Question 2 of 4: Term Pairs

Question B: Term Pairs (20 marks — source asks for 5 of 10; all 10 answered)

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.

(B1) Stress vs. Strain. Stress is the internal force per unit area a body carries in response to applied loads — a tensor quantity (MPa), the cause. Strain is the resulting fractional change in shape and/or size — a dimensionless geometric quantity, the measured response. A rock can carry stress with zero permanent strain (elastic loading below yield).

(B2) Traction vs. Stress Tensor. Traction is the stress vector acting across one specific plane through a point (magnitude + direction, resolved into a normal and a shear component). The stress tensor is the complete, plane-independent description — it specifies the traction on every possible plane through that point, and has 6 independent components. A traction is a single "slice" of the full tensor.

(B3) Fold Axis vs. Fold Hinge. The fold axis is an abstract line which, when translated parallel to itself, generates the folded surface — it is a geometric property of the whole cylindrical fold, not tied to any one layer. The fold hinge (hinge line) is the actual line of maximum curvature on one specific folded surface; every folded layer has its own hinge line, and for a truly cylindrical fold all of those hinge lines are parallel to the single fold axis.

(B4) Fault Slip vs. Fault Separation. Slip is the true 3-D relative-displacement vector of two points that were adjacent before faulting (magnitude and direction, measured on the fault surface itself). Separation is the apparent offset of a displaced marker measured in some chosen reference plane or direction (e.g. map separation, dip separation) — it depends on the marker's own orientation relative to the slip vector and, except in special cases, is not equal to the true slip.

(B5) Coaxial vs. Non-coaxial Strain. In coaxial strain (pure shear), the principal strain axes keep a fixed orientation relative to an external reference frame throughout the deformation history — no net rotation. In non-coaxial strain (e.g. simple shear), the principal axes progressively rotate relative to that external frame as strain accumulates.

(B6) Point Defect vs. Line Defect. A point defect is a zero-dimensional lattice flaw confined to a single lattice site (a vacancy, interstitial, or substitution). A line defect (dislocation — edge or screw) is a one-dimensional flaw extending along a line through the crystal, around which the lattice is elastically distorted; slip on dislocations is the main mechanism of intracrystalline plastic flow.

(B7) Homogeneous vs. Heterogeneous Strain. In homogeneous strain, originally straight lines stay straight and originally parallel lines stay parallel — a single strain ellipse describes the deformation identically at every point in the body. In heterogeneous strain, the magnitude and/or orientation of strain varies from point to point (e.g. strain intensifying toward the centre of a shear zone, as in C2 below); a heterogeneously strained body can be approximated as many small homogeneous domains.

(B8) Joint vs. Shear Fracture. A joint is a fracture across which there is no discernible displacement parallel to the fracture surface (Mode I, dilational opening). A shear fracture shows measurable displacement of markers parallel to the fracture plane (Mode II/III) — it is a small fault.

(B9) Finite Strain vs. Incremental Strain. Finite strain is the total, cumulative strain comparing the final deformed state directly to the original undeformed state — it integrates the entire deformation path. Incremental strain is the strain accumulated during one small step of that progressive path; the increments sum (as tensors) to the finite strain, but in non-coaxial deformation an individual increment's principal axes generally differ in orientation from the finite strain's own principal axes.

(B10) Trend vs. Rake. Trend is the compass bearing (0–360°, referenced to true north) of the vertical projection of a linear feature onto the horizontal plane. Rake (pitch) is the angle measured within an inclined plane, between the linear feature and that plane's strike line (0–90°). Trend is a horizontal-plane, north-referenced measurement; rake is an in-plane, strike-referenced measurement — the same line has one trend but its rake depends on which containing plane is used.