04-BS-14 · December 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams December 2017, 04-BS-14 Geology, 3 hours, CLOSED BOOK (Casio/Sharp approved calculators only). Three questions constitute a complete exam (candidates must answer Questions 1 to 3); every printed sub-part is answered, including the 5 of 9 items in Question 3 the exam does not require.
Reference texts: Marshak, Earth: Portrait of a Planet (general/structural/surficial geology); Goodman, Engineering Geology: Rock in Engineering Construction (engineering-geology, permafrost, mass wasting); Freeze & Cherry, Groundwater (aquifers, wells, water table).
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.
Given. Figure 1: a block diagram of layered sedimentary rock with a stratovolcano at the surface, a dark lava field on the surface, and two large intrusive bodies at depth feeding sheets and cross-cutting bodies, with six leader lines (a)–(f). Figure 2: a forested hill with a house, a shallow well and a deeper "Unsuccessful well", a lens-shaped body under the hillside, four blank boxes and two boxes marked "(Zone)".
Find. The geologic name of each feature the arrows point to.
[Figure not reproduced: Source figure, item 21 part 1. See the official exam paper or the cited reference text.]
| Label | Where the arrow ends | Feature |
|---|---|---|
| (a) | Summit of the steep, layered cone at left, where the central conduit reaches the surface under the eruption cloud | Volcano (composite cone / stratovolcano) |
| (b) | The dark sheet spread over the land surface, fed from below by a cross-cutting feeder that reaches the surface | Lava flow (fissure-fed flow) |
| (c) | A thin dark sheet running parallel to the bedding just below the upper layers | Sill (concordant tabular intrusion) |
| (d) | A thin dark body cutting steeply across the layers from the dark pluton towards the surface | Dike (discordant tabular intrusion) |
| (e) | Inside the large dark mass at the base of the block that cuts across all the lower layers | Batholith (pluton) |
| (f) | The lens-shaped body with a flat floor that bows the overlying layers up into a dome | Laccolith |
The four intrusive forms are told apart by their shape and how they meet the bedding. A sill (c) and a laccolith (f) both follow the layers; the sill stays a thin, flat sheet, but the laccolith swells and lifts its roof into a dome. A dike (d) cuts across the layers. The batholith (e) is the large, deep, discordant parent body that feeds them. Magma that reaches the surface builds the cone (a) at a central vent or spreads out as a lava flow (b) from a fissure.
[Figure not reproduced: Source figure, item 21 part 2. See the official exam paper or the cited reference text.]
| Box (position on the figure) | Feature |
|---|---|
| Box on the left, arrow to the hillside where water seeps out and runs down to the stream | Spring. The perched water table meets the hillside here and groundwater comes out at the surface. |
| Box in the upper middle, leaders to the top of the saturated lens and to the shallow well | Perched water table. This is the top of a small saturated zone held up by the lens, well above the main water table. The shallow well gets its water here. |
| Small box below the lens, arrow up to its base | Aquitard (aquiclude). This is the low-permeability lens, such as clay, that catches downward-seeping water. |
| Long box at the bottom, arrow up to the boundary between the light upper ground and the darker lower ground | Main (regional) water table |
| Upper "(Zone)" box | Zone of aeration (unsaturated or vadose zone). Its pores hold both air and water. |
| Lower "(Zone)" box | Zone of saturation (phreatic zone). All of its pores are filled with water. |
The labelled well is "unsuccessful" because it stops inside the zone of aeration. It misses the perched lens, and it does not go deep enough to reach the main water table. So there is no saturated ground at its bottom to supply water. The shallow well nearby does produce, because it ends in the perched zone above the aquitard. A perched supply like this is small and can dry up in a drought. A dependable well has to go below the main water table.
Given. Five labelled block diagrams (a)–(e) of stream-network patterns in plan view, one over resistant "Ridges of resistant rock".
Find. The drainage-pattern name for each.
| Panel | Pattern | Control |
|---|---|---|
| a | Dendritic | Uniform, flat-lying rock with no structural control — random, tree-like branching at acute angles. |
| b | Rectangular | A regular joint/fault grid gives streams preferential, mutually perpendicular lines of weakness. |
| c | Trellis | Alternating resistant/weak folded or tilted strata (ridges of resistant rock) force short tributaries at right angles into parallel main streams. |
| d | Radial | Streams diverge outward from an isolated conical/domal high point, e.g. a volcano. |
| e | Deranged | Irregular, recently glaciated (or otherwise disrupted) terrain with numerous disconnected lakes/swamps and no integrated drainage network. |
Given. A Pacific-centred world map with the plate boundaries drawn as thick black lines. The figures 1, 2 and 3 mark plate interiors and the letters A and B mark boundary segments.
Find. (a) the names of plates 1, 2 and 3; (b) the boundary type at A and at B.
[Figure not reproduced: Source figure, item 23. See the official exam paper or the cited reference text.]
| Label | Identification |
|---|---|
| Plate 1 | North American Plate. The label is on the United States and Canada. |
| Plate 2 | African Plate. The label is on the African continent at the left edge. |
| Plate 3 | South American Plate. The label is on southern South America. |
| Boundary A | Divergent (mid-ocean ridge). A lies on the line running north–south down the middle of the North Atlantic, which is the Mid-Atlantic Ridge between the North American and Eurasian/African plates. The plates spread apart here and new oceanic lithosphere forms. |
| Boundary B | Convergent (subduction). B lies on the boundary that runs along New Guinea and the Solomon/Vanuatu arcs and then south towards Tonga–Kermadec. It separates the Indo-Australian Plate from the Pacific Plate, and oceanic lithosphere is subducted there beneath trenches and volcanic island arcs. |
Given. The cross-section shows these units and features:
Find. The geologic events, oldest to youngest.
[Figure not reproduced: Source figure, item 24. See the official exam paper or the cited reference text.]
| Order | Event | Evidence / principle |
|---|---|---|
| 1 (oldest) | Deposition of J, then K, then F, then conglomerate H, in that order | Original horizontality and superposition. The beds now dip left, so the stratigraphically lowest (oldest) bed is at the lower right (J) and the youngest is at the upper left (H). |
| 2 | Tilting (folding/uplift) of the J–H sequence | Beds laid down flat now dip steeply. |
| 3 | Erosion that bevels the tilted beds flat | The flat base of B cuts across the tilted beds, which makes it an angular unconformity. |
| 4 | Deposition of limestone B | Superposition. B lies flat on the erosion surface. |
| 5 | Intrusion of dike G | Cross-cutting relationships. G cuts H, F and B, so it is younger than all three. |
| 6 | Erosion that truncates the top of G and B | G ends flat at the base of E and does not continue into it. The B/E contact is therefore an erosion surface (a disconformity). |
| 7 | Deposition of E | Superposition |
| 8 | Normal faulting on D | D cuts and offsets E and B. The hanging wall on the right side is dropped down. |
| 9 | Intrusion of the large igneous body I, with contact metamorphism (the dark baked margin) | Cross-cutting relationships. I cuts J, K, the tilted beds, B, E and fault D, so it is younger than all of them. |
| 10 | Uplift and erosion that expose the top of I | C rests directly on the eroded intrusion, which makes this contact a nonconformity. |
| 11 | Deposition of C, with fragments of the eroded rock at its base | Principle of inclusions. The oval fragments in C came from the rock beneath, so I is older than C. D stops at the base of C. |
| 12 (youngest) | Deposition of A | Superposition |
