Bull. Volcanol. Soc. Japan
Vol. 58 (2013) No. 1, pp. 59-76

ArticleComparative Study of Proximal Eruptive Events in the Large-scale
Eruptions of Sakurajima Volcano: An-ei Eruption vs. Taisho Eruption
Maya YASUI＊, Masaki TAKAHASHI＊, Jun SHIMADA＊,
Daisuke MIKI＊＊ and Kazuhiro ISHIHARA＊＊＊(Received January 4, 2011 ; Accepted December 4, 2012)
The An-ei eruption (1779-1782 A. D.) and Taisho eruption (1914-1915 A. D.) were large-scale eruptions ofth3
Sakurajima Volcano. The latter, the largest eruption in Japan during the 20 century, produced about 1.5 km DRE ofandesitic magma. In both cases, flank eruptions from two sides of the volcano caused pumice fall and lava flows. The
An-ei eruption occurred on the northeastern and southern flanks (An-ei NE and An-ei S eruptions, respectively) and the
Taisho eruption occurred on the western and eastern flanks (Taisho W and Taisho E eruptions, respectively).
In the An-ei NE, two fissures are recognized from the alignments of craters: the main fissure (5 km long) and aminor fissure (1 km long). A large pyroclastic cone consisting of welded pyroclastic materials was formed along thesefissures on the upper to middle flank slopes. Old drawings of the An-ei eruption show that a large amount ofpyroclastic materials fell from the eruption column in the proximal area. Thus, the cone was considered to have formed
simultaneously with the Plinian eruption. The presence of many cracks and a horse-shoe shaped depression on the cone
is attributed to the deformation and collapse of the pyroclastic cone due to gravitational instability on the flank slope ofthe volcano. A stratigraphy of the eruption products shows that many lava lobes were formed after the initial Plinian
eruption. In the An-ei S, the existence of a deformed pyroclastic cone sticking on the steep upper slope below thesummit crater and clastogenic lava flows on the downslope indicate the syn-Plinian deposition of pyroclasitc materials
on the steep slope. After then, effusion of lavas and some explosions occurred from the chains of craters on the middle
flank slope.
The An-ei eruption progressed continuously in three stages. The initial Plinian eruption for up to two days on the
upfissure (Stage 1) was followed by lava flows on the downfissure (Stage 2). Then, submarine eruption occurredintermittently for about two years on the NE offshore (Stage 3). In the case of the Taisho eruption, initial Plinian
eruption (Stage 1) was followed by lava flows associated with the intermittent Vulcanian eruptions (Stage 2) and then,gentle lava outflow continued for more than 1.5 years in the Taisho E in Stage 3. In both eruptions, progress from an
explosive pyroclastic eruption at a higher flank in Stage 1 to the effusion of lava at a lower flank in Stage 2 could be
explained by the propagation of a radial dyke.
Although the pyroclastic cone of the An-ei NE and Taisho W are large in scale, the syn-Plinian clastogenic flow
and cone collapse of the Taisho W were extensive. On the other hand, the cone of the An-ei S and Taisho E are small inscale. The cone of the An-ei S was deformed on the steep slope. From these variations, the intensity and duration of
the Plinian eruption and the gradient of the flank slope would result in the various morphologies of the resultant cone.
Concerning the growth of the edifice of Sakurajima Volcano, the proximal process in which the pyroclastic
materials plaster the flank slope in Stage 1 does not contribute to the growth of the summit area, but to that of the flankslopes. The intense proximal deposition of pyroclastic materials during the initial vigorous Plinian eruption and thefollowing lava flow mean that a rapid response against fires caused by pyroclast fallout and lava flows from multiple
craters on eruptive fissures is needed to minimize damage in future eruptions.
Key words : Plinian eruption, proximal deposition, pyroclastic cone, clastogenic lava

1．Introduction
Sakurajima Volcano is located in southern Kyushu. It is＊

Department of Geosystem Sciences, College of Humanities
and Sciences, Nihon University, 3-25-40, Sakura-josui,
Setagaya-ku, Tokyo 156-8550, Japan.
＊＊
Sakurajima Volcano Research Center, Disaster Prevention Research Institute, Kyoto University, Sakurajima-composed of two volcanic edifices, Kitadake (1117 m a.s.l.)and Minamidake (1040 m) (Fig. 1). There have been
Yokoyama, Kagoshima-shi, Kagoshima 891-1419, Japan.
2-1-34, Nanryo-cho, Uji, Kyoto 611-0028, Japan.

＊＊＊

Corresponding author: Maya Yasui
e-mail: yasui.maya@nihon-u.ac.jp

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Maya YASUI, Masaki TAKAHASHI, Jun SHIMADA, Daisuke MIKI and Kazuhiro ISHIHARA

Fig. 1. Index map of Sakurajima Volcano and the distributions of the eruption products of the An-ei and Taisho eruptions.
Isopach maps of pyroclastic fall deposits of the two eruptions by Kobayashi (1986) are shown. Thickness in cm. Basedon the bathymetric map (Japan Coast Guard, 1990) and Ishihara et al. (1981), the areas of submerged lava flows are also
shown. The squares on the map show the areas in Figs. 2 and 3. Abbreviation of the place-names are as follows; FJ:
Fujino, T: Take, KO: Koike, H: Hakamagoshi, Y*: Yokoyama, A* Akobaru, K: Karasu-jima (buried island), F:
Furusato, AR*: Arimura, WK*: Waki, ST*: Seto, KR: Kurokami, SJ: Shinjima, SO: Sonoyama. *: Buried villages in1914.

many large-scale eruptions of Sakurajima Volcano. In
historic times, four large-scale eruptions occurred, i.e., in764-766 A. D. (Tenpyo era), 1471-1476 (Bunmei era),
1779-1782 (An-ei era), and 1914-1915 (Taisho era)
(Kobayashi, 1982, 2009). In particular, in the case of the
Taisho eruption, lava spread extensively, causing Sakurajima island to become connected to the Osumi Peninsula in1914 (Fig. 1). In 1946, lava issued from the eastern upper
flank slope of Minamidake in 1946 (Kobayashi, 1982).
Ash-emitting eruptions with or without detonations havebeen occurring from the crater of Minamidake since 1955
(Kobayashi et al., 1988). This type of eruption has beenfrequently observed in the 1946 crater since 2008 (Iguchi
et al., 2010).
Generally, near-vent processes are crucial for understanding eruption phenomena. Geologic investigations oferuption products in a proximal area give us detailed
information on how eruption phenomena progressed along
an eruptive activity. In this study, we focus on the proximal products of the An-ei and Taisho eruptions. In bothcases, flank eruptions occurred on two sides of the volcano
generating pumice fall and lava flows (Fig. 1). They areboth documented eruptions and their eruption products are
well-preserved. Furthermore, drilling core samples fromlava fields for both eruptions enable us to investigate thestratigraphy of deposits. Therefore, the two eruptions aregood examples for the case study of the large-scale eruption of Sakurajima Volcano.
Geologic features of the eruption products, eruption
sequence and eruption style of the Taisho eruption havealready been summarized by Yasui et al. (2007). Thus,
we will present those of the An-ei eruption. Then, we willcompare the eruption sequences and styles of the two
eruptions. In particular, we will examine the phenomena
in a proximal area in the initial phase and discuss theirimplications in detail. Hereafter, we will call the flankslopes in the An-ei and Taisho eras An-ei NE and An-ei S,
and Taisho W and Taisho E, respectively.
2．Previous studies of the two large-scale eruptions
2-1 Eruption sequence and eruption style of the
Taisho eruption
The Taisho eruption of Sakurajima Volcano was thelargest eruption in Japan in the 20th century, which produced andesitic magma of about 1.5 km3 DRE (dense rockequivalent) (Ishihara et al., 1981). Many previous studiesof the eruption have been reported. Studies based oneyewitness accounts of the eruption are important for reconstructing the eruption sequence (e. g., Koto, 1916;
Omori, 1914, 1916a, b, 1920a, b, 1922; Yamaguchi, 1967,

Comparative Study of Proximal Eruptive Events in the Large-scale Eruptions of Sakurajima Volcano :An-ei Eruption vs. Taisho Eruption

61

Fig. 2. Distributions of eruption products of the Taisho eruption. (a) Stage 1 on the western side. (b) Stage 2 on the westernside. (c) Stages 1 and 2 on the eastern side. (d) Stage 3 on the eastern side. (e) Duration and eruption styles of each stage(modified from Fig. 18 of Yasui et al., 2007). For details of the stratigraphy, see Yasui et al. (2007). Lava units arelabeled as W1 and E1. Craters are labeled as C1 and C2. Assumed fissures are shown in Figs. 2a and 2c.

1975). Yasui et al. (2006) compiled these records andYasui et al. (2007) gave a geological description of theeruption products and established a stratigraphy of thedeposits. The following is a summary of their results.
The uppermost crater in the Taisho W locates at analtitude of approximately 450 m (C1 in Fig. 2a) and that in
the Taisho E at an altitude of 400 m (C1 in Fig. 2c). The
length of the fissure in the Taisho W is estimated to beapproximately 2 km and that on the Taisho E is 2.3 km.
The eruption products of the Taisho eruption are composedof many eruptive units of pyroclastic deposits and lava
flows. Thick pyroclastic materials formed pyroclastic
cones in the proximal area on both sides. 17 units of lava

flow on the western side and 21 units on the eastern side
can be defined.
Yasui et al. (2007) showed that the eruption style of the
Taisho eruption changed with time, and they divided theactivity into three stages (Fig. 2e). Fig. 2 shows the distributions of pyroclastic cones and lava flows that formedin each stage. The eruption occurred from eruptive fissureson the western and eastern flanks (Fig. 1). The initial,
vigorous Plinian eruption started at approximately 10 am
on January 12th, 1914 and lasted for about 36 h (Stage 1).
The photographs taken at that time show that a large
amount of pyroclastic materials from the Plinian eruption
column fell into the proximal area. Several villages (KR,62

Maya YASUI, Masaki TAKAHASHI, Jun SHIMADA, Daisuke MIKI and Kazuhiro ISHIHARA

ST, AR, and WK in Fig. 1) were burnt by falling pyroclasts
within several hours from the onset of the Plinian eruption(Omori, 1916a). According to the records, minor pyroclastic flows occurred on several occasions, setting housesof several villages (Y, KO, A, T, and FJ in Fig. 1) on fire inStage 1. Thereafter, lava extruded for about 20 days on
both sides (Stage 2). Lava spread widely on the seafloor
on the Taisho E in Stage 2 (Fig. 2c). Intermittent ashemitting eruptions with or without detonations occurred inStage 2 indicating occasional Vulcanian eruptions simultaneous with lava flows. Subsequently, the number of ashemitting eruptions decreased and a gentle lava outflow
continued for more than 1. 5 years on the eastern side
(Stage 3). Consequently, vast lava fields, which consistedof a number of flow units, formed on both sides of the
volcano. Small-scale lava deltas also developed on theeastern side (Fig. 2d). Eyewitness accounts reported thatlava entered the ocean at several points starting fromseveral months after the start of Stage 3. On the basis of
the descriptions of the eruption and the topographic
features, Yasui et al. (2007) deduced that lava tube
systems were formed to feed lava and lava deltas formedon the southern coast (Fig. 2d).
Yasui et al. (2007) discussed the type of lava of the
Taisho eruption mainly on the basis of petrographic features. In particular, they focused on the broken crystal
content that is useful for distinguishing coherent lava fromclastogenic lava. Since broken crystals are dominant infree crystals of pumice fall deposits (Yasui and Suganuma,
2003), they are probably formed during explosive eruptions such as a Plinian eruption (Yasui, 2012). To investigate the type of lava of the Taisho eruption, the proportionof broken plagioclase phenocrysts [volume of broken plagioclase phenocrysts/total volume of plagioclase phenocrysts×100 (refered to as BPL hereafter)] was measured
for 81 thin sections of lava samples from the Taisho
eruption (Figs. 16 and 17 in Yasui et al., 2007).
The following features indicate that the lava flows inStage 1 on both sides are of clastogenic origin. The lava
(e.g. W4 and W10 in Fig. 2a, and E4 in Fig. 2c) shows ahigh BPL content of approximately 70 % and a distinct
eutaxitic texture under a microscope. It is the same as adrilling core sample that represents a 30-m-thick lava
(open circle in Fig. 2a). The surface blocks of the lava
flows consist of pyroclastic materials. Various degrees of
welding are recognized even within a single block. The
nonwelded part of a single block consists of porous,
reddish-brown, oxidized pyroclasts that are reasonably
well sorted. Some of the flow units on the western side
can be traced upstream to a collapsed pyroclastic cone thatis indicated by a depression surrounded by a horse-shoe
shaped scarp (Fig. 2a). From these facts, it is consideredthat cone growth, partial collapse of the cone, andgeneration of the clastogenic lava occurred simultaneouslywith Plinian eruption in Stage 1 in the Taisho W.

In contrast to the lava in Stage 1, the lava in Stage 3
contains much smaller amounts of broken crystals, with a
BPL of approximately 20 % and shows no eutaxitic
texture, indicating a gentle outflow of coherent lava.
The lava in Stage 2 contains broken crystals with a BPL
of approximately 50 %. Its flows show a distribution that
resembles ginkgo leaves (Figs. 2b and 2c) suggesting thatthey are coherent flows that overflowed from the craters.
Since these lava flows were associated with frequent ashemitting eruptions, magma in the conduit was thought tohave experienced repeated fragmentation and coalescence
due to intermittent explosions prior to the outflow.
2-2 Eruption sequence of the An-ei eruption
The distribution of eruption products of the An-ei
eruption has been described in previous studies (e. g.,
Fukuyama, 1978; Kobayashi et al., 1988). Imura (1998a)
examined historical records of the An-ei eruption in detailand reconstructed its eruption sequence. According toImura (1998a), a Plinian eruption started at around 2 pm on
November 8th , 1779 on the southern flank of Minamidake
and on the northeastern flank of Kitadake in succession(Fig. 1). The Plinian eruption on both flanks became
intense, reaching its climax from the evening of November
8th to the following morning. On the night of November
10th or the following morning, the eruption on the southernside ceased and lava reached the southern coast of
Sakurajima. The eruption on the northeastern side ceased
at around 2 pm on November 10th and lava reached theocean on that day. Aramaki and Kobayashi (1986) pointed
out that the lava flows were generated from a chain ofcraters on both sides. However, it is not clear when thelava started flowing on both sides. According to Kobayashi(2009), submarine eruption started on the northeasternoffshore just before the lava reached the ocean on thenortheastern coast. Intermittent submarine eruptions continued for more than a year and eight islands collectively
named the An-ei islets (Fig. 1) had appeared by 1782.
These islands are on a plateau-like rise with a diameter ofabout 4 km on the sea floor (Fig. 2 in Kobayashi, 2009).
These islands are composed of lava or uplifted sedimentson the sea floor. Kobayashi (2009) also examined old
documents and concluded that a submarine cryptodome
first intruded below the seafloor and then phreatomagmatic
eruptions continued in the shallow waters. Takahashi etal. (2011) discussed the difference between the wholerock chemical compositions of terrestrial deposits andsubaqueous drilled lava samples. They concluded thatmagma that erupted into Sakurajima and the magma that
intruded below the seafloor were not derived from thesame parental magma. Thus, the magma with a different
chemical composition is thought to have intruded to form asubmarine cryptodome after the generation of lava flows
on the northeastern flank in Stage 2. According to Aramaki
and Kobayashi (1986), the volume of the terrestrial deposits including the lava and pumice fall is about 0.83 km3Comparative Study of Proximal Eruptive Events in the Large-scale Eruptions of Sakurajima Volcano :An-ei Eruption vs. Taisho Eruption

63

Fig. 3. Topographic features of the eruption products of the An-ei eruption.
(a) Northeastern side. Solid square shows the cliff mentioned in section 3-1-1. Spatial distribution of the cone X, Y, andZ is also shown.

DRE. Ishihara et al. (1981) estimated the total volume of
the An-ei eruption as about 1. 7 km3 DRE including
subaqueous lava.
3．Description of the eruption products of the An-ei
eruption
For convenience, we divide the flank slope into three:
upper, middle, and lower flanks (Figs. 3a and 3b). The
upper flank corresponds to the steep slope below the craterrim of summit craters and the gradient of this slope isbetween 20 and 30 degrees. The altitude between theupper and middle flanks is about 500 meters and thatbetween the middle and lower flanks is about 100 several
meters. The gradient of the middle-flank slope is between
10 and 20 degrees and that for the lower flank slope is lessthan 10 degrees. The lower flank on the southern side isnarrow (Fig. 3b), indicating that the average gradient of theslope is steeper on the southern side than on thenortheastern side.

3-1

Eruption products of An-ei eruption on thenortheastern side
3-1-1 Topographic features
Pyroclastic cones, craters, and lavas are observed inaerial photographs in the An-ei NE (Fig. 3a).
Pyroclastic cones
Three pyroclastic cones are recognized: X, Y, and Z (Fig.
3a). Cone X is the largest extending from the upper to the
middle flank slope. Many cracks develop on its surface on
the middle flank slope (labeled A in Fig. 3a). Similarcracks also exist on the eastern edge of the cone (labeled B
in Fig. 3a). Cone Y is the medium-sized one, distributing
the boundary part of the middle to lower flank slopes(labeled Y in Fig. 3a). Although a large area is covered bylava flows, the scattered flat areas and a cliff indicate aplateau-like morphology. Cone Z is the smallest; it has asemicircular ridge suggesting it to be a remnant of aconical body.

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Maya YASUI, Masaki TAKAHASHI, Jun SHIMADA, Daisuke MIKI and Kazuhiro ISHIHARA

Fig. 3. Continued.
(b) Southern side. The contact relationship between
lava flows was determined using aerial photographs.

Craters
Many depressions with diverse shapes are observed,
most of which are bowl-shaped craters. They are labeled 1
to 12 (Fig. 3a). Crater 3 on the upper flank slope at thealtitude of about 610 m is the largest circular crater with adiameter of 600 m (Fig. 3a). Mortar- and bowl-shaped
craters (5, 6, 8, 9, and 10-12) with diameters between 60
and 130 m are scattered on the middle to lower flanks.
Since their localities continue to lava lobes, they are
possible sources of lava flows. This will be discussed indetail in section 4-1-2. Crater 7 is a tiny crater on the lava
lobe. There is a deep gully (labeled D1 in Fig. 3a)connecting two bowl-shaped craters, i.e., craters 1 and 2(Fig. 3a). D2 is a long and narrow depression that
corresponds to the present valley bottom. D3 is a shallow
depression surrounded by a horse-shoe shaped scarp (Fig.
3a). From their shapes and localities, D1 and D2 may
represent eruptive fissures and D3 may represent a
collapsed topography. Origin of D3 will be discussed in
section 4-1-2.
Lava
Lava is subdivided into lava lobes and depression-filledlava. Most of the lava shows lobelike distribution.

Multiple lava lobes (e. g., a, g, i, and o in Fig. 3a) are
observed on the middle to lower flank slopes. The lengthsof lobes range from several hundred meters to 1 km. A
circular arclike folded texture that is concave in the
downward direction developed on the surface of the lava
lobes, indicating the flow direction of each lobe. Distinct
lava levees are observed on lobes (a, g, and i in Fig. 3a).
Mounds are present on the surface of lobes h, l, m, and o(Fig. 3a). A cliff, shown by a solid square in Fig. 3a, isobserved midstream of g and i. It appears that these flows
crept down the cliff and spread downstream. The cliff
corresponds to the edge of cone Y. In the coastal area, lobec spread to form a lava delta (Fig. 3a). The distribution of
the lava shown by the bathymetric map (Japan Coast
Guard, 1990) indicates that some of the lava entered theocean and spread on the ocean floor less than 1 km fromthe present coastline (Fig. 1). Although the ocean entry
sites are unclear from the aerial photographs, lobes a, c, e
and f are possible candidates of the submerged flows,
judging from their distributions on-land.
Lavas n, p and q are depression-filled lavas (Fig. 3a).
Lava n fills D3. Circular arclike cracks that are concave inthe upward direction develop on the smooth surface of lava
n. Lava q is dome-shaped and occupies the bottom ofcrater 3. This is clearly recognized in the aerial photograph taken in 1974. Lava p is a massive lava with a flat
surface burying D2.
3-1-2 Geological features
The geological features of the An-ei eruption productson the upper flank were observed in this study on thenortheastern side. A thick pyroclastic deposit composedof many units with various occurrences is seen on theupper flank. However, it was difficult to observe thesurface features on the middle to lower flank because of
thick vegetation and cultivation.
A section of the thick pyroclastic materials consisting ofcone X is observed at localities 1 and 2 (Fig. 4a). At
locality 1 (Fig. 3a), nonwelded pyroclastic materials of 18
m thickness overlie a welded part of more than 45 m
thickness (Fig. 4b). The main constituents are reddishbrown to grayish vesicular pyroclasts with glassy lithic
fragments. The deposits are mostly matrix-supported.
However, clast-supported parts and layers are sometimes
observed in the deposit. The pyroclastic deposit often
contains many coarse grains, suggestive of proximalballistic origin. Fractured surface of the welded part is
exposed on the valley bottom (labeled C in Fig. 4b).
Welded pyroclastic materials are exposed on thenorthern wall of D2 at locality 2 (Fig. 4c). The degree of
welding changes within a single outcrop. The occurrenceis sometimes chaotic, containing a patchy massive part anda nonwelded well-sorted part (Fig. 4d). A distinct
eutaxitic texture and many broken crystals are observed in
the densely welded rock under a microscope (Fig. 5).
Stratified pyroclastic deposits with a total thickness ofComparative Study of Proximal Eruptive Events in the Large-scale Eruptions of Sakurajima Volcano :An-ei Eruption vs. Taisho Eruption

65

Fig. 4. Photographs showing the proximal pyroclastic materials of the An-ei eruption at localities 1, 2, and 3 on thenortheastern side. For localities, see Fig. 3a. (a) Upper flank slope seen from locality 2. (b) Thick pyroclastic deposit at
locality 1. (c) Thick welded pyroclastic deposit at locality 2. (d) Closeup view of the chaotic occurrence at locality 2.
Flattened blocks are seen. Scale: 33 cm. (e) Lower half of the pyroclastic deposit at locality 3. Layers [2] to [5] are seen.
For details, see the columnar section in Fig. 6. Scale: 1 m.

about 180 cm are exposed on the gully wall at locality 3(Fig. 4e). About five layers with scoriaceous and pumiceous components are observed (Fig. 6). Some layers are
lenticular, becoming thin in the horizontal direction.
Layers 1 and 3 are composed of scoriaceous blocks and areclast supported. Angular scoriaceous blocks with a diameterof about 20 cm comprise layer 1. Glassy lithic fragments
are found in layer 3. Layers 2, 4, and 5 are not wellsorted. Large scoriaceous blocks concentrate in the upper
part of layer 4 and are densely welded. Layer 5 contains

charcoal. Most of the layers are assumed to be smallscale pyroclastic flow deposits.
The pyroclastic deposits described above consist ofvesicular pyroclastic materials, suggesting that they wererelated to a pyroclastic eruption of Plinian style.
3-1-3 Drilling core samples
A drilling core exists in the area of lobe g (Fig. 3a). It
was drilled by the Sakurajima Volcano Research Center of
Kyoto University (SVO) to install devices for monitoring
the volcano. The drilling site is located at an altitude of66

Maya YASUI, Masaki TAKAHASHI, Jun SHIMADA, Daisuke MIKI and Kazuhiro ISHIHARA

Fig. 5. Photomicrograph of the densely welded part of the
pyroclastic materials at locality 2. For locality, see Fig.
3a. A markedly eutaxitic texture can be observed.
Lines denote the broken surfaces of plagioclase. Opennicol.

126 m above sea level. The total depth of the core is 122
m. Vertical changes in the occurrence, apparent density,
grain size of the groundmass and chemical composition are
observed throughout the core.
The core from the surface up to a depth of 2 m iscomposed of ash (Fig. 7a) mostly derived from the recent
Vulcanian activity. Below 2 m, a lava continues up to adepth of about 100 m. It is reddish gray from depths of 2
m to 30 m, and dark-gray and glassy from depths of 30 m
to 45 m and 82 m to 100 m. The core between 45 and 82 m
is pinkish, pale-gray lava with platy joints. Porous
breccias with approximately 5 cm long are intercalated at
depths of 34, 36, 41, 42.5 and 46 m. The cores at depths
between 6 and 14 m and between 52 and 58 m are reddish,suggesting oxidation. No core samples were recovered at
depths between 102 and 109 m. Miki et al. (2003) showed
the paleomagnetic features of the core samples, indicatingthat the core below 109 m is made up of the products of
Kitadake Volcano. This is also confirmed in this study on
the basis of the chemical composition. Four lava fragments 20 to 40 cm long were recovered at depths between
92 and 96 m. These fragments also have a chemical composition identical to that of Kitadake Volcano.
Samples at depths greater than 20 m have apparent
densities of approximately 2. 5 g/cm3, whereas those at
depths less than 20 m have smaller densities (Fig. 7b).
The drilling core samples are heterogeneous to thenaked eye regardless of depth. In particular, at depths ofapproximately 8 and 21 m, reddish-brown lenses are
randomly distributed in the gray matrix. A eutaxitic
texture and broken crystals are observed under a microscope. BPL varies from 35 to 87 % for nine core samples
and samples with a high BPL content more than 60 % are

Fig. 6. Columnar section of the eruption products observed in locality 3. For locality, see Fig. 3a. Details
are described in section 3-1-2.

dominant (Fig. 7c).
The phenocryst contents of 32 thin sections show anarrow range of approximately 16 vol.% throughout thedepth (Fig. 7d). The grain size of the groundmass changes
with depth. From the surface, the average grain size
gradually increases toward the center and then decreases
with depth (Fig. 7e).
The SiO2 content of the drilling core samples (Takahashi
et al., 2011) has a wide vertical variation and changes
systematically with depth. The SiO2 contents range from63. 3 to 66. 6 wt. %. From the bottom, the SiO2 content
increased sharply from approximately 63.8 to 66.6 wt.%
and then decreased to 63.6 wt.% (Fig. 7f).
3-2 Eruption products of An-ei eruption on thesouthern side
The area covered by the eruption products is small on
the southern side compared with that on the northeasternComparative Study of Proximal Eruptive Events in the Large-scale Eruptions of Sakurajima Volcano :An-ei Eruption vs. Taisho Eruption

67

Fig. 7. Columnar section and down-hole variations of the drilling core at the Shirahama observation well in the An-ei NE.
The drilling site is shown in Fig. 3a. (a) Columnar section and lithology. A: volcanic ash, RG: reddish-gray lava, DG:
dark-gray lava, PG: pinkish, pale-gray lava, BR: breccia, LF: lithic fragment, N: no core sample, open circle: vesicle
concentrated zone, solid square: lithic fragment of Kitadake lava, arrow: oxidized zone. (b) Density (g/cm3). To measure
the apparent density, drilling core samples were cut into columns whose volume and weight were measured. (c) Broken
plagioclase (BPL) content (%). (d) Phenocryst content (vol.%). (e) Average grain size of groundmass plagioclase (μm).
The length of the long axis of groundmass plagioclase was measured for 28 thin sections. Here the average groundmass
size was determined by measuring the long axis of ten grains of plagioclase here. (f) Bulk SiO2 content (wt.%).

side (Fig. 1). A pyroclastic cone (cone A) is on the upper
flank slope (Fig. 3b). Many cracks develop on the surfaceof the cone and the talus deposit covers the foot of cone. A
smaller cone (cone B) is on the lower edge of cone A.
The uppermost bowl-shaped crater, crater I, is locatedon the steep slope of 27 degrees at an altitude of about 760
m a.s.l., which is just below the crater rim of Minamidake.
This crater I is surrounded by cone A, whereas crater III issurrounded by cone B. A chain of small craters, i. e.,
craters V, VI, and VII, are on the lava field, forming enechelon alignments (Fig. 3b). They extend to the NNESSW directions. Craters II, III, and IV also make analignment in the same direction.
At least five lava lobes are recognized: lobes 1〜4, and6. Their contact relationships are shown in Fig. 3b. The
upstream regions of lobes 1 and 2 continue to cone A. The
source craters of each lobe are assumed as follows: crater
II for lobes 3 and 6, and crater IV for lobe 4 (Fig. 3b).
Lava 5 seems to be a small-scale dome. The preservationof the surface morphology is ideal for lobe 4 and dome 5,
indicating that their effusion occurred last among the
products described above. On the other hand, the surfaceof lobe 3 is flat suggesting that it was buried by ash.

A sample from lobe 1 (locality a in Fig. 3b) shows a
distinct eutaxitic texture and a high BPL of 68 %,indicating its origin to be a densely welded pyroclasticrock.
3-3 Bulk-rock chemistry of An-ei eruption products
Here, bulk-rock chemical compositions are comparedamong the eruption products from the pumice fall deposit,
proximal pyroclastic materials, on-land lava (Fig. 8) anddrilling core samples. The data are from Takahashi et al.
(2011). The SiO2 contents of the overall eruption productsof the An-ei eruption range from 63.3 to 66.6 wt.%. These
data show a single trend on a variation diagram (Fig. 9).
The SiO2 content of the on-land lava has an intermediate
composition range from 64. 9 to 65. 7 wt. %; that of theproximal pyroclastic materials of cone X has a range from
64.7 to 66.6 wt.%. The SiO2 content of pumice grains of
the An-ei pumice fall deposit has a rather narrow range
centered at approximately 65 wt.%. On the other hand, theSiO2 content of the drilling core samples has a wide range
from 63.3 to 66.6 wt.%. (Fig. 9).
4．Discussion
Regarding the An-ei eruption, the erupted volume on the68

Maya YASUI, Masaki TAKAHASHI, Jun SHIMADA, Daisuke MIKI and Kazuhiro ISHIHARA

Fig. 8. SiO2 content of lava lobes in the An-ei NE. Unitsand craters are the same as those in Fig. 10.

northeastern side is larger than that on the southern side, as
determined from the areas of the lava flows (Fig. 1). Sincemuch information is available from the An-ei NE, we
discuss the activity on the northeastern side predominantlyin the first half of this chapter. In the second half, we
compare the An-ei eruption to the Taisho eruption anddiscuss the implication of the proximal deposition of
pyroclastic materials.
4-1 Eruption sequence and eruption style of theAn-ei eruption on the northeastern side
4-1-1 Stratigraphy of eruption products on theAn-ei NEThe eruption order in the An-ei NE is discussed from thestratigraphy in this section. On the basis of the contact
relationship (Fig. 10), we deduced the stratigraphy of theAn-ei eruption products on the northeastern side (Fig.
11a). The lobes on the middle flank slope (lobes j〜o) areat the upper level of the succession, while those on thelower flank slope (e.g., lobes a〜i) are at the lower level(Fig. 11a). This indicates that the lobes on the middle
flank slope were formed in a later stage, while those on thelower flank were formed in an early stage.
Furthermore, the drilling core gives us important
information on the stratigraphy beneath the surface. As
described in section 3-1-3, an approximately 100-m-thickdeposit from the An-ei eruption was confirmed at thedrilling site (Fig. 7). The chemical composition of thedeposit changes vertically throughout the core (Fig. 7f).
This indicates that the whole deposit is not a single flow
unit of lava but a pile of several units of lava and/or
pyroclastic materials. The drilling site is in the area oflobe g (Fig. 3a). The thickness of lobe g is estimated to beapproximately 20 m from its topography. The chemicalcomposition of the upper 20 m of the drilling core is almost
constant (Fig. 7f). Therefore, the upper 20 m of thedrilling core corresponds to lobe g. Stratigraphically, coresamples at depths between 20 and 50 m may correspond to

Fig. 9. SiO2 vs. CaO diagram for the eruption products of
the An-ei eruption. The raw data are summarized inTable 1 (No.123-190) in Takahashi et al. (2011). See
Appendix C, D, F, and G in Takahashi et al. (2011) forthe localities. Drilling core samples and proximal
pyroclasts are of in the An-ei NE. For on-land lava,
samples are taken from the northeastern and southernside. Pumice fall deposits are taken from a locality
about 1. 8 km ESE from locality 1. See Fig. 3a for
locality 1.

the underlying cone Y. The core samples at depths
between 50 and 82 m correspond to lobe c because they
have similar compositional ranges to the lava of lobe c(Figs. 7f and 8). Core samples at depths between 82 and100 m have SiO2 contents of less than 65 wt.%, indicating
the presence of buried products (? in Fig. 11a) with a more
mafic composition than lobe c.
4-1-2 Proximal processes on the An-ei NE
The An-ei NE craters distribute along two linear lines.
Two distinct fissures are inferred; namely, the southernand northern fissures (Fig. 10) which we consider as the
major and minor fissures, respectively. The main fissure
is about 5 km long and the minor one is 1 km long. Thelong and narrow depressions D1 and D2 are also indicativeof the main fissure (Figs. 3a and 10).
Upper to middle flank slopes
In some old drawings (Photos 2 and 7 in Imura 1998b),
abundant pyroclasts falling from a vertical eruption
column into a proximal area in the initial Plinian phasewere shown. This is consistent with the existence of a
thick proximal pyroclastic deposit along the fissure at
localities 1 and 2. The existence of a thick, densely
welded part of pyroclastic materials (Fig. 4c) suggests a
continuous pyroclastic eruption. The stratification of theproximal deposit indicates intermittent eruptions and/or
temporal variations in eruptive intensity during thecontinuous pyroclastic eruption. We conclude that coPlinian proximal deposition played an important role in theformation of pyroclastic cones (hereafter referred to as the“co-Plinian fountain”). Cone X is considered to haveformed throughout the initial Plinian eruption. Its
elongated shape implies that the Plinian eruption occurredComparative Study of Proximal Eruptive Events in the Large-scale Eruptions of Sakurajima Volcano :An-ei Eruption vs. Taisho Eruption

69

Fig. 10. Distribution of individual units of lava, pyroclastic cones and craters in the An-ei NE. The contact relationshipbetween lava flows was determined using aerial photographs.

from the fissure on the upper to the middle flank slopes.
Imura (1998b) considered that the Plinian eruption started
from a point directly below the summit of Minamidake on
the southern side, and then the Plinian eruption on thenortheastern side followed immediately after. Therefore,
the upper half of the main fissure (Fig. 10) is thought to be
the eruptive center of the initial Plinian eruption. A
semicircular, large crater, crater 3 between D1 and D2
might be a point of the intense eruption on the fissure (Fig.
10).
Middle flank slope
Cracks develop on the smooth outer surface of cone X(labeled A in Fig. 3a). In particular, the development of
cracks is distinct on the northeastern edge of cone X (label
B in Fig. 3a) that covers the thick lava from Kitadake
Volcano (Fig. 12a). It is considered that these cracks
originated from the deformation of the inner, densely
welded part of the cone due to gravitational instability.
This is because the pyroclastic cone was formed on theflank slope of the volcano. The occurrence at the locality

1 that the fractured welded part hanging down (labeled C
in Fig. 4b) is consistent with the deformation of the cone.
Furthermore, circular arclike cracks that are concave inthe upward direction develop in lobe n. The upstream
region of lobe n corresponds to a shallow depression, D3,
and a folded texture develops in the downstream region.
These occurrences are similar to those observed in thecollapsed pyroclastic cone shown by Sumner (1998) andShimano and Koyaguchi (2001). It is concluded that thegravitational instability of cone X on the slope caused thepartial collapse of cone X to form D3.
Crater 5 is surrounded by a semicircular ridge (cone Z in
Fig. 10). Judging from its shape, it is a small-scale
pyroclastic cone that developed around crater 5. Lobe o,
one of the later flows in the An-ei NE (Fig. 11a), continues
to crater 5. These facts suggest that a cone-building
eruption occurred at crater 5 and that the cone wasdestroyed by the flow of lobe o. It is also possible tointerpret that a syn-eruptive cone collapse occurred togenerate lobe o.

70Maya YASUI, Masaki TAKAHASHI, Jun SHIMADA, Daisuke MIKI and Kazuhiro ISHIHARA

Fig. 11. Diagram showing stratigraphic relationships
between the units of lava flows and pyroclastic conesof the An-ei eruption. The tie lines between blocks are
stratigraphic relations based on aerial photos.
(a) Northeastern side.

Lower flank slope
The existence of multiple craters and parallel flows on
the lower flank slope indicates the activities on the fissure.
There is a tendency for a lava lobe to continue to a certain
crater (Figs. 3a and 10), indicating its source. For
example, the source of lobes g and i is inferred to be crater
12 (Fig. 10). The following are correlations of lobes tocraters; lobe m to crater 10, lobe o to crater 5, lobe l to
crater 11, lobe e to crater 8, and lobe f to crater 9. The
sources of lobes a, b, c, d, and j are not known. Lobes a
and b might have originated somewhere between craters 6
and 7. On the other hand, since crater 7 opens throughlobe g, it is considered to have developed after theemplacement of the flow.
For the drilling core samples, the groundmass
plagioclase is coarser in the central part of the drilling corethan in the upper and lower parts (Fig. 7e). This fact
implies that the drilling core samples correspond to asingle cooling unit. In other words, the accumulation of
the deposits at the drilling site on the lower flank is
considered to have been completed within a short period.
To discuss the origin of lava flows, Yasui et al. (2007)
focused on the petrographic features of the eruption
products including broken crystals, as reviewed in section
2-1. The core sample of lobe g has the nature of a welded
pyroclastic rock with a eutaxitic texture and abundant
broken crystals, meaning that lobe g is very likely to beclastogenic lava. Lava fragments with the chemicalcomposition of Kitadake Volcano appear at depths greaterthan 80 m. Since such fragments are unlikely to be

Fig. 11. Continued.
(b) Southern side.

entrained in coherent lava during the effusion, they were
probably trapped throughout the pyroclastic eruption.
Since the drilling site is nearly on the main fissure (Fig.
10), materials at the depths between 82 and 100 m may be
pyroclastic deposits (label ? in Fig. 11a). This suggeststhat there is a buried pyroclastic cone. The timing offormation of the buried cone might have overlapped withthat of the cone X (Fig. 11a).
4-1-3 Eruption sequence and eruption style in theAn-ei NEThe eruption sequence on the An-ei NE can be divided
into three stages mainly on the basis of the stratigraphy(Fig. 11a): Stage 1, syn-Plinian stage; Stage 2, postPlinian stage; and Stage 3, subsequent subaqueous eruption.
Stage 1: A vigorous Plinian eruption started at around 2
p.m. on November 8th and continued for up to 48 hours(Imura, 1998a). At the same time, co-Plinian fountaining
generated an intense fallout of pyroclasts in the proximal
area to form a large-scale pyroclastic cone (cone X) on theupper and middle flank slopes. Small-scale pyroclastic
flows were generated on the middle flank slope in thisstage from the occurrence of outcrop at locality 3 (Fig. 6).
The Plinian eruption ceased on the evening of November
10th (Imura, 1998a).
Stage 2: Stage 2 is divided into two substages on thebasis of the stratigraphy and the difference in the altitudeof the flow field of lava. In Stage 2-1, many lava lobes
were generated to form parallel flows on the lower flank(lobes a〜i). These flows were from chains of craters
along the fissures (Figs. 10 and 12a). As discussed above,
pyroclastic materials (cone Y and ? in Fig. 11a) are inter-

Comparative Study of Proximal Eruptive Events in the Large-scale Eruptions of Sakurajima Volcano :An-ei Eruption vs. Taisho Eruption

71

Fig. 12. Birdʼs-eye views showing the distribution of theAn-ei eruption products. To make these figures, thesoftware “Kashmir 3D” was used. The verticalexaggeration is about two times.
(a) Northeastern side seen from the northeast.

calated with lava lobes at the drilling site and the entire
deposition at the drilling site is thought to have occurred
within a short period. The eruption style in Stage 2-1 isthought to be a dominant lava flow associated with conebuilding fountaining from the fissures. According to Imura(1998a), the lava reached the NE coast on November 10th.
On the basis of the distribution, lobes a and c are thought
to have entered the ocean. Therefore, the lava lobes (i.e.,lobes d, g, and i) that overlie lobes a and c are considered
to have flowed down after November 10th.
The following Stage 2-2 is characterized by lava flows
on the middle flank slope (lobes j〜o) (Figs. 10 and 12a).
These flows are shorter than those on the lower flank.
They traveled less than 1 km and small mounds are there
on their surface (Fig. 3a). These features are similar to
those of rootless flows in Kilauea Volcano (Wolfe et al.,1988). Since these flows are on the steep slope of theterminal cliff of a thick lava from Kitadake Volcano (Fig.
12a), they are considered to be clastogenic flows derived
from cone X due to gravitational instability. As discussed
in 4-1-2, the partial collapse of cone X is evident from theexistence of D3. The inner welded part of the cone wassufficiently hot and fluidal to flow, suggesting that Stage 2
immediately followed after Stage 1. A small-scale conebuilding activity and a collapse also occurred at crater 5.

Fig. 12. Continued.
(b) Southern side seen from the southwest. D3 in theAn-ei NE is also seen.

Some explosions occurred at crater 7 after the emplacement of lobe g. Crater 6 also opened in the area of theassumed pyroclastic cone Y (Fig. 10). These craters arereminiscent of the activity of the Vulcanian eruptions that
occurred in Stage 2 of the Taisho eruption (Yasui et al.,2007). According to Kobayashi (2009), the explosive
eruptions continued on the flank craters until 1785 and it isdifficult to tell when the eruptive activity ended.
The existence and shape of lava q reveals that a lava
dome has appeared on the bottom of crater 3. The lava pfilled D2. Although there is no information on the timingof the extrusion of these lavas (q and p), they might haveappeared in the later stage.
Stage 3: Kobayashi (2009) concluded that an intrusionof a submarine cryptodome and subsequent phreatomagmatic eruptions on the northeastern offshore formed theAn-ei islets (Kobayashi, 2009). Lava appeared on the sea
surface on November 12th. There is no information on the
duration of Stage 2. The formation of the cryptodome might
have overlapped with onland activity in Stage 2. As reviewed in section 2-2, Takahashi et al. (2011) showed that

72

Maya YASUI, Masaki TAKAHASHI, Jun SHIMADA, Daisuke MIKI and Kazuhiro ISHIHARA

the subaqueous drilled lava samples have a different
chemical composition from the terrestrial deposits and concluded that they were derived from different magma systems.
Examining the chemical composition of the magma
erupted during Stages 1 and 2, we detected no systematic
temporal variation. The SiO2 content of the erupted
magma in Stage 1 is between 64. 5 and 66. 5 wt. % asdetermined from the compositions of the proximal pyroclasts on the upper flank slope (Fig. 9). The SiO2 content
of the erupted magma in Stage 2 is between 63.2 and 66.6
wt. % including the compositions of the drilling coresamples. Part of the temporal variation in the chemicalcomposition of the erupted magma in Stage 2-1 isindicated by the vertical change in the SiO2 content of thedrilling core samples throughout the entire depth. Fromthe bottom, the SiO2 content increased sharply fromapproximately 63.7 to 66.6 wt.% and then decreased to 63.
2 wt.% (Fig. 7f).
4-2 Eruption sequence and eruption style of theAn-ei eruption on the southern side
Stratigraphy of the eruption products on the southernside is shown in Fig. 11b. The following eruption sequenceis considered on the basis of the stratigraphy and topographic features described in section 3-2.
Stage 1: Plinian eruption started at the uppermost crater(I in Fig. 3b) at approximately 2 pm on November 8th andcontinued for about 20 hours (Imura, 1998a). Co-plinian
fountaining formed a pyroclastic cone, cone A, on the steep
slope and syn-eruptive clastogenic lava flowed down toform lava lobes 1 and 2. Cone A deformed downward
owing to the steepness of the slope resulting in cracks on
its surface. A partial collapse of the edge of cone A
possibly generated the upper part of lobe 2. Aramaki andKobayashi (1986) described the pumice avalanche layersintercalating in the pyroclastic deposits on the An-ei
southern side. There is a possibility of triggering such anavalanche from the cone built on a steep slope.
Stage 2: The eruption associated with an eruption cloudceased in the afternoon on November 9th (Imura, 1998a).
Lava lobe 3 was generated from crater II. Then, crater III
was active to form cone B. Since the surface of lobe 3 isflat, pyroclastic fall material from crater III is considered
to have paved the lava surface. Then, lobe 4 was
generated from crater IV. It was followed by the effusionof lava dome 5. A small-scale lobe, lobe 6, might havegenerated from crater II at the same period. Although theexact timing is unclear, small-scale explosions occurredfrom the chain of craters V, VI, and VII on the lava field
after emplacement of lobes 1, 2, and 3. There is no
information on how long the activity on the southern side
continued.
4-3 Similarities and differences between the An-ei
and Taisho eruptions
With the aim of comparing the past two large-scale
eruptions of Sakurajima Volcano, the similarities anddifferences between the An-ei and Taisho eruptions will beshown in this section. Features of the two eruptions are
summarized in Table 1. There are many similarities, especially in the initial phase (Stage 1). Durations of Stage 1
for the two eruptions are similar, i.e., less than two days.
Some old drawings of the An-ei eruption show abundant
pyroclasts falling from an ascending Plinian eruption
column in the proximal area (photos 2, 3, 5, and 7 in
Imura, 1998b). In the case of the Taisho eruption, theintense fallout of pyroclasts from the Plinian eruption
column also appears in photographs of the initial Plinian
eruption. The pyroclastic cones develop along theeruptive fissures (Fig. 1). Therefore, the co-Plinian, proximal deposition of pyroclastic materials characterizes
Stage 1 of these eruptions. There are differences in detailed
phenomena in Stage 1 (A〜E in Table 1). For the Taisho
W, cone growth, partial cone collapse, and generation of
the clastogenic lava occurred simultaneously with thePlinian eruption as reviewed in section 2-1. The cone
growth and deformation might have continued after thegeneration of clastogenic flows in the An-ei S. A smallscale cone collapse occurred in Stage 2-2 in the An-ei NE.
Small-scale pyroclastic flows occurred in the Taisho W
and the An-ei NE.
The Plinian eruption in Stage 1 was followed by thefissure eruption associated with lava flows on the lower
flank in Stage 2-1 in the An-ei NE. The small-scale conebuilding eruption at crater 5 in Stage 2-2 is characteristicof the An-ei NE eruption and was not observed in the
Taisho eruption. Stage 2 of the Taisho eruption ischaracterized by the dominance of lava flows, especiallyon the eastern side. In the same period, Vulcanian
explosions from the craters on the fissure occurred
repeatedly to generate a heavy ash fall in the Taisho
eruption (Yasui et al., 2007). In the An-ei eruption,
explosive activities like those in the Taisho eruption are
indicated by the existence of craters. According to
Kobayashi (2009), explosions have continued for longtime until 1799 from the flank craters. He mentioned that
it is difficult to determine when the activity ended.
The subsequent courses of the two eruptions wereconsiderably different. That is, an on-land gentle effusionof lava in the Taisho E was the main event in Stage 3 of the
Taisho eruption. On the other hand, on-land lava effusion
might have ceased in the An-ei eruption. Instead, submarine eruptions took place on the northeastern offshore toform the An-ei islets. Takahashi et al. (this issue)
concluded that the magma chamber subsystem of thepresent volcanic activity started with the An-ei subaqueous
eruption on the basis of their comparative study of thechemical compositions of historical lava flows of Sakurajima Volcano.
The magma composition of the Taisho eruption is more
mafic than that of the An-ei eruption. The SiO2 contents
in Stages 1 and 2 are higher than 61 wt.%, whereas that inComparative Study of Proximal Eruptive Events in the Large-scale Eruptions of Sakurajima Volcano :An-ei Eruption vs. Taisho Eruption
Table 1.

73

Summary and comparison of the An-ei and Taisho eruptions of Sakurajima Volcano.

*1: Vigorous Plinian eruption associated with various phenomena. A: Pumice fall, B: Formation of pyroclastic cone, C:
Collapse of pyroclastic cone, D: Clastogenic lava flow, E: Small-scale pyroclasitc flow *2: According to Kobayashi (2009),
small-scale eruptions from the flank craters continued for long period at least until 1785. *3: See Kobayashi (2009) for
detail. *4: The data cited from Takahashi et al. (2011).

Stage 3 is lower than 61 wt.% in the Taisho eruption (Fig.
12 in Yasui et al., 2007). On the other hand, there is no
systematic temporal variation in magma composition in
the An-ei eruption. Judging from the above results, there
may be no relationship between the timing of the change inchemical composition of the erupted magma and thetiming of the change in eruption style, at least for these two
eruptions.
4-4 Features of the activities on individual eruptive
fissures from the two eruptions
We compare the features of the activities on individual
eruptive fissures and discuss the factors generating thevariation.
The area of the pyroclastic cone formed in Stage 1 in theAn-ei NE is larger than that in the An-ei S (Fig. 1). The
deformed pyroclastic cone in the An-ei S is stuck on thesteep slope and clastogenic lava flows are distinct. According to Imura (1998a), the duration of the Plinian
eruption in the An-ei S was shorter than that in the An-ei
NE. The shorter duration of the Plinian eruption and thesteepness of the slope are considered to have prevented astable cone building in the An-ei S. In the Taisho W, thesyn-Plinian cone collapse and clastogenic flows were
dominant in Stage 1. On the other hand, most of the74

Maya YASUI, Masaki TAKAHASHI, Jun SHIMADA, Daisuke MIKI and Kazuhiro ISHIHARA

pyroclastic cones were stable in the An-ei NE except in thearea on the steep terminal cliff of lava from Kitadake
Volcano. This indicates that the intensity and deposition
rate of the co-Plinian fountain of the Taisho W were higher
than those in the An-ei NE. Therefore, the intensity andduration of the Plinian eruption and the gradient of theslope are thought to be clues to cone growth and thegeneration of clastogenic lava in these cases.
The craters on the upper slope are larger than the others
for both eruptions. In the case of the An-ei S, a semicircular crater (Crater I) just below the summit remains(Figs. 1 and 12b). Such craters are inferred to be centersof the initial vigorous Plinian eruption. The altitudes ofthese craters are different between the two eruptions.
They are higher than 600 m a.s.l. for the An-ei eruption,
but lower than 450 m a.s.l. for the Taisho eruption. Thismeans that the localities where the Plinian eruption started
in the An-ei eruption were close to the summit area of
Sakurajima Volcano compared with those in the Taisho
eruption. In the case of the An-ei NE, the initial Plinian
eruption from the upfissure and the following lavadominated eruption from the downfissure are probably
explained by the propagation of a radial dyke from acentral conduit system. This is interpreted to mean that theinitial pyroclastic explosive eruption on the upper flankbecame less explosive, corresponding to a downward dyke
propagation to generate lava flows on the lower flank. In
the Taisho E, a shorter duration of the Plinian eruption
resulted in a smaller-scale pyroclastic cone. On the other
hand, the effusion of lava in Stage 2 in the Taisho E waslarge in scale (Figs. 1 and 2c). There may a relationshipbetween the altitude of the initial Plinian eruption and thevolume of coherent lava. The altitude of crater of theinitial Plinian eruption may be reflected by the volatile
content of supplied magma which also controls the ratio of
the coherent lava that appears on the downfissure.
Relatively large-scale effusion of lava in the Taisho E
suggests that degassed magma has supplied to the downfissure in the Taisho E.
4-5 Implications of the proximal deposition of pyroclastic materials in the large-scale eruptions of
Sakurajima Volcano
It is revealed that there were large-scale proximal
depositions of pyroclastic materials due to the co-Plinian
fountaining during Stage 1 in the two eruptions of
Sakurajima Volcano. We discuss the implication of suchproximal depositions in this section.
Concerning the growth of the edifice of Sakurajima
Volcano, the following are common to the two eruptions.
For both eruptions, there are pyroclastic cones on the upper
to middle flanks along eruptive fissures. There was almost
no change in the topography in the summit area caused by
the large-scale eruption, but the flank slope was
extensively plastered and hardened by pyroclastic
materials. In other words, proximal deposition in a large-

scale eruption does not contribute to the growth of thesummit area, but to that of the flank slopes of Sakurajima
Volcano. On the other hand, a pyroclastic cone wasformed on the summit through a climactic Plinian stage inthe 1783 eruption of Asama Volcano (Yasui and Koyaguchi,
2004). In the case of the 1929 eruption of Hokkaido
Komagatake Volcano, it is reported that large amounts ofpyroclastic materials fell into the proximal area, burying
the pre-existing crater through the Plinian eruption (Katsui
et al., 1975). These cases suggest that the growth of a
volcanic edifice by co-Plinian fountaining is an important
aspect of some andesitic eruptions.
In future large-scale eruptions of Sakurajima Volcano,fissures could propagate downward to the cultivated land
and inhabited areas immediately after the onset of theeruption. The followings are expected to occur in areas
close to eruptive fissures. First, the intense fallout of
pyroclastic materials from the Plinian eruption column will
occur in Stage 1, setting buildings on fire. Small-scale
pyroclastic flows will also occur in this stage. Moreover,clastogenic lava will start to flow downward in Stage 1.
This means that lava flows will appear immediately after
the onset of the Plinian eruption. Second, repeated lava
flows from multiple craters will continue after Stage 1. In
the case of the Taisho eruption, the houses in the villages
close to the fissure were burnt within several hours fromthe onset of the eruption and several villages were buried
by lava within two days. In order to avoid serious
disasters due to the fallout of pyroclastic materials and lava
flows, a rapid evacuation is needed, especially for
Sakurajima Volcano.
5．Summary1) The eruption sequence of the An-ei and Taisho
eruptions was divided into several stages. For both largescale eruptions, the eruption styles were similar, at least in
the initial stage of up to two days, characterized by avigorous Plinian eruption with an intense proximal deposition of pyroclastic materials. Consequently, the formation of pyroclastic cones along the eruptive fissures andgeneration of clastogenic lava flows occurred.
2) After the Plinian eruption, lava continued to flowfrom multiple craters on eruptive fissures to form vast lava
fields for both eruptions. However, these two eruptions
took different courses afterwards. Thus, forecasting aneruption sequence in a future eruption will be particularly
difficult for a post-Plinian stage.
3) In the case of a large-scale eruption of Sakurajima
Volcano, an intense co-Plinian proximal deposition of
pyroclastic materials contributes to the considerable growthof the volcanic edifice. It occurs not on the summit area
but on the flank slopes along the fissure system.
Acknowledgements
We thank Dr. Tetsuo Kobayashi and Dr. Shigeo

Comparative Study of Proximal Eruptive Events in the Large-scale Eruptions of Sakurajima Volcano :An-ei Eruption vs. Taisho Eruption

Aramaki for discussions in the field. Drilling core samples
were analyzed in 2001 and 2002 by the interuniversity
research of Sakurajima Volcano Research Center, Disaster
Prevention Research Institute, Kyoto University. Constructive comments by the two anonymous reviewers and Dr.
Masao Ban are greatly appreciated.
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(Editorial handling Masao Ban)

桜島火山の大規模噴火における火口近傍プロセスの比較研究
─安永噴火と大正噴火─
安井真也・高橋正樹・島田純・味喜大介・石原和弘

桜島火山の歴史時代の大規模噴火である安永噴火（1779-1782 年）と大正噴火（1914-1915 年）の噴出物の岩相や層序，地形，噴火当時の記録を比較した．両噴火では山頂をはさんだ両側山腹で割れ目火口列が活動した．割れ目の推定の長さは大正噴火で約 2.3 km，安永噴火では 5 km に及ぶ．噴火開始後数 10 時間の大正
噴火と安永噴火の噴火様式は共通しており，プリニー式噴煙柱から火口近傍への大量の火砕物降下により斜面上に火砕丘を形成しながら火砕成溶岩をもたらした．引き続く数週間には両噴火とも溶岩流出が繰返されて溶岩原が形成された．その後は，大正噴火が陸上での溶岩流出を主としたのに対し，安永噴火では北東沖で海底噴火が起きて安永諸島を形成した点で大きく異なる．両噴火とも噴火初期に割れ目火口近傍へ著しい火砕物降下があることが特徴的である．これは火山体形成の観点からは，両噴火では山頂部の地形変化はほとんどないが，山腹斜面が成長したことを意味する．また桜島の大規模噴火の減災という観点では，居住地域近くまで到達しうる割れ目火口の活動への迅速な初期対応の重要性を示している．