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.