Explosive fragmentation criteria and velocities for vesicular magma

a r t i c l e i n f o We present a new criterion for primary fragmentation of natural rock samples in shock-tube experiments, and new formulae for the effective strength of natural rock samples, for the size of primary fragments, and for fragmentation velocities. These formulae and the fragmentation criterion are given in terms of the physical properties of the rock and in terms of experimental parameters. The formulae and criterion are derived from numerical solutions and asymptotic analytic solutions to a novel recently published mathematical model for the explosive fragmentation of vesicular magma in shock-tube experiments. This model is singularly successful in accounting for the length-scales observed in these experiments. The criterion and formulae provide good matches to data from shock-tube experiments on natural samples. Explosive volcanic activity can take a wide range of forms, ranging from Hawaiian fire fountaining and Strombolian eruptions to highly energetic Vulcanian and Plinian eruptions. Fragmentation types may be roughly divided into two end-members depending on magma viscosity. In low-viscosity magma, bubbles can easily expand, ascend and coalesce, so that proposed fragmentation mechanisms include bursting bubbles and foam instability On the other hand, in high-viscosity magma, vesicles are not very mobile and bubble growth is constrained by viscous forces resulting in overpressurised vesicles. This magma tends to fragment in a brittle manner when the strength of the magma is exceeded, and this is usually taken to be due to the A number of experiments have been designed to explore in detail the fragmentation processes underlying explosive eruptions either using analogue material (Mader et al. with natural samples where fragmentation was achieved by the sudden release of high-pressure gas in brittle magma. The experiments were conducted in an apparatus based on the shock-tube principle, built by (Alidibirov and Dingwell, 1996) and subsequently optimised and adapted (Spieler et al., 2004a; Scheu et al., 2008), yet still based on the same basic principles. The setup consists of a high pressure section (autoclave, made from Nimonic™ stainless steel or acrylic glass) which is sealed off from a de-compression chamber (a 3 m long steel tank of volume 0.38 m 3) by a system of rupture discs. A cylindrical porous rock sample is tightly mounted in the autoclave. The autoclave is slowly pressurised by argon or nitrogen resulting in the pressurisation of the connected pore space of the rock sample. After pressure equilibration, and (where relevant) …