04-BS-14 · December 2019
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. National Exams, December 2019 — 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 first four answers appearing in the answer book are marked. Marks: Q1 = 20 (1 mark/item, 20 MC items), Q2 = 10 (11 True/False items), Q3 = 30 (item 32 = 10, item 33 = 5, item 34 = 15), 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. Each item is settled by returning to the defining relationship behind the term — plate setting for arcs and trenches, process for weathering and erosion, geometry for folds, and stress state for faulting — rather than by recognising a familiar phrase.
1. (b) Subduction of an oceanic plate under a continental plate. A volcanic arc (e.g. the Cascades, the Andes) is the chain of stratovolcanoes built above a subducting oceanic slab, where dehydration of the slab lowers the melting point of the overlying mantle wedge and generates magma that rises through the continental crust. Option (a) reverses the geometry, and (c)/(d) describe collision and a non-existent process respectively.
2. (d) At subduction zones. The deepest points in the ocean are the trenches that mark subduction zones — the Mariana Trench, at nearly 11 km, sits above the Pacific plate descending beneath the Mariana arc. Mid-ocean ridges are the shallowest oceanic features, being newly formed, hot, buoyant lithosphere.
3. (b) Erosion of overlying rocks due to uplift. A pluton crystallises kilometres underground; it is only brought within reach of the surface once tectonic uplift raises the region and erosion strips away the overlying cover, a process called unroofing. No "igneous inversion" or ongoing post-solidification intrusion exists.
4. (c) Basalt. Mafic rocks (roughly 45–53 per cent silica, rich in Fe/Mg minerals) are represented here by basalt, the fine-grained volcanic equivalent of gabbro. Granite and rhyolite are felsic; andesite is intermediate.
5. (a) Tide. The map printed with the item is labelled "Bangladesh" and "Bay of Bengal", with a legend of older surfaces, delta plain, tidal flat and tidal sand bar. It is the Ganges–Brahmaputra delta, the textbook tide-dominated delta. Its seaward edge is broken into elongate sand bars and islands aligned with the tidal currents, separated by funnel-shaped tidal channels. A stream-dominated delta such as the Mississippi instead builds a bird's-foot of long levee-bounded distributaries. A wave-dominated delta such as the Nile or São Francisco has a smooth, arcuate shoreline of beach ridges.
6. (a) P-wave → S-wave → L-wave. Body waves radiate from the focus and travel through the Earth's interior, with the faster P-wave (compressional) always arriving before the slower S-wave (shear); surface waves, including Love (L) waves, travel only along the surface and arrive last because their path is longer and their velocity lower. Option (d) invents a non-existent "T-wave."
7. (b) Basal slip. Glaciers move by internal (plastic) deformation of the ice itself, by basal slip — sliding on a lubricating film of meltwater at the bed — and by deformation of soft, water-saturated subglacial sediment. Frost heaving, morainal sliding and crevassal slip are not recognised glacier-flow mechanisms.
8. (a) Folded anticlines and synclines. Orogenic (mountain-building) compression shortens layered rocks horizontally, buckling them into alternating upfolds (anticlines) and downfolds (synclines). Horsts and grabens are products of extension, not compression.
9. (d) Biologic activity, expansion from unloading, frost wedging. "In place" restricts the answer to mechanical weathering processes that break rock apart without moving it: root and burrowing action, sheeting joints opened as overburden is removed, and repeated ice growth in fractures. Option (a) describes erosion, which by definition involves transport.
10. (b) Pleistocene Epoch. The most recent "Ice Age," including the glacial advances that shaped most of Canada, falls within the Pleistocene, roughly 2.6 Ma to 11.7 ka. The Pliocene precedes it, the Permian is Paleozoic, and the Proterozoic hosts far older (Snowball Earth) glaciations.
11. (a) Regional metamorphism. Regional metamorphism affects vast volumes of rock at convergent plate boundaries and during orogeny, so it produces by far the largest exposed area of metamorphic rock — the schist and gneiss belts of continental shields. Contact and hydrothermal metamorphism are localised around intrusions; burial and impact metamorphism affect comparatively small volumes.
12. (b) A fold in which older flanking strata dip toward the axis. A syncline closes downward, with the youngest beds in the core, so the older beds on its flanks dip inward toward the axial trace. Option (d) exchanges old and young and describes an anticline instead.
13. (c). A mineral is defined by a regular, repetitive internal atomic structure; a rock is a lithified or consolidated aggregate of one or more minerals. Options (a), (b) and (d) all swap or garble this pairing.
14. (c) A few metres. Coseismic slip in a large earthquake is metre-scale — the 1964 Great Alaska earthquake and 2011 Tohoku earthquake both produced slip in the tens of metres at most. Kilometre-scale offsets accumulate only over many earthquake cycles and geological time.
15. (d) Erosion. Erosion is the entrainment and removal of weathered material by a mobile agent (wind, water or ice). Ablation is loss of glacier mass by melting/sublimation, and solifluction is the slow downslope flow of saturated soil over permafrost — neither is the general term the question asks for.
16. (b) Esker. An esker is the sinuous ridge of stratified sand and gravel deposited in an ice-walled or ice-roofed meltwater tunnel, left standing when the surrounding ice melts away. Drumlins are streamlined till hills, kettles are collapse depressions from buried ice blocks, and valley trains are outwash bodies confined between valley walls rather than tunnel fills.
17. (a) Dissolution, hydrolysis, and oxidation. Dissolution removes soluble minerals such as calcite and halite; hydrolysis breaks down silicates such as feldspar into clay minerals, releasing dissolved silica and cations; oxidation attacks iron-bearing minerals and sulphides. Carbonation and hydration are usually treated as special cases of dissolution and hydrolysis rather than separate major processes.
18. (a) Warmer temperatures and high confining pressures. Ductile (plastic) behaviour is favoured by high temperature, high confining pressure, low strain rate and, typically, greater depth — conditions that let minerals deform by crystal-plastic mechanisms rather than fracturing. Cool, shallow, low-pressure conditions favour brittle failure instead.
19. (d) Very much slower; vastly more. Alpine glaciers advance at metres to a few hundred metres per year, orders of magnitude slower than streamflow in the same valley, yet ice can support and transport clasts of any size without hydraulic sorting, so a glacier carries a far greater and far coarser sediment load than the stream that preceded or follows it.
20. (a) Horizontally directed, extensional stresses. Faulting of an otherwise undeformed, flat-lying sedimentary sequence — without folding — is the classic signature of brittle extension: horizontal stretching produces normal faults that drop blocks down relative to their neighbours, as in the Basin and Range province, without first buckling the strata. Horizontal compression (option d) more commonly folds a layered sequence before, or instead of, faulting it; option (b) is a garbled, non-physical phrase.
| Item | Answer | Item | Answer |
|---|---|---|---|
| 1 | b | 11 | a |
| 2 | d | 12 | b |
| 3 | b | 13 | c |
| 4 | c | 14 | c |
| 5 | a | 15 | d |
| 6 | a | 16 | b |
| 7 | b | 17 | a |
| 8 | a | 18 | a |
| 9 | d | 19 | d |
| 10 | b | 20 | a |
