Bull. Volcanol. Soc. Japan
Vol. 58 (2013) No. 1, pp. 191-212

ArticleTemporal Variations of the Petrological Features of the Juvenile
Materials during 2006 to 2010 from Showa Crater,
Sakurajima Volcano, Kyushu, Japan
Akiko MATSUMOTO＊, Mitsuhiro NAKAGAWA＊, Mizuho AMMA-MIYASAKA＊ and Masato IGUCHI＊＊(Received August 19, 2011 ; Accepted February 10, 2012）
On June 2006, Sakurajima volcano in Japan resumed its eruptive activity at Showa crater in Minamidake. We
investigated dated ash samples, including lapilli ejecta of several eruptions, to evaluate the temporal relationshipbetween their petrological features and eruptive activity. The activity is characterized by low frequency of explosive
eruptions accompanied with small, periodic inflation from June 2006 to August 2009. No juvenile materials existed in
the ash during this period (1st period). Frequent explosive eruptions accompanied with large, continuous inflation hadcontinued from September 2009 until March 2010 (2nd period). Juvenile materials such as unaltered scoria and pumicewith fresh glass have been recognized since late September 2009. The whole-rock chemistry of these juvenile materials
is similar to those from AD 1955 to AD 2000 but is the most mafic (SiO2=58.5-59.1 wt.%), indicating that the magma
system, in which mafic magma was injected into silicic magma, has not changed. The matrix glass compositions of thejuvenile materials are dacitic (SiO2=67.2-72.7 wt.%). During the 2nd period, the proportion of juvenile materials in
eruptive ash increased and the SiO2 contents of glass decreased with time. These temporal changes suggest that thehigh level of eruptive activity during the 2nd period was caused by the increase of mafic components in eruptive
magma. During April to May 2010 (3rd period), the number of eruption had become small without inflation, and theproportion of juvenile materials also decreased. From June 2010 (4th period), although the mode of crustal deformation
has changed to deflation, eruptive activity has increased again and the juvenile materials have been mainly ejected. The
glass compositions during this period have become slightly higher in SiO2 content. These temporal changes of eruptive
style and glass compositions suggest that the eruptive activity since AD 2006 has not been directly affected by theaddition of mafic components in the magma system, but has occurred because of the silicic (andesitic) magma alreadysupplied beneath the volcano. In this way, the monitoring of the petrological features of dated eruptive materials could
provide us useful information to evaluate ongoing eruptive activity as well as geophysical monitoring.
Key words : Sakurajima volcano, volcanic ash, Showa crater, juvenile material, temporal variation

1．Introduction
Sakurajima volcano, located in southern Kyushu, is oneof the most active volcanoes in Japan (Fig. 1). Four large
historical eruptions have been recorded at Sakurajima (AD764-766, AD 1471-1476, AD 1779, and AD 1914-1915),
in each of which vigorous lava flows had effused after aplinian eruption (Fukuyama and Ono, 1981; Kobayashi,1982; Kobayashi and Ishihara, 1988). After the AD 19141915 eruption, the eruptive style of the volcano changed.
Small scale of eruptions occurred at the east-southeast
flank of Minamidake during AD 1935-1945, followed bylava effusion in AD 1946 from Showa crater, which issituated on the eastern slope of Minamidake, ca. 500
meters southeast of Minamidake crater (Fig. 1). Since AD＊

Department of Natural History Sciences, Graduate Schoolof Science, Hokkaido University, N10 W8, Kita-ku,
Sapporo 060-0810, Japan.
＊＊
Sakurajima Volcano Research Center, Disaster Prevention Research Institute, Kyoto University, 1722-19, Sakura

1955, small but explosive eruptions (vulcanian explosions)have frequently occurred at Minamidake crater. The
number of explosive eruptions increased especially in theAD 1970s and 1980s. In the 21st century, however, thefrequency of explosions has decreased drastically (Iguchiet al., 2008b).
On June 2006, a small phreatic eruption occurred at
Showa crater. Thereafter, small eruptions intermittently
took place until AD 2008. Since AD 2009, explosive
eruptions have often occurred, and their frequency has
been increasing significantly until now. These activities
have been monitored by a well-developed geophysicalmonitoring network in the area of Sakurajima volcano toobtain high-quality data sets (e. g., Iguchi et al., 2008a,
jima Yokoyamacho, Kagoshima 891-1419, Japan.
Corresponding author: Akiko Matsumoto
e-mail: a-matsu@mail.sci.hokudai.ac.jp