Weathering vs. Erosion What is weathering?What is weathering? –Breakdown of earth materials What...

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Weathering vs. ErosionWeathering vs. Erosion

• What is weathering?What is weathering?– Breakdown of earth materialsBreakdown of earth materials

• What is erosion?What is erosion?– Removal of earth material Removal of earth material

• GlaciersGlaciers• WindWind• WaterWater

Types of WeatheringTypes of Weathering

• There are two primary types of There are two primary types of weathering:weathering:– Physical – Breaking a material into Physical – Breaking a material into

smaller particlessmaller particles– Chemical – Changing the materials Chemical – Changing the materials

compositioncomposition

Physical WeatheringPhysical Weathering

• Physical or Physical or mechanical mechanical weatheringweathering– Breaking a rock or Breaking a rock or

mineral into mineral into smaller piecessmaller pieces

– No change in No change in compositioncomposition

• There are six types of physical There are six types of physical weatheringweathering– AbrasionAbrasion– Frost wedgingFrost wedging– Salt crackingSalt cracking– Organic activityOrganic activity– Pressure reliefPressure relief– Thermal expansionThermal expansion

1)1) Abrasion – Abrasion – – A rock is pelted by a stream of small A rock is pelted by a stream of small

particlesparticles– Particles carried by wind or waterParticles carried by wind or water– Think of sandblastingThink of sandblasting

2)2) Frost Frost wedging – wedging – – Water fills a crackWater fills a crack– Freezing water Freezing water

expands cracksexpands cracks– Destroys rockDestroys rock

3)3) Salt cracking – Salt cracking – – Similar to frost wedgingSimilar to frost wedging– Most common in coastal regionsMost common in coastal regions

4)4) Organic Organic activity – activity – – Animals burrowingAnimals burrowing– Plants grow and the Plants grow and the

roots spread roots spread

5)Pressure relief 5)Pressure relief cracking – cracking – – Depth equals Depth equals

higher pressurehigher pressure– Pressure makes a Pressure makes a

rock smallerrock smaller– Pressure is Pressure is

released, and the released, and the rock expands and rock expands and crackscracks

• Pressure relief Pressure relief (cont’d)(cont’d)– Cracks are called Cracks are called

jointsjoints– Joints - Joints - rocks rocks

separate but don’t separate but don’t movemove

• Pressure relief Pressure relief (cont’d)(cont’d)– Within igneous Within igneous

bodies, the joints bodies, the joints tend to form tend to form parallel to the parallel to the surfacesurface

– This is called This is called exfoliationexfoliation

6)6) Thermal expansion – Thermal expansion – – Heating and cooling = expansion and Heating and cooling = expansion and

contractioncontraction– Different minerals expand and contract Different minerals expand and contract

differentlydifferently– Weakens a rockWeakens a rock

Chemical WeatheringChemical Weathering

• Chemical weathering – Chemical weathering – – A rock is broken down and it’s A rock is broken down and it’s

composition changescomposition changes– Exposure to air or water initiate Exposure to air or water initiate

chemical changeschemical changes

• Chemical weathering can be broken Chemical weathering can be broken into a number of categoriesinto a number of categories– OxidationOxidation– HydrolysisHydrolysis– Weathering by solutionWeathering by solution

1)1) Oxidation – Oxidation – – Elements reacting with free oxygenElements reacting with free oxygen– Oxygen combines with elements, Oxygen combines with elements,

removing them from the mineralremoving them from the mineral– RustingRusting

• 4 Fe (+3) + 3 O4 Fe (+3) + 3 O22 (-4) = 2 Fe (-4) = 2 Fe22OO33 (Hematite) (Hematite)

• Oxidation (cont’d) Oxidation (cont’d) – – – Pyrite (FeSPyrite (FeS22) is ) is

commonly oxidized commonly oxidized when exposed to when exposed to oxygen at the oxygen at the surfacesurface

2)2) Hydrolysis – Hydrolysis – – Hydrogen ions (H+) replaces other Hydrogen ions (H+) replaces other

positively charged ions positively charged ions – Happens within slightly acidic waterHappens within slightly acidic water

• Hydrolysis (cont’d) – Hydrolysis (cont’d) – – Acidic water travels across a rock, Acidic water travels across a rock,

replacing cations with hydrogen replacing cations with hydrogen – Common reaction:Common reaction:

• Potassium feldspar + hydrogen ions + Potassium feldspar + hydrogen ions + water = Muscovite mica + potassium ions + water = Muscovite mica + potassium ions + dissolved silica dissolved silica

• 12H12H22O + O + 2H2H++ + 3KAlSi + 3KAlSi33OO88 = = KAlKAl33SiSi33OO1010(OH)(OH)22 + + 2K2K++ + 6H + 6H44SiOSiO44

3)3) Weathering by solution –Weathering by solution –– Also called dissolutioningAlso called dissolutioning– Ions pulled into solutionIons pulled into solution– Halite (salt) and calciteHalite (salt) and calcite

• Weathering by Weathering by solution (cont’d) –solution (cont’d) –– Water is dipolar Water is dipolar – Pure, neutral water Pure, neutral water

can weather by can weather by solutionsolution

• Weathering by Weathering by solution (cont’d) –solution (cont’d) –– Water attaches to Water attaches to

the ionsthe ions– The salt is The salt is

dissolveddissolved

• Weathering by solution (cont’d) –Weathering by solution (cont’d) –– Water isn’t normally neutralWater isn’t normally neutral– Natural water is usually slightly acidicNatural water is usually slightly acidic

– HH22O + COO + CO22 = H = H22COCO33 = H = H++ ion + HCO ion + HCO3)3)

• Excess HExcess H++

• Weathering by Weathering by solution (cont’d) –solution (cont’d) –– Carbonic acid Carbonic acid

reacts with calcite reacts with calcite (CaCO(CaCO33)) in in limestones limestones

– Calcite dissolves Calcite dissolves into solutioninto solution

– CaCOCaCO33 + H + H22COCO33 = = Ca + 2HCOCa + 2HCO33

• Acid Rain – Acid Rain – – Defined – Precipitation with a pH < 5.0Defined – Precipitation with a pH < 5.0– Excess acid rain can:Excess acid rain can:

• Rapidly weathers certain rock types Rapidly weathers certain rock types • Effects groundwater qualityEffects groundwater quality

• Acid rain (cont’d) – Acid rain (cont’d) – – Result of water and Result of water and

industrial wastes industrial wastes mixingmixing

– Wastes from Wastes from burning fossil fuelsburning fossil fuels

• Acid rain (cont’d) - Acid rain (cont’d) - – Waste products include:Waste products include:

• SO2SO2• NO2NO2

– These wastes create:These wastes create:• Nitric acid - Nitric acid - NONO22 + O + O22 + H + H22O = HNOO = HNO33

• Sulfuric acid -Sulfuric acid - SO SO22 + O + O22 + H + H22O = HO = H22SOSO44

• Acid rain (cont’d) –Acid rain (cont’d) –– Acid rain dissolves marble or limestone Acid rain dissolves marble or limestone

(weathering by solution)(weathering by solution)– Excess acid rain pollutes ponds, lakes Excess acid rain pollutes ponds, lakes

or reservoirs or reservoirs – Acid rain can leach into the Acid rain can leach into the

groundwatergroundwater

Rate of WeatheringRate of Weathering

• Variables influencing the rate of Variables influencing the rate of weathering include weathering include – ClimateClimate– Rock typeRock type– Particle sizeParticle size– Particle shapeParticle shape

1)1) Climate – Climate – – Very important Very important

consideration re: consideration re: weatheringweathering

– MoistureMoisture

Physical weathering

Chemical weathering

Cool, dry climate

Cool, moist climate x xWarm, dry climate xWarm, moist climate x x

2)2) Rock type – Rock type – – Different rock weather at different ratesDifferent rock weather at different rates– Granite weathers slowlyGranite weathers slowly

• Mineral strengthMineral strength

– Limestone weathers quicklyLimestone weathers quickly• CalciteCalcite

3)3) Particle size – Particle size – – Rock or particle size influences rate of Rock or particle size influences rate of

weatheringweathering– More surface area exposed = quicker More surface area exposed = quicker

weatheringweathering

• Particle size Particle size (cont’d) – (cont’d) – – Small particles Small particles

have more surface have more surface area exposed area exposed

– More surface area More surface area = faster weathering= faster weathering

4)4) Particle shape –Particle shape –– A particle with numerous edges or A particle with numerous edges or

corners will weather quicklycorners will weather quickly– A particle with few edges or corners A particle with few edges or corners

will weather slowlywill weather slowly

• Particle shape Particle shape (cont’d)(cont’d)– A corner weathers A corner weathers

quickestquickest– An edge weathers An edge weathers

slowerslower– A face weathers A face weathers

slowestslowest

Weathering vs. Erosion What is weathering?What is weathering? –Breakdown of earth materials What...

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