04-BS-14 · December 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams December 2015 — 04-BS-14, Geology. Closed-book, 3 hours. Five questions constitute a complete paper: Questions 1-4 are mandatory and one of Questions 5-7 must be chosen; every question (5, 6, and 7) is answered here as a complete study resource. Marks for each sub-part are printed as given in the source; totals are transcribed as printed, not forced to a uniform 20.
Reference texts: Marshak, Earth: Portrait of a Planet (structural geology, relative dating, weathering, glacial and fluvial landforms, mineralogy/igneous classification); Goodman, engineering-geology mapping methods (strike and dip, cross-cutting relationships); Freeze & Cherry, Groundwater (water table, zones of aeration/saturation, well hydraulics).
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.
All ten pairs are defined below; five would satisfy the exam's own instruction.
Foliated texture is a planar or layered fabric in metamorphic rock produced by the parallel alignment of platy or elongate minerals (mica, chlorite, amphibole) perpendicular to the direction of maximum compressive stress — visible as slaty cleavage (slate), schistosity (schist) or gneissic banding (gneiss). Non-foliated texture lacks any such planar fabric, typically because the rock is composed dominantly of equant (blocky), non-platy minerals (calcite in marble, quartz in quartzite) that do not develop a preferred alignment even under directed stress, or because metamorphism was dominantly thermal (contact) rather than directed (e.g. hornfels).
A metamorphic facies (e.g. greenschist, amphibolite, granulite, blueschist, eclogite) is a characteristic mineral assemblage that forms from a given parent-rock composition under a specific range of pressure-temperature conditions — it is a P-T diagnostic, read from the minerals present, regardless of what caused the metamorphism. A type of metamorphism (regional, contact, burial, hydrothermal, dynamic/cataclastic) instead classifies the process/setting that produced the metamorphic rock — e.g. regional (orogenic heat + directed pressure over large areas) versus contact (heat from an adjacent pluton, localized). The two classifications are independent: a given facies can, in principle, arise from more than one type of metamorphism.
Porosity (n, dimensionless, expressed as %) is the fraction of a rock or sediment's total volume occupied by void (pore) space, n = Vvoids/Vtotal × 100% — a measure of how much fluid a material can hold. Permeability is a measure of how easily a fluid can flow through the connected pore network (governed by pore size, shape and interconnection, not just total pore volume). A material can have high porosity but low permeability if pores are poorly connected or very fine (e.g. clay, with porosity often >40% but permeability near zero due to tiny, poorly connected pores), while well-sorted, coarse sand has both high porosity and high permeability.
A depositional (conformable) contact is a bedding surface across which sedimentation was essentially continuous — no significant time gap, no erosion, layers deposited one on top of the next without interruption. An unconformity is instead a surface representing a gap in the geologic record from non-deposition and/or erosion; the three types (as in Q4.1) are distinguished by what lies beneath the gap: nonconformity (sedimentary rock over eroded igneous/metamorphic rock), disconformity (parallel sedimentary sequences, no angular discordance), and angular unconformity (younger strata overlie tilted/truncated older strata at an angle).
Physical (mechanical) weathering disaggregates rock into smaller pieces without changing mineral chemistry: frost wedging (repeated freeze-thaw expansion of water in cracks), exfoliation/unloading (sheet fracturing as overburden is removed and confining pressure released), thermal expansion/contraction (daily heating-cooling cycles, notably in deserts), salt crystallization (growing salt crystals in pores force grains apart), and biological/root wedging. Chemical weathering decomposes minerals into new, more stable compounds: hydrolysis (water reacts with silicates, e.g. feldspar → clay), oxidation (Fe²⁺ → Fe³⁺ in ferromagnesian minerals, forming rust-coloured oxides), dissolution (soluble minerals like calcite or halite dissolve directly in water, especially acidic water), and carbonation (CO₂ dissolved in water forms carbonic acid, which attacks carbonate and silicate minerals).
Amphibole (hornblende is the common dark, rock-forming variety) is a ferromagnesian (mafic) double-chain silicate (two parallel chains of SiO₄ tetrahedra cross-linked, with Fe/Mg/Ca/Al filling interstitial sites), typically dark green to black, with two cleavage planes intersecting at ~56°/124°. Feldspars (plagioclase and K-feldspar/orthoclase) are framework (tectosilicate) aluminosilicates, the single most abundant mineral group in the crust, typically light-coloured (white/grey/pink), with two cleavages at ~90°. Amphibole crystallizes in the discontinuous branch of Bowen's series; plagioclase feldspar is the continuous branch (see part g).
As detailed in Question 2 Part 1: the discontinuous branch of Bowen's Reaction Series is a stepwise sequence of structurally distinct ferromagnesian minerals — olivine → pyroxene → amphibole → biotite — each reacting with the melt to form the next as temperature falls. The continuous branch is the single plagioclase feldspar mineral family, whose composition changes smoothly (Ca-rich → Na-rich) via continuous ionic substitution without a structural change. Both branches converge into the final felsic sequence (K-feldspar → muscovite → quartz).
Mafic rocks (from Magnesium + Ferric) are Fe/Mg-rich, silica-poor (<52% SiO₂), dark-coloured and dense (e.g. basalt, gabbro), crystallizing from the discontinuous branch + Ca-rich plagioclase early in Bowen's series. Felsic rocks (Feldspar + Silica) are Si/Al-rich, high-silica (>65% SiO₂), light-coloured and less dense (e.g. granite, rhyolite), crystallizing last, from K-feldspar, muscovite and quartz. Intermediate rocks (andesite/diorite, ~52–65% SiO₂) fall between the two end-members.
A fracture is any break in rock — a joint or crack — across which no significant displacement has occurred (the rock on either side has not moved relative to the other, beyond perhaps minor opening). A fault is a fracture (or fracture zone) along which measurable displacement has occurred, parallel to the fracture surface — classified by the sense of that displacement (normal = extension, reverse/thrust = compression, strike-slip = lateral shear). Every fault is a fracture, but not every fracture is a fault.
A syncline is a trough-shaped (downward-closing, U- or V-shaped) fold in which the youngest strata are preserved in the core and both limbs dip inward, toward the fold axis. An anticline is an arch-shaped (upward-closing) fold in which the oldest strata are exposed in the core (having been folded up and, often, partially eroded off the crest) and both limbs dip outward, away from the axis. The two fold types alternate along a single compressed sequence, as illustrated by Q6 item 2 (“folded anticlines and synclines”).
