HR: 11:20h
AN: V31G-05    [PDF]
TI: An Experimental Contribution to the Dynamics of Explosive Volcanism. A Case Study of Unzen Volcano and Soufri\`{e}re Hills Volcano, Montserrat.
AU: * Scheu, B
EM: betty@min.uni-muenchen.de
AF: Department of Earth \& Environmental Science, University of Munich, Theresienstr.41, Munich, 80333 Germany
AU: Spieler, O
EM:
AF: Department of Earth \& Environmental Science, University of Munich, Theresienstr.41, Munich, 80333 Germany
AU: Dingwell, D B
EM:
AF: Department of Earth \& Environmental Science, University of Munich, Theresienstr.41, Munich, 80333 Germany
AB: Knowledge of the dynamics of magma fragmentation is necessary for a better understanding of the explosive behavior of silicic volcanoes. Here we have measured the fragmentation speed and the fragmentation threshold of five dacitic field samples (6.7 vol.% to 53.5 vol.% open porosity) from Unzen volcano, Kyushu, Japan and four andesitic-dacitic field samples of Soufri\`{e}re Hills Volcano, Montserrat, West Indies (2.5 vol.% to 70 vol.%). The measurements were carried out using a shock-tube based fragmentation apparatus modified after Alidibirov \& Dingwell (1996a, b). The results of the experimental work confirm the dominant influence of porosity on fragmentation dynamics. The velocity of the fragmentation front increases, and the value of the fragmentation threshold decreases, with increasing porosity. Further, we observe that the fragmentation speed is strongly influenced by the initial pressure difference and the texture of the dacite. E.g. at the Unzen samples the fragmentation speed varies at an initial pressure difference of 30 MPa from 34 m/s for the least porous sample (6.7 vol.%) to 100 m/s at 53.5 vol.% porosity. During field work at Unzen we observed porosity layering in single large blocks within the pyroclastic flow deposits as well as in dome material of Mt. Unzen. The existence of layered structures may be the sites for the initiation of the explosive events at Unzen. Here we address the influence of the layered structure on the fragmentation behavior and deduce a speed propagation model for fragmentation in a layered medium. These results are evaluated in terms of their application to the 1990 - 1995 eruptive activity of Unzen Volcano, Japan and the 1997 eruption activity of Soufri\`{e}re Hills Volcano, Montserrat. The different explosivity and eruption style of these both volcanoes can be analyzed by means of the fragmentation behavior. Hereby we highlight the influence of the fragmentation dynamics on dome collapse and vulcanian events. In order to explain long lasting collapse events which occurred on Montserrat as well as on Unzen, we propose a fragmentation-induced collapse scenario.
DE: 3944 Shock wave experiments
DE: 5100 PHYSICAL PROPERTIES OF ROCKS
DE: 8400 VOLCANOLOGY
DE: 8414 Eruption mechanisms
DE: 8494 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting