Questions: Lava Rheology and Planetary Eruptive Styles

Question 1

Two magma batches have identical dissolved water content of 3 wt%. Magma A is basaltic (50% SiO₂, 1200°C); Magma B is rhyolitic (70% SiO₂, 800°C). What eruption style do you predict for each, and what drives the difference?

• A. Both erupt explosively, because identical volatile content means identical gas pressure at the surface
• B. Magma A erupts effusively; Magma B erupts explosively — because viscosity determines whether gas bubbles can rise and escape before the magma fragments
• C. Magma B erupts effusively because its higher silica content chemically bonds the water and prevents volatile exsolution
• D. Magma A erupts explosively because basaltic melt is denser, trapping gas bubbles more effectively than the lighter rhyolite

Answer: (B)

Volatile content determines how much gas is available, but viscosity determines whether that gas can escape. In low-viscosity basaltic magma, bubbles nucleate and rise easily through the melt, releasing gas progressively at the surface — an effusive eruption. In high-viscosity rhyolitic magma, bubbles are trapped because they cannot rise through the stiff, polymerized melt. Pressure builds until the magma fragments explosively. Same water content, opposite eruption style — because viscosity, not volatiles alone, is the master variable.

Question 2

Scientists find glassy volcanic beads in Apollo 17 lunar samples (the 'orange soil'), indicating explosive eruption. Yet lunar magmas are low-silica basalts — the same composition that erupts gently at Kilauea. What best explains explosive volcanism from low-viscosity lunar basalt?

• A. The Moon's lower gravity compresses basaltic magma during ascent, building enough pressure for explosive fragmentation
• B. Lunar basalts are actually more silica-rich than Hawaiian basalts, making them more viscous and explosive
• C. The Moon's near-vacuum surface pressure allows even trace dissolved volatiles to flash instantly to vapor, fragmenting even low-viscosity melt
• D. Lunar eruptions are not truly explosive — the orange beads formed by meteorite impact, not volcanic processes

Answer: (C)

On Earth, a basaltic magma's dissolved gases exsolve gradually as pressure drops during ascent, producing quiet lava fountains. On the Moon, the surface pressure is essentially zero — any dissolved volatile reaching the surface flashes explosively to vapor regardless of melt viscosity. The composition of the magma barely matters; the absence of atmospheric back-pressure means even the gentlest basalt can erupt violently. This illustrates that eruption style depends on viscosity AND atmospheric pressure — the planetary context can override the composition effect entirely.

Question 3

A rhyolitic magma with the same dissolved water content as a basaltic magma is more likely to erupt explosively, because its higher viscosity prevents gas bubbles from rising and escaping through the melt before eruption.

• T. True
• F. False

Answer: True

This is the core mechanism. Rhyolitic viscosity can exceed 10⁸ Pa·s — comparable to glass — while basaltic viscosity is around 10–100 Pa·s. Bubbles that would easily percolate to the surface in basalt are completely immobilized in rhyolite. The result is that gas pressure builds within the trapped bubbles until it exceeds the tensile strength of the magma, fragmenting it catastrophically. Volatile content is necessary but not sufficient to predict explosivity — you need to know if those volatiles can escape.

Question 4

On Venus, with its 90-atmosphere surface pressure, explosive eruptions are more common than on Earth because the extreme atmospheric pressure compresses gas bubbles and raises volatile saturation pressure, causing larger explosive releases when eruption finally occurs.

• T. True
• F. False

Answer: False

High atmospheric pressure suppresses volatile exsolution — dissolved gases remain dissolved in the melt to much greater depths and surface pressures because the external pressure opposes bubble nucleation and growth. This strongly favors effusive eruptions, even from magmas that would be explosive on Earth. Venus's thick atmosphere is one reason its surface appears dominated by vast flood basalt plains rather than explosive calderas. The reasoning in the false statement reverses cause and effect: high pressure discourages, rather than stores up energy for, explosive eruption.

Question 5

Why does viscosity — not volatile content alone — determine whether a volcanic eruption is explosive or effusive?

Model answer: Volatiles dissolved in magma exsolve as bubbles when pressure drops during ascent. Whether those bubbles can escape determines eruption style. In low-viscosity basaltic magma, bubbles nucleate and rise freely through the melt, venting gas gradually at the surface — effusive eruption. In high-viscosity silicic magma, bubbles are trapped because they cannot move through the stiff, silica-polymerized network. Gas pressure builds inside trapped bubbles until it exceeds the magma's tensile strength, causing explosive fragmentation. Two magmas can have identical volatile content but opposite eruption styles if their viscosities differ.

The key concept is that volatiles must both be present AND be unable to escape for an explosive eruption to occur. This is why the same volcano can erupt effusively when magma is hot and gas-poor but explosively when it is cooler, more crystalline, and volatile-rich. Volcano monitoring tracks both volatile emissions and lava viscosity indicators for this reason. The silica-viscosity link is the molecular explanation: SiO₄ tetrahedra polymerize into chains and networks that dramatically increase resistance to flow, trapping the very bubbles that would otherwise make the eruption gentle.