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What is it?
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What is it? (Quiz)
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Announcements Fault paper- keep the first draft and submit with final draft!!!!!!!!!! If you take into consideration the comments, you will get a higher grade- if you disregard comments, you will get a lower grade. general suggestion: follow the guidelines closely!
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Today: Deformation mechanisms Reading: Chapter 4 of Davis and Reynolds
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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
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Crystal Structure Crystal Lattice: systematic array of atoms or molecules Bonds (covalent, ionic, metallic) are due to attractive forces between atoms/molecules
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Elastic Deformation of a Lattice
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Crystal Structure and Strength Different minerals/rocks have different strengths- due to differences in bonds, atom spacing and charge, and crystal symmetry
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Observed strength of rocks/minerals in lab is order of magnitude lower than theoretical strength- WHY??
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Imperfections lead to beautiful structures - but unfortunately, still termed "flaws" and "defects": deviations from ideal crystalline structure
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Deformation mechanisms take advantage of defects, but vary as a function of , T, P, and other
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(1) fracturing and frictional sliding- due to the linking of microcracks
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(2) Kinking – bending rather than breaking the crystal lattice- takes advantage of planes of weakness
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(3) Pressure solution (a.k.a., Dissolution creep)– distortion by dissolution and recrystallization of material- aided by presence of fluid activated by pressure
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(4) Grain boundary diffusion : Same as pressure solution, but purely in solid state (no fluid) activated by temperature
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(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
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(6) Dislocation creep : deformation by slip within the crystal lattice Dislocations: linear defects of pent up energy
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Dislocation creep cont.: at any single moment, only portions of slip plane are slipping- hence "creep"
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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
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"annealing" after deformation dynamic recrystallization during deformation undulatory extinction
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Relationships between deformation and metamorphism time
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Next Lecture: Deformation, Metamorphism, and Time
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