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 had
continued 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 the
proportion 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 the
AD 1970s and 1980s. In the 21st century, however, thefrequency of explosions has decreased drastically (Iguchi
et 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

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2008b; Iguchi, 2010; Iguchi et al., 2010; Iguchi et al.,2011; Iguchi et al., in this issue; Yokoo et al., 2009).
Although there have been some petrological investigations
on volcanic ash from Showa crater since 2006 (Miyagi etal., 2010; Shimano et al., 2010), we have yet to clarify the
petrological features of the juvenile materials produced by
the series of eruptions since AD 2006. In this study, we
describe the petrological features of dated eruptive materials generated from Showa crater to reveal the characteristics of the erupted magma and its temporal variations.
In addition, the relationship between these petrologicalfeatures and the geophysical data, especially geodesy, is
discussed.

increased. Explosive eruptions were intermittently repeatedfrom February to April 2009 and almost every day from
July to September 2009. The number of explosive eruptions in a day has increased considerably since October
2009. The total number of eruptions, accompanied with
air shocks, was 896 times in a year until March 2010,
according to the monitoring data from the Japan Metrological Agency. However, the eruptive activity suddenly
ceased since April 2010. This resumed in the middle ofJune 2010, and since then explosive eruptions continued
until September 2010.
The total weight of erupted volcanic ash was less than
1×105 ton/month in AD 2006-2008 when the eruptive
activity occurred intermittently (Iguchi, 2010). Since July
2009, the total weight has increased significantly withincreasing the number of explosions. The monthly weightof volcanic ash was 6.6×105 tons in November 2009 andmore than 1×106 tons from January to March 2010. However, as the explosive activity became less frequent from
April to the middle of June 2010, the weight also decreased
during this period (<5×105 ton/month). Although themonthly weight of volcanic ash immediately increased
again from late June to July 2010 (1×106 tons), the weight
constantly decreased until September 2010 (less than 4×
105 tons).
Iguchi et al. (2011; in this issue) reported a tilt change inthe radial component of the underground tunnel located in
the southern area of Sakurajima volcano. Grounddeformation related to eruptive activity has been observed.
Although small inflation had been observed periodically,
the remarkable tilt change had not been detected until AD
2008. After the number of explosions increased in July
2009, clear deflation had been detected. In contrast, continuous inflation had obviously appeared at the end of
September 2009 and had continued until early April 2010.
The change in ground deformation became much smaller
in May 2010. The deflation started after the explosions in
the middle of June 2010, and the considerable deflation has
been detected until September 2010. The estimated pressure source of the observed ground deformation is locatedbeneath Minamidake at a depth of 4 km (Iguchi, 2011).

2．Volcanic activity at Showa crater during AD 2006
to 2010At the beginning of the 21st century, the level of eruptive activity at Sakurajima volcano was low (Iguchi et al.,2008b; in this issue). However, a small eruption occurredat Showa crater on June 4, 2006, making the resumption of
eruptive activity at the crater after 58 years of dormancy.
Fig. 2 shows the monthly number of explosions, weight oferuptive ash, and changes of ground deformation.
Small-scale eruptions continued for two weeks to one
month in each year from AD 2006 to AD 2007, andexplosive eruptions occurred 18 times in AD 2008. Since
February 2009, the level of eruptive activity gradually

3．Sampling, preparation and analytical procedures
The eruptive products were collected around the volcanic edifice in almost real-time since AD 2006 (Table 1).
Because the eruptive products in the activity on 2008 andJune-August 2010 could not be collected unfortunately, we
did not analyze these samples in this study. The collected
samples are bimodal in size. Most of the collected samples
are ash size (0.1-1 mm), whereas lapilli-size pumice, scoria,
and lithic samples (5-30 mm) were also collected on
November 17, 2009, and on April 6 and June 12, 2010
(Fig. 3).
All the preparation and analyses for these samples werecarried out at Hokkaido University. The samples were

Fig. 1.

Locality map of Showa crater of Sakurajima volcano.

Temporal Variations of the Petrological Features of the Juvenile Materials
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Fig. 2. The monthly variations of geophysical data. Top: number of explosions; Second: weight of volcanic ash; Third andbottom: tilt change at the Arimura underground tunnel (located in the southern area of Sakurajima volcano) and volume
of pressure source beneath Minamidake at a depth of 4 km. These data are taken from Iguchi (2010) and Iguchi et al. (inthis issue). Shaded area shows the non-treated period in this study.

washed using ultrasonic equipment, and thin sections wereprepared for microscopic observation. To identify thejuvenile materials, we observed the back-scattered electron
image (BEI) using a JEOL-8800R electron probe microanalyser, in addition to the microscopic observation. The
juvenile materials represented by lapilli-size pumice andscoria on November 2009 and April 2010 can be identified
on the BEIs with the adjusted brightness on plagioclase.

Because of this, the glassy materials with the obviouslybrighter matrix glass are considered as the juvenile
materials from each eruption (“Juvenile-A” type), and thematerials showing darker glass are regarded as the magmatic products related to previous eruptions (“Juvenile-B”type). Details are described in Chapter 4.
We carried out the component analysis of ash-size
samples on thin section, counting more than 1000 grains

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Akiko MATSUMOTO, Mitsuhiro NAKAGAWA, Mizuho AMMA-MIYASAKA and Masato IGUCHI
Table 1.

Summary of the volcanic ash and lapilli samples generated from Showa crater used in this study.

per thin section. For the calculation of the proportion of
the Juvenile-A type, we also counted these grains on the
BEIs. The modal compositions of microlite in the representative high-Si and low-Si Juvenile-A type materials
were determined on the BEIs (at 500-1000 magnification).
The areas of microlite and interstitial glass were measured
using an image processing software package (Image J).
We determined the whole-rock compositions of representative volcanic ash as well as lapilli-size samples by an

X-ray fluorescence (XRF) using a Spectris MagiX PRO
system with Rh tube. A few clasts of lapilli-size materials
were powdered as one sample, and the bulk samples of
volcanic ash with and without washing were also analyzed.
The matrix glass compositions (interstitial glass) were
determined using a JEOL-8800R electron probe microanalyser (WDS). The operating conditions were 15 kV
accelerating voltage and 10 nA beam current using a 10
µm beam. All the analyses were corrected by a ZAF cor-Temporal Variations of the Petrological Features of the Juvenile Materials
during 2006 to 2010 from Showa Crater, Sakurajima Volcano, Kyushu, Japan

Fig. 3. Photographs of lapilli-size materials from three
eruptions at Showa crater. (a) November 17, 2009; (b)April 6, 2010; and (c) June 12, 2010. The boldfacedbars in each photograph represent 2 cm. Abbreviations:lt, lithic; sc, scoria; pm, pumice; and alt: altered material (reddish lithics). There are some strongly altered,
reddish lithics (probably high temperature oxidation)in the samples from the April 6, 2010 eruption,
whereas most of the lapilli are fresh specimens.

rection method. The matrix glass and whole-rock compositions of the representative samples are listed in Table 2.

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4．Petrographical features of eruptive products
On the basis of microscopic and BEI (back-scatteredelectron image) observations, ash- and lapilli-size samples
can be grouped into several types, as described below.
The previous study reported that the BEIs were useful forthe identification of altered materials (Miyagi and Tomiya,2002). In addition, they determined the juveniles on thebasis of the compositions of magnetite microphenocrysts.
In contrast, many ash-size samples in this study donʼt include magnetite microphenocrysts. Therefore, we recognized the possible juvenile materials among eruptive
materials in each eruption on the basis of the brightness on
the BEIs (adjusted brightness on plagioclase) in addition to
the petrographical features under microscopy.
4-1 Volcanic ash
The volcanic ash, ranging from 0.2 to 0.8 mm in average
size, comprises four types of materials: Juvenile-A, JuvenileB, strongly altered materials, and isolated crystals. The
petrographical features of these are as follows:(a) Juvenile-A type: These materials, including pumice,scoria, and dense lithics (Figs. 4a-c), show an angular shapeand are intersertal in texture. Their matrixes are composed
mainly of plagioclase, with small amounts of pigeonite andmagnetite, and colorless or brown glass. Pumice and scoria
are well vesiculated and include a smaller amount of microlite in their matrix compared with those in dense lithics.
Regardless of the type of material, the matrix glass is lightcolored on the BEIs, and therefore the plagioclase microlites are clearly distinguishable from matrix glass (Figs.
4a-c). According to these features, this type of materials
can be considered as juvenile ejecta in each eruption.
(b) Juvenile-B type: These materials are similar to JuvenileA types in shape and texture on microscopy (Figs. 4d-e).
However, their matrix glass is dull-colored and sometimes
shows a small amount of silica minerals. On the BEIs,
these materials have more microlites and darker matrix
glass compared with those of Juvenile-A types, similar toplagioclase on brightness. The matrix glass of these materials usually shows more silicic compositions than those ofJuvenile-A types (Appendix 1). These features suggestthat Juvenile-B types are magmatic materials not of each
eruption, but probably of the previous eruptions since AD
2006. Many microlites might reflect the slow cooling.
We call them “Juvenile-B type” in order to distinguishthem from the juvenile materials from eruptions before AD
2006.
(c) Strongly altered type: This type of material is strongly altered, and consists of two subtypes: reddish-toyellowish lithics and silicified lithics (Figs. 4f-g). The
reddish-to-yellowish lithics are usually glassy, and rarely
include plagioclase microphenocrysts. Their matrix glasses
show much darker than those of juvenile materials since
AD 2006 on the BEIs (Fig. 4f). They are clearly different
from the Juvenile-A and Juvenile-B types in glass compositions (Appendix 1). The silicified lithics have many

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Table 2. Representative matrix glass of Juvenile-A types and whole-rock compositions of the
volcanic ash and lapilli samples from Showa crater.

silica minerals (probably cristobalite) in their matrix,
showing much lower brightness on the BEIs (Fig. 4 g).
These are so-called accessory and accidental ejecta.
(d) Isolated crystals: Plagioclase, orthopyroxene, clinopyroxene, and Fe-Ti oxides are commonly found as isolated crystals. Olivine is sometimes identified. Some ofthese crystals are angular. In addition, most of the samples
include some cristobalite crystals.
4-2 Lapilli-size materials
The lapilli-size samples, ranging from 5 to 30 mm in size,
can be grouped into three: Juvenile-A, Juvenile-B, andstrongly altered types, as in the case of ash-size samples.
On the basis of microscopic and BEI observations, it canbe estimated that each type of lapilli was fragmented to
produce the same types of ash samples. The isolated
crystals in the ash samples must be also fragments of thelapilli samples (except for cristobalite).
(a) Juvenile-A type: This type consists of scoria andpumice, collected in November 2009 and April 2010, witha fresh and angular shape. The mineral assemblage is thesame in all the samples: plagioclase, orthopyroxene, clinopyroxene, Fe-Ti oxides, and sometimes olivine in scoria(Fig. 5c). Their groundmass is intersertal in texture andcomposed mainly of plagioclase, with small amounts ofpigeonite and magnetite, and light brown (scoria) orcolorless (pumice) glass (Figs. 5a, 5c-d). On the BEIs, theTemporal Variations of the Petrological Features of the Juvenile Materials
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Fig. 4. Photomicrographs and back-scattered electron images (BEIs) of the representative component materials of the
volcanic ash from Showa crater. (a) Scoria-type Juvenile-A, (b) pumice-type Juvenile-A with olivine phenocrysts (OL),(c) dense lithic-type Juvenile-A, (d) and (e) dense lithic-type Juvenile-B, (f) strongly altered (reddish lithic: arrow), and(g) strongly altered (silicified lithic). All Juvenile-A type materials can be distinguished from Juvenile-B by the higher
brightness of matrix glass compared with plagioclase on the BEIs, whereas both materials appear similar features under
microscopy.

matrix glass of this type is bright, and therefore the matrix
glass is distinctive from plagioclase microlite (Figs. 5gh). Considering these microscopic and BEI features, thesematerials can be considered as the juvenile products ofeach eruption.

(b) Juvenile-B type: Dense lithics (from all three eruptions) and some scoria and pumice (from the eruptions in
April 2010 and June 2010) are classified under the JuvenileB type. Most of the petrographical features of this type arenearly the same as those of the Juvenile-A type. However,198

Akiko MATSUMOTO, Mitsuhiro NAKAGAWA, Mizuho AMMA-MIYASAKA and Masato IGUCHI

Fig. 4.

Continued.

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Fig. 5. Photomicrographs and back-scattered images (BEIs) of the lapilli-size materials during AD 2009 to 2010 fromShowa crater. (a) and (e) Scoria of November 17, 2009 (Juvenile-A); (b) and (f) dense lithics of November 17, 2009
(Juvenile-B); (c) and (g) scoria of April 6, 2010 (Juvenile-A); (d) and (h) pumice of April 6, 2010 (Juvenile-A). The
phenocrystic minerals consist of plagioclase, orthopyroxene, clinopyroxene, magnetite, and sometimes, olivine. The
microlites are composed of plagioclase and small amounts of orthopyroxene and magnetite. On the BEIs, the matrix
glass of dense lithics is darker than those of scoria and pumice and shows partial leaching (such as lacking of matrix
interstitial glass). Therefore, most scoria and pumice are considered as juvenile materials, while dense lithics areprobably accessory materials.

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Akiko MATSUMOTO, Mitsuhiro NAKAGAWA, Mizuho AMMA-MIYASAKA and Masato IGUCHI

Fig. 6. Photomicrographs and BEIs of the volcanic ash at Showa crater from each period during AD 2006 to 2010. (a)March 2, 2009 (1st period); (b) February 19, 2010 (2nd period); (c) April 6, 2010 (3rd period); and (d) August 27, 2010
(4th period). Red squares in each photomicrograph shows the area of BEI. Abbreviations: A, Juvenile-A type; B,
Juvenile-B type; Red, reddish lithic; and Si, silicified lithic.

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Fig. 7. The result of component analysis of the representative ash-size samples from Showa crater since AD 2006. Asterisk(*) shows the samples used for the bulk chemical analysis.

the matrix glass of Juvenile-B materials exhibits a dullcolor under microscope (Fig. 5b). In addition, the matrix
glass is darker on the BEIs and shows partially leachedfeatures, such as lacking of matrix interstitial glass in somesamples (Fig. 5f). Thus although Juvenile-B materials areslightly altered, their petrographical similarity to JuvenileA materials suggests that they can be considered asmagmatic materials related to the series of eruptive activities since AD 2006.
(c) Strongly altered type: This type consists of strongly
altered lapilli. Such materials were observed only in April
2010 (Fig. 3b). They show a rounded shape and are
reddish in color. Their matrix glass is glassy, showing amuch darker in the BEIs. This type might be much older
eruptive products not related to the activity since AD 2006.
4-3 Temporal variations of the petrographicalfeatures of volcanic ash
The volcanic ash from Showa crater show peculiar
characteristics by following four periods: from June 2006
to August 2009; from September 2009 to March 2010;
from April to May 2010; and from August to September
2010 (Figs. 6 and 7).

From June 2006 to August 2009 (the 1st period)
The volcanic ash ejected during this period ischaracterized by the absence of Juvenile-A type materials(Figs. 6a and 7) (although the samples from the activities
in AD 2008 lacking). The ash comprises Juvenile-B-typematerials (ca. 40-60 %), strongly altered materials (mainly
silicified) (ca. 10-35 %), and isolated crystals (ca. 15-30%). Olivine crystals are not found.
From September 2009 to March 2010 (the 2nd period)
The main components of the volcanic ash of this period
are Juvenile-A and Juvenile-B types (ca. 50-70 %), andisolated crystals composed of plagioclase, orthopyroxene,
clinopyroxene, Fe-Ti oxides, and a small amount of olivine(ca. 20-30 %) (Figs. 6b and 7). The strongly altered materials in this period are much fewer than the samples of the
1st period (ca. 5-20 %). Although Juvenile-A type materials
are minor components in the ash of September 2009, theirratio in the eruptive ash had increased since late October
2009, from less than 5 % to 15 % (Fig. 7). The ratio ofmaterials having a slightly smaller amount of microlites(Figs. 8a-b) seems to have increased in the ash samples
ejected during the later phase of this period.

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Fig. 8. The back-scattered electron images (BEIs) of the representative Juvenile-A type materials in the volcanic ash
samples. (a) Volcanic glass from October 21, 2009 (2nd period, high-Si); (b) Volcanic glass from March 3, 2010 (2ndperiod: low-Si); (c) Volcanic glass from April 6, 2010 (3rd period: low-Si); and (d) Volcanic glass from September 13,
2010 (4th period: high-Si). The abundance of microlite of high-Si volcanic glass seems to be slightly larger than those of
low-Si one.

From April 2010 to May 2010 (the 3rd period)
The volcanic ash of this period is characterized by thepresence of more strongly altered materials: reddish lithics(Figs. 6c and 7). These ash-size samples consist mainly ofJuvenile-A and Juvenile-B types (ca. 60 %), strongly alteredmaterials (mainly reddish) (ca. 10-15 %), and isolated crystals (ca. 20-30 %). Olivine is sometimes present as an
isolated crystal. The grain size of these ash samples issmaller than those in the earlier periods (Fig. 6c). Althoughmany Juvenile-A type scoria and pumice with moderate
microlite content are also found in the ash of April 2010(ca. 14 %) (Fig. 8c), these components are much fewer inthe volcanic ash of May 2010 (less than 3 %) (Fig. 7). In
contrast, the ratio of reddish lithics in strongly alteredmaterials increases in the sample of May 2010.
From August to September 2010 (the 4th period)
The proportion of Juvenile-A type materials increases

from this period again (ca. 5-10 %) (Figs. 6d and 7). The
groundmass of these materials exhibits moderate microlite
content, slightly higher content than those of the 2nd and3rd periods (Fig. 8d). The amount of strongly alteredmaterials is less than those in the other periods (ca. 5-8 %).
Olivine is sometimes present as an isolated crystal.
5．Geochemical features of the lapilli and volcanic ash
5-1 Whole-rock chemistry
The chemical compositions of the lapilli-size samples ofJuvenile-A and Juvenile-B type materials show a narrow
range of SiO2 content (58.5-59.5 wt. %) irrespective of
their type, such as scoria and lithic (Fig. 9). These samples
are plotted on the areas of silica-poor end of the trends
defined by the juvenile materials from Minamidake crater
since AD 1955, consistent with our conclusion based on
petrographical observation that both Juvenile-A andTemporal Variations of the Petrological Features of the Juvenile Materials
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Fig. 9. Representative plots of oxides against SiO2 for the whole-rock chemistry of the lapilli-size materials (Juvenile-A andJuvenile-B types) and volcanic ash since AD 2006 from Showa crater. The compositions of the 1955-2000 juveniles
from Minamidake crater are also shown (Nakagawa et al., 2011). The lapilli-size samples from Showa crater exhibit themost mafic compositions among the juveniles since AD 1955, agreeing with the compositional trends in 20th-century
juveniles. The compositions of the volcanic ash are clearly different from the trends found in juvenile lapilli.

Juvenile-B types of materials are essential (juvenile) ejecta
in the activities since AD 2006.
We also determined the whole-rock chemical compositions of volcanic ash samples from five eruptions (March
2, 2009; February 19, 2010; March 18, 2010; April 6,
2010; and August 27, 2010). Although samples treatedthrough different preparation methods (with or withoutwashing with water) were analyzed, no obvious differencein chemical compositions due to the distinct preparation
techniques was found. Compared with the whole-rockchemistry of lapilli-size juvenile clasts from AD 1955 to
AD 2010, these samples displayed different trends in SiO2
variation diagrams, showing higher TiO2 and FeO* andlower Na2 O and K2 O contents (Fig. 9). Also, there is aclear compositional gap in SiO2 content between the sample
from March 2009 and the other samples. This might beconsistent with the appearance of Juvenile-A type materials in addition to the decrease of strongly altered materials(silicified lithics) in the volcanic ash samples since September 2009 (Fig. 7).
5-2 Glass chemistry
The chemical compositions of volcanic glass in JuvenileA type materials from the ash and lapilli-size samples,such as pumice, are dacitic (SiO2=67-72 wt.%; Fig. 10).
The glass compositions in the volcanic ash are similar ineach eruption, irrespective of the color and type of materials.
Focusing on temporal variation, the average of SiO2

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Fig. 10. Representative plots of oxides against SiO2 for the matrix glass compositions of the juvenile materials in lapilli andvolcanic ash from Showa crater. Arrows show the compositional trends of the results of mass balance calculations forthe effect of microlite crystallinity on the basis of microlite modal compositions in the Juvenile-A type materials of theash-size sample (plagioclase, pigeonite and magnetite: Table 3). Although there are no differences in size, the 2010
lapilli are slightly less silicic than the 2009 ones. Also, mass balance calculations cannot explain the compositionalvariations of matrix glass by the microlite crystallization, especially SiO2, TiO2 and FeO* contents.

Temporal Variations of the Petrological Features of the Juvenile Materials
during 2006 to 2010 from Showa Crater, Sakurajima Volcano, Kyushu, Japan

content of glass compositions of each sample decreases
from SiO2=71-70 wt.% in September 2009 to SiO2<69wt.% in early April 2010 (Fig. 11). In contrast, the glass
compositions of August to September 2010 seem tobecome more silicic with time. The average SiO2 content
of matrix glass of the lapilli-size juveniles of November
2009 and April 2010 is the same as those of juvenile
materials in volcanic ash from eruptions in the same
period. The matrix glass of the 2010 lapilli-size samples ismore mafic than that of the 2009 samples.
6．Discussion
6-1 Comparison between the temporal variationsof petrological features and geophysical data
Four periods defined by the temporal change in the
proportion of the component types in volcanic ash would
correspond to the monitoring data (Fig. 12). In the first
period from June 2006 to September 2009, the level of
eruptive activity had been lower than in the following
periods, whereas inflation had gradually progressed, withsmall inflations and deflations occurring periodically.
Since September 2009, remarkable inflation had been
detected continuously, and the number of explosive
eruptions and the weight of ejected volcanic ash increased
with time until March 2010 (2nd period). This high levelof eruptive activity until March 2010 would be consistent
with the appearance of Juvenile-A-type materials during
the period. In the third period from April to May 2010,
explosive eruptions had almost ceased without grounddeformation. The proportion of Juvenile-A type materials
in volcanic ash also decreased. From June 2010 (4thperiod), the eruptive activity resumed with clear deflation.
However, the number of explosive eruptions and theweight of ejected volcanic ash had become much less thanthose during the second period.
The temporal variations in the glass compositions arealso consistent with the above division of eruptive activity.
During the second period, the glass compositions graduallybecame less silicic, from SiO2=71.0 to 68.8 (in average).
In the third period, the volcanic glass showed the mostmafic composition (SiO2=68.7 wt.%). We were unable toinvestigate the temporal change in volcanic glass compositions from June 2010, but the four samples from
August 2010 became slightly silicic again (Figs. 11 and
12).
The matrix glass composition reflects the crystallinity of
microlite as well as the variations in the whole-rock composition and phenocryst mode of original magma. That is,differences in the crystallinity of the matrix affect compositional variations in the matrix glass (e.g., Hammer et al.,2000). Thus, we examined the possibility of the effect of
microlite crystallinity to matrix glass compositions on thebasis of the observations. Indeed, as described above,
especially in the 2nd period, the abundance of microlite
decreases in the later samples (Fig. 8), and the silica con-205

tent of matrix glass increases with time (Fig. 11). We performed mass balance calculations on the basis of the
microlite modal compositions of representative high-Si
and low-Si Juvenile-A type materials (Appendix 2), andwe concluded that the matrix glass showed the compositional variations beyond the effect of microlite crystallinity (Fig. 10). For example, the difference of microlite
crystallinity induces an increase of SiO2 less than 2 wt.%,
inconsistent with the range of matrix glass compositions(ca. 4 wt.%). On plots of Al2 O3, CaO, Na2 O, and K2 O
against SiO2, the compositional trends by microlite
crystallization might correspond to the measured data. On
the other hand, TiO2 and FeO* contents increase withincreasing of SiO2: this feature disagrees with the compositional variations of matrix glass. In addition, the sumof the squared residuals (R2) shows low reproducibility inthis calculation, and there are more than 10 % differences
between the calculated and measured values on several
elements (Table 3). Therefore, it is concluded that thecompositional variations of matrix glass reflect the effectof mafic component, not being largely affected by the
degree of microlite crystallinity.
In summary, the synchronized temporal changes in both
the geophysical features of the eruptive activity and thepetrological changes in the eruptive materials suggest thatmagmatic (in the chamber) and eruption (in the conduit)
processes have temporally changed across the four periods
since AD 2006. During each period, similar magmatic anderuption processes had continued, and had changed at thetransition between periods. We note that during the 2nd
period, magmatic and eruption processes might havechanged gradually.
6-2 Deep processes during four episodes since AD
2006
It has already been indicated by previous studies thatmagma mixing was the main magmatic process during thehistorical activity on Sakurajima volcano: mafic magma
had recharged into silicic magma (Miyasaka et al., 2010;
Nakagawa et al., 2010, 2011; Yanagi et al., 1991). The
whole-rock chemistry of the juvenile materials is consistent with the compositional variations in those from AD
1914 to AD 1999 (Fig. 9). Thus, it can be assumed that theeruptive magma since AD 2006 was also a magma-mixing
product, as in the case of the AD 1914 and 1946 eruptions.
If magma mixing has occurred, the temporal variations in
the glass chemistry could be related to the mixing ratio
between mafic and silicic end-member magmas as discussed above. If so, the temporal change in glass chemistry
since the 2nd period suggests that the effect of the mafic
component gradually increased during the 2nd and 3rd
periods and then decreased in the 4th period. In otherwords, the temporal change in eruptive activity (e. g.,number of explosion and eruptive volume) might be related to the degree of the effect of the mafic component in
the magma system. As mentioned before, each eruptive

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Akiko MATSUMOTO, Mitsuhiro NAKAGAWA, Mizuho AMMA-MIYASAKA and Masato IGUCHI

Fig. 11. Temporal change in the SiO2 content of the matrix glass compositions of the Juvenile-A type materials in the
volcanic ash and lapilli from Showa crater. The compositions of the lapilli-size samples also agree with the systematic
variations of the volcanic ash. The temporal change in the glass compositions of the ash samples seems to be consistent
with the eruptive episodes since AD 2006.

Temporal Variations of the Petrological Features of the Juvenile Materials
during 2006 to 2010 from Showa Crater, Sakurajima Volcano, Kyushu, Japan

207

Fig. 12. Comparison between the magma supply rate beneath the Minamidake and Showa craters (Iguchi et al., 2013, in thisissue) and the glass compositions of juveniles from Showa crater. The eruptive activity at Showa crater can be divided
into four periods on the basis of the petrological features of the eruptive products as well as the geophysical data. The
eruptive periods might be related to the injection of mafic magma. The 1st period: Because addition of mafic magma
was small, andesitic magma was not supplied substantially, resulting in low eruptive activity without juveniles. The 2nd
period: Mafic magma had significantly injected into magma system; therefore, a large amount of andesitic magma was
supplied beneath the volcano, accompanied by remarkable inflation, an increase in the number of eruptions, and adecrease in the SiO2 content of the volcanic glass of juveniles because of the increase of the effect of the mafic
component. The 3rd period: The volcano had almost ceased erupting because the mafic magma recharge was much too
small, and the supply volume of andesitic magma had decreased. The 4th period: The andesitic magma that had
accumulated until the 3rd period had erupted, increasing the silica content of volcanic glass because of the absence ofmafic magma recharge.

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Akiko MATSUMOTO, Mitsuhiro NAKAGAWA, Mizuho AMMA-MIYASAKA and Masato IGUCHI
Table 3.

Result of mass balance model for crystallization of microlite on the basis of modal compositions.

period from the 1st to the 3rd periods is characterized by adistinct mode of crustal deformation around the volcano.
Thus, it is probable that the monitored deformationreflected the temporal change of the addition (injection) of
mafic component to the magma plumbing system beneaththe volcano.
In contrast, the eruptive activity has continued sinceJune 2010 (4th period), whereas the crustal deflation has
been monitored. In addition, the matrix glass of juvenile
materials has become more silicic. These suggest that theaddition of mafic component has decreased or ceased during
the period. Thus, the eruptive activity during this period
was not directly caused by the addition of mafic component but by a more open condition of the conduit, where
already supplied silicic (andesitic) magma was easily tapped to produce the explosive activities. This explanation
might be consistent with the much lower level of eruptive
activity in the 4th period than that in the 2nd period.
Based on long-term geophysical monitoring, especially
geodesy, magma accumulation has continued beneath theAira caldera adjacent to Sakurajima volcano (Eto et al.,
1997; Iguchi et al., 2008a; Kamo, 1978; Kamo et al., 1997).
It has also been suggested that part of the accumulated
magma had moved into the volcano during previous
eruptive activity, resulting in the frequent explosive
eruptions from AD 1955 to the end of the 20th century(Kamo, 1978). In addition, the whole-rock compositionsof the 2009-2010 juvenile materials are concordant withthe compositional variations in juvenile materials from the20th-century activity (Fig. 9). Therefore, it is inferred thatthe andesitic magma system since AD 2006 is nearly thesame as that during the 20th activity and that andesitic
magma has been supplied from the inflated pressure source
beneath the Aira caldera to Sakurajima volcano since AD
2006.

However, the previous model based on geophysicalmonitoring did not consider the effect of the mafic component on the andesite magma migrating from the Airacaldera toward Sakurajima volcano. In order to re-evaluate the geophysical data and to revise the model of themagma plumbing system of the volcano, additional petrological and geochemical investigations for the rocks fromrecent eruptive activity especially after AD 1970s must be
carried out, because dense and excellent geophysicalmonitoring has continued until now.
6-3 Evaluation of the chemical compositions of
volcanic ash
The chemical compositions of volcanic ash are easily
determined and must be correlated with the features of thejuvenile materials. However, the whole-rock compositionof volcanic ash is usually different from that of adequatesize juvenile materials. This is because the ratio of mineral to melt in volcanic ash is different from that in thederived magma due to the separation of mineral and melt
by fragmentation and transportation. In addition, the ash
sample often includes materials that are not juveniles (e.g.,altered rocks and basement sediments). Even though these
problems, we tentatively evaluate the bulk chemical compositions of volcanic ash from several eruptions in order todevelop conventional methods of monitoring of eruptive
materials.
Comparing the whole-rock chemical compositions ofash samples and lapilli-size ones (Juvenile-A and JuvenileB type materials), the ash samples are higher in SiO2,
TiO2, FeO*, and MgO, and lower in Na2O, K2O, and Al2O3 contents. This suggests that the compositional differences between lapilli-size and ash-size samples are related
to the separation of crystals and melt by fragmentation.
The isolated crystals in the volcanic ash are plagioclase,
orthopyroxene, clinopyroxene, Fe-Ti oxides, and some-Temporal Variations of the Petrological Features of the Juvenile Materials
during 2006 to 2010 from Showa Crater, Sakurajima Volcano, Kyushu, Japan

times olivine. If any combinations of these isolated crystals accumulate, the whole-rock composition of volcanicash becomes lower in silica content. Thus, higher SiO2
content in the whole-rock compositions of the volcanic ash
would be produced by the accumulation of the strongly
altered materials (silicified lithics and cristobalite).
Among the ash-size samples, the chemical compositionsof the sample from the 1st period show higher SiO2 andlower Na2O contents than those of juvenile materials, probably reflecting a dominant proportion of altered materials,
especially silicified lithics (Fig. 7). The SiO2 contents ofthe volcanic ash from the 2nd to the 4th periods are lower
than that in the 1st period, consistent also with the microscopic observation that these ash samples are mainlycomposed of Juvenile-A and -B type materials with lessamounts of strongly altered materials than the samples in
the 1st period (Figs. 7 and 9). Although the change in
SiO2 content of ash samples from the 2rd to 4th periods
might reflect the proportion of components of silicified
lithics, we are unable to evaluate because of the paucity ofthe data (only 5 samples).
In summary, there exists a discrepancy in chemicalcomposition between the coarse juvenile materials and ash
samples even in the 2nd to the 4th periods, when JuvenileA type materials were ejected (Fig. 9). Even so, the large
differences in the chemical compositions of ash samples
between the 1st and the other periods can be recognized.
The difference of the 1st period from the others is consistent with the existence of a large amount of silicified
lithics in the ash samples. Interestingly, the proportion ofsilicified lithics in the ash samples from Showa crater
during AD 2006 to September 2010 correlates with thepresence of Juvenile-A type materials: the ash samples in
the 1st period that include more silicified lithics are in theabsence of Juvenile-A types (Fig. 7). Therefore, the silica
content of the whole-rock compositions of ash samples canbe a good index to identify the eruption type: the presenceor absence of magmatic materials related to magma in each
eruption. Although we used only five samples in this examination and the components of volcanic ash samples
might depend on their grain size (Miyagi, unpublisheddata), at least, the chemical analysis of ash samples might
become a tool for monitoring in ongoing eruptions at
Sakurajima volcano.
7．Concluding remarks
We revealed the petrological features of the dated
volcanic ash and lapilli-size materials since AD 2006
collected from Showa crater. Also, we revealed the relationships between the petrological features of eruptive
products and the geophysical data set, and indicated theeffect of the magma migration (or supply) on the mode oferuption in the activity at Showa crater since AD 2006.
For evaluation of future eruptive activity, it is necessary to
combine the geophysical and petrological observations in

209

detail. The eruptive activity at Sakurajima volcano is
ongoing, and therefore, we must continue the petrologicalmonitoring of the eruptive materials.
Acknowledgements
We express our gratitude to T. Kobayashi and H.
Miyamachi for helping in the collection of samples. We
wish to thank H. Nomura and K. Nakamura for preparing
the thin sections. We appreciate to H. Sato and I. Miyagi
for valuable and constructive comments. We also thankM. Ban for editing our paper.
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Appendix 1, 2
Appendix 1. Representative matrix glass compositions ofJuvenile-B type and reddish lithics in the volcanic ash
samples.

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during 2006 to 2010 from Showa Crater, Sakurajima Volcano, Kyushu, Japan

211

Appendix 2. Modal compositions of microlite of the matrix with high-Si and low-Si glass (Juvenile-A types).

(Editorial handling Masao Ban)

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Akiko MATSUMOTO, Mitsuhiro NAKAGAWA, Mizuho AMMA-MIYASAKA and Masato IGUCHI

桜島火山，2006〜2010 年の昭和火口噴出物の岩石学的特徴の時間変化
松本亜希子・中川光弘・宮坂瑞穂・井口正人

九州南部に位置する桜島火山は，2006 年 6 月に昭和火口における噴火活動を再開した．我々は，2006 年 6月〜2010 年 9 月に噴出した火山灰及び火山礫について岩石学的特徴を明らかにし，マグマの特徴とその時間変化，そして噴火活動との関係を検討した．その結果，2006 年以降の昭和火口の活動が以下の 4 つの活動期に区分されることが明らかになった．第 1 期（2006 年 6 月〜2009 年 8 月）: この期間は，爆発的噴火が少なく，小さな山体膨張・収縮が繰り返された．噴出物中に本質物は認められない．第 2 期（2009 年 9 月〜2010 年 3 月）: この期間は，爆発的噴火の頻度・規模ともに大きくなり，明瞭な山体膨張が継続した．噴出物中には本質物（スコリア・軽石）が認められる．火山礫の全岩化学組成は，1955 年以降の噴出物の組成ト
レンドと調和的で，且つ最も苦鉄質な組成を示す (SiO2=58.5-59.1 wt.%)．これは，桜島火山では 1955 年以降同じマグマ系が活動しており，珪長質マグマに苦鉄質マグマが注入していることを示唆している．また，火山灰中の本質物の量比は時間とともに増加し，その石基ガラス組成は時間とともに SiO2量が減少している．従って，第 2 期では，噴出マグマにおける苦鉄質マグマの割合の増加が活動規模を拡大させていると考えられる．第 3 期（2010 年 4 月〜5 月）: この期間は噴火頻度が極端に低下し，山体膨張もほぼ停止した．噴出物中に本質物は認められるが，その量は少なく，変質岩片が増加する．第 4 期（2010 年 6 月〜2010 年 9月）: この期間になると，明瞭な山体収縮が始まり，火山爆発も再び活発化した．噴出物中に本質物が多く認められるようになるが，その石基ガラス組成はやや珪長質であることから，この時期の噴出マグマは苦鉄質
マグマの影響が小さいと言える．つまり，2006 年以降の桜島火山では，マグマ系に注入する苦鉄質マグマそのものが噴出しているのではなく，既に火山下に供給されている珪長質マグマが主体となって活動していると考えられる．