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18-Geol-A1 Mineralogy and Petrology · December 2019

Question 6 of 12: Ignimbrites — Formation by Eruption-Column Collapse

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

EGBC National Exam — Geological Engineering, 18-Geol-A1 Mineralogy and Petrology, 2019-Dec. Closed book; no calculator permitted.

Reference texts: Klein & Dutrow, Manual of Mineral Science, 23rd ed. (silicate structural classification, mineral chemistry and substitution, crystal systems, sulfide/carbonate ore mineralogy); Winter, Principles of Igneous and Metamorphic Petrology, 2nd ed. (magmatic differentiation, Bowen's reaction series, tectonic settings of magmatism, metamorphic/metasomatic processes, volcanic and pyroclastic processes, plate-tectonic cycle).

Question 6: Ignimbrites — Formation by Eruption-Column Collapse (Part 2 – 10 marks)

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 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.

Formation

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 collapsing material forms 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 one flow unit (or, in large eruptions, several stacked units).

Ignimbrite formation by eruption-column collapse buoyant column (before collapse) column too dense to stay buoyant → collapses pyroclastic density current (PDC) pre-eruption ground surface densely welded (fiamme) thinner, non-welded ignimbrite deposit (distal) vent / caldera Column collapse converts a buoyant eruption plume into a dense, ground-hugging PDC that thins and cools away from the vent.
Column collapse generates the pyroclastic density current whose deposit is the ignimbrite; welding is thickest and most intense near the vent, where heat and load are greatest.

Away from the vent, the current progressively decelerates, loses its coarsest pyroclasts and cools, so a single flow unit typically thins and fines distally while its internal welding grade tracks the balance of retained heat against heat lost to the substrate and atmosphere: the thick, insulated interior of a proximal, thick flow unit welds most strongly (producing dense, glassy, eutaxitic rock with flattened fiamme visible in outcrop), the chilled basal contact and the thin, rapidly-cooled upper surface weld least (remaining as loose, friable ash-flow tuff), and a single caldera-forming eruption can deposit several such flow units stacked as separate cooling packages if the eruption itself pulses rather than proceeding as one continuous current.