18-Geol-A1 Mineralogy and Petrology · Undated paper
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. National Exam — 18-Geol-A1 Mineralogy and Petrology. 3 hours, closed book, no calculator permitted. Two parts, twelve ten-mark short-answer questions in total: Part 1 (Q1–5) requires all five questions (50 marks); Part 2 (Q6–12) is printed as "answer 5 of the 7" on one page and "answer 5 of the 5" on another (the paper's own instructions disagree on the count) — every question in both parts is solved in full below so this set also serves as a complete study reference. This sitting is treated as undated because the paper is internally inconsistent about its own date: the first-page footer reads "May 2018" (matching the 18-Geol-A1 code, in use from December 2018 onward) while a later page's footer reads "19-Geol-A1 / May 2019". No exam date is asserted.
Reference texts: Klein & Dutrow, Manual of Mineral Science, 23rd ed. (silicate/oxide structural classification, mineral chemistry, solid solution and exsolution, crystal systems); Winter, Principles of Igneous and Metamorphic Petrology, 2nd ed. (magmatic differentiation, Bowen's reaction series, tectonic settings of magmatism and melting, ophiolites, LIPs, anatexis, contact/thermal metamorphism).
There is no numeric given data anywhere in this qualitative/descriptive paper.
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.
An ophiolite is a slice of oceanic lithosphere — crust plus uppermost mantle — that has been tectonically obducted (thrust up, rather than subducted) onto continental crust, preserving the full oceanic stratigraphic sequence intact and exposed on land.
From base to top the sequence is: (1) tectonized (depleted) mantle peridotite — harzburgite/dunite, the melt-depleted residue left after MORB extraction; (2) ultramafic–mafic cumulates — layered gabbro formed by crystal settling in the base of the ridge magma chamber; (3) massive (isotropic) gabbro, crystallized higher in the same chamber; (4) the sheeted dike complex — vertical, one-way-chilled diabase dikes that fed the eruptions above; (5) pillow basalt, the submarine lava extruded at the ridge axis; and (6) a thin pelagic sediment cap (chert and/or limestone) deposited after the crust moved off-axis. The sequence forms at a mid-ocean ridge by the same seafloor-spreading process that generates MORB, and is later obducted — typically during subduction initiation or arc–continent/continent–continent collision, when buoyant crust jams the subduction zone — thrusting a slice of the downgoing plate onto the overriding plate rather than allowing it to subduct.
