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.
A lava flow's surface morphology and runout are controlled by one physical property — effective viscosity — but that viscosity is itself set jointly by composition, temperature and (through cooling/degassing/crystallization along the flow path) proximity to the vent, so the four variables named in the question are not independent.
Viscosity rises steeply with $\text{SiO}_2$ content because silica tetrahedra polymerize into long chain/network structures in the melt that resist flow; basaltic melt ($\approx 45$–$52\,\text{wt\%}$ $\text{SiO}_2$) is one to several orders of magnitude less viscous than rhyolitic melt ($\approx 70\,\text{wt\%}$ $\text{SiO}_2$) at the same temperature. Dissolved volatiles (chiefly $\text{H}_2\text{O}$) depolymerize the melt and lower viscosity, so a degassed flow is stiffer than the same melt while still volatile-rich.
Viscosity increases roughly exponentially as a melt cools, both because the melt itself stiffens and because cooling drives crystallization — a growing crystal cargo raises the bulk (melt + crystals) viscosity sharply once crystallinity exceeds roughly 30–40 vol.%, near the rheological "lock-up" point at which a flow effectively stops advancing.
Because a flow cools, degasses and crystallizes continuously as it travels, distance from the vent is really a proxy for elapsed time since eruption: a flow is hottest, most fluid and most gas-rich immediately at the vent and becomes progressively cooler, more degassed and more viscous down-flow. This is why the same basaltic flow commonly shows smooth, ropy pahoehoe texture near the vent (hot, low-viscosity, low shear rate, crust dragged into ropes) transitioning irreversibly to rough, clinkery a'a texture toward the flow front (cooled, degassed, higher effective viscosity and shear rate, crust broken into rubble). Lava tubes, which insulate flowing lava from heat loss, let pahoehoe travel unusually far from source while staying hot and fluid; without that insulation, runout distance is limited by how quickly the advancing flow front cools and stiffens to the point it can no longer be pushed forward.
