Question 3 of 4: Short Answer

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. Labelled igneous/structural cross-section

Method. (The AI-vision caption in the extraction called (b) a sill, (e) a fault and (f) a fold; the arrowheads do not support that reading.)

• (a) The leader runs down from the upper-left blank to the summit vent of the cone under the eruption cloud: a volcano (a composite cone fed by the central conduit).
• (b) The vertical arrow comes straight down onto the dark sheet spread across the land surface beside the volcano. That sheet sits on top of the strata, so it is extrusive: a lava flow.
• (c) The arrowhead ends on the thin dark sheet running parallel to the bedding inside the upper layers, a tabular body that follows the layering (concordant): a sill.
• (d) The arrowhead ends on the thin, steep sheet that cuts across the horizontal layers from the pluton roof up to the surface, a tabular body that crosses the layering (discordant): a dike.
• (e) The arrowhead ends in the igneous body directly beneath the layers that are arched upward on the right. It is concordant at its base and domes its roof: a laccolith.
• (f) The arrowhead ends deep inside the large pale mass that fills the bottom of the block, cuts across the layering and has no visible floor: a batholith (pluton).

32. Drainage patterns

Pattern identification follows from the network geometry and the implied underlying rock/structure. (a) Dendritic — a random, tree-like branching network with tributaries joining at acute angles; develops on flat-lying, uniform (homogeneous) rock with no structural control. (b) Rectangular — streams follow a grid of near-right-angle bends, exploiting a regular joint or fault network in the bedrock. (c) Trellis — parallel main streams with short tributaries entering at right angles; the "ridges of resistant rock" annotation confirms alternating resistant/weak, folded or tilted strata forcing streams into parallel valleys with short cross-cutting tributaries. (d) Radial — streams diverge outward from a central high point in all directions, typical of an isolated conical or domal landform such as a volcano. (e) Deranged — an irregular, chaotic network with numerous small lakes and swamps and no consistent orientation, characteristic of recently glaciated terrain where the drainage system has not yet organized itself on the disrupted, hummocky glacial landscape.

33. Tectonic plates and boundary types

a. Plate names: Plate 1 carries Europe and most of Asia and is the Eurasian Plate. Plate 2 carries India, the Indian Ocean and Australia and is the Indo-Australian Plate (often split into the Indian and Australian plates). Plate 3, the oceanic plate that fills the Pacific basin inside the ring of boundaries, is the Pacific Plate.

b. Boundary types: Boundary A is the Aleutian arc and trench, where the Pacific Plate moves north and subducts beneath the North American Plate. It is a convergent (subduction) boundary, marked by a deep trench, the Aleutian volcanic island arc and large megathrust earthquakes such as the 1964 Alaska earthquake (M 9.2). Boundary B is the Mid-Atlantic Ridge between the South American and African plates. It is a divergent (spreading) boundary: the plates move apart, basaltic magma wells up to form new oceanic lithosphere, and the ridge carries shallow earthquakes and symmetric magnetic stripes. B is the ocean-centre line, not the Andean margin on the west coast of South America, which is a separate convergent boundary.

34. Geologic history from a cross-section

Approach. Apply the relative-dating principles in turn. By superposition, lower layers are older. By cross-cutting relationships, a fault or intrusion is younger than everything it cuts. By inclusions, a rock is younger than the fragments it encloses. An unconformity is younger than everything it truncates and older than what rests on it. Folding post-dates every layer it bends, and any structure that is not folded post-dates the folding.

1. Formation of G. G is the oldest rock. E encloses detached blocks of it.
2. Intrusion of E into G. E has an irregular intrusive contact with G and carries G inclusions, so E is younger (principle of inclusions).
3. Faulting on fault 2. It offsets both G and E but stops at the base of K, so it is younger than E and older than K.
4. Uplift and erosion of G/E, forming a nonconformity. The sedimentary pile rests directly on the eroded igneous/crystalline G–E surface.
5. Deposition of K, D, L, A, M, B, I in that order (superposition). K is the oldest and I the youngest layer of the folded sequence.
6. Folding by horizontal compression. K through I and the nonconformity beneath them are all folded together.
7. Intrusion of dike N. It cuts the folded layers and E and is not folded itself, so it is post-folding.
8. Intrusion of the dark Y-shaped dike. It cuts the folded layers and E, is unfolded, and is truncated at the base of H. N and this dike never cross, so the figure does not fix their order relative to each other. Both are younger than the folding and older than the H unconformity. N is listed first here by convention.
9. Normal faulting on fault 1. The east (hanging-wall) side is down: the conglomerate B and stippled L sit lower east of the fault. Fault 1 offsets the dark dike and stops at the base of H.
10. Uplift and erosion, forming an angular unconformity. The erosion surface bevels the folds, both dikes and fault 1.
11. Deposition of H, then C, then F as flat-lying layers on the unconformity.
12. Normal faulting on fault 3. It cuts H, C and F, so it is the youngest structure. The east (hanging-wall) block moved down, which is why C and F survive only on that side.
13. Erosion to the present land surface. Erosion removed C and F from the upthrown west block and levelled the surface, leaving H exposed west of fault 3.