04-BS-14 · Undated paper

Question 4 of 4: Answer four (4) of the following questions (40 marks)

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Question 4: Answer four (4) of the following questions (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.

Approach. All twelve items are answered at the depth appropriate to a ten-mark essay; an examinee selects four.

13) Three types of landslides (10 marks)

"Landslide" is used loosely for any rapid, gravity-driven mass movement; formally it is classified by the type of moving material and by the mechanism of motion.

Rockfall. Detachment and free fall, bouncing or rolling of individual rock blocks from a steep or overhanging rock face, typically triggered by freeze–thaw wedging, root growth or undercutting along pre-existing joints. Motion is extremely rapid and the material is not water-saturated; accumulated debris forms a talus cone at the base of the slope.

Rotational slump. Movement of a coherent mass along a concave-upward failure surface, so the top of the slide block rotates backward while its toe bulges outward — common in thick, homogeneous clay or till slopes such as the Leda (Champlain Sea) clay bluffs of the Ottawa/St. Lawrence valleys. Slumps are moderate in velocity and often occur as retrogressive, multiple, headward-stepping failures.

Debris flow. A water-saturated slurry of soil, rock fragments and organic debris that moves as a viscous fluid, often confined to and accelerated within a channel or gully. Debris flows are among the fastest and most destructive mass movements, triggered by intense rainfall or rapid snowmelt on steep terrain, and are common in mountainous British Columbia after wildfire has removed slope-stabilising vegetation.

14) Three types of unconformities (10 marks)

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 lay above base level (or simply received no sediment) and during which an unknown thickness of rock may have been removed. The three types are classified by the relationship between the rocks below and above the surface.

Angular unconformity. Tilted or folded strata below, flat-lying strata above. Records the fullest cycle: deposition, deformation, uplift, erosional bevelling, subsidence and renewed deposition. Hutton's Siccar Point is the type example.

Disconformity. Parallel strata above and below; the bedding is concordant across the gap, and only an irregular erosion surface, a palaeosol, a basal lag of resistant clasts, or a break in the fossil record reveals the missing time. Because the geometry gives nothing away, disconformities are the easiest of the three to overlook.

Nonconformity. Sedimentary strata above, igneous or metamorphic (crystalline basement) rock below. Requires the crystalline rock to have been unroofed by deep erosion before burial, implying a very large hiatus — the Precambrian basement beneath much of the Canadian Shield's Paleozoic cover is a continental-scale example.

15) Four drainage patterns and their underlying geology (10 marks)

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.

16) Five factors that influence mass wasting (10 marks)

Mass wasting is 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.

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 simultaneously. Undercutting by a river, wave action or an excavation is a common trigger because it steepens the toe.

Water content and pore-water pressure. Water adds weight, and more importantly raises pore pressure, which reduces effective normal stress and therefore frictional strength. Most Canadian landslides occur during or after heavy rain or rapid snowmelt.

Material type, structure and discontinuities. The strength of the mass — cohesive clay, cohesionless sand, or jointed rock — sets the resisting side. In rock, discontinuity orientation usually matters more than intact strength: a joint set dipping out of a face steeper than its friction angle can fail even where the rock itself is strong. Sensitive Leda clays of the Ottawa/St. Lawrence valleys lose most of their strength on remoulding.

Vegetation cover. Roots reinforce soil in tension, and canopy interception/transpiration reduce infiltration. Wildfire or clear-cut logging elevates landslide frequency for years until roots decay and replacements establish.

Triggering events (seismicity, freeze–thaw, human loading). Earthquakes add cyclic inertial load and can liquefy loose saturated sand; freeze–thaw cycling, blasting, and loading the crest of a slope with fill or a structure all act as triggers on slopes already near limiting equilibrium.

17) Three erosional and three depositional glacial features (10 marks)

Erosional — cirque. The steep-walled, amphitheatre-shaped hollow excavated at the head of an alpine glacier by plucking at the headwall and abrasion of the floor. Adjacent cirques growing headward leave arêtes between them and a horn where three or more meet (e.g. the Matterhorn, Mount Assiniboine).

Erosional — U-shaped (glacial trough) valley. A glacier occupying a former V-shaped river valley erodes floor and walls, straightening the course, truncating interlocking spurs, and leaving a broad, flat-floored, steep-sided trough with hanging tributary valleys above it. A drowned trough becomes a fiord, as along coastal British Columbia.

Erosional — roche moutonnée / striations. Ice sliding over bedrock abrades a gently polished and striated up-ice (stoss) face and plucks a steeper, irregular down-ice (lee) face, producing an asymmetric bedrock knob that also records the direction of ice flow.

Depositional — moraine. Accumulations of unsorted, unstratified till deposited directly by ice: terminal/end moraines mark the ice's maximum extent, recessional moraines mark stillstands during retreat, lateral moraines flank a valley glacier, and ground moraine is the sheet of till left as a till plain.

Depositional — esker. A long, sinuous ridge of sorted, stratified sand and gravel deposited by meltwater flowing in an ice tunnel, left standing when the ice melts; eskers cross valleys and hills alike because the water flowed under hydraulic pressure.

Depositional — drumlin. A streamlined, elongate hill of till, blunt and steep on its up-ice (stoss) end and tapering gently down-ice, moulded subglacially and, like the roche moutonnée, usable as an ice-flow indicator.

The key contrast between erosional and depositional features is between removal and accumulation, and within depositional features between direct ice deposition (till: unsorted, unstratified) and glaciofluvial deposition (sorted, stratified).

18) Three types of glaciers (10 marks)

Alpine (valley) glaciers. Confined within a pre-existing mountain valley, fed by snow accumulating in a cirque at its head and flowing down-valley under gravity. They actively erode and reshape the valley (cirques, arêtes, horns, U-shaped troughs) and are the type most sensitive to modern climate change because of their small mass and steep mass-balance gradient. Examples: the Athabasca and Illecillewaet glaciers, Canadian Rockies.

Continental ice sheets. Vast (>50,000 km²) domes of ice that bury the underlying topography entirely and flow outward from a central accumulation zone under their own weight, independent of the terrain beneath. The Laurentide Ice Sheet covered most of Canada during the Pleistocene; today only the Antarctic and Greenland ice sheets remain, and their thickness (up to several kilometres) and volume dominate global sea level.

Piedmont glaciers. Formed where one or more alpine glaciers emerge from confining mountain valleys onto an adjacent lowland and spread out into a broad, lobate ice apron unconstrained by valley walls, as in the Malaspina Glacier of Alaska. They represent an intermediate case, morphologically between valley and continental glaciation.

19) Bowen's Reaction Series (10 marks)

Bowen's Reaction Series, established experimentally by N. L. Bowen in the early twentieth century, describes the order in which minerals crystallize from a cooling basaltic magma, and simultaneously predicts the sequence of reactions between early-formed crystals and the remaining melt as cooling continues.

Discontinuous branch. A sequence of distinct ferromagnesian minerals, each stable over a narrow temperature range: olivine crystallizes first at the highest temperature, and as the melt cools further, olivine reacts with the remaining liquid to form pyroxene, which in turn reacts to form amphibole, then biotite mica. Each reaction is discontinuous because it involves an abrupt change in crystal structure from one mineral to the next.

Continuous branch. Plagioclase feldspar crystallizes throughout the same temperature interval, but its composition changes continuously (without a structural break) from calcium-rich at high temperature to sodium-rich at low temperature, as Na progressively substitutes for Ca (with coupled Si for Al substitution) in the crystal lattice as cooling proceeds.

Convergence and late-stage minerals. The two branches converge at the base of the series, and the last minerals to crystallize from the residual, silica-enriched melt are potassium feldspar, muscovite mica, and finally quartz — the felsic assemblage of a granite.

The series has two major practical uses: it explains the sequence of igneous rock compositions (peridotite/gabbro → diorite → granite) that can be produced from a single parent basaltic magma by fractional crystallization (removal of early-formed crystals from the remaining liquid), and it predicts which minerals in a rock are most susceptible to weathering — the "Goldich weathering series" is the mirror image of Bowen's series, because minerals that crystallized furthest from surface temperature and pressure (olivine, Ca-plagioclase) are least stable at the surface and weather first, while quartz, crystallizing closest to surface conditions, is the most weathering-resistant common rock-forming mineral.

20) Five types of volcanoes (10 marks)

Volcano form is controlled primarily by magma composition and viscosity, which in turn control eruption style (effusive versus explosive).

Shield volcano. Very large in basal diameter, low, broad, gently sloping (2–10°) profile built almost entirely of low-viscosity basaltic lava flows erupted effusively with little explosive activity. Mauna Loa/Kilauea (Hawaii) are the type examples; shields can be enormous in volume because fluid lava travels far from the vent before solidifying.

Composite volcano (stratovolcano). Large, steep-sided, symmetrical cone built of alternating layers of viscous andesitic-to-dacitic lava flows and pyroclastic material, produced by eruptions that alternate between effusive and violently explosive. Mount St. Helens and Mount Rainier are examples; their steepness and gas-rich, viscous magma make them the most hazardous volcano type (pyroclastic flows, lahars).

Cinder cone. Small (rarely >300 m), steep-sided (up to the angle of repose of loose cinder, ~30–40°) cone built from a single short-lived, highly explosive eruption of basaltic-to-andesitic gas-charged magma that ejects cinder/scoria fragments which fall back and accumulate around the vent. Parícutin (Mexico) is the classic example; often occur on the flanks of larger volcanoes.

Caldera. A large, roughly circular depression, commonly kilometres across, formed by collapse of a volcanic edifice into a partially emptied magma chamber following a catastrophic explosive eruption — not a constructional landform at all but a collapse feature. Crater Lake (Oregon) and Yellowstone are examples; the surrounding rim is often built of the same composite-volcano material that later collapsed.

Lava dome (volcanic dome). A steep-sided, bulbous mass built by the slow extrusion of extremely viscous, silica-rich (rhyolitic/dacitic) lava that is too stiff to flow far and piles up over and around the vent; growth is often accompanied by dangerous dome-collapse pyroclastic flows. The post-1980 dome within the Mount St. Helens crater is a well-documented example.

21) Permafrost profile and seasonal temperature–depth graph (10 marks)

Permafrost is ground (soil or rock) that remains at or below 0 °C for at least two consecutive years; its presence is a thermal definition, independent of whether the ground is ice-bonded.

Active layer. The uppermost layer, thawing each summer and refreezing each winter; its thickness (typically 0.3–several metres, thinner in fine-grained, ice-rich soils) is the design-critical depth for shallow foundations, pipelines and roads in the North, because freeze–thaw cycling and thaw-related settlement (thaw consolidation, frost heave) occur only within it.

Permafrost table. The upper boundary of the permafrost, i.e. the maximum depth of summer thaw — the base of the active layer.

Permafrost. Perennially frozen ground extending from the permafrost table down to the base of permafrost; in continuous permafrost zones of the Canadian Arctic this can exceed several hundred metres.

Base of permafrost / geothermal transition. The depth at which the geothermal gradient (heat flowing from the Earth's interior) finally raises the ground above 0 °C; below this, ground is unfrozen year-round.

Temperature–depth curves. The two curves bound the annual temperature range at each depth, and they form a "trumpet" that narrows downward. The summer (maximum) curve is above 0 °C in the active layer and falls to exactly 0 °C at the permafrost table, which is how that boundary is defined. The winter (minimum) curve is well below 0 °C at the surface and stays colder than the summer curve at every depth. The two meet at the depth of zero annual amplitude, typically 10–20 m. That meeting point is below 0 °C, at the mean annual ground temperature. Below it, a single curve follows the geothermal gradient, warming with depth until it reaches 0 °C at the base of the permafrost.

22) Structure of the Earth's interior (10 marks)

The Earth is a layered sphere, its major divisions distinguished both compositionally (crust/mantle/core) and mechanically (lithosphere/asthenosphere/mesosphere/outer core/inner core), established from the refraction and reflection of seismic body waves.

Crust. The thin, rigid, compositionally distinct outer rind — continental crust is thick, old, low-density, granitic (felsic); oceanic crust is thin, young, denser, basaltic (mafic). The Mohorovičić discontinuity (Moho), marked by an abrupt seismic-velocity jump, is the crust–mantle boundary.

Mantle. By far the largest layer by volume, composed of ultramafic peridotite; although solid, it flows plastically over geological time (mantle convection), driving plate tectonics. Its uppermost, weak, partially molten portion, the asthenosphere, allows the overlying rigid lithosphere (crust + uppermost mantle) to move as plates.

Outer core. Liquid iron–nickel alloy; its convective, electrically conductive flow, combined with the Earth's rotation, generates the geomagnetic field by the geodynamo mechanism. Its liquid state is inferred because S-waves (which cannot travel through a fluid) do not propagate through it, producing the seismic "S-wave shadow zone."

Inner core. Solid iron–nickel alloy despite temperatures exceeding that of the outer core, because the immense pressure at the Earth's centre raises the melting point of iron above the local temperature. It slowly grows as the Earth cools and the liquid outer core crystallizes onto it.

23) Three types of tectonic plate boundaries (10 marks)

Every plate boundary is classified by the relative motion of the two plates across it, which in turn determines whether lithosphere is created, destroyed, or conserved there.

Divergent (constructive) boundary. Plates move apart; upwelling asthenospheric mantle partially melts as pressure drops, and the new magma solidifies as new oceanic crust, building a mid-ocean ridge (e.g. the Mid-Atlantic Ridge, spreading ~2–3 cm/yr) or, on continents, an actively rifting zone such as the East African Rift.

Convergent (destructive) boundary. Plates move together; the denser plate (normally oceanic) is subducted beneath the other, generating a deep oceanic trench, a zone of intense seismicity along the dipping slab (Wadati–Benioff zone), and, from partial melting of the mantle wedge above the slab, a volcanic arc (e.g. the Cascades above the subducting Juan de Fuca plate). Continent–continent convergence instead produces collisional mountain belts (e.g. the Himalaya) with little subduction.

Transform (conservative) boundary. Plates slide horizontally past one another; lithosphere is neither created nor destroyed, and the boundary is marked by a narrow zone of shallow, often large, strike-slip earthquakes with no volcanism. The San Andreas Fault (California) and the offshore Queen Charlotte Fault (British Columbia) are transform boundaries.

24) Six principles of relative dating (10 marks)

Relative dating establishes the order of geologic events without assigning numerical ages, using a small set of logical principles applicable to any outcrop or cross-section.

1. Superposition. In an undisturbed sequence of sedimentary or volcanic layers, each bed is younger than the bed beneath it and older than the bed above it, because material can only be deposited on top of what already exists.

2. Original horizontality. Sediment is deposited under gravity in essentially horizontal layers; a bed now found tilted or folded must have been deformed after it was deposited, so the tilt itself is a datable event.

3. Lateral continuity. Sediment layers extend continuously in all directions until they thin against the edge of their depositional basin or are cut off by later erosion or faulting; correlative layers on either side of a valley or fault can therefore be matched as originally one continuous bed.

4. Cross-cutting relationships. A geologic feature that cuts across another — an intrusion, a fault, a fracture — must be younger than the rock it cuts, because the cutting feature could not have formed until the host rock already existed.

5. Inclusions. A fragment of rock enclosed within another rock (a xenolith in an intrusion, or a clast eroded from a nearby unit and incorporated into a younger sedimentary bed) must be older than the rock that contains it.

6. Faunal (and floral) succession. Fossil assemblages succeed one another through geologic time in a fixed, non-repeating, recognisable order because evolution is irreversible; strata containing a given assemblage can therefore be correlated in relative age with other strata containing the same or an equivalent assemblage, even across great distances and dissimilar rock types.

A seventh principle, unconformities, is often added: a buried erosion or non-deposition surface represents a gap, so the rocks above it are separated from those below by a missing interval of time recorded nowhere in the local rock record.