A model of vulcanian explosions Original Research Article

We present a model of the initial stages of the explosive eruption of magma from a volcanic conduit as occurs in Vulcanian style eruptions. We assume there is a volatile rich (1–10 wt%) mixture of magma, vaporised groundwater and exsolved volatiles, trapped at high pressure (1–100 atm) just below a plug in a volcanic conduit. If the plug disrupts, there is an explosive eruption in which a rarefaction wave propagates into the conduit allowing the volatile rich mixture to expand and discharge into the atmosphere ahead of the vent. Typically, the explosions are so rapid that coarse grained ejecta (> 0.5 mm) do not remain in thermal equilibrium with the gas, and this leads to significantly lower velocities and temperatures than predicted by an equilibrium model. Material may erupt from the vent at speeds of 100–400 m s−1 with an initial mass flux of order 107–109 kg s−1, consistent with video observations of eruptions and measurements of the ballistic dispersal of large clasts.