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

1A2_2H-O3Room A1

Date/Time: July 21 11:15-11:30

Petrographic assessment on possible drainback processes of the andesitic lava filled in
the crater of Shinmoedake, 2011 eruption of Kirishima volcano
Hiroaki Sato1 , Keiko Suzuki2 , Eiichi Sato3 , Kyohei Sano4 , Keiji Wada5 , Ryusuke Imura6
1CIREN, Shizuoka University, Japan, 2 Dept of Earth Plan Sci, Kobe University, Japan, 3 Instit forPromotion of Higher Education, Kobe University, Japan, 4 Dept of Earth Plan Sci, Kyushu University,Japan, 5 Hokkaido University of Education at Asahikawa, Japan, 6 Dept Earth Sci, Kagoshima University,Japan
E-mail: hsato47@gmail.com

Drainback of emplaced lava in vent and crater is common phenomena for basaltic magmas (e.g., EPR, Kilauea,and Izu-Oshima). This is consistent with small supercooling and viscosity increase of basaltic magmas by magma
degassing and crystallization during extrusion. In contrast, dacitic and rhyolitic magmas tend to show large increaseof viscosity by degassing and crystallization during eruption, thus prohibiting magma drainback to the conduit andchambers. (e.g., Mt. Unzen, Mt.St.Helens). This work examines the case of andesitic lava emplaced in the 850m
diameter crater of Shinmoedake, Kirishima volcano after three subplinian eruptions in January, 2011 eruption ofShinmoedake, Kirishima volcano group. The emplaced andesitic lava is ca. 600m in diameter and ca. 130m
thick. We analyzed the blocks thrown from the crater-filled lava by subsequent Vulcanian explosions to evaluate thepossible drainback processes of the crater-filled lava. The andesitic blocks have SiO2 contents of ca 57-58 wt%
coinciding with the brown pumice of the subplinian ejecta, both of which are the mixing products of more maficand silicic magmas. The SiO2 contents of the matrix glass in the block is 71-73 wt%, slightly higher comparedwith 67-69 wt% of the matrix glass of the brown pumice of the subplinian eruption. Crystal contents of the blockis 20-35 vol% for phenocryst and 40-45 vol% for microlite. The temperatures of the lava blocks were estimatedby the compositions of pyroxenes and iron-titanium oxides giving values from 950 to 1020 degree C. The watercontent of the matrix glass was estimated to be 0.1-0.4 wt% from the measurements of bulk water contents. Theviscosity of the crater-filled magma just after the extrusion was estimated to be ca. 109-11 Pas. The conduit radius isconstrained by the eruption rate of the sub-Plinian eruptions at ca. 2-3*106 kg/s to be 12-16m by using the conduit
model of CONFLOW. Simple drainback rate of the lava by their weight including the thickness of the extruded lavas
is estimated at 5.9-13.6*10-2 m3 /s. This rate of drainback causes less than 1.6 percent of the total amount of theeffused lava within a month. SAR observation did not show evidence of drainback after the eruption, possibly
partly sustained by refilling of the chamber as observed by GNSS measurement of the volcano by NIED. This worksuggests that degassing and crystallization of the andesitic magma during emplacement may cause increase of themagma viscosity by 6 orders of magnitude, prohibiting drainback of the crater-filled lava after the emplacement.

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