Unsteady explosive activity: Vulcanian eruptions

Overview Vulcanian eruptions are named for the 1888–90 eruptions of Vulcano, Aeolian Islands, Italy (Mercalli, 1907), and are defined here as short-lived, discrete explosions resulting from sudden decompression of a volcanic conduit caused by disruption of a sealing plug or dome. Resulting eruptions characteristically last only seconds to minutes and may produce buoyant columns, pyroclastic density currents, or both. They may occur as single events or in a sequence of discrete explosions. The short duration and unsteady vent conditions of vulcanian eruptions make them distinct from sustained plinian or subplinian eruptions. Pre-eruption pressures can reach 10 MPa, vent velocities may approach 400 m s⁻¹, eruption plumes typically rise to < 10 km, but in some cases may reach nearly 20 km, and the amount of magma erupted is typically < 10¹¹kg. This chapter reviews mechanisms associated with vulcanian eruptions and discusses several relevant conceptual and quantitative models. Topics include plug formation and disruption, magma fragmentation, calculation of vent flux, the production and propagation of shock waves, the dynamics of pyroclastic jets and plumes ascending from unsteady sources, and ballistic analysis. This chapter also addresses important questions regarding controls on the scale and duration of such short-lived explosions, as well as transitions in eruptive style.