Questions: Magma Composition and Physical Properties

Question 1

Two magmas have identical silica content. Magma A contains 5% dissolved water; Magma B is anhydrous (no water). Both remain at depth under high pressure. Which magma is more viscous?

• A. Magma A — dissolved water adds mass to the silicate network, increasing resistance to flow
• B. Magma B — without water, the silicate network is more completely polymerized and resists flow more strongly
• C. Both are equally viscous — dissolved water has no effect on silicate melt viscosity
• D. Magma A — water molecules insert into silicate chains, creating new cross-links that stiffen the melt

Answer: (B)

Dissolved water breaks Si-O-Si bridges by inserting OH groups into the silicate network, disrupting polymerization and dramatically lowering viscosity. Magma B, being anhydrous, has an intact, fully polymerized network — the silicate tetrahedra are maximally cross-linked and the melt is far more viscous. This is counterintuitive but critical: water makes magma *less* viscous at depth. The dangerous consequence comes when pressure drops during ascent and that water exsolves, generating explosive gas pressure.

Question 2

A basaltic volcano rarely produces explosive eruptions primarily because:

• A. Basaltic magma contains very few dissolved volatiles, so little gas forms during ascent
• B. Basaltic magma's low viscosity allows exsolving gas bubbles to migrate freely through the melt and escape at the surface before pressure builds
• C. Basaltic magma erupts at cooler temperatures than rhyolitic magma, which reduces gas expansion
• D. Basalt solidifies quickly at the vent, sealing off gas before it can accumulate to explosive pressure

Answer: (B)

The decisive factor is viscosity, not volatile content. Basaltic magmas can contain significant volatiles, but their low viscosity (as low as 10¹ Pa·s, similar to warm honey) allows gas bubbles to rise freely through the melt and degas gradually at the surface — like bubbles in a thin soup. In high-viscosity rhyolitic magma (up to 10⁸ Pa·s), the same gas bubbles cannot migrate; pressure builds within them until the magma fragments catastrophically. It is the viscosity contrast, driven by silica content, that determines eruptive style.

Question 3

In rhyolitic magmas, both high silica content and low eruption temperature independently increase viscosity, so the two effects compound each other.

• T. True
• F. False

Answer: True

Silica content drives viscosity up through polymerization: more Si-O-Si linkages create a tangled molecular network. Temperature drives viscosity down by providing thermal energy to break those bonds. Rhyolitic magmas are doubly disadvantaged: they have the highest silica content (~65–75% SiO₂) AND erupt at the lowest temperatures (~700–900°C, versus ~1100–1250°C for basalt). These two factors amplify each other, producing viscosities up to 10⁸ Pa·s — roughly seven orders of magnitude higher than basalt. This compounding effect explains why felsic volcanoes produce the most dangerous eruptions.

Question 4

Dissolved water in magma at depth increases viscosity because water molecules bond to and extend the silicate network.

• T. True
• F. False

Answer: False

Dissolved water does the opposite: it decreases viscosity. Water molecules react with bridging oxygen atoms in the silicate network (Si-O-Si + H₂O → 2 Si-OH), breaking the cross-links that cause polymerization and viscosity. The more water dissolved in the melt, the more disrupted the network and the lower the viscosity. This is why volatile-rich rhyolitic magmas at depth are less viscous than their dry counterparts — but the danger emerges during ascent when that water exsolves and forms trapped gas bubbles.

Question 5

Explain why the same dissolved water that decreases magma viscosity at depth can contribute to violent explosive eruptions as magma rises toward the surface.

Model answer: At depth, high confining pressure keeps water dissolved in the melt, where it disrupts silicate polymerization and reduces viscosity. As magma ascends and pressure drops, water's solubility decreases — it exsolves from solution and forms gas bubbles (vesiculation). In low-viscosity basaltic magma, these bubbles rise freely and degas at the surface. In high-viscosity rhyolitic magma, the thick melt prevents bubbles from migrating; gas pressure builds inside them. When the pressure exceeds the tensile strength of the melt, the magma fragments explosively into ash, pumice, and pyroclastic flows. Water's role therefore flips: a viscosity-reducer at depth becomes a fragmentation driver at shallow levels.

The key concept is that dissolved and exsolved water are fundamentally different in their effects. The phase transition from dissolved to exsolved — driven by pressure decrease during ascent — is the trigger for explosive behavior. High-silica magma's viscosity prevents the pressure release that low-viscosity magma achieves through gentle degassing.