18-Geol-A4 Structural Geology · December 2018
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.
Given. Twenty True/False statements spanning stress and strain theory, fold and fracture classification, and crystal-defect terminology, followed by eight fill-in-the-blank items on the same topics.
Find. The correct True/False call for each of the 20 statements, with the one-line reasoning that would earn the mark, plus the missing term for each of the 8 blanks.
| # | Statement | Answer | Reasoning |
|---|---|---|---|
| 1 | Rocks that have undergone purely strike slip faulting can show evidence of dip separation. | True | Separation is the apparent offset measured on an arbitrarily-oriented marker, not the true slip vector. A purely strike-slip net slip can still produce an apparent vertical (dip) offset where it crosses a marker plane that is not vertical — separation and slip coincide only for a marker parallel to the net slip. |
| 2 | Axial planar cleavage forms during the heterogeneous strain stage of fold development. | False | Buckling/amplification is the heterogeneous (layer-parallel-shortening then flexural) stage. Axial-planar cleavage is imposed by a later, approximately homogeneous flattening strain oriented perpendicular to the axial surface — that is why it fans only gently and stays sub-parallel to the axial surface across the whole fold. |
| 3 | The fold hinge marks a point on the folded layer where the curvature changes from convex to concave or vice versa. | False | That describes the inflexion point. The hinge is the point of maximum curvature on the folded surface, not the point where curvature changes sign. |
| 4 | In theory, buckling involving flexural flow folding produces Class 2 folds. | False | Flexural flow (and flexural slip) folding preserves orthogonal layer thickness around the fold — the defining geometry of Ramsay Class 1B (parallel folding), not Class 2 (similar folding, which requires internal flow/shear). |
| 5 | Stress traction refers to stress on a plane. | True | Traction is the stress vector resolved on one specific plane through a point; it is distinct from the stress tensor, which describes the complete state of stress (tractions on every possible plane). |
| 6 | The Deformation Path must represent the shortest distance from initial to final position of a material point in a deforming volume. | False | The deformation path is the actual, generally curved trajectory a material point follows through the full strain history (e.g. under progressive or polyphase deformation); only the net displacement, not the path itself, is a straight line. |
| 7 | Mode 3 fractures are produced by a shear stress acting parallel to the plane of the crack and parallel to the crack front. | True | Mode III (tearing) fracture is anti-plane shear: displacement of the crack faces is parallel to both the crack plane and the crack front, distinguishing it from Mode II (shear perpendicular to the front). |
| 8 | Rigid body deformation involves translation and distortion. | False | Rigid-body motion is translation and rotation with no distortion (no change of shape) — distortion is precisely the component excluded from a rigid-body description. |
| 9 | For ideally elastic material strain is linearly related to stress. | True | This is Hooke's Law, the defining constitutive relation of ideal (linear) elasticity: \(\sigma = E\varepsilon\). |
| 10 | Griffith's law of failure refers to transtensional tensile behavior during deformation. | False | Griffith theory explains brittle fracture initiating from tensile-stress concentration at the tips of pre-existing microcracks — a micro-mechanical fracture criterion, not a description of the transtensional (combined extension + strike-slip) kinematic regime. |
| 11 | Solid state diffusion involving Coble creep occurs along grain boundaries. | True | Coble creep is grain-boundary diffusion creep by definition, as distinct from Nabarro-Herring creep, which is volume (lattice) diffusion through the grain interior. |
| 12 | Hooke's Law describes the ratio of lateral strain to longitudinal strain. | False | That ratio defines Poisson's ratio. Hooke's Law relates stress to strain through the elastic modulus, \(\sigma=E\varepsilon\). |
| 13 | Elastic deformation involves some component of non-recoverable deformation. | False | Elastic deformation is fully recoverable by definition — permanent (non-recoverable) strain is plastic/ductile deformation, not elastic. |
| 14 | Lines that represent the principal strain axes were perpendicular before the strain. | True | The (Lagrangian) principal strain axes are the eigenvectors of the symmetric strain tensor evaluated in the undeformed state, and the eigenvectors of a symmetric tensor are always mutually orthogonal — so the material lines that become the principal strain axes were already perpendicular before straining began, whether or not the deformation is coaxial. |
| 15 | For coaxial strain, the instantaneous shortening axes are inclined at \(45^{\circ}\) to the shear plane. | False | A 45° instantaneous axis fixed relative to a shear plane is the hallmark of non-coaxial (simple shear) strain. In coaxial (pure shear) deformation the instantaneous and finite principal axes coincide and stay fixed in the material, parallel to the applied principal stresses, throughout. |
| 16 | The stress tensor is a vector quantity that considers magnitude of force in relation to the area of the surface it acts upon. | False | The stress tensor is a rank-2 tensor (nine components, six independent), not a vector. The traction — force per unit area resolved on one plane — is the vector quantity; the tensor maps any plane's normal to its traction. |
| 17 | An edge dislocation is oriented parallel to the Burgers vector. | False | An edge dislocation's line is perpendicular to its Burgers vector; a line parallel to the Burgers vector is the defining property of a screw dislocation. |
| 18 | Hydrostatic stress is characterized by the absence of shear stress in all directions. | True | Hydrostatic (isotropic/mean) stress is equal normal stress in every direction with zero shear stress resolved on any plane — by definition. |
| 19 | Principal strain axes for pure shear are always irrotational. | True | Pure shear is the coaxial strain path: the principal strain axes do not rotate relative to a fixed external reference frame at any stage of the deformation — this irrotational behaviour is exactly what distinguishes it from simple shear (non-coaxial, rotational). |
| 20 | Deviatoric stress is the non-hydrostatic component of stress that tends to produce distortion. | True | Total stress decomposes as hydrostatic (mean, volume-change) plus deviatoric (shape-change) components; the deviatoric part is precisely what drives distortion. |
| # | Item | Answer |
|---|---|---|
| 1 | Name two types of line defects ____ and ____. | Edge dislocation and screw dislocation |
| 2 | ____ is the study of the response of rocks to stress. | Rheology |
| 3 | ____ is the term for the line that connects points of maximum curvature on a folded surface. | Hinge line |
| 4 | ____ is a fault melt that freezes to glass. | Pseudotachylyte |
| 5 | ____ are offsets or steps in the land surface that coincide with locations of faults. | Fault scarps |
| 6 | ____ is the apparent relative displacement related to a fault. | Separation |
| 7 | When rocks fail in tension by a combination of Mode 1 and Mode 2 fracturing, this is called ____. | Mixed-mode (hybrid) fracture |
| 8 | Name two types of point defects: ____ and ____. | Vacancies and interstitials |
