Structures with Hammy

Steno’s Laws

Steno’s Laws

law of superposition, law of original horizontality, law of cross-cutting relationships, and law of lateral continuity. Also important: Fossil Succession

Folds

a stack of originally planar surfaces that are bent or curved during permanent deformation

Fault

a planar fracture or discontinuity in a volume of rock across which there has been significant displacement

Fracture

any separation in a geologic formation, such as a joint or a fault that divides the rock into two or more pieces

Primary Structure

Structures that go back to the formation of the rock

Secondary Structure

Structures that were created after the formation of the rock

Fracture Set

A group of fractures

Fracture Modes

Opening, Sliding, Tearing, Hybrid

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

Elastic Deformation

a temporary deformation of a material's shape that is self-reversing after removing the force or load

Plastic Deformation

the permanent distortion that occurs when a material is subjected to stresses that exceed its yield strength

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

Poisson’s Ratio

v = -((dy/y)/(dx/x)) = -(e_yy/e_xx)

Sections of the Failure Envelope

Tensile Failure, Frictional Sliding, Von Mises

Conjugate Sets

Faults organized into two intersecting sets with opposite shear sense formed simultaneously

Dihedral Angle

Acute angle between fracture sets

Mode I Fractures

Opening. Displacement is perpendicular to the fracture face. Tension

Mode II Fractures

Sliding. Displacement is parallel to the fracture face. Shear

Mode III Fractures

Tearing. Displacement is parallel to the fracture face. Shear

Hybrid

Mode I + Mode II or III

Joints

Mode I fractures that are barely opened with no displacement along fracture plane

Fissure

Mode I fractures more open than joints

Regional Geometries

Longitudinal, Transverse, Oblique

Longitudinal Regional Geometry

Parallel to tectonic trend

Transverse Regional Geometry

Cuts tectonic trend at a high angle

Oblique Regional Geometry

Cuts tectonic trend at a low angle

Fracture Geometries

Radial, Ring, En Echelon, Pinnate, Columnar, Exfoliation

Radial Fracture Geometry

Form from point stesses, something rising up or falling down

Ring Fracture Geometry

Form from point stresses

En Echelon Fracture Geometry

Laterally offset, stepwise geometry, may indicate shear

Pinnate Fracture Geometry

Form in association with faults. Motion of the fracture is in direction of acute to obtuse angle

Columnar Fracture Geometry

Usually hexagonal in cross-section, cooling of igneous rock

Exfoliation Fracture Geometry

Expansion of rock due to decompression and/or weathering

Joint Intersection Geometries

Y, T, X

Hackles

Radiate outward from origin point and converse along line of propogation

Ribs

Concentric around origin point and indicate location where fracture propagation slowed or halted

Fringe

Outer margin of joint face where energy was low

Fracture Kinks

Result of mixed-mode loading

Hybrid Fracture Tip Types

Abrupt, wing-tip, hook, en echelon segments

Stress

Force over an area

Normal Stress

Stress perpendicular to faces

Shear Stress

Stress along faces

Shear Tensor

the state of stress for a unit volume consisting of stresses on all surfaces

Normal Fault Stress

Vertical stress is the largest

Thrust Fault Stress

Vertical stress is the smallest

Strike-Slip Fault

Max and min stresses are both horizontal

Strain

the relative displacement or deformation of a material due to an applied stress

Brittle

Material that breaks under 5% strain

Mohr Circle Diameter

sigma_1 - sigma_3

Mohr Circle Center

Average of max and min stress

Differential Stress

Max stress (sigma_1) - min stress (sigma_3)

Frictional Sliding Equation

t = Co + sig_ntan(phi)

Frictional Coefficient Equation

tan(phi) = mu

Listric Fault

a fault with a curved fault plane, flattens at depth

Yield Stress

the point at which a material begins to deform plastically

Ultimate Stress

The highest possible stress after strain hardening

Strain Hardening

irreversible deformation where yield strength increases with strain

Strain Softening

irreversible deformation where yield strength decreases with strain

Hooke’s Law

sig = kE

Young’s Modulus

linear proportion of stress along one axis to strain in the same direction

Shear Modulus

linear proportion of shear stress to strain in the same direction

Bulk Modulus

Linear proportion of hydrostatic stress to volume change

Hysteresis

Strain is fully recovered when stress is released, but not perfectly reversible

Types of Elasticity

Linear, perfect, hysteresis

Anelastic

The time delay between an applied stress and the resulting strain

Effect of pore pressure

Weakens the material

Von Mises

The highest yield strength where a material cannot support shear

Viscous Behavior

the yield stress depends on the strain rate

Strain Rate

the amount of time over which strain occurs

Primary Creep

Plastic behavior takes over from elastic; strain hardening and strain softening

Secondary Creep

Strain rate approaches a steady state

Tertiary Creep

Microfractures grow or crystallographic changes allow strain rate to increase, followed by fracture

Anisotropy

Difference in properties as a function of direction/orientation

Griffith Theory of Fracture

Stress is concentrated at the tips of a void

Borehole Breakouts

Shear fractures occur on the sides of a circular void where there is the min stress

Fluid Pressure Impact on Mohr Circle

Shifts the circle to the left relative to fluid pressure. The circle will be outside the envelope

Induced Extension Fractures

Extensional fractures that open in the direction of maximum compression

Deformation Bands

small faults with very small displacements

Point Defects

Vacancies, impurities, substitutions, interstitial ions. Move by solid-state diffusion processes

Line Defects

Line, Edge, Screw. Move by slip or creep and my result in twinning

Cataclasis

the crushing or fracturing of rocks and minerals due to stress

Microcrack Types

Intragranular, intergranular, transgranular

Intragranular

Cracks inside a grain

Intergranular

Cracks between grains

Transgranular

Cracks across multiple grains

Dilatancy

an increase in volume, can be the result of cataclasis

Kink Banding

Lattice bent along a discrete plane through the crystal

Volume Diffusion

Vacancy diffusion through crystals, aka Nabarro-Herring Creep, temperature dependent. Lower crust and mantle

Grain Boundary Diffusion

Preferential diffusion of vacancies along crystal boundaries, aka Coble Creep. Middle and upper crust

Syntaxial Vein

Crystals grow from the middle

Antitaxial Vein

Crystal grow from the vein wall(s)

Crack-and-Seal Texture

The result of a repeating pattern of fracture and fill