Island Arc (2010) 19, 470–488
Research Article
Evolution of Mount Fuji, Japan: Inference from drilling into the
subaerial oldest volcano, pre-Komitake
iar_722
470..488
MITSUHIRO YOSHIMOTO,1* TOSHITSUGU FUJII,2 TAKAYUKI KANEKO,2 ATSUSHI YASUDA,2
SETSUYA NAKADA2 AND AKIKAZU MATSUMOTO3
1
Department of Natural History Sciences, Faculty of Science, Hokkaido University, N10 W8, Kita-ku, Sapporo,
060-0810, Japan (email: m-yoshi@mail.sci.hokudai.ac.jp), 2Earthquake Research Institute, University of Tokyo,
1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan, and 3Geological Survey of Japan, National Institute of Advanced
Industrial Science and Technology, Tsukuba Central 7, 1-1, Higashi 1-Chome, Tsukuba, Ibaraki 305-8567, Japan
Abstract A buried, old volcanic body (pre-Komitake Volcano) was discovered during drilling into the northeastern flank of Mount Fuji. The pre-Komitake Volcano is characterized
by hornblende-bearing andesite and dacite, in contrast to the porphyritic basaltic rocks of
Komitake Volcano and to the olivine-bearing basaltic rocks of Fuji Volcano. K-Ar age
determinations and geological analysis of drilling cores suggest that the pre-Komitake
Volcano began with effusion of basaltic lava flows around 260 ka and ended with explosive
eruptions of basaltic andesite and dacite magma around 160 ka. After deposition of a thin
soil layer on the pre-Komitake volcanic rocks, successive effusions of lava flows occurred at
Komitake Volcano until 100 ka. Explosive eruptions of Fuji Volcano followed shortly after
the activity of Komitake. The long-term eruption rate of about 3 km3/ka or more for Fuji
Volcano is much higher than that estimated for pre-Komitake and Komitake. The chemical
variation within Fuji Volcano, represented by an increase in incompatible elements at
nearly constant SiO2, differs from that within pre-Komitake and other volcanoes in the
northern Izu-Bonin arc, where incompatible elements increase with increasing SiO2. These
changes in the volcanism in Mount Fuji may have occurred due to a change in regional
tectonics around 150 ka, although this remains unproven.
Key words: basalt, drilling core, eruptive history, Fuji Volcano, hornblende dacite, Komitake Volcano, pre-Komitake Volcano.
INTRODUCTION
Mount Fuji is situated in a complex tectonic setting
near the junction of three plates (North American
Plate in the east, Eurasian (or Amur) Plate in the
west, and Philippine Sea Plate in the south), under
two of which the Pacific Plate subducts from east to
west (Fig. 1a). It is one of the largest stratovolcanoes in the Japan arc with high, long-term eruption rates (5 km3/ka), as compared with typical
subduction-related volcanoes (0.1–0.01 km3/ka;
Tsukui et al. 1986). Fuji Volcano, the youngest part
*Correspondence.
Received 25 April 2009; accepted for publication 23 April 2010.
© 2010 Blackwell Publishing Asia Pty Ltd
of Mount Fuji, has grown since about 100 ka
through dominant eruptions of basaltic lava flows
and tephra. The geochemistry is characterized by
a wide variation of incompatible elements with
little change in SiO2. It differs from other volcanoes of the northern Izu-Bonin arc (Fig. 1), which
show a coupled variation between incompatible
elements and SiO2 (Tsuya 1971; Fujii 2001).
The unique chemical characteristics of Fuji
Volcano have been discussed in relation to its tectonic setting by several investigators (e.g. Arculus
et al. 1991; Takahashi 2000; Fujii 2007). Fujii (2007)
proposed that the chemical variations were formed
by fractional crystallization of a hydrous magma in
a deep magma chamber. That is, in hydrous basalt
magma at high pressures, pyroxenes become the
doi:10.1111/j.1440-1738.2010.00722.x
Evolution of Mount Fuji
471
Fig. 1 Tectonic setting and location of Mount Fuji and drilling sites. (a) Tectonic setting of the study area. PHS, Philippine Sea Plate; EUR, Eurasian Plate;
NAM, North American Plate; PAC, Pacific Plate. (b) Quaternary volcanoes around Mount Fuji. Solid circles are Higashi-Izu Monogenic volcanoes (c)
Locations of drilling sites, ERI-FJ-1 to -5 of the Earthquake Research Institute (ERI), University of Tokyo. Maps, ‘Fujisan’ and ‘Subashiri’, published by the
Geographical Survey Institute, Japan, were used. Dotted lines indicate a horseshoe-shaped crater.
major phase to crystallize, instead of olivine and
plagioclase. He proposed that the Fuji Volcano
magma chamber is located at a depth of 15–25 km,
based both on petrology and on seismological
investigations (Lees & Ukawa 1992; Ukawa 2005;
Nakamichi et al. 2007). On the other hand, Takahashi (2007) proposed an upwelling of mantle
material through a split within the subducted Phil-
ippine Sea Plate just beneath Mount Fuji, based on
current GPS measurements and on the absence of
earthquakes at the subducting slab surface under
Mount Fuji (Yamaoka 1995). However, the geometry of the subducted slab and seismological evidence (Seno 2005) do not support this proposal. To
explore the tectonic background controlling the
high eruption rate and chemical evolution of this
© 2010 Blackwell Publishing Asia Pty Ltd
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M. Yoshimoto et al.
volcano, detailed temporal, geologic and chemical
changes in this volcano, including older volcanic
activity hidden by the younger edifice, need to be
examined, along with the relationships of the surrounding volcanoes.
Scientific drilling, a good way to investigate the
temporal, geological and chemical development of
Mount Fuji in detail, was sponsored by the Ministry of Education, Culture, Sports, Science and
Technology, Japan, and was carried out during
2001–2003, soon after the occurrence of a spate of
deep-seated, low frequency earthquakes. In this
paper, we describe lithological features, wholerock chemistry and K-Ar ages of the drilled core
samples, aiming to provide new constraints on the
volcanological and geochemical evolution of Mount
Fuji.
TECTONIC SETTING AND GEOLOGY OF THE FUJI
VOLCANIC FIELD
Collision of the Philippine Sea Plate and the North
American Plate resulted in accretion of the upper
crust onto the North American Plate and subduction of the lower crust beneath the North American Plate (e.g. Seno 2005). The boundary between
the North American and Philippine Sea Plates
jumped to the south periodically during repeated
accretion episodes. The upper crust accreted from
the Philippine Sea Plate forms the present Izu
Peninsula. Mount Fuji is located just behind the
present plate boundary, which runs from west of
Ashitaka Volcano to the northeast of Hakone
Volcano (Fig. 1b). Deep depressions suggesting
possible remnants of an older plate boundary are
found to the west and north of Mount Fuji (Amano
1991).
Fuji Volcano has grown to cover a part of Ashitaka Volcano in the south and most of Komitake
Volcano in the north (Tsuya 1971). Results from
drilling on a saddle between Mount Fuji and Ashitaka indicate that olivine basalt of Fuji Volcano
covers olivine-two-pyroxene andesite of Ashitaka
Volcano at about 200 m above sea level (a.s.l.)
(Tsuya 1962). At present, Komitake Volcano is
exposed on the north flank of Mount Fuji (Fig. 1c).
These volcanoes had been active before the initial
eruptions from Fuji Volcano at about 100 ka.
ASHITAKA VOLCANO
Ashitaka Volcano, located on the southeastern foot
of Mount Fuji, is a dissected stratovolcano. Its
© 2010 Blackwell Publishing Asia Pty Ltd
activity initiated with eruption of basalt to basaltic
andesite lava flows and pyroclastics at around
400 ka and ended with eruption of a hornblendebearing dacite lava dome around 100 ka (Yui &
Fujii 1989). Its rocks are within the range of the
high-alumina basalt and pigeonitic rock series of
Kuno (1968). The andesite and dacite fall within
the calc-alkaline hypersthenic rock series.
KOMITAKE VOLCANO
The edifice of Komitake Volcano is exposed only in
a horseshoe-shaped crater that opens to the northeast on the northern middle slope (about 2300 m
a.s.l.) of Mount Fuji (Fig. 1). Lava flows, each
0.3–1 m thick, are exposed inside the crater. The
center of Komitake Volcano, estimated from dips
of the lava flows, is 0.7 km east-northeast of
the present peak of Komitake (Tsuya 1971). Its
rocks are plagioclase-phyric, olivine-bearing, twopyroxene basalt and basaltic andesite with
51–53 wt% SiO2 (Tsuya 1968). Its activity is
believed to have started at the same time as Ashitaka Volcano, in the middle-Pleistocene, based on
the similar extent of erosion of the two volcanoes
(Tsuya 1971). Tsuya (1971) also considered Komitake and Ashitaka volcanoes to be chemically
similar.
FUJI VOLCANO
Fuji Volcano consists of Older-Fuji and YoungerFuji stages (Machida 1964; Tsuya 1971; Miyaji
1988). Older-Fuji Volcano began its activity at
about 100 ka; a silicic tephra marker called ‘On
Pm-I’ has been identified just below the lowest
part of the Older-Fuji ejecta (Machida 1964).
Eruptions of Older-Fuji are thought to have been
explosive owing to wide-spread basaltic air-fall
deposits with a volume up to 250 km3 that covered
abroad area from the eastern flank of Mount Fuji
to the south Kanto Plain, including Tokyo (Miyaji
et al. 1992). Mudflow deposits are plentifully distributed on the eastern and western flanks of
Mount Fuji (Machida 2007). These Older-Fuji
ejecta and the mudflow deposits are covered by the
Fuji black soil (FB), dated to 10-5 ka (Machida
1964). Younger-Fuji Volcano has been active since
20 ka with eruptions of basaltic lava flows (Miyaji
1988; Yamamoto et al. 2005). Its volcanism is subdivided into six stages with different eruption
styles (Miyaji 1988). Dacite and andesite tephra
was formed during recent explosive activities, as
997.2
1487
138°50′55″E
138°48′00″E
35°25′10″N
35°22′04″N
ERI-FJ-4
ERI-FJ-5
C, using a core pack sampler with a thin plastic tube; W, using an HQ size (core diameter of 61 mm) triple tube wireline core barrel.
0–129
115–365
352–650
0–75
0–200
129
365
650
75
200
1406.7
138°46′38″E
35°24′36″N
ERI-FJ-3
35°24′40″N
ERI-FJ-2
FJ-3
FJ-3B
FJ-3C
FJ-4
FJ-5
0–100
75–414
100
414
FJ-2
FJ-2A
1441.8
35°23′42″N
ERI-FJ-1
138°44′59″E
0–100
100
FJ-1
1722.7
Hole
Elevation (m)
Longtitude
Latitude
Site
Table 1 Location, drilling depth and cored range of the boreholes in Mount Fuji
Drilling was conducted on the northern and
eastern flanks of Mount Fuji at five sites (ERIFJ-1 to -5) (Nakada et al. 2004) (Fig. 1, Table 1). To
monitor the volcanic activity of Fuji Volcano,
sensors were installed at the three sites (FJ-1 to
-3) on the north to northeastern flank: seismometers (all three sites), strainmeter (FJ-2), and
tiltmeter and thermometer (FJ-3). The other two
sites, FJ-4 and -5, on the eastern flank, were
drilled to further understand the eruptive history
of Fuji Volcano. FJ-1 (1722.7 m a.s.l.) was drilled to
a depth of 100 m. FJ-2 (1441.8 m a.s.l.) consists of
two holes 100 m and 414 m in depth. Geophysical
logging, consisting of normal resistivity, spontaneous potential, acoustic, density, neutron, seismic
velocity, natural gamma ray, temperature, Fullbore
Formation MicroImager (FMI, microresistivity
formation images) and borehole caliper, was
carried out in both holes for depth intervals of
1–100 m and 75–414 m, respectively. FJ-3
(1406.7 m a.s.l.) consists of three holes 129 m,
365 m and 650 m in depth. Geophysical logging was
carried out for depth intervals 0–129 m, 115–365 m
and 352–650 m, respectively. FJ-4 (997.2 m a.s.l.)
and FJ-5 (1487 m a.s.l.) were drilled to 75 m and
200 m, respectively. Core samples from shallower
depths were obtained using a core pack sampler
with a thin plastic tube, and those from deeper
depths were obtained using an HQ size (core diameter of 61 mm) core barrel (Nakada et al. 2004)
(Table 1). Diameters of the recovered core samples
range from 61 to 70 mm. Total core recovery was
more than 94%.
We describe herein characteristics of rocks and
deposits observed in drillholes FJ-1 to FJ-3. FJ-4
and FJ-5 are not described since they are composed of only younger rocks (Fig. 2). Photographs
of core samples with typical lithofacies are shown
Drilling depth (m)
CORE DESCRIPTION FROM DRILL HOLES IN
MOUNT FUJI
138°45′58″E
Cored range (m)
Method (m)
observed in the Hoei eruption (AD 1707) and the
Zunasawa eruption (c. 3 ka).
The dominant magma type for both Older- and
Younger-Fuji volcanoes is high-alumina basalt (48–
52 wt% SiO2), in which phenocrysts are mainly
plagioclase and olivine with a minor amount of clinopyroxene. Plagioclase-phyric basalt without
olivine phenocrysts is rarely observed (Tsuya
1971). The dacites contain orthopyroxene phenocrysts without hornblende.
473
W: 0–27
C: 27–100
C
C: 89–200
W: 75–89, 200–414
C
C
W
C
C
Evolution of Mount Fuji
© 2010 Blackwell Publishing Asia Pty Ltd
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M. Yoshimoto et al.
Fig. 2 Lithostratigraphy of the boreholes (ERI-FJ-1 to -5). The locations of the drilling sites are indicated in Figure 1.
in Figure 3. Core depths (m) for lavas given in the
text indicate those at the top of flows.
ERI-FJ-1 (FJ-1)
FJ-1 is composed of lava flows, mudflow deposits
and unspecified deposits, lacking both soil layers
and layers of well sorted and monolithologic
© 2010 Blackwell Publishing Asia Pty Ltd
angular scoria, classified as air-fall deposit
(Fig. 2). Five lavas are observed, each with a
massive center and fractured upper and lower
margins, sometimes reddish in color due to
oxidation. Rocks are basalt with about 15 vol.%
plagioclase, olivine, clinopyroxene and rare orthopyroxene phenocrysts. Mudflow deposits are
subdivided into two types: a moderately sorted
Evolution of Mount Fuji
475
Fig. 3 Photographs of recovered drilling cores from ERI-FJ-2 and -3, showing typical occurrences for ERI-FJ-1 to -3. (a) Scoriaceous mudflow (slush
flow) deposit from 25 to 30 m depth of the FJ-3. (b) Basaltic lava flow, orthopyroxene-bearing olivine basalt with 15 vol.% phenocrysts and an underlying
burned red scoria fall deposit from 90 to 95 m in FJ-2. (c) Plagioclase-phyric (35 vol.%) basaltic andesite lava flows and alternation of scoria fall deposits
and weathered ashy soils from 170 to 175 m in FJ-2. (d) Hornblende dacite lava clasts, which are bluish-gray in color and less than 12 cm long, and
two-pyroxene andesite and basaltic andesite in the mudflow deposit from 185 to 190 m in FJ-2. (e) Andesitic pyroclastic flow deposit from 190 to 195 m
in FJ-2. (f) Massive basaltic andesite lava flow with about 30 vol.% phenocrysts consisting mainly of plagioclase and a small amount of clinopyroxene,
orthopyroxene and olivine from 215 to 220 m in FJ-2. (g) Typical polymict mudflow deposit from cores at 405 to 410 m in FJ-2. The mudflow is composed
of large blocks of lava in a fine-grained matrix of volcanic material.
scoriaceous one and a poorly sorted polymictic
one. The former consists of rounded scoria
without fine ash, and resembles a deposit from a
mixture of snow and scoria commonly observed
during seasons of snow melting at Mount Fuji
(Anma 2007). The latter contains abundant lava
clasts up to 10 cm across and fine ash particles in
the matrix. The unspecified deposits consisting of
well-sorted clasts of lava and scoria are either
fragmented clinker from lava flows or coarse
clasts of mudflows, in which fine particles were
washed out during the drilling operation.
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M. Yoshimoto et al.
Fig. 4 Photomicrographs showing
typical rocks in the recovered cores: (a)
Olivine basalt from 64 m in FJ-2; (b)
Plagioclase-phyric basaltic andesite
from 131 m in FJ-2; (c) Hornblende
dacite from 177 m in FJ-2; (d)
Clinopyroxene-hornblende
andesite
from 447 m in FJ-3; (e) Two-pyroxene
basaltic andesite from 355 m in FJ-3;
and (f) Two-pyroxene olivine basaltic
andesite from 376 m in FJ-2. ol, olivine;
cpx, clinopyroxene; opx, orthopyroxene;
hbl, hornblende; pl, plagioclase.
ERI-FJ-2 (FJ-2)
Three units are defined in FJ-2, whose depth intervals are 0–95 m, 95–174 m, and 174–414 m, based
on the petrographical and lithological signatures
of the core (Fig. 2). FJ-2 unit-1 (0–95 m) consists
of seven lavas. Lavas at depths of 5, 61.9 and
90.4 m are olivine basalt containing 15–25 vol.%
phenocrystals, with or without clinopyroxene
(Figs 3b,4a). Lavas at depths of 21, 25 and 26.8 m
are basalt containing less than 10 vol.% phenocrysts, rarely with olivine. The lava at a depth of
15.5 m is plagioclase-phyric (30 vol.% phenocrysts)
basalt with a small amount of olivine. The lava at a
depth of 61.9 m is 13 m thick, while the other six
are less than 3 m thick. Fuji black soil (FB)
(Machida 1964) of 10 to 5 ka is observed at a depth
of 9.2 to 9.3 m. Tephra ‘On Pm-I’, characterizing
the base of the Older-Fuji volcano, was not found.
More than ten layers of well sorted scoria fall
deposits were found.
© 2010 Blackwell Publishing Asia Pty Ltd
The middle FJ-2 unit-2, at a depth interval of
95–174 m, is composed of plagioclase-phyric basalt
(30–35 vol.%) containing small amounts of clinopyroxene, orthopyroxene and olivine phenocrysts
(Figs 3c,4b). The maximum length of plagioclase
phenocryst reaches 3 mm. Lavas with thickness of
1 to 4 m are found at depths of 98.5, 103.8, 122,
125.9, 129.8, 134.8, 139.1, 155, 165.4 and 170.5 m
(Fig. 2). These are intercalated with poorly sorted
lava clast-rich mudflow deposits, scoriaceous
mudflow deposits and unspecified deposits. The
unspecified deposits are composed of lapilli of lava
fragments without fine particles.
The lowest FJ-2 unit-3, from a depth of 174 m
to the bottom of the hole at a depth of 410 m, is
characterized by the occurrence of two-pyroxene
andesite and hornblende dacite. Lavas and lava
clasts of basalt and basaltic andesite are intercalated with small amounts of two-pyroxene andesite
and hornblende dacite (Figs 3,4). At the top of this
unit (174–176.6 m), there are three layers of scoria
Evolution of Mount Fuji
fall deposits separated by soil layers (Fig. 3c). At
depths of 176.6–190 m, a poorly sorted mudflow
deposit includes clasts of hornblende dacite, and
two-pyroxene andesite and basaltic andesite
(Fig. 3d). Some of the basaltic andesite here is
characterized by large phenocrysts of pyroxene.
At depths of 190–201 m, two layers of very poorly
sorted and monolithologic pyroclastic flow deposits
of two-pyroxenes andesite are found, the bottoms
of which are oxidized (Fig. 3e). The lowest section
(at a depth interval of 201 m to 414 m) consists of
nine lavas and several polymict mudflow deposits.
The latter contain large, massive blocks up to
100 cm across (Fig. 3g). Lavas and most clasts in
the mudflow deposits are plagioclase-phyric olivine
basalt and basaltic andesite, with or without clinopyroxene, and orthopyroxene basaltic andesite,
with or without olivine and/or clinopyroxene,
both of which contain 25–40 vol.% phenocrysts.
Lavas at depths of 210 m, composed of olivine-twopyroxene basaltic andesite, and 350 m, composed
of clinopyroxene-bearing olivine basalt, are 17 m
and 10 m thick, respectively. Oxidized, reddish
scoria fall deposits more than 1 m thick lie beneath
these lavas (Fig. 3f). The other lavas at depths of
202.5, 338, 365.7, 375, 378, 388.8 and 393.6 m are
less than 2 m thick.
ERI-FJ-3 (FJ-3)
FJ-3 is composed mainly of mudflow deposits
intercalated with six lavas and scoria fall layers.
Three different units are also recognized in FJ-3 at
depths of 0–75 m, 75–315 m, and 315–650 m, based
on the occurrence of mudflow deposits and the petrographical features of rocks (Fig. 2). Although
deposits at depths of 315–328 m were grouped into
the upper unit of FJ-3 by Nakada et al. (2007), they
are regrouped petrographically into FJ-3 unit-3.
FJ-3 unit-1 (at depth interval of 0–75 m) and unit-3
(at depth interval of 315–650 m) are similar in
lithofacies to those of FJ-2 units-1 and -3, respectively; however, FJ-3 unit-2 (at depth interval of
75–315 m) differs from FJ-2 unit-2 both petrographically and lithologically.
FJ-3 unit-1 is composed of two lavas, twenty
scoria-fall deposits and a pyroclastic flow deposit,
separated by weathered soil layers and moderately sorted scoriaceous mudflow deposits. The
pyroclastic flow deposit found at depths of 8–9 m
consists of poorly sorted basaltic scoria and fine
ash, and contains charcoals. Fuji black soil (FB) is
observed at depths of 14.2–14.7 m. Lavas at depths
477
of 31.8 and 56 m are olivine basalt with 15–25 vol.%
phenocrysts.
FJ-3 unit-2, at a depth interval of 75–315 m, is
almost entirely composed of moderately to poorly
sorted scoriaceous mudflow deposits, which differ
from FJ-2 unit-2 petrographically and lithologically. The mudflow deposits occasionally contain
lava clasts up to 10 cm across, which are similar
to rocks of FJ-2 units-1 and -3 without hornblende. Plagioclase-phyric basaltic andesite, characterizing FJ-2 unit-2, was merely found as a
clast in a mudflow deposit at a depth of 225.5 m.
Lavas of olivine basalt, containing 10–12 vol.%
phenocrysts with a small amount of pyroxenes,
are observed at depths of 209.8 and 213 m. At
depths of 243–245 m, thin well-sorted scoria-fall
deposits with brown-colored weathered layers are
observed.
FJ-3 unit-3 at a depth interval of 315–650 m
is mainly composed of poorly sorted polymic
mudflow deposits and two massive lavas. The top
of this unit, at depths of 315–328.5 m, consists
of alternating layers of well-sorted scoria and
brown-colored, weathered soil with a thickness of
about 4 m. More than ten scoria fall deposits are
present at this depth interval. One of them, at
a depth of 327 m, contains hornblende-bearing
scoria. Massive lava flow (25 m thick) of
hornblende-two-pyroxene basaltic andesite with
more than 40 vol.% phenocrysts is found at a
depth of 390 m. Olivine-bearing two-pyroxene
basaltic andesite (4 m thick) with more than 40
vol.% phenocrysts is found at a depth of 584.4 m.
Most clasts in a mudflow deposit below the depth
of 328.5 m are two-pyroxene basaltic andesite and
andesite with or without olivine phenocrysts.
Many of them are characterized by large (2 to
4 mm long) clinopyroxene phenocrysts (Fig. 4e),
similar to the two-pyroxene basaltic andesite and
andesite from FJ-2 unit-3. Some andesitic clasts
also contain hornblende phenocrysts (Figs 2,4d).
Thus, FJ-3 unit-3 has common signatures with
FJ-2 unit-3.
Two types of mudflow deposits are recognized in
both FJ-2 and -3 (Fig. 3a,g): scoriaceous mudflows
and polymict mudflows containing abundant large
lava clasts. A scoriaceous mudflow deposit is moderately sorted and is mainly composed of lapilli and
coarse ash particles of basaltic scoria. It commonly
lacks fine-ash particles, although it occasionally
contains lava clasts up to 10 cm in length. On the
other hand, a polymict mudflow deposit is very
poorly sorted, and contains abundant fine ash
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M. Yoshimoto et al.
particles in the matrix. It also contains about 20%
block-sized clasts of basalt, andesite and dacite.
The two types of mudflow deposits are separated
by layers of scoria fall and brown weathered soil at
the depths of 174–176.7 m in FJ-2 and 315–328.5 m
in FJ-3 (Fig. 2). The lithological difference between the two types of mudflow deposits reflects a
difference in source materials; one is dominantly
scoria-rich tephra, while the other is rich in basalt,
andesite and dacite lava flows with subordinate
pyroclastic materials.
WHOLE ROCK CHEMISTRY
Major and trace elements of lavas and the lava
clasts in mudflow deposits from the drill cores
were measured using an X-ray fluorescence spectrometer, Philips PW 2400 (Philips, Almelo, the
Netherlands) of Earthquake Research Institute,
University of Tokyo. Fifty-two samples from 37
lavas and 190 lava clasts from mudflow deposits
were selected. The lava clasts were collected
throughout mudflow deposits as evenly as possible
and represent about 20–30% of all clasts larger
than 10 cm in length. Representative analyses of
core samples are listed in Table 2. Variation
diagrams of the analyzed samples are shown in
Figures 5 and 6.
Whole rock chemistry of the core samples falls
into three groups, corresponding to the stratigraphically defined units of FJ-2 units-1 to -3
(Table 3). These groups are clearly distinguished
on variation diagrams of MgO vs. SiO2, and TiO2 vs.
MgO (Figs 5,6) and in the stratigraphic plot
(Fig. 7). Rocks of Group 1 found in FJ-2 unit-1 and
Group 2 found in FJ-2 unit-2 show a remarkably
narrow range in SiO2 (49 to 53 wt%). In contrast,
rocks of Group 3, found in FJ-2 unit-3 range from
50 to 70 wt% in SiO2. Although rocks from the FJ-2
units-1 and -2 are similar in SiO2, they are distinct
in other elements. For example, FJ-2 unit-2 rocks
are higher in Al2O3 and lower in TiO2, FeO, MgO,
K2O, P2O5, Ba, Co, Cr, Ni, Zn, Y, and Rb than FJ-2
unit-1. Rocks of FJ-2 unit-3 show chemical trends
that differ from those of FJ-2 units-1 and -2; that
is, Na2O, K2O, Ba, Zr, and Rb increase with increasing SiO2, whereas TiO2, Al2O3, FeO, MnO, MgO,
CaO, Co, V, Sc, and Y decrease. Basalt and basaltic
andesite from FJ-2 unit-3 have lower TiO2, K2O
and P2O5 and higher Al2O3 than those from FJ-2
unit-1. Rocks in FJ-2 units-2 and -3 can be discriminated in the variation diagrams of MgO vs.
SiO2 and TiO2 vs. MgO.
© 2010 Blackwell Publishing Asia Pty Ltd
Rocks of FJ-1 and two lavas from FJ-3 unit-1
are similar in chemistry to those from Group 1.
Samples from FJ-3 unit-3 range from 50 to 63 wt%
SiO2 and show the same compositional variation as
Group 3. On the other hand, samples from lavas
and lava clasts in mudflow deposits from FJ-3
unit-2 do not show chemical characteristics similar
to Group 2, though they have compositions that fall
into Groups 1 and 3; they are low in K2O and have
a narrow range of TiO2. Only one sample from
FJ-3 unit-2 with plagioclase-phyric texture at
225.5 m plots near the compositional range of
Group 2.
Samples below 285 m in depth of FJ-2 are
basalt and basaltic andesite, having narrow
ranges in SiO2 and the other major elements. In
contrast, samples from the upper part of FJ-2
unit-3, 174–285 m deep, show a wide range in
SiO2; SiO2 increases with decreasing depth from
basalt to dacite. At depths from 95 to 174 m in
FJ-2, major elements, including SiO2, remain
nearly constant. Finally, above 95 m in FJ-2
unit-1, SiO2 remains in a narrow range, though
the other elements vary widely and unsystematically. On the other hand, almost all major elements in the FJ-3 samples below 75 m gradually
increase or decrease with depth except K2O,
which shows an abrupt jump across the boundary
between units-2 and -3.
K-Ar AGE DETERMINATIONS
K-Ar age determinations were carried out at
Geological Survey of Japan, National Institute
of Advanced Industrial Science and Technology
(AIST). Six rock samples were collected from lava
flows and mudflow deposits in the deeper part of
FJ-3. The samples were crushed using an iron
pestle and sieved to 0.25–0.50 mm in diameter.
Phenocrysts were removed as much as possible
using an isodynamic separator. The groundmassrich fractions were prepared for argon isotope
analyses, and the rest of samples were further pulverized for potassium analyses using an agate
mortar.
The concentrations of potassium were determined by flame emission spectrometry, in which
peak integration and lithium internal standard
methods were adopted (Matsumoto 1989). The
concentrations of radiogenic 40Ar were determined
on a VG Isotopes 1200C mass spectrometer by the
conventional isotope dilution method using a 38Ar
51.43
1.14
16.63
10.46
0.18
6.87
10.20
2.48
0.60
0.19
100.18
188
41
175
160
36
335
75
93
13
66
331
22
+
tr
tr
+
51.73
1.29
16.80
10.54
0.18
6.06
10.06
2.70
0.67
0.23
100.26
198
38
111
203
36
374
55
97
13
77
391
23
+
+
SiO2 (wt%)
TiO2
Al2O3
FeO†
MnO
MgO
CaO
Na2O
K2O
P2O5
Total
Ba (ppm)
Co
Cr
Cu
Sc
V
Ni
Zn
Rb
Zr
Sr
Y
Phenocryst
Olivine
Clinopyroxene
Orthopyroxene
Hornblende
Plagioclase
FJ-1
75.6
FJ1 U1
Lava
FJ-1
45.0
FJ1 U1
Lava
Drilling site
Depth (m)
Stratigraphic unit
Occurrence
+
+
++
51.46
1.24
17.30
10.42
0.18
5.90
10.05
2.55
0.71
0.25
100.06
207
36
72
167
37
349
40
95
15
81
388
25
FJ-2
5.5
FJ2 U1
Lava
tr
++
51.48
1.43
19.16
9.62
0.16
3.87
10.17
3.03
0.82
0.32
100.06
267
30
39
204
29
323
32
95
17
100
437
27
FJ-2
17.1
FJ2 U1
Lava
tr
+
51.48
1.81
16.40
12.07
0.20
4.50
9.05
3.03
0.98
0.37
99.89
294
35
26
274
36
430
31
120
21
109
404
33
FJ-2
25.6
FJ2 U1
Lava
+
50.76
1.93
15.85
13.14
0.21
4.87
8.91
2.93
1.00
0.38
99.98
296
40
19
273
36
490
32
129
21
112
385
35
FJ-2
27.5
FJ2 U1
Lava
+
+
+
51.84
1.25
16.95
10.34
0.17
6.03
10.14
2.71
0.66
0.23
100.32
216
38
110
168
33
365
53
96
14
76
399
22
FJ-2
64.1
FJ2 U1
Lava
+
tr
++
50.85
1.02
20.23
9.40
0.16
3.90
10.44
2.66
0.35
0.13
99.14
155
30
12
134
32
336
10
82
5
50
423
17
FJ-2
99.9
FJ2 U2
Lava
Table 2 Representative major and trace element compositions and mineral assemblages for core samples
+
tr
++
50.70
1.07
19.06
9.87
0.18
4.04
10.01
2.73
0.42
0.14
98.22
131
29
6
90
30
326
9
84
7
53
399
19
FJ-2
156.4
FJ2 U2
Lava
+
+
69.50
0.31
16.30
2.60
0.18
0.78
3.79
4.85
1.06
0.13
99.50
312
3
3
4
7
82
17
110
406
18
FJ-2
177.5
FJ2 U3
md
+
+
++
61.44
0.65
17.20
5.40
0.11
3.08
6.48
3.91
0.89
0.15
99.31
253
20
46
35
15
149
34
65
15
80
467
13
FJ-2
200.2
FJ2 U3
md
+
+
+
++
54.19
0.96
18.83
8.87
0.18
3.81
8.83
3.24
0.49
0.17
99.57
186
24
4
28
22
208
6
82
8
58
491
18
FJ-2
214.9
FJ2 U3
Lava
+
tr
++
50.98
0.96
18.04
10.29
0.18
6.35
9.76
2.63
0.45
0.14
99.78
136
41
48
75
32
318
39
85
7
47
376
17
FJ-2
338.1
FJ2 U3
Lava
+
+
++
52.88
0.96
19.16
8.77
0.15
4.47
9.12
3.13
0.59
0.16
99.39
170
31
21
106
25
260
23
80
10
60
466
18
FJ-2
393.0
FJ2 U3
Lava
Evolution of Mount Fuji
479
© 2010 Blackwell Publishing Asia Pty Ltd
© 2010 Blackwell Publishing Asia Pty Ltd
+
++
+
++
+
tr
+
49.99
1.25
16.93
10.96
0.19
6.94
10.46
2.53
0.66
0.23
100.14
203
40
126
189
35
390
62
95
14
66
403
23
FJ-3
60.0
FJ3 U1
Lava
tr
tr
tr
++
50.61
1.07
18.30
10.44
0.19
5.77
10.23
2.59
0.48
0.16
99.84
132
38
48
114
31
329
32
86
8
55
362
21
FJ-3
211.3
FJ3 U2
Lava
tr
tr
tr
++
53.00
0.90
21.47
7.65
0.15
2.77
10.33
3.10
0.47
0.14
99.98
167
23
6
77
27
229
6
77
7
55
447
18
FJ-3
225.5
FJ3 U2
md
+
+
+
++
59.98
0.63
18.06
6.03
0.11
2.89
6.55
3.70
0.91
0.11
98.97
236
20
31
64
16
144
32
97
16
82
401
20
FJ-3
342.2
FJ3 U3
md
+
+
+
++
51.61
0.98
18.10
9.01
0.16
5.97
10.00
2.90
0.60
0.19
99.52
185
35
69
65
26
298
40
86
11
53
545
15
FJ-3
349.3
FJ3 U3
md
tr
+
+
+
++
54.95
0.75
18.62
6.49
0.12
4.31
8.79
3.35
0.55
0.16
98.09
149
24
65
74
21
181
43
68
8
65
493
14
FJ-3
399.9‡
FJ3 U3
Lava
tr
+
+
++
59.39
0.67
17.99
6.05
0.13
2.86
7.01
3.66
0.84
0.17
98.77
188
17
9
33
14
161
12
74
14
79
550
15
FJ-3
425.9‡
FJ3 U3
md
+
+
++
54.50
0.94
17.18
8.03
0.14
5.70
9.02
3.27
0.68
0.15
99.61
219
30
103
110
25
267
56
78
11
60
516
17
FJ-3
428.9‡
FJ3 U3
md
+
+
+
++
63.78
0.50
15.64
4.62
0.09
3.33
5.74
3.80
0.98
0.12
98.60
265
20
90
52
19
121
40
60
18
76
392
12
FJ-3
447.0
FJ3 U3
md
+
+
+
++
55.53
0.82
17.36
7.50
0.14
5.89
8.53
3.26
0.61
0.16
99.80
186
30
176
38
21
203
76
77
10
69
457
16
FJ-3
586.3‡
FJ3 U3
Lava
+
+
tr
++
57.84
0.74
18.10
7.02
0.13
3.38
7.55
3.53
0.75
0.17
99.21
241
23
8
42
18
185
11
80
13
74
531
15
FJ-3
611.3‡
FJ3 U3
md
+
+
+
++
60.10
0.66
17.76
6.20
0.13
2.95
6.96
3.72
0.86
0.16
99.50
279
20
7
31
15
168
10
73
15
81
515
14
FJ-3
619.0‡
FJ3 U3
md
FJ1 U1, FJ-1 unit-1; FJ2 U1, FJ-2 unit-1; FJ2 U2, FJ-2 unit-2; FJ2 U3, FJ-2 unit-3; FJ3 U1, FJ-3 unit-1; FJ3 U2, FJ-3 unit-2; FJ3 U3, FJ-3 unit-3; md, lava clast in mud flow deposit; ++, more
than 10%; +, 1 to 10%; tr, trace; -, absent.
†
Total iron as FeO; ‡ Sample for K-Ar dating.
50.45
1.15
17.42
10.39
0.19
6.66
10.86
2.42
0.51
0.19
100.24
158
38
111
162
39
335
49
89
11
61
373
23
52.54
0.98
18.66
8.67
0.15
5.30
9.20
3.13
0.60
0.16
99.39
174
36
41
117
28
283
37
80
10
58
469
14.7
SiO2 (wt%)
TiO2
Al2O3
FeO†
MnO
MgO
CaO
Na2O
K2O
P2O5
Total
Ba (ppm)
Co
Cr
Cu
Sc
V
Ni
Zn
Rb
Zr
Sr
Y
Phenocryst
Olivine
Clinopyroxene
Orthopyroxene
Hornblende
Plagioclase
FJ-3
34.3
FJ3 U1
Lava
FJ-2
412.6
FJ2 U3
md
Drilling site
Depth (m)
Stratigraphic unit
Occurrence
Table 2 Continued
480
M. Yoshimoto et al.
Evolution of Mount Fuji
481
Fig. 5 Harker variation diagrams for drill core samples from FJ-1 and -2 (left diagrams) and FJ-3 (right diagrams).
© 2010 Blackwell Publishing Asia Pty Ltd
482
M. Yoshimoto et al.
Fig. 6 MgO-TiO2 of drill core samples
from FJ-1 and -2 (left) and FJ-3 (right).
Fig. 7 Stratigraphic variations of major elements in the FJ-2 and -3 boreholes. Filled diamonds are lava flows; open diamonds are lava clasts from
mudflow deposits; filled squares are juvenile clasts from pyroclastic flow deposits.
© 2010 Blackwell Publishing Asia Pty Ltd
Group 3
Phenocryst content; ‡ Rocks of FJ-3 unit-2 contains those within the compositional range of Groups 2 and 3.
†
FJ-2 unit-3
174–414
–
FJ-3 unit-3
315–615
FJ-2 unit-2
95–174
–
483
spike. The analytical procedures and the estimation of errors were based on the procedures
described by Matsumoto and Kobayashi (1995) and
Uto et al. (1995). The decay constants used in this
study are lb = 4.96 ¥ 10-10/y, le = 0.581 ¥ 10-10/y and
40
K/K = 0.01167% (Steiger & Jäger 1977).
The results of K-Ar dating analyses are shown in
Table 4. The oldest age of 267 ⫾ 16 ka for ERIFJ3-04 is from the deepest lava in FJ-3 (586.9 m).
Two rocks from mudflow deposits at depths of
611.3 and 618.8 m in FJ-3 unit-3 show ages of
200 ⫾ 50 ka and 240 ⫾ 50 ka, respectively. These
three samples correspond to lavas whose chemistry
falls into Group 3. The youngest age is 160 ⫾ 40 ka
for a lava clast (ERI-FJ3-02) in a mudflow deposit
from FJ-3 unit-3 (425.9 m) that also belongs to
Group 3. Analyses of ERI-FJ3-01 and -03 resulted
in no significant ages, as the proportions of non
radiogenic 40Ar were more than 99% and errors for
ages were estimated to be more than 80%.
Positively correlated with SiO2
50–70
Polymictic
Very poorly sorted
Lava block rich
Fine-graind matrix
Group 2
Invariant
51–53
Olivine-bearing plagioclasetwo-pyroxene basaltic
andesite (30–35 vol.%†)
Upper part: hornblendebearing andesite and dacite
lower part: plagioclaseolivine basaltic andesite or
basalt with or without
pyroxene (25–40 vol.%†)
Poorly sorted
Lava block rich
Group 1
Wide variation with little
change in SiO2
49–52
Scoriaceous moderately
sorted
Olivine basalt with or without
pyroxene (15–25 vol.%†)
aphyric basalt
FJ-3 unit-1
0–75
FJ-3 unit-2
75–315‡
–
FJ-2 unit-1
0–95
FJ-1 unit-1
0–100
Core description
Dominant occurrence
of mudflow
Lithology
FJ-3
Hole depth (m)
FJ-2
FJ-1
Table 3 Summary of core descriptions and whole rock chemistry for drill cores
SiO2 (wt%)
Whole rock chemistry
Incompatible elements
Group
Evolution of Mount Fuji
DISCUSSION
DEFINITION OF PRE-KOMITAKE VOLCANO
Figure 8 shows chemical compositions of the Fuji
and Komitake volcanic rocks (Kaneko et al. 2004;
Fujii 2007; Nakada et al. 2007). The Fuji volcanic
rocks show wide variations in K2O and Ba with a
small range of SiO2 (49 to 52 wt%). The Komitake
volcanic rocks also have a narrow range in SiO2,
from 51 to 53 wt%. In spite of the similarity in SiO2,
the Komitake and Fuji volcanic rocks are clearly
distinguished from each other; Komitake is richer
in Al2O3, and poorer in TiO2, FeO, MgO, K2O and
P2O5 than Fuji.
It is evident that Group 1 of the core samples
has chemical ranges identical to those of the Fuji
volcanic rocks (Fig. 8). Group 2 has chemical compositions similar to those of Komitake volcanic
rocks. Group 3 differs from both the Komitake
and Fuji volcanic groups (Fig. 8). The wide variation in SiO2 coupled with incompatible elements
found in Group 3 has not been observed in the
Fuji and Komitake volcanic rocks. However, it is
common to the chemical trends in volcanic rocks
from the Izu-Bonin arc, including those from
Ashitaka and Hakone volcanoes (Yui & Fujii 1989;
Takahashi et al. 2006; Fujii 2007; Fig. 9). Hornblende dacite, characterizing Group 3, is also
found in products from a late stage of Ashitaka
Volcano (exposed on c. 1507 m a.s.l.). The existence of a ridge of basement rocks (c. 900 m a.s.l.)
© 2010 Blackwell Publishing Asia Pty Ltd
484
M. Yoshimoto et al.
Table 4 K-Ar ages of core samples from deeper part of FJ-3
Sample I.D.
Drilling
site
Depth
(m)
Occurrence
Sample wt.
(g)
K2O
(%)
ERI-FJ3-01
ERI-FJ3-02
ERI-FJ3-03
ERI-FJ3-04
ERI-FJ3-05
ERI-FJ3-06
FJ-3
FJ-3
FJ-3
FJ-3
FJ-3
FJ-3
401.85
425.90
428.90
586.30
611.30
618.80
Lava flow
lava clast in mudflow
lava clast in mudflow
Lava flow
lava clast in mudflow
lava clast in mudflow
1.001
1.011
1.003
1.001
1.005
1.008
0.881
0.677
0.921
0.742
0.846
1.00
Total 40Ar Rad. 40Ar
(10-9STP ml/g)
370
135
480
69
258
305
1⫾2
3.5 ⫾ 0.8
4⫾3
6.2 ⫾ 0.4
5.5 ⫾ 1.5
7.8 ⫾ 1.7
Non rad.
40
Ar (%)
K-Ar age
(ka)
99.7
97.4
99.3
91.0
97.9
97.4
30 ⫾ 80
160 ⫾ 40
120 ⫾ 90
267 ⫾ 16
200 ⫾ 50
240 ⫾ 50
Fig. 8 Major element variation diagrams for the products issued from Fuji
and Komitake volcanoes, plotted with
compositional ranges of the core
samples except those of FJ-3 unit-2.
Open circles represent both the Younger
and Older Fuji volcanic rocks. Data
sources for the Fuji products are Fujii
(2007), Nakada et al. (2007) and Kaneko
et al. (2004).
separating Ashitaka and Komitake volcanoes
indicates that the Group 3 dacite (174 m in FJ-2
unit-3; c. 1250 m a.s.l.) drilled in this study was
not derived from Ashitaka.
As mentioned previously, there is no Group 2
sequence in FJ-3. We observe that the site of FJ-3
is located at the northeast of the northeast-facing
amphitheater of Komitake (Fig. 1C) and suggest
that the absence of this sequence results from a
sector collapse. FJ-3 unit-2 contains rocks of
Groups 1, 2 and 3 as clasts in the mudflow deposits
(Figs 5–7) suggesting that those of Groups 2 and 3
were captured by scoriaceous mudflow deposits.
We judge FJ-3 unit-2 to be correlated with the
Group 1 sequence.
© 2010 Blackwell Publishing Asia Pty Ltd
We infer that the rocks of Group 3 are derived
from an unknown volcanic body that was buried
by Komitake Volcano, which has not been reported.
We name the Group 3 sequence ‘pre-Komitake’
Volcano. The drilled sequences show that earlier
activities of pre-Komitake Volcano were effusive
eruptions of basalt and basaltic andesite and
that the activities ended with explosive eruption of
pyroclastic falls and flows of basaltic andesite to
dacite.
GROWTH HISTORY OF MOUNT FUJI
K-Ar age determination in this study suggests
that the activity of pre-Komitake Volcano began,
Evolution of Mount Fuji
Fig. 9 SiO2-K2O and SiO2-Ba variation diagrams for ejecta from Mount
Fuji, Ashitaka and Hakone volcanoes. Data sources for Hakone Volcano
are from Arculus et al. (1991) and Takahashi et al. (2006).
at least, about 270 ka and continued to about
160 ka, a period of activity similar to that of Ashitaka Volcano (Tsuya 1971; Yui & Fujii 1989). In
drill cores, the boundary between pre-Komitake
and Komitake is characterized by an alternation
of a scoria fall layer and thin brown-weathered
soil layer at a depth of 174–176.6 m in FJ-2
(Fig. 3c). No soil layer is found within FJ-2 unit-2
or within the sequence of Komitake lava flows
and pyroclastic layers exposed in the northern
middle slope of Mount Fuji. The thin basal soil
layer suggests that the activity of Komitake
began shortly after that of pre-Komitake. The
activity of Komitake continued without any hiatus
long enough for soil deposition, and the absence
of thick soil layers or reworked deposits between
the FJ-2 units-1 and -2 indicates successive activ-
485
ity, even through a change in chemistry (OlderFuji Volcano) around 100 ka.
A schematic vertical section through Mount
Fuji is shown in Figure 10. The peak of preKomitake is considered to have been located
south to southeast of FJ-3, as inferred from the
dips and strikes of planar structures in the mudflows showing paleo-flow direction, which were
measured by FMI Fullbore Formation MicroImager (microresistivity formation images) as part
of the logging data (Nakada et al. 2004). The
summit of Komitake is considered to have been
northeast of the present summit of Mount Fuji
(Tsuya 1971).
We can roughly estimate the total volume of
the pre-Komitake and Komitake volcanoes by
assuming the total volume of Mount Fuji as
400 km3 (Tsukui et al. 1986), the analogous conical
bodies for pre-Komitake/Komitake (peaked at
2300 m a.s.l.) and Mount Fuji (3776 m a.s.l.), and
the basement level of the both volcanic bodies at
300 m a.s.l. The volumes of pre-Komitake/
Komitake and Fuji volcanoes can be calculated
as approximately 76 and 324 km3, respectively.
Long-term eruption rates of pre-Komitake/
Komitake and Fuji volcanoes are 0.45 and
3.3 km3/ka. The former value is within the range
of normal subduction-related volcanoes (Tsukui
et al. 1986). Uesugi (2003) suggested that the
total volume of eruption products from Mount
Fuji exceeds 700 km3, considering the volume of
tephra layers in the Kanto Plain. If we accept this
value, the long-term eruption rate of Fuji Volcano
is roughly doubled. Although uncertainty remains
for the volume of pre-Komitake/Komitake, we
believe that the eruption rate of Mount Fuji has
greatly increased since 100 ka.
Although some episodic high eruption rates for a
period of 1 to 10 kyr are known even in normal
subduction-related volcanoes (Table 6 of Hildreth
2007), a high eruption rate for periods on the order
of 100 kyr is rare. Over 300 km3 of the magma
volume corresponds to those of large-scale,
caldera-forming eruptions, though the latter is
intermediate to felsic in composition. For example,
the volume of pyroclastic flow deposit, Aso-4 in
central Kyushu (erupted 90 ka), is more than
600 km3 (Machida & Arai 2003), which is >360 km3
as DRE (dense-rock-equivalent). This volume of
magma had been stored in a magma chamber
beneath Aso Volcano for only 35 kyr after the
eruption of Aso-3 (120 ka). After the eruption, a
large caldera formed in this volcano; however, no
caldera has formed in the Fuji Volcano. This simply
© 2010 Blackwell Publishing Asia Pty Ltd
486
M. Yoshimoto et al.
Fig. 10 Schematic north-south cross-section through Mount Fuji, showing the relation of volcanic piles (Younger- and Older-Fuji, Komitake, preKomitake and Ashitaka volcanoes) with age (ka) and SiO2 (wt%).
means that mafic magma had oozed nearly continuously from Mount Fuji.
The period between 160 ka, when pre-Komitake
Volcano ceased its activity, and 100 ka, when
Fuji Volcano became active, is characterized by
changes in the mode of eruptions in other volcanoes near Mount Fuji. Hakone Volcano changed
from caldera-forming eruptions to central cone
development around 150 ka (Takahashi et al.
2006). In the Izu Peninsula, volcanic activity
changed from polygenetic to monogenetic around
150 ka, reflecting a change in tectonic stress field
(Koyama & Umino 1991; Hasebe et al. 2001). The
drastic change in the volcanic system of Mount
Fuji, from normal subduction-related volcanism to
basalt-dominant Fuji volcanism, may have been
caused by a change in tectonic framework.
CONCLUDING REMARKS
In scientific drilling on the northeastern flank
of Mount Fuji, a new volcanic series (‘preKomitake’ Volcano) was found beneath rocks of the
Komitake Volcano, previously considered to be the
oldest volcano within the Mount Fuji edifice. An
updated history of Mount Fuji starts with eruptions of pre-Komitake volcano from >about 270 to
160 ka, followed by eruptions of Komitake Volcano
from about 160 to 100 ka, Older-Fuji from 100 to
20 ka and Younger Fuji < 20 ka. Pre-Komitake,
Komitake and Fuji Volcanoes are easily distinguished
in variation diagrams of MgO vs. SiO2, and TiO2
vs. MgO. The pre-Komitake volcanic rocks are
characterized by hornblende-bearing andesite and
dacite together with low-K2O basalt and basaltic
andesite.
Incompatible element abundances increase with
increasing SiO2 in the pre-Komitake rocks, which
© 2010 Blackwell Publishing Asia Pty Ltd
is commonly found in volcanoes in the northern
Izu-Bonin arc, in contrast to decoupling of incompatible elements with SiO2 in Fuji Volcano. The
long-term eruption rate at Mount Fuji drastically
increased around 100 ka, when Fuji Volcano
became active. The change in volcanism of Mount
Fuji may have been induced by a change in the
tectonic framework related to the subduction of
the Philippine Sea Plate.
ACKNOWLEDGEMENTS
The authors thank Drs S. Aramaki, T. Shimano,
S. Nakano, Y. Ishizuka, N. Miyaji and D. Miura
for discussions about Mount Fuji. Thanks also to
the Geothermal Engineering Co., Ltd, particularly chief engineer Mr S. Sakuma, Kawasaki
Geological Engineering Co. Ltd, and drilling
operators. XRF analysis was supported by K.
Komori and S. Kitazawa. An early version of the
manuscript was improved by Dr T. Wright. We
also thank Drs A. Takada, S. Umino and an
anonymous reviewer for their constructive comments. Drilling was financially supported by a
Special Coordination Fund for Promoting Science
and Technology from the Ministry of Education,
Culture, Sports, Science and Technology of
Japan.
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