Del Seppia, D.

We have performed a parametric study on the dynamics of trachytic (alkaline) versus rhyolitic (calc-alkaline) eruptions by employing a steady, isothermal, multiphase non-equilibrium model of conduit flow and fragmentation. The employed compositions correspond to a typical rhyolite and to trachytic liquids from Phlegrean Fields eruptions, for which detailed viscosity measurements have been performed. The investigated conditions include conduit diameters in the range 30–90 m and total water contents from 2 to 6 wt%, corresponding to mass flow rates in the range 106–108 kg/s. The numerical results show that rhyolites fragment deep in the conduit and at a gas volume fraction ranging from 0.64 to 0.76, while for trachytes fragmentation is found to occur at much shallower levels and higher vesicularities (0.81–0.85). An unexpected result is that low-viscosity trachytes can be associated with lower mass flow rates with respect to more viscous rhyolites. This is due to the non-linear combined effects of viscosity and water solubility affecting the whole eruption dynamics. The lower viscosity of trachytes, together with higher water solubility, results in delayed fragmentation, or in a longer bubbly flow region within the conduit where viscous forces are dominant. Therefore, the total dissipation due to viscous forces can be higher for the less viscous trachytic magma, depending on the specific conditions and trachytic composition employed. The fragmentation conditions determined through the simulations agree with measured vesicularities in natural pumice clasts of both magma compositions. In fact, vesicularities average 0.80 in pumice from alkaline eruptions at Phlegrean Fields, while they tend to be lower in most calc-alkaline pumices. The results of numerical simulations suggest that higher vesicularities in alkaline products are related to delayed fragmentation of magmas with this composition. Despite large differences in the distribution of flow variables which occur in the deep conduit region and at fragmentation, the flow dynamics of rhyolites and trachytes in the upper conduit and at the vent can be very similar, at equal conduit size and total water content. This is consistent with similar phenomenologies of eruptions associated with the two magma types.

Carbon dioxide is the second most abundant volatile species in magmas after water (Johnson et al., 1994) but its role on eruption dynamics is still largely unknown. The effects of the presence of CO2 in the Agnano Monte Spina eruption (4100 BP, Phlegrean Fields, Italy) are here evaluated by simulating the eruption dynamics from the base of the volcanic conduit up into the atmosphere. The numerical simulations consider multiphase flow dynamics and couple the steady-state, one-dimensional magma ascent model of Papale (2001) and the transient, axisymmetric pyroclast dispersal model of Neri et al. (2003). The main input parameters of the models were based on eruptive conditions estimated from the deposits. A parametric study has been performed on H2O and CO2 concentrations in the erupted magma. The addition of CO2 results in increased volatile saturation pressure and complex non-linear changes in the conduit flow. Nonetheless, within the range of conditions explored, this volatile scarcely affects the eruption style and dynamics in the atmosphere, which are principally controlled by the H2O content. The different roles of the two volatiles in the large-scale eruption dynamics are mostly the result of the competing changes induced by CO2 on vent conditions.