Modelling Surtseyan Ejecta
Surtseyan ejecta are formed in shallow sub-aqueous volcanic eruptions. They occur when water, containing a slurry of previously erupted material, is washed into the volcanic vent. This slurry is incorporated into the magma and ejected from the volcano inside a ball of magma. These magma bombs containing entrained material are called, Surtseyan ejecta or Surtseyan bombs. At the time of entrainment there is a large temperature difference between the magma (at approximately 1000°C) and the slurry (at approximately 20°C). As the inclusion temperature increases, the water contained in the slurry evaporates, causing an increase in the pressure at the boundary of the entrainment. This pressure increase is offset by the vapour diffusing through the pores of the magma. If the pressure exceeds the tensile strength of the surrounding magma the Surtseyan ejecta will rupture. The volcanological question of interest is whether the magma ruptures. There is evidence of intact ejecta so it can be concluded that rupture does not always occur. We have developed a set of equations that transiently model the changes in temperature and pressure in Surtseyan ejecta. Numerical solutions show that the pressure rapidly increases to a stable value. Because the pressure reaches equilibrium a steady-state solution can be used to determine the maximum pressure and a criterion for rupture.