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Structural Geology II
Midterm 2
Undergraduate 3

Additional Geology Flashcards




What is the difference between a body force and a contact force?
• 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
What is the difference between normal and shear stress?
• 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
What's the difference between primary, secondary, and tertiary creep?
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
What's the difference between elastic, plastic, and viscous responses to deviatoric stress?
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
What's the difference between Young's modulus and Poisson's ratio?
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
What's the difference between brittle and ductile deformation?
• 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
What's the difference between 0D, 1D, and 2D defect structures?
• 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
What's the difference between recrystallization and neocrystallization?
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
What's the difference between strain hardening and strain softening?
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
What's the difference between dynamic recrystallization and annealing?
o Dynamic recrystallization: occurs during deformation
o Annealing: occurs after deformation
What's the difference between dissolution creep, grain boundary diffusion creep, volume diffusion creep, and dislocation creep?
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
What is force?
• Vector quantity
• One Newton (N) = 1 kgm/s2
What is stress?
• 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
What is traction?
• Traction: force per unit area acting on a surface
What is the Mohr circle and how is it used?
• 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
What is strain rate?
• 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
What is viscosity?
• Viscosity (η): relates shear stress in fluids to shear strain rate and normal stress to elongation rate
What's the difference between a Newtonian fluid and a Bingham fluid?
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
What is plastic behaviour in relation to deformation?
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
What is recrystallization?
• 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
What is grain boundary migration recrystallization?
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
What is recovery?
• Recovery:
o Involves the rearrangement of dislocations into low-energy configurations (walls), and/or their destruction
What is dislocation climb?
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
What is creep?
• Creep: slow processes that occur at differential stresses well below the yield (rupture) strength of the rock
What is mechanical twinning?
• 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
What determines the prevailing deformation mechanism?
• 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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