18-Geol-A1 Mineralogy and Petrology · December 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 18-Geol-A1 Mineralogy and Petrology, 2018-Dec. Closed book; no calculator permitted.
Reference texts: Klein & Dutrow, Manual of Mineral Science, 23rd ed. (silicate/oxide structural classification, mineral chemistry and formulas, crystal systems); Winter, Principles of Igneous and Metamorphic Petrology, 2nd ed. (magmatic differentiation, Bowen's reaction series, metamorphic agents/facies, volcanic and pyroclastic processes, partial melting); Boggs, Petrology of Sedimentary Rocks, 2nd ed. (carbonate mineral diagnostics).
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.
