law of superposition, law of original horizontality, law of cross-cutting relationships, and law of lateral continuity. Also important: Fossil Succession
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Folds
a stack of originally planar surfaces that are bent or curved during permanent deformation
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Fault
a planar fracture or discontinuity in a volume of rock across which there has been significant displacement
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Fracture
any separation in a geologic formation, such as a joint or a fault that divides the rock into two or more pieces
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Primary Structure
Structures that go back to the formation of the rock
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Secondary Structure
Structures that were created after the formation of the rock
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Fracture Set
A group of fractures
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Fracture Modes
Opening, Sliding, Tearing, Hybrid
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Plumose Structures
aggregates of gentle curvilinear undulations that radiate from the point where the joint originated and fan outward from a generally straight axial line, then resembling the shape and imprint of a feather
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Elastic Deformation
a temporary deformation of a material's shape that is self-reversing after removing the force or load
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Plastic Deformation
the permanent distortion that occurs when a material is subjected to stresses that exceed its yield strength
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Poisson Effect
In an elastic body, volume change is resisted and therefore there will be longitudinal strain as well. E.g. things thin as stretched or widen as compressed
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Poisson’s Ratio
v = -((dy/y)/(dx/x)) = -(e_yy/e_xx)
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Sections of the Failure Envelope
Tensile Failure, Frictional Sliding, Von Mises
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Conjugate Sets
Faults organized into two intersecting sets with opposite shear sense formed simultaneously
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Dihedral Angle
Acute angle between fracture sets
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Mode I Fractures
Opening. Displacement is perpendicular to the fracture face. Tension
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Mode II Fractures
Sliding. Displacement is parallel to the fracture face. Shear
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Mode III Fractures
Tearing. Displacement is parallel to the fracture face. Shear
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Hybrid
Mode I + Mode II or III
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Joints
Mode I fractures that are barely opened with no displacement along fracture plane
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Fissure
Mode I fractures more open than joints
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Regional Geometries
Longitudinal, Transverse, Oblique
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Longitudinal Regional Geometry
Parallel to tectonic trend
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Transverse Regional Geometry
Cuts tectonic trend at a high angle
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Oblique Regional Geometry
Cuts tectonic trend at a low angle
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Fracture Geometries
Radial, Ring, En Echelon, Pinnate, Columnar, Exfoliation
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Radial Fracture Geometry
Form from point stesses, something rising up or falling down
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Ring Fracture Geometry
Form from point stresses
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En Echelon Fracture Geometry
Laterally offset, stepwise geometry, may indicate shear
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Pinnate Fracture Geometry
Form in association with faults. Motion of the fracture is in direction of acute to obtuse angle
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Columnar Fracture Geometry
Usually hexagonal in cross-section, cooling of igneous rock
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Exfoliation Fracture Geometry
Expansion of rock due to decompression and/or weathering
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Joint Intersection Geometries
Y, T, X
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Hackles
Radiate outward from origin point and converse along line of propogation
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Ribs
Concentric around origin point and indicate location where fracture propagation slowed or halted
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Fringe
Outer margin of joint face where energy was low
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Fracture Kinks
Result of mixed-mode loading
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Hybrid Fracture Tip Types
Abrupt, wing-tip, hook, en echelon segments
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Stress
Force over an area
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Normal Stress
Stress perpendicular to faces
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Shear Stress
Stress along faces
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Shear Tensor
the state of stress for a unit volume consisting of stresses on all surfaces
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Normal Fault Stress
Vertical stress is the largest
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Thrust Fault Stress
Vertical stress is the smallest
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Strike-Slip Fault
Max and min stresses are both horizontal
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Strain
the relative displacement or deformation of a material due to an applied stress
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Brittle
Material that breaks under 5% strain
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Mohr Circle Diameter
sigma_1 - sigma_3
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Mohr Circle Center
Average of max and min stress
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Differential Stress
Max stress (sigma_1) - min stress (sigma_3)
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Frictional Sliding Equation
t = Co + sig_ntan(phi)
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Frictional Coefficient Equation
tan(phi) = mu
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Listric Fault
a fault with a curved fault plane, flattens at depth
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Yield Stress
the point at which a material begins to deform plastically
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Ultimate Stress
The highest possible stress after strain hardening
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Strain Hardening
irreversible deformation where yield strength increases with strain
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Strain Softening
irreversible deformation where yield strength decreases with strain
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Hooke’s Law
sig = kE
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Young’s Modulus
linear proportion of stress along one axis to strain in the same direction
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Shear Modulus
linear proportion of shear stress to strain in the same direction
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Bulk Modulus
Linear proportion of hydrostatic stress to volume change
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Hysteresis
Strain is fully recovered when stress is released, but not perfectly reversible
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Types of Elasticity
Linear, perfect, hysteresis
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Anelastic
The time delay between an applied stress and the resulting strain
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Effect of pore pressure
Weakens the material
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Von Mises
The highest yield strength where a material cannot support shear
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Viscous Behavior
the yield stress depends on the strain rate
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Strain Rate
the amount of time over which strain occurs
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Primary Creep
Plastic behavior takes over from elastic; strain hardening and strain softening
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Secondary Creep
Strain rate approaches a steady state
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Tertiary Creep
Microfractures grow or crystallographic changes allow strain rate to increase, followed by fracture
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Anisotropy
Difference in properties as a function of direction/orientation
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Griffith Theory of Fracture
Stress is concentrated at the tips of a void
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Borehole Breakouts
Shear fractures occur on the sides of a circular void where there is the min stress
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Fluid Pressure Impact on Mohr Circle
Shifts the circle to the left relative to fluid pressure. The circle will be outside the envelope
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Induced Extension Fractures
Extensional fractures that open in the direction of maximum compression
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Deformation Bands
small faults with very small displacements
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Point Defects
Vacancies, impurities, substitutions, interstitial ions. Move by solid-state diffusion processes
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Line Defects
Line, Edge, Screw. Move by slip or creep and my result in twinning
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Cataclasis
the crushing or fracturing of rocks and minerals due to stress
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Microcrack Types
Intragranular, intergranular, transgranular
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Intragranular
Cracks inside a grain
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Intergranular
Cracks between grains
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Transgranular
Cracks across multiple grains
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Dilatancy
an increase in volume, can be the result of cataclasis
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Kink Banding
Lattice bent along a discrete plane through the crystal
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Volume Diffusion
Vacancy diffusion through crystals, aka Nabarro-Herring Creep, temperature dependent. Lower crust and mantle
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Grain Boundary Diffusion
Preferential diffusion of vacancies along crystal boundaries, aka Coble Creep. Middle and upper crust
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Syntaxial Vein
Crystals grow from the middle
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Antitaxial Vein
Crystal grow from the vein wall(s)
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Crack-and-Seal Texture
The result of a repeating pattern of fracture and fill