Rheological properties of magma from the 1538 eruption of Monte Nuovo (Phlegrean Fields, Italy): an experimental study

The rheology of natural, partially crystallized samples from Monte Nuovo (1538 AD) eruption have been investigated using a high temperature, high pressure internally heated pressure vessel. Within the temperature interval of investigation the samples exhibited Newtonian rheology at deformation strain rates up to 10 − 4 s − 1 . At higher strain rates a decrease of viscosity with increasing strain rate (shear thinning), eventually terminating in a sudden viscosity drop (stress weakening), was observed. Stress weakening occurred at higher applied strain rates and stresses with increasing temperature. The microstructural analyses of the samples revealed that stress weakening resulted from strain localization, which occurred in brittle and viscous regimes at the lowest and highest temperatures investigated respectively. The different rheological behaviors of the investigated material is likely responsible for the changes in the dynamics of magma ascent within the volcanic conduit during the eruption of Monte Nuovo volcano. We infer that the occurrence of syn-eruptive decompression driven crystallization, observed in previous studies, induced strong variations in the rheological properties of magma, which in turn was responsible for the changes of eruptive style observed during the Monte Nuovo eruption.