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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
Structure and
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 s, 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
(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
(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
Relationships between deformation and metamorphism
Next Lecture: Deformation, Metamorphism, and Time