18-Geol-A4 Structural Geology · Undated paper
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Reference texts: Davis & Reynolds, Structural Geology of Rocks and Regions (3rd ed.); Fossen, Structural Geology (2nd ed.); Marshak & Mitra, Basic Methods of Structural Geology.
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
Approach. Each statement is judged independently against the underlying rock-mechanics or structural-geology principle it tests; the blanks are answered with the single standard term the definition points to.
| # | Statement | Answer | Reasoning |
|---|---|---|---|
| 1 | Layer thickness is the primary factor that dictates fold wavelength. | True | Biot–Ramberg buckling theory gives dominant wavelength ∝ layer thickness × (viscosity contrast)1/3; for a given competence contrast, thickness is the first-order control on wavelength — thicker layers buckle into longer wavelengths. |
| 2 | Strain results from rigid-body deformation either through translation and/or rotation. | False | Rigid-body translation and rotation change position/orientation only, with no change in size or shape — by definition they produce zero strain. Strain requires distortion and/or dilation (non-rigid-body deformation). |
| 3 | Byerlee's law demonstrates a non-linear correlation between confining pressure and rock strength. | False | Byerlee's law is essentially piecewise-linear (τ≈0.85σn below ≈200 MPa; τ≈50+0.6σn above), and famously holds across almost all rock types — its defining feature is that linear frictional strength is nearly rock-independent, not that it is non-linear. |
| 4 | Purely dip slip faulting can show evidence of strike separation. | True | Separation is the apparent offset of a marker measured in an arbitrarily-oriented reference surface, not the true slip vector. A purely dip-slip net slip can still produce apparent strike separation where the fault crosses a non-vertical marker surface — separation and slip coincide only when the marker is parallel to the net-slip vector. |
| 5 | Mode 2 fractures are produced by a shear stress acting parallel to the plane of the crack and parallel to the crack front. | False | That describes Mode III (tearing/anti-plane shear). Mode II (in-plane shear) fractures are produced by a shear stress acting perpendicular to the crack front, within the fracture plane. |
| 6 | Nonrigid deformation involves dilation and/or distortion. | True | Deformation = rigid-body translation + rigid-body rotation + nonrigid deformation. The nonrigid part is, by definition, what changes the body’s size (dilation) and/or shape (distortion) — i.e. it is strain. |
| 7 | Axial planar cleavage forms during the homogeneous strain stage of fold development. | True | Layer buckling/amplification is the earlier, heterogeneous (flexural) stage. Axial-planar cleavage is imposed by the later, approximately homogeneous flattening strain oriented perpendicular to the axial surface — which is why it stays sub-parallel to the axial surface across the whole fold instead of fanning strongly with position. |
| 8 | For ideal viscous material, strain rate is linearly related to stress. | True | This is the defining property of a Newtonian (ideal) viscous fluid: σ=ηέ̇, a linear stress–strain-rate relationship with constant viscosity η. |
| 9 | In theory, buckling involving flexural slip folding produces Class 1b folds. | True | Flexural-slip (parallel) folding preserves orthogonal layer thickness around the fold, which is exactly Ramsay's Class 1B (parallel-fold) geometry. |
| 10 | von Mises criterion refers to transtensional tensile behavior during deformation. | False | Von Mises is a ductile/plastic yield criterion based on the second deviatoric stress invariant (distortional strain energy) — it describes the onset of plastic flow, not brittle tensile fracture (that is Griffith theory). |
| 11 | Hydrostatic stress is characterized by the absence of shear stress in all directions. | True | Hydrostatic (isotropic) stress has σ1=σ2=σ3, so every plane through the point is a principal plane and the shear stress on every plane is exactly zero. |
| 12 | Solid state diffusion involving Nabarro-Herring creep occurs along grain boundaries. | False | Nabarro–Herring creep is lattice (volume) diffusion through the grain interior. Grain-boundary diffusion creep is Coble creep, the distinct companion mechanism. |
| 13 | Poisson's ratio describes the ratio of lateral strain to longitudinal strain. | True | By definition ν=−εlateral/εlongitudinal under uniaxial stress. |
| 14 | Stress tensor is a vector quantity that considers the magnitude of a force per unit area. | False | Stress is a second-rank tensor, not a vector — a full description needs both the force (magnitude and direction) and the orientation of the surface it acts on, which a single vector cannot encode. |
| 15 | Lines that represent the principal strain axes were perpendicular before the strain. | True | For any homogeneous strain — coaxial or non-coaxial — the principal strain axes are the one set of material lines that were mutually perpendicular before the strain and are still perpendicular after it (they are the principal axes of the reciprocal strain ellipse in the undeformed state). In non-coaxial strain those lines have rotated, but they were still perpendicular before straining. |
| 16 | For non-coaxial strain, the principal strain axes have no net shear strain. | True | By definition the principal strain axes are the (instantaneously orthogonal) directions of zero shear strain in the current state, regardless of whether the deformation history was coaxial or non-coaxial. |
| 17 | The stress tensor is a vector quantity that considers magnitude of force in relation to the area of the surface it acts upon. | False | Same error as statement 14: stress is a tensor, not a vector, because it also depends on the orientation of the surface. |
| 18 | A screw dislocation is oriented parallel to the Burgers vector. | True | Defining property: a screw dislocation's line is parallel to its Burgers vector (an edge dislocation's line is perpendicular to its Burgers vector). |
| 19 | Principal strain axes for simple shear have zero net shear strain. | True | Simple shear is the classic non-coaxial deformation, yet its (instantaneous) principal strain axes still have zero shear strain by definition — the same logic as statement 16. |
| 20 | Differential stress is the non-hydrostatic component of stress that tends to produce distortion. | True | Differential stress σ1−σ3 is exactly the deviatoric (non-hydrostatic) part of the stress state, which is what drives shape change (distortion); the hydrostatic/mean part drives only volume change. |
| # | Blank(s) | Answer |
|---|---|---|
| 1 | Name two types of microcracks: ____ and ____. | Intragranular (intracrystalline) microcracks and intergranular (grain-boundary) microcracks — transgranular microcracks, which cut across several grains, are also a standard answer |
| 2 | ____ involves the combination of grain boundary sliding and grain boundary diffusion. | Superplastic flow (superplasticity) |
| 3 | ____ represents the intersection between the axial surface and any other surface. | Axial trace |
| 4 | ____ connect points of equal inclination on the outer and inner bounding surfaces of a folded layer. | Dip isogons |
| 5 | Curved limb segments of opposing convexity join at locations known as ____. | Inflection points (inflexion points) |
| 6 | ____ is the actual, relative displacement related to a fault. | Slip (net slip) |
| 7 | When rocks fail in tension by a combination of Mode 1 and Mode 2 fracturing, this is called ____. | Hybrid fracture (hybrid extensional-shear fracture) |
| 8 | Name two types of line defects: ____ and ____. | Edge dislocation and screw dislocation |
