Bull Volcanol (2005)
DOI 10.1007/s00445-005-0042-5
RESEARCH ARTICLE
Fukashi Maeno · Hiromitsu Taniguchi
Silicic lava dome growth in the 1934–1935 Showa Iwo-jima
eruption, Kikai caldera, south of Kyushu, Japan
Received: 17 May 2004 / Accepted: 5 October 2005
C Springer-Verlag 2005
Abstract The 1934–1935 Showa Iwo-jima eruption
started with a silicic lava extrusion onto the floor of the
submarine Kikai caldera and ceased with the emergence of
a lava dome. The central part of the emergent dome consists of lower microcrystalline rhyolite, grading upward
into finely vesicular lava, overlain by coarsely vesicular
lava with pumice breccia at the top. The lava surface is
folded, and folds become tighter toward the marginal part
of the dome. The dome margin is characterized by two
zones: a fracture zone and a breccia zone. The fracture
zone is composed of alternating layers of massive lava and
welded oxidized breccia. The breccia zone is the outermost
part of the dome, and consists of glassy breccia interpreted
to be hyaloclastite. The lava dome contains lava with two
slightly different chemical compositions; the marginal part
being more dacitic and the central part more rhyolitic. The
fold geometry and chemical compositions indicate that the
marginal dacite had a slightly higher temperature, lower
viscosity, and lower yield stress than the central rhyolite.
The high-temperature dacite lava began to effuse in the
earlier stage from the central crater. The front of the dome
came in contact with seawater and formed hyaloclastite.
During the later stage, low-temperature rhyolite lava effused subaerially. As lava was injected into the growing
dome, the fracture zone was produced by successive fracturing, ramping, and brecciation of the moving dome front.
In the marginal part, hyaloclastite was ramped above the sea
surface by progressive increments of the new lava. The central part was folded, forming pumice breccia and wrinkles.
Editorial Responsibility J. McPhie
F. Maeno ()
Institute of Mineralogy, Petrology, and Economic Geology,
Graduate School of Science, Tohoku University,
Aramaki-Aza-Aoba, Aoba-ku, Sendai 980-8578, Japan
e-mail: [email protected]
Tel.: +81-22-795-7552
Fax: +81-22-795-6272
H. Taniguchi
Center for Northeast Asian Studies, Tohoku University,
Kawauchi, Aoba-ku, Sendai 980-8576, Japan
Subaerial emplacement of lava was the dominant process
during the growth of the Showa Iwo-jima dome.
Keywords Showa Iwo-jima volcano . Kikai caldera .
Submarine eruption . Silicic lava . Dome growth .
Emplacement of lava . Hyaloclastite
Introduction
Various modes of emplacement for submarine silicic lava
flows or domes are well recognized, based on geological
(Pichler 1965; De Rosen-Spence et al. 1980; Yamagishi
1987; Cas et al. 1990; Kano et al. 1991; Goto and McPhie
1998; DeRita et al. 2001; Kano 2003), theoretical, and
experimental studies (Griffiths and Fink 1992; Gregg and
Fink 1995). Almost all of this knowledge is limited to
cases where the entire process took place under the sea.
If a submarine dome continues to grow and emerge above
the sea surface, the cooling dynamics and the mode of
emplacement of the lava, in governing the surface and
internal structures of the dome, will reflect the combination
of submarine and subaerial settings. The detailed process
of emergent dome growth is, however, rarely described
in modern oceans (the 1953–1957 eruption of Tuluman
volcano, Reynolds et al. 1980; the 1952–1953 eruption
of Myojinsho volcano, Fiske et al. 1998) and in ancient
volcanic terrains (De Rosen-Spence et al. 1980; Cas et al.
1990; DeRita et al. 2001).
This paper describes the partly emergent Showa Iwo-jima
lava dome produced by a submarine eruption in 1934–1935
of the Kikai caldera, Kyushu, Japan. The eruption was
observed directly by Tanakadate (1935a,b), and the surface
and internal structure of this silicic dome are well exposed
and preserved, providing a good opportunity for examining
lava dome growth.
Geological setting
Showa Iwo-jima lava dome exists on the northern rim of
Kikai caldera, 40 km southwest of Cape Sata, southern
Fig. 1 a The location of Kikai
caldera, south of Kyushu, Japan.
b The location of Showa
Iwo-jima dome. c Submarine
topography around the Showa
Iwo-jima dome
Kyushu, Japan (Fig. 1a). Kikai caldera is 17 km wide and
20 km long, and almost completely submerged. It is located
at the southern end of a volcano-tectonic depression along
the volcanic front of southwestern Japan. This caldera was
produced at 6.5 ka by the catastrophic eruption of the Koya
ignimbrite, which covered southern Kyushu Island. Showa
Iwo-jima and the adjacent Satsuma Iwo-jima were formed
by post-caldera eruptions at the caldera rim. Satsuma
Iwo-jima comprises Iwo-dake (rhyolitic volcano) and
Inamura-dake (basaltic volcano) (Fig. 1b). Inamura-dake
was produced at 3.5–2.8 ka, and Iwo-dake has been active
since 5.6 ka (Ono et al. 1982; Okuno et al. 2000; Kawanabe
Fig. 2 a Submarine eruption of
Showa Iwo-jima with a plume
of steam, viewed from the
summit crater of Iwo-dake
volcano (September 1935). The
diameter of the plume was not
described, but was probably less
than 1 km. b The blocks of
pumice (arrows) floating on the
sea (September 1935). The size
of the largest block reached
about 10 m in length. c New
lava islet, viewed from the top
of Iwo-dake volcano (January
1935). d Sketches of the Showa
Iwo-jima dome (January 21 and
March 31, 1935) based on direct
observation by Tanakadate and
modified from Tanakadate
(1935a,b). The cone was made
of pyroclastic deposits. e Map
of the Showa Iwo-jima dome
before erosion by wave action
(July 1935; Matumoto 1936)
and at present. Photos by
Tanakadate in Matumoto (1943)
and Saito 2002; Maeno and Taniguchi 2005). Showa Iwojima was erupted in 1934–1935 from a vent on the caldera
floor, 300 m deep and 2 km east of Satsuma Iwo-jima.
Showa Iwo-jima eruption in 1934–1935
The 1934–1935 Showa Iwo-jima eruption was described
by Tanakadate (1935a,b) and Matumoto (1936). The
eruption is divided into the following four stages.
The first stage was characterized by submarine activity. Floating pumices (Kano 2003) were first noticed in
September 1934 and were accompanied by earthquakes
(Fig. 2a, b). The second stage started around December
8 when a pyroclastic cone first became visible above the
sea level and emitted ‘white smoke’ from its crater. During
this stage, there were numerous explosive eruptions repeated at intervals of 1–2 min. Each eruption ejected enormous cauliflower-shaped ‘dark smoke’ through the middle
of the ‘white smoke’. The pyroclastic cone was destroyed
by a strong explosion on December 30. The third stage
was characterized by lava effusion, accompanied by some
phreatomagmatic eruptions which generated cock’s tail jets
repeated at intervals of less than a few minutes. In early
January 1935, new lava emerged on the western side of the
islet. On January 8, a new pyroclastic cone was visible on
the lava (Fig. 2c). On January 21, the height of the new
cone exceeded 12 m above high tide level. The volcanic
islet had a maximum length of 300 m in the NE direction
and was about 150 m across (Fig. 2d). In the fourth stage
from late January to March, new silicic lava effused and a
dome grew. On February 10, a new small islet, composed
of lava, appeared 50 m northwest from the main islet. In
early March, small explosions were sometimes observed.
The central crater of the main islet widened and the crater
rim collapsed. Later, effusion of a large amount of lava
buried the entire crater. The former pyroclastic cone was
covered with the lava. On March 26, a new main islet,
the present Showa Iwo-jima lava dome, was observed with
little ‘smoke’. All activity seemed to decline at this time.
The dome was about 300 m in length in the NS direction
and 530 m across, and its height was 55 m above the sea
level (Fig. 2d, e). Part of the other new islet near the main
dome disappeared in 1936 as a result of erosion by wave
action.
By assuming an elliptical plate shape (height, long
and short axis lengths) for the dome, the total volume of
effused lava from 21 January (12 m × 250 m × 150 m)
to 26 March in 1935 (55 m × 530 m × 270 m) can
be estimated at about 6.2×106 m3 , and the average
effusion rate during this period to be about 1×105 m3 /day,
comparable with 3×105 m3 /day in the first 6 months of
activity (May 1991–November 1991) of the 1991–1996
Mt. Unzen eruption (Nakada and Fujii 1993).
Structure of Showa Iwo-jima dome
The Showa Iwo-jima dome is presently about 270 m wide
(NS direction) and 500 m long (EW direction), and its
height is 20 m above the sea level (Fig. 3a, b). The dome is
on top of the submarine edifice which rises 300 m from the
seafloor (Fig. 1b, c) and which was produced by the first
stage of the eruption from September to December 1934.
The Showa Iwo-jima lava dome produced between January
and March 1935 subsided about 30 m in 3 months just after
the eruption (from April to July in 1935), and has been
reduced in area by wave erosion (Fig. 2e).
The dome consists of two main parts: a central part and
a marginal part. The marginal part is also characterized
by two zones: a fracture zone and a breccia zone. The
fracture zone consists of alternating massive lava (ML)
and welded oxidized breccias (WOB). The breccia zone
consists of hyaloclastite. Figure 4 shows a cross-section of
the southwestern dome (X–Y line in Fig. 3b).
Structure of the central part
The central part of the Showa Iwo-jima lava dome contains
a central crater about 50 m across and an eastern crater
about 20 m across. Lava in both craters has multiple crease
structures (Anderson and Fink 1992; Fig. 3a, b) and is finely
vesicular with curviplanar surfaces. Microcrystalline rhyolite (MRHY) occurs at depths of 3–5 m in deep fractures
(Fig. 5c). The rock faces exposed by crease structures are
striated, perhaps due to the scraping of lava on lava during
emplacement.
Around the central crater, in the western sector, the dome
surface is wrinkled, and the dome consists of lower microcrystalline rhyolite (MRHY) with a density of 2,200–
2,400 kg/m3 , grading upward into finely vesicular lava
(Fig. 5b; FVL) with a density of 1,100–1,400 kg/m3 and
upper coarsely vesicular lava (Fig. 5a; CVL) with a density of 500–600 kg/m3 . These parts are coherent. The surface consists of a mixture of finely vesicular (FVPb) and
coarsely vesicular (CVPb) pumice breccia. Coarsely vesicular pumice breccia is dominant on the surface of the western sector (Fig. 5d). Pumice clasts of the surface breccia
are blocky and a few tens of centimeters to a few meters
in length. The densities of pumice clasts in the FVPb and
CVPb are 1,000–1,200 kg/m3 and 500–600 kg/m3 , respectively. The eastern sector of the dome consists of finely
vesicular lava (FVL) and pumice breccia (FVPb) (Fig. 5e),
and minor coarsely vesicular lava (CVL) and pumice breccia (CVPb). Although the densities are variable, almost all
of the lava in the central part has 16–18 vol.% phenocryst
contents. The interior of the dome (MRHY) is exposed
on the southern and northern coasts at Locations A and
D (Fig. 3), and shows an onion-like structure defined by
flow-banding (Fig. 6a, b). Lava wrinkles are increasingly
tight toward the marginal part (Fig. 6c, d).
From the wavelength (L) and amplitude (H) of lava wrinkles determined at 20 and 6 locations along the western and
eastern cross-sections, respectively (Fig. 7a, b), the H/L ratio increases from 0.2 (4 m/18 m) to 1.3 (4 m/3 m) toward
the margin of the lava dome (Table 1, Fig. 8). Here, we use a
normalized distance (D/D0 ) from one of the craters to each
lava wrinkle, because the dome shape is not a perfect circle
but a distorted ellipse (Fig. 7c). D0 is the distance from
the center of one of the craters to a margin before erosion,
crossing each lava wrinkle. For example, for the wrinkle 7
(W7 ), D is the distance from the central crater (C) to P7, D0
is the distance from the center of central crater (C) to the
margin (Q7). For the eastern lava wrinkles, D/D0 was measured from the eastern crater. Any relationship between the
orientation of flow-banding in the surface wrinkles and in
the interior is obscure, due to vesiculation and brecciation
of the lava.
Fig. 3 a Aerial photograph of
Showa Iwo-jima dome taken by
the Geographical Survey
Institute of Japan in 1977. b
Geological map of Showa
Iwo-jima dome. The outer
margin is well-exposed due to
the erosion of wave action.
Locations A–H are
representative outcrops
investigated. Solid and dashed
lines show the crests and
troughs of wrinkles, and thick
solid lines show the axes of
crease structures. Arrows show
the strike of subvertical
flow-banding. A cross-section
along X–Y is shown in Fig. 4.
Closed squares are sampling
localities. Modal analyses and
whole-rock compositions of
numbered samples are listed in
Tables 2 and 3. FVL, finely
vesicular lava; FVPb, finely
vesicular pumice breccia;
CVPb, coarsely vesicular
pumice breccia
Structure of the marginal part
Fracture zone
The fracture zone is exposed on an erosion surface immediately above the sea level (Locations B, C, D, E and F),
grading outward into the breccia zone (Fig. 3). In this zone,
massive lava (ML) is interlayered with welded oxidized
breccias (WOB, Fig. 9a). Welded oxidized breccias occur in layers concordant with the subvertical flow-banding
in the massive lava, and some breccia layers are lenticular. Both ML and WOB are a few tens of centimeters
to about 1 m thick. The massive lava (Fig. 9b) is poorly
vesicular, grayish, glassy dacite with abundant phenocrysts
Fig. 4 X–Y cross section (Fig. 3b) of the southwestern part of
Showa Iwo-jima dome. The dome has two main parts: a central part
with the original surface preserved and a marginal part with an erosion surface. The marginal part is characterized by a fracture zone
and a breccia zone. The fracture zone is composed of massive lava
(ML) and welded oxidized breccias (WOB). The central part consists
Fig. 5 a Coarsely vesicular
lava, b finely vesicular lava, and
c microcrystalline rhyolite in
the central part of Showa
Iwo-jima dome. The surface
consists of finely and coarsely
vesicular pumice breccias. d
Coarsely vesicular pumice
breccia (CVPb) is dominant on
the western dome surface. e
Finely vesicular pumice breccia
(FVPb) is dominant on the
eastern dome surface
of lower microcrystalline rhyolite (MRHY), middle finely vesicular
lava (FVL), and upper coarsely vesicular lava (CVL); finely vesicular pumice breccia (FVPb) and coarsely vesicular pumice breccia
(CVPb) cover the surface. Dashed lines show the approximate boundary of lithofacies in the stratigraphic section (left) and on the cross
section
Fig. 6 Photograph a and
sketch b show an onion-like
structure in the central part of
Showa Iwo-jima dome. The
foreground in the photo is the
central part near Location A
(Fig. 3), and the background is
the marginal part (fracture zone
and breccia zone) at Location B
(Fig. 3). Photograph c and
sketch d show folded lava in the
western side of the central part
(at Location E; Fig. 3). Arrows
show the orientation of flow
banding. MRHY,
microcrystalline rhyolite; FVL,
fine vesicular lava; FVPb, fine
vesicular pumice breccia
Fig. 7 a A photo of a cross
section of Showa Iwo-jima
dome at Location A (Fig. 3) is
shown on the left. Definitions of
the amplitude (H) and
wavelength (L) of a lava wrinkle
are given on the right. Arrows
point to crests of examples of
measured wrinkles. b
Distribution of measured lava
wrinkles along the western
(circles) and eastern (squares)
parts of the dome. c Definition
of normalized distance (D/D0 )
from one of the craters to each
lava wrinkle. D0 is the distance
from the center of one crater to
the margin before erosion,
crossing each lava wrinkle. For
example, for the wrinkle 7 (W7 ),
D is the distance from the
central crater (C) to P7, D0 is
the distance from the center of
central crater (C) to the margin
(Q7). For the eastern lava
wrinkles, D/D0 was measured
from the center of the eastern
crater
Table 1 Wavelength, amplitude, and normalized distance from
sources for wrinkles in the central part of Showa Iwo-jima lava dome
Distance
Fig. 8 Results of wavelength analysis for lava wrinkles in the western sector (closed circles) and the eastern sector (closed squares)
of Showa Iwo-jima dome. H/L is the ratio of the amplitude (H) to
wavelength (L) of a lava wrinkle. D/D0 is the normalized distance
(>20 vol.%) and sparse spherulites 2–5 mm across. The
flow bands are a few to 30 mm thick, brown and light grayish bands. The strike of the flow bands is shown as arrows in
Fig. 3.
Welded oxidized breccias (Fig. 9c) comprise reddish,
coarsely vesicular pumice clasts and nonvesicular (glassy
or crystalline) lava fragments that are a few centimeters to
1 m in length. These breccias are welded to various degrees.
The matrix of the WOB comprises fragments of vesicular or
nonvesicular lava (>1 cm) and reddish to grayish “tuffisite”
(Macdonald 1972), which is composed of glass particles
(less than a few mm) and broken phenocrysts. The broken
phenocrysts have various shapes, and most of them are
subhedral (Fig. 10b). In contrast, the phenocrysts in the
near-vent massive lava are euhedral (Fig. 10a). The spaces
between the broken phenocrysts are filled with a clear to
brownish-tan glassy material that appears isotropic under
crossed polars, and most likely has a hyalopilitic texture.
Large, elongate crystals in the tuffisite are oriented parallel
to the ML layers.
In the southeastern part (Location B), the degree of welding compaction in the oxidized breccias is higher than at
other breccias, and fragments of coarsely vesicular pumice
are lens-like in shape. Tongues of massive lava that extend into the WOB (arrows in Fig. 11a) have striated surfaces. Lenticular-shaped cavities (a few centimeters to a few
tens of centimeters in length) with vesicular surfaces occur
at the hinges of the folds in the massive lava (Fig. 11b).
Spherulites (Fig. 11c) composed of albite-rich plagioclase,
cristobalite (Fig. 11d), and Opal-CT, commonly occur on
the surfaces of cavities, similar to lithophysae (e.g. McPhie
et al. 1993).
West
W1
W2
W3
W4
W5
W6
W7
W8
W9
W10
W11
W12
W13
W14
W15
W16
W17
W18
W19
W20
East
E1
E2
E3
E4
E5
E6
D (m)
Normalized Wavelength Amplitude
distance
L (m)
H (m)
D/D0
H/L
36
69
84
86
90
109
129
95
112
123
131
142
156
190
206
210
41
66
75
64
0.16
0.30
0.37
0.60
0.40
0.48
0.57
0.51
0.60
0.66
0.58
0.63
0.69
0.84
0.91
0.97
0.14
0.70
0.81
0.57
18
14
11
9
7
10
7
9
8
7
7
6
7
4
3
2
18
5
3
4
4
4
3
3
2
2
3
3
3
3
3
2
2
3
4
2
4
3
3
3
0.22
0.29
0.27
0.33
0.29
0.20
0.43
0.33
0.38
0.43
0.43
0.33
0.29
0.75
1.33
1.00
0.22
0.60
1.00
0.75
72
73
101
112
134
180
0.64
0.55
0.70
0.78
0.59
0.73
4
7
6
4
4
5
2
3
3
2
2
5
0.50
0.43
0.50
0.50
0.50
1.00
Breccia zone
A breccia zone forms the seaward edge of the dome (Figs. 3
and 4), and is characterized by breccia and tuffisite, which
partially overlie the massive glassy or microcrystalline
dacite lava (Fig. 12a). In the southeastern part (Location B)
and southwestern part (Location F), this zone is distributed
from the sea level to a few meters in height at the
margin. The breccia in this zone is made up of polyhedral
fragments, ranging from a few centimeters to 1 m in length,
of poorly vesicular, glassy to microcrystalline dacite, and a
small amount of glassy pumiceous dacite. Most clasts are
black in color and have many fractures on their surfaces;
some have contraction cracks (Fig. 12b; Yamagishi 1994).
The clast shapes and the grain size of the glassy breccia
suggest that it is insitu hyaloclastite (Pichler 1965).
Tuffisite occurs in the fractures in the massive lava and the
coarse breccia of this zone (Fig. 12c). The components are
glass particles (less than a few mm) and broken phenocrysts
(less than a few mm in length), and the tuffisite is grayish
in color.
Fig. 9 a Fracture zone at
Location E (Fig. 3) in Showa
Iwo-jima dome. Massive lava
(ML) is interlayered with
welded oxidized breccias
(WOB). b Massive lava is
poorly vesicular, glassy dacite
with brown and light grayish
bands. The orientation of the
flow banding is parallel to the
contacts of the ML and WOB
layers. c Welded oxidized
breccias comprise reddish,
coarsely vesicular pumice
clasts, nonvesicular (glassy or
crystalline) lava fragments, and
tuffisite
Fig. 10 Photomicrographs of
volcanic rocks of Showa
Iwo-jima. Phenocrysts are
plagioclase (Pl), hypersthene
(Hyp), augite (Aug), and Fe-Ti
oxide (Ox). Scale bar is 1 mm. a
Massive lava in the central part,
characterized by euhedral
phenocrysts. The groundmass
shows a hyalopilitic texture. b
Tuffisite in the fracture zone,
characterized by broken
phenocrysts. c Broken
phenocrysts in tuffisite (plane
polarized light). The tuffisite
has a grainy, clear to
brownish-tan glassy matrix that
appears isotropic under crossed
polars, and is most likely a
hyalopilitic texture (arrows
show fragmented crystals)
Fig. 11 a Sketch of fracture
zone, comprising glassy
massive lava (ML) and welded
oxidized breccias (WOB) at
Location B (Fig. 3) in Showa
Iwo-jima dome. Arrows point to
tongues of lava extending from
the main massive lava. Dashed
lines show the flow-banding,
with vertical foliation. b
Lenticular-shaped cavities (a
few tens of centimeters to about
1 m length) with vesicular
surfaces at the hinges of folded
massive lava. c Spherulites (Sp),
composed of albite-rich
plagioclase, commonly occur
near the cavities. d SEM image
of cristobalite (Cr) on the
surface of a cavity
Petrography and chemical composition
The modal proportions of phenocrysts were measured for
seven representative samples by point counting (6,000
points per sample, Table 2). Sampling localities are shown
in Fig. 3. Phenocrysts (plagioclase, hypersthene, augite,
and Fe-Ti oxide) are more than 200 µm (Fig. 12a) in size,
including free crystals and microphenocrysts aggregating
as fine-grained mafic inclusions. For the massive lava in
the dome center (SiW1r, SiW2s, SiW5Q, and SiW7Ob),
the total phenocryst contents are 16–18 vol.%, excluding
the vesicles (<10 vol.%), and for the massive lava in the
dome margin (SiE18E, SiW9L, and SiE16F), they are 23–
25 vol.%, excluding the vesicles (<10 vol.%). The groundmass of both parts shows a hyalopilitic texture comprising
plagioclase microlites. Microlites are less abundant in the
marginal part than the central part. The higher total phenocryst content of the dome margin is due to the presence
of a large number of fine-grained mafic inclusions. The
inclusions are also present in the central part, but are less
abundant than the marginal part.
Whole-rock and glass major element compositions of the
same samples are listed in Table 3. The whole-rock SiO2
contents of dome samples range from 67 to 73 wt.%, and
the glass SiO2 contents range from 76–79 wt.% (Fig. 13a,
Table 3). The samples from the marginal part (SiE18E,
SiW9L, and SiE16F) are poor in SiO2 , whereas those from
the dome center (SiW1r, SiW2s, SiW5Q, and SiW7Ob) are
rich in SiO2 (Fig. 13b).
Saito et al. (2002) suggested that a magma chamber is
present beneath Showa Iwo-jima and that the magma is
stratified upward from a lower basaltic layer through a thin
middle layer of andesite to an upper rhyolitic layer. They
concluded that multiple injections of very similar basaltic
magma have occurred since the growth of the neighboring
Iwo-dake dome, based on the chemical variation of the
Iwo-dake and Showa Iwo-jima mafic inclusions. Although
the linear chemical trend (Fig. 13a) and petrography of the
Showa Iwo-jima dome probably reflect the heterogeneity of
the magma before emplacement, there is little difference in
the physical properties (e.g. viscosity) of the end-members
compositions, as discussed below.
Physical properties of lava
The morphology of the silicic lava domes both in air and
under the water is controlled mainly by temperature, viscosity, yield stress and effusion rate, according to theoretical,
experimental and geological studies (subaerial: Huppert
et al. 1982; Blake 1990; Griffiths and Fink 1993; Fink and
Griffiths 1998; Nakada et al. 1999; submarine dome:
Griffiths and Fink 1992; Gregg and Fink 1995).
The viscosity of the central and marginal parts of the
dome can be estimated by using the surface-folding model
of Fink and Fletcher (1978) and Fink (1980). We consider
here the folding of a planar flow, subjected to a uniform
compressive strain rate. For the central rhyolitic part, given
the parameters 20 m for the dominant wavelength of wrinkles, 5 m for the approximate thickness of the brittle crust
(the depth of fractures), and 1011 Pa S for the viscosity of the
lava surface at the glass-transition temperature, the calculated viscosity of the dome interior is 108 –109 Pa S using the
model of Hess and Dingwell (1996). The glass-transition
temperature (Tg ) for the Showa Iwo-jima lava is estimated
at 710–760◦ C using the equation Tg (◦ C) = 778−223 WH2 O
(Taniguchi 1981 ), where WH2 O is the water content. A water content of 0.1–0.3 wt.% was used, based on the results
of FTIR analyses of the groundmass glasses for 11 samples from the central to marginal part (Maeno, unpublished
Fig. 12 a Hyaloclastite in the
breccia zone, ramped to about
5 m in height above the sea
surface and overlying massive
lava at Location B (Fig. 3) in
Showa Iwo-jima dome. b In the
breccia zone, glassy clasts are
characterized by contraction
cracks. c Many fractures
developed on the surface of a
lava fragment are filled with
tuffisite (arrows)
data). For the outer part, given the parameters 10 m for the
dominant wavelength of wrinkles, 3 m for the approximate
thickness of the brittle crust, and 1011 Pa S for the viscosity
of the lava surface at the glass-transition temperature, the
calculated viscosity of the dome interior is 107 –108 Pa S.
The dome contains lava with two slightly different chemical compositions: the less silicic lava makes up the outer
margins and the more silicic lava forms the more proximal
portions (Fig. 13). However, composition has little influence on the viscosity (less than one order of magnitude)
at temperatures higher than the glass-transition temperature, as calculated by the model of Shaw (1972) using
the groundmass glass composition. The effect of the phenocryst content on viscosity was also calculated by the
Einstein–Roscoe equation η = η0 (1 − Rϕ)−2.5 , where η
and η0 are the bulk and liquid phase viscosities, respectively, R is constant set to be 1.67 (Marsh 1981 ), and ϕ is
Table 2 Modal abundance of
representative samples of
Showa Iwo-jima lava
Sampling localities are shown in
Fig. 3
Plagioclase
Hypersthene
Augite
Fe-Ti oxide
Total
Groundmass
the volume fraction of crystals. The viscosity of the central
part with a phenocryst content of 16–18 vol.% is estimated
at only 0.2-0.3 orders in Pa S higher than that of the
marginal lava with a phenocryst content of 22–25 vol.%,
when the temperature and the water content are the same.
The result is that the difference of phenocryst contents has
little influence on the viscosity.
We can estimate the temperature of the dome interior
using the calculated viscosity from the surface-folding
model (Fink and Fletcher 1978; Fink 1980). The inner lava
in the central part with a viscosity of 108 –109 Pa S should
have a temperature of 830–900◦ C for a water content of
0.3 wt.%, and one of 910–990◦ C for a water content of
0.1 wt.%, when calculated according to the method of
Hess and Dingwell (1996). Therefore, we suggest that
the temperature of the inner lava in the central part was
830–990◦ C. For the outer lava for which the viscosity is
1
SiW1r
2
SiW2s
3
SiW5Q
4
SiW7Ob
5
SiE18E
6
SiW9L
7
SiE16F
14.4
0.9
1.0
1.3
17.6
82.4
14.5
0.3
1.2
0.9
16.9
83.1
12.6
0.7
1.7
1.3
16.2
83.8
14.9
1.2
1.1
0.7
18.0
82.0
17.6
1.5
1.8
1.2
22.0
78.0
21.4
1.8
1.1
1.1
25.5
74.5
18.4
2.7
1.2
1.4
23.7
76.3
Table 3
SiO2
TiO2
Al2 O3
Fe2 O3
FeO
MnO
MgO
CaO
Na2 O
K2 O
P2 O5
H2 O+
H2 O−
Total
Whole-rock and glass major element composition (wt.%) of Showa Iwo-jima lava
Whole-rock major element composition
1
2
3
4
SiW1r
SiW2s
SiW5Q
SiW7Ob
5
SiE18E
6
SiW9L
7
SiE16F
Groundmass glass composition
1
7
SiW1r
SD
SiE18E
SD
72.00
0.55
13.52
1.25
1.83
0.09
0.72
2.53
4.22
2.49
0.11
0.30
0.03
99.65
69.80
0.63
14.39
1.98
2.13
0.11
1.11
3.38
4.10
2.27
0.14
0.17
0.11
100.32
68.99
0.65
14.69
2.00
2.27
0.11
1.18
3.74
4.24
2.10
0.16
0.21
0.12
100.46
68.12
0.65
14.52
2.08
2.26
0.12
1.15
3.70
4.23
2.08
0.16
0.19
0.26
99.52
79.05
0.41
11.73
–
1.21
0.06
0.08
0.84
3.41
3.31
–
–
–
100.08
1.67
0.08
0.20
–
0.26
0.05
0.06
0.20
0.18
0.17
–
–
–
(n=10)
71.87
0.55
13.73
1.29
1.94
0.09
0.77
2.66
4.21
2.44
0.11
0.25
0.08
99.98
71.63
0.57
13.69
1.38
2.13
0.10
0.84
2.73
4.20
2.40
0.12
0.24
0.14
100.15
70.70
0.59
13.86
1.58
2.15
0.10
0.96
3.02
4.11
2.35
0.13
0.38
0.10
100.05
0.27
0.05
0.08
–
0.04
0.04
0.01
0.05
0.07
0.04
–
–
–
(n=11)
75.30
0.61
12.17
–
1.58
0.07
0.10
0.80
2.65
5.60
–
–
–
98.87
Sampling localities are shown in Fig. 3. All iron calculated in FeO for groundmass glass composition
Analytical procedures: Major element compositions were determined by X-ray fluorescence analysis (XRF) as described in Yajima et al.
(2001). Ferric/Ferrous and ignition loss were determined following the method of Tiba (1970). Groundmass glasses were analyzed by
electron microprobe (JEOL JSM-5410 with wavelength dispersive solid state detector of Oxford Link ISIS) using defocused electron beam
of 10–20 m in diameter, accelerating voltage of 15 keV, and selecting random points (number of parentheses; n) in each thin section
SD standard deviation of electron probe micro-analyses
estimated to be 107 –108 Pa S, the temperature is estimated
at 50–100◦ C higher than that of the inner part. Saito et al.
(2001, 2002) estimated a water content of less than 1 wt.%
by FTIR analysis of melt inclusions in plagiolclase and a
temperature of 970◦ C using pyroxene geothermometry for
pre-eruption Showa Iwo-jima magma.
Another parameter, yield stress (σ ), strongly depends
on temperature. We use the equation for yield stress,
σ = σl × eb(Tl −T ) (Doragoni et al. 1992), where T and Tl
are the temperature of lava and the liquidus temperature,
respectively, σl is the yield stress at T=Tl and b is the
constant. A 50–100◦ C temperature difference results in a
difference of 2–4 orders of magnitude in the yield stress.
Hence, the high-temperature lava of the outer part probably
had a lower yield stress, and the low-temperature lava of
the central part probably had a higher yield stress.
Discussion
Morphology of Showa Iwo-jima dome
The morphology of a lava dome is controlled by the physical properties of the actively moving lava: temperature,
viscosity, yield stress, and effusion rate. These physical parameters directly control the thickness and aspect ratio of
the dome (Huppert et al. 1982; Blake 1990; Griffiths and
Fink 1993; Fink and Griffiths 1998; Nakada et al. 1999).
Some silicic lava domes exhibit a spiny surface morphology, due to high viscosity (e.g. Unzen, Montserrat and
Redoubt; Fink and Griffiths 1998). Spiny domes tend to be
high with very steep sides, and their tops are commonly
punctured by one or more subvertical spines with smooth
and curving sides (Fink and Griffiths 1998). Other silicic
lava domes, such as the Showa Iwo-jima lava dome, have
an axisymmetric shape (Fig. 3). Axisymmetric domes have
regular outlines, relatively low relief, and surfaces that tend
to be covered by abundant small (a few tens of centimeters
to a few meters in length) fragments (Fink and Griffiths
1998). Although the average effusion rate (1×105 m3 /day)
for Showa Iwo-jima is almost the same as that of the earlier
stage of Mount Unzen (3×105 m3 /day; Nakada and Fujii
1993), the dome shape is very different. The volume of
the dome of Showa Iwo-jima is 6.2×106 m3 in the total 2
months of activity, and the volume of Unzen is 23×106 m3
in the first 6 months of activity (Nakada and Fujii 1993),
respectively. The difference between their morphologies is
derived from the deformation behavior that is transitional
between Bingham plastics (spiny shape) and viscous fluids
(axisymmetric shape). The viscosity of the Mount Unzen
lava was estimated to be over 1011.5 Pa S by brittle failure condition analysis (Goto 1999), or 2–4×1010 Pa S,
based on the analysis using moving lobe conditions (Suto
et al. 1993), and the temperature was 780–880◦ C (Nakada
and Motomura 1999). On the other hand, the lava for the
Showa Iwo-jima dome had a lower viscosity (107–9 Pa S)
and lower yield stress than that of Mount Unzen, due to
its inferred high temperature (830–990◦ C), resulting in its
axisymmetric shape.
Subaerial growth of Showa Iwo-jima dome
The aerial photographs of Showa Iwo-jima show surface
folds that have continuous, arcuate fold axes, roughly parallel to the outer part. The surfaces of some noneroded
subaerial silicic lava flows or domes are commonly composed of vesicular pumice fragments that were jostled and
ground against each other while the active flow advanced
and its surface was alternately extended and compressed
(Fink 1980, 1983; Castro et al. 2002). The decrease in
density from lessvesicular lava in the deeper part, grading
up into morevesicular lava on the surface of dome, suggests that magmatic volatiles diffused outward from the
dome interior. It is suggested that upon final decompression at the dome surface, rhyolite lava inflated to vesicular
pumice, and a cooling contraction during movement fractured the pumice (e.g. Fink 1980, 1983; Anderson et al.
1998). Compression and extension during lava emplacement should further disrupt the dome carapace, resulting
in pumice breccia. Since the upper surface of the dome
was unconfined, folds grew vertically as the dome progressively enlarged. Wrinkles in the outer part have shorter
wavelengths than wrinkles near the central crater (Figs. 4
and 8). Outer lava may have had a lower viscosity because
of its higher temperature and it also experienced the most
Fig. 14 Model for the formation of the fracture zone in Showa
Iwo-jima dome (cross sections of the marginal part). a As lava was
repeatedly injected into the growing dome, fractures and ramp structures moved outwards. With time, each fraction of fractured and
ramped lava became progressively attenuated, due to stretching and
shearing. b When the active dome surface was alternately extended
and compressed, the fractures propagated into the interior of the
viscous lava, resulting in thin layers of massive lava (ML layers).
The surface of each ML layer would have vesiculated and brecciated
insitu, due to a temperature gradient as shown in the right figure.
c Newly produced breccias were embedded in the lava, and were
reheated and welded when the fractures closed, resulting in massive lavas (ML) interlayered with welded oxidized breccias (WOB).
Closed arrows show the direction of stress
Fig. 13 a SiO2 -K2 O diagram for Showa Iwo-jima dome samples.
Sample locations are shown in Fig. 3. b Compositional variation
of Showa Iwo-jima lava dome for normalized distance (D/D0 ) from
the vents. The samples from the marginal part are SiO2 -poor (67–
70 wt.%), whereas those from the dome center are SiO2 -rich (70–
73 wt.%)
strain for the longest time. The evidence from the surface and internal structure suggests that the dome surface
of the central part, immediately inside the marginal part,
was more folded than the near-crater part, as the leading
edge almost ceased flowing due to the formation of a rigid
margin.
Fracturing and brecciation in the marginal part
of Showa Iwo-jima dome
Formation of the fracture zone
The structures and components of the dome margin reflect the conditions of lava emplacement. The fracture zone
in the Showa Iwo-jima dome is interpreted as the product of the actively moving lava in the dome margin. Once
the surface of the dome margin was cold, it would have
behaved in a brittle manner, and fracturing and ramping
could then be important (Macdonald 1972). As lava was
repeatedly injected into the growing dome, the fractures
and ramp structures moved outwards, and the orientation
of the flow foliation in the marginal part became predominantly subvertical. With time, each fraction of the fractured
and ramped lava became progressively attenuated, due to
stretching and shearing (Fig. 14a). While the active dome
surface was alternately extended and compressed, the fractures along the foliation propagated into the viscous interior
lava, resulting in thin layers of massive lava, that is, ML
layers (Fig. 14b). The surface of each ML layer would
have undergone insitu vesiculation and brecciation, due to
a temperature gradient between the hot interior lava and
the surface of the open fractures (right figure in Fig. 14b).
When the fractures closed, newly produced breccias were
embedded in the lava, reheated and welded, resulting in
massive lava (ML) interlayered with welded oxidized breccia (WOB) (Fig. 14c). Tongues of massive lava extended
into the WOB (ML with striated surface) and lenticularshaped cavities formed at the hinges of the folded massive
lava, indicating pull-apart of the lava by internal shear (e.g.
Castro et al. 2002).
Formation of a fine matrix in the WOB was also related to
movement of the active lava. Manley (1996) suggested that
pumice at the surface of domes is comminuted, producing
loose shards, bits of pumice, chips of dense glass, and fragments of phenocrysts. This debris sifts down around loose
blocks and into open fractures deeper in the flow, where
it can be reheated, compressed, and annealed to varying
degrees. Fine matrix may have been locally remobilized
by the escaping gas shortly after emplacement. Secondary
minerals crystallized on the surfaces of cavities, showing
that gas continued to escape from the lava after the emplacement.
Formation of the breccia zone
Breccia in the outer margin of the dome is interpreted to
be hyaloclastite generated by brittle spalling of hot lava
that was rapidly cooled by seawater during lava emplacement (e.g. Pichler 1965; De Rosen-Spence et al. 1980;
Yamagishi 1987; Kano et al. 1991). While the breccia was
being generated, a large amount of steam was produced
and rose up from the margin, as observed by Tanakadate
(1935a,b). Tuffisite in the breccia zone may have been produced by quenching of the lava under the sea. Hyaloclastite
breccia, observed in the subaerial parts of the dome at
present, may have been uplifted during the lava emplacement. If the dome front became rigid and almost stopped
advancing, further output of lava from the vent could have
squeezed earlier-erupted lava and breccia outwards, resulting in ramping of breccia above the sea surface. The breccia
zone is limited to the outer margin near Locations B and F,
implying that there was very little uplifted breccia, or else
it has been almost entirely eroded.
Formation of Showa Iwo-jima Island
The Showa Iwo-jima eruption in 1934–1935 is divided
into four stages; (1) the formation of a submarine edifice
with floating pumice, from September to December in
1934, (2) phreatomagmatic explosions and pyroclastic
cone-formation in December, (3) lava effusion and
formation of a new pyroclastic cone on the western side
of the lava, in early January 1935 (Figs. 2d and 15a) and
followed by (4) effusion of the new silicic lava and dome
growth from January to March (Figs. 2d and 15b–d).
The growth of the dome from late January is summarized
in the following scenario: The high-temperature dacite lava
began to effuse from the central crater using two separate
vents (Fig. 15b). Judging from some photographs (example: Fig. 2c), during the earlier dome growth stage, a large
amount of steam was produced and rose up from the margin. During the later stage, low-temperature rhyolite lava
effused subaerially, and flowed out to the shallow seafloor
on the eastern side and onto formerly emplaced lava on the
western side (Fig. 15c). Although Tanakadate (1935a,b) reported only one central crater, our observation indicates that
another vent also exists in the center of the eastern sector.
The breccia zone was formed by rapid cooling when lava
entered the sea. In March 1935, the central part of the dome,
composed of rhyolite lava, grew up subaerially, resulting in
an axisymmetric shape (Fig. 15d). In the marginal part, the
fracture zone was produced by lava being successively fractured, ramped, and brecciated. Newly produced breccias
were embedded in the lava and reheated when the fractures
closed, resulting in the formation of welded oxidized
breccias (WOB). During this stage, hyaloclastite breccia
was also ramped above the sea surface by progressive emplacement of increments of new lava. The central part was
folded, and pumice breccia and wrinkles developed. The
small area of breccia shows that subaerial emplacement
was the dominant process during the growth of the exposed
parts of the Showa Iwo-jima dome. Post-emplacement
subsidence and wave erosion of the marginal part resulted
in the present Showa Iwo-jima dome (Fig. 15e).
Conclusion
The Showa Iwo-jima dome consists of two main parts; a
central part and a marginal part. The marginal part is also
characterized by two zones; a fracture zone and a breccia zone. The lava dome contains lava with two slightly
Fig. 15 Model for the emplacement of Showa Iwo-jima silicic lava
dome (cross sections; directions are southwest (SW) and east (E), as
shown in the insert). a Pyroclastic cone formation and minor lava effusion with phreatomagmatic eruptions in early January 1935. b The
dacitic lava effused from the crater of the pyroclastic cone. c Rhyolite
lava effused subaerially using two separate vents, and flowed out to
the shallow seafloor on the eastern side and onto formerly emplaced
lava on the southwestern side, from late January, 1935. d The fracture
zone, characterized by the ML-WOB layers, was produced by fracturing and ramping during movement of the lava. Hyaloclastite at the
dome margin was ramped above the sea surface. The central part of
the dome remained subaerial, resulting in the Showa Iwo-jima lava
dome in late March, 1935. e Post-emplacement subsidence and wave
erosion of the marginal part produced the present Showa Iwo-jima
lava dome. Dashed lines show the approximate boundary of dense
lava (MRHY) and vesicular lava (FVL, CVL)
different chemical compositions, one of them being more
dacitic in the marginal part and the other being more rhyolitic in the central part. The high-temperature dacite lava
began to effuse from the central craters and flowed onto the
shallow seafloor. The surface of the dome came into contact
with seawater and brecciated, forming hyaloclastite. During the later stage, low-temperature rhyolite lava effused
subaerially, and the fracture zone was produced by successive fracturing, ramping, and brecciation of the actively
moving dome front. At the southern margin, hyaloclastite
breccia was ramped above the sea surface by progressive increments of new lava. The central part was folded, forming
a surface layer of pumice breccia. Subaerial emplacement
of lava was a dominant process during the growth of the
Showa Iwo-jima dome. The process of dome growth in a
shallow sea can be elucidated by this evidence.
Acknowledgements We acknowledge A Goto, T Miyamoto, M
Ichihara, and A Yokoo for their logistical advice. We also thank H
Fujimaki for the XRF analyses. We are grateful to Geographical
Survey Institute of Japan for permission to use the aerial photograph
of Showa Iwo-jima, and to Mishima village, Kagoshima, Japan, for
help with our field survey. We thank J McPhie, K Kano, and K Dadd
for constructive reviews of the manuscript. This research was partly
supported by a Grant-in-aid for Scientific Research from the Ministry
of Education, Culture, Sports, Science and Technology, Japan (No.
14080203).
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hyaloclastites in southwest Hokkaido. Rep Geol Surv Hokkaido
59:55–117
Yamagishi H (1994) Subaqueous volcanic rocks: atlas and glossary.
Hokkaido University Press