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.
An ignimbrite is the deposit of a pyroclastic density current (PDC) — a hot, gravity-driven, ground-hugging ash-and-gas flow generated by a highly explosive, typically silicic (dacitic–rhyolitic) eruption. Depending on emplacement temperature and overburden load it ranges from a loose ash-flow tuff to a densely welded tuff, in which hot glass shards and pumice fragments are flattened and sintered together into fiamme.
Ignimbrites form when an explosive eruption column becomes too dense to remain buoyant in the atmosphere — either through column collapse (eruption rate too high, or gas content too low, for the column to entrain enough air to stay convective) or by direct boiling-over from a vent or caldera ring-fracture. The result is a dense current of hot ash, pumice and gas that sweeps across the ground surface at tens to hundreds of m/s, following topographic lows, and comes to rest as a flow unit (or, in large eruptions, several stacked units).
