chapter 3- weathering processes two major categories mechanical (or physical) processes physical...
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Chapter 3- Weathering ProcessesTwo major categories
Mechanical (or Physical) processes
physical breakdown of larger particles into smaller particles
Chemical processes
Chemical reactions that facilitate soil profile development
Removal of chemical elements
Addition of altered chemical elements
Addition of new chemical elements
Chapter 3- Weathering ProcessesTwo major categories
Mechanical (or Physical) processes
physical breakdown of larger particles into smaller particles
Chemical processes
Chemical reactions that facilitate soil profile development
Removal of chemical elements
Addition of altered chemical elements
Addition of new chemical elements
Mechanical processes aid Chemical processes
Increases surface area to volume ratio
Chemical processes dominate soil development
Mechanical processes
breakage can occur along bedding planes
Along stress induced fractures or joints
Micro or macro scale
Along crystal boundaries
Mechanical processes aid Chemical processes
Increases surface area to volume ratio
Chemical processes dominate soil development
Mechanical processes
breakage can occur along bedding planes
Along stress induced fractures or joints
Micro or macro scale
Along crystal boundaries
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Pressure releaseCaused by unroofing of deeply buried rock- typically igneous, but can be any kind of rock
Pressure releaseCaused by unroofing of deeply buried rock- typically igneous, but can be any kind of rock
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Sheet joints due to pressure release create an exfoliation dome known as Enchanted Rock in Enchanted Rock State Park, Texas
Sheet joints due to pressure release create an exfoliation dome known as Enchanted Rock in Enchanted Rock State Park, Texas
Freeze ThawWater, when frozen, expands and can fracture rock if the cracks have the appropriate geometry and the climate is suitable.
Salt Crystal GrowthSimilar to freeze thaw fracturing, crystal growth occurs as chemicals dissolved in the water precipitate out as water evaporates- creating salt crystals that pry the rock apart
e.g., K-horizons with high percentage of salt in sandy soils push sand grains apart so they are held in salt suspension
Can also induce chemical reactions on mineral grains- chemical weathering
Freeze ThawWater, when frozen, expands and can fracture rock if the cracks have the appropriate geometry and the climate is suitable.
Salt Crystal GrowthSimilar to freeze thaw fracturing, crystal growth occurs as chemicals dissolved in the water precipitate out as water evaporates- creating salt crystals that pry the rock apart
e.g., K-horizons with high percentage of salt in sandy soils push sand grains apart so they are held in salt suspension
Can also induce chemical reactions on mineral grains- chemical weathering
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This boulder in Death Valley, California has disintegrated in situ due to saline waters entering fractures, and prying apart the cracks
Best developed early in pedogenesis
Thermal FracturingImplication of freeze thaw expansion and contraction due to changes in temperature and thermal properties of the parent materialAlso relates to fire related actions as well
Spalling occurs when fire superheats the surfaceAlso vaporizes water which exerts force
Can be influenced by micro-environmental conditions
Thermal FracturingImplication of freeze thaw expansion and contraction due to changes in temperature and thermal properties of the parent materialAlso relates to fire related actions as well
Spalling occurs when fire superheats the surfaceAlso vaporizes water which exerts force
Can be influenced by micro-environmental conditions
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Influence of fire and environmental conditions on weathering of bouldersInfluence of fire and environmental conditions on weathering of boulders
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Sand in the bottom of a 5 m deep weathering pit at Cookie jar Butte in Utah
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13 m deep weathering pit in Utah
Likely formed from a variety of mechanical and chemical processes
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Sand accumulated at the bottom of a 18 m deep weathering pit in Utah.
Chemical weathering is dominant factor in pedogenesis
Requires mechanical weathering to facilitate continued chemical reactionsSurface area to volume ratio…
Chemical weathering is dominant factor in pedogenesis
Requires mechanical weathering to facilitate continued chemical reactionsSurface area to volume ratio…
Congruent vs Incongruent weatheringCongruent- no by products from the chemical reaction in an water-based solution
i.e., the stuff goes into solution and completely dissolves
e.g., salt (NaCl) or limestone (Calcite {CaCO3}
NaCl + H2O ------------->Na+ & Cl-
Crystalline & liquid aqueous aqueous
CaCO3 + CO2 + H20 ----->Ca2+ + 2HCO3-
Crystalline & gas & liquid aqueous aqueous
Solubility of calcite is dependent upon CO2 and H+ concentration
Congruent vs Incongruent weatheringCongruent- no by products from the chemical reaction in an water-based solution
i.e., the stuff goes into solution and completely dissolves
e.g., salt (NaCl) or limestone (Calcite {CaCO3}
NaCl + H2O ------------->Na+ & Cl-
Crystalline & liquid aqueous aqueous
CaCO3 + CO2 + H20 ----->Ca2+ + 2HCO3-
Crystalline & gas & liquid aqueous aqueous
Solubility of calcite is dependent upon CO2 and H+ concentration
Soil environment is loaded with acids and gases that can supply the either of the two things that drive the dissolution of calcite
organics create organic acids
CO2 is abundant in soils and is greater than in the actual atmosphere
The reactions are self sustaining up to a point
saturation of solution with respect to CaCO3
Soil environment is loaded with acids and gases that can supply the either of the two things that drive the dissolution of calcite
organics create organic acids
CO2 is abundant in soils and is greater than in the actual atmosphere
The reactions are self sustaining up to a point
saturation of solution with respect to CaCO3
Soils over limestone are influenced by
insoluble materials in the rock
eolian influx
biological components
Complete dissolution of pure limestone produce very little in the way of soil. Incomplete dissolution creates a special kind of material called terra rosa
red earth
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Terre Rosa fills sinkholes in karst terrane
processes of chemical weatheringprocesses of chemical weathering• • Oxidation/ReductionOxidation/Reduction
- - a function of available free oxygen and the Eh (redoxa function of available free oxygen and the Eh (redoxpotential)potential)
- - Oxygen strips electrons away from other atomsOxygen strips electrons away from other atoms
• • the process is reversiblethe process is reversible
- - often related to water levels in streams andoften related to water levels in streams andgroundwater systemsgroundwater systems
Incongruent chemical weathering
creates weathering by-products or materials
Especially commonplace when considering silicates
Why? - silica is only slightly soluble in most aqueous solutions with lower pH values (0.0007 g/liter or 7 ppm in most solutions less than pH of 9)
That means that some other elements will dissolve but in so doing cause secondary chemical reactions that create new minerals, especially those that can react with the silicate structures
Examples clay minerals; oxidation processes
Incongruent chemical weathering
creates weathering by-products or materials
Especially commonplace when considering silicates
Why? - silica is only slightly soluble in most aqueous solutions with lower pH values (0.0007 g/liter or 7 ppm in most solutions less than pH of 9)
That means that some other elements will dissolve but in so doing cause secondary chemical reactions that create new minerals, especially those that can react with the silicate structures
Examples clay minerals; oxidation processes
OxidationOxidation• • interaction between substances and oxygen (andinteraction between substances and oxygen (and
in some cases water)in some cases water)
• • E.G., E.G., 4FeO + O4FeO + O22 2Fe 2Fe22OO33
The iron goes from the ferrous reducedThe iron goes from the ferrous reducedstate (Fe2+) to the ferric state (Fe2+) to the ferric ((Fe3+) stateFe3+) state
MgFeSiOMgFeSiO44 + 2H + 2H22O Mg(OH)O Mg(OH)2 2 + H+ H22SiOSiO33 + FeO + FeOolivineolivine magnesiummagnesium
hydroxidehydroxidesilicicsilicicacidacid
2FeS2FeS22 + 7H + 7H22O + 15O 2Fe(OH) + 4HO + 15O 2Fe(OH) + 4H22 SOSO44
pyritepyrite sulfuricsulfuricacidacid
Clay mineralsClay minerals
• • occur as layers or sheets arranged in a variety ofoccur as layers or sheets arranged in a variety of
waysways
- - layers consist of sheets of aluminum and silica thatlayers consist of sheets of aluminum and silica that
alternate between discrete layersalternate between discrete layers
> > 1:1 - clay minerals include Kaolinite, Allophane, and Halloysite1:1 - clay minerals include Kaolinite, Allophane, and Halloysite
> > 2:1 - clay minerals include Illite, Smectite, Montmorillonite, and2:1 - clay minerals include Illite, Smectite, Montmorillonite, and
GlauconiteGlauconite
• • usually derived from the decomposition of silicateusually derived from the decomposition of silicate
minerals like feldspars, micas, and hornblendeminerals like feldspars, micas, and hornblende
• • Can have unique properties that are important forCan have unique properties that are important for
good constructiongood construction
> > shrink-swell clays like Smectiteshrink-swell clays like Smectite
Solubility of materials determines which things remain to be able to create alternative minerals in the soil, most notably clay minerals
Polynov’s Ion mobility series
Cl- > SO42- > Na2+ > Ca2+ > Mg2+ > K2+ > Si2+ > Fe3+,2+ >
Al2+
Phase I--><--II--><---III---><--IV--><---V----
Solubility of materials determines which things remain to be able to create alternative minerals in the soil, most notably clay minerals
Polynov’s Ion mobility series
Cl- > SO42- > Na2+ > Ca2+ > Mg2+ > K2+ > Si2+ > Fe3+,2+ >
Al2+
Phase I--><--II--><---III---><--IV--><---V----
Other factorsOther factors• • Mobility-how susceptible atoms are to movementMobility-how susceptible atoms are to movement
due to normal chemical processesdue to normal chemical processes
• • LeachingLeaching
- - removal of elements from upper portions of theremoval of elements from upper portions of the
weathering profile to locations lower in the weatheringweathering profile to locations lower in the weathering
profileprofile
• • Chelation- mobilization of relatively immobileChelation- mobilization of relatively immobile
ions by encapsulating them in a complex molecularions by encapsulating them in a complex molecular
structurestructure- - common way to move things like Aluminum, Iron and Calciumcommon way to move things like Aluminum, Iron and Calcium
- - usually involves organic compounds that fix the metalusually involves organic compounds that fix the metal
into a mobile compoundinto a mobile compound
Abrasion pH’s for common minerals
As cations are released in the grinding process, the solution, which begins as a neutral pH becomes increasingly more basic
However, in most environments, this tendency is offset by production of more H+ cations
As cations are released in the grinding process, the solution, which begins as a neutral pH becomes increasingly more basic
However, in most environments, this tendency is offset by production of more H+ cations
EtchingEtching• • related to chemical weathering acting andrelated to chemical weathering acting and
pieces breaking off along cleavage surfacespieces breaking off along cleavage surfaces
- - keeps rates from achieving some steady state keeps rates from achieving some steady state
processes of chemical weatheringprocesses of chemical weathering• • solutionsolution
- - removal of atoms from mineral structuresremoval of atoms from mineral structures
- - causes minerals to fall apartcauses minerals to fall apart
• • related to pH of the solventrelated to pH of the solvent
- - typically the watertypically the water
chemical processeschemical processes• • Ion exchangeIon exchange
- - usually a function of a polarized molecularusually a function of a polarized moleculararrangementarrangement> > causes a propensity to try and balance the charges bycauses a propensity to try and balance the charges by
replacing ions with other ions through a process calledreplacing ions with other ions through a process calledadsorptionadsorption
> > esp. Cationsesp. Cations
- - often measured as CEC (Cation Exchange Capacity)often measured as CEC (Cation Exchange Capacity)> > a measure of the adsorption ability of a surfacea measure of the adsorption ability of a surface
- - particularly common in clay mineralsparticularly common in clay minerals
Measurement of weatheringTotal chemical analysis is one way
given as oxides- oxygen is a common balancing ion