04-BS-14 · December 2018

Question 3 of 4: Short Answer (30 marks)

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

Question 3: Short Answer (30 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.

31 (a) — Figure Q3-1: igneous intrusive structures (10 marks, part)

Given. A block diagram of a flat-lying layered sedimentary sequence. A small volcanic cone sits on the surface at the centre. In the cut face, from left to right, the four leader lines point to: a dome-roofed, flat-floored body inflating the strata just below the surface; a thin tabular body lying parallel to the bedding; a thin tabular body cutting steeply across the bedding down to a large mass at depth; and, on the right, that same large crystalline mass exposed at the surface in rugged, deeply eroded terrain.

Find. The name of the intrusive structure indicated by each of the four blanks.

[Figure not reproduced: Figure 31-1 — The four intrusive structures indicated in Figure Q3-1, keyed left to right as printed on the exam paper. See the official exam paper.]

Box 1 — Laccolith. A concordant intrusion fed from below whose viscous magma was too stiff to spread far, so it inflated a blister with a flat floor and an arched roof, bowing the overlying strata upward. Laccoliths are shallow and typically intermediate to felsic in composition.

Box 2 — Sill. A tabular concordant intrusion emplaced along a bedding plane, so its contacts are parallel to the layering. Sills are injected where the magma pressure exceeds the weight of the overburden and a weak bedding plane offers the easiest path.

Box 3 — Dike. A tabular discordant intrusion that cuts across the layering, here rising from the pluton at depth and feeding the volcano at the surface. Dikes exploit fractures opened normal to the least principal stress.

Box 4 — Batholith. The large discordant plutonic mass at depth, exposed at the right where erosion has stripped the cover. By convention a pluton with an exposed area greater than 100 km² is a batholith and a smaller one is a stock; the Coast Plutonic Complex of British Columbia is the Canadian type example.

31 (b) — Figure Q3-2: igneous rock classification (10 marks, part)

Given. A “Mineral Composition” chart. The centre column is labelled Andesitic (Intermediate). Two blank classification boxes flank it, left and right. The rock-colour scale beneath runs light (< 15 per cent dark minerals) — intermediate (15–40 per cent) — dark (> 40 per cent), against a 0–15–40–100 per cent gradient bar. The three specimen photographs are coarse-grained, with crystals large enough to be identified by eye. Two further blanks ask for the name of rock for the light and the dark specimen.

Find. The two missing compositional classifications and the two missing rock names.

Left classification box — Granitic (Felsic). The compositional series runs granitic–andesitic–basaltic–ultramafic with decreasing silica and increasing iron and magnesium. Because the printed centre column is andesitic and the left column carries the specimen with fewer than 15 per cent dark minerals, the left column must be the silica-rich, feldspar- and quartz-dominated end: granitic, equivalently felsic.

Right classification box — Basaltic (Mafic). The right column holds the specimen with more than 40 per cent dark minerals, dominated by pyroxene, olivine and calcium-rich plagioclase: basaltic, equivalently mafic.

Light specimen — Granite. The photograph shows interlocking crystals large enough to see unaided, so the texture is phaneritic and the rock cooled slowly at depth. A coarse-grained rock of granitic composition is granite.

Dark specimen — Gabbro. By the same logic, a coarse-grained rock of basaltic composition is gabbro. Its fine-grained volcanic equivalent would be basalt, and the coarse-grained intermediate rock in the centre column, which the paper does not ask for, is diorite.

The pairing of composition with texture is the whole content of the chart: composition sets the column and cooling history sets the row, so that granite and rhyolite are the same magma cooled at different rates, as are diorite and andesite, and gabbro and basalt.

32 — Plates and plate boundaries (6 marks)

Given. A Pacific-centred world map with plate boundaries drawn as thick lines. Label 3 lies in the middle of the largest ocean basin; label 2 is printed on the African continent; label 1 is printed on the North American continent. Letter A lies on the boundary running along the western coast of North America, between labels 3 and 1; letter B, at the top right beside Greenland, sits on the boundary that runs north–south down the middle of the Atlantic, separating the Americas from Europe and Africa.

Find. (a) the names of plates 1, 2 and 3; (b) the type of each boundary A and B.

(a) The three plates. Label 3 occupies the whole of the largest ocean basin on the map and is therefore the Pacific plate, the largest plate on Earth and almost entirely oceanic. Label 2 is printed on Africa itself, on the plate that carries the African continent together with the eastern South Atlantic and western Indian Ocean floor: the African plate. Label 1 is printed on the North American continent, on the far side of boundary A from the Pacific plate, and therefore marks the North American plate, which extends from the western margin of the continent east to the Mid-Atlantic Ridge and includes Greenland and the western North Atlantic floor.

(b) The two boundary types. Boundary A runs along the northeastern edge of the Pacific plate, where the Pacific and North American plates slide past one another; there is no ridge and no trench along it and lithosphere is neither created nor destroyed, so it is a transform (conservative) boundary — the San Andreas and Queen Charlotte systems are its expression on land and offshore British Columbia. Boundary B runs down the axis of the Atlantic Ocean, midway between the Americas on one side and Europe and Africa on the other, tracing the mirror-image fit of those coastlines. It is the Mid-Atlantic Ridge, a divergent (constructive) boundary at which new oceanic lithosphere is created by seafloor spreading and the Atlantic widens by roughly 2–3 cm per year.

33 — Chronological sequence of geologic events (14 marks)

Given. A cross-section, read from the printed figure of page 8. From the top down, flat-lying units A, C, E and B lie one on another. B has a brick ornament, indicating limestone. Beneath B is a set of tilted beds that dip toward the left and are truncated at the base of B. From upper left to lower right, and therefore from top to bottom of that tilted pile, they are H (pale, with pebble ornament), F, K and J. A small wedge of H also shows between G and F. A black steep dike G cuts H, F and the lower tilted beds and passes up through B, but it stops at the base of E. A large speckled crystalline mass I, with a dark rim along all its contacts, occupies the lower right. It cuts J, K and B and sends a tongue up through E that reaches the base of C. On the right, D labels a straight line that cuts B and E. The B–E contact is lower on its right-hand (hanging-wall) side than on its left, the line stops at the base of C, and the tongue of I cuts across it. Oval inclusions of speckled rock lie in the base of C directly above the tongue of I.

Find. The geologic events that produced this configuration, in order from oldest to youngest.

Approach. Four principles decide the whole sequence: original horizontality (beds are deposited flat, so tilted beds were tilted after deposition), superposition (in an undisturbed flat-lying pile the lower bed is older), cross-cutting relationships (a body that cuts another is younger than the one it cuts), and inclusions (a fragment enclosed in a rock is older than the rock enclosing it). Each unconformity records an interval of uplift and erosion between the units it separates.

1. Deposition of J, K, F and H, in that order. The tilted beds were laid down horizontally, one on another. They now dip toward the left, so the stratigraphic top of the pile faces the upper left. J, at the lower right, is therefore the oldest, and H, at the upper left, is the youngest. Reading the letters left to right as oldest to youngest reverses the true order.
2. Tilting of the sequence. By original horizontality the uniform dip was imposed after all four units were in place. Nothing above them shares that dip.
3. Uplift and erosion, cutting the first unconformity. The tilted beds are bevelled along a planar surface. That required the pile to be raised above base level and planed off, removing an unknown thickness of rock and an unknown span of time.
4. Deposition of B. Limestone B was laid flat across the bevelled edges of H, F, K and J. The discordance between flat B and the dipping beds beneath it is an angular unconformity. The brick ornament indicates a return to marine, carbonate-producing conditions.
5. Intrusion of dike G. G cuts H, F and the lower tilted beds and passes through B, so it is younger than all of them. It ends flat at the top of B and does not enter E, so it was intruded before E was deposited. The flat top suggests the dike and the top of B were planed off together before E accumulated.
6. Deposition of E. E lies flat on B and across the top of G.
7. Normal faulting along D. Fault D offsets both B and E. The B–E contact stands lower on the right-hand side, which is the hanging wall of a fault dipping to the right. The hanging wall has moved down, so D is a normal fault and records extension. It cuts E, so it is younger than E.
8. Intrusion of the pluton I. I cuts J, K, B and E. Its tongue crosses the line of fault D without being offset, so I is younger than the fault. The dark rim along every contact of I is the chilled margin and baked contact aureole. It shows that the host rocks were already in place when the magma arrived.
9. Uplift and erosion, cutting the second unconformity. The top of E, the upper end of fault D and the top of the tongue of I are all planed off along one surface. Where this surface crosses the plutonic rock of I it is a nonconformity. Elsewhere it is a disconformity between E and C.
10. Deposition of C. C was deposited across that surface. The oval inclusions at its base sit directly above the tongue of I and are fragments of it. By the principle of inclusions, I is older than C, which agrees with the truncation of I.
11. Deposition of A. A is the uppermost and therefore youngest unit.