Question 10 of 10: Question 5, Option 3: Term Pairs

Question 5, Option 3: Term Pairs (10 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.

a) Joints vs. faults. Both are fractures in rock, but a joint shows NO observable shear displacement parallel to the fracture surface, while a fault shows measurable displacement of the rock on one side relative to the other. Joints commonly form in regularly-spaced sets (cooling contraction, unloading, regional stress); faults are classified by their sense of relative motion (normal, reverse, strike-slip).

b) Unconformity vs. non-conformity. Unconformity is the general term for ANY surface representing a gap in the rock/time record (erosion and/or non-deposition). Non-conformity is one specific TYPE of unconformity: sedimentary rock deposited directly on top of eroded igneous or metamorphic (crystalline) basement rock – distinguished from an angular unconformity (tilted beds below, flat beds above) and a disconformity (parallel beds both above and below) by the fact that the rock type itself changes across the surface from crystalline to sedimentary.

c) Polymorphism vs. isomorphism (minerals). Polymorphic minerals share the IDENTICAL chemical formula but crystallize in different crystal structures/forms depending on pressure-temperature conditions (e.g. graphite and diamond, both pure carbon; calcite and aragonite, both CaCO₃). Isomorphic minerals share the SAME crystal structure but can substitute different (usually similarly-sized/charged) ions into the same atomic sites, forming a solid-solution series (e.g. the plagioclase feldspar series, albite NaAlSi₃O₈ to anorthite CaAl₂Si₂O₈, or the olivine series forsterite–fayalite).

d) Elastic vs. plastic strain. Elastic strain is temporary/recoverable deformation: the material returns exactly to its original shape once the applied stress is removed (governed by Hooke's law, $\sigma=E\varepsilon$, up to the yield point) – the regime in which seismic (earthquake) energy accumulates and is released. Plastic strain is PERMANENT deformation that persists after the stress is removed, occurring once stress exceeds the material's yield strength; rocks deep in the crust, at high confining pressure and temperature, deform plastically (folding) rather than elastically or by brittle fracture.

e) Dyke vs. sill. Both are tabular (sheet-like) intrusive igneous bodies that cut into existing (host) rock while magma was still liquid, but a dyke is DISCORDANT – it cuts ACROSS the host rock's bedding/layering (often near-vertical, exploiting a fracture) – while a sill is CONCORDANT – it intrudes PARALLEL to existing bedding/layering (often near-horizontal in flat-lying strata), physically wedging layers apart rather than cutting across them.