Chapter 3- Weathering Processes Two major categories Mechanical (or Physical) processes

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



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



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



Influence of fire and environmental conditions on weathering of bouldersInfluence of fire and environmental conditions on weathering of boulders



Sand in the bottom of a 5 m deep weathering pit at Cookie jar Butte in Utah



13 m deep weathering pit in Utah

Likely formed from a variety of mechanical and chemical processes



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



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

Examples of incongruent chemical weathering

Examples of incongruent chemical weathering

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

Chelation structures

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

H+ ions attack weaker spots in the crystalline structure of aluminosilicate minerals

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

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Chapter 3- Weathering Processes Two major categories Mechanical (or Physical) processes