An arching orange lava fountain erupts from a vent on Kīlauea amid dark cooling lava.

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Model scale

Idealised forms. relative geometry. cross-section exposes the conduit.

Viscosity 0.25 · gas drive 0.45 · broad shield. The advancing lava front follows the selected process progress. Gas, water interaction and eruption conditions also control explosivity; basalt can erupt explosively.

Volcanoes

Explain how magma viscosity influences volcanic form and eruption style, using shield and composite volcanoes as examples.

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Why it happens

Magma composition, gas content and the conditions of ascent influence eruption style. Comparing volcanic forms helps connect those processes with lava flows, fragmental deposits and the structure built over time.

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Kīlauea, 25 February 1983. USGS photographed a lava fountain approximately 10 metres high; molten rock is at the surface, not throughout the mantle.

Try a comparison

Switch to Fissure vents. A fissure is a crack, not a viscosity class. Open the cross-section and identify the conduit.

Reading the result

The displayed eruptions illustrate selected mechanisms; volcano shape alone cannot predict the next eruption.

A common misunderstanding

A volcano’s size alone predicts how explosive its next eruption will be.

Eruption behaviour depends on magma properties, gas, ascent conditions and possible interaction with water or ice. Large shield volcanoes often produce fluid lava flows, but basaltic eruptions can also be explosive. The existing landform records earlier activity without determining every future event.

Cases

Exam language

Viscosity

A measure of a fluid’s resistance to deformation and flow. Magma viscosity depends on composition, temperature and crystal content. More viscous magma can hinder gas escape, but eruption style also depends on gas supply, ascent and interaction with external water.

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Kīlauea, 25 February 1983. USGS photographed a lava fountain approximately 10 metres high; molten rock is at the surface, not throughout the mantle.

Shield volcano

A broad volcano with gentle slopes built mainly by fluid lava flows, commonly basaltic. Mauna Loa is a classic example.

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Hawaii from the ISS. The volcanic chain records Pacific plate motion relative to a long-lived hotspot; the youngest main island is at the southeast end.

Stratovolcano

A commonly steep-sided volcano built from successive lava flows and fragmental deposits. Its magma composition and eruption style can vary; many form above subduction zones.

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Mount Fuji from Lake Kawaguchi, Japan. Repeated eruptions built a stratovolcano with layers of lava and fragmental deposits.

Fissure eruption

An eruption in which magma emerges along an elongated crack rather than only from a single central vent. Lava fountains may form a line and then become concentrated at a few outlets. Basaltic fissure eruptions are common in Iceland and Hawaiʻi.

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USGS, 5 March 2011: lava erupts along a newly opened fissure between Puʻu ʻŌʻō and Nāpau, Kīlauea. Several fountains mark the linear opening.

Basalt

A fine-grained volcanic rock rich in iron- and magnesium-bearing minerals. Basaltic magma is generally less viscous than silica-rich magma.

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NOAA, Galápagos Rift, 21 July 2011. Lava quenched in seawater forms pillows close to a spreading centre, where new oceanic crust is built.

Conduit

A passage through which magma and volcanic gases rise towards an eruptive vent.

Pyroclastic

Describing fragmented volcanic material ejected during an explosive eruption, including ash, lapilli and larger blocks. Pyroclastic deposits differ from coherent lava flows. Particle size and deposit structure help reconstruct how material was transported.

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USGS photograph of Pinatubo's Marella River valley, 1 July 1991. These are deposits left by pyroclastic flows, not a view of a flow in motion.

Process0.40Viscosity0.25Gas drive0.45

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