Volcano Eruption Types

Effusive vs explosive eruptions, magma viscosity, and volcano shapes

Volcanic eruption style is primarily determined by magma viscosity and gas content. Basaltic magma (low SiO₂, ~50%) has low viscosity, allowing gas to escape easily — producing gentle effusive eruptions with lava flows (Hawaii-type shield volcanoes). Andesitic magma (intermediate SiO₂, ~60%) produces moderate explosive eruptions (Strombolian/Vulcanian). Rhyolitic magma (high SiO₂, ~70%) is extremely viscous, trapping gas until pressure builds to catastrophic levels — producing violent explosive eruptions with pyroclastic flows and ash columns (Plinian, like Mt. St. Helens). Temperature also matters: hotter magma is less viscous. Water content reduces viscosity but increases explosive potential as it flashes to steam during eruption.