Term
What is the difference between a body force and a contact force? |
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Definition
• Body forces act on the mass of a body in a way that depends on the amount of material in the body, but not on the forces created by surrounding material • Contact forces act across real or imaginary surfaces of contact |
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Term
What is the difference between normal and shear stress? |
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Definition
• Normal stress (σN) are oriented perpendicular to the plane of interest. o Tend to inhibit sliding on the plane o + if compressive; - if tensile • Shear stress (σS) are oriented parallel to the plane of interest. o + if acting towards the right; - if acting towards the left o Maximum angle of 45* to the surface of interest |
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Term
What's the difference between primary, secondary, and tertiary creep? |
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Definition
o Primary creep: initial rapid strain that slows due to strain hardening o Secondary creep: near-constant strain rate due to offsetting of strain hardening by strain softening o Tertiary creep: rapid strain due to necking phenomena; leads to failure |
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Term
What's the difference between elastic, plastic, and viscous responses to deviatoric stress? |
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Definition
o Elastic: application of stress invokes an immediate change in shape, which is recoverable • Shown by linear plot of stress vs strain • Hookean behaviour: stress is proportional to strain o Plastic: a certain threshold must be surpassed before applied stresses invoke change, which is permanent o Viscous: applies to fluids; even tiny stresses will invoke flow in Newtonian fluids, which is not recoverable |
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Term
What's the difference between Young's modulus and Poisson's ratio? |
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Definition
o Young’s modulus (E): describes the stiffness of an isotropic medium. • Measure of its tensile elasticity, so by tradition, is a negative number • Hooke’s law: σ = Eε; describes the relationship between stress and strain in elastic bodies; measured in Pa • Corresponds to the slope of the stress/strain curve • Greater E value = stiffer rock (resistant to deformation) o Poisson’s ratio (v): describes the extent to which an object bulges as it is compressed • Determined by measuring bulging rock core as it shortens • Rocks with low values of v tend to burst: stress gets stored, and is released suddenly, rather than being accommodated by significant barrelling |
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Term
What's the difference between brittle and ductile deformation? |
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Definition
• Brittle: o microfracturing o Intragranular: form within a single grain, often controlled by cleavage o Intergranular: form along grain boundaries o Transgranular: cut across neighbouring grains o Cataclastic flow: grains slide past one another • Ductile: o Twinning/kinking o Creep mechanisms o Defect structures: perfect crystals are difficult to deform, therefore strain in crystals is facilitated by their presence |
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Term
What's the difference between 0D, 1D, and 2D defect structures? |
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Definition
• 0D (point defects): vacancies, extraneous atoms • Can help linear defects (dislocations) migrate through crystal lattices • 1D (dislocations): a disturbed region between two substantially perfect parts of a crystal • linear defect around which some of the atoms are misaligned • Movement of dislocations require that only a portion of the bonds break at any given time. Movement in this manner thus requires a much smaller force than breaking all the bonds along a plane through the lattice simultaneously • Dislocations are classified according to their orientation relative to slip direction: o Edge dislocations: oriented perpendicular to slip direction o Screw dislocations: oriented parallel to slip direction o Mixed dislocations: oblique to direction of slip • 2D (planar defects): stacking faults, mechanical twins, subgrains, grain boundaries |
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Term
What's the difference between recrystallization and neocrystallization? |
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Definition
o Recrystallization: change in size, number, or shape of existing minerals in rock o Neocrystallization: crystals of new minerals form o Grain boundary migration recrystallization: occurs as a grain grows at the expense of its strained neighbours • Atoms hop across grain boundaries; requires the stored strain energy associated with dislocations and other defect structures |
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Term
What's the difference between strain hardening and strain softening? |
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Definition
o Strain hardening: dislocations that impede one another become tangled, making the crystal hard to deform o Strain softening (recovery): crystals that contain a large amount of stored strain energy can be healed by recovery and recrystallization processes |
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Term
What's the difference between dynamic recrystallization and annealing? |
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Definition
o Dynamic recrystallization: occurs during deformation o Annealing: occurs after deformation |
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Term
What's the difference between dissolution creep, grain boundary diffusion creep, volume diffusion creep, and dislocation creep? |
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Definition
o Dissolution creep: pressure solution • Occurs at low differential stress and T conditions • Relies on the dissolution of the mineral along boundaries of impinging grains, the diffusion of dissolved material along grain boundaries or microfractures, and the reprecipitation of the dissolved material • Occurs in the presence of a fluid phase o Grain boundary diffusion creep: Coble creep • Dry pressure solution • Moderate T and differential stresses • Material migrates from domains of high stress to those of lower stress • Rates of migration exceed those related to volume diffusion creep o Volume diffusion creep: Nabarro-Herring creep • Presence of vacancies allows material to migrate through crystals being stressed at high T • Given differential stress, vacancies migrate toward domains of high compressive stress • Atoms migrate in the opposite direction, thereby causing the crystal to change its shape o Dislocation creep • Higher differential stress • Rather than break all bonds simultaneously on a slip plane, only part of it is active at any given moment (like a wave, sand dune, wrinkle in carpet) • Dislocation propagates along the slip plane by dislocation creep • Only possible if dynamic recrystallization keeps pace with strain hardening |
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Term
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Definition
• Vector quantity • One Newton (N) = 1 kgm/s2 |
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Term
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Definition
• σ = force per unit area = F/A • One Pascal (Pa) = 1 N/m2 • Lithostatic stress (pressure): increases with depth; varies with rock density o Average increase is 26.5 MPa/km of depth o Plith = ρgh = rock density * acceleration due to gravity * height of column of rock • Tecronic forces impart a lateral confining pressure that is relieved vertically by uplift or subsidence |
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Term
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Definition
• Traction: force per unit area acting on a surface |
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Term
What is the Mohr circle and how is it used? |
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Definition
• Mohr circle: resolves paired values of σS and σN operating on all orientations of planes within a body subjected to known values of stresses σ1 and σ3 o Centre of circle corresponds to mean stress: (σ1+ σ3)/2 • Hydrostatic stress; can cause dilation o Radius of circle corresponds to deviatoric stress: (σ1- σ3)/2 • Non-hydrostatic stress; can cause distortion • The greater the deviatoric stress, the greater the likelihood that rocks will be distorted o Diameter of circle corresponds to differential stress: σ1- σ3 |
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Term
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Definition
• The rate at which a rock is stretched or shortened affects how it deforms • Expressed as the elongation (-) or shortening (+) per unit of time: έ = extension/time = ε/t o Geologic strain rates are typically ~ 10-12 to 10-15/second (ie, very slow) • Relatively large amounts of stress are required to deform rocks at high strain rates • The strength of rocks decreases as a function of long-sustained deviatoric stress |
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Term
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Definition
• Viscosity (η): relates shear stress in fluids to shear strain rate and normal stress to elongation rate |
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Term
What's the difference between a Newtonian fluid and a Bingham fluid? |
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Definition
o Newtonian fluids are linear viscous materials: σs = ηγ o Bingham fluids only strain once a certain value of shear stress (the yield stress) is achieved • Most magmas behave as Bingham fluids because they contain solid crystals and gas bubbles |
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Term
What is plastic behaviour in relation to deformation? |
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Definition
Pastic behaviour: • Plastic media will not deform until a certain stress threshold is surpassed • Non-ideal plastic behaviour is shown by a gradual rise in stress-strain curve o Its slope is a measure of the amount of strain hardening occurring in the medium o It shows how more and more stress is required to invoke strain in the substance • Plastic deformation is permanent • Strain (work) softening: o Grain size reduction during mylonitization o Formation of weak metamorphic minerals o Increase in T |
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Term
What is recrystallization? |
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Definition
• Recrystallization: o Occurs as recovery continues to the point that dislocations are removed or annihilated • Occurs as dislocations exit the lattice, arrange themselves into low energy configurations, cancel one another o Causes strained crystals to be replaced by relatively strain-free grains o Two-dimensional arrays of dislocations form as strained grains reduce their stored strain energy • These walls of dislocations separate subgrains |
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Term
What is grain boundary migration recrystallization? |
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Definition
o Grain boundary migration recrystallization: occurs as a grain grows at the expense of its strained neighbours • Atoms hop across grain boundaries; requires the stored strain energy associated with dislocations and other defect structures |
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Term
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Definition
• Recovery: o Involves the rearrangement of dislocations into low-energy configurations (walls), and/or their destruction |
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Term
What is dislocation climb? |
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Definition
o Dislocation climb: dislocations migrate to higher or lower slip planes as point defects diffuse in the opposite direction • Allows linear defects to exit the grain as they reach the grain boundary, or annihilate one another as they encounter other dislocations provided their half-planes don’t overlap |
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Term
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Definition
• Creep: slow processes that occur at differential stresses well below the yield (rupture) strength of the rock |
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Term
What is mechanical twinning? |
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Definition
• Mechanical twinning: o kinking of crystal lattice o Produces deformation twins o Form where shear stress is high, usually at 45* to σ1 o Common in plagioclase and calcite o Can provide info on stress field o Discontinuous, so can be distinguished from other crystallographic twins |
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Term
What determines the prevailing deformation mechanism? |
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Definition
• The deformation mechanism that prevails depends on differential stresses and temperature; prevailing deformation mechanisms for different stress-T regimes are shown on deformation maps |
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