IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

1W_2H-P13

Date/Time: July 21 Poster

Syn-Plinian vigorous lava fountain in andesitic volcanoes: Case study of the Sakurajima
1914 and 1779 eruptions and the Asama 1783 eruption, Japan
Maya Yasui
Geosystem Sciences, College of Humanities and Sciences, Nihon University, Japan
E-mail: yasui.maya@nihon-u.ac.jp

Concrete examples of complex behaviors of erupting magma during explosive eruption will be reported on thebasis of geological observations. The eruptive products of the Sakurajima 1914 and 1779 eruptions and the Asama
1783 eruption include evidence of a large-scale deposition of pyroclastic materials in the proximal areas during
these Plinian eruptions and suggest syn-Plinian lava fountaining.
The geology, eruptive sequence, and eruption styles have been investigated for the above three eruptions (e.g.
Yasui and Koyaguchi, 2004, Yasui et al., 2013). The initial Plinian eruption occurred from flank fissures on twosides of Sakurajima volcano in 1914 and lasted for about 36 hours (Stage 1). It was followed by the outflow oflava (Stages 2 and 3). The 1779 eruption followed a similar sequence as the 1914 erupiton. Old drawings andphotographs of these eruptions show that a large amount of pyroclastic materials fell from the Plinian column in theproximal area. Pyroclastic cones are recognized along the fissures on the upper to middle flank slopes. Thus, thecones were considered to have formed simultaneously with the Plinian eruptions. Extensive clastogenic lava flows
are also recognized on the slope. These cones and lava flows are composed of welded pyroclastic materials andare characterized by eutaxitic textures and abundant highly broken crystals.
In the case of the Asama 1783 eruption, the activity culminated in a climactic, explosive eruption after theintermittent Vulcanian eruptions of about three months. The eruptive style of the climactic stage of the Asama
1783 eruption is quite similar to that of Stage 1 of the Sakurajima 1914 and 1779 eruptions.
These eruptions have the common features of the formation of proximal cones and generation of clastogenic flows
as well as dispersal of the pumice fall in the distal areas. Concerning the Asama 1783 climactic eruption, the volumeof pyroclastic materials that fell onto the proximal area is estimated to be 20 times as large as that entrained in thePlinian column. This indicates that these eruptions have similar aspects to the high fountaining with minor tephra
observed at Kilauea and Etna. The coexistence of a Plinian column and lava fountain indicates a complex behaviorof erupting magma in the conduit. Annular, misty flow in the conduit may be a possible explanation in the case ofthe Asama 1783 eruption. That is, a gas-rich center is surrounded by a pyroclasts-rich lining and a Plinian column
may originate from the center part. In the cases of the eruptions in Sakurajima, the progress from an explosive
eruption at a higher flank in Stage 1 to the effusion of lava at a lower flank in Stage 2 could be explained by thepropagation of a radial dyke. The condition of misty flow in the dyke system may be similar to that in Asama 1783
eruption.

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