# Shared Flashcard Set

## Details

Structural Geology II
Midterm 2
26
Geology
11/05/2013

Term
 What is the difference between a body force and a contact force?
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
Term
 What is the difference between normal and shear stress?
Definition
 • Normal stress (σN) are oriented perpendicular to the plane of interest. o Tend to inhibit sliding on the planeo + 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 lefto Maximum angle of 45* to the surface of interest
Term
 What's the difference between primary, secondary, and tertiary creep?
Definition
 o Primary creep: initial rapid strain that slows due to strain hardeningo Secondary creep: near-constant strain rate due to offsetting of strain hardening by strain softeningo Tertiary creep: rapid strain due to necking phenomena; leads to failure
Term
 What's the difference between elastic, plastic, and viscous responses to deviatoric stress?
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 straino Plastic: a certain threshold must be surpassed before applied stresses invoke change, which is permanento Viscous: applies to fluids; even tiny stresses will invoke flow in Newtonian fluids, which is not recoverable
Term
 What's the difference between Young's modulus and Poisson's ratio?
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
Term
 What's the difference between brittle and ductile deformation?
Definition
 • Brittle: o microfracturingo Intragranular: form within a single grain, often controlled by cleavageo Intergranular: form along grain boundarieso Transgranular: cut across neighbouring grainso Cataclastic flow: grains slide past one another• Ductile: o Twinning/kinkingo Creep mechanismso Defect structures: perfect crystals are difficult to deform, therefore strain in crystals is facilitated by their presence
Term
 What's the difference between 0D, 1D, and 2D defect structures?
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 directiono Screw dislocations: oriented parallel to slip directiono Mixed dislocations: oblique to direction of slip• 2D (planar defects): stacking faults, mechanical twins, subgrains, grain boundaries
Term
 What's the difference between recrystallization and neocrystallization?
Definition
 o Recrystallization: change in size, number, or shape of existing minerals in rocko Neocrystallization: crystals of new minerals formo 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
Term
 What's the difference between strain hardening and strain softening?
Definition
 o Strain hardening: dislocations that impede one another become tangled, making the crystal hard to deformo Strain softening (recovery): crystals that contain a large amount of stored strain energy can be healed by recovery and recrystallization processes
Term
 What's the difference between dynamic recrystallization and annealing?
Definition
 o Dynamic recrystallization: occurs during deformationo Annealing: occurs after deformation
Term
 What's the difference between dissolution creep, grain boundary diffusion creep, volume diffusion creep, and dislocation creep?
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 phaseo 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 creepo 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 shapeo 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
Term
 What is force?
Definition
 • Vector quantity• One Newton (N) = 1 kgm/s2
Term
 What is stress?
Definition
 • σ = force per unit area = F/A• One Pascal (Pa) = 1 N/m2• Lithostatic stress (pressure): increases with depth; varies with rock densityo Average increase is 26.5 MPa/km of deptho 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
Term
 What is traction?
Definition
 • Traction: force per unit area acting on a surface
Term
 What is the Mohr circle and how is it used?
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 σ3o Centre of circle corresponds to mean stress: (σ1+ σ3)/2• Hydrostatic stress; can cause dilationo 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 distortedo Diameter of circle corresponds to differential stress: σ1- σ3
Term
 What is strain rate?
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/to 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
Term
 What is viscosity?
Definition
 • Viscosity (η): relates shear stress in fluids to shear strain rate and normal stress to elongation rate
Term
 What's the difference between a Newtonian fluid and a Bingham fluid?
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
Term
 What is plastic behaviour in relation to deformation?
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 curveo Its slope is a measure of the amount of strain hardening occurring in the mediumo 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 mylonitizationo Formation of weak metamorphic mineralso Increase in T
Term
 What is recrystallization?
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 anothero Causes strained crystals to be replaced by relatively strain-free grainso Two-dimensional arrays of dislocations form as strained grains reduce their stored strain energy• These walls of dislocations separate subgrains
Term
 What is grain boundary migration recrystallization?
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
Term
 What is recovery?
Definition
 • Recovery:o Involves the rearrangement of dislocations into low-energy configurations (walls), and/or their destruction
Term
 What is dislocation climb?
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
Term
 What is creep?
Definition
 • Creep: slow processes that occur at differential stresses well below the yield (rupture) strength of the rock
Term
 What is mechanical twinning?
Definition
 • Mechanical twinning: o kinking of crystal latticeo Produces deformation twinso Form where shear stress is high, usually at 45* to σ1o Common in plagioclase and calciteo Can provide info on stress fieldo Discontinuous, so can be distinguished from other crystallographic twins
Term
 What determines the prevailing deformation mechanism?
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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