Magma composition, viscosity and eruption style

Overview

Effusive and an explosive eruption and what a composite volcano would look like in cross-section.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is viscosity, and why is it the key property of a magma?

   Model answer

   Viscosity is a fluid's resistance to flow: a low viscosity magma is runny like syrup, a high viscosity one is stiff like toothpaste. It is the key property because it controls whether gas can escape freely from the magma. Everything else about an eruption, how violent it is, how far the lava travels, what shape the volcano takes and which hazards are produced, follows from that.

2. 2

   How does silica content control the viscosity of a magma?

   Model answer

   Silica in a melt forms tetrahedral units that link together into chains and networks. The more silica present, the more extensively these link up, and the more tangled the melt becomes internally, so the harder it is for it to flow. A basaltic magma with roughly 45-52 per cent silica is therefore very fluid, an andesitic magma with roughly 52-63 per cent is noticeably stiffer, and a rhyolitic magma with around 70 per cent is extremely viscous.

3. 3

   Set out the chain of reasoning from silica content to eruption violence.

   Model answer

   High silica means more linking of silica units, which means high viscosity. High viscosity means dissolved gases cannot bubble out freely as the magma rises and pressure falls, so the bubbles stay trapped and the magma foams while remaining stiff. Pressure therefore builds until it exceeds the strength of the rock confining it, and the magma is blasted apart, fragmenting into ash and pumice. Low silica magma does the opposite: it is runny, gas escapes continuously, pressure never accumulates and the magma simply pours out. Silica content controls viscosity, viscosity controls gas escape, and gas escape controls violence.

4. 4

   Why does temperature also affect viscosity, and which magmas are hottest?

   Model answer

   Higher temperature breaks down the linking between silica units and makes a melt runnier, so hotter magma flows more easily. Basaltic magma erupts at around 1000-1200 degrees Celsius while rhyolitic magma erupts at roughly 650-900 degrees. Composition and temperature therefore work in the same direction: the silica-poor magmas are also the hottest, making them doubly fluid, and the silica-rich magmas are also the coolest, making them doubly stiff.

5. 5

   Why does gas come out of solution as magma rises?

   Model answer

   Water vapour, carbon dioxide and sulphur dioxide are dissolved in magma at depth because the confining pressure is enormous. As the magma rises the pressure on it falls, and the gases can no longer stay dissolved, so they exsolve as bubbles, exactly as bubbles appear when a fizzy drink is opened. The bubbles expand rapidly as pressure continues to drop, and in a viscous magma this expansion is what drives the explosion.

6. 6

   Distinguish between an effusive and an explosive eruption.

   Model answer

   An effusive eruption is the relatively quiet outpouring of fluid lava from a vent or fissure, with gas escaping continuously and sometimes lava fountaining, producing flows that can travel many kilometres. An explosive eruption is the violent fragmentation of viscous, gas-charged magma, blasting ash, pumice and rock fragments upwards in an eruption column and outwards in pyroclastic flows, with little or no lava flow. Effusive eruptions are associated with basaltic magma, explosive ones with andesitic and rhyolitic magma.

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