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During a fieldtrip in central Java in November 1996, in collaboration with the Volcanological Survey of Indonesia, samples were obtained from the 1994 pyroclastic flow of Merapi at Kali Boyong. The samples were selected in order to obtain as wide a range of density (porosity) and crystal content as possible for fragmentation experiments.The samples exhibit porosities between 5 and 60 vol.% (2D image analysis measurements on thin sections). Plagioclase und pyroxene phenocrysts make up 45 vol%. The crystal portion is expressed as a fraction of the vesicle-free volume. 18 samples were chosen for fragmentation-experiments. On these samples XRF and Electron Microprobe analyses were performed and the results are presented in Fig. 3.7-5.
 

Fig. 3.7-5: Electron Microprobe and XRF analyses of basaltic trachyandesites from Merapi. The interstitial glass covering the fields trachyte, trachydacite-rhyolite was analysed by microprobe to estimate the viscous behaviour of the sample matrix during the fragmentation experiments. XRF whole rock analyses plotted as basaltic andesite to basaltic trachyandesite were performed in the Institut für Mineralogie, Universität Freiburg.

Whole rock analyses from 17 samples show basaltic andesitic to basaltic trachyandesitic compositions. Microlite-free groundmass analysed by Electron Microprobe using a defocused beam reveals trachytic to rhyolitic glass compositions indicating a high viscosity of the molten groundmass. The range in wt% H₂O determined by Karl-Fischer-Titration is 0.14-0.26 (±0.05). Optical characterisation reveals a high degree of vesicle alignment with respect to phenocrysts. Fractures will thus more likely follow the vesicle alignment mantling a phenocryst than break the phenocryst. This is confirmed by observations of the surfaces of experimentally-generated pyroclasts and by features of their particle-size-distribution curves (Fig. 3.7-6).
 

Fig. 3.7-6: Particle-size-distribution curves of fragmentation experiments on Merapi andesite.

The distribution curves show a "second" peak at ~ 0, in the size range of the large undisturbed phenocrysts. First experiments on sample MRP9603 reveal a fragmentation pressure limit P_o of 45 bar at 18°C. Further investigation will be concentrated on understanding the influence of microlites on the fragmentation process.