Basaltic eruptions are commonly regarded as the weakest form of explosive volcanism, as the low viscosity of the magma allows a large fraction of primary gas phases to escape quietly. Nevertheless, basaltic eruptions of highly-explosive nature have occurred in historic times or the near past (e.g. Etna 2001, 2002-03; Stromboli 2003, 2007) and brought harm to populations and infrastructures. These occurrences draw attention on poorly-defined volcanic processes, which may cause a shift of the eruption style of basaltic volcanoes into explosive behaviours proper of their silicic counterpart. Observed rapid fluctuations in the eruptive behaviour of basaltic volcanoes suggest that the eruption style is modulated by variable two-phase flow (melt-bubbles) conditions and the mechanism by which the magma fragments. What governs the transition from intermittent (Hawaiian and Strombolian) eruptions to more violent and possibly sustained (sub-Plinian and Plinian) activity remains largely uncertain. Textural observations of tephra suggest that increased viscosity and brittle fragmentation of microlite-rich magma may be crucial for such transition. To define the constitutional conditions for violent basaltic explosive activity, we will experimentally investigate the influence of rheological changes in the rising magma on conduit flow, fragmentation, and, ultimately, the eruptive style. The fragmentation mechanism of the magma will be investigated on the base of its rheological behaviour modelled as a non-Newtonian multiphase suspension (melt-bubbles-crystals). This will be achieved through a two-fold approach: 1) state-of-the-art rheological characterization and 2) rapid decompression (shock-tube) experiments on both analogues materials and natural samples. Comparing features of natural pyroclasts and deposits will provide key parameters to be used to scale the fragmentation experiments and place them in the field of modelling of eruption dynamics.
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