GEOLOGY 101GEOLOGY 101Today: Chapter 6Today: Chapter 6

““Weathering and Erosion”Weathering and Erosion”

Instructor:Instructor:Professor Matt FouchProfessor Matt Fouch

Email: Email: fouch101@asu.edufouch101@asu.edu

Office: PSF-540 965-9292Office: PSF-540 965-9292

Course Website: Course Website: http://fouch101.asu.eduhttp://fouch101.asu.edu

TA:TA:Ayelet BlattsteinAyelet Blattstein

Email: Email: ayelet@asu.eduayelet@asu.edu

Office: PSH-452Office: PSH-452

How do we move material How do we move material to lower elevations?to lower elevations?

• WeatheringWeathering– PhysicalPhysical – smaller pieces, but composition isn’t – smaller pieces, but composition isn’t

alteredaltered– ChemicalChemical – composition altered – composition altered

• ErosionErosion– Transfer of material by water, wind, iceTransfer of material by water, wind, ice

• Mass WastingMass Wasting– Transfer of rock & soil downhill via gravityTransfer of rock & soil downhill via gravity

WeatheringWeathering

• PhysicalPhysical and and chemicalchemical changes changes that occur in sediments and rocks that occur in sediments and rocks when they are exposed to the when they are exposed to the atmosphere and biosphereatmosphere and biosphere

• Not the same as erosion!Not the same as erosion!

Why Care About Weathering and Why Care About Weathering and Erosion?Erosion?

• Among other Among other reasons, they reasons, they produce produce soilsoil

• Important natural Important natural resourceresource– Supports plant life which Supports plant life which

support ussupport us– Acts as a storage site for Acts as a storage site for

COCO22

• Causes of landslidesCauses of landslides• Septic system designSeptic system design• Building foundation Building foundation

designdesign• Landfill designLandfill design• Climate historyClimate history

TheThe RockRock CycleCycle

What Controls What Controls Weathering?Weathering?

• Rock propertiesRock properties– Hardness, composition, solubility, zones of weaknessHardness, composition, solubility, zones of weakness

• ClimateClimate– Wide variations in temperature and moisture Wide variations in temperature and moisture

accelerate weatheringaccelerate weathering

• Soil and vegetationSoil and vegetation– Exposes rock to variations in moisture and chemistryExposes rock to variations in moisture and chemistry

• Length of exposureLength of exposure

Rock corners are eroded more rapidly than sides

(more surface area at corners)

Weathering Changes the Surface Weathering Changes the Surface Area to Volume RatioArea to Volume Ratio

Physical WeatheringPhysical Weathering

• Physical forces break rock into Physical forces break rock into smaller pieces w/out changing smaller pieces w/out changing mineral compositionmineral composition

Types of physical weatheringTypes of physical weathering• Frost wedgingFrost wedging• UnloadingUnloading• Expansion and contractionExpansion and contraction• Biological activityBiological activity

Repeated cycles of freezing & thawing can break rock into smaller fragments

How?How?

Water expands ~ 9% upon freezing(Other minerals also expand/contract)

Where?Where?

Regions w/ daily freeze/thaw cycles

Frost wedgingFrost wedging

Frost wedgingFrost wedging

Talus slope

Frost wedgingFrost wedging

Talus “cones”

Frost Frost wedgingwedging

Expansion of rock from removal/erosionof overlying rock (“sheeting”)

Why?Why?

Reduction in pressure (since lessoverlying weight)

Manifestation:Manifestation:

Slab-like layers/sheets break loose

UnloadingUnloading

Continued weathering causesrock slabs to separate & fall

HALF DOMEYOSEMITE NATIONAL PARK

(California)

“exfoliation domes”

UnloadingUnloading

Another “sheeting” example

““Devil’s Marbles” Devil’s Marbles” AustraliaAustralia

Spheroidal weatheringSpheroidal weathering

Expansion/contraction of rock

from

heating/cooling

Thermal expansionThermal expansion

Thermal expansion of rock can also occur without water or minerals present

Weathering of rock from activities of

organisms plants burrowing animals humans

Biological activityBiological activity

Chemical WeatheringChemical Weathering

• Processes that break rock Processes that break rock components and internal structures components and internal structures of minerals, making new mineralsof minerals, making new minerals

Ways to chemically weather rockWays to chemically weather rock• Oxidation (add oxygen)Oxidation (add oxygen)• Dissolution (dissolve)Dissolution (dissolve)• Hydration (add water)Hydration (add water)

Chemical WeatheringChemical Weathering

• All minerals are chemically All minerals are chemically unstable once removed from unstable once removed from where they were formedwhere they were formed

• Water is the primary agent of Water is the primary agent of chemical weatheringchemical weathering

Products of Chemical Products of Chemical WeatheringWeathering

• ClaysClays• OxidesOxides• SaltsSalts

– HaliteHalite

• Silica and quartz sandSilica and quartz sand

Chemical reaction that causes loss of electronsChemical reaction that causes loss of electrons

ExampleExample

Iron (Fe) + water (H2O) oxygen (O) combines with Fe to form hematite (Fe2O3)

Oxidation decomposes important minerals: mineral formula where

olivine (Mg,Fe)2SiO4 Earth’s mantle

pyroxene (Mg,Fe)SiO3 Earth’s mantle

hornblende Ca2(Fe,Mg)5Si8O22(OH)2 Continental rocks

OxidationOxidation

Dissolving minerals by aliquid agent (such as water)

Dissolving minerals by aliquid agent (such as water)

How?How?

Many minerals are water-soluble example: Halite (salt)

Adding acid (H+) increases corrosiveness example: carbon dioxide and rain

DissolutionDissolution

DissolutionDissolution

• Rainfall in the atmosphere produces Rainfall in the atmosphere produces carbonic acidcarbonic acid (H (H22COCO33) which can dissolve ) which can dissolve carbonates and decompose mineralscarbonates and decompose minerals– The pH of “natural” rain is 5.6 to 5.8The pH of “natural” rain is 5.6 to 5.8

• pH of 7 is neutralpH of 7 is neutral• pH of acid rain can be as low as 4.5pH of acid rain can be as low as 4.5

(10-15 times more acidic than natural rain)(10-15 times more acidic than natural rain)• pH of ketchup is ~ 3.9pH of ketchup is ~ 3.9• pH of CokepH of Coke®® is ~ 2.7 is ~ 2.7

The reaction of any substance w/ water.The reaction of any substance w/ water.

ExampleExample

Silicates primarily decomposed by hydration

Water molecules break down from H2O into H+ and (OH-)

HydrationHydration

Granite (mainly quartz + potassium feldspar) +carbonic acid (water + carbon dioxide)

Decomposition of graniteDecomposition of granite

Liberates potassium from feldspar - important plant nutrient

makes kaolinite - this left over mineral is clay - very stable - main inorganic part of soil

quartz - very stable, glassy appearance - carried to sea: beach sand, sand dunes - cements to form sandstone

Hydration ExampleHydration Example

Formation of SoilFormation of Soil

• Product of both physical and Product of both physical and chemical weatheringchemical weathering

Made of: decomposed rock + decayed animal/plant life (“humus”) + water + air

45%

5%

25%

25%

“good soil:”

Important Soil-Forming Important Soil-Forming FactorsFactors

• ClimateClimate– Temperature & precipitationTemperature & precipitation

• TimeTime– Longer time = thicker soilLonger time = thicker soil

• Plants/AnimalsPlants/Animals– Organic matterOrganic matter

• SlopeSlope– If too steep, little/no soilIf too steep, little/no soil

Soils and ClimateSoils and Climate

• Soil formation is directly linked to Soil formation is directly linked to climate that soil forms inclimate that soil forms in

Three major groupsThree major groups• LateriteLaterite

– Wet climateWet climate

• PedalferPedalfer– Temperate climateTemperate climate

• PedocalPedocal– Dry climateDry climate

Soil ProfileSoil Profile

LateriteLaterite

A horizonA horizon

C horizonC horizon

PedalferPedalfer

PedocalPedocal

Soil ErosionSoil Erosion

• The biggest problem facing the The biggest problem facing the developing world because of developing world because of deforestationdeforestation– Much of Madagascar’s soils has been lost Much of Madagascar’s soils has been lost

due to deforestationdue to deforestation

• Read Box 6.1 (pp. 132-133)Read Box 6.1 (pp. 132-133)