Weathering

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Weathering

• Geomorphic processes operate on surficial
  materials not just bedrock
• Definition  Weathering is the disintegration and
  decomposition of rocks and minerals at or near
  the earth's surface as a result of physical,
  chemical, and biological processes.  No transport
  or entrainment is considered.
• Surficial materials     Bedrock     new
  materials created by weathering     resistant
  materials     organic debris
• We divide weathering into two principal process
  types, although they do not work independently
• Physical (or mechanical) weathering
  -disaggregation with no change in chemistry 
  creates surface area
• Chemical weathering -alteration to cause
  chemical or mineralogic changes  weakens rocks
• Primary controls on weathering  climate
  (temperature and precipitation) and geology (rock
  type and distribution).
• Secondary controls  topography (relief and
  aspect) and vegetation (changes chemistry and is
  vigorous physically).

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http//www.oneworldhull.co.uk
Coffee making as analogy for physical and
chemical weathering
http//www.peets.com/
http//www.peets.com/
http//www.peets.com/
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Increase surface area
http//www.gly.fsu.edu/7Esalters/GLY1000/10Weathe
ring_Erosion/Slide10.jpg
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http//www.gly.fsu.edu/7Esalters/GLY1000/10Weathe
ring_Erosion/Slide6.jpg
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Environmental control on weathering
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Chemical weathering

• -Decomposition by chemical processes to cause
  chemical and mineralogic changes
• -Disequilibrium response  surface of the earth
  is much different than the environment in which
  most rocks form.
• -Genesis of material to be weathered is most
  important. What are the differences in physical
  and chemical conditions?

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http//www.gly.uga.edu/railsback/GeologicalDiagram
s2.html
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Many plant species have a symbiotic partnership
with a unique group of soil organisms called
mycorrhizal fungi. They enhance chemical
weathering of rock for the benefit of plants
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edutel.musenet.org8042/.../close_lamb.JPG
wps.prenhall.com/.../1268/1298711/stone-6.gif
http//www.env.duke.edu/eos/geo41/wea027.gif
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http//www.gly.fsu.edu/7Esalters/GLY1000/10Weathe
ring_Erosion/Slide12.jpg
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Relative resistance Two simple ways to
anticipate resistance to chemical weathering
1) Rocks Igneous and metamorphic rocks (least
stable) V Sedimentary rocks V
Weathering products (most stable environment
most similar to surface)
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http//www.gly.uga.edu/railsback/GeologicalDiagram
s2.html
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Relative resistance Two simple ways to
anticipate resistance to chemical weathering
2) Empirical formulation for minerals (Goldich,
1938 compared parent and resulting minerals)
(inverse Bowen's reaction series more shared
Si-O bonds means more resistance to weathering)
Quartz Muscovite Biotite Amphibole K
feldspar Pyroxene Na-Plag Olivine Ca-Plag
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Chemical weathering
Processes of decomposition

• Oxidation and reduction
• Solution
• Hydrolysis
• Mobility
• Leaching
• pH
• Chelation

Occurs on mineral surfaces (see figure 3.3 from
Ritter et al.)
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Oxidation-Reduction

• Oxidation occurs rapidly above the water table
  (wet, but interstitial spaces are filled with
  air) red colors.
• Element loses electrons to an oxygen ion.
• "Weathering of iron-bearing minerals"
• Reduction occurs below water table or in other
  conditions of low oxygen (anoxic) black colors.
• Reactions

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http//www.gly.fsu.edu/7Esalters/GLY1000/10Weathe
ring_Erosion/Slide18.jpg
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Solution

• Extract ions from crystal lattice and eventually
  the mineral falls apart.  The freed ions go into
  solution and may subsequently precipitate.
• Variations in mineral solubility are strongly
  dependent on crystal structure (esp. crystalline
  versus amorphous) and pH (4-9 is typical for soil
  waters)
• Remaining insoluble materials might be Fe3 or
  Al2O3 (Bauxite)
• Most important rx

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Hydrolysis

• Reaction between cations which are replaced by H
  from disassociated water. Important as basic way
  in which many silicates are attacked and
  decomposed.

Ion exhange Substitution for ions in solution for
those held by mineral grains -often onto surface
of clay particles -generates acids, and pulls
some cations (Ca, Na, K, others) out of solution
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http//www.gly.fsu.edu/7Esalters/GLY1000/10Weathe
ring_Erosion/Slide14.jpg
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Hydrolysis

• Will keep going until solvent is saturated with
  respect to cations
• What keeps the reaction going?
• Leaching--movement of groundwater H supplied
  and cations (K, Ca2, Na, Mg2) removed.
• Acids  often come from rainfall (CO2 in the
  atmosphere) and from organic material in the
  soil  humic acid
• CO2 (gas) H2O (water) lt-gt H2CO3 (carbonic
  acid)  lt-gt H (hydrogen ion)  HCO3-
  (bicarbonate ion)

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http//www.gly.uga.edu/railsback/1121WeatheringChe
mCaliche.jpeg
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Leaching

• Movement of water through weathering zone
• 1) removes dissolved minerals 2) adds fresh H
  (keeping things in solution) 3)  moves material
  within weathering zone possibly allowing
  precipitation of new minerals

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Mobility

• Controls extent of alteration

pH H concentration as pH goes down, mobility
goes up
http//www.gly.uga.edu/railsback/GeologicalDiagram
s2.html
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Chelation

• Normally immobile metal ions form more than one
  bond with molecules of the chelating agent
  resulting in the formation of a ring structure
  incorporating the metal ion.
• Often a means of accelerated chemical weathering
  by plants.
• Plants are capable of putting into solution or
  absorbing substantial portions of the total mass
  of the rock on which they grow.

http//en.wikipedia.org/wiki/ImageLichen_squamulo
se.jpg
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General comments about chemical weathering
Latitudinal control on weathering
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Clays are key weathering products
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http//www.gly.uga.edu/railsback/1121PeriodicTable
3.jpeg
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Karsted landscape in China
http//daac.gsfc.nasa.gov/geomorphology/GEO_7/geo_
images_KL-1/FigKL-1.3.gif
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Travertine-cemented colluvium on Colorado River
slope in the Grand Canyon
Vasey's Paradise in the Grand Canyon--lots of
dissolved ions in that water from dissolution and
chemical weathering of carbonates.
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Birkeland
Ecological role of weathering
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Lots of Ca in these desert plants!