04-BS-14 · December 2018

Question 2 of 4: True and False (10 marks)

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

Notes on this paper

Paper format. National Exams, December 2018 — 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 candidate chooses four items from the bank of items 34–44. Marks: Q1 = 20, Q2 = 10, Q3 = 30 (item 31 = 10, item 32 = 6, item 33 = 14), Q4 = 40 (four items at 10 marks each). Total = 100 marks.

Scope of this solution. Every printed item is answered, including all eleven Question 4 items (34–44), not merely the four an examinee would select. The complete set is far more useful as a study resource.

Reference texts.

Marshak, S., Earth: Portrait of a Planet — general physical geology (rock cycle, plate tectonics, structural geology, glaciation, mass wasting).
Tarbuck, E. J. & Lutgens, F. K., Earth: An Introduction to Physical Geology — igneous classification chart and igneous structures figures of the type reproduced in Question 3.
Goodman, R. E., Engineering Geology: Rock in Engineering Construction — engineering-geological description of rock masses, weathering and discontinuities.
Freeze, R. A. & Cherry, J. A., Groundwater — aquifers, aquitards/aquicludes, springs and the subsurface limb of the hydrologic cycle.
Fletcher, G. & Karrow, P. F. (Geological Association of Canada), Glacial Geology reference material — Canadian glacial landform terminology (drumlin, esker, roche moutonnée, till plain).

Question 2: True and False (10 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. Each statement is tested against the formal definition of the term it uses; where a statement contains two clauses, both must hold for the statement to be true.

21. False. The active layer is defined as the near-surface layer that thaws each summer and refreezes each winter, and it exists over both continuous and discontinuous permafrost. What never thaws is the permafrost beneath it. In continuous permafrost across the Canadian Arctic the active layer is simply thin, commonly 0.3–1 m in fine-grained soils, and its seasonal thaw is precisely what governs foundation design, pipeline bedding and slope stability in the North.

22. False. The order is reversed. Silicates are by far the most abundant mineral group in the crust, making up roughly 90 per cent of it, because silicon and oxygen are the two most abundant crustal elements. Carbonates are volumetrically minor by comparison, however conspicuous they are in sedimentary basins.

23. False. The two definitions have been exchanged. An aquifer is a saturated unit permeable enough to transmit usable quantities of groundwater to a well or spring; an aquiclude (aquitard) is the low-permeability unit that confines it. As written, the statement makes the aquifer impermeable and the aquiclude transmissive, which is exactly backwards.

24. False. A spring is a point where the water table or a confined potentiometric surface intersects the ground surface so that groundwater discharges out of the ground. Flow into the ground is recharge or infiltration.

25. True. As a meander loop grows, the narrowing neck is eventually breached during a flood, the stream adopts the shorter course, and the abandoned loop is sealed off by deposition at both ends, leaving a crescent-shaped oxbow lake that gradually fills with fine sediment and organic matter.

26. False. The composition half of the statement is correct — a drumlin is built of till and a roche moutonnée is bedrock — but the shapes are not the same, because their asymmetry is reversed. A drumlin is blunt and steep on its up-ice (stoss) end and tapers gently down-ice, whereas a roche moutonnée is gently abraded and striated on its up-ice side and steep and irregular on its down-ice side where ice plucked jointed blocks away. That opposite asymmetry is exactly what allows either landform to be used as an ice-flow direction indicator, so treating them as the same shape would reverse the reconstructed flow.

27. True. In a normal fault the hanging wall moves down relative to the footwall, which lengthens the section and requires a horizontal least principal stress, that is, extension. In a reverse (or thrust) fault the hanging wall rides up, shortening the section under horizontal compression.

28. True. This is the well-known asymmetry of the Hjülström diagram. Medium sand, around 0.2–0.5 mm, has the lowest entrainment velocity of any grain size. Coarser grains resist entrainment because of their weight; finer silt and clay resist it because of electrostatic cohesion and because they lie inside the viscous sublayer, so a higher velocity is needed to erode them than to erode sand — even though, once suspended, they stay in suspension at vanishingly low velocities.

29. False. Quartz is correctly described: it is a framework (tectosilicate) structure in which every oxygen is shared between two tetrahedra. Biotite, however, is a sheet silicate (phyllosilicate) built of continuous two-dimensional layers of tetrahedra sharing three oxygens each, which is why it cleaves into elastic flakes. Chain silicates (inosilicates) are the pyroxenes and amphiboles.

30. False. Streak — the colour of the powdered mineral — is reasonably diagnostic, but colour is one of the least reliable properties, because trace substitutions and inclusions produce many colours in one species. Quartz alone ranges from colourless through purple, pink, grey and black, and pyrite, chalcopyrite and gold are all brassy yellow yet have quite different streaks. The word “always” makes the statement false on its own.

Question 2 — answer key
Item	21	22	23	24	25	26	27	28	29	30
Answer	F	F	F	F	T	F	T	T	F	F

Check: item 26. The answer given is False on the strength of the reversed stoss-and-lee asymmetry, which is the distinction every text draws between the two landforms. A marker who read “same overall shape” loosely, as meaning only “both are streamlined, ice-moulded hills,” could accept True. Either way, the reasoning about asymmetry and ice-flow direction is the substance of the mark.

Topic. Definitions in permafrost engineering, hydrogeology, fluvial and glacial geomorphology, and mineralogy.

Key relations. Active layer thaws seasonally, permafrost does not. Aquifer transmits, aquiclude confines. Springs discharge groundwater. Hjülström: entrainment velocity is minimum for medium sand and rises for both coarser and finer material. Silicate structural classes run from isolated tetrahedra (olivine) through chains (pyroxene, amphibole) and sheets (mica, clay) to frameworks (quartz, feldspar).

Why this works. A true/false item in this subject almost always turns a real definition by one hinge — a swapped pair of terms, a reversed order of abundance, or an absolute qualifier such as “always” or “never.” Locating the hinge, rather than judging the sentence as a whole, produces both the answer and the one-line justification that earns the mark.

Common pitfall. Accepting a compound statement because one of its clauses is right. Items 26 and 29 each pair a correct clause with a wrong one, and a statement is true only if every clause is true.

Source. Freeze & Cherry, Groundwater, ch. 2 (aquifers, aquitards, springs); Marshak, Earth: Portrait of a Planet, chapters on minerals, streams and glaciers.

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