Physical weathering

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Physical weathering

• Disintegration and breakup of surficial materials
  without chemical changes.
• Decrease particle size, thus helping to produce
  regolith available for transport
• Increase surface area of particles and increase
  potential for chemical weathering.
• Important processes
• Expansion and contraction
• Unloading or exfoliation or sheeting
• Hydration and swelling
• Crystal growth
• Organic processes

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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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Expansion and contraction

• Driven by thermal processes may produce rock
  fracture or fatigue (many cycles) which will
  weaken and ultimately break rock. 1)  Rock is
  poor conductor, so most effects are concentrated
  near rock surface 2)  Different coefficients of
  thermal expansion for minerals sets up stresses
  in rock 3)  Daily thermal changes (can be up to
  50C). Rock surface temperatures can be as high as
  80C. These effects are very strong with no other
  sources of stress (no confinement or burial).
• Really intense heat -gt forest fires and lightning
  may crack rocks

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Expansion and contraction

• Famous experiment Griggs, 1936 heat and cooled
  cubes of granite 140C to 30C for the equivalent
  of 240 years of daily fluctuations, but nothing
  happened. That was dry, with wetting, in 2.5
  years they fell apart. Why?

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Expansion and contraction

• Famous experiment Griggs, 1936 heat and cooled
  cubes of granite 140C to 30C for the equivalent
  of 240 years of daily fluctuations, but nothing
  happened. That was dry, with wetting, in 2.5
  years they fell apart. Why? Effect of the water
  for subcritical crack growth and corrosion in
  general. The samples may have been very clean
  and thus flaw free Thorough heating?

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Unloading or exfoliation or sheeting

• Change in confining pressure  overburden is
  removed and rock expands.  Fractures form
  parallel to surface and the fractured shells
  spall off. Fracture spacing decreases as we get
  closer to the free surface.
• Principal stresses are parallel or perpendicular
  to the free surface and the fractures are pure
  opening, so they grow parallel to s1 and
  perpendicular to s3

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Bedrock domes
http//uts.cc.utexas.edu/rmr/E-rock/Aerial4.html
Enchanted Rock in Texas
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http//uts.cc.utexas.edu/rmr/E-rock/Aerial5.html
Bedrock domes
Enchanted Rock in Texas
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"hoodoos" or "mushroom stones" or "pedestal
rocks-weathered sheets
http//uts.cc.utexas.edu/rmr/E-rock/hoodoo.html
Bedrock domes
Enchanted Rock in Texas
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Yosemite National Park, Sierra Nevadas area
glacial and exfoliation features
http//virtual.yosemite.cc.ca.us/ghayes/roadside.h
tm
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Yosemite National Park, Sierra Nevadas area
glacial and exfoliation features
http//virtual.yosemite.cc.ca.us/ghayes/roadside.h
tm
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South Canyon, Grand Canyon
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Hydration and swelling

• Grus or saprolite (dissaggregated granite) and
  some soils  Swelling absorption by clays,
  especially bentonite which will absorb 140x its
  own mass in water
• absorption of water into mineral structures
  causing expansion mechanical stress as water
  forces mineral structures apart

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Expansive soils
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http//www.geol.vt.edu/profs/js/weathered.html
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Crystal growth

• Freeze-thaw of water 9 increase in volume.
• In a confined situation (like a frozen cap), you
  can get 30,000 lbs/inch
• Frost cracking with full saturation, rapid
  freezing, and frequent cycling, one can  move
  materials around by ice growth and even have
  frost heaving.

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http//www.gly.uga.edu/railsback/1121Lxr20.html
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http//piru.alexandria.ucsb.edu/geog3/concept_db_
illus/collection/mike/Weathering/freeze_thaw.jpg
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Felsenmeer chaotic block fields typically
developed above the tree line scree general
term for chaotic blocky rubble Talus a scree
slope formed at the base of a cliff Talus cone
conical scree slope
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Crystal growth

• Salt crystal growth sulfates, carbonates,
  chlorides
• Salt crystals precipitate and they too can induce
  stresses in the material (as well as induce
  chemical weathering) by changes in temperature
  and hydration.
• Need arid to semi-arid conditions usually

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Tafoni
http//user.tninet.se/bgb354w/home/Tunisien.htm8
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Tafoni
http//user.tninet.se/bgb354w/home/Tunisien.htm8
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Organic processes

• trees open and hold open fractures and also
  root/tree/saguaro throw animals lots of
  churning
• earthworms termites and ants Burrowing critters
  Humans

http//www.nicholas.duke.edu/eos/geo41/wea.htm
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Mass balance equation for soil
Heimsath et al., Nature 388, pp 358-361 (1997)
accumulation
production
transport