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.
Thermal metamorphism (contact metamorphism, its most common expression) is metamorphism driven overwhelmingly by heat conducted outward from a nearby igneous intrusion into cooler country rock, at roughly constant, relatively low lithostatic pressure and with little differential stress. It is localized in a metamorphic aureole surrounding the pluton, in sharp contrast to regional (dynamothermal) metamorphism, where elevated pressure and sustained directed stress act together with heat over a much larger area.
Because differential stress is minimal, thermal metamorphic rocks are characteristically unfoliated: pelitic country rock recrystallizes to a fine-grained, granoblastic or decussate (randomly oriented) hornfels rather than a foliated schist. Grain size and texture typically grade outward from the contact — coarser, more thoroughly recrystallized hornfels near the intrusion, grading through spotted (poikiloblastic) hornfels or slate at intermediate distance, to unmetamorphosed protolith at the aureole's outer edge.
Peak temperature falls systematically outward from the contact, producing a sequence of metamorphic facies zones concentric to the pluton: pyroxene hornfels facies (highest T, nearest the contact — anhydrous assemblages such as pyroxene + plagioclase), through hornblende hornfels facies (intermediate T — hornblende + biotite + andalusite in pelites), to albite–epidote hornfels facies (lowest-grade, outermost — albite, epidote, chlorite, essentially transitional to the unmetamorphosed protolith). If the intruded country rock is carbonate and reactive fluid is present at the contact, the same setting instead produces metasomatic skarn rather than isochemical hornfels.
Individual index minerals also mark this same outward temperature gradient in a pelitic protolith: cordierite and andalusite (both low-pressure, high-temperature aluminosilicate indicators) appear nearest the contact, giving way outward to biotite- and then chlorite-grade assemblages as peak temperature falls toward the unmetamorphosed shale beyond the aureole — the same low-pressure aluminosilicate polymorph (andalusite, rather than kyanite or sillimanite) is itself diagnostic of the low-pressure, heat-dominated regime characteristic of thermal metamorphism.
