Volcanic Eruption Types: 18-Geol-A1 Mineralogy and Petrology, December 2016, Q12

Question 12 (Part 2, Q6): Volcanic Eruption Types (Part 2 – 10 marks)

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Eruption style is controlled primarily by magma viscosity (which scales with $\text{SiO}_2$ content and crystal load) and volatile (gas) content: low-viscosity, low-gas magma degasses passively (effusive), while high-viscosity, gas-rich magma traps rising bubbles until they fragment the melt explosively.

Volcanic eruption styles
Type	Controlling process	Typical rock type	Tectonic setting
Hawaiian	Very low-viscosity, low-gas basaltic magma degasses passively; effusive lava flows and low lava fountains.	Basalt	Oceanic hotspot (intraplate) or mid-ocean-ridge/rift setting.
Strombolian	Mildly viscous basaltic–andesitic magma; discrete gas slugs rise and burst rhythmically at a shallow, often-open conduit, producing intermittent small explosions.	Basalt–basaltic andesite	Small stratovolcanoes/scoria cones at convergent-margin arcs or hotspots.
Vulcanian	More viscous andesitic–dacitic magma repeatedly plugs and seals the conduit; pressure builds until short, violent, ash-laden explosions clear the plug.	Andesite–dacite	Subduction-zone stratovolcanoes.
Plinian	Highly viscous, volatile-rich silicic magma fragments almost completely at depth in the conduit, driving a sustained, buoyant eruption column tens of km high that can collapse into pyroclastic density currents.	Dacite–rhyolite	Large subduction-zone stratovolcanoes/caldera systems (continental arcs).
Peléan	Very viscous, degassed dacitic–andesitic magma extrudes as a growing lava dome/spine; gravitational or explosive collapse of the dome generates fast-moving pyroclastic density currents (nuées ardentes) rather than a sustained column.	Dacite–andesite	Subduction-zone stratovolcanoes with dome-forming silicic magma.
Surtseyan / phreatomagmatic	Rising magma (commonly basaltic) interacts explosively with external water; rapid steam generation fragments the magma far more finely than magmatic gas alone would.	Basalt (most common), but any composition where magma meets external water	Shallow submarine/subaerial vents at any tectonic setting where rising magma intersects a water table or sea floor.

Topic. Volcanic eruption style classification and its controls.

Key relations. Magma viscosity increases with SiO$_2$ content and decreasing temperature; explosivity increases with viscosity and volatile content together, because high viscosity prevents efficient bubble escape while high volatile content supplies the gas that eventually fragments the melt.

Why this works. Because magma composition correlates strongly with tectonic setting (basaltic at spreading ridges/hotspots, calc-alkaline andesite–dacite–rhyolite at subduction arcs), the eruption-style spectrum from effusive Hawaiian to violently explosive Plinian is, at root, the same viscosity/volatile control expressed across the compositional spectrum that also defines the major tectonic settings.

Common pitfall. Treating eruption style as solely a function of tectonic setting rather than of the magma's actual viscosity/volatile state at the time of eruption; a subduction-zone volcano can erupt effusively if temporarily degassed, and phreatomagmatic activity can occur in any setting wherever magma meets external water.

Source. Winter, Principles of Igneous and Metamorphic Petrology, 2nd ed., §5 (volcanic eruption styles, magma viscosity and volatile controls).

Question 12 of 13: Volcanic Eruption Types

Question 12 (Part 2, Q6): Volcanic Eruption Types (Part 2 – 10 marks)