04-BS-14 · Undated paper

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 structures (10 marks, part)

Given. Four white boxes sit above the block; their leader lines, traced from left to right, end as follows. Box 1 has two leaders: one ends on the domed roof of a flat-floored, lens-shaped body just beneath the surface, fed from below by a vertical conduit, and the other ends on the ground surface this body arches up. Box 2's leader ends on a thin sheet running parallel to the bedding, branching sideways from a second vertical conduit. Box 3's leader ends on a thin sheet cutting steeply across the layers from the deep mass up to the surface. Box 4's leader ends on the right-hand part of the block, where the large mass itself is exposed at the surface in rugged, coarse-textured ground. A small volcanic cone sits on the surface at the back of the block.

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

[Figure not reproduced: Figure 3.1a — 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.

Box 2 — Sill. A tabular concordant intrusion emplaced along a bedding plane, so its contacts run parallel to the layering.

Box 3 — Dike. A tabular discordant intrusion cutting across the layering, here rising to feed the surface volcano.

Box 4 — Batholith. A very large, irregularly shaped plutonic body, here exposed at the surface only because a great thickness of overlying rock has since been removed by erosion.

The key rests on geometry: concordant with a domed roof and flat floor = laccolith; concordant and tabular = sill; discordant and tabular = dike; the large deep mass itself = batholith.

31(b) — Figure Q3-2: alpine glacial features (10 marks, part)

Given. The landscape shows sharp peaks joined by knife-edge ridges, bowl-shaped hollows on the upper slopes, small lakes in those hollows, a waterfall where a side valley meets the main valley, and a broad valley floor with a meandering stream. Six boxes are to be filled in. Their leaders, traced from the drawing:

• Box A (far left, lower): ends on a small light-coloured pool on the floor of a hollow on the left-hand mountain.
• Box B (upper left): ends on the sharp summit of the left-hand mountain.
• Box C (top centre): ends inside a steep-walled, bowl-shaped hollow on the flank of the left-hand mountain.
• Box D (centre): ends on the floor of the side valley between the left-hand and central peaks, above the level of the main valley floor.
• Box E (upper right, two leaders): the two leaders end on the narrow ridges descending from the central and right-hand peaks.
• Box F (lower right): ends on the broad, flat floor of the main valley, where the meandering stream flows.

Find. The glacial feature indicated by each of the six blanks.

Reading note: the ends of leaders D and E fall on small features that are hard to make out in the printed figure. Boxes A, B, C and F are unambiguous. For D and E the key below is the best reading of the leader ends; the six names, drawn from the textbook panel this figure reproduces, are the answer set either way.

Box A — Tarn. A small lake occupying the rock basin of a cirque floor, where plucking and abrasion over-deepened the bed beneath the head of the former glacier.

Box B — Horn. A sharp, pyramidal peak left where three or more cirques have cut headward into a mountain from different sides (the Matterhorn is the type example; Mount Assiniboine in the Canadian Rockies is a Canadian one).

Box C — Cirque. A steep-walled, amphitheatre-shaped basin at the head of a glacial valley, excavated by frost wedging and plucking beneath the accumulation zone of a valley glacier.

Box D — Hanging valley. A tributary glacial valley whose floor is left perched above the floor of the deeper, more powerfully eroded main trough; after the ice melts its stream enters the trough over a waterfall, as the figure shows.

Box E — Arêtes. Narrow, knife-edged ridges left where adjacent cirques or glacial troughs erode headward toward one another from opposite sides of a divide.

Box F — Glacial trough (U-shaped valley). The broad, flat-floored, steep-walled main valley that the trunk glacier carved from an originally V-shaped stream valley, now occupied by a post-glacial meandering stream.

11 — Tectonic plates and boundary types (6 marks)

Given. A Pacific-centred world map, reproduced below from page 8. The map is located by its continents: Asia and Europe in the upper left, Africa at the left edge, Australia below the centre-left, and North and South America on the right. Label 1 sits on Asia. Label 2 sits on Africa. Label 3 sits on Australia. Label A sits on the thick boundary line that runs along the west coast of South America. Label B sits in the South Atlantic beside the boundary line running north–south down the middle of the ocean, midway between South America and Africa.

Find. (a) The names of plates 1, 2 and 3. (b) The type of boundary at A and at B.

(a) Plates.

1 — Eurasian Plate. The label sits on the Asian landmass, which, with Europe, forms the continental part of the Eurasian Plate.

2 — African Plate. The label sits on the African continent.

3 — Indo-Australian Plate (Australian Plate). The label sits on Australia, and the plate includes the Indian Ocean floor north-west of it and, in the classical scheme, India.

(b) Boundaries.

A — Convergent (ocean–continent subduction) boundary. Along the west coast of South America the oceanic Nazca Plate is subducting beneath the continental South American Plate at the Peru–Chile Trench. The result is the Andean volcanic arc, the Andes mountain belt and very large megathrust earthquakes, such as the 1960 M9.5 Chile earthquake.

B — Divergent (spreading) boundary. This is the Mid-Atlantic Ridge, where the South American and African plates are moving apart and new oceanic lithosphere forms by sea-floor spreading. It is marked by a central rift valley, shallow earthquakes, basaltic volcanism, and magnetic stripes that are symmetric about the ridge.

12 — Chronological sequence of geologic events (14 marks)

Given.

• Folded lower sequence. From the bottom up: limestone H (brick ornament), sandstone G (dots), shale F (dashes), limestone E (brick) and conglomerate D (pebbles). They are folded into an anticline on the left, with H in its core, and a syncline on the right, with D in its core.
• Fault Y. A thick straight line dipping steeply to the right. It cuts H, G, F and E, and stops at the flat base of C.
• Dike A. A broad band with a "v" ornament and short contact-alteration ticks along both margins. It rises from lower left to upper right through G, F and E, and ends at the flat base of C. Where A and Y cross, the fault line is not drawn through the dike, and the fault segments above and below the dike lie on one straight line. The dike is therefore not offset.
• Flat erosion surface. A straight, horizontal contact truncates the folded sequence, fault Y and dike A alike.
• Upper sequence. Flat-lying shale C rests on that surface, and flat-lying sandstone B rests conformably on C.
• II. A black dot at the lowest point of the present land surface, in a valley eroded into B, marked with an arrow. It is the channel of a present-day stream.

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

Approach. Superposition and original horizontality order the layers. Cross-cutting relationships place the fault and the dike, since anything that cuts a rock is younger than it, and a body that crosses another without being offset is the younger of the two. A feature truncated at an erosion surface is older than that surface.

1. Deposition of H, G, F, E and D, in that order. They were laid down as horizontal layers (limestone, sandstone, shale, limestone, conglomerate), with H the oldest by superposition.
2. Folding. Horizontal compression folded the whole H–D package into the anticline (H in the core) and syncline (D in the core). The upper sequence C–B is flat, so the folding predates it.
3. Faulting along Y. Y cuts the folded layers H to E, so it is younger than the folding. It stops at the base of C, so it is older than the erosion surface.
4. Intrusion of dike A. A cuts G, F and E and bakes its margins. It crosses fault Y without being offset, and Y is not drawn through it, so A was intruded after the faulting. It too is truncated at the base of C.
5. Uplift and erosion. This cut the flat surface that bevels the folds, fault Y and dike A. Folded layers below a flat erosion surface with flat layers above form an angular unconformity.
6. Subsidence and deposition of shale C on the unconformity.
7. Deposition of sandstone B on C (superposition).
8. Uplift and present-day erosion. This produced the modern rolling land surface on B. The valley at II is being cut by a present-day stream, the youngest event shown.