HR: 09:00h
AN: V51B-05    [PDF]
TI: Fragmentation Efficiency via Fine Particle Analysis of \\Experimental Pyroclasts
AU: * Kueppers, U
EM: ulli@min.uni-muenchen.de
AF: Earth and Environmental Sciences, University of Munich, Theresienstrasse 41/III, Munich, 80333 Germany
AU: Spieler, O
AF: Earth and Environmental Sciences, University of Munich, Theresienstrasse 41/III, Munich, 80333 Germany
AU: Dingwell, D B
AF: Earth and Environmental Sciences, University of Munich, Theresienstrasse 41/III, Munich, 80333 Germany
AB: Internal overpressure can fragment porous volcanic rocks at pressure differentials mainly depending on the porosity. With increasing pressure, an increasingly effective generation of fine particles is expected. To date, experimental data on the fragmentation efficiency of natural multiphase samples has been absent. Knowledge of this fragmentation efficiency is however necessary for modelling explosive eruption behaviour and dome stability scenarios. To this end, a series of hot (850 $\deg$C) rapid-decompression experiments has been performed in a modified fragmentation bomb above the fragmentation threshold (i.e. the critical gas overpressure required to fragment volcanic materials when decompressed rapidly). We used cylindrical samples (d = 25 mm, l = 60 mm) with different porosity and known phenocryst content drilled from natural samples. The fragmentation depends on open porosity and permeability of the specific sample as these parameters are defining how much energy (the pressurised gas) can be stored inside the sample and how fast it can escape. \\The artificially generated pyroclasts are sampled, dried and prepared for grain-size analysis and surface area measurements. Grain-size analysis includes dry sieving (wt.%) and wet laser particle sizing (vol.%). Grain size results are limited by the sample size and instrumental limitations (Coulter LS230, measuring range 0.375 - 2000 $\mu$m). \\Laser particle Sizer results are cast for inclusion in a wt.%-grain size histogram. Preliminary results of experiments with dense sample from Unzen volcano (5 % average open porosity) at pressures up to twice as high as the sample's threshold (22.5 MPa) reveal a relatively similar grain size distribution with increasing pressure. The most abundant fraction (8$<$x$<$5.6 mm) represents 25 $\pm$ 2.5 wt.% of all particles. \\The surface area is determined through Ar absorption after the BET method (Micromeritics Gemini). The evaluated results (m$^{2}$/g) are corrected by the known pre-fragmentation surface area and multiplied with the weight of the fine fraction to get the actual increase in surface area (m$^{2}$) in this size class. The m$^{2}$/g-results of the fine particles are up to 10 times higher than the value of the cylinders. The newly generated surface of the fine particles may exceed the total surface of the cylinders although representing only a small weight percentage. The newly generated surface seems to be correlated positively with pressure.
DE: 5100 PHYSICAL PROPERTIES OF ROCKS
DE: 8400 VOLCANOLOGY
DE: 8404 Ash deposits
DE: 8414 Eruption mechanisms
DE: 8494 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting