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.
Anatexis is the in-situ partial melting of pre-existing continental crustal rock (typically metasedimentary or metaigneous), producing crustal-derived ("S-type") granitic magma, as opposed to a mantle-derived ("I-type") magma. It occurs where crustal rock is heated — by advected heat from underplating mantle-derived basaltic magma, by crustal thickening during orogeny burying radiogenic heat-producing rock, or by decompression during exhumation — and/or fluxed by water released from progressive dehydration-melting reactions (muscovite breakdown, then biotite breakdown) as temperature rises past the crustal solidus (broadly $650$–$700\,{}^{\circ}\text{C}$ at mid-crustal pressure).
Partial melting first produces a migmatite: a light, quartzofeldspathic melt fraction (leucosome) segregated from a darker, refractory residue of biotite/garnet/sillimanite (melanosome/restite). If melt fraction and buoyancy are sufficient, the leucosome segregates and rises to form a discrete granitic pluton or, if it reaches the surface, erupts as high-silica rhyolitic/dacitic magma.
Because anatectic melt is silicic, viscous and volatile-rich, eruption is explosive rather than effusive: pyroclastic fall deposits (ash and pumice tephra) and pyroclastic density currents depositing ignimbrite (welded, eutaxitic tuff), the same explosive/PDC product family generated by any silicic magma, here sourced from crustal melting rather than purely mantle-derived, fractionated magma.
The Taupo Volcanic Zone, New Zealand, is a well-documented example: continental crust above a zone of active rifting/extension (back-arc extension behind the Hikurangi subduction margin) is heated by mantle-derived basalt underplating and by the elevated regional heat flow of extension, driving crustal anatexis and generating the voluminous rhyolitic ignimbrites (e.g. the Oruanui and Taupo eruptions) for which the zone is famous, erupted from large, repeatedly reactivated caldera systems.
