3.5 2.a - Volcanoes, magma and eruption styles

Explosive eruptions at convergent boundaries

Magma is molten or partly molten rock below the surface, containing liquid, crystals and dissolved gases. Once erupted as flowing molten material it is called lava. Viscosity is resistance to flow: highly viscous magma deforms less readily than low-viscosity magma under comparable conditions. Composition, temperature and crystal content all matter, rather than viscosity being an unchanging property of every volcano.

At many subduction settings, fluids from a descending slab promote partial melting above it. Magma can evolve through crystallisation, mixing and interaction with crust, producing intermediate or silica-rich compositions such as andesite, dacite and rhyolite. Silica-rich liquids generally have more interconnected molecular structures and higher viscosity than hot basaltic liquids. Cooling and additional crystals can further resist flow. USGS's magma comparison explains these controls.

As magma rises and pressure decreases, dissolved gases can form and expand as bubbles. Where escape is restricted by viscous magma or a blocked conduit, gas pressure can fragment the magma and surrounding material, producing an explosive eruption. Fragmentation ejects ash, pumice and larger particles; a high eruption column or pyroclastic density current may result. Explosion is therefore not simply “thick lava moving fast”.

Convergent boundaries commonly host explosive stratovolcanoes, but this is a tendency, not a rule for every eruption. Gas supply, storage, ascent rate and external water can alter behaviour. A volcano may alternate explosive and effusive phases. USGS's eruption-style account explains why the setting alone cannot predict the exact next event.

Effusive eruptions at divergent boundaries

Effusive eruptions mainly release lava that flows or accumulates at the surface, rather than fragmenting most material into a large explosive cloud. At spreading boundaries, mantle rises and partially melts as pressure decreases. The resulting basaltic magma is commonly hot and relatively low in silica, giving it low viscosity compared with more evolved, cooler magma.

Gas can escape more readily, and lava can travel away from a fissure or vent. Repeated flows build new crust and layered lava fields; sustained accumulation can form broad volcanic edifices. Underwater eruptions can produce pillow-shaped masses where lava surfaces chill rapidly. On land, extensive fissures can feed lava sheets and fields rather than one steep cone. These mechanisms help explain volcanic landscapes along exposed spreading environments such as Iceland.

Low viscosity does not mean an absence of force, heat or hazard. Fountains can eject gas and clots of lava, flows can bury infrastructure, and water interaction can cause explosive fragmentation even where basalt dominates. Conversely, some highly viscous lava can emerge slowly as a dome if conditions allow gas loss. The USGS Hawaiian-eruption explanation links viscosity and degassing while recognising varied behaviour.

Kīlauea's 2018 lower East Rift Zone eruption and summit collapse demonstrate the connection between lava leaving one part of a volcanic system and changes at its summit. Effusion can transform settled landscapes even where lava mobility permits broad flows. A basaltic volcano's low-viscosity magma therefore supports a process explanation, rather than a judgement that its eruptions are harmless. USGS account of the 2018 event.

An effective explanation therefore connects setting to magma generation, magma properties to gas behaviour, and eruption products to landforms. Simply replacing “destructive” with “explosive” and “constructive” with “gentle” misses the mechanisms and exceptions.

Hotspots and continental rifting

Volcanism is not confined to mapped plate edges. A hotspot is a relatively persistent region of magma supply, often explained through anomalously hot mantle rising beneath moving lithosphere. In the Hawaiian example, the Pacific Plate moves broadly north-westwards over the source region. Volcanoes carried away lose their main magma supply, become less active and are eroded and submerged, while younger volcanoes form nearer the active south-eastern end.

The age progression of islands and seamounts is evidence of relative motion between plate and source. It does not require every island to have the same size, lifetime or erosion rate, nor every hotspot to remain absolutely fixed. USGS's Hawaiʻi geology account explains the Hawaiian sequence, while its hotspot overview notes that hotspots can also occur near plate boundaries, including Iceland. Hotspot activity and boundary activity can interact.