Three Controlling Factors on Magma Viscosity: 18-Geol-A1 Mineralogy and Petrology, May 2017, Q12

Question 12: Three Controlling Factors on Magma Viscosity (Part 2 – 10 marks)

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Three controls on magma viscosity
Factor	Effect on viscosity	Geological example
SiO₂ content (degree of polymerization)	Higher $\text{SiO}_2$ builds a more extensively polymerized Si–O–Si network in the melt, sharply raising viscosity.	Rhyolitic magma forming steep, slow-moving obsidian domes (e.g. Mono–Inyo Craters, California) vs. highly fluid basaltic lava at Kilauea.
Temperature	Higher $T$ increases thermal disruption of the polymerized network and ionic mobility, lowering viscosity; cooling raises it.	Fresh, hot ($\approx1150\,{}^{\circ}\text{C}$) pahoehoe at Kilauea flows readily, while its cooling, degassed flow front develops a stiff, blocky a'a crust.
Dissolved volatile (H₂O) content	Dissolved $\text{H}_2\text{O}$ breaks Si–O–Si bridging bonds (depolymerizes the melt), lowering viscosity while in solution; exsolution on ascent removes this effect and can raise bulk (apparent) viscosity via bubble nucleation.	Water-rich rhyolitic magma is comparatively fluid at depth, but the degassed lava dome it forms after eruption (e.g. Mount St. Helens 1980–86 dome) is stiff and nearly immobile.

Topic. Physical and chemical controls on silicate melt viscosity.

Key relations. Viscosity increases with $\text{SiO}_2$ content and decreases with temperature and dissolved $\text{H}_2\text{O}$; degassing during ascent removes the depolymerizing effect of water.

Why this works. All three factors act on the same underlying property — the degree of polymerization of the Si–O network — either by changing how much of it there is ($\text{SiO}_2$ content), how strongly it resists thermal disruption (temperature), or how much of it is broken by dissolved network-modifying species (H₂O); this is why the same magma can behave very differently at depth (hot, hydrous, fluid) versus after eruption (cooled, degassed, viscous).

Common pitfall. Treating crystal content as irrelevant; while not one of the three classical compositional/thermal factors, a high crystal fraction (not requested here) also raises apparent (bulk) viscosity independently of melt chemistry.

Source. Winter, Principles of Igneous and Metamorphic Petrology, 2nd ed., §3–4 (magma properties, viscosity controls).

Question 12 of 13: Three Controlling Factors on Magma Viscosity

Question 12: Three Controlling Factors on Magma Viscosity (Part 2 – 10 marks)