04-BS-14 · December 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. National Exams, December 2018 — 04-BS-14 Geology. Three hours, closed book, one approved Casio or Sharp calculator. Four questions constitute a complete paper: Questions 1, 2 and 3 are mandatory, and on Question 4 the candidate chooses four items from the bank of items 34–44. Marks: Q1 = 20, Q2 = 10, Q3 = 30 (item 31 = 10, item 32 = 6, item 33 = 14), Q4 = 40 (four items at 10 marks each). Total = 100 marks.
Reference texts.
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.
Approach. All eleven items are answered at the depth appropriate to a ten-mark essay. An examinee would select four; the remaining seven are provided as well.
An unconformity is a buried erosion or non-deposition surface separating rocks of appreciably different age. It represents missing time — a hiatus during which the region was uplifted above base level, or at least received no sediment, and during which an unknown thickness of rock may have been removed. Unconformities are classified by the relationship between the rocks below the surface and those above it.
An angular unconformity separates tilted or folded strata below from flat-lying strata above. Its history is deposition, deformation, uplift, erosional bevelling, subsidence and renewed deposition, so it records the most complete tectonic cycle of the three. Hutton’s classic exposure at Siccar Point is the type example, and the surface beneath unit B in item 33 of this paper is another.
A disconformity separates parallel strata; bedding above and below is concordant, and only an irregular erosion surface, a palaeosol, a lag of resistant clasts or a break in the fossil record reveals the gap. Because the geometry gives nothing away, disconformities are the easiest to overlook and are usually identified biostratigraphically, from the missing zones.
A nonconformity separates sedimentary strata above from igneous or metamorphic basement below. It requires the crystalline rocks to have been unroofed by deep erosion before burial, so it implies a very large hiatus — the surface at the base of the Paleozoic cover across much of the Canadian Shield is a continental-scale example, and the surface above the eroded pluton in item 33 is a local one.
Some texts add a fourth category, the paraconformity, for a disconformity so planar that no erosion surface is visible at all. In engineering practice these surfaces matter because they are commonly weathered, weak and permeable, and because they concentrate groundwater flow along the contact.
A drainage pattern is the plan-view arrangement of a stream network, and it is inherited directly from the erodibility and structure of the substrate. Four common patterns are as follows.
Dendritic. An irregularly branching, tree-like network with tributaries joining at acute angles. It develops where the substrate is essentially uniform in resistance and has no strong structural grain — flat-lying sedimentary rocks, massive crystalline rock, or thick till. It is the default pattern, and its presence is evidence against strong structural control.
Trellis. Long parallel main streams with short tributaries entering at right angles. It develops on tilted or folded sequences of alternating resistant and weak beds: the main streams occupy the strike valleys eroded in the weak units, and the short tributaries drain the ridges of resistant rock. Folded belts such as the Valley and Ridge, or the folded Foothills of the Canadian Rockies, produce it.
Rectangular. Streams meeting at sharp right-angle bends and following straight segments. It reflects a substrate cut by two intersecting sets of joints or faults, so the channels exploit the pre-existing fracture grid. It is common on jointed granite and on faulted terrain of the Canadian Shield.
Radial. Streams flowing outward in all directions from a central high. It develops on an isolated conical or domal topographic high — a stratovolcano, an intrusive dome, or a structural dome. The inverse pattern, centripetal, converges inward on a closed basin, crater or sinkhole; an annular pattern of concentric streams develops on a dissected dome where alternating resistant and weak beds crop out in rings.
Mass wasting is the downslope movement of rock, soil and debris under gravity, without a transporting medium such as flowing water or ice. It occurs whenever the driving shear stress on a potential failure surface exceeds the available shear strength, so every controlling factor acts on one side of that inequality or the other.
Slope angle and relief. The gravitational shear component along a potential slip surface increases with slope angle while the normal stress that generates frictional resistance decreases, so steepening a slope raises the driving stress and lowers the resistance at the same time. Undercutting by a river, wave action or an excavation is one of the commonest triggers precisely because it steepens the toe.
Water content and pore-water pressure. Water adds weight, and far more importantly it raises pore pressure, which reduces the effective normal stress and therefore the frictional strength on the failure surface. Most landslides in Canada occur during or immediately after heavy rain or rapid snowmelt, and rapid drawdown of a reservoir has the same effect on the submerged part of a slope.
Material type, structure and discontinuities. The strength of the mass, whether it is cohesive clay, cohesionless sand or jointed rock, sets the resisting side. In rock the orientation of bedding, joints and faults usually matters more than the intact strength: a slope with discontinuities dipping out of the face at an angle greater than their friction angle can fail even where the rock itself is strong. Sensitive marine clays, such as the Leda (Champlain Sea) clays of the Ottawa and St. Lawrence valleys, lose almost all strength on remoulding and produce large retrogressive flowslides.
Vegetation cover. Root systems reinforce soil in tension, and canopy interception and transpiration reduce the water reaching and remaining in the slope. Removing vegetation by wildfire or clear-cut logging therefore raises landslide frequency for the several years until roots decay and before replacement roots establish.
Triggering events and ground shaking. Earthquakes impose cyclic inertial loads that add directly to the driving stress and can liquefy loose saturated sand, and freeze–thaw cycling, volcanic activity, blasting and construction loading all act as triggers on slopes already close to limiting equilibrium.
Also frequently listed and equally acceptable are climate, because it governs weathering depth and moisture regime, and human activity such as loading the crest of a slope with fill or a structure, which raises the driving moment directly.
Erosional feature 1 — the cirque. A cirque is the steep-walled, amphitheatre-shaped hollow excavated at the head of an alpine glacier by a combination of plucking at the headwall, where meltwater freezes into joints and pulls blocks away as the ice moves, and abrasion of the floor. Continued growth of adjacent cirques produces arêtes between them and a horn where three or more meet, as at the Matterhorn or Mount Assiniboine.
Erosional feature 2 — the U-shaped (glacial trough) valley. A glacier occupying a former V-shaped river valley erodes its floor and both walls, straightening the course, truncating interlocking spurs and leaving a broad, flat-floored, steep-sided trough with hanging tributary valleys above it. Where such a trough is later drowned by the sea it becomes a fiord, as along the British Columbia coast.
Depositional feature 1 — the moraine. Moraines are accumulations of unsorted, unstratified till deposited directly by ice. A terminal or end moraine is the ridge built at the maximum extent of the ice; recessional moraines mark stillstands during retreat; lateral moraines flank a valley glacier, and a medial moraine forms where two lateral moraines merge at a confluence. Ground moraine is the sheet of till left beneath the ice as a till plain.
Depositional feature 2 — the esker. An esker is a long, sinuous ridge of sorted and stratified sand and gravel deposited by meltwater flowing in a tunnel within or beneath the ice and left standing above the surrounding ground when the ice melts. Eskers cross valleys and hills alike, because the water was flowing under hydraulic pressure rather than under gravity alone, and they are important aggregate sources across the Canadian Shield.
The key contrast between the two depositional features is sorting: till deposited directly from ice is unsorted and unstratified because ice carries all grain sizes at once, whereas glaciofluvial deposits such as eskers, kames and outwash plains are sorted and stratified because running water separates grain sizes by competence.
The hydrologic cycle is the continuous, closed circulation of water among ocean, atmosphere, land surface and subsurface, driven by solar energy and by gravity. Over the Earth as a whole, and averaged over a long enough period, it is a closed system: total precipitation equals total evaporation, roughly 500,000 km³ per year.
Evaporation and transpiration. Solar radiation evaporates water from the ocean, which supplies roughly 85 per cent of atmospheric moisture, and from lakes, soil and snow. Plants add water drawn from the root zone by transpiration; the two are usually lumped as evapotranspiration because they are difficult to separate in practice.
Condensation, transport and precipitation. Rising moist air cools adiabatically to its dew point, water vapour condenses on nuclei to form cloud, and the droplets or ice crystals grow until they fall as rain, snow, hail or freezing rain. Atmospheric circulation carries this moisture from ocean to continent, which is what makes the cycle a net transfer of water onto land.
The land phase. Precipitation reaching the ground is partitioned. Some is intercepted by the canopy and re-evaporated; some infiltrates; some is held in depression storage; the remainder becomes overland flow, which concentrates into channels as runoff. In cold regions a large fraction is stored seasonally as snowpack and released in the spring freshet, and a small fraction is stored for millennia in glaciers and ice caps.
The subsurface phase. Infiltrating water passes through the unsaturated (vadose) zone to the water table, recharging the saturated zone. Groundwater moves slowly under hydraulic gradient from recharge areas to discharge areas, emerging in springs, wetlands, streams and directly into the ocean. This groundwater discharge is what sustains streamflow between storms as baseflow, and its residence time ranges from days in a shallow aquifer to many thousands of years in a deep confined one.
Storage. The reservoirs are grossly unequal: the oceans hold about 97 per cent of all water, ice caps and glaciers about 2 per cent, groundwater most of the remainder, and lakes, rivers, soil moisture and the atmosphere together well under 0.1 per cent. The atmosphere, the smallest reservoir, turns over roughly every ten days, which is why the cycle is fast even though most of the water is locked in slow storage.
A glacier is a perennial mass of ice, formed by the compaction and recrystallisation of snow, that deforms and flows under its own weight. Glaciers are classified chiefly by their size and by whether the underlying topography controls their flow.
Valley (alpine) glaciers. These occupy pre-existing mountain valleys and flow down them, so their shape and flow direction are entirely controlled by topography. They are long and narrow, typically a few kilometres to tens of kilometres, they head in cirques, they carry conspicuous lateral and medial moraines, and their surfaces are broken by crevasses where the ice passes over convexities. They are the principal agents of alpine erosion, producing troughs, arêtes and horns. The Athabasca Glacier in the Columbia Icefield is a Canadian example. A valley glacier that spreads out onto a lowland beyond the mountain front forms a lobate piedmont glacier.
Ice caps and icefields. These are dome-shaped masses that bury the underlying relief over an area conventionally taken as less than 50,000 km², flowing radially outward from a central high rather than following valleys, and commonly feeding outlet glaciers that descend through gaps in the surrounding mountains. Where the highest peaks still project through the ice as nunataks and the ice remains partly topographically controlled, the mass is termed an icefield — the Columbia Icefield and the Penny Ice Cap on Baffin Island illustrate the two cases.
Continental ice sheets. These are the largest form, covering more than 50,000 km² and burying all but the highest terrain, kilometres thick, and flowing outward from one or more central domes independently of the buried topography. Only Antarctica and Greenland survive today, together holding most of the world’s fresh water, but the Laurentide ice mass covered most of Canada at the last glacial maximum and is responsible for the till plains, drumlin fields, eskers and isostatic rebound of the Canadian landscape. Where such an ice body flows out over the sea and floats while remaining attached, it forms an ice shelf from which tabular icebergs calve.
In every case the mass balance controls behaviour: the glacier gains mass by snowfall in the accumulation zone above the equilibrium line and loses it by melting and calving in the ablation zone below, and the terminus advances or retreats according to the sign of the net balance even though the ice itself always flows downglacier.
This item extends item 37; the cirque, U-shaped valley, moraine and esker described there are repeated here in summary and three further features are added, so that three of each type are given in full.
Erosional 1 — cirque. Amphitheatre-shaped hollow at a glacier head, excavated by headwall plucking and floor abrasion; commonly holds a tarn after deglaciation.
Erosional 2 — U-shaped glacial trough. Broad, flat-floored, steep-walled valley with truncated spurs and hanging tributary valleys; drowned examples are fiords.
Erosional 3 — roche moutonnée and striated pavement. A bedrock knob smoothed, polished and striated by abrasion on its up-ice side and steepened by plucking on its down-ice side. Striations and the asymmetry together record the direction of ice flow, which is the primary field evidence used to reconstruct former ice movement across the Canadian Shield.
Depositional 1 — moraine. Ridges and sheets of unsorted, unstratified till laid down directly by ice: terminal, recessional, lateral, medial and ground moraine.
Depositional 2 — esker. Sinuous ridge of sorted, stratified sand and gravel deposited in a subglacial or englacial meltwater tunnel; a major aggregate resource.
Depositional 3 — drumlin. A streamlined hill of till, tens of metres high and hundreds of metres to a few kilometres long, blunt and steep on the up-ice end and tapering down-ice. Drumlins occur in fields of hundreds and are moulded beneath actively flowing ice, so their long axes give ice-flow direction. Also acceptable in this group are the outwash plain (a broad sheet of sorted sand and gravel deposited by braided meltwater streams beyond the ice margin), the kame (a mound of stratified sediment let down from a hollow in the ice), the kettle (a depression left where a buried ice block melted, often now a pond) and glaciolacustrine varves.
The unifying principle is again sorting. If the deposit is unsorted and unstratified, ice placed it; if it is sorted and stratified, meltwater placed it. That single test also predicts the geotechnical behaviour of the deposit, which is why it matters in practice as well as in the exam.
Bowen’s Reaction Series, established experimentally by N. L. Bowen in the early twentieth century, describes the order in which silicate minerals crystallise from a cooling basaltic magma, and equally the order in which they melt on heating. It explains how a single parent magma can produce a range of igneous rock compositions by fractional crystallisation, and it also predicts the relative weathering stability of the common rock-forming minerals.
The discontinuous branch comprises the ferromagnesian minerals olivine, pyroxene, amphibole and biotite. It is discontinuous because each mineral, once formed, reacts with the remaining melt at a definite temperature to produce a structurally different mineral: olivine (isolated tetrahedra) reacts to pyroxene (single chains), pyroxene to amphibole (double chains), and amphibole to biotite (sheets). Each step shares more oxygens between tetrahedra and incorporates more silica, water and alkalis.
The continuous branch is the plagioclase feldspar solid-solution series. Crystals that form first are calcium-rich (anorthite); as the melt cools they react continuously with it, exchanging Ca and Al for Na and Si, and become progressively sodium-rich (albite). The change is continuous because the crystal structure is unchanged throughout — only the composition shifts — and if crystals are removed or armoured by rims, zoned plagioclase with calcic cores and sodic margins results.
The common tail. Below about 800 °C the two branches converge and the residual melt, now enriched in silica, potassium and water, crystallises potassium feldspar, then muscovite, and finally quartz at the lowest temperature.
Consequences. First, fractional crystallisation: if early-formed crystals settle out or are otherwise isolated from the melt, the residual liquid becomes progressively more felsic, so a basaltic parent can yield andesitic and ultimately granitic derivatives. Second, minerals far apart on the series rarely occur together, which is why olivine and quartz are almost never found in the same igneous rock. Third, the sequence predicts weathering stability — the reverse of the Goldich stability series: minerals that crystallised at the highest temperature, furthest from surface conditions, break down fastest, so olivine and calcic plagioclase weather quickly while quartz, formed at the lowest temperature, survives to dominate mature sands.
Volcano form is governed chiefly by magma composition, which sets viscosity and gas content, and therefore by eruption style.
Shield volcano. Built almost entirely of low-viscosity basaltic lava flows that travel far before congealing, so the edifice is very broad with gentle flank slopes of only a few degrees, but can be enormous in volume. Eruptions are effusive rather than explosive because gas escapes easily from fluid magma. Mauna Loa is the type example; shields characterise hotspots and mid-ocean ridges.
Cinder (scoria) cone. A small, steep-sided cone, generally under 300 m high, built of loose pyroclastic scoria ejected ballistically from a single vent and piled at the angle of repose, about 30–35 degrees. It usually has a large summit crater relative to its size, is monogenetic (erupting once, over months to years) and is commonly basaltic. Parícutin in Mexico is the classic example.
Composite volcano (stratovolcano). A large, steep-sided, symmetrical cone built of alternating layers of lava flows and pyroclastic material, generally of andesitic composition. The intermediate, gas-rich, viscous magma produces both effusive and violently explosive eruptions, so composite cones are the most hazardous type, generating pyroclastic flows and lahars. They are the characteristic volcano of subduction-zone arcs — Mount St. Helens, Mount Fuji, and Mount Garibaldi and Mount Meager in British Columbia.
Caldera. A large, roughly circular depression, kilometres to tens of kilometres across, formed when the roof of a shallow magma chamber collapses after a very large eruption evacuates the chamber. Calderas are associated with highly viscous, gas-rich rhyolitic magma and with the largest known eruptions; Crater Lake and Yellowstone are examples. They are a volcanic landform produced by collapse rather than by construction, which is what distinguishes them from the cones above.
Lava dome (volcanic dome). A steep-sided, bulbous mound of highly viscous rhyolitic or dacitic lava too stiff to flow away from the vent, so it piles up over it, often within the crater of a composite volcano. Domes grow by endogenous inflation and by extrusion of spines, and they are dangerous because their gravitational collapse generates block-and-ash pyroclastic flows — the dome-building phases of Mount St. Helens after 1980 are the standard example.
A sixth form sometimes listed is the fissure eruption and associated flood-basalt plateau, in which basaltic lava issues from a linear fracture rather than a central vent and builds no cone at all, producing instead vast plateaus such as the Columbia River Basalts.
Permafrost is ground — soil, sediment or rock — that remains at or below 0 °C continuously for at least two consecutive years. The definition is purely thermal: it says nothing about ice content, and dry permafrost with no ice at all is possible, although in Canadian fine-grained soils permafrost is usually ice-rich.
The active layer is the uppermost layer, which thaws each summer and refreezes each winter. Its thickness ranges from roughly 0.3 m in ice-rich fine-grained soil under thick organic cover in the high Arctic to several metres in coarse, well-drained gravel further south. Everything that matters to engineering happens here: seasonal frost heave as ice lenses grow on freezing, thaw settlement and loss of bearing capacity on thawing, and solifluction on slopes. Foundations in permafrost are therefore either taken through the active layer to bear in the permafrost, on piles with an air gap or thermosyphons to keep the ground frozen, or designed to tolerate the seasonal movement.
The permafrost body lies beneath the permafrost table, the surface at the base of the active layer. Its temperature is below 0 °C year-round and its thickness is set by the balance between mean annual ground surface temperature at the top and geothermal heat flux from below. In Canada it ranges from a few metres at the southern fringe of the discontinuous zone to several hundred metres in the continuous zone, exceeding 500 m in parts of the high Arctic. Unfrozen zones called taliks may persist within it beneath lakes and rivers that do not freeze to the bed.
The unfrozen ground below begins at the permafrost base, where the geothermal gradient brings the temperature back up through 0 °C.
The temperature–depth curves. Plotting ground temperature against depth for mid-summer and mid-winter produces the classic “trumpet” envelope. Near the surface the two curves are far apart, because the surface temperature swings widely between seasons; the summer curve is warmest at the surface and the winter curve coldest. Both curves swing back toward the mean as depth increases, because the seasonal wave is damped exponentially and lagged as it diffuses downward. At the depth of zero annual amplitude, typically 10–20 m, the seasonal variation dies out and the two curves converge on the mean annual ground temperature. Below that depth a single, nearly linear geothermal gradient of roughly 25 °C per kilometre applies, warming with depth until it crosses 0 °C at the permafrost base. Note that the summer curve crosses 0 °C at the permafrost table — that intersection is the definition of the active-layer thickness — whereas the winter curve is below 0 °C throughout.
The interior is known chiefly from the travel times of seismic waves, supplemented by the Earth’s mean density of about 5.5 Mg/m³, its moment of inertia, meteorite compositions and laboratory work at high pressure. Two complementary subdivisions are used: one by composition, giving crust, mantle and core, and one by mechanical behaviour, giving lithosphere, asthenosphere, mesosphere, outer core and inner core. The Earth’s mean radius is 6371 km.
The crust is the thin, chemically distinct outer unit, separated from the mantle by the Mohorovičić discontinuity, at which P-wave velocity jumps from about 7 to about 8 km/s. Oceanic crust is only 5–10 km thick, basaltic to gabbroic in composition and dense at about 3.0 Mg/m³; continental crust is 30–70 km thick, granitic to dioritic in average composition and lighter at about 2.7 Mg/m³. That density contrast is why continents stand high and oceanic lithosphere subducts. The crust is a trivial fraction of the Earth by volume — under one per cent.
The mantle extends from the Moho to 2900 km depth, so it is about 2865 km thick and makes up some 84 per cent of the Earth’s volume. It is solid ultramafic rock, peridotite — olivine and pyroxene — with density rising from about 3.3 to about 5.5 Mg/m³ and with phase transitions at 410 and 660 km marking the transition zone between upper and lower mantle. Although solid, the mantle creeps over geological time and convects, and that convection drives plate motion.
The outer core extends from 2900 to 5150 km, a thickness of 2250 km. It is a liquid alloy of iron and nickel with about ten per cent of lighter elements such as sulphur and oxygen, with density near 10 Mg/m³. That it is liquid is proved directly by the S-wave shadow zone: shear waves cannot propagate through a liquid, so they stop at 2900 km, while P-waves are refracted sharply at the boundary. Convection of this conducting liquid, organised by the Earth’s rotation, generates the geomagnetic field by dynamo action.
The inner core extends from 5150 km to the centre at 6371 km, so its radius is 1221 km. It is solid iron–nickel at about 13 Mg/m³ and roughly 5000 °C; it is solid despite that temperature because pressure at the centre, some 360 GPa, raises the melting point above the local temperature. It is inferred from a weak P-wave arrival refracted through it and grows slowly as the outer core freezes onto it, releasing latent heat and light elements that help to drive the dynamo.
The mechanical subdivision cuts across the compositional one at shallow depth, which is the point most often missed. The lithosphere — crust plus the uppermost, coolest mantle, together about 100 km thick under oceans and up to 250 km under old cratons — behaves rigidly and is broken into the tectonic plates. Beneath it the asthenosphere, roughly 100–350 km deep, is hot enough to be near its solidus and deforms readily, allowing the plates above to move and isostatic rebound to proceed. Below that the mesosphere is stiffer again because of pressure, though still solid and convecting.
