Changes the chemical composition of rocksChanges the chemical composition of rocks bybyremoving and/or adding ionsremoving and/or adding ions

Mechanical weatheringMechanical weathering Breaks rocks into smaller pieces withoutBreaks rocks into smaller pieces without

changing their compositionchanging their composition


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Chemical WeatheringChemical Weathering

Chemical weathering removes and/or adds ions byChemical weathering removes and/or adds ions by

dissolution, hydrolysis, and oxidationdissolution, hydrolysis, and oxidation DissolutionDissolution

Is removal soluble ions NaIs removal soluble ions Na++, K, K++, Ca, Ca 2+2+, Mg, Mg 2+2+,,

FeFe2+2+

, SiO, SiO22 Is enhanced by acids: HIs enhanced by acids: H22COCO33, H, H22SOSO44, HNO, HNO33, and, and

HClHCl

HydrolysisHydrolysis is addition of water as the OHis addition of water as the OH-- ionion

Forms clays from olivine, augite, hornblende,Forms clays from olivine, augite, hornblende,biotite, feldsparsbiotite, feldspars

Forms HForms H44SiOSiO44, a cementing agent, from quartz, a cementing agent, from quartz

OxidationOxidation is addition of oxygenis addition of oxygen

Iron + oxygen yields hematite (if dry), limoniteIron + oxygen yields hematite (if dry), limonite(if wet)(if wet)

Water is the most effective chemical weatheringWater is the most effective chemical weatheringagentagent


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Effects of Chemical WeatheringEffects of Chemical Weathering

ACID RAINACID RAIN

Carbon dioxideCarbon dioxide,, nitrogen dioxidenitrogen dioxide, and, and

sulfur dioxidesulfur dioxide produced by burning ofproduced by burning offossil fuels react with rain to formfossil fuels react with rain to form

carboniccarbonic ,, nitricnitric , and, and sulfuric acidssulfuric acids ,,

acid rainacid rain

Chemical WeatheringChemical Weathering

This statue is composed ofThis statue is composed ofmarblemarble

Marble is composed ofMarble is composed ofCaCOCaCO33,, which dissolved bywhich dissolved byacid solutionsacid solutions

The statueThe statues facial featuress facial featureswere dissolved by acid rainwere dissolved by acid rain


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Effects of Chemical WeatheringEffects of Chemical WeatheringChemical WeatheringChemical Weathering

Spheroidal weathering occursSpheroidal weathering occurs

because the corners and edges ofbecause the corners and edges ofrocks are more readily chemicallyrocks are more readily chemically

weathered than their flat sidesweathered than their flat sides

Spheroidally weathered graniteSpheroidally weathered granite

Spheroidal WeatheringSpheroidal Weathering

C


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In arid regions,In arid regions,

iron in the rocksiron in the rocks

reactsreacts

with Owith O22 to formto form

hematite, Fehematite, Fe22OO33, red, red

rustrust

Oxidation of IronOxidation of Iron

Ch i l W h iCh i l W th i


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Oxidation and hydrolysisOxidation and hydrolysisof pyrite in the rocksof pyrite in the rocks

produces redproduces red--coloredcolored

water rich in sulfuric acidwater rich in sulfuric acid

Acid mine drainage is aAcid mine drainage is a

big problem anywherebig problem anywhere

pyrite is present in rockspyrite is present in rocksexposed at the surface,exposed at the surface,

particularly around, coalparticularly around, coal

mines and gold minesmines and gold mines

2FeS2FeS22 + 7O+ 7O22 + 2H+ 2H22OO 2Fe2Fe++++ (aq) +(aq) + 4H4H++ (aq)(aq) + 4SO+ 4SO44

22-- (aq)(aq)

Acid Mine DrainageAcid Mine Drainage

Ch i l W th iCh i l W th i


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Intense hydrolysis ofIntense hydrolysis of

minerals by hot acidicminerals by hot acidic

groundwatergroundwatermovingmoving

through the bedrockthrough the bedrock

formed the clays in thisformed the clays in this

boilingboiling mudpotmudpot

Hydrolysis of Bedrock by Hydrothermal WaterHydrolysis of Bedrock by Hydrothermal Water



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Water percolatingWater percolating

through soils formsthrough soils forms

clays from theclays from the

feldsparsfeldspars

by hydrolysis andby hydrolysis and

carries away solublecarries away soluble

ions and silicaions and silica

Hydrolysis of Feldspars in SoilsHydrolysis of Feldspars in Soils



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Chemical weathering by organismChemical weathering by organism

Ch i l W th iChemical Weathering


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Negative charges on the flatNegative charges on the flat

surfaces of clay mineralssurfaces of clay minerals

attract positive ends of waterattract positive ends of water

molecules, which in turnmolecules, which in turn

Keeps the soil moistKeeps the soil moist

Enables plants to absorbEnables plants to absorbmoisture and exchangemoisture and exchange

ions with the soilions with the soil

Hydrolysis of Feldspars in SoilsHydrolysis of Feldspars in Soils

Mechanical WeatheringMechanical Weathering


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Mechanical weathering breaks rock into smallerMechanical weathering breaks rock into smallerpieces without changing the compositionpieces without changing the composition

Mechanical weathering is caused byMechanical weathering is caused by

Frost wedging:Frost wedging: Expansion of water duringExpansion of water duringfreezingfreezing

Unloading:Unloading: Pressure reduction due to removalPressure reduction due to removalof overburden causes formation of sheet jointsof overburden causes formation of sheet joints

and exfoliation domesand exfoliation domesThermal expansion or contraction:Thermal expansion or contraction: ExtremeExtreme

changes in temperature cause cracks to form inchanges in temperature cause cracks to form in

rocksrocksOrganic fracturing:Organic fracturing: Due to root wedging,Due to root wedging,

burrowing by animals, mining activities, etc.burrowing by animals, mining activities, etc.

Erosional agents:Erosional agents: moving water, wind, icemoving water, wind, ice



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Rain water enters joints,Rain water enters joints,

cracks in the rockscracks in the rocks

As the water freezes (formsAs the water freezes (formsfrost) it expands, wedges thefrost) it expands, wedges the

cracks further open, makescracks further open, makes

them widerthem wider

Frost WedgingFrost Wedging

M h i l W th iMechanical Weathering


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Weight of the overburdenWeight of the overburdenOverburden

Pluton

(the crust and soil above(the crust and soil above

the pluton) exerts greatthe pluton) exerts great

pressure on the plutonpressure on the pluton

keeps it from expandingkeeps it from expanding

UpliftUplift andand erosionerosion greatlygreatlyreduce pressure exertedreduce pressure exerted

on the top of the pluton,on the top of the pluton,

cause formation ofcause formation ofsheetPluton

sheet

jointsjoints ,, exfoliationexfoliation , and, andexfol iation domesexfol iation domes

UnloadingUnloading



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Sheet joints produced bySheet joints produced by

unloadingAn exfoliating granite domeAn exfoliating granite dome unloading

UnloadingUnloading



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Plants take advantage of cracks in rocks, wedge the cracks widerPlants take advantage of cracks in rocks, wedge the cracks wider

as their their roots grow largeras their their roots grow larger

Root WedgingRoot Wedging

Factors That Infl ence the Rate of WeatheringFactors That Influence the Rate of Weathering


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Factors That Influence the Rate of WeatheringFactors That Influence the Rate of Weathering

StructureStructure Structures such as fractures, foliation,Structures such as fractures, foliation,

cleavage, bedding enhance the rate at whichcleavage, bedding enhance the rate at whichrocks weatherrocks weather

Mineral compositionMineral composition Mafic minerals weather more rapidly than felsicMafic minerals weather more rapidly than felsic

mineralsminerals

ClimateClimate A warm moist climate most effectivelyA warm moist climate most effectively

enhances weathering: hydrolysis, dissolution,enhances weathering: hydrolysis, dissolution,oxidationoxidation

TopographyTopography Steep slopes weather less rapidly thanSteep slopes weather less rapidly than

horizontal surfaces because most of the rainhorizontal surfaces because most of the rainruns off instead of soaking inruns off instead of soaking in



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Presence ofPresence ofstructurestructure

increases theincreases the

surface area ofsurface area of

rocks, which inrocks, which inturn increases theturn increases the

rates at whichrates at which

they weatherthey weather

Effect of StructureEffect of Structure



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Mafic mineralsMafic minerals

are much moreare much more

susceptible tosusceptible to

oxidation,oxidation,

dissolution, anddissolution, andhydrolysis thanhydrolysis than

felsic mineralsfelsic minerals

Effect of Mineral CompositionEffect of Mineral Composition



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Marble (right, composed ofMarble (right, composed of

calcium carbonate) weatherscalcium carbonate) weathers

more readily than slate (left, richmore readily than slate (left, rich

in clays)

Mudstone (bottom, rich in clays)Mudstone (bottom, rich in clays)

weathers more readily thanweathers more readily than

sandstone (top, rich in quartz)sandstone (top, rich in quartz)

in clays)




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Where underlain by theWhere underlain by the

same rocksame rock--type, steeptype, steep

slopes weather more slowly,slopes weather more slowly,

are covered by thinner soils,are covered by thinner soils,

than horizontal surfaces

Exposed part of basalt flow 1 isExposed part of basalt flow 1 is

covered by a thicker layer of soilcovered by a thicker layer of soil

because it has been exposed tobecause it has been exposed to

soilsoil--forming processes longerforming processes longer

than flow 2 than horizontal surfacesthan flow 2

Time and TopographyTime and Topography

SOILSOIL


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SOILSOIL

SoilSoil A layer of weathered unconsolidated material consistingA layer of weathered unconsolidated material consisting

ofofmineral mattermineral matter,, organic matter (humus)organic matter (humus), and, and porepore

spacesspaces

LoamLoam

A fertile soil consisting of equal amounts ofA fertile soil consisting of equal amounts ofsandsand ,, siltsilt ,,

clay, andclay, and organic matterorganic matter

TopsoilTopsoil The darkThe dark--colored upper portion of a soilcolored upper portion of a soil

SubsoilSubsoil

Infertile stony organicInfertile stony organic--poor soil usually underlyingpoor soil usually underlyingtopsoiltopsoil

RegolithRegolith

Loose unconsolidated rock material resting on bedrockLoose unconsolidated rock material resting on bedrock

SOILSOILD d l hi


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Soil HorizonsSoil Horizons

O Organic matter

A Organic matter mixed withmineral matter

B Mineral matter mixed with fine claysand colloids washed down from

the top soil

C Rock fragments mechanicallyweathered from bedrock mixedwith partially decomposed rock

SOILSOIL

D Bedrock

Downward leaching

of ions and clays due

to percolating water


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Factors That Control Soil FormationFactors That Control Soil Formation

Composition of the bed rockComposition of the bed rockMafic rocks weather more rapidly than felsicMafic rocks weather more rapidly than felsic

rocksrocks

TimeTime Longer time leads to more soil formationLonger time leads to more soil formation

ClimateClimate

A warm moist climate is most effectiveA warm moist climate is most effective TopographyTopography

Horizontal surfaces weather more rapidlyHorizontal surfaces weather more rapidly

Plants and AnimalsPlants and Animals Plants supply nutrients, form acids, and fixPlants supply nutrients, form acids, and fix

nitrogennitrogen

Burrowing animals increase porosity, mixBurrowing animals increase porosity, mixmaterialsmaterials


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What will be happenedWhat will be happenedif no weatheringif no weathering

processprocessin our planet?in our planet?

Modul 15 - Weathering

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