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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