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.
The rock cycle is the continuous set of processes that transform Earth materials among the three rock classes — igneous, sedimentary and metamorphic — and back to magma, with no fixed starting point. Igneous rock forms when magma (molten rock generated by partial melting in the mantle or lower crust) cools and crystallizes, either slowly at depth (intrusive/plutonic rock, e.g. granite, with coarse interlocking crystals) or rapidly at/near the surface (extrusive/volcanic rock, e.g. basalt, fine-grained or glassy). Once exposed at the surface by uplift and erosion, any rock is broken down by weathering (mechanical disintegration and chemical decomposition) into loose sediment, which is then eroded and transported by water, wind or ice and deposited in layers. Burial under successive layers causes compaction (grains pressed together, pore space reduced) and cementation (minerals precipitated from pore water bind the grains); together these lithification processes turn loose sediment into sedimentary rock (e.g. sandstone, shale, limestone). Any rock — igneous, sedimentary or already metamorphic — that is subjected to elevated heat and pressure without melting (typically from deep burial or tectonic activity) recrystallizes in the solid state into metamorphic rock (e.g. slate, schist, gneiss, marble), developing new mineral assemblages and often a foliated fabric. If temperature rises enough for partial or complete melting, the rock returns to magma, closing the cycle. Any pathway may be interrupted at any stage (e.g. a sedimentary rock can be uplifted and re-weathered without ever being metamorphosed), which is why the cycle is drawn with multiple cross-linking arrows rather than a single loop.
Bowen's Reaction Series (N.L. Bowen, based on experimental crystallization of basaltic magma) describes the systematic order in which silicate minerals crystallize from a cooling melt, and explains why certain minerals commonly occur together while others rarely do. It has two branches that merge at their low-temperature end:
Discontinuous (mafic/ferromagnesian) branch: minerals crystallize in a stepwise sequence, each reacting with the remaining melt to form the next mineral as temperature falls — olivine (isolated SiO4 tetrahedra) → pyroxene (single chains) → amphibole (double chains) → biotite mica (sheets). Each step is a distinct mineral with a different crystal structure ("discontinuous" because the structure changes abruptly at each reaction).
Continuous (plagioclase feldspar) branch: a single mineral family, plagioclase feldspar, changes composition smoothly (continuously) as it crystallizes, from Ca-rich (anorthite) at high temperature, through intermediate compositions, to Na-rich (albite) at lower temperature, via continuous ionic substitution (Ca2+/Al3+ ↔ Na+/Si4+) without a change in crystal structure.
The two branches converge and continue as a single sequence at lower temperature: K-feldspar → muscovite mica → quartz, the last minerals to crystallize. The series predicts both crystallization order in an igneous rock and weathering stability in the reverse sense (Goldich's weathering series): minerals that crystallize first, at the highest temperature and furthest from surface conditions (olivine, Ca-plagioclase), are least stable and weather fastest at the Earth's surface, while quartz, crystallizing last, is the most chemically stable and weathering-resistant.
