04-BS-14 · May 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams May 2015 — 04-BS-14, Geology. Closed-book, 3 hours. Five questions constitute a complete paper: Questions 1-4 are mandatory and one of Questions 5-7 must be chosen; every question (5, 6, and 7) is answered here as a complete study resource. Each question is worth 20 marks.
Reference texts: Marshak, Earth: Portrait of a Planet (structural geology, relative dating, weathering, glacial and fluvial landforms); Goodman, engineering-geology mapping methods (strike and dip, three-point problem); Freeze & Cherry, Groundwater (Darcy flow, piezometers).
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.
| # | Item | Options | Answer | Why |
|---|---|---|---|---|
| 1) | The physical removal of dissolved or disaggregated rock from the site of weathering by wind, water, or ice is termed | a) ablation b) recidivism c) solifluction d) erosion | d | This is the definition of erosion — the physical removal and transport of weathered material away from its source, as distinct from weathering itself (which only breaks the rock down in place). |
| 2) | is the dissolution or decomposition of minerals and rocks. | a) Mechanical weathering b) Chemical weathering c) Hydrolysis d) Rendering | b | Chemical weathering is the general term for dissolution/decomposition of minerals through chemical reactions (hydrolysis, oxidation, carbonation, etc.); hydrolysis (option c) is only one specific chemical-weathering reaction, so it is too narrow to be the correct general term. |
| 3) | In nature, where does the acidity come from to speed up chemical weathering? | a) plutonism b) nuee ardentes from explosive volcanic eruptions c) Bowen's reaction series d) organic acids from decayed plants, acid rain, and sulphuric acid from oxidation of pyrite | d | Natural acidity that accelerates chemical weathering comes from biological/atmospheric/mineralogical sources: humic/organic acids from decaying vegetation, dissolved CO2 forming carbonic acid (acid rain), and sulphuric acid generated by the oxidation of sulphide minerals such as pyrite. |
| 4) | The principal causes of mechanical fragmentation of rocks in place are | a) erosion and transport by moving wind, water, or ice b) the relentless actions of Sisyphus c) always inscrutable because they happened at some time in the past d) biologic activity, expansion from unloading, frost wedging | d | Mechanical (physical) weathering breaks rock apart in place without transport; its principal agents are root growth and burrowing organisms (biologic activity), pressure-release/sheeting joints as overburden is removed (unloading), and repeated freeze-thaw cycles (frost/ice wedging). |
| 5) | The three major processes involved in chemical weathering are | a) dissolution, hydrolysis, and oxidation b) precipitation, ion exchange reactions, and degasification c) carbonation, dissimulation, and salinization d) recrystallization, pitting, and rinsing | a | The three principal chemical-weathering reactions acting on common rock-forming minerals are dissolution (minerals dissolve directly into water, e.g. calcite), hydrolysis (water reacts with silicate minerals, e.g. feldspar to clay), and oxidation (Fe2+-bearing minerals oxidize to iron oxides/hydroxides). |
a) Oxbow lake. A crescent-shaped lake formed when a meandering river cuts through the narrow neck of a tight meander loop during a flood, taking a shorter, straighter course; the abandoned meander loop is severed from the active channel and left as a standing body of water that gradually fills with fine sediment and organic matter.
b) Meander. A sinuous, looping bend in the course of a river, produced by lateral (helicoidal) flow that erodes the outer, faster-flowing bank (cut bank) while depositing sediment on the inner, slower-flowing bank (point bar); meanders migrate laterally and downstream over time and are typical of low-gradient rivers flowing over fine, erodible floodplain sediment.
c) Levee. A natural (or artificial) raised bank running along a river channel; natural levees build up from coarser sediment that drops out of floodwater first, immediately adjacent to the channel, as the flood loses velocity and depth on spilling over the bank, leaving progressively finer sediment further from the channel on the floodplain.
d) Tide-dominated delta. A delta whose form is controlled primarily by strong tidal currents reworking river-supplied sediment (rather than by river or wave energy); tidal scour elongates the distributary channels into funnel-shaped, tapering mouths and reworks the sediment into linear, shore-normal tidal sand ridges/bars, giving the delta a comb-like or estuarine plan shape rather than the smooth arcuate/bird's-foot forms of wave- or river-dominated deltas.
Dendritic — an irregular, tree-branching network of tributaries joining the main channel at varied (typically acute) angles, developing on flat-lying rock of uniform resistance (e.g. horizontal sedimentary strata or a homogeneous crystalline mass) where the underlying geology exerts no structural control on channel direction.
Trellis — a rectangular, comb-like pattern of long parallel main streams joined by short tributaries entering at close to right angles, developing where alternating bands of resistant and non-resistant, tilted or folded strata guide the main channels along the strike of the weak rock while short tributaries cut directly down the dip slopes of the resistant ridges.
Radial — a pattern of streams flowing outward in all directions from a central high point, like spokes on a wheel, developing on isolated conical or dome-shaped topography (most classically a volcano, but also a resistant dome or laccolith) where every flow path is simply the local downslope direction away from the summit.
