geology 101 today: chapter 6 “weathering and erosion” instructor: professor matt fouch email:...
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GEOLOGY 101GEOLOGY 101Today: Chapter 6Today: Chapter 6
““Weathering and Erosion”Weathering and Erosion”
Instructor:Instructor:Professor Matt FouchProfessor Matt Fouch
Email: Email: [email protected]@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: [email protected]@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
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
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
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 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)