Chapter 3- Weathering Processes
Two 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
Pressure release
Caused 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
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Freeze Thaw
Water, when frozen, expands and can fracture rock
if the cracks have the appropriate geometry and the
climate is suitable.
Salt Crystal Growth
Similar 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 grainschemical 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
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Thermal Fracturing
Implication of freeze thaw
expansion and contraction
due to changes in
temperature and thermal
properties of the parent
material
Also relates to fire related
actions as well
Spalling occurs when
fire superheats the
surface
Also 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 boulders
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.
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Chemical weathering
is dominant factor in pedogenesis
Requires mechanical weathering to facilitate continued
chemical reactions
Surface area to volume ratio…
Congruent vs Incongruent weathering
Congruent- 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+ &
Crystalline & liquid
aqueous
Claqueous
CaCO3 + CO2 + H20 ----->Ca2+ + 2HCO3Crystalline & 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
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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
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processes of chemical weathering
• Oxidation/Reduction
- a function of available free oxygen and the Eh (redox
potential)
- Oxygen strips electrons away from other atoms
• the process is reversible
- often related to water levels in streams and
groundwater 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
Oxidation
• interaction between substances and oxygen (and
in some cases water)
4FeO + O2
2Fe2O3
The iron goes from the ferrous reduced
state (Fe2+) to the ferric (Fe3+) state
MgFeSiO4 + 2H2O Mg(OH)2 + H2SiO3 + FeO
• E.G.,
olivine
magnesium
hydroxide
2FeS2 + 7H2O + 15O
pyrite
silicic
acid
2Fe(OH) + 4H2 SO4
sulfuric
acid
Clay minerals
• occur as layers or sheets arranged in a variety of
ways
- layers consist of sheets of aluminum and silica that
alternate between discrete layers
> 1:1 - clay minerals include Kaolinite, Allophane, and Halloysite
> 2:1 - clay minerals include Illite, Smectite, Montmorillonite, and
Glauconite
• usually derived from the decomposition of silicate
minerals like feldspars, micas, and hornblende
• Can have unique properties that are important for
good construction
> shrink-swell clays like Smectite
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----
Other factors
• Mobility-how susceptible atoms are to movement
due to normal chemical processes
• Leaching
- removal of elements from upper portions of the
weathering profile to locations lower in the weathering
profile
• Chelation- mobilization of relatively immobile
ions by encapsulating them in a complex molecular
structure
- common way to move things like Aluminum, Iron and Calcium
- usually involves organic compounds that fix the metal
into 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
H+ ions attack weaker spots in the crystalline structure of
aluminosilicate minerals
Etching
• related to chemical weathering acting and
pieces breaking off along cleavage surfaces
- keeps rates from achieving some steady state
processes of chemical weathering
• solution
- removal of atoms from mineral structures
- causes minerals to fall apart
• related to pH of the solvent
- typically the water
chemical processes
• Ion exchange
- usually a function of a polarized molecular
arrangement
> causes a propensity to try and balance the charges by
replacing ions with other ions through a process called
adsorption
> esp. Cations
- often measured as CEC (Cation Exchange Capacity)
> a measure of the adsorption ability of a surface
- particularly common in clay minerals
Measurement of weathering
Total chemical analysis is one way
given as oxides- oxygen is a common balancing ion