HR: 1340h
AN: V43D-1442    [Abstracts]
TI: Fragmentation efficiency of explosive volcanic eruptions: a study of experimentally generated pyroclasts
AU: * Kueppers, U
EM: ulli@min.uni-muenchen.de
AF: Earth & Environmental Sciences, University of Munich, Theresienstrasse 41/III, Munich, 80333 Germany
AU: Scheu, B
AF: Earth & Environmental Sciences, University of Munich, Theresienstrasse 41/III, Munich, 80333 Germany
AU: Spieler, O
AF: Earth & Environmental Sciences, University of Munich, Theresienstrasse 41/III, Munich, 80333 Germany
AU: Dingwell, D B
AF: Earth & Environmental Sciences, University of Munich, Theresienstrasse 41/III, Munich, 80333 Germany
AB: Volcanic ash, one product of magma fragmentation, can pose various severe threats. Systematic evaluation of the mechanisms and the consequences of volcanic fragmentation is very difficult as not all volcanic processes are observable directly. Laboratory experiments on volcanic fragmentation using natural samples open the possibility of substantial advances in understanding quantitatively fragmentation processes and the generation of pyroclastic materials. We performed a series of rapid decompression experiments on differently porous natural samples from Unzen volcano, Japan (7.0, 20.5, 35.5 vol.% open porosity, respectively). Investigation of experimentally generated pyroclasts permits precise characterization and quantification of the fragmentation efficiency and its dependence on changing material properties and the physical conditions at fragmentation. The analysis comprised grain-size analysis and surface area measurements. The grain-size analysis is performed by dry sieving for particles larger than 250 $\mu$m and wet laser refraction for smaller particles. For all three sets of samples the grain-size of the most abundant fraction decreases and the weight fraction of newly generated ash particles (up to 40 wt.%) increases with experimental pressure (potential energy for fragmentation). This energy can be estimated from the volume of the gas fraction and the applied pressure. The specific surface area was determined through Argon adsorption. Results show that the fragmentation efficiency and the generated surface are positively correlated with the potential fragmentation energy. Results from experiments with dense samples show a large scatter but the surface increase is clearly positively correlated with the potential energy for 20.5 and 35.5 vol.% porosity samples. It turned out clear that the newly generated surface is approximately twice as high for the 20.5 vol.% sample compared to the 35.5 vol.% sample. We speculate that this is related to a decrease of bubble wall thickness with increasing porosity.
DE: 8400 VOLCANOLOGY
DE: 8404 Ash deposits
DE: 8414 Eruption mechanisms
DE: 8494 Instruments and techniques
DE: 5100 PHYSICAL PROPERTIES OF ROCKS
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting