18-Geol-A1 Mineralogy and Petrology · May 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 04-Geol-A1 Mineralogy and Petrology, 2017-May. Closed book; no calculator permitted. Part 1 requires all five 10-mark short-answer questions (50 marks); Part 2 lists eight questions with instructions to answer "5 of the 7" (a source discrepancy noted on the exam page itself).
Reference texts: Klein & Dutrow, Manual of Mineral Science, 23rd ed. (silicate structural classification, mineral chemistry/formulas); Winter, Principles of Igneous and Metamorphic Petrology, 2nd ed. (magmatic differentiation and mixing, metamorphic agents/facies, volcanic processes, phase equilibria and AFM projections, magma viscosity, layered intrusions, tectonic melting mechanisms).
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.
Effusive eruptions occur when magma is sufficiently low in viscosity and dissolved volatiles that gas can escape passively as the magma rises, so the melt simply flows out onto the surface rather than fragmenting. Example: Kilauea, Hawaiʻi (ongoing basaltic shield volcanism). Products: smooth, ropy pahoehoe and blocky, clinkery a'a lava flows, lava lakes and lava tubes — coherent, low-vesicularity to moderately vesicular flow rock.
Explosive eruptions occur when high-viscosity, volatile-rich magma cannot degas passively; volatiles exsolve and expand faster than the melt can deform, fragmenting it violently into pyroclasts. Example: Mount Pinatubo, Philippines, 1991 (Plinian, dacitic). Products: pyroclastic fall deposits (ash and pumice tephra blanketing a wide area), pyroclastic density currents that deposit ignimbrite, and voluminous co-ignimbrite ash clouds — unconsolidated to welded fragmental rock, in sharp contrast to the coherent flow rock of an effusive eruption.
The controlling variable is magma viscosity and volatile content (governed chiefly by $\text{SiO}_2$ content, temperature and dissolved $\text{H}_2\text{O}$): low-viscosity, low-volatile basaltic magma degasses effusively, while high-viscosity, volatile-rich silicic magma fragments explosively.
