True / False: 18-Geol-A4 Structural Geology, May 2018, Q1

Question A — True / False (20 marks)

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 (dislocation) terminology.

Find. The correct True/False call for each statement, with the one-line reasoning that would earn the mark on a national exam.

#	Statement	Answer	Reasoning
1	Rocks that have undergone purely dip slip faulting can show evidence of strike separation.	True	Separation is measured on an arbitrarily-oriented marker surface (e.g. a dipping bed), not along the true slip vector. A purely dip-slip net slip can still produce an apparent horizontal (strike) offset where it crosses a non-vertical, non-slip-parallel marker — separation and slip are only the same thing for a marker line parallel to net slip.
2	Axial planar cleavage of a fold forms during the homogeneous strain stage of fold development.	True	Buckling itself is a heterogeneous (layer-parallel-shortening then flexural) process; axial-planar cleavage is imposed by a later, approximately homogeneous flattening strain oriented perpendicular to the axial surface, which is why cleavage fans only slightly and is sub-parallel to the axial surface across the whole fold.
3	The fold axis connects points of maximum curvature for non-cylindrical folds.	False	That describes the hinge line, which exists for any fold. A true fold axis — a single line direction that can be translated parallel to itself to generate the entire folded surface — is only defined for CYLINDRICAL folds; non-cylindrical folds have no single axis direction.
4	In theory, buckling involving flexural slip folding produces only Class 1B folds.	True	Flexural slip (and flexural flow) folding preserves orthogonal layer thickness around the fold (parallel folding), which is the defining geometry of Ramsay Class 1B.
5	Stress traction refers to stress at a point.	False	Traction is the force per unit area resolved on ONE specific plane through a point. The complete state of stress at a point requires the stress tensor (tractions on every possible plane), not a single traction vector.
6	Mode 1 fractures only form perpendicular to \(\sigma_1\).	False	Mode I (opening) fractures form perpendicular to \(\sigma_3\) (parallel to \(\sigma_1\)-\(\sigma_2\)) — they open against the least compressive stress, not perpendicular to the greatest.
7	Mode 2 fractures have displacement perpendicular to the fracture front.	True	Mode II is in-plane shear: the two crack faces slide past each other in the fracture plane, in the direction perpendicular to the propagating crack front (Mode III sliding is parallel to the front).
8	A Mode I fracture may form from either positive or negative normal stresses.	True	A joint/vein opens under net tension, but hydraulic fracturing shows Mode I opening can also occur under a nominally compressive \(\sigma_3\) once elevated pore pressure drives the EFFECTIVE normal stress to the tensile failure condition.
9	For ideally plastic material strain is linearly related to stress.	False	Ideal plasticity means strain increases at CONSTANT stress once yield is reached — the opposite of a proportional (linear, elastic/Hookean) stress-strain relation.
10	An intersection lineation between bedding and cleavage provides the orientation of the axial surface.	False	A bedding-cleavage intersection lineation is PARALLEL TO THE FOLD AXIS (a single line), not the axial surface (a plane, needing strike and dip).
11	Solid state diffusion involving Nabarro-Herring creep occurs along grain boundaries.	False	Nabarro-Herring creep is diffusion THROUGH the grain interior (lattice/volume diffusion); diffusion along grain boundaries is Coble creep.
12	The same bedding contact can intersect the axial planar cleavage of a fold only once.	False	A bedding surface threading through a periodic fold train (or around both limbs of one fold) crosses the fanning axial-planar cleavage once per hinge zone it passes through, i.e. potentially many times.
13	An intersection lineation between bedding and cleavage provides the orientation of the axial surface. (repeated in source)	False	Same statement as item 10 above (a duplicate in the printed paper — flagged rather than silently answered differently); the reasoning is identical: an intersection lineation gives the fold-axis line, not the axial-surface plane.
14	Lines that represent the principal strain axes were perpendicular before the strain.	False (in general)	True only for COAXIAL (pure shear) strain, where material lines that become principal axes never rotate. In non-coaxial strain (e.g. simple shear) the material lines that end up as the finite principal strain axes were NOT mutually perpendicular before straining, because they rotate progressively through the deformation.
15	Finite strain represents the total accumulated strain for a given period of time.	True	Finite strain is the TOTAL strain from the undeformed to the final state, as distinct from incremental (instantaneous) strain accrued during one small step of the deformation path.
16	The stress tensor can be fully defined by 6 different components of stress.	True	The 3×3 stress tensor is symmetric (\(\sigma_{ij}=\sigma_{ji}\)), so only 3 normal + 3 shear = 6 independent components are needed.
17	A screw dislocation is oriented parallel to the Burgers vector.	True	By definition: a screw dislocation's line is parallel to \(\mathbf{b}\); an edge dislocation's line is perpendicular to \(\mathbf{b}\).
18	The yield point in a rock deformation experiment is the onset of inelastic deformation.	True	The yield point marks the transition from recoverable elastic strain to permanent (plastic or brittle) deformation.
19	Principal strain axes for simple shear have some component of net angular shear.	True	Simple shear is non-coaxial: the material lines that end up parallel to the finite principal strain axes rotate throughout the deformation path (unlike pure shear, where principal-axis material lines never experience angular shear), so tracked over the whole history they record a net angular shear.
20	Plane strain is a product of non-coaxial strain.	False	Plane strain is a GEOMETRIC classification of the strain ellipsoid (one principal strain = 0) and is completely independent of the deformation PATH — both pure shear (coaxial) and simple shear (non-coaxial) commonly produce plane strain.

Check: item 13 is a verbatim repeat of item 10 in the source paper (a printed duplication, not a transcription error here) — both are answered identically and consistently below.

Topic: Stress/strain theory, fold and fracture classification, crystal defects

Key relations: Coulomb/Byerlee friction; Ramsay fold classes; Mode I/II/III fracture mechanics; coaxial vs non-coaxial strain; Nabarro-Herring vs Coble creep; edge vs screw dislocations.

Why this works: These twenty items sample the whole first-year structural-geology toolkit rather than one topic, so each answer follows from a single definition (e.g. \(\mathbf{b}\) parallel to the line = screw; perpendicular = edge) rather than a calculation — the exam is testing precise recall of definitions that are easy to state loosely but that have one strict textbook meaning.

Common pitfall: The most common slip is confusing a LINE property with a PLANE property (fold axis vs. axial surface; Nabarro-Herring/lattice vs. Coble/grain-boundary diffusion) or assuming a geometric classification (plane strain) implies something about deformation PATH (coaxial vs non-coaxial), which it does not.

Source: Davis & Reynolds Ch.3 (stress), Ch.4 (strain), Ch.5 (fracture); Fossen Ch.12 (rock rheology, dislocation creep).

Question 1 of 5: True / False

Question A — True / False (20 marks)