Question 1 of 9: Term Pairs

Question 1: Term Pairs (40 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) Foliated vs. non-foliated texture. Foliation is the planar/layered fabric produced in a metamorphic rock when platy or elongate minerals (mica, chlorite, amphibole) grow with their long axes aligned perpendicular to the maximum principal compressive stress during regional metamorphism; it appears as visible layering, banding or a sheen of aligned grains (slate’s slaty cleavage, schist’s schistosity, gneiss’s gneissic banding). Non-foliated rocks (marble, quartzite, hornfels) form either from minerals that crystallize as equant grains with no preferred shape (calcite, quartz) or under stress fields without strong differential (directed) stress, typically in contact metamorphism around an igneous intrusion – the result is a granular, massive texture with no platy fabric even though the rock is fully recrystallized.

b) Metamorphic facies vs. types of metamorphism. A metamorphic facies (e.g. greenschist, amphibolite, granulite, blueschist, eclogite) is a set of pressure–temperature conditions identified by a characteristic mineral assemblage that a given parent-rock composition produces at equilibrium – it is a P–T "zone," read off a metamorphic facies diagram, independent of what caused the metamorphism. Types of metamorphism, by contrast, classify the geologic process/setting: regional (large-scale, directed stress + heat at convergent margins, produces foliated rocks across many facies), contact (localized heating from an igneous intrusion, produces non-foliated hornfels), dynamic (cataclasis along fault zones, mostly stress with little heat), and hydrothermal/burial metamorphism. A single facies (say greenschist) can be reached by more than one type of metamorphism.

c) Porosity vs. permeability. Porosity $n = V_v/V_T$ is a scalar STORAGE property – the fraction of a rock or soil’s total volume that is void space, controlling how much water/oil it can hold. Permeability ($k$, or hydraulic conductivity $K$ for a specific fluid) is a TRANSMISSION property – how well those voids are interconnected, controlling how easily fluid flows through the medium, and it enters directly into Darcy’s law $q = -K\,dh/dl$. A rock can have high porosity but low permeability if the pores are isolated or poorly connected (e.g. shale: porosity often >30% but permeability near zero because the pores are tiny and poorly connected; compare a well-sorted, well-connected sandstone or gravel, which can have both high porosity and high permeability).

d) Depositional contacts vs. unconformities. A depositional (conformable) contact is a boundary between two sedimentary units deposited continuously, one on top of the other, with no significant time gap or erosion in between – layers stay parallel and record an unbroken depositional history. An unconformity is a surface representing a gap in the rock/time record – either non-deposition or erosion (or both) removed part of the section before deposition resumed. Three types: angular unconformity (older beds tilted/folded and eroded flat before younger, flat-lying beds were deposited on top – the two sets of bedding cross at an angle); disconformity (both sets of beds are parallel, but an erosional surface with relief/channeling between them shows a time gap, often recognized by missing fossil zones); and nonconformity (sedimentary rock deposited directly on eroded igneous or metamorphic basement).

e) Physical vs. chemical weathering. Physical (mechanical) weathering breaks rock into smaller pieces of the same mineral composition – frost/ice-wedging, thermal expansion–contraction cycling, salt-crystal growth, unloading/exfoliation, root-wedging, and abrasion. Chemical weathering changes the mineral composition itself via reactions with water, oxygen, and acids – hydrolysis (feldspar → clay), oxidation (iron-bearing minerals → rust/iron oxides), dissolution (carbonate rocks + carbonic acid), and hydration. The two are complementary: physical breakup increases surface area, which accelerates chemical attack, and chemical weathering weakens grain boundaries, which accelerates physical disintegration.

f) Moraines vs. eskers. A moraine is an unsorted, unstratified accumulation of glacial till (a poorly sorted mix of clay to boulders) bulldozed, carried, or dumped directly by glacial ice – terminal moraines mark the ice’s farthest advance, lateral moraines flank a valley glacier’s sides, and ground moraine blankets the deglaciated terrain. An esker is a long, sinuous ridge of well-sorted, stratified sand and gravel deposited by a subglacial or englacial meltwater stream flowing within or beneath the ice in an ice-walled tunnel; when the ice melts, the stream’s channel-fill sediment is left standing as a ridge tracing the old tunnel’s path. The key contrast is process (ice-deposited vs. meltwater-deposited) and the resulting sediment texture (unsorted till vs. sorted, stratified sand/gravel).

g) Apparent dip vs. true dip. True dip is the maximum angle a bedding (or other planar) surface makes with the horizontal, measured in the vertical plane perpendicular to strike. Apparent dip is the (always smaller) angle the same plane appears to make with the horizontal when measured in any other vertical section – e.g. along a road cut or borehole cross-section that is not perpendicular to strike. The two are related by $\tan\delta' = \tan\delta \cdot \sin\theta$, where $\delta$ is the true dip, $\delta'$ the apparent dip in a section making angle $\theta$ with the true-dip direction (equivalently, $(90^\circ-\theta)$ from strike); a section cut exactly along strike ($\theta=0$) shows zero apparent dip, and a section cut exactly along the true-dip direction ($\theta=90^\circ$) recovers the true dip.

Worked check: for a true dip of $40^\circ$, a section cut $60^\circ$ from the true-dip direction shows an apparent dip of $\delta' = \arctan(\tan 40^\circ \cdot \sin 60^\circ) \approx \boxed{36.0^\circ}$ – smaller than the true dip, as required.

h) Aquifer vs. aquitard. An aquifer is a saturated geologic unit permeable enough to transmit and yield usable quantities of groundwater to a well or spring (sand, gravel, sandstone, fractured/karstic rock) – high $K$. An aquitard is a unit with low enough permeability that it significantly retards (but does not necessarily stop) vertical or lateral groundwater flow, acting as a confining/semi-confining layer between aquifers (silt, clay, shale, unfractured crystalline rock) – low $K$. The distinction is relative and scale-dependent: whether a unit "is" an aquifer or an aquitard depends on the contrast in $K$ with its neighbours and on the pumping demand being asked of it.