04-BS-14 · December 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams December 2017, 04-BS-14 Geology, 3 hours, CLOSED BOOK (Casio/Sharp approved calculators only). Three questions constitute a complete exam (candidates must answer Questions 1 to 3); every printed sub-part is answered, including the 5 of 9 items in Question 3 the exam does not require.
Reference texts: Marshak, Earth: Portrait of a Planet (general/structural/surficial geology); Goodman, Engineering Geology: Rock in Engineering Construction (engineering-geology, permafrost, mass wasting); Freeze & Cherry, Groundwater (aquifers, wells, water table).
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.
Mass wasting is the downslope movement of rock, regolith and soil under gravity, and its likelihood/style is controlled by several interacting factors. Slope angle/gradient: steeper slopes have a larger gravitational shear-stress component and less frictional resistance margin, so they fail more readily and more violently (falls/slides) than gentle slopes (creep). Material type and cohesion: well-cemented, jointed bedrock fails differently (rock falls, planar/wedge slides along discontinuities) than weakly cohesive soil or unconsolidated regolith (slumps, flows). Water content and pore pressure: water adds weight, lubricates failure surfaces, and — critically — raises pore-water pressure, which reduces the effective normal stress and thus the frictional shear strength (per Mohr–Coulomb, τf = c + (σ − u)tanφ); heavy rainfall or snowmelt is the single most common trigger of slope failure. Vegetation cover: roots mechanically reinforce soil and intercept/transpire water, so removal (fire, logging, development) increases failure susceptibility. Geologic structure and weathering: bedding, joints, faults and foliation oriented parallel to (and dipping out of) a slope face create ready-made failure planes; weathering progressively weakens rock toward the surface. Seismic shaking adds transient dynamic stress that can trigger failure on slopes already near their strength limit. Human activity — slope undercutting (road cuts, river erosion), added loading (fill, structures), and altered drainage — frequently pushes a marginally stable slope past failure.
The hydrologic cycle is the continuous circulation of water between the ocean, atmosphere, and land, driven by solar energy and gravity. Solar heating drives evaporation of ocean (and land surface) water into water vapour, supplemented by transpiration from plants (together, "evapotranspiration"). As moist air rises and cools — often forced upward over mountains — water vapour undergoes condensation into cloud droplets, which grow and fall as precipitation (rain, snow). On reaching land, precipitation either becomes surface runoff, flowing downslope into streams and rivers that eventually return it to the ocean, or undergoes infiltration into the soil and underlying rock, becoming groundwater that flows slowly (governed by Darcy's law) through the subsurface, discharging to streams, springs, wetlands or directly back to the ocean. Some water is temporarily stored as snow/ice, lakes, or groundwater, but over geologic and hydrologic timescales the system is a closed loop with no net gain or loss.
Alpine (valley) glaciers form in mountainous terrain, confined within and flowing down a pre-existing river valley from a high-elevation accumulation zone (often a cirque); they are relatively small, erode distinctive U-shaped valleys, cirques, arêtes and horns, and their flow direction is topographically controlled by the valley walls. Continental ice sheets are vast (sub-continental scale, e.g. Antarctica, Greenland, or the former Laurentide Ice Sheet) domes of ice that bury underlying topography entirely and flow outward from a central thick region under their own weight, independent of pre-existing valleys; they produce regional-scale erosional and depositional landscapes (till plains, drumlin fields, outwash plains). Piedmont glaciers form where one or more alpine glaciers emerge from confining mountain valleys onto an adjacent lowland and spread out laterally into a broad, fan-like lobe of ice (e.g. the Malaspina Glacier, Alaska), combining characteristics of both valley confinement upstream and unconfined spreading downstream.
Erosional features — produced by glacial abrasion and plucking: Cirques, amphitheatre-shaped hollows carved at the head of an alpine glacier by rotational plucking and abrasion; arêtes and horns, knife-edge ridges and pyramidal peaks left where cirques on opposing sides of a divide erode headward toward each other; and U-shaped (glacial trough) valleys, formed when a glacier widens and deepens a former V-shaped stream valley, truncating interlocking spurs.
Depositional features — produced where a glacier deposits till or meltwater deposits outwash: Moraines (terminal, lateral, and ground moraines), ridges or blankets of unsorted till marking the former margin, sides, or base of the ice; drumlins, streamlined, elongated hills of till moulded parallel to ice-flow direction beneath moving ice; and eskers, sinuous ridges of stratified sand and gravel deposited in subglacial or ice-marginal meltwater tunnels.
Bowen's Reaction Series (N.L. Bowen, early 20th century) describes the sequence in which silicate minerals crystallize from a cooling, differentiating basaltic magma, and it has two parallel branches that converge at the bottom. The discontinuous branch consists of distinct ferromagnesian (Fe/Mg-bearing) mineral species that crystallize in sequence as temperature falls and each reacts with the remaining melt to form the next: olivine → pyroxene → amphibole → biotite mica, each step representing an abrupt change in crystal structure. The continuous branch consists of plagioclase feldspar, which crystallizes continuously across the same temperature range but with a smoothly changing composition — from Ca-rich (anorthite) at high temperature to Na-rich (albite) at low temperature, as Ca²⁺ is progressively replaced by Na⁺ in the crystal lattice. Both branches converge at low temperature into the final minerals to crystallize: K-feldspar, muscovite mica, and finally quartz, the last mineral to form. The series both predicts crystallization order in a cooling magma (controlling igneous rock textures such as zoned plagioclase and reaction rims) and — because minerals crystallizing at higher temperature/further from surface equilibrium are also chemically and structurally the least stable at Earth-surface conditions — parallels the Goldich weathering-stability series: olivine weathers fastest, quartz slowest.
Plate tectonics is the unifying theory that Earth's rigid outer layer (the lithosphere, comprising the crust and uppermost mantle) is broken into a mosaic of large and small plates that move relative to one another, riding atop the weaker, more ductile asthenosphere beneath. Plate motion is driven primarily by mantle convection, and more specifically by ridge push (gravitational sliding away from the elevated, buoyant mid-ocean ridge) and slab pull (the greater density of cold, subducting oceanic lithosphere dragging the trailing plate along), with basal traction from convecting mantle playing a secondary role. Plates interact at three boundary types: divergent boundaries (mid-ocean ridges, continental rifts), where plates move apart and new oceanic lithosphere forms by seafloor spreading; convergent boundaries, where plates collide — oceanic–continental or oceanic–oceanic convergence produces subduction (one plate descends beneath the other, generating volcanic arcs and deep-focus earthquakes), while continent–continent convergence produces crustal thickening and mountain building (e.g. the Himalaya); and transform boundaries, where plates slide horizontally past one another (e.g. the San Andreas Fault), producing shallow, often large strike-slip earthquakes with no significant creation or destruction of lithosphere. The theory unifies and explains the distribution of earthquakes, volcanism, mountain belts, and the magnetic striping and age progression of the seafloor, superseding the earlier, mechanistically incomplete idea of simple continental drift.
Shield volcanoes are broad, gently sloping domes built almost entirely of low-viscosity basaltic lava flows (e.g. Mauna Loa); because the magma is fluid and gas-poor, eruptions are predominantly effusive rather than explosive, producing a wide, low profile. Composite volcanoes (stratovolcanoes) are tall, steep-sided cones built of alternating layers of viscous, intermediate-to-felsic (andesitic–dacitic) lava flows and explosive pyroclastic material (e.g. Mount Fuji, Mount St. Helens); the higher-viscosity, gas-rich magma produces episodic explosive eruptions interspersed with lava flows, and these are the most hazardous volcano type. Cinder cones are small, steep, conical hills built from loose pyroclastic fragments (cinders/scoria) ejected from a single vent during a short-lived, moderately explosive eruption of gas-rich basaltic-to-andesitic magma; they are the simplest and shortest-lived volcano type, often forming in a single eruptive episode. Lava domes form when highly viscous, silica-rich (rhyolitic–dacitic) magma is too stiff to flow far and instead piles up directly over the vent as a bulbous mass; because gas cannot escape easily through the viscous rock, lava domes are prone to sudden, violent collapse and explosive disintegration (pyroclastic flows).
From the surface downward, a typical permafrost profile has three zones. The active layer is the shallow surface zone (commonly <0.5–2 m) that thaws each summer and refreezes each winter; nearly all biological activity, plant roots, and seasonal frost heave and thaw settlement occur here. Below the permafrost table (the top of the perennially frozen ground) lies the permafrost itself — ground that has remained at or below 0°C for at least two consecutive years, ranging from a few metres to hundreds of metres thick in continuous-permafrost regions; it typically contains segregated ground ice (lenses, wedges) in addition to frozen pore water. Below the base of permafrost the geothermal gradient has warmed the ground above 0°C, and unfrozen ground (talik) continues downward. On the temperature–depth graph, drawn to the same depth scale as the profile, the summer curve is warm at the surface and cools with depth, crossing 0°C exactly at the permafrost table (this crossing defines the base of the active layer). The winter curve is far below 0°C at the surface and warms with depth but stays below 0°C, because the whole active layer is refrozen in winter. The two curves form a "trumpet" that narrows with depth: the seasonal swing dies out, and both curves meet at the depth of zero annual amplitude (roughly 10–20 m) at the mean annual ground temperature, which is below 0°C. Below that point a single curve warms along the geothermal gradient and crosses 0°C at the base of the permafrost. This is why foundations, pipelines and roads in permafrost terrain use thermosyphons and insulation. They keep the permafrost table from dropping and so prevent thaw settlement, which is consistent with item 11 of Question 1.
Both curves begin with the same straight-line elastic segment, whose slope is the material's Young's modulus; deformation in this region is fully recoverable if the load is removed. At the elastic limit (yield point) the two materials diverge. The brittle sample (e.g. cold, unconfined, near-surface rock) has essentially no capacity for permanent (plastic) strain: stress rises to a peak just past the elastic limit and then the sample fractures suddenly, with the stress curve dropping vertically to near zero — failure occurs at low total strain and with little warning. The ductile sample (e.g. rock deformed at depth, under high confining pressure and/or high temperature, or a metal) yields at a similar or lower stress but then continues to accumulate strain at roughly constant or slowly changing stress (a plastic "flow" plateau) before eventually failing at a much larger total strain. The area under each curve represents the energy absorbed prior to failure (toughness); the ductile curve encloses substantially more area, which is why ductile materials/structures give warning (visible deformation) before failing, while brittle materials do not.
