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454 lecture 2
WEATHERING
the interacting physical, chemical & biological processes that
progressively alter the original lithologic character of rocks to
produce secondary minerals (e.g. clays) & unconsolidated
regolith
Weathering – breakdown of rock materials
Erosion – transport of broken-down materials
Physical weathering – disintegration operate simultaneously &
Chemical weathering – decomposition affect each other
Physical weathering: the transformation of rocks into
unconsolidated debris; occurs when stress is exerted along
zones of weakness within the original material (bedding,
fractures, mineral boundaries)
454 lecture 2
Stress field originates via expansion of rocks or minerals, or
pressure caused by growth of foreign substance in voids:
thermal expansion: low thermal conductivity of rocks allows them
to expand at surface, but not at depth, creating differential
stress & spalling
unloading: confining pressure decreases when overlying rocks
are removed, & sheets form perpendicular to the direction of
pressure release, mirroring surface topography
hydration: hydration of salts or mineral expansion causes
exfoliation at the large scale, spheroidal weathering at
small scale
growth in voids: openings may expand at different rates or in
different directions, creating differential pressure
frost action plant roots
minerals (percolating fluids evaporate, supersaturate, & precipitate;
e.g. sulfates & carbonates in arid/semiarid regions)
454 lecture 2
unloading
Granite, Yosemite National Park,
California
Sandstone, Arches National Park, Utah
Granite, Poudre drainage, Colorado
454 lecture 2Sandstone, central AustraliaGranite, central Australia
Granite, Missouri
454 lecture 2
Physical weathering dominates where mean annual T < 0° C,
& there is precipitation
454 lecture 2
Chemical weathering
Rainwater is mildly acidic; chemical weathering is a neutralizing
process in which minerals assimilate H ions and/or water, &
release cations to the soil liquid
Other anions, cations, gases & organic acids are introduced by
chemical impurities in rainwater, groundwater, & organic
processes (metabolism of microrganisms & decay of vegetation)
Two stages of chemical weathering
Inorganic processes: geochemical weathering produces
saprolites (“rotten rock”)
Organic processes: biologically controlled, pedochemical
weathering produces soils
454 lecture 2
Roadcut in highly weathered granite,
Sequoia National Park, California
454 lecture 2
As precipitation percolates into exposed rock & regolith, it
chemically reacts with minerals & alters the atomic structure of
the minerals & the pH of the water
Reactions involved in chemical weathering:
mottling, nw Australia
mottled laterite, India
oxidation & reduction
in oxidation, an element loses an electron to an oxygen ion
(Zn + O = ZnO)
occurs spontaneously above the water table, where atmospheric oxygen
is readily available
in reduction, an element gains an electron; occurs below the water table
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solution
atoms are dissolved from a mineral & the structure becomes unstable
CaCO3 + H2CO3 = Ca2+ + 2(HCO3)-
the most common elements & minerals are soluble to some extent in
groundwater – exceptions are aluminum oxide and ferric iron
hydrolysis
reaction between mineral elements & H ion of dissociated water
works on silicates by removing metallic cations from the mineral structure
2KAlSi3O8 + 2H+ + 9H2O = H4Al2Si2O9 + 4H4SiO4 + 2K+
orthoclase kaolinite
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solution
Sandstone, Zion Canyon National Park, Utah
Sandstone, Uluru, central Australia
454 lecture 2
ion exchange
substitution of ions in solution for those held by mineral grains
most effective in clay minerals
relative mobility (ease of removal) of cations:
Ca2+, Mg2+, Na+ > K+ > Fe2+ > Si4+ > Ti4+ > Fe3+ > Al3+
increasing
lichens, Arizona Havasu Creek, AZ
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Death Valley, California
western
Arizona
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joint-controlled weathering Joints in sandstone, Canyonlands NP, Utah
Sandstone,
Escalante basin,
Utah
Sandstone, Rainbow Arch,
northern Arizona
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joint-controlled weathering
Rhyolite, Chiricahua National
Monument, Arizona
Sandstone, Zion National Park,
Utah
454 lecture 2
joint-controlled weathering
Granite, Wyoming
Granite, Sinai peninsula, Egypt
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joint-controlled weathering
Granite, Wind River Range,
Wyoming
Granite, Half Dome, Yosemite
Granite, Loch Vale, Colorado
454 lecture 2
joint-controlled weathering
Sandstone, Zion National Park, Utah
Granite,Yosemite, California
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desert varnish
Sandstone, northern Arizona Basalt, northern New Mexico
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Weathering stability of common silicate minerals
1 olivine, anorthite least stable
pyroxenes, Ca-Na plagioclase
amphiboles, Na-Ca plagioclase
biotite, albite
K-feldspars
muscovite
7 quartz most stable
1 high-temperature minerals, first to crystallize, least stable
in surface environment, weather rapidly
7 last to crystallize, form under lowest temperature conditions,
most stable at surface
454 lecture 2
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