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

4W_2I-P5

Date/Time: July 24 Poster

Petrological monitoring of volcanic ash and evaluation of on-going eruptive activity ofSakurajima volcano, Japan: Characterization of juvenile magma and its evolution since
2006
Akiko Matsumoto1 , Mitsuhiro Nakagawa1 , Mizuho Amma-Miyasaka1 , Masato Iguchi212

Department of Natural History Science, Graduate School of Science, Hokkaido University, Japan,
Sakurajima Volcano Research Center, Disaster Prevention Research Institute, Kyoto University, Japan
E-mail: a-matsu@mail.sci.hokudai.ac.jp

Sakurajima volcano restarted vulcanian eruptions in 2006 after a quiet period of nearly ten years. Recently, ourpetrological examination of juvenile materials of major eruptions during 20th century indicated that basaltic magma
had repeatedly input in silicic mixed magma and that the level of eruptive activity had corresponded to the scaleof basaltic injection (Ebihara et al., in this meeting). Here, we reveal the relationship between eruptive activity anderupted magma since 2006 to clarify whether magma plumbing system has changed or not. Based on the temporalchange of the magmatic system, we also evaluate the geophysical monitoring to forecast future activity. Collected
eruptive materials since 2006 consist mainly of volcanic ash, and sometimes lapilli. Lapilli usually have pyroxene
phenocrysts with reverse zoning, and sometimes olivine ones with normal zoning. Its whole-rock chemistry shows
the most mafic (SiO2 = 58.5-59.8 wt.%), being consistent with the compositional trends of the 20th century’s
juveniles. These features suggest that the 20th century’s magma plumbing system accompanied with basalt input
has continued until now. Based on microscopic and BEI observation, juveniles in ash samples can be identified andare subdivided into two types: Juvenile-A and -B. Juvenile-A are magmatic, essential materials with fresh matrix
in each eruption. On the other hand, Juvenile-B are similar to Juvenile-A, but usually contains dull colored matrix.
Thus, we recognized that Juvenile-B are slightly altered materials which were related to the previous eruptions
since 2006. We focus on Juvenile-A materials to investigate temporal change of magmatic materials. Juvenile-B
are occurred in all the samples since 2006, whereas juvenile-A has occurred since September, 2009. The matrix
glass compositions of Juvenile-A are wide (SiO2 = 65-73 wt.%) and become more mafic from Sep. 2009 to Apr.
2010. Then, those have become more silicic again until Sep. 2010. Although such compositional fluctuation has
repeated four times until now, those have gradually changed to be silicic as a whole.
If matrix glass compositions of juveniles could reflect the increase of the basaltic input, we can recognize four
periods with the increase of basaltic magma (Sep. 2009-Apr. 2010; Nov. 2010-Feb. 2011; Aug-Sep. 2011;
Dec. 2011-Feb. 2012). In these periods, the weight of volcanic ash as well as the volume of expansion of possiblepressure source beneath the volcano became larger. Thus, we conclude that the scale of the basalt input correlates
to the level of eruptive activity as in the case of activity during 20th century. Petrological and geophysical monitoringsuggests that the eruptive activity since 2006 reached maximum in the period from Sep. 2009 to Apr. 2010.
Although eruptive activity has continued since then, the temporal change of petrological features of volcanic ash
suggests that there exists no evidence indicating the increase of the level of eruptive activity.

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