Die Fragmentierung hochviskoser Magmen

The fragmentation of highly viscous magma by rapid decompression is
a young technique. For the first time an experimental tool permits
the analysis of the influence of pressure, temperature and texture
on the fragmentation of magma. The physical range of the
experiments is close to the natural conditions of dome collapse.
The experiments were performed at temperatures up to 950°C. The
initial pressure difference in the range of 10 to 250bar is in good
agreement to the natural conditions. In addition to the analysis of
the fragmentation threshold, the rapid decompression experiments
permit analysis of the primary particle distribution above the
fragmentation level of an eruption. Further analyses were aimed at
the influence of crystals on the particle distribution. An
experimental set-up was used to investigate the speed of the
propagating fragmentation wave, initiated by the rapid
decompression of porous magma samples. The fragmentation speed
experiments were performed at ~20°C using two dynamic pressure
transducers. A new technique was designed to extend the temperature
range of the fragmentation speed experiments up to 950°C. Next to
the fragmentation experiments a comparison of analytical methods
for the characterisation of the experimental pyroclasts was
necessary. The applicability of the characterisation method to both
experimental and fieldwork was of major interest. The experiments
were performed on Dacite samples form Mt. St. Helens (Washington,
USA), Fugendake (Unzen volcano complex, Japan) and basaltic
Andesites of the 1994 eruption of Merapi (Java, Indonesia). The
experimental investigation, the methodological and technical
development are scope of the presented dissertation.