Bulletin of Volcanology

, Volume 69, Issue 2, pp 175–187 | Cite as

Dynamics of explosive volcanism at Unzen volcano: an experimental contribution

  • Bettina ScheuEmail author
  • Oliver Spieler
  • Donald B. Dingwell
Research Article


Knowledge of the dynamics of magma fragmentation is necessary for a better understanding of the explosive behaviour of silicic volcanoes. Here we have measured the fragmentation speed and the fragmentation threshold of five dacitic samples (6.7–53.5 vol% open porosity) from Unzen volcano, Kyushu, Japan. The measurements were carried out using a shock-tube-based fragmentation apparatus modified after Alidibirov and 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. At an initial pressure difference of 30 MPa, the fragmentation speed varies from 34 m/s for the least porous sample to 100 m/s for the most porous sample. These results are evaluated by applying them to the 1990–1995 eruptive activity of Unzen volcano. Emplacements of layered lava dome lobes, Merapi-type pyroclastic flows and minor explosive events dominated this eruption. The influence of the fragmentation dynamics on dome collapse and Vulcanian events is discussed.


Fragmentation Volcanology Unzen volcano Magma Experiments Porosity Dome collapse Vulcanian 



This work was partially funded by the project DI 431-20-1 from the German Science Foundation (DFG). The fieldwork and sampling were carried out in 2000 and 2001 together with Ulrich Kueppers. Joachim Gottsmann (Institute of Earth Sciences “Jaume Almera”, Barcelona, Spain) drilled the layered block during the 2000 field campaign. We would like to thank Ben Kennedy, Ulli Kueppers, Sebastian Mueller and Alex Nichols for helpful discussions, Setsuya Nakada and Hiroshi Shimizu for providing us with qualitative information. Shimabara Earthquake and Volcano Observatory (SEVO) permitted access to the Unzen National Park and provided logistic support during fieldwork. Two anonymous reviewers and Andy Woods contributed helpful suggestions that improved the manuscript.


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Copyright information

© Springer-Verlag 2006

Authors and Affiliations

  • Bettina Scheu
    • 1
    • 2
    Email author
  • Oliver Spieler
    • 1
  • Donald B. Dingwell
    • 1
  1. 1.Earth and Environmental SciencesUniversity of MunichMunichGermany
  2. 2.Earthquake Research InstituteUniversity of TokyoTokyoJapan

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