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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.

Determining the state of magma reservoirs is essential to mitigate volcanic hazards. However, geophysical methods lack the spatial resolution to quantify the volume of eruptible magma present in the system, and the study of the eruptive history of a volcano does not constrain the current state of the magma reservoir. Here, we apply a novel approach to Nevado de Toluca volcano (Mexico) to tightly constrain the rate of magma input and accumulation in the subvolcanic reservoir. We show that only a few percent of the supplied magma erupted and a melt volume of up to 350 km3 is currently stored under the volcano. If magma input resumes, the volcano can reawake from multi-millennial dormancy within a few years and produce a large eruption, due to the thermal maturity of the system. Our approach is widely applicable and provides essential quantitative information to better assess the state and hazard potential of volcanoes., This study makes use of the total spread of zircon ages and trace elements to study the thermal evolution of magmatic systems. Applied to Nevado de Toluca, the authors determine the size of its subvolcanic magma reservoir and assess its potential of re-activation.