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Today: Deformation mechanisms
Reading: Chapter 4 of Davis and Reynolds
Rocks may behave in an elastic or plastic manner- but how?
Deformation mechanisms: Processes that permit rocks to deform at the microscopic and atomic scale
Crystal Structure
Crystal Lattice: systematic array of atoms or molecules
Bonds (covalent, ionic, metallic) are due to attractive forces between atoms/molecules
Elastic Deformation of a Lattice
Crystal Structure and
Strength
Different minerals/rocks have different strengths- due to differences in
bonds, atom spacing and charge, and
crystal symmetry
Observed strength of rocks/minerals in lab is order of magnitude lower than theoretical
strength- WHY??
Imperfections lead to beautiful structures
- but unfortunately, still termed "flaws" and "defects": deviations from ideal crystalline structure
Deformation mechanisms take advantage of defects, but vary as a function of , T, P, and other
(1) fracturing and frictional sliding- due to the linking of microcracks
(2) Kinking – bending rather than breaking the crystal lattice- takes advantage of planes of weakness
(3) Pressure solution (a.k.a., Dissolution creep)– distortion by dissolution and recrystallization of
material- aided by presence of fluid
activated by pressure
(4) Grain boundary diffusion : Same as pressure solution, but purely in solid state (no fluid)
activated by temperature
(5) Volume diffusion : movement of material
within a crystal by taking advantage of point defects: missing
(vacancies), extra (interstitial), or terrorist (impurities) atoms in a
lattice
(6) Dislocation creep : deformation by slip within the crystal lattice
Dislocations: linear defects of pent up energy
Dislocation creep cont.: at any single moment, only portions of slip plane are slipping- hence "creep"
What happens when differential stress decreases at high temperatures?
Recovery and Recrystallization- ahhh
releases stored energy in crystal and rock
Recovery: rearranges and removes dislocations
Recrystallization: smaller grains into larger grains and straightening of grain boundaries
"annealing" after
deformation
dynamic recrystallization
during deformation
undulatory extinction
Relationships between deformation and metamorphism
time
Next Lecture: Deformation, Metamorphism, and Time
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