The Rheological Behaviour of Vesuvius Magmas

Somma-Vesuvius is considered one of the highest-risk volcanoes of the world due to its high population density and the large variability of its past explosive activity. Eruptive style at Vesuvius largely varies from effusive to explosive and is strongly controlled by the evolution of the physical and chemical properties of the magma. However, with the exception of previous investigations of the 1631 eruption [1,2], rheological properties of Vesuvius products are still unconstrained. Here, we investigate the viscosity of dry and hydrous remelted glasses from the Mercato (plinian) and 1906 (violent strombolian) eruptions that differ for size, eruption styles and chemistry (phonolite vs. phonolitic tephrite). In addition, we also investigate the rheological properties of the total rocks and matrices of the products of the Pollena and the Pompei eruptions. Preliminary results on the rheology of the liquid+crystals mixtures were obtained at high-T using concentric cylinder. Low T viscosities were measured using micropenetration viscometry. The results from this study were parameterized by a modified Vogel-Fulcher-Tammann equation, accounting for the effect of water and composition, and compared with previous measurements [1,2]. The results show that the high T viscosities differ by as much as 2.5 orders of magnitude and that it varies from close to Arrhenian to significantly non- Arrhenian depending on composition. Hydrothermal syntheses were performed for the Mercato and the 1906 eruption. The results show that the viscosity strongly decreases with water content, the decrease being more marked at low water contents. Isothermal holds measurements on Vesuvian melts suggest that non-Newtonian rheology occurs even at low crystal contents. Isothermal holds viscosity measurements can also define a time-temperature-viscosity window over which crystallization occurs. Results indicate that below 1150°C a very rapid viscosity increase occurs due to a high crystallization rate. [1] Romano et al., 2003, Chem. Geol. 202, 23-38; [2] Giordano et al., 2006, Chem Geol. 229,42-56