18-Geol-A1 Mineralogy and Petrology · December 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 04-Geol-A1 Mineralogy and Petrology, 2017-Dec. Closed book; no calculator permitted. Part 1 requires all five 10-mark short-answer questions (50 marks); Part 2's page-1 header says "5 of the 8" while the page-3 instructions say "5 of the 7" and list exactly 7 questions (a source discrepancy noted on the exam page itself).
Reference texts: Klein & Dutrow, Manual of Mineral Science, 23rd ed. (silicate/sulfide/carbonate structural classification, mineral chemistry and formulas); Winter, Principles of Igneous and Metamorphic Petrology, 2nd ed. (magmatic differentiation, metamorphic agents/facies, volcanic processes, layered intrusions and cumulates, partial melting, ophiolites); 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.
Both are surface textures of the same basaltic lava; the difference between them is not composition but the physical state (temperature, degassing, effective viscosity, shear rate) of the flow at the moment its solidifying crust is deformed.
Forms from hotter ($\approx1150$–$1200\,{}^{\circ}\text{C}$), relatively low-viscosity, still gas-rich basaltic lava advancing at a low, steady shear rate. A thin, still-plastic surface skin is continuously dragged and folded by the fluid, insulated interior beneath it, producing the diagnostic smooth, billowy, ropy surface. Sustained flow is commonly organized into insulated lava tubes, which minimize heat loss and let pahoehoe travel efficiently far from the vent while staying hot and fluid.
Forms from cooler, more degassed, higher-effective-viscosity basaltic lava advancing at a higher shear rate. The crust that forms is too thick and rigid to be dragged into smooth ropes; instead it is continuously broken, sheared and tumbled by the still-moving core, producing a rough, clinkery, spinose rubble surface (a blocky clinker layer over a denser, more massive flow interior).
Because pahoehoe transforms irreversibly into a'a as a flow cools, degasses and its shear rate increases (the reverse transition is never observed), the pahoehoe-to-a'a transition along a single flow records the flow's thermal and rheological history: pahoehoe dominates near the vent (hottest, most fluid, lowest shear rate) and a'a dominates toward the flow front and distal reaches (cooled, degassed, higher effective viscosity and shear rate as the flow is forced to move faster relative to its now-stiffer rheology). A flow that is entirely a'a from very near the vent instead signals an initially higher effusion rate/higher shear rate or a more viscous, more crystal-rich or more degassed parent magma from the outset.
