Magma fragmentation may occur either in the volcanic conduit during magma ascent (leading to plinian columns) or in domes (leading to pyroclastic flows and surges). Although several studies (analytical and numerical calculations, experiments at room pressure and temperature using analogue materials in shock-tube type apparatus) have been performed, the fragmentation mechanisms are poorly understood. However, it is known that the fragmentation process is directly influenced by the magma physical properties and the decompression rate. In order to simulate natural eruption conditions, we are performing systematic experiments up to 950°C and 200 bars in the fragmentation bomb, using materials of different vesicularities and melt water contents. Vesiculation and fragmentation under rapid decompression are achieved in a single experiment, therefore avoiding bubble collapse.
The starting materials are haplogranitic (HPG8-Qz36Ab39Or25) glass cylinders (either 20 mm in diameter, 2.5 cm long or 8 mm in diameter and 2 cm long), hydrated up to 4 wt% in an internally-heated pressure vessel (CNRS-CRSCM, Orléans). Three sets of experiments are being performed, simulating vesiculation + fragmentation processes leading to pyroclastic flows, surges and plinian columns.
Preliminary nucleation experiments have shown that we can control the nucleation
process and define the vesicularity that should be reached before fragmentation.
After vesiculation + fragmentation experiments, the porosity of the fragments
is measured by image analysis, the melt water content is measured either
by infrared spectroscopy or Karl-Fischer titration and the melt viscosity
is calculated. The difference between calculated and measured porosity
gives the water overpressure in the bubbles. These characteristics and
the textural study of the fragments by SEM images (Fig. 3.7-4) allow to
define the fragmentation mechanisms (Annual Report 1996).
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