NivaarExam PrepOfficial exam papers ↗

18-Geol-A1 Mineralogy and Petrology · May 2017

Question 6 of 13: Ignimbrites — Formation and Internal Stratigraphy

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

Notes on this paper

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 6: Ignimbrites — Formation and Internal Stratigraphy (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, 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 are flattened and sintered together.

Formation

Ignimbrites form when an explosive eruption column becomes too dense to remain buoyant — through column collapse (eruption rate too high, or gas content too low, to sustain convective entrainment of air) or by direct boiling-over from a vent or caldera ring-fracture — releasing 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 (or, in large eruptions, several stacked) flow unit(s).

Internal stratigraphy

  1. Basal ground/surge layer Thin, fines-depleted, sometimes cross-bedded, deposited from the dilute turbulent head of the current ahead of the main dense flow.
  2. Lower non-welded zone Rapidly chilled against the cool ground; pumice and shards retain their original vesicular shape.
  3. Densely welded zone (with basal vitrophyre) The thick, insulated flow interior retains heat longest; shards and pumice are flattened into aligned fiamme, giving a eutaxitic texture, often with a glassy basal vitrophyre where rapid quenching locks in unrecrystallized glass.
  4. Upper non-welded zone Cools faster (open to air) under lighter load, grading back to loose, unwelded ash/pumice.
  5. Capping co-ignimbrite ash-fall layer Fine ash lofted from the current top by convective plumes settles over a wider area, forming a thin, well-sorted, normally-graded cap.
co-ignimbrite ash fall upper non-welded (loose ash/pumice) densely welded, eutaxitic (fiamme) -- hottest, thickest part basal vitrophyre (quenched glass) lower non-welded zone basal ground/surge layer pre-eruption ground surface welding intensity max here Single ignimbrite flow unit — vertical zonation Welding grade tracks retained heat and load: the thick, insulated interior welds most; the chilled base and open upper surface weld least.
Idealized cross section through one ignimbrite cooling unit, base at bottom. Welding peaks in the thick, insulated interior and dies out toward both the chilled base and the free upper surface.