SVC47-01

Room:303

Time:May 26 09:00-09:15

Precursory eruption for a large ignimbrite eruption; example of Osumi pumice fall deposit
from Aira caldera
GESHI, Nobuo1∗ ; MIYABUCHI, Yasuo2
1

Inst Earthquake Volcano Geology, Geological Survey of Japan, AIST, 2 Kumamoto University

Caldera collapse and emission of voluminous ignimbrite follow the decompression process of magma chamber during precursory eruption. Rapid withdrawal of massive magma from a chamber causes decompression of magma chamber and results
collapse of the roof of magma chamber. A massive eruption from Aira caldera at 29 calBP produced a massive ignimbrite
which covered the southern half of Kyushu Island, and formed the structure of Aira caldera. A voluminous pumice fall deposit(Osumi pumice fall deposit, ˜100 cubic kilometer in bulk volume; Kobayashi et al. 1983) was erupted prior to the emission of
Ito ignimbrite and caldera collapse. The eruption of Osumi pumice fall caused the decompression of the magma chamber toinduce the collapse. We are analyzing the sequential change of pumice fall deposit to understand the mechanism of large scale
eruption associating caldera formation and voluminous ignimbrite. Osumi pumice fall deposit shows clear upward coarsening
as pointed by Kobayashi et al. 1983, and is directly covered by the ignimbrite. An outcrop at ˜15km from the vent along the
distribution axis shows ˜10 m thick of the deposit. The lowest part within ˜2 m from the base is finer than the overlying unit, andthin finer bed exist at ˜4.5 m from the base. Other part is homogeneous and no clear fall unit is recognized. Maximum size of
pumice is 3 cm at the base and increases up to 8 cm at the top of the deposit. Total amount of lithic fragments is ˜5 % in volume.
The lithic fragments consist of shale and sandstone derived from the basement (Shimanto Group) and Quaternary volcanic rocks(rhyolite-dacite lava, and aidesite lava and scoria). The ratio of basement fragments against the surface volcanic rock decreases
from the base to the top. These observations suggests the model that the enlargement of the conduit caused the increase of theeruption rate and results the rapid decompression of the magma chamber to induce the collapse of the magma chamber roof.
Keywords: Ignimbrite eruption, caldera-forming eruption, Aira caldera, Eruption

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SVC47-02

Room:303

Time:May 26 09:15-09:30

Mixing, end-member components and origin of felsic and mafic magmas erupted by Airacaldera-forming eruption
HINATA, Hironobu1∗ ; TATSUMI, Yoshiyuki1 ; SUZUKI-KAMATA, Keiko1 ; TANI, Kenichiro2 ;KIMURA, Jun-Ichi3 ; CHANG, Qing3
1

Department of Earth and Planetary Sciences, Graduate School of Science, Kobe University, 2 Department of Geology andPaleontology, National Museum of Nature and Science, 3 Department of Solid Earth Geochemistry, Japan Agency for MarineEarth Scienc and Technology
Aira caldera, located in southern Kyushu, is a collapse caldera with ˜20 km in diameter and was formed by a caldera-forming
eruption occurred at ˜29 ky ago. The Aira eruption effused voluminous white pumices basically formed from homogeneous
felsic magma. They show the following disequilibrium petrographic features: (1) dark pumice (Arakawa et al., 1998) andbanded pumice erupted along with the white pumice, and (2) cores of the plagioclase phenocrysts possess wide compositional
range in both An content and Sr isotope composition. These suggest that the Aira eruption was caused not only by the felsicmagma but also by the mafic magma, generated from different sources.
The An content of the plagioclase cores ranges from An33 to An88 . These phenocrysts can be divided into two types suchas high-An (type-A: An >70), low-An (type-B: An <60) based on An contents of their cores and rims. Sr isotope ratios of thetype-A and B phenocrysts coincide with those of the dark pumice and the white pumice, respectively. These values are distinct
from Shirahama basalt which is assumed to be derived from upper mantle and basement rocks such as the Shimanto sedimentaryrocks and the Takakumayama granite. Therefore, the mafic and felsic magmas which crystallized type-A and type-B plagioclase
are not simply derived from upper mantle and basement rocks, respectively.
U-Pb dating of the zircon crystals in the white pumice shows concordant ages ranging from 249 to 2517 Ma. The age range
is identical to those of the zircons from the basement rocks (Shimanto sedimentary rocks). However, no older zircons in white
pumice show overgrowth structure. It is thus likely that older zircon is not source material but one of end-member components.
End-member components for magmas were estimated using element partitioning data (Bindeman et al., 1998; Bindeman andDavis, 2000). The mafic magma (SiO2 = 59 wt.%, Sr = 391, 87 Sr/86 Sr = 0.7066) which crystallized the type-A plagioclase
can be derived from mixing between the basement rocks (Shimanto sedimentary rocks) and the basaltic magma (Shirahama
basalt) derived upper-mantle. The felsic magma (SiO2 = 75 wt.%, Sr = 103, 87 Sr/86 Sr = 0.7060) which crystallized the type-B
plagioclase was derived from the middle-lower crustal rocks based on the Sr isotope ratio. The composition of the dark pumicein the Ito ignimbrite can be explained by mixing between the mafic and the felsic magmas. Therefore, it is possible to proposethat a cryptic magma should contribute to felsic magma production in association with Aira eruption.

Keywords: Aira caldera, caldera-forming eruption, magma mixing, end-member components, zircon, plagioclase

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SVC47-03

Room:303

Time:May 26 09:30-09:45

Cooling process of mafic magma by texture analysis of mafic inclusions in An-ei lava,
Sakurajima volcano
YAMASHITA, Shunsuke1∗ ; TORAMARU, Atsushi2
1

Department of Earth and Planetary Sciences, Graduate School of Sciences, 33 Kyushu University, 2 Department of Earth andPlanetary Sciences, Faculty of Sciences, 33 Kyushu University
The origin of mafic inclusions is interpreted as relics of mafic end-member magma associated with magma mixing. So, themafic inclusions record the important information about mafic end-member magma and the mixing processes. Yanagi et al.
(1991) suggested binary magma mixing process in the historical eruptions of Sakurajima volcano, according to the fact thatplagioclase phenocrysts show a bimodal compositional distribution with two distinctive peaks at about An58 and An85, andreported the presence of mafic inclusions in An-ei lava of 1779 eruptions. Therefore, in order to obtain insights into the behaviorof mafic inclusion (especially, magma cooling process) during the magma mixing processes in Sakurajima volcano, we conduct
sampling, petrographic description and texture analyses of mafic inclusion in An-ei lava.
Most of mafic inclusions in the An-ei lava are elliptic in shape, the length of the major axis is about 10-20cm. The boundary
to the host An-ei lava is sharp. The mafic inclusions show porphyritic texture, the phenocrysts are mainly plagioclase, orthopyroxene, clinopyroxene, oxide and olivine in minor amounts. The microlites in mafic inclusions are plagioclase, orthopyroxene,
clinopyroxene and oxide. The host An-ei lava also show porphyritic texture, the phenocrysts are mainly plagioclase, orthopyroxene, clinopyroxene and oxide, but not olivine. The microlites in host An-ei lava are mainly plagioclase and orthopyroxene.
Two types of plagioclase phenocrysts are found in mafic inclusions and host An-ei lava: (1) honeycomb plagioclase phenocrysts
with large inclusions in heterogeneous mozaic cores with An75-90 and An55-70; and (2) clear plagioclase phenocrysts without
any inclusions in homogeneous core from grain by grain, and this plagioclase phenocrysts show an unimodal compositional
distribution with a major single peak at about An85, and small fraction of about An60, whereas those in host An-ei lava show abimodal compositional distribution with two similar peaks at about An60 and An85 plagioclase. The length of plagioclase andpyroxene microlites in mafic inclusions are ∼300µm, whereas those in the host An-ei lava are ∼50µm.
We conduct microlite number density analyses in order to quantitatively estimate the cooling rate by using plagioclase microlite number density(MND)-cooling rate meter. As a result of application to measured data, the cooling rate (dT/dt) is calculated
as 0.19 to 4.18×10−4 K/s. We estimate initial and final temperatures during microlite crystallization with core and rim compositions for the plagioclase- and alkali feldspar-liquid thermobarometer. By assuming a constant cooling rate, we calculate thecrystallization-time-scale of microlites in mafic inclusions as several days to dozen days. In addition, as a result of crystal size
distribution (CSD) analysis, the typical CSD plot of plagioclase microlites in mafic inclusion shows a log-linear trend. This result
suggests that the annealing process is not effective and the time interval from the termination of microlite crystallization to theeruption is negligibly short. Thus, we conclude that the time scale from initial microlite crystallization in mafic inclusion to theeruption is about several days to dozen days.

Keywords: Sakurajima volcano, mafic inclusion, texture analysis, microlite number density, magma mixing process, magma
cooling process

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SVC47-04

Room:303

Time:May 26 09:45-10:00

Eruptive history of Koshikidake Volcano of the Kirishima volcanic group - A study on
growing stratovolcanoesTAJIMA, Yasuhisa1∗
1

Nippon Koei Co., Ltd.

The Kirishima volcano group has over 20 craters within a 25 km WNW-ESE and 15 km NNE-SSW expanse. Volcanic activity
in the northwest area of this group has been reported for the recent 30,000 years by Imura (1992), Imura and Kobayashi (2001),
and Tajima et al. (2014). We studied the Koshikidake Volcano, which comprises 11 tephra layers (Tajima and Kobayashi,2011), of the Kirishima volcano group. These tephra layers are named Koshikidake-Shiratorishimoyu 1 to 11 tephra (Ks-Ss1
to Ks-Ss11) in this study because their type locality was around the Shiratori-Shimoyu Onsen. The first-stage five tephra layers
indicate occurrence of small to medium vulcanian and scoria fall eruptions. The largest scoria fall (Ks-Ss6) among all Ks-Ss
tephra was recorded in the sixth eruption event, which produced over 2 km3 of lava flow to the northern part of Koshikidake
Volcano. Subsequently, Koshikidake volcano erupted again, indicating repeated vulcanian and scoria fall eruptions. Peat was
found in Ks-Ss7a and Ks-Ss7b, and lake deposits in Ks-Ss7b and Ks-Ss8 tephra at Jogasaki. Ko-Kakuto lake in the Kakuto
Caldera had been pounded at the Ito-pyroclastic flow eruption from the Aira Caldera (Aramaki, 1968). This study shows that
Ko-Kakuto lake existed before Ks-Ss5 tephra, but the lake disappeared after Ks-Ss9 or 10 at Jogasaki. A peat layer deposited in
Ks-Ss7a and Ks-Ss7b and lahars deposited above Ks-Ss8, indicate shallow water conditions at the Jogasaki location.
Koshikidake Volcano has ejected the greatest volume of products in the Kirishima volcano group, and its activity has been
divided into the first small or medium eruption, second climax eruption with huge lava and scoria fall, and third small to large
eruption stages, similar to those of the developing Takachihonomine Volcano.

Keywords: Kirishima volcanic group, Koshikidake volcano, strato volcanoe, lava, Ko-Kakuto lake

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SVC47-05

Room:303

Time:May 26 10:00-10:15

The genesis of the Minami-Shimabara basalts erupted at the pre-stratovolcao stage of
Unzen
MASHIMA, Hidehisa1∗
1Center for Obsidian and Lithic Studies, Meiji University

Hydrous components derived from the subducting slab, such as aqueous fluids and hydrous melts, are generally believedto play an essential role in subduction zone magmatism. From the viewpoint of this petrologic concept, the genesis of the
Unzen magmatism in west Kyushu, southwest Japan is an enigma. The Wadachi-Beniof Zone beneath Kyushu indicate thatthe subducted Philippe Sea Plate does not extend to Unzen, which indicates that petrologic models emphasizing the role of slabderived hydrous components cannot explain the genesis of the Unzen magmatism. In other words, the Unzen magmatism gives us
new insights into our understanding of the subduction zone magmatism. The petrogenesis of Unzen, however, has not been wellunderstood yet, since the modern Unzen volcanism is dominated by eruptions of dacite and is devoid of primitive basalt lavas. It
is, therefore, impossible to constrain physicochemical conditions of primitive magma genesis using petrologic observations forpresent magmatic products there. The Minami-shimabara basalts (MSBs) distributed at the southern foot of Unzen erupted at the
pre-stratovolcao stage from 4.6 to 1.0 Ma. The genesis of the MSBs would give us some insights into our understanding of the
Unzen magmatism.
The MSBs do not show meaningful correlations on the major element oxide vs. MgO diagrams, indicating that magmatic
processes the MSBs experienced were complicated. The behaviors of compatible elements such as Ni and Cr, however, indicatethat magmatic processes in the mantle would have played essential role in the compositional features of the MSBs. The MgFe-Ni compositions indicate that the MSBs could have been in equilibrium with Fe-rich mantle olivines with Fo = 80 -87. On
the normative olivine-quartz-Jd+Ca diagram, the MSB are plotted parallel to the adiabat of melting anhydrous peridotite. Thesefeatures indicate that multi-stage partial melting at 1.5-0.5GPa would essentially have formed compositional variations of the
MSBs. The low pressure where the primitive melts were last in equilibrium with the source mantle is consistent with seismic
observations of Unzen suggesting that crustal thinning occurs there.
The normative compositions also indicate that the source mantle would have had relatively low temperature (Tp ≈ 1300 ℃).
The estimated low potential temperature indicates the MSB magmatism would have caused by a passible upwelling induced by
the subsidence of the Shimabara Basin. Forcal mechanisms and GPS displacements indicate that dextral transtensional strain of
the Amakusa-nada Graven controls these geologic phenomena.

Keywords: the Minami-Shimabara basalts, multi stage partial melting, tectonics, dextral

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SVC47-06

Room:303

Time:May 26 10:15-10:30

Zonal structure of Aso-4 magma reservoir as estimated from compositions of plagioclase
and melt inclusions
YAMASAKI, Hideto1 ; HASENAKA, Toshiaki1∗ ; YASUDA, Atsushi2
1

Graduate School of Science & Technology, Kumamoto University, 2 Earthquake Research Institute, The University of Tokyo

We measured compositions of plagioclase and melt inclusions of four flow units of Aso-4 pyroclastic flow. They are Oyatsu
pumice flow, Koei ash flow, Yame pumice flow, and Benri scoria flow with pumice and banded pumice, as defined by Watanabe(1978). They represent the first cycle of Aso-4 eruption sequence.
Plagioclase in pumice shows compositional variation changing from one peak (Koei) to two peaks (Oyatsu and Yame), thenback to one peak (Benri pumice). Range of compositional variation becomes wider toward later stage, with the widest peakfound in Benri pumice and scoria. Median value of the larger peak is An=35 (Koei), An=45 (Oyatsu, Yame and Benri pumice),and An=90 (Benri scoria). All the samples contain small amount of An55 plagioclase. Difference between core and rim was notfound among analyzed samples.
Plagioclase in pumice shows compositional variation changing from one peak (Koei) to two peaks (Oyatsu and Yame), thenback to one peak (Benri pumice). Range of compositional variation becomes wider toward later stage, with the widest peakfound in Benri pumice and scoria. Median value of the larger peak is An=35 (Koei), An=45 (Oyatsu, Yame and Benri pumice),and An=90 (Benri scoria). All the samples contain small amount of An55 plagioclase. Difference between core and rim was notfound among analyzed samples.
Bulk composition of pumice is SiO2=68˜70 wt.% for Yame pumice, SiO2=67˜70 wt.% for Oyatsu pumice, and SiO2=66˜67wt.% for Benri pumice. Distinct composition is found between melt inclusions hosted by plagioclase and orthopyroxene between
Oyatsu and Koei pumice. Koei melt inclusions show composition in a narrow range of SiO2=73˜74%, whereas Oyatsu pumiceshow that of SiO2=71˜74%. Shiihara (2014) reported distinct composition of volcanic glass among tephra of Aso-4A in the eastarea from Aso caldera. She named them as Group-1 glass (SiO2=73-74%) and Group-2 glass (SiO2=71-72.5%), and argued thatGroup-1 correspond to glass of Koei ash flow deposit, and Group-2, that of Yame pumice flow deposit. Because the compositionof our Koei and Oyatsu melt inclusions and those of Shiihara’s (2014) Koei and Yame glass are nearly the same, we concluded
that Oyatsu and Yame glass represent the same melt. Thus Oyatsu pumice flow that flowed to the west of the caldera and Yamepumice flow that flowed to east and northwest possiblly originated from the same or similar source in the magma supply system.
Water content of Koei melt inclusions is more than 4 wt.%, and that of Oyatsu melt inclusions is 2-4 wt.%. Chlorine content
of Koei melt inclusions is higher than that of Oyatsu, whereas, sulfur content of the former is lower than that of the latter.
In summary, we estimate that Koei pumice flow represent one magmatic melt, and Oyatsu and Yame pumice flow represent
two magmatic melt. Plagioclase of An55 composition represent mafic magma in equilibrium with this plagioclase. This melt
was not found as melt inclusions, however, is a possible end member of mixing event prior to the huge eruption. Benri scoria
and An90 plagioclase, the final product of the first cycle Aso-4 activity, represent the most mafic magma of the magma supply
system. We propose an existence of zoned stratified magma reservoir for Aso-4 magma system, from which magma was mixed,
and was evacuated from the upper zones.
Keywords: Aso-4 pyroclastic flow, magma reservoir, melt inclusion, plagioclase

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SVC47-07

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Time:May 26 11:00-11:15

Evolution of magmatic plumbing system and tectonics of Fuji and adjacent volcanoes
since 0.4Ma.
TAKAHASHI, Masaki1∗ ; UKAWA, Motoo1
1

College of Humanities and Sciences, Nihon University

The evolution of magmatic plumbing system of Fuji and adjacent volcanoes, the Ashitaka, Hakone and Izu-Tobu, since 0.4Ma,
has been controlled by the collisional tectonics in the northern tip of the Philippine Sea plate. The evolution of magmatic plumbing system of Fuji and adjacent volcanoes comprises the Stage-1 to Stage-3. In the Stage-1 (0.40 to 0.27Ma), the Ashitaka,
Pre-Komitake, Hakone and Amagi basaltic to andesitic stratovolcanoes were active. The Philippine Sea plate, where the Hakone
and Amagi andesitic stratovolcanoes are located, subducted along the Suruga trough and its northern extension beneath the Tanzawa block on which the Ashitaka and Pre-Komitake basaltic stratovolcanoes are situated. In the Stage-2 (0.27 to 0.13Ma), theAshitaka, Pre-Komitake and Hakone volcanoes continued their volcanic activities. The Hakone volcano in this stage was characterized by the voluminous felsic pyroclastic eruptions, related calderas, and andesitic to felsic independent monogenetic volcanogroup; the NNW-SSE trending graben was probably formed in the Hakone volcano under the ENE-WSW extensional tectonics.
The tensional strain caused by the westward subduction of Philippine Sea plate was probably released by the extension of thegraben of Hakone, because the subduction of the Philippine Sea plate along the Kannawa fault was stopped and it completely
stacked to the Tanzawa block. In the Stage-3 (0.13Ma to present), the volcanism of Komitake, Fuji, central cone of Hakone,
and Izu-Tobu monogenetic volcano group were active. The amalgamation of Philippine Sea plate with Tanzawa block along the
Kannawa fault was resulted in the formation of the Tanna-Hirayama left lateral strike-slip fault. The central cone of Hakone
was developed in the pull-apart portion of the Tanna-Hirayama strike slip fault. The northward movement of the eastern blockof the Tanna-Hirayama fault caused the extensional tectonics in the Amagi volcano, bringing the volcanic activity of Izu-Tobu
monogenetic volcano group. The strain caused by the subduction of Philippine Sea plate along the Suruga trough gave rise to the
opening of the deep fracture in the Philippine Sea slab beneath the Fuji volcano, which is probably the cause of the extraordinary
voluminous eruption of basaltic magma of Fuji volcano.
Keywords: Fuji volcano, magmatic plumbing system, tectonics

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SVC47-08

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Time:May 26 11:15-11:30

Multistage magma mixing determined by phenocryst composition and zoning of the Sessho
lava, Kusatsu-shirane Volcano
OKAMOTO, Naoya1 ; OSHIO, Kazuki1 ; UEKI, Kenta2∗ ; INUI, Mutsuko1 ; NOGAMI, Kenji3
1

Kokushikan University, 2 Japan Agency for Marine-Earth Science and Technology, 3 Tokyo Institute of Technology

We present a detailed study of the texture and chemical zoning of phenocrysts in the Sessho lava, andesite lava flow from
Kusatsu-shirane volcano, Central Japan, in order to reconstruct cooling and crystallization processes of andesite magma. The
Sessho lava is estimated to have been erupted 5 ka from the Moto-shirane cone (Yoshimoto et al., 2013) and exhibits andesitic
composition with SiO2 content of 60˜63 wt. % (e.g., Ueki and Terada, 2012). We sampled 5 different samples to cover the wholearea of the Sessho lava, and conducted textural and chemical analysis of the samples. Phenocryst assemblage of the Sessho lava
is plagioclase, clinopyroxene, orthopyroxene, magnetite and rare olivine with glassy groundmass.
Disequilibrium olivine (Fo=˜80) with reaction rim suggests the mixing between basaltic and evolved magmas during evolutionof the Sessho lava. Orthopyroxene-magnetite symplectite, which is estimated to be formed by olivine breakdown by oxidization(e.g., Goode, 1974), coexists with the olivine phenocryst.
Pyroxene thermometry (Lindsley, 1983) based on rim compositions of opx and cpx phenocrysts suggests the mixing between
high temperature (1000 degree) and low temperature (700 degree) magmas. The rim compositions of higher temperature opx
show higher Al content and rim compositions of the lower temperature opx and core compositions of opx show lower Al content,suggesting these pyroxenes have been derived from different magmas both in terms of composition and temperature. The hightemperature pair and the low temperature pair coexist in some samples.
We observed four different types of plagioclase phenocrysts in a single rock sample, in terms of zoning profiles, internal textures and external shape. Type 1 plagioclase exhibits euhedral shape, clear and homogeneous core with An content of 55˜65.
Type 2 plagioclase exhibits euhedral shape with oscillatory zoned core between An content of 50˜80. Type 3 plagioclase exhibits
rounded shape, rough interface and dusty core with An content of 80˜90. Type 4 plagioclase exhibits clear core with An# of 55˜80
surrounded by dusty mantle zone with An# of 80˜90, and rough interface. All types of plagioclases have thin (˜50µm) rim with
An content of 60˜80. This rim may represent a crystallization during upwards migration, because plagioclase microphenocrystin groundmass exhibits similar An content with the rim. Dusty core of type 3 and dusty mantle of type 4 show higher MgO
and FeO contents than the clear and oscillatory parts and type 1 and type 2 plagioclases, suggesting the dusty core and mantle
crystallized in higher FeO and MgO content magmas. Type 1 and type 2 plagioclases are larger (˜1.5mm) than the type 3 and
4 (˜1mm). Crystal size distribution analysis of plagioclase phenocryst suggests that type 1 and 2, and type 3 and 4 crystallized
in different physical conditions. Clear plagioclase (type 1) may represent equilibrium crystallization process in relatively low
temperature evolved magma, whereas dusty plagioclase (Type 3) may represent crystallization in relatively mafic and high temperature magma. Oscillatory zoning (Type 2) and dusty mantle (Type 4) may represent interaction between the high temperatureand low temperature magmas.
Based on the above observations, we propose multi-stage magma mixing and crystallization processes during the evolutionof andesite magma of the Kusatsu-shirane volcano; high temperature basaltic magma with olivine, and calcic plagioclase wasrecharged periodically in a crystallized lower temperature silicic magma chamber, and inhomogeneous mushy magma chamber
was formed.

Keywords: lava flow, two pyroxene geothermometry, crystal size distribution, andesite, eruption, active volcano

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SVC47-09

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Time:May 26 11:30-11:45

Low tempter Pyroclastic flow (pyroclastic density current) from phreatic eruption
OIKAWA, Teruki1∗
1

GSJ/AISTIn this study, is carried out restoration of Yakedake Taishoike eruption from newly discovered record and old photographs. The
6 June 1915 Yakedake, active volcano in the central Japan, eruption produced “cold” pyroclastic flow. The pyroclastic flow was
spread over an area of ca.1 km2 . The eruption products are clay rich tephra and lacking any juvenile component, there shows
the characteristics of phreatic eruption. At the same time with the eruption, the lahar flowed in the gullies. The lahars deposits
dammed up the Azusa River in the foot of Yakedake, and made the dam lake of length 1.9km. The dam lake is named Taishoike.
Thickness of the eruption tephra is significantly thicker at near the crater; but significantly thin at distal area of the crater. Itsfeature is also similar to the Ontakesan 2014 eruption that low-temperature pyroclastic flows occurred. Therefore, significantlythicker phreatic eruption tephra at near the crater, there is a possibility that the low-temperature pyroclastic flow occurred during
the eruption.
Since the 19th century, steam eruption that low temperature pyroclastic flow occurs in Japan, pyroclastic flow occurs in theinitial. From these eruptions sequence, it is suggested that the plume cannot obtained the buoyancy, because contains a large
amount of low-temperature rock fragments in ejecta by new crater formation.

Keywords: Yakedake, phreatic eruption, Taishoike, Low tempter pyroclastic flow, pyroclastic density current, 1915

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SVC47-10

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Time:May 26 11:45-12:00

Sedimentary facies changes of lahar deposits in distal area, Chokai volcano, NE JapanMINAMI, Yusuke1∗ ; OHBA, Tsukasa2 ; HAYASHI, Shintaro3 ; KATAOKA, Kyoko S.4
1

Graduate School of Engineering and Resource Science, Akita University, 2 Faculty of International Resource Science, Akita
University, 3 Faculty of Education and Human Studies, Akita University, 4 Research Institute for Natural Hazards and Disaster
Recovery, Niigata University
Chokai volcano is an andesitic stratovolcano in northeast Japan. The sector collapse of the volcano occurred about 2,500
years ago. The Kisakata debris avalanche deposit was formed by the collapse to the northern foot of Chokai volcano (Ohsawa
et al. , 1982). The post-collapse fan (partly volcaniclastic apron) deposits, largely distributed in the northern foot of the volcano,overlies the Kisakata debris avalanche deposit. From geological survey in the proximal area, Minami et al. (2015) reported
that the post-collapse fan deposits accumulated by a series of debris flows and hyperconcentrated flows, and then concluded
that the deposits are originated from several lahar events. However, change in sedimentary facies in the distal area was notwell studied. Furthermore the depositional processes changes between the proximal and distal areas of these lahar deposits
have not been well understood. This study aims to understand transition of depositional processes with distance, and relationshipbetween geomorphology formed by these lahar deposits and their depositional processes. The volcanic fan can be topographicallysubdivided into four areas; the steeply-sloping area, the moderately-sloping area, the gently-sloping area, and the very-gentlysloping area. We trenched and cored (by handy geoslicer; Takada et al. , 2002) in the distal of volcanic fan (gently sloping area,and very gently sloping area) at a total of 12 sites. Each set of trench and core by the depth of one to two meters from the surface
were observed. In the distal area, the lahar deposits are composed of debris flow, hyperconcentrated flow and streamflow facies.
These flows are mostly originated from lahar events. The facies variation with distance implies that lahars flowed down as debris
flows in proximal areas. Then, they transformed into hyperconcentrated and stream flows, although some reached to the distal
area as debris flow. Some lahar reached to the coastline that is 20 km distant from the volcanic edifice as stream flow, where
highly populated towns are distributed.
Keywords: Lahar, Chokai volcano

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SVC47-11

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Time:May 26 12:00-12:15

Relationship between eruptive style and fragmentation derived from the all grain size
analysis for juvenile fragments
HIROI, Yoshimi1∗ ; MIYAMOTO, Tsuyoshi1
1

CNEAS, Tohoku Univ.

1. Introduction
Phreatoplinian eruption is one of the phreatomagmatic eruption that occurs by the contact of vesiculated and fragmented felsicmagma with external water. This eruption produces extremely fine-grained ash particles (Self and Sparks,1978), it is typicalcharacteristics. On the other, Hayakawa(1985) makes a point that plinian eruption, the magmatic eruption, produces fine-grained
ash same as phreatoplinian eruption, though a large quantity of fine particles have been lost.
Although, there are many of all grain size analysis with a number of eruption, it does not come to a conclusion (i.e. Walker,1980;1981).
However, these all grain size analysis intendes all components of eruptive products, containing accidental lithic fragments. So
there are some inaccuracy to degree of fracturing for juvenile fragments.
This study compare the degree of fracturing between magmatic eruption and phreatomagmatic eruption by only juvenile fragments.
2. Candidate and Method
Heian eruption, the latest silicic volcanism at Towada volcano, began with magmatic eruption, thereafter repeated magmaticand phreatomagmatic eruptions alternately within a short period, approximately a half days (Hiroi and Miyamoto, 2010). The
products mainly consist of pumice fragments. Because there are very few blocky glass shards, magma has already vesiculated
and fragmented before contacting external water.
This study analyzed the unit OYU-1 and OYU-2b in Heian eruptive sequence. OYU-1 is the magmatic plinian pumice fall
deposit. It contains many accidental lithic fragments. Lithofacies such as grainsize and component are homogeneous, though
eruptive rate is estimated to constant. Eruptive volume is about 0.21km3 . OYU-2b is the phreatomagmatic base-surge deposit,
proceeded from OYU-1 almost continuous. It contains many of fine-grained ash, and eruptive volume is about 0.27km3 .
This study based on detail field surveys. We made isopach map of OYU-1 and distribution map of OYU-2b. Samples are
sieved to each grain size and separated to juvenile fragments, accidental lithic fragments, and free crystals. We made isograde
maps for juvenile fragments and free cryastals about each grain size, and calculated the eruptive weight about them. Fine grained
ash lost by the diffused process in air are estimated by crystal method (Walker,1980), and we got all grain size distribution only
juvenile fragments.
3. Result and Discussion
The lost fine grained ash are six times as many as existent coarse fragments left in OYU-1 deposits and twice in OYU-2b,
and the amount of ash finer than 1mm in total eruptive weight is about 89% both of OYU-1 and OYU-2b. It shows that there islittle difference between magmatic and phreatomagmatic eruption for the amount of produced fine grained ash. Yamamoto(1994)
pointed out that the large volume production of fine grained ash in phreatoplinian eruption have to be deny, and this result supports
this opinion. And Hayakawa(1985) pointed out that the fine grained ash are predisposed to deposit by aggregation of external
water in phreatoplinian eruption, and this result is consistant with this opinion. The eruption of Mt. St. Helens in 1980 occurred
plinian eruption in rainfall, and the deposit is rich in fine grained ash as phreatoplinian eruption (Carey and Sigurdsson,1982).
This grain size feature is the very important instance to confirm these previous indications and this study.
Keywords: magmatic eruption, phreatomagmatic eruption, extent of fragmentation, all grain size analysis

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Try to draw the volcanic eruptions and earthquake activity in the same figure around Japan
Part 1: in and around JapanISHIKAWA, Yuzo1∗
1

Geological Survey of Japan, AISTThe relation between the volcanic eruption and the occurrence of large earthquakes were well known. It was the famous
eruption of Mt. Fuji in 1707 in Japan. It was followed 49 days after the occurrence of M9 Hoei earthquake. The 2004 Sumatra
M9 earthquake followed some eruptions, too. But 2011 off Tohoku, Japan M9 earthquake did not follow eruptions, yet. The
seismicity was commonly analyzed in time and space, using several database, for example ISC and PDE catalogs. But the dataof volcanic eruptions were not popular and needed to analyze the relation.
The data of eruptions in and around Japan was inputted in order to search the relation between eruptions of volcanoes andoccurrence of large earthquakes. The database of the Global Volcanism Program by Smithsonian Institute (http://volcano.si.edu/)
was adopted. The histories of volcanoes of Onkakesan, Asosan, Kuchinoerabujima, Nishinoshima, Izu-Torishima, Kikai, loto,
Kirishimayama, Miyakejima, Fukutoku-Oka-no-Ba, Asamayama, Akan, Suwanosejima, Tokachidake, Kita-loto, HokkaidoKomagatake, Toya, Niigata-Yakeyama, Akita-Yakeyama, Adatarayama, Unzendake, Minami-Hiyoshi, Kujusan, Yakedake, IzuOshima, Izu-Tobu, Kusatsu-Shiranesan, Myojinsho, Kaitoku Seamount, Fukujin, Shikotsu, Nikko, Azumayama, Kasuga, IzuTorishima, Sofugan, Chokaisan, Io-Torishima, Nasudake, Kita-Fukutokutai, Aira, Nikko-Shiranesan, Kurikomayama, Nakanoshima,
Akagisan(no), Shiretoko-lozan, Iwatesan, Submarine Volcano NNE of Iriomotejima, Akita-Komagatake, Maruyama, Bandaisan,
Esan, Midagahara, Fujisan, Yokoate-jima, Kuttara, and (Smisujima) were checked and only ’Confirmed’ eruptions were adopted.
The format of eruption data is same as that of hypocenters. The location of the volcano, not craters, is the epicenter. The heightof volcano is minus depth in 10m of hypocenter, and the volcanic explosivity index (VEI) is used as the magnitude of the earthquake. The origin time is the date of the eruption and is assumed on 00:00:00 time. The eruption was assumed every day fromthe start of the eruption to the end. If the start date and the stop date were only known in year, Jan. 1 was assumed for the start
day and Dec. 31 for the stop date. If there was no information of the stop date, only the start date was inputted. Totally morethan 153,000 eruptions were inputted from AD 20 to 2014.
Mixing this data and earthquake catalog, we can get some relations between eruptions and earthquake occurrences. One is thatthe 1922 M7.6 earthquake in Okinawa followed VEI 4 eruption near Miyako island in 1924. The other is some large earthquakes
occurred before and after the large eruptions in NE Japan in 17 century.

Keywords: volcano, eruption, earthquake, database

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Roles of Basaltic Lower Crust for a Periodically Refilled Magma Chamber
YANAGI, Takeru1∗
1

Department of Earth and Planetary Sciences, Kyushu University

A series of primary magmas supplied from the mantle seems to be transformed by crystallization in a refilled chamber intoa magma with the average composition of upper continental crust, via a series of calc-alkaline magmas. One of mechanicalmechanisms that have such power is a system of coupled magma chambers. It consists of a lower chamber lying at the base ofcrust and an upper chamber lying at the middle of crust. They are connected by a cylinder and a plug lying between them. Thismechanism is important in a sense that crystallization in it has a power to give good explanation for the genesis of calc-alkaline
igneous rock series and continental crust. However, its presence has not yet been well confirmed. Therefore it is necessary toaccumulate the evidence through geological surveys, and chemical and physical examination of the system.
It is well understood that those, which carry evidence, are volcanic rocks. However, Outlook of what type of investigation
should be done is not easy, due to complexity of chemical differentiation of magma in a refilled chamber. One way to help it is to
study characteristics and differentiation trends of magma composition through simple simulation of a simplified refilled chambersystem. Here, we assumed primary magma and lower crust are well deleted in water and alkali elements, and also assumed
that a heat transfer coefficient between magma and the surrounding crust, a melting point of the crust and a thickness of crust
are all variables and then examined variations of magma temperature, upper chamber position in the depth direction and size ofmagma, in a stirring state, in a refilled chamber lying in the lower crust. Four of results are as follows. 1.High melting points
is necessary for the magma chamber to grow. 2.The magma chamber rises up as the result of settling of crystalline materials
formed by crystallization from the magma and digestion of the ceiling crust of the chamber by the magma. Upper and lower
limits of the temperature that varies with time in a saw-tooth fashion depends on magnitude of heat transfer coefficient, melting
point of ceiling crust and thickness of overlying crust. The points are that lower limit rises more, though at decreasing rate withtime, depending on higher melting point of crust and thinner overlying crust and finally becomes constant. This is confirmedby the evidence observed on compositional variations of volcanic rocks plotted on MgO vs. K2O diagram. 4. If the overlying
crust becomes too thin, then the digestion of crust comes to stop. This is confirmed by the corresponding variation of strontium
isotopic composition of volcanic rocks.
Keywords: refilled chamber, calc-alkaline volcanic rocks, continental crust, thermal chamber evolution

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Magmatic processes for somma-lavas from Usu Volcano
KURITANI, Takeshi1∗ ; TANAKA, Mayumi2 ; YOKOYAMA, Tetsuya3 ; NAKAGAWA, Mitsuhiro1 ;MATSUMOTO, Akiko1
1

Graduate School of Science, Hokkaido University, 2 Kyoto City Office, 3 Graduate School of Engineering, TITECH

Usu volcano formed mainly by eruption of basaltic and andesitic magmas (somma lavas) at 10-20 ka, followed by intermittent
eruptions of felsic magmas after A.D. 1663. Magmatic processes for the historical felsic magmas have been intensively examinedby e.g. Tomiya and Takahashi (1995) and Matsumoto and Nakagawa (2010), while studies on the somma lavas have been limited.
Ohba (1964) and Fujimaki (1986) suggested that the geochemical variation of the somma lavas can be explained principally by
fractional crystallization. However, the number of samples which they examined were limited, and the processes were not wellconstrained by high-quality geochemical data.
In this study, we have performed a petrological and geochemical analysis on samples from the somma lavas to understandmagmatic processes. Whole-rock major element compositions were determined for ˜90 samples, and trace element and Pb
isotopic data were also obtained for 40 samples, as well as lower crustal xenoliths from Ichinomegata volcano. Whole-rock SiO2
content of the lavas ranges 49.6-54.9 wt.%, and they are divided into basaltic samples (SiO2 <52.0 wt.%) and andesitic samples(SiO2 >52.4 wt.%). The andesitic samples can further be subdivided into high-P2 O5 (0.13-0.19 wt.%) type and low-P2 O5 (0.080.13 wt.%) type. The phenocryst assemblage of the basalt is olivine + cpx + opx + pl, and that of the andesite is cpx + opx +pl. The phenocryst content is variable, ranging from ˜10 to ˜35%. P2O5 contents of the somma lavas correlate negatively with
206
Pb/204 Pb ratios, and the ratio decreases from 18.63 to 18.53 with increasing P2O5 content. The lower crustal xenoliths are
significantly lower in 206 Pb/204 Pb and 208 Pb/204 Pb ratios than those of the somma lavas.
We have performed a principal component analysis (PCA) for the whole-rock major element data of the lavas to understandwhat processes were involved in the evolution of the somma lavas. The analysis shows that some elements including SiO2 andP2 O5 are important in PC1, while two elements, Al2 O3 and CaO, play a dominant role in PC2. The contribution of PC1 and PC2
is 58% and 24%, respectively, and these two components sum up to >80% of the total contribution. We found that PC1 shows agood correlation with Pb isotopic ratios and La/Yb ratios, and PC2 correlates positively with the modal abundance of plagioclase
phenocryst. These results suggest that PC1 reflects a mixing process between a less radiogenic component and a more radiogenic
component, whereas PC2 reflects separation and/or accumulation processes of plagioclase phenocrysts.
The high-PC1 end-member component is likely to be a less differentiated basaltic magma because of the low P2 O5 featureof the component. On the other hand, the low-PC1 end-member component has a differentiated feature (i.e. high P2 O5 ), but
it has less radiogenic Pb isotopic composition than the somma lavas. Therefore, it is plausible that the low-PC1 component
would be partial melt of the lower crust. This scenario is supported by the observation that the lead isotopic data of the lower
crustal xenoliths plot mostly on the linear extension of the trend formed by the lava data in a 207 Pb/204 Pb-206 Pb/204 Pb and a
208
Pb/204 Pb-206 Pb/204 Pb compositional space. For these considerations, we conclude that the somma magma evolved throughmixing of a less differentiated basalt magma and partial melt of the lower crust, followed by differentiation and re-distribution ofplagioclase phenocrysts in a crustal magma reservoir.
Keywords: Usu Volcano, Somma lava, Magma process, Lower Crust

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Formation history and magma evolution of Asahidake Volcano of Taisetsu volcanic field,
central Hokkaido, JapanISHIGE, Kosuke1∗ ; NAKAGAWA, Mitsuhiro1
1

Department of Natural History Sciences, Graduate School of Science, Hokkaido University

Taisetsu volcanic field, located in the northern part of Taisetsu-Tokachi volcanic chain, central Hokkaido, consists of Quaternary volcanoes. Its activity started at 1.0 Ma accompanied with voluminous lava flows and many lava domes and cones widely.
The youngest activity has occurred in the center of the volcanic field as follows: Ohachidaira caldera-forming eruption occurredat 35 ka, and after that, the volcanic edifices, namely Ushiro-Asahidake, Kumagatake and Asahidake, have been formed at thesouthwestern rim of the caldera (Katsui et al . 1979). There are several previous studies about eruptive history and magma plumbing system of Asahidake, which is the youngest active cone (e.g. Sato & Wada 2007). However, the stratigraphic relationshipbetween Asahidake and the other volcanoes (Kumagatake and Ushiro-Asahidake) is still unclear. In addition, the petrologicalstudy of these volcanoes has not been carried out sufficiently. Therefore, we performed geological and petrological study ofAsahidake Volcano (Asahidake, Kumagatake, and Ushiro-Asahidake), in order to reconstruct the formation history and to reveal
the magma evolution of Asahidake volcano.
Asahidake (2,291 m) consists of a pyroclastic cone formed above the 1600 m altitude and many lava flows on the west side.
The horseshoe-shaped explosion crater called Jigokudani crater exists on the west side of the cone and the fumaroles still active.
Ushiro-Asahidake lava dome (2,216 m) and Kumagatake pyroclastic cones (2,210 m) locate in the 1 km to the east from the summit of Asahidake and both are covered with the edifice of Asahidake. Based on the difference in volcanic edifice, the volcanic
activity can be divided into 3 stages: Kumagatake, Ushiro-Asahidake, and Asahidake stages, in ascending order.
Kumagatake stage is characterized by the formation of pyroclastic cone with multiple craters. The eruptive materials are subdivided into 3 units by the difference in crater. The total eruptive volume of Kumagatake stage is 0.35 km3 DRE.
In Ushiro-Asahidake stage, lava flowed down southward. After that, the lava dome was built on the summit. Total volume ofthis stage is 0.33km3 DRE.
Asahidake stage is divided into two substage by the difference in eruption style. Early substage is mainly composed of magmatic eruptions. They are subdivided into lower and upper units on the basis of the stratigraphic relationship and magma type.
The lower and upper units can be respectively subdivided into L1 - L3 and U1 - U5 subunits. The latest magmatic eruption of
Asahidake is ca. 5 ka (Okuno 2005). The total volumes of lower and upper units are 4.50 and 0.99 km3 DRE, respectively.
Asahidake late substage is characterized by phreatic explosions forming Jigokudani crater. The last small phreatic explosion
might occur in 250 years ago (Wada et al . 2003).
The rocks of three stages are 2px basaltic andesite to dacite, often contain mafic inclusions. They sometimes include olivine
and hornblende phenocrysts. On whole-rock chemistry, the SiO2 contents of host rocks are 54.7-65.4 wt.% . Mafic inclusions
show 54.0-59.0 wt.%. Dacitic rocks exhibits little variation in petrological features through all the stages. In contrast, mafic
rocks are distinguishable according to stage and unit. Kumagatake stage can be clearly distinguished from the other stages byhigh Ni and low Cr contents. Ushiro-Asahidake stage and the lower unit of early Asahidake stage show similar, lower Ni and Cr
contents. In contrast, the mafic rocks of upper unit of early Asahidake stage exhibit much wider variations
Heterogeneous textures suggest that magma mixing is the main magmatic process in the activities of Asahidake volcano, as
mentioned by previous studies. This study revealed that magmatic compositions, especially mafic compositions, has changed inevery stages, and units. That is, the mafic end-member magmas has replaced in each volcanic edifice and craters.
Keywords: Asahidake, Formation history, Transition magma, geology, petrology, Taisetsu volcanic field

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Slab-mantle thermal structure beneath northeast Kamchatka Peninsula constrained fromhigh-Mg basalts and andesites
NISHIZAWA, Tatsuji1∗ ; NAKAMURA, Hitomi2 ; CHURIKOVA, Tatiana3 ; BORIS, Gordeychik4 ;ISHIZUKA, Osam5 ; IWAMORI, Hikaru2
1

Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2 Japan Agency for Marine-Earth Science andTechnology, 3 Institute of Volcanology and Seismology, FED, RAS, 4 Institute of Experimental Mineralogy, RAS, 5 Geological
Survey of Japan, AISTThe Kamchatka Peninsula is one of the most active volcanic arcs and corresponds to subduction of slab near the northwesternedge of the Pacific Plate. The northeastern part of Kamchatka Peninsula involves (i) triple junction including the edge of the
Pacific Plate, and (ii) subduction of the seamount chain. As a result, the world-most active volcanic group (Kliuchevskoy
Volcanic Group; KVG) with a wide across-arc volcanic zone (~ 400 km) is seen in this area, associated with systematic spatial
variations in rock type and chemistry, including adakite near the slab edge (Portnyagin and Manea, 2008; Bryant et al., 2011).
This study aims at understanding the physical-chemical conditions beneath the northeastern Kamchatka Peninsula, by studying
monogenetic volcanoes and the primitive lavas near the northeastern end of the volcanic chain.
We have performed field survey and sampling of the monogenetic volcanic group (East Cones; EC) in the Kumroch Range
where the Eastern Volcanic Front (EVF) terminates. EC are located above the slab of 50-80 km depth (i.e., in a fore-arc region)(Gorbatov et al., 1997) and 60-100 km south from the slab edge. About 15 monogenetic cones are distributed along the eastern
cast over ~ 60 km distance. The samples from eight cones have been newly analyzed to yield the first data set on major-traceisotopic compositions and, the K-Ar age.
The mineral assemblage of EC lavas is uniform: olivine, clinopyroxene, plagioclase, and magnetite. Based on the SiO2 ,MgO, and Al2 O3 contents, the EC lavas are classified into 5 rock-types: high-Mg basalt (HMB), high-Al basalt (HAB), highMg andesite (HMA), and other basalts (B) and basaltic andesites (BA). Most of the EC lavas have primitive compositions(FeO/MgO<1, Mg#>0.63) except for HAB.
All EC lavas have the common geochemical feature of the subduction-related magmas (e.g., HFSE depletion relative to LILE),
suggesting important roles of water. Based on hydrous melting experiments of HMB and HMA (Tatsumi, 1982), the watercontents for individual primitive magmas are estimated as follows: HMB: 2 wt.%, HAB:4 wt.%, HMA: 4-7 wt.%, B: 2.6 wt.%,
BA: 3.3 wt.% respectively. Melting temperature of these primitive melts in the mantle (1.5 GPa) is estimated to be 1100-1200 ℃ ,based on modeling for HREE abundances and the water contents estimated above, using the hydrous melting relations (Iwamori,1998). The melting temperature is comparable to numerical model with an average thermal condition in arc setting (Iwamori
and Zhao, 2000; Manea and Manea, 2007). Combining these information with the previously published genetic PT and waterconditions in the neighboring areas, horizontal heat transport from the slab edge region (i.e., roughly from north to south) as
was suggested in the previous studies (Yogodzinski et al., 2007) is not supported. We also estimate the slab surface temperatureby using H2 O/Ce thermometer (Cooper et al., 2012). The estimated slab surface temperature is 620 ~ 730 ℃. Unifying theestimated slab surface temperature and the mantle melting temperature, together with trace element and isotopic characteristics,
we propose that a subducted seamount which inherited a local thermal anomaly could have enhanced the flux of slab-derived
fluid (not slab melt), and caused flux-melting to produce the high-Mg andesite and basalts, in the for-arc region.
Keywords: mantle thermal structure, arc volcano, high-Mg andesite, slab, subducting seamount

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Eruption history of Nyos volcano, northwestern Cameroon
HASEGAWA, Takeshi1∗ ; MIYABUCHI, Yasuo2 ; KOBAYASHI, Tetsuo3 ; AKA, Festus4 ; BONIFACE, Kankeu4 ;,, Issa4 ; MICHE, Linus4 ; FILS, Salomon4 ; KANEKO, Katsuya5 ; OHBA, Takeshi6 ; KUSAKABE, Minoru7 ;GREGORY, Tanyileke4 ; JOSEPH, Hell4
1

Ibaraki University, 2 Kumamoto University, 3 Kagoshima University, 4 IRGM, 5 Kyoto University, 6 Tokia University, 7 Toyama
University
Lake Nyos (2 x 1.2 km), a maar volcano in northwestern Cameroon, exploded a large amount of CO2 that killed 1746 people
1986. We performed field survey in and around Lake Nyos to establish its eruptive history and formation processes.
Eruptive deposits of Nyos maar lie on Pre-Cambrian granitic basement and can be seen at the north to eastern lakeshore.
These include tuff breccia (Unit A-1), scoria fall (A-2), lava flow (A-3) and base surge (A-4), in ascending order. We see no
evidence of time breaks between these units as can be typified by paleosol and reworked deposits. A-1 is rich in lithic fragments,
such as basaltic bombs with chilled margins (juvenile) and granitic (crustal) and peridotitic (mantle) xenoliths. Its limited areal
distribution in the eastern lakeside indicates a nearby vent location (Vent 1). A-2 is clast-supported and mainly composed of wellsorted basaltic scoria. The thickness increases from east to north lakeshore. A-3 is deposited 20 m above lake level with covering
basement rock and A-2 in the north lakeshore. Its depositional level decreases to the lake level in other parts surrounding thelake. These evidences suggest that the vent that erupted A-2 and -3 was located at the north part of the lake (Vent 2). Distributionof A-3 extends northeastward for more than 10 km along valleys from Lake Nyos. A-4 (most voluminous) is base surge deposits
characterized by cross-laminated and fines-poor facies. It consists of dominant basalt, isolated crystals and accidental lithics.
The thickness is more than 30 m at the lakeside and it makes a depositional surface 1 km around Vent 2. The distal facies shows
well-sorting and parallel laminations containing accretionary lapilli, which can be interpreted as air fall deposits.
A scoria cone is situated 1.5 km northeast of Lake Nyos. Ejecta from the cone (scoria fall) overlies A-4 around the lake,suggesting that activity of the cone started immediately after the Nyos maar-forming event. The cone is not a single cone but acomplex structure. The main cone structure is divided into west and east, and the southern slope is collapsed. Several hummocks,
which might be debris avalanche deposits from the sector collapse are found at the south to southwestern foot of the main cone.
A remarkable crater (NE Vent), cutting part of the main cone is located near the northeast side. Products from the scoria coneconsist of scoria fall (B-1), volcanic bombs (B-2) and lava flow (B-3). B-1 is characterized by highly vesiculated basaltic scoria
whose thickness and diameter increase towards the main cone. B-2, made of basaltic bombs, was emplaced on B-1 and scattered
within about 500 m around the cone. There are two types of bombs: xenolith-poor and -rich. Xenolith-poor bombs are a few
meters in size and occur near the main cone and on the hummocks. Xenolith-rich bombs are abundant near the NE vent. Thiskind of distribution suggests that xenolith-poor bombs were produced from the main cone and xenolith-rich bombs from the NE
Vent. B-3 is a small, blocky lava flow less than 500 m long on the southwest side of the main cone.
Existence of accretionary lapilli and chilled margins of juvenile materials in Unit A suggest that phreatomagmatic eruption
occurred during the Nyos maar-forming eruption. Although conditions of conduit changed from wet to dry during A-1 to A-3
with shifting of the vent locations (Vent 1 to 2), catastrophic base surge caused by interaction of water and voluminous magma
formed Lake Nyos consequently. After that, the locus of eruption moved to the scoria cone. B-1 formed the main part of thescoria cone, and then the cone collapsed to generate debris avalanche deposits on the southern foot. Finally, eruptive activity
terminated with small effusion of lava flow from the collapsed cone and formation of NE crater ejecting xenolith-rich bombs.
The volcanic activity is characterized by various products and styles of eruptions with changes in vent locations northeastwards.
Keywords: Lake Nyos, Eruption history, Maar, Scoria cone, Phreatomagmatic eruption

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Petrology of Rinjani volcano, Indonesia: The magmatic processes before and during AD
1257 caldera-forming eruption
NAKAGAWA, Mitsuhiro1∗ ; TAKAHASHI, Ryosuke1 ; AMMA-MIYASAKA, Mizuho1 ; KURITANI, Takeshi1 ;WIBOWO, Haryo1 ; FURUKAWA, Ryuta2 ; TAKADA, Akira2
1

Department of Earth & Planetary Science, Hokkaido University, 2 Geological Survey of Japan

In order to compare distinct types of caldera volcanoes, we start to study the AD 1257 caldera-forming eruption of Rinjani
volcano, Indonesia, which occurred at the summit of a large startovolcanoto. On the other hand, there exist no a large preceding
stratovolcano in Japan. Thus, the volcano must be different type. The volcano started its activity ca. 0.10 Ma. The activity canbe divided into four major stages, stratovolcano-building, low activity (pre-caldera), caldera-forming and post-caldera stages.
Eruption rate had decreased since the low activity stage from 0.6 km3 /ky to 0.15km3 /ky. The caldera-forming stage occurredin AD 1257. The total eruptive volume during the stage is estimated to be more than 10 km3 DRE. Activity of the postcaldera stage has continued within the caldera. The rocks of the stratovolcano-building stage, ranging from SiO2 =44.8 to 63.7%,
are mainly basaltic andesite. Type of rocks has clearly changed since the low-activity stage and are amphibole dacite. The
rocks of the caldera-forming one are less silicic compared with those of the low activity one. The rocks of the post-caldera
stage are olivine-pyroxene andesite (SiO2 ˜55). Based on whole-rock chemistry, the rocks of the volcano can be grouped into
two, stratovolcano-building and post-caldera stages, and low activity and caldera-forming ones. The two groups can be easily
distinguished according to two distinct chemical trends in many SiO2 variation diagrams for major (Al2 O3 , MgO, FeO, CaO andK2 O) and trace (V, Rb, Y, Zr, Ba and Th) elements. Ratios of incompatible elements and Sr isotope ratios of the rocks fromtwo groups are also distinct. These suggest that the dacitic magmas from the low activity and caldera-forming stages could be
produced not by crystallization differentiation of the basaltic magma but by additional processes, such as crustal melting and/or
AFC process. Although the rocks from low activity and caldera-forming stages are similar amphibole dacite, these rocks can be
distinguished in terms of two distinct trends in many SiO2 variation diagrams. Contents of LIL elements, such as K2 O, Rb andBa, of the rocks from each stage increase with increasing of whole-rock SiO2 . Thus, nearly parallel, positive trends are formedin such SiO2 variation diagrams. Contents of LIL elements of the rocks from caldera-forming stage are lower contents thanthose from low activity one at the same SiO2 content. On the other hand, HFS elements, such as Nb, Zr and Y, show different
variations of the rocks between two stages with increasing of whole-rock SiO2 contents. Although the elements of the rocks
from the caldera-forming stage show positive correlation with SiO2 in SiO2 variation diagrams, those from the low activity stage
show negative one. Thus, the contents of LIL elements from two stages are similar in silicic dacite, whereas those are largely
different in andesitic dacite. The andesitic dacite of the low activity stage is characterized by high contents of HFS elements.
Considering these chemical difference in dacites between low activity and caldera-forming stages, these rocks cannot be formedby simple crystallization differentiation and/or contamination from the same primary magma. According to the temporal changeof volcanic activity and erupted magma since late Pleistocene, it would be possible to consider the low activity stage as preceding
stage for caldera-forming eruption. During the stage, eruption rate had rapidly decreased and magma type has also changed to be
dacitic one, which is similar to the magma of the caldera-forming stage. Thus, it seems that caldera-forming, voluminous dacitic
magma had been formed and accumulated during the low activity stage. However, our study revealed that the dacitic magmafrom these two stages are different. Thus, this would not be the case that the same dacitic magma had been continuously formed
and accumulated until the caldera-forming stage.
Keywords: caldera, caldera-forming eruption, magma process, silicic magma, Rinjani volcano

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Time:May 26 18:15-19:30

The history of volcanic eruption based on widespread tephra correlation at Pliocene andearly Pleistocene in Japan
TAMURA, Itoko1∗ ; YAMAZAKI, Haruo1
1

Dep. Geography, Urban Environmental Sc. Tokyo Met. Univ.

The frequency and the scale of the large volcanic eruption in the Japanese Islands are one of the important problems in theforecast of the long-term geological features change. The restoration of the history of the large volcanic eruption in the late
Pleistocene has been examined by the correlation with the pyroclastic flow deposit and the distal fine ash (For example, the Itopyroclastic flow deposit and the comparison with the AT volcanic ash of 30ka: Machida, Arai, and 1974, etc ). The eruptionof VEI 6 was generated eight times and the VEI 7 was generated nine times for the past 125,000 years (Machida, Arai, and2003). It is clarified that the extremely large volcanic eruption in the Japanese Islands is the frequency once about 7000-8000
year. However, an older age, the volcanic edifice and geographical features in the caldera etc. disappear, a lot of source volcano
cannot be specified, and the evaluation at the scale and age of a volcanic eruption are difficult.
In tephrochronology, identification and the comparison of the tephra based on the chemical composition analysis of mayor
element (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, P) of the volcanic glass from the 1990’s. As a result, the comparison accuracy ofan older pyroclastic flow deposit and the tephra in the remote place improves. The Ebs-Fukuda tephra (1.75Ma: Yoshikawa etal., 1996) and the Omn-Sk110 tephra (1.65Ma: Nagahashi, 1998), etc. were found. It has come to obtain information on thelarge volcanic eruption the widespread tephra correlation before early Pleistocene. Moreover, Kikkawa (1990) showed that traceelement such as La, Ba, Sr, and Y was identified from the difference of the element characteristic even by the tephra to which the
major elemental composition of the volcanic glass was similar. In addition, Mizuno (2001) showed that the source volcano areaof the widespread tephra was definable to Chubu Sangaku, Kyushu, and Tohoku region from the element characteristic of a traceelement of volcanic glass of the pyroclastic flow deposit to some degree. We do trace elemental chemical composition analysesof the volcanic glass of the index tephra in around 2000, and have been examining the comparison and tephrochronology of theindex tephra of central Japan based on the characteristic. It has reported on a petrology feature, the eruption scale, the source andthe age of the tephra about 36 widespread tephra layers of 5Ma-1Ma that did trace analyses.
Among 36 tephra layers, Tohoku type ( La/Y : <0.5, Ba/La >30 ) are eight tephra layers, i.e. the In1-B25 tephra (3.1Ma :Tamura et al., 2014 ), the TmgR4-HSC tephra ( 2Ma : Shimogama and Suzuki, 2006 ), the Kry1-HAS tephra ( 1.9Ma : Tamura
et al., 2006 ), the Kd44-Nk tephra ( 1.9Ma : Nakayama and Suzuki, 2007 ; Tamura et al., 2008 ), the Kumado-Kd22Utephra (Murata and Suzuki, 2011 ), the Akai-Kd18 tephra( Murata and Suzuki, 2011 ), the Ashino-Kd8-CH13 tephra ( 1.3Ma : Murata
and Suzuki, 2011 ; Tamura et al.) and the Ysm-CH3 tephra ( 1Ma : Tamura et al). It concentrates on 2Ma-1Ma. Tephra layers
presumed to be a Kyushu origin are the Hbt1-MT2 tephra (2.8-2.9Ma: Tomita, Kurokawa, 1999), the Ass-Tmd2 tephra (2.6Ma: Tamura et al., 2008), the Skt-Kd16 tephra (1.4Ma: Mizuno, 2007), and the Ss-Pnk tephra (1.02Ma: Machida and Arai, 2003).
As for the Trb1-Ya4 tephra (4.2Ma: Tamura and Yamazaki, 2004) and the Ksg-An77 tephra (4Ma: Tamura and Yamazaki, 2004)
have the possibility of the Ryohaku mountain origin from the grain degree tendency to the tephra particle. Tephra layers of the
Chubu Sangaku origin are 20 layers. The tephra of the Chubu Sangaku origin begins to appear because of Hgs-An129 tephra(Satoguchi and Nagahashi, 2012) of about 3.6Ma. Moreover, it concentrates between 3Ma-1.5Ma with 16 tephras.
The Tohoku origin tephra layers are concentrated on 2Ma-1Ma and the Chubu Sangaku origin tephra concentrates on 3Ma1.5Ma.

Keywords: Widespread tephra correlation, Pliocene, early Pleistocene, Histoly of volcanic eruption, Japanese Islands

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SVC47-P02

Room:Convention Hall

Time:May 26 18:15-19:30

Emplacement processes of Takayubaru lava flow which actived before Aso-4 pyroclastic
flow
SHIIHARA, Kousuke1∗ ; HASENAKA, Toshiaki1 ; MORI, Yasushi2
1

Graduate School of Science and Technology, 2 Kitakyushu Museum of Natural History and Human History

Eruption of Omine volcano and effusion of associated Takayubaru lava, just before large-scale Aso-4 pyroclastic eruption,
formed a pyroclastic cone 200 m high, and lava plateau of block lava with ca. 100 m thickness and an area of 28 km2 . BothAso-4 pumice and Takayubaru lava are dacite in composition and are similar, however, the former made an explosive eruption,
whereas, the latter, an effusive eruption.
We made a cross section of Takayubaru lava plateau, using the drill-core data of Kumamoto Office of River and National
Highway (1994). We found that the basement is inclined from the base of Omine cone (200 m in elevation) toward 3 km west
(15 m in elevation), where depression occurs. The thickness of Takayubaru lava becomes maximum of 140 m at the depression,and decreases toward further west to become minimum of 10 m at the western end. The surface of lava is nearly flat. The N-Scrossed section shows that the land surface is inclined toward south. The southern end is 50 to 100 m lower than the northernend, after 3.5 km in distance. This tilt was supposedly caused by Futagawa active fault.
The drilling core include, from the top to bottom, soil, Aso-4 tephra, (no soil), upper auto-brecciated part of lava (15 m thickin the middle part), massive part of lava (80 m thick), and lower auto-brecciated part of lava (2 m thick), which were underlain
by Futa Formation.
Massive part is homogeneous, and has no interstitial auto-brecciated part. Modal composition of phenocryst minerals does notshow much difference from top to bottom. Vesicles are observed in all positions at the fore front of lava flow, however, they are
only observed in the upper and bottom part of the lava from the middle and near-source area. Silica content of bulk lava fromall position varies within 2 wt.%. It is especially homogeneous (<1 wt.% SiO2 ) in the middle massive part. Plagioclase crystals
show lineation. The standard deviation in angle does not change from top to bottom. Plagioclase crystals show characteristic
honey-comb texture, or melting texture. The degree of melting is greater at the upper portion. All these observations strongly
suggest that Takayubaru lava flow does not represent multiple flows but a single flow unit.
We estimated lava effusion rate from the equation relating single lava flow length (L) and effusion rate (E), i.e. L=10ˆ3.11∗Eˆ0.47(Calvari & Pinkerton, 1998). The length of Takayubaru lava flow, 7.5 km gives the effusion rate of 42 m3 /s. Because the volumeof Takayubaru lava flow is 2.0 km3 , the period of flowing event was calculated to be 1.4 year. If we assume a constant rate offlow, then the velocity was ca. 0.6 m/hr.

Keywords: Aso, Omine volcano, Takayubaru lava flow, Effusion rate

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SVC47-P03

Room:Convention Hall

Time:May 26 18:15-19:30

The occurrence and grain-size characteristics of distal facies of Aso-4 ignimbrite in Yamaguchi Prefecture
TSUJI, Tomohiro1∗ ; IKEDA, Michiharu1 ; FURUSAWA, Akira2 ; NAKAMURA, Chisato3 ; NISHIZAKA, Naoki4 ;ONISHI, Kozo4
1

Shikoku Research Institute Inc., 2 Furusawa Geological Survey Inc., 3 Ehime University, 4 Shikoku Electric Power Company Inc.

Aso-4T ignimbrite is a low-aspect ratio ignimbrite which distributed widespread from Aso caldera via Seto Inland Sea to
Yamaguchi Prefecture. Though the mechanisms of transportation and deposition of that are important issue of volcanology, thefundamental data such as the depositional distribution and grain-size characteristics were not studied enough. Then we surveyed
geologically and analyzed the grain-size of Aso-4T ignimbrite in Yamaguchi Prefecture in addition to Aso-4A ignimbrite andAso-4 ash-fall in the other localities for its comparison.
Aso-4T ignimbrite distributes widespread in central to western region of Yamaguchi Prefecture with relatively thin (2-3 m)
thickness in contrast with Aso-4A ignimbrite which fills valley to a thickness of more than 40 m in Oita Prefecture. Aso-4 ash-falldeposits as 15 cm-thick and partially modified by water current and resedimented at Miyoshi city in Tokushima Prefecture.
Aso-4T ignimbrite is significantly altered and rich in clay. Oppositely, Aso-4A ignimbrite is coarse, poorly sorted and includes
small amount of clay. Aso-4 ash is fine , relatively sorted and rich in clay. The maximum size of pumice in Aso-4T at 130-160
km from the source caldera is 1.0 to 1.2 cm, that in Aso-4A at 50-70 km is 20-30 cm and that in Aso-4 ash-fall at 314 km is 5mm. The maximum length of hornblende in Aso-4T at 50 km and 130-160 km is 3.8 mm and 2.8-3.1 mm. That in Aso-4A at 50
km is 3.4 mm and in Aso-4 ash-fall at 314 km and 680 km is 2.5 mm and 0.9 mm.
The aspect ratio of hornblende in Aso-4T is 1.6-8.0, that in Aso-4A at 50-70 km is 1.5-8.0 and that in Aso-4 ash-fall at 314-682
km is 1.0-6.0.
Hornblende is resistant to alteration so that it suites to the study of grain-size characteristics of significantly altered ignimbritesuch as Aso-4T. We recognized the tendency of the grain-size characteristics as follows:
1) the grain-size of hornblende in Aso-4A ignimbrite is slightly larger than that in Aso-4T ignimbrite2) longitudinal variations of maximum size of hornblende in Aso-4A and 4T ignimbrites are relatively homogeneous than thatin Aso-4 ash-fall3) aspect ratios of hornblende in both ignimbrites are high than that in Aso-4 ash-fall.

Keywords: Aso-4 ignimbrite, Yamaguchi Prefecture, grain-size characteristics

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SVC47-P04

Room:Convention Hall

Time:May 26 18:15-19:30

Geological Map pf Kuju Volcano; More accurate eruptive history and magma eruption
rate
KAWANABE, Yoshihisa1∗ ; HOSHIZUMI, Hideo1 ; ITOH, Jun’ichi1 ; YAMASAKI, Seiko1
1Geological Survey of Japan, AIST

Geological Survey of Japan, AIST has published a new Kuju volcano geological map. In this ”Geological Map of Kuju
Volcano”, we re-examined the stratigraphy of Kuju volcano by tephra study and many K-Ar and 14 C dating. Kuju volcano
is composed of an aggregation of small stratovolcanoes and lava domes of dacite from basaltic andesite. Approximately old
volcanic body exists in the west, new volcanic body are distributed to the east. Around the Kuju volcano, there are volcanic fan
made of block and ash flow deposits, debris avalanche deposits and debris flow deposits. Large-scale pyroclastic flows, such as
the Handa pyroclastic flow deposits, also distributed around Kuju volcano. We divided the activity of Kuju volcano into fourstages, the 1st stage (from 200ka to 54ka), the 2nd stage of the Handa pyroclastic flow eruption occurred (54ka), the 3rd stage(from 54ka to 15ka) mainly occurred at the central area of Kuju volcano, the 4th stage (from 15ka to present), characterized byeruption of mafic magma. For the fourth stage, we performed a detailed tephra description including a relatively small (magma)
steam eruption. We were re-examined the magma eruption rate of the 3rd and the 4th stage. The magma eruption rate of the 4thstage is about 0.45km3 /1000 years (DRE) and has increased from the 3rd stage of about 0.29km3 /1000 years.
Keywords: active volcano, Kuju volcano, eruptive history, geological map of volcano, eruption rate, dating

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SVC47-P05

Room:Convention Hall

Time:May 26 18:15-19:30

Volcanic History of Ogasawara Ioto (Iwo-jima)
NAGAI, Masashi1∗ ; KOBAYASHI, Tetsuo1
1

National Research Institute for Earth Science and Disaster Prevention, 2 Kagoshima University

Ioto (Iwo-Jima; Sulphur Island) is a volcanic island located at the volcanic front of the south end of Izu-Bonin arc. The
island consists of a central cone and southwest rim of a submarine caldera with a diameter of about 10 km. High rates ofgeothermal activity and crustal uplift have been observed, which are considered to be related to magma intruding at a shallow
depth. Therefore, Ioto volcano is considered to be an active resurgent dome. However, eruptive history, including the process
and timing of caldera formation, has not been clarified. Eruptive history based on our recent field survey, dating, and chemical
analysis is as follows. A pre-caldera edifice was formed by volcanic activity of trachyandesite-trachyte magma in a subaerial andsubaqueous environment. The K-Ar ages of the lavas on the Western rim represents about 0.07-0.08 Ma, but these results havelarge error. The magma composition and types of eruption were similar to those of the post-caldera edifice. It is still unclear
when the caldera was formed. The caldera floor, which was a sedimentary basin with shallow marine sediments and a subaqueous
lava flow, has been present at least since 2.7 kBP. Furthermore, a small volcanic island covered with trees used to exist in theMotoyama area. The complicated sequence of the Motoyama 2.7 kBP eruption is described as follows. First, on the volcanicisland or in the surrounding shallow water, an explosive phreatomagmatic eruption occurred that formed subaqueous welded tuff
(Hinodehama ignimbrite) and a subsequent thick subaqueous lava flow (Motoyama lava). While the Motoyama lava was still
hot, the eastern part collapsed. The collapsed mass was quenched to form large blocks similar to pillow lava. A subsequent large
phreatomagmatic eruption occurred, destroying the hot Motoyama lava, the older edifice, and the marine sediment. The resultant
subaqueous pyroclastic flow generated the Motoyama pyroclastic deposit. Then, the eruption center shifted to the Suribachiyama
area, which is just outside the southwest caldera rim. Deposits from three different eruption periods have been identified-lower,
middle, and upper pyroclastic deposits-and a lava flow that erupted during the middle pyroclastic period. The lower unit wasformed by a subaqueous eruption at a deeper level; the middle deposit was formed by a phreatomagmatic explosion at a shallow
depth; and, the following lava emission generated a lava island. The upper pyroclastic deposit was generated by a combinationof phreatomagmatic and Strombolian eruptions. Although the ages of these eruptions are not obvious, the first phase of theeruption occurred during the period between 2.7 kBP and 0.8-0.5 kBP, which is estimated from the age of the reworked deposit
sandwiched between Motoyama pyroclastic deposit and the upper marine terrace X (Kaizuka et al. , 1983) deposit. The eruptionof the upper deposit occurred before AD 1779 (ca. 0.2 kBP). The eruptive products described so far are covered with younger
sediment from marine terraces and spits. Recently, small-scale deposits from phreatic explosions accompanied by geothermal
and uplift activities have been found in various places on the island, but juvenile material has not been confirmed to exist in the
products.
Keywords: caldera, subaqueous pyroclastic flow, subaqueous lava

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SVC47-P06

Room:Convention Hall

Time:May 26 18:15-19:30

Investigation into transition magma after Caldera forming, Izu-Oshima Volcano, easternJapan
YAMAGUCHI, Azusa1∗ ; NAKAGAWA, Mitsuhiro1 ; KURITANI, Takeshi11Department of Natural History Science, Graduate school of Science, Hokkaido University

Izu-Oshima is a volcanic island located at the northernmost end of Izu-Ogasawara islands. The volcanic activity has been divided into ”pre-caldera stage”, ”caldera forming stage”, and ”post-caldera stage” (Nakamura, 1964). Twelve significant eruptions
occurred after the caldera formation (about 1500 years ago), and these ejecta are called ”Younger Oshima Group” (referred to
as YOG) (Nakamura, 1964).There have been several petrological studies that investigated temporal evolution of YOG magmas,
showing that Mg# of the whole rocks tends to decrease monotonously with time after the caldera formation (e.g., Fujii et al.,1988). It has also been suggested that the magmas were essentially derived from a single evolving magma chamber without
significant replenishment of a primitive magma (Kawanabe, 1991). In this study, to understand magmatic processes in moredetail, we are performing a geochemical analysis on YOG products. As a preliminary report, here we present petrography andwhole-rock compositions of eruptive products from YOG. We also evaluate the magmatic system with respect to alignment ofcraters from which YOG magmas were ejected.
In this study, 44 rocks were collected from YOG, as well as from products of the 1950 and 1986 eruptions, according to the geological map of the volcano (Kawanabe, 1998). The samples are lavas and scoria, and are basalt ˜basaltic andesite un composition.
The phenocryst content ranges 1.5˜10 vol.%. The phenocryst assemblage consists of plagioclase, orthopyroxene, clinopyroxene,
and magnetite. The samples can be petrographically divided into those including many magnetite microphenocrysts (Type1),
those including plagioclase glomerocrysts (Type2), and those without magnetite microphenocrysts and plagioclase glomerocrysts (Type3). SiO2 contents range from 52 to 58 wt.%, but the contents of the samples other than those from the 1986 flank
eruption are limited to 52˜54 wt.%. In some Type2 samples, Al2 O3 contents correlate positively with the modal abundance ofthe plagioclase phenocryst, but most samples have limited range in Al2 O3 contents (14˜15 wt.%) regardless of the abundance ofthe plagioclase phenocryst. The concentration ratios of incompatible trace elements (e.g., Ba/Zr) are essentially constant. In ahistogram of the An content of the cores of the plagioclase phenocryst, a remarkable peak is observed at An=90 throughout theeruption stages in YOG. On the other hand, the An content of the rims of the plagioclase phenocrysts tends to decrease fromolder to younger samples. The plagioclase phenocrysts with reverse zoning are commonly observed in samples from the caldera
forming stage, while they are scarce in samples from the following stages.
The ratios such as Ba/Zr are constant in the YOG samples, which suggests that the magmas were derived principally froma single primary magma. The observation that reversely-zoned plagioclase phenocrysts occur especially in samples from thecaldera forming stage may suggest that magma mixing was dominated in the early stage of YOG, but it did not play an importantrole in most of the evolution after the caldera formation. On the flank of the main edifice, craters are aligned on two different
NW-SE lines, and the flank craters of the 1986 eruption seem to be aligned on a line between them. We found that whole-rockcompositions of the ejecta differ with respect to craters from the three lines. This observation may suggest that three discrete
magma chambers with variable degrees of differentiation have been present beneath each line, while they were derived from asingle primary magma.

Keywords: Izu-Oshima, petrology, volcanology, magma-plumbing system

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SVC47-P07

Room:Convention Hall

Time:May 26 18:15-19:30

Magma mingling of the Kagusa lava in the Kusatsu-Shirane volcano: preliminary resultsfrom analyses of a boring core
USHIODA, Masashi1∗1

Volcanic Fluid Research Center, Tokyo Tech

Three borehole-type seismometer and tiltmeter were installed around the Yugama crater in the Kusatsu-Shirane volcano by the
Kusatsu-Shirane volcano observatory, Tokyo Tech. NE observation well was penetrated into the Kagusa lava which thickness
was about 50 m (Uto et al., 2004). Hayakawa (1983) reported that Kagusa lava flowed from neighborhood of the Mizugama
crater to east valley along the Oosawa river about 7,000 years ago. Whole-rock chemical compositions of Kagusa lava whichsamples were collected subaerially were analyzed by many studies (e.g. Hayakawa 1983, Uto et al. 1983, and Takahashi et al.
2010) and range from andesite to dacite (from 58 to 65 wt.% SiO2 ). Kagusa lava often displays banded texture of andesite (graycolor) and dacite (white color) in a flow unit at intervals of dozens of centimeter and flowed with mingling andesite and dacite
together (Uto et al., 1983). In order to understand the time and space scale of magma mingling of eruption of Kagusa lava, in thisstudy, depth profile of whole-rock compositions for this boring core was researched for every several meters based on descriptionof this core (Uto et al., 2004).
Core samples were cut into homogeneous fractions which had no banded texture and each fractions were powdered. Somecore samples which had complex mingling texture were powdered as a whole. Eleven times diluted glass beads were preparedfor each sample using Li2 B4 O7 . Major elements of whole-rock compositions were analyzed using XRF (RIGAKU RIX2100)
which was installed at Tokyo Tech. In the results, two groups were recognized in the compositions of most samples; group (1)corresponded to ∼60 wt.% SiO2 (andesite) and group (2) corresponded to ∼65 wt.% SiO2 (dacite). In view of the whole trend,intermediate samples which compositions corresponded to 62-63 wt.% SiO2 lay along mixing trend between group (1) and (2).
Core samples above ∼35 m in depth were dominated by dacite and those below ∼35 m in depth were dominated by andesite.
This profile was consistent with core description of NE observation well (Uto et al. 2004). Some lower core samples which wascollected below ∼50 m in depth sometimes displayed banded texture of andesite and dacite. Judging from this results, regarding
compositions, magmas of group (1) and (2) were mingled together immediately before eruption and mingled magma had no time
to homogenize. In order to discuss mingling time scale in detail, I will carried out petrological study; phenocryst assemblage,
composition, and zoning profile of phenocryst in the regions of contact will be researched.

Keywords: magma mingling, magma mixing, Kusatsu-Shirane volcano

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SVC47-P08

Room:Convention Hall

Time:May 26 18:15-19:30

Geological map of Zao volcano
BAN, Masao1∗ ; OIKAWA, Teruki2 ; YAMASAKI, Seiko2
1

Dept. Earth&Environmental Sci. Yamagata Univ., 2 Geological Survey of Japan, AIST

We present new geological map of Zao volcano, as Geological Map of Volcanoes 18 by GSJ.
Zao volcano is a Quaternary stratovolcano located in the middle part of the volcanic front of northeast Japan arc. The volcanic
activity started at ca. 1 Ma, and has continued to the present. The activity can be divided into six stages.
Stage I: The eruption products of ca. 1 Ma are hyaloclastites and dikes. These eruptions would have taken place under lakewater. Rocks are tholeiitic basalt to andesite. Stage II: Around 0.5 Ma, activities of mainly lava effusion were taken place in thenorthern part. Stage III: During ca. 0.35 to 0.25 Ma, several small to middle sized andesitic to dacitic edifices were formed at thecentral part. At the end of this stage, small sized ediffice of basaltic andesite lavas was formed. Stage IV: During ca. 0.25 to 0.20
Ma, andesitic to dacitic lavas were swelled out in the southern area, and formed middle sized volcanic ediffice. Stage V: Fromca. 0.13 to 0.04 Ma, andesitic lava flows with pyroclastic materials erupted from several vents in the northern to central part,which constitute middle sized volcanic ediffice. Stage VI: The most recent stage of the Zao volcano began at ca. 35 ka, when
the horseshoe-shaped erosion caldera (1.7 km in diameter) was formed in the central part. Numerous small- to medium-sized
explosive eruptions of calc-alkaline basaltic andesite magmas have occurred since then. The youngest small ediffice Goshikidake
building activity started at about 2ky. In this stage, small sized lava flows were rarely effused.
Systematic temporal change in petrologic characteristics can be observed. All eruption products of stage I belong to low-K
tholeiitic series, while the others belong to medium-K calc-alkaline series. The potassium levels of stage II products are lower
than those of stage IV and V products. Geologic units with both of those two potassium levels can be observed in stage III. The
stage VI products show compositional trends crossing the boundary between low and medium-K series. More in detail, temporaland special variation in compositional trends within each stage can be observed. Especially, the variations are relatively intensein stage IV and V products.
From the record of AD 1230, many historic activities were recorded in written accounts. All were generated at the Okama
crater lake. From 17th to 19th century, the eruptions continued for ca. 100 years with intermittent dormancies. In 17th, theeruptions have occurred in 1620-1625, 1641, 1668-70, 1694. From late 18th to 19th, the eruptions were taken place in 1794-96,
1809, 1831-33, 1894-1897. The activities are characterized by outbreak of lahar. Among the activities from AD 1894 to 1897,
phreatic eruptions occurred. The latest volcanic activity in Okama occurred during 1939 to 1943.
Keywords: Zao, Volcano, Geological map

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SVC47-P09

Room:Convention Hall

Time:May 26 18:15-19:30

Magma evolution and time scales of magma mixing of the Kumanodake agglutinate of
Zao volcano, northeastern Japan
TAKEBE, Yoshinori1∗ ; HIROKAMI, Chihiro1 ; BAN, Masao1
1

Faculty of Science, Yamagata Univ.

To understand the evolution of magma system for the activity of newest stage (c. 33 ka to present) of Zao volcano in northeastern Japan, we have investigated the eruption products of the Komakusadaira activity (c. 33 ka to 12 ka). In this study, we willinfer the evolution of magma system based on detailed petrological features of the Kumanodake agglutinate which was formed
during initial period of the activity.
The Kumanodake agglutinate drapes the summit area of Mt. Kumanodake whose inner part is comprised by the older stage
products. This unit piles up successively without any unconformity or secondary deposits. The succession is ca. 30 m thick, comprising stratified pyroclastic layers (agglutinate, agglomerate, tuff breccia, and lapilli tuff). The layers include various amount of
black scoriae (˜1m), gray andesitic bombs (˜20cm), and subordinate amounts of gray andesitic lapillus to volcanic block in thematrix of reddish brown scoriaceous ash. The explosivity is inferred to have increased over time because the abundance of thescoriae increased. Futhermore, the large scoriae are usually found in the top part.
The rocks are mixed medium-K calc-alkaline olv-cpx-opx basaltic-andesite (55.2-56.2% SiO2 , 0.82-0.85% K2 O). From thebase to the top, SiO2 and Zr contents decreased gradually, whereas the CaO and MgO contents increased.
Based on petrologic features, we deduced the products were formed by magma mixing of felsic magma and mafic magma.
The estimated felsic magma (59-62% SiO2 , 956±17 degrees Celsius in the lower part and 967±22 degrees Celsius in the upperpart) with orthopyroxene (Mg# = 60-69), clinopyroxene (Mg# = 65-71), and An-poor plagioclase (Anca.60−70 ) was stored in ashallower region. The mafic magmas are further divisible into two types: less and more differentiated, designated respectively
as mafic magma-1 and mafic magma-2. The less differentiated mafic magma-1 was olivine (Fo˜84 ) basalts (ca. 49-51% SiO2 ,1110-1140 degrees Celsius). The differentiated mafic magma-2 was basalt (1070-1110 degrees Celsius) having olivine (Foca.80
with reverse zoned part of Fo˜84 ) and An-rich plagioclase (Anca.90 ) phenocrysts was the basalt formed occasionally at 3-6 km
depth. The mafic magma-1 was the dominant mafic magma because of Fo81−84 olivine phenocrysts are more common than
Fo76−80 ones.
We estimated the time scales from magma mixing to the eruption on the basis of zoning analysis of olivine (Fo˜84 ) phenocrysts
rim (˜50 µm from phenocrystic optical edge) and diffusion calculations. The zoning analysis revealed that a significant mixing
process occurred 250 days to 3.5 years before the eruption in the lower part, while 15 to 130 days in the upper part. In the lower
part, up to 40% An-poor plagioclase and orthopyroxene phenocrysts in the same thin section have multiple dusty zones or oscillatory zoning part inside the phenocrystic rim. The abundance of those phenocrysts decreases 15% up-section. These phenocrysts
are antecrysts formed by injection of mafic magma prior to the eruption. Thus, the duration of the time scale correlates to theamount of antecrysts. Consequently, in the early part of the activity, the erupted magmas had more antecrysts.
The erupted magma composition became more mafic, which reflects increased percentage of mafic magma involved in mixing.
At the beginning of the activity, the mafic magma also acted as a heat source for activation of the cold felsic magma chamber,thereby suppressing the volume percentage of mafic magma in the mixing, and also resulted in longer residence time before theeruption. As the activity proceeded thereafter, the shallow felsic chamber would become more mobile, consequently the mafic
would be able to mix with felsic magmas more easily, resulted in higher percentage of the mafic magma in the mixing, andprompt eruption.

Keywords: Magma evolution, Magma mixing, Time scale, Kumanodake agglutinate, Zao volcano, NE Japan

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SVC47-P10

Room:Convention Hall

Time:May 26 18:15-19:30

Petrologic characteristics of early part of the Okama-Goshikidake activity of the Zaovolcano.
NISHI, Yuki1∗ ; BAN, Masao2 ; OIKAWA, Teruki3 ; YAMASAKI, Seiko4
1

Yamagata Univ, 2 Yamagata Univ, 3 GSJ, AIST, 4 GSJ, AIST

Introduction: Zao volcano is representative active stratovolcano in NE Japan. The volcanic activity commenced at about 1Ma
and has continued to present. The Goshikidake, the youngest cone in the central part, started its activity from ca. 2ka. The
present crater lake Okama is in the western part of the Goshikidake. We examined the geologic and petrologic features of theeruption products in the initial period of the Okama-Goshikidake activity.
Stratigraphy: The eruption products can be subdivided into Furikodaki lava, Goshikidake-nanpo lava and pyroclastics,
Goshikidake-nanbu pyroclastics, Goshikidake-toubu pyroclastics. The Furikodaki lava flowed down from the northeastern baseof the Goshikidake cone along a stream. The length and width of lava are ca. 750m and 20-30m, respectively. The Goshikidakenanpo lava and pyroclastics cropped out in a narrow area of ca. 650m south from the summit of Goshikidake. This unit
is composed of upper brecciated lava with coarser lateral and finer vertical joints, and lower hyaloclastite-like tuff breccia.
Goshikidake-nanbu pyroclastics distributes ca. 500m southward from the summit, consisted of pyroclastic surge deposit andvent breccia. The former consists of lapilli tuff to volcanic breccia with ca. 10m in thickness. The latter consists of three veinselongated from northeast to southwest with 2-8m width and 5-8m length. These intrude nearly vertically into surge deposits.
They are poorly sorted tuff breccia with minor amount of ˜20cm rounded bombs. The Goshikidake-toubu pyroclastics distributein ca. 300m eastward from the summit. The thickness is ca. 6-35m. They are fairly stratified tuff - lapilli tuff - tuff breccia
including various amounts of volcanic bombs.
Petrography: All rocks are cpx-opx andesites. Most of plagioclase phenocrycrysts has dissolution textures such as dusty zone
and/or patchy zoning. Plagioclase of the Furikodaki lava, the Goshikidake-nanpo lava and pyroclasts lack dusty zones. Opx
and cpx phenocrysts show usually euhedral to subhedral texture except for some rounded cpx phenocrysts. About half of thepyroxene phenocrysts include sparsely glass inclusions. Cpx content in the bombs of the vent breccia of the Goshikidake-nanbu
pyroclastics is higher than other products.
Whole rock chemistry: All products belong to medium-K, calc-alkaline series, with 56 to 58% SiO2 . Looking at in detail,the range of SiO2 contents of Furikodaki lava, Goshikidake-nanpo lava and pyroclasts is 57.5-58wt%, while that of the otherproducts is 56-57.7wt%. Most of the products are plotted in same variation trends in silica variation diagrams, but the bombs ofthe vent breccia have higher FeO, TiO2 contents and lower MgO contents than the other products.
Mineral chemistry: In Furikodaki lava, Goshikidake-nanpo lava and pyrocrastics, the core composition of opx phenocrycrysts
is 64-65 Mg#. In the pyroclastic surge deposits, that is around 65 Mg#. In the bombs, that has a range of Mg#62-69. The cpx corecomposition shows a range of ca. 64 to 70 Mg# in all products. The core compositions of plagioclase phenocrycryst show wide
range of An62-92. In Furikodaki lava, Goshikidake-nanpo lava and pyrocrastics, the main peak compositions are in An68-70 andaround An78, and subordinate peak is in An90. The pyroclastic surge deposits have main peak compositions of An64-66 andAn76-78, subordinate peak composition of An90. The bombs have main peak compositions of An62-66 and An74-76.

Keywords: zao volcano, andestic lava, pyroclastic surge, agglutinate, calc-alkaline

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SVC47-P11

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Time:May 26 18:15-19:30

Petrological characteristics of rocks from Chokai A.D.1800-1804 activities, NE JapanKAMIYA, Chika1∗ ; BAN, Masao2 ; HAYASHI, Shintaro3 ; OHBA, Tsukasa4 ; SATO, Takanori5
1

Graduate School of Science and Engineering, Yamagata University, 2 Department of Earth and Environmental Sciences, Facultyof Science, Yamagata University, 3 Department of Earth Sciences, Faculty of Education and Human Studies, Akita University,
4
Department of Earth Science and Technology, Faculty of Engineering and Resource Science, Akita Unive, 5 AIST
Chokai volcano is a Quaternary stratovolcano located at rear arc of the Northeast Japan. The activity is divided into threestages (ca. 0.6 Ma ; Stage Ⅰ, ca. 0.16Ma to 20ka; Stage Ⅱ, 20ka to present; Stage Ⅲ). In historical age, magmatic and phreatic
eruptions occurred at least three and four times, respectively. One of the historical magmatic eruption occurred two years after the
big earthquake of AD 869. Thus, this volcano has a potential to erupt after the 2011 off the Pacific coast of Tohoku Earthquake,but no precursory phenomena have been detected yet. To reveal the magma feeding system, we examined geologic and petrologic
features of youngest (AD1800-1804) magmatic eruption products.
The AD1800-1804 eruption products are composed of Shinzan lava (SL), Shinzan pyroclastic fall deposits (SPFD) and Shinzan
dome lava (SLD) in ascending order. SL is consisted of mainly two lava flows, one flowed to the southward and the other tonorthward from SLD. Maximum thicknesses of southern and northern lobe are 50m and 25m, respectively. Total volume is ca.
7.3×10−3 km3 . These are massive lavas and the surfaces are composed of the blocky lava. Mafic inclusions can be observed inthese lavas. SPFD drapes the SL within ca. 50m from SLD. The thickness varies and reaches ca. 30cm near the base of SLD. The
estimated volume is 9.0×10−7 km3 . The deposit is lapilli tuff to volcanic breccia, composed of mainly angular lithic fragments
in coarse ash matrix. Most of the fragments have shiny surfaces. Subangular to subrounded pumice is rarely observed, whose
diameter reaches ca. 5cm. Volcanic bombs are observed in the area of ca. 500 m radius from SLD. These usually cover the
SPFD, thus we include them into the SPFD. These are mainly breadcrust bomb and subsequently breccia bomb, which looks likea fragment of the explosion breccia. Mafic inclusions are generally seen in breadcrust bombs, but rarely breadcrust bombs are
made of only mafic inclusion. Breccia bombs are consisted of angular rubbles of several centimeters to one meter in weak welded
matrix. SLD forms a peak dome of Chokai volcano. Relative elevation of SLD is 50m, volume is 9.0×10−4 km3 . Columnar andplaty joints can be observed inner of dome. Mafic inclusions are seen.
AD1800-1804 eruption products belong to medium-K to high-K, calk-alkaline series. Phenocryst assemblage is plagioclase,
orthopyroxene, clinopyroxene, opaque ± olivine and hornblende. Plagioclases with resolved texture such as dusty zone andhoney comb texture dominate than the clear type. Orthopyroxene and olivine sometimes have a reaction rim, but hornblendedoesn’t have it. Volumes of total phenocrysts in the hosts are high in SL (48-50vol.%) than the others (26-36vol.%). Those ofmafic inclusions are 20-30 vol.%. Groundmass texture of host lava is hyaloophitic, while that of inclusion is dikty-taxitic. Sizeof microlites in the inclusions varies by samples. SiO2 contents of host lavas are ca. 60-62wt.% (SL, 60.7-60.8 wt.%; SPFD
pumice 60.7-61.2 wt.%; SPFD lithic fragments, 60.8-61.1 wt.%; SPFD volcanic bomb, 60.5-61.5 wt.%; SPFD breccia bomb,
60.7-61.0 wt.%; SLD, 61.2-62.2 wt.%) and those of mafic inclusions are ca. 52-57wt.% (SL 53.3 wt.%; SPFD lithic fragments,
54.2 wt.%; SPFD volcanic bomb, 56.5 wt.%; SPFD breccia bomb, 52.6-54.8 wt.%; SLD 54.0-55.8wt.%). All eruption products
are depicted on the same linear trends in silica variation diagrams.
Keywords: Chokai volcano, eruption history

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SVC47-P12

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Time:May 26 18:15-19:30

Eruptive sequence and magma system of the latest large silicic eruption of Kuttara volcano(Kt-1), southwestern Hokkaido
NISHIOKA, Hirotaka1∗ ; NAKAGAWA, Mitsuhiro1
1

Department of Natural History Sciences Hokkaido University

Kuttara volcanic group, located in southwestern Hokkaido, started its eruptive activity ca. 80 ka. The volcano has repeated thelarge silicic eruptions several times. The youngest large silicic eruption (Kt-1) occurred ca. 42 ka, forming the present Kuttara
caldera (Yamagata, 1994; Moriizumi, 1998). Total volume of Kuttara volcanic group is more than 100 km3 , being comparable tothose of Shikotsu and Toya caldera volcanoes that locate near Kuttara volcanic group. However, it has not been still understood
the reason for the multiple eruptions of voluminous silicic magma. In order to resolve this problem, it is important to reveal theeruptive sequence and the evolution of magma system. Therefore, we has carried out the geological and petrological studies about
Kuttara volcanic groups. In this presentation, we reexamine the eruptive sequence of Kt-1 eruption and discuss the relationshipbetween eruptive sequence and magma system.
Moriizumi (1998) recognized that Kt-1 pyroclasts are composed of pyroclastic falls (Kt-1pfa), pyroclastic flow (Kt-1pfl), andpyroclastic surge (Kt-1ps). Kt-1pfa is divided into lower and upper units (Kt-1 pfa1 and Kt-1 pfa2, respectively) on the basis ofscoria fall deposits (Kt-1sfa). Kt-1 pfa1 includes hornblende-bearing pumices, whereas Kt-1 pfa2 is absent from such pumices.
Kt-1 pfl has also no hornblende in pumices, and therefore, it is interpreted that Kt-1 pfa2 and -pfl occurred simultaneously.
In this study, we can divide pyroclastic falls into five units according to the time gaps on the basis of weathered layers of the
top of each unit: from Kt-1A to Kt-1E in ascending order. In Kt-1A, -1B and -1C, there are some pumices including hornblende,
whereas pumices in Kt-1D to Kt-1E have no hornblende. This suggests that units from Kt-1A to Kt-1C and from Kt-1D to Kt-1E
respectively correspond to Kt-1 pfa1 and Kt-1 pfa2 defined by Moriizumi (1998). In addition, we recognize the pyroclastic flow
and surge deposits, agreeing with Kt-1pfl and Kt-1ps, respectively. The pyroclastic falls comprise of white and banded pumices,and pyroclastic flow includes white pumice, scoria, and banded pumice˜scoria. Phenocrystic minerals of all the juveniles arequartz, plagioclase, orthopyroxene, clinopyroxene, and opaque minerals. On whole-rock chemistry, the juvenile materials of
Kt-1 eruption show 59.2-74.1 wt.% in SiO2 . We identify four compositional trends, being clearly different in many Harker
diagrams. On the basis of the compositional trends as well as mineral assemblage, we classified Kt-1 juveniles into four types asfollows. Type1 is the most silicic rhyolite including hornblende. Type2 is a hornblende-free rhyolite and rhyodacite rock. Type3is dacite having a small amount of hornblende. Type4 is characterized by wide compositional variation from rhyolite to andesite
without hornblende. The compositional trends of Type1, -2, and -3 are clearly distinct at both silicic and mafic sides. In contrast,
Type4 shows the compositional trend converging to Type2 at silicic side.
Considering the temporal change of juvenile types, components change with the progress of eruption. Type1 and -3 are present
in Kt-1A. In Kt-1B and -1C, however, Type1 disappears and Type2 and -3 are found. In Kt-1D and -1E, only Type2 occurs. On
the other hand, in Kt-1pfl, there is only Type4 juveniles. The difference in types of juveniles between pyroclastic falls andflow suggests that pyroclastic flow did not occur simultaneously with Kt-1D and -1E (Kt-1pfa2). The compositional trends forwhole-rock chemistry of Type2 and -4 converge at silicic side. This indicates that pyroclastic flow generated after pyroclastic fall
eruptions.
We recognize four types of juvenile materials in Kt-1 eruption. That is, four types of magma erupted in Kt-1 eruption. Thesemagmas had been active in turn as the eruption progressed, suggesting the relationship between eruptive sequence and magma
system.

Keywords: Kuttara volcano, caldera volcano, caldera-forming eruption, silicic magma, Petrological features, magma chemistry

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SVC47-P13

Room:Convention Hall

Time:May 26 18:15-19:30

Petrology of pre-caldera eruption of Shikotsu volcano (Shadai pyroclastic flow), Southwestern Hokkaido
AMMA-MIYASAKA, Mizuho1∗ ; NAKAGAWA, Mitsuhiro1 ; HASE, Ryuichi1
1

Sci., Hokkaido Univ.

Shikotsu volcano started large-scale eruption at 60ka, and erupted scoria fall and scoria flow (Shadai pyroclastic flow=Shadaipfl) deposits. The volcano repeated explosive eruptions every several thousands years, and after 10ky dormancy, caldera-forming
eruption took place at 40ka. Eruptive volume of pre-caldera eruption is less than 50km3 and that of caldera-forming eruption
is ca. 400km3 (Yamagata, 1994). We collect samples of Shadai pfl from continuous boring core, 5km-south of lake Shikotsu(JMA-V05), and clarify petrological characters to compare with those of caldera-forming eruption.
The boring core consists of soil (0-1.8m), reworked deposits (1.80-2.60m depth), Shadai pfl (2.60m-3.40m), reworked deposits (3.40m-4.25m), Shadai pfl (4.25m-18.15m), reworked deposits (18.15m-19.90m) and Shadai pfl (19.90m-101.00m), andthe lowest part of pfl cannot be seen (Takarada and Furukawa, 2010). Shallow part of Shadai pfl (<18.15m) is non-welded.
Juvenile materials consist mainly of banded pumice and small amount of white pumice. Weakly to strongly welded parts exist in
the deeper part (53.40m-72.00m and >80.90m are strongly welded). Most of the juveniles of these parts are scoria and banded
pumice.
Juvenile materials of Shadai pfl are SiO2 =53-62wt% andesite and dacite. They contain 20-40vol% phenocrysts of plagioclase,
orthopyroxene, clinopyroxene and oxides (sometimes accompanying olivine). Banded pumice is heterogeneous in microscopic
order. They usually make linear trends on Harker diagrams. SiO2 contents of white pumice are SiO2 >60wt%, and most of scoria
are SiO2 <56wt%. Furthermore, SiO2 contents changes with depth. Rocks of <18.15m consist only of SiO2 =60-62wt% dacites,
but SiO2 -poor materials appear from 19.90m. Andesites of SiO2 <53wt% are often found in 53.40m-72.00m and compositional
range becomes the maximum. Then, SiO2 contents concentrate to 55-62wt% in >72.00m. REE contents increase with SiO2 ,however chondrite-normalized REE patterns are scattered in LREE. Ratios of MREE/LREE and HREE/LREE, together with
Y/Rb, Zr/Rb and Ba/Rb, decrease as increasing SiO2 . Isotopic ratios of Sr and Nd are nearly constant for wide SiO2 range.
Existence of banded pumice, heterogeneous texture under microscope and linear compositional trends suggest that Shadai pfl
is produced mainly by magma mixing of mafic and felsic magmas. Difference of compositional range of each depth reflects thatmixing ratios has changed with time. According to the constant isotopic ratios and decreasing of MREE-HREE/LREE, Y/Rb,
Zr/Rb, Ba/Rb with increasing SiO2 , end-member magmas cannot be produced by simple crystal differentiation but by different
degree of partial melting from the same crustal material.
Caldera-forming eruption of 40ka started with phreato-magmatic eruption and then occurred plinian eruption to flowed down
the pyroclastic flow. The juveniles are classified into 2 types; dacitic to rhyolitic A-type (<5vol% phenocrysts, SiO2 =74-78wt%)
and andesitic to dacitic P-type (7-45wt% phenocrysts, SiO2 =57-72wt%). A-type rocks can be seen throughout the calderaforming eruption, but P-type exists only in the upper unit of pyroclastic flow (Nakagawa et al., 2013). Phenocryst assemblageis plagioclase, orthopyroxene, clinopyroxene, hornblende and oxides. There are evidences of magma mixing for both A and P
types, however end-member magmas are different from each other. Nakagawa et al. (2010) shows that felsic magma of Shadai
pfl is similar to P-type dacite of caldera-forming eruption, and suggests that the felsic magma mixes with mafic magma of low
Sr isotope. As we reanalyze REE compositions and isotopic ratios of caldera-forming eruption, Sr isotopic ratio of Shadai pfl isslightly lower and Nd ratio is higher than caldera-forming eruption and Nd isotope of these eruptions make parallel trends. Thesesuggest that origin of these eruptions is different and new magma should be produced after pre-caldera eruption.
Keywords: Shadai pyroclastic flow, pre-caldera eruption, caldera-forming eruption, magma mixing, origin of magmas

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SVC47-P14

Room:Convention Hall

Time:May 26 18:15-19:30

The sequence of caldera-forming eruption of Shikotsu caldera inferred from component
analysis
TOMIJIMA, Chiharu1∗ ; NAKAGAWA, Mitsuhiro1
1

Department of Natural History Sciences, Graduate School of Science, Hokkaido University

Shikotsu caldera, located at southeastern Hokkaido, was formed ca. 42ka. The caldera forming eruption has been studied indetail. The eruption started with phreatomagmatic eruption, followed by plinian eruption to deposit. In final, pyroclastic flows(Spfl) were produced to form the Shikotsu caldera (Katsui, 1959). Yamagata (1992) revealed the detail sequence of the calderaforming eruption and emphasized the presence of lag-breccia in the upper part of Spfl to discuss the formation of the caldera.
In addition, Nakagawa et al. (2006) revealed temporal variation of magma type during the caldera-forming eruption to discuss
the relationship between caldera formation and magmatic processes. However, the relationship between eruption sequence andtemporal variations of eruptive materials has not been well revealed. Recently, good outcrops have occurred at the southern areaof Shikotsu caldera, where the whole sequence of the caldera-forming eruption can be observed. We describe these deposits
and carry out component analysis of representative samples. Based on these data, we discuss the eruptionsequence and caldera
formation processes. In newly formed outcrops, we can observe tephra layers for 60 ky. The tephra of the caldera- forming
eruption of Shikotsu caldera can be divided into 5 eruptive phases, based on the mode of eruption and emplacement. Phase1 isdefined as phreatomagmatic deposits. After that, a seriesof plinian eruptions occurred(Phase 2). After possible erosional gap,
large scale and high energy pyroclastic flow erupted (Phase3). The lowermost flow sometimes include the block of deposits of
the Phase 2, suggesting that the flow was energetic to erode the surface. Following pyroclastic flows of Phase 4 are characterized
by the presence of thick lithic concentration layer. After that, the scale of eruption had decresed to depost thin surge deposits andpumice fall deposits (Phase5).
We collected sampls for the component analysis from 40 horizons. Lithic fragments of each unit composed of sedimentaryrocks (shale and sandstone), volcanic rocks (two pyroxene andesite), and altered rocks. And rarely contain fragments of plutonic
rocks and minerals. The weight ratio of lithic in these layers is usually less than 30%. Not only the weight ratio of lithic breccia
but also the ratio of lithic types temporally change. The weight ratio of lithic breccia becomes quite high in the upper layer ofPhase 2. These suggest the vent widening and/or migration during early stage of the caldera-forming eruption. In Phase 3, thetypes of lithic breccia had not changed. The lithic concentration layer in the deposts of Phase 4 is most voluminous among all
deposits. We will investigate the temporal variation of juvenile materials based on our newly revealed stratigraphy.
Phase1 is characterized by high content of volcanic rocks but, in early plinian eruption, that generally as high as 50 wt.%. And,in the plinian eruption medium phase, volcanic rock on behalf of the sedimentary rocks content increases. This suggest vent is
moved from phase1 through phase2, after that column became unstable by vent expansion. And when phase2 late dropped toabout 20 wt.%, became higher of content of volcanic rocks and altered rocks , and there is also unit reaches 70%. Therefore,in phase2 later, expansion and movement of the vent is suggested on a large scale. Lithic fragments type in phase2 later feature
is followed by phase 3. That suggests a large scale vent movement and expansion was not over the pyroclastic flow eruption in
Phase 3. Lithic breccia layer containing more than 70% of the lithic fragments in phase 5 is the most large scale in this outcrop,
also is observed new volcanic rock type. This shows activities from a new vent and the vent expansion on a large scale. Thus,
this phase was a mature stage of caldera-forming. In the future, in addition to the process of caldera-forming eruption to attempt
to elucidate the detailed magma transition.

Keywords: caldera-forming eruption, tephrastratigraphy, component analysis, eruption sequence

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SVC47-P15

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Time:May 26 18:15-19:30

Magma mixing processes and origin of mafic inclusions for the Mikurasawa lava in the
Taisetsu volcano, Hokkaido, Japan
BABA, Hikaru1 ; WADA, Keiji1∗
1

Hokkaido University of Education at Asahikawa

The Mikurasawa lava, in the Taisetsu volcano is a lava flow erupted from the outside in southeastern the Ohachidaira caldera.
Most of the lava consist of andesite (SiO2 =60.6-57.3 wt. %). Some parts of the lava consist of dacite (SiO2 =63.6-62.4 wt. %)and banded lava constructed with andesite and dacite. In addition, the andesite has some mafic inclusions (SiO2 =58.0-51.3 wt.
%). The Mikurasawa lava is a rare because of coexisting banded lava and mafic inclusion. This study elucidates magma mixing
processes and origin of mafic inclusions of the Mikurasawa lava by petrological method.
I considered that the andesite of the Mikurasawa lava formed by magma mixing on account of existing of banded lava andmafic inclusions, coexisting of plagioclase which have different composition and showing reverse zoning in plagioclase phenocryst. Many plagioclase in the andesite show resorbed texture in core and reverse zoning in rim. It indicates that the andesite
experience several times of injection of mafic magma. I estimate that a period from initial inject to eruption was about 60-600
years by element diffusion rate. The phenocrysts in the andesite are classified into two crystal groups (type-A and -B) by thecore composition. The phenocrysts of each crystal groups derived from same magmas because they form crystal clots eachother. Type-A crystal group consists of high-An plagioclase, high-Mg] orthopyroxene, high-Al2 O3 clinopyroxene and olivine.
These crystals were derived from mafic magma. Type-B crystal group consists of low-An and low-MgO plagioclase, low-Mg]
orthopyroxene and low-Al2 O3 clinopyroxene. These crystals were derived from felsic magma. I estimated the temperature of
the mafic and felsic magmas by two-pyroxene thermometer. The results are about 1000 ℃ and 900 ℃.
The mafic inclusions in the Mikurasawa lava show various texture and composition. I classified them into three types (Type-1,-2 and -3) by size of plagioclase. Type-1 mafic inclusion has fine-grained acicular plagioclase in groundmass. Type-2 mafic
inclusion has coarse-grained tabular plagioclase in groundmass. Type-3 mafic inclusion consists of bigger plagioclase than theother mafic inclusions most of which are phenocrysts.
I considered the origin of each types of mafic inclusions from texture and composition. The type-1 mafic inclusion is characterized by fine-grained crystals and these compositions which are similar to those of microphenocrysts of the andesite. Therefore,
it seems that this mafic inclusion formed by quenching of hybrid layer magma between mafic and felsic magma in the reservoir.
The type-2 mafic inclusion is characterized by coarse-grained groundmass crystals and these compositions which are similar to
those of crystals of dacite. Therefore, it seems that this mafic inclusion formed by disruption and inclusion of crystal mush layerof felsic magma during eruption. The type-3 mafic inclusion is formed by inclusion of mafic magma because of being composedof type-1 crystal group.
I considered magma mixing processes in magma reservoir of the Mikurasawa lava based on above date. First, felsic magma
reservoir having crystal mush layer was injected by mafic magma. This injection occurred partial melt of plagioclase in felsicmagma, forming resorbed texture. Furthermore, at a boundary between felsic and mafic magma, hybrid layer was formed bymagma mixing. After 60-600 years, reinjection of mafic magma occurred eruption. During eruption, andesitic magma formedby mixing of mafic and felsic magmas. Felsic magma which was not affected by mafic magma formed dacite and banded lava.
Hybrid layer and mafic magma included in andesitic magma, forming Type-1 and Type-3 mafic inclusions respectively. Crystal
mush layer was disrupted during magma ascent. The fragment was included in andesitic magma, forming Type-2 mafic inclusion.

Keywords: magma mixing, mafic inclusion, Taisetsu volcano, Mikurasawa lava, plagioclase phenocrysts

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SVC47-P16

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Time:May 26 18:15-19:30

The formation process of obsidian; Insights from the application of TTT diagrams to
Tokachi-Ishizawa obsidian
SANO, Kyohei1∗ ; TORAMARU, Atsushi2 ; WADA, Keiji3
1

Engaru Town Hall, 2 Department of Earth and Planetary Sciences, Faculty of Sciences, Kyushu University, 3 Hokkaido University of Education at Asahikawa
Why obsidian contains low crystal amount, namely mostly glass, though it experiences the effective undercooling and long
crystallization time during magma ascending and cooling, has been a big problem in volcanology. In generally speaking, glasscan be formed because of a high effective undercooling and a short cooling time. However, ascent rates of obsidian lava eruption
are expected to be relatively small.
In order to evaluate the development of glass formation in obsidian lava eruption, we applied Time- Temperature- Transformation (TTT) diagram for natural obsidian lava, and estimated critical cooling rate to form the obsidian. The TTT diagram has beenused to predict the cooling rate to form the glass (Uhlmann, 1982; Weinberg and Uhlmann, 1989; Rao 2002). The TTT diagram
is a contour map of crystallized volume fraction as function of crystallization temperature and time. A contour line for a given
crystallization fraction has the cone shape with its “nose” which corresponds to a minimum time and a temperature required forthe time. We simulated the 2 types of situations; decompression-induced crystallization and cooling-induced crystallization after
decompression. The nucleation and growth rate were calculated based on the classical theory (James, 1985; Hammer, 2004;
Rao, 2002). The crystallized fraction, under the assumption that nucleation and growth rate are constant for time, is given by the
Avrami-Johnson-Mehl equation. We applied TTT diagram for Tokachi-Ishizawa obsidian lava, and estimated the critical coolingrate. Based on the calculation results, critical cooling rates are highly dependent on interfacial energy and the pre-exponential
factor of nucleation rate.
We compared estimated critical cooling rates with cooling rates estimated from the microlites number density (Sano et al.,2015). Our calculation results show that obsidian glass can be formed during decompression and cooling, especially under thehigh interfacial energy. We investigated the glass forming conditions based on parameters such as interfacial energy and preexponential factor, and suggest that obsidian can be formed although it experienced the low effective undercooling and long
crystallization time during eruption.

Keywords: obsidian lava, glass, rock texture, Hokkaido, Shirataki

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SVC47-P17

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Time:May 26 18:15-19:30

Factors governing fragmentation of submarine lava - mechanism of hyaloclastite formation
UMEZAWA, Yumi1∗ ; UMINO, Susumu1 ; KANAYAMA, Kyoko1 ; KUSANO, Yuki1
1

Department of Earth Sciences, Kanazawa University

Hyaoclastite is produced by fragmentation of lava when stress accumulates on solid lava faster than it relaxes and ultimately
reaches the mechanical strength of the lava. Thermal stress, shear stress and tensile stress acculumating on the lava crust are relaxed by viscous flow of lava, which is governed by viscosity. Therefore, fuidal basalt lava tends to form coherent flows without
fragmentation, whereas viscous lava such as andesite and dacite tends to form hyaloclastite. Hyaloclastite with a wide compositional range spanning from andesite to rhyolite is associated with pillow flows and dikes in the Eocene submarine volcanic
strata on Chichijima, Ogasawara Archipelago (Umino and Nakano, 2007). These volcanic ejecta are ideal to assess the effect ofvarying lava composition on the factors that govern the fragmentation of lava.
Quenched glass from chilled margins of pillow and hyaloclastite were collected and analyzed by EPMA for major elements
and by SIMS and FTIR for water contents. The amount of primary, magmatic water was discerned from secondary hydration
by differential thermal analysis. Eruption temperatures were estimated by the clinopyroxene-liquid geothermometer of Putirka(1999). Crystal number densities of groundmass plagioclase and clinopyroxene were determined on COMPO images and modal
abundance of constituent minerals was determined on elemental distribution maps of EPMA. Bulk viscosity of lava was estimatedby the methods of Giordano et al. (2008) and Pinkerton and Stevenson (1992).
Andesite consists of clinopyroxene, orthopyroxene, plagioclase and magnetite as phenocrysts set in a groundmass of clinopyroxene and plagioclase microlites, magnetite and glass. In Nagasaki, pillow lava coexists with hyalcoastite. In transition zones
from pillow lava to hyaloclastite, pillow lobes are scattered in hyaloclastite. Hyaloclastite is higher in crystal number density,mode of groundmass plagioclase and vesicle number density than the associated pillow lava. Hyaloclastite glass is lower in
Al2 O3 than associated pillow glass, indicating plagioclase fractionation. However, the cpx-saturated melt temperatures show
little difference between pillow lava and hyaloclastite. Water contents in glass were determined by using FTIR, which are almost
identical in pillow and hyaloclastite. However, primary water contents estimated by differential thermal analysis are lower inpillow lava than in hyaloclastite. Therefore, degassing either within the conduit or during flowage through lava tubes induced plagioclase crystallization that raised the bulk viscosity of lava and stress relaxation time, resulted in the formation of hyaloclastite.
However, higher water contents in hyaloclastite suggest that the hyaloclastite was formed by spalling off of chilled margin glassfrom pillows.
Dacite has phenocrysts of clinopyroxene, orthopyroxene, plagioclase and magnetite in the groundmass of clinopyroxene andplagioclase microlites, magnetite and elongate or spherical vesicles. The dacite shows little difference in melt composition,
eruption temperature, crystal number density between pillow lava and hyaloclastite. In Ogamiyama and north of Manjumisaki,
lower water contents in hyaloclastite indicate fragmentation occurred due to degassing and increase in bulk viscosity. On theother hand, in Zonahanasaki and Sakaiura, dacite pillow lava has lower water contents than the associated hyaloclastite, which
may have formed by spalling off of chilled margins or by fragmentation of pillow lava crust due to higher shear stress.
Keywords: hyaloclastite, viscosity, submarine lava, the Bonin Islands Chichijima

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SVC47-P18

Room:Convention Hall

Time:May 26 18:15-19:30

Emplacement and solidification process of off-axis large submarine lava field from the
Oman Ophiolite
OTSUKA, Ryo1∗ ; KUSANO, Yuki1 ; UMINO, Susumu1 ; KANAYAMA, Kyoko1
1

Earth Science Course School of Natural System, College of Science and Engineering, Kanazawa Universi

Large submarine lava flows more than 100 m in thickness and a volume exceeding a few cubic kilometers are not uncommon
volcanic constructs of mid-ocean ridges and Hawaiian volcanoes, yet details of the physical processes of emplacement of suchlarge flows are poorly constrained because of their inaccesibilty under deep water and lack of direct observations. The V3 Volcanics of the Oman Ophiolite extruded at 90 Ma far off the paleospreading axis as thick lava flows with a minimum areal extent
of >11 km by 1.5 km and the maximum thickness >270 m, yielding a minimum estimated volume of several cubic kilometers.
The V3 flow was fed through a thick feeder dike in the SW of the flow field and buried off-axial fault-bounded basins with a thicksedimentary cover in ca. 40 days. The basic structure of the V3 flows consists of massive lava sandwiched between columnar
jointed lava crusts, similar to that of subaerial flood basalt. The upper crust comprises piled up flow lobes forming dome-likestructures with occasional inflation cracks, which are interpreted as welded aggregates of coalesced and inflated flow lobes. V3
flow is roughly divided into the Upper and the Lower flow by the presence of pillow lava with interstitial mudstone. Thicknessof individual lobes varies from 2 to 20 m. The uppermost 35 m comprises at least eight welded flow lobes, averaging 3.4 m inthickness.
Low-T hydrothermal alteration and weathering slightly modified the bulk compositions as indicated by moderately albitizedplagioclase, completely replaced olivine by clay minerals and partially replaced titanomagnetite and augite by titanite and actinolite, respectively. However, HFSEs and REEs show mutual positive correlations and relatively good correlations with some major
elements besides LILEs and Pb, indicating that these elements were less mobile and preserve primary characteristics. V3 flowis hawaiitic-mugearitic dolerite and has intermediate characteristics between OIB and E-type MORB. TiO2 shows a moderateincrease with decreasing MgO from 8 to 5 wt%, and then decreases with the decrease in MgO down to 4 wt%, whereas Yb ranges
from 2.12 to 4.56 ppm.
Whole-rock major and trace element variations through a stratigraphical transect at a distance of 8.7 km from the feeder dikeindicate fractionation of augite, plagioclase and magnetite. By contrast, other V3 samples show highly scattered whole-rockcompositions, suggesting internal mixing of variably differentiated magmas. Yb concentrations of the basal crust increase downflow to a distance of 4.5 km from the feeder dike, and then decrease further downflow with a spike at 7 km. Because the basal
crust is the first lava that came to rest at that place, samples farther away from the feeder were extruded and emplaced later inthe eruptive event. The downflow variations show extrusion of differentiated lava in the middle stage of the eruption and lessdifferentiated lava in early and late stages.
Width/length ratio of groundmass plagioclase at 6 km from the feeder, where V3 flow is thickest, is higher in the Upper flow
than in the Lower flow. Stratigraphic variations of Yb shows a decrease from the basal crust to a height of 26 m in the core, andthen increase to a height of 83 m in the upper crust and decrease to the top of the Lower flow. The minimum Yb in the core isclose to that of the latest lava shown by the basal crust. This can be reconciled with the model that the core is formed by the last
intruded lava. On the contrary, the variation in Yb from the height of 83 m to the top of the Lower flow is correlatable to that of
the basal crust at distances from 6 km to 8 km, suggesting that the upper crust consists of piled-up and welded lava lobes.

Keywords: Oman Ophiolite, V3, Large Lava Flow, emplacement process, chemical variation, geochemistry

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SVC47-P19

Room:Convention Hall

Time:May 26 18:15-19:30

K-Ar ages of Samalas-Rinjani volcano cluster, Lombok: pre-caldera variation from migrating to stationary activity.
TOSHIDA, Kiyoshi1∗ ; TAKADA, Akira2 ; KITSUKAWA, Takashi3
1

CRIEPI, 2 AIST, GSJ, 3 Ceres, Inc.

K-Ar dating of Pliocene to Quaternary lavas consisting large volcanoes in Lombok are preformed to determine the active periods of Samalas-Rinjani volcano complex (VC) prior to the caldera eruption in 1257 CE. In order to accurately date the hand
specimen of young lavas, non-spiked argon ratios are analysed to estimate the amount of mass fractionation of argon isotopes at
the time of solidification of the lavas. Consistent ages were obtained for replicate analyses of the four young lavas from Samalasin the range of 0.08-0.04 Ma. Two samples are estimated to have initial argon ratios that are fractionated from atmosphericvalues. The ages form distinct groups that correspond to the active periods of volcano clusters: 2.7 Ma and 2.0-1.8 Ma for West
Lombok VC, 0.5-0.4 Ma for East Lombok VC and 1.0 Ma to present for Samalas-Rinjani. Samalas-Rinjani system should be
defined as single volcano complex based on the relative duration of each active period. Rinjani and the current activity of Segara
Anak caldera are correlated to the younger stages of Samalas-Rinjani. The location of volcanism has been relatively stable forthe past 0.4 million years in both Lombok and Sumbawa, which hosts Tambora volcano. Caldera-forming eruptions of the tworegions (the 1257 eruption and the 1815 eruption at Tambora) occurred at the volcanoes with 1000 km3 class edifice that hadformed through 0.1 to 0.2 million years of volcanic activity. This contrasts clearly with the migration of volcanic activity from 5
to 0.7 million years BP in the two regions.
Keywords: Quaternary, caldera, Indonesia, Sunda arc, radiometric dating, mass fractionation correction method

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