Geology

1. WEATHERINGWEATHERING
CHAPTER 6CHAPTER 6

2. The Rock CycleThe Rock Cycle
 SedimentarySedimentary
Rocks areRocks are
formed fromformed from
the lithificationthe lithification
of sediments.of sediments.

3. The Sediments that Make upThe Sediments that Make up
Sedimentary Rocks areSedimentary Rocks are
Produced by:Produced by:
Mechanical & Chemical Weathering
Weathering isWeathering is
the response ofthe response of
Earth’sEarth’s
materials to amaterials to a
changingchanging
environment.environment.

4. WeatheringWeathering
 ChemicalChemical alterationalteration
(decomposition) of(decomposition) of
rock at or near therock at or near the
Earth's surface.Earth's surface.
 PhysicalPhysical breakdownbreakdown
(disintegration)(disintegration)

5. Products ofProducts 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 MineralsClay Minerals
 Bentonite, MontmorilloniteBentonite, Montmorillonite

6. ErosionErosion
 IncorporationIncorporation andand transportationtransportation (physical(physical
removal) of material by mobile agents underremoval) of material by mobile agents under
the influence of gravity.the influence of gravity.
 Mobile agents include:Mobile agents include:
 waterwater
 windwind
 iceice

7. Mass WastingMass Wasting
 Rapid transfer of rock material downslopeRapid transfer of rock material downslope
under the influence of gravity.under the influence of gravity.

8. Mechanical WeatheringMechanical Weathering

9. Mechanical WeatheringMechanical Weathering
 Mechanical WeatheringMechanical Weathering – physical– physical
forces that break rock into smallerforces that break rock into smaller
pieces without altering the rock’spieces without altering the rock’s
mineral composition.mineral composition.
 Produces lithic fragments.Produces lithic fragments.
 Aids further breakdown byAids further breakdown by chemicalchemical
weatheringweathering..

10. Mechanical WeatheringMechanical Weathering
 Types of Mechanical Weathering:Types of Mechanical Weathering:
 Frost WedgingFrost Wedging
 UnloadingUnloading
 Thermal ExpansionThermal Expansion
 Biological ActivityBiological Activity
 Root WedgingRoot Wedging
 BurrowingBurrowing
 Decaying OrganismsDecaying Organisms
 Human ActivitiesHuman Activities

11. Frost WedgingFrost Wedging
 Freeze-ThawFreeze-Thaw
Cycles:Cycles:
 Liquid water expandsLiquid water expands
approximately 9%approximately 9%
upon freezing.upon freezing.
 Freezing water exertsFreezing water exerts
outward pressure onoutward pressure on
the walls of a confinedthe walls of a confined
space.space.
 Expands and enlargesExpands and enlarges
fractures in rock.fractures in rock.
 Example – potholes inExample – potholes in
roadways.roadways.

12. Frost WedgingFrost Wedging

13. Frost WedgingFrost Wedging
Insert Animation # 65:Insert Animation # 65:
Physical WeatheringPhysical Weathering

14. Frost WedgingFrost Wedging
 Freeze-Thaw Cycles:Freeze-Thaw Cycles:
 Aided by regular patterns ofAided by regular patterns of
jointsjoints ((fracturesfractures in rock alongin rock along
which no movement occurs).which no movement occurs).
 Joints are produced duringJoints are produced during::
 magma contractionmagma contraction duringduring
cooling/crystallization,cooling/crystallization,
 unloadingunloading, and, and
 tectonic eventstectonic events (tensional or(tensional or
compressional).compressional).
 Joints alsoJoints also allow water toallow water to
penetrate to depthpenetrate to depth and allowand allow
the process of weathering tothe process of weathering to
begin before the rock isbegin before the rock is
exposed.exposed.

15. UnloadingUnloading
 Large masses ofLarge masses of
igneous bodiesigneous bodies
(particularly granite)(particularly granite)
exposed by erosion.exposed by erosion.
 Great reduction inGreat reduction in
pressure from thepressure from the
removal of overlyingremoval of overlying
rocksrocks ((unloadingunloading).).
 Causes outer layers toCauses outer layers to
expand and separateexpand and separate
from the remainingfrom the remaining
igneous body.igneous body.
 FormingForming expansionexpansion
fracturesfractures (tensional(tensional
jointsjoints).).

16.  Expansion fractures generate onion-likeExpansion fractures generate onion-like
layers of concentric slabs (layers of concentric slabs (sheetingsheeting))
that separate andthat separate and spallspall off formingoff forming
exfoliation domesexfoliation domes..
SheetingSheeting

17. ExfoliationExfoliation
DomesDomes

18. Thermal ExpansionThermal Expansion
 Repeated heating and cooling.Repeated heating and cooling.
 CausesCauses expansionexpansion andand contractioncontraction ofof
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 ofNot a significant form of
mechanical weathering.mechanical weathering.
 Example: Glass breaks ifExample: Glass breaks if
cooled to quickly aftercooled to quickly after
being hot and vise versa.being hot and vise versa.

19. Biological ActivityBiological Activity
 Weathering accomplished by theWeathering accomplished by the
activities of organisms including:activities of organisms including:
 plantsplants,,
 burrowing animalsburrowing animals,,
 decaying organismsdecaying organisms, and, and
 humanshumans..

20. BiologicalBiological
ActivityActivity
 Plant roots in searchPlant roots in search
of nutrients growof nutrients grow
into fractures andinto fractures and
wedge them apartwedge them apart
((root wedgingroot wedging).).

21.  Burrowing animalsBurrowing animals bring fresh materialbring fresh material
to the surface where it will be moreto the surface where it will be more
quickly weathered.quickly weathered.
Biological ActivityBiological Activity
www.prairiedoglover.com/relocation.htm
www.coralsprings.org/.../gopher.cfm

22.  Decaying organismsDecaying organisms produce acids (humicproduce acids (humic
acids) that enhance chemical weathering.acids) that enhance chemical weathering.
Biological ActivityBiological 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
www.spiritlightphotography.com/Gallery1.asp
www.mobot.org/.../cordillera/images.shtml

23.  HumanHuman construction,construction,
mining, agricultural,mining, agricultural,
deforestationdeforestation
activities.activities.
BiologicalBiological
ActivityActivity

24. Chemical WeatheringChemical Weathering

25. Chemical WeatheringChemical Weathering
 Chemical transformation (decomposition)Chemical transformation (decomposition)
of unstableof unstable minerals at or near the Earth’sminerals at or near the Earth’s
surface.surface.
 Residual MaterialsResidual Materials::
 Formation of one or more newFormation of one or more new residual mineralsresidual minerals thatthat
areare stablestable in surface environments.in surface environments.
 Stable MineralsStable Minerals –– Quartz, Orthoclase, MuscoviteQuartz, Orthoclase, Muscovite
 Rust MineralsRust Minerals –– Hematite, GoertiteHematite, Goertite
 Clay MineralsClay 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.

26. Chemical WeatheringChemical Weathering
 Most important agent isMost important agent is waterwater
 Responsible forResponsible for transport of ionstransport of ions andand
molecules involved inmolecules involved in chemicalchemical
processesprocesses..
 Carries naturally occurringCarries naturally occurring acidsacids..

27. Chemical WeatheringChemical Weathering
 Types of Chemical Weathering:Types of Chemical Weathering:
 DissolutionDissolution
 OxidationOxidation
 HydrolysisHydrolysis

28. DissolutionDissolution
 Minerals that are dissolved in water.Minerals that are dissolved in water.
 Aided by small amounts ofAided by small amounts of acidacid in water.in water.
 Various organic acids areVarious organic acids are
released into the soil byreleased into the soil by
decaying organisms –decaying organisms – HumicHumic
AcidsAcids..
 Sulfuric AcidSulfuric Acid produced byproduced by
weathering of pyrite.weathering of pyrite.
 Carbonic AcidCarbonic Acid is produced whenis produced when
COCO22 in the atmosphere isin the atmosphere is
dissolved into precipitation (maydissolved into precipitation (may
be enhanced by CObe enhanced by CO22 in soil).in soil).
 CaCOCaCO33 + (H+ (H++
+ HCO+ HCO33
--
)) ⇒⇒ CaCa2+2+
+ 2HCO+ 2HCO33
--
 calcite + carbonic acidcalcite + carbonic acid ⇒⇒ calcium ion + bicarbonate ioncalcium ion + bicarbonate ion

29. OxidationOxidation
 Any reaction when electrons areAny reaction when electrons are
lost from one element.lost from one element.
 Important in theImportant in the decomposingdecomposing
ferromagnesian silicate mineralsferromagnesian silicate minerals
– olivine, pyroxene, and– olivine, pyroxene, and
hornblende.hornblende.
 Oxidation can only occur afterOxidation can only occur after
the iron is freed from the silicatethe iron is freed from the silicate
structure bystructure by hydrolysishydrolysis..
 4Fe4Fe+2 (Ferrous Iron)+2 (Ferrous Iron)
+ 3O+ 3O22 ⇒⇒ 2Fe2Fe+3 (Ferric Iron)+3 (Ferric Iron)
22OO33
 iron + oxygeniron + oxygen ⇒⇒ iron oxide (iron oxide (hematite and limonitehematite and limonite))

30. 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

31. HydrolysisHydrolysis
 The reaction of any substance withThe reaction of any substance with
water.water.
 Water molecules dissociateWater molecules dissociate to form hydrogen (Hto 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) + H22OO ⇒⇒ AlAl22SiSi22OO55(OH)(OH)44 +2K+2K++
+ 2HCO+ 2HCO33
--
+ 4SiO+ 4SiO22
 Potassium Feldspar + carbonic acid + waterPotassium Feldspar + carbonic acid + water ⇒⇒ kaolinite (residualkaolinite (residual
clay) + [potassium ion + bicarbonate ion + silica (in solution)]clay) + [potassium ion + bicarbonate ion + silica (in solution)]

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

34.  RoundingRounding of corners or edgesof corners or edges
((spheroidal weatheringspheroidal weathering).).
Successive shells loosening as the weathering processSuccessive shells loosening as the weathering process
continues to penetrate deeper into the rock.continues to penetrate deeper into the rock.
Physical Alterations CausedPhysical Alterations Caused
by Chemical Weatheringby Chemical Weathering

35. Factors Influencing Rates ofFactors 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 CharacteristicsRock Characteristics
 Mineral composition and stabilityMineral composition and stability
 Mineral solubilityMineral solubility
 Fractures, bedding planes, voidsFractures, bedding planes, voids
 ClimateClimate
 TopographyTopography
 Differential WeatheringDifferential Weathering

36. Increase in Surface AreaIncrease in Surface Area

37. Rock CharacteristicsRock Characteristics
 Mineral composition and stabilityMineral composition and stability
 Rates of chemical weathering of silicate minerals from leastRates of chemical weathering of silicate minerals from least
resistant to most resistantresistant 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 theSilicate minerals decompose in the same order as their ordersame order as their order
of crystallization due to the number of Si-O bonds within theof crystallization due to the number of Si-O bonds within the
silicate structuresilicate structure..

38. 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) readilyRocks containing calcite (marble and limestone) readily
dissolve in weakly acidic solutions = high solubilitydissolve in weakly acidic solutions = high solubility

39. MineralMineral
StabilityStability

40. ClimateClimate
 Warm temperatures and abundant moistureWarm temperatures and abundant moisture
promote mechanical and chemical weathering.promote mechanical and chemical weathering.
 Climates withClimates with frequent freeze-thaw cyclesfrequent freeze-thaw cycles
promote mechanical weathering.promote mechanical weathering.
 Temperature and moisture dictateTemperature and moisture dictate vegetationvegetation
type and amounttype and amount..
 Amount of decaying organic matter producingAmount of decaying organic matter producing
carboniccarbonic andand humic acidshumic acids..
 Human activitiesHuman activities can affect the composition ofcan affect the composition of
the atmosphere impacting rates of chemicalthe atmosphere impacting rates of chemical
weathering (increased carbon dioxide andweathering (increased carbon dioxide and
sulfur and nitrogen oxides –sulfur and nitrogen oxides – acid rainacid rain).).

41. TopographyTopography
 Weathering andWeathering and
erosion increases aserosion increases as
elevation increases.elevation increases.

42. Differential WeatheringDifferential Weathering
 Rocks do not weather uniformly due toRocks do not weather uniformly due to
regional and local factors.regional and local factors.
 Results in many unusual and spectacularResults in many unusual and spectacular
rock formations and landforms.rock formations and landforms.
DifferentialDifferential
Weathering –Weathering –
Bryce Canyon,Bryce Canyon,
National Park,National Park,
UtahUtah