Silicic Lava Dome Growth in the 1934–1935 Showa Iwo-Jima Eruption, Kikai Caldera, South of Kyushu, Japan

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 Subaerial emplacement of lava was the dominant process started with a silicic lava extrusion onto the floor of the during the growth of the Showa Iwo-jima dome. submarine Kikai caldera and ceased with the emergence of a lava dome. The central part of the emergent dome con- Keywords Showa Iwo-jima volcano . Kikai caldera . sists of lower microcrystalline rhyolite, grading upward Submarine eruption . Silicic lava . Dome growth . into finely vesicular lava, overlain by coarsely vesicular Emplacement of lava . Hyaloclastite lava with pumice breccia at the top. The lava surface is folded, and folds become tighter toward the marginal part Introduction of the dome. The dome margin is characterized by two zones: a fracture zone and a breccia zone. The fracture Various modes of emplacement for submarine silicic lava zone is composed of alternating layers of massive lava and flows or domes are well recognized, based on geological welded oxidized breccia. The breccia zone is the outermost (Pichler 1965; De Rosen-Spence et al. 1980; Yamagishi part of the dome, and consists of glassy breccia interpreted 1987; Cas et al. 1990; Kano et al. 1991; Goto and McPhie to be hyaloclastite. The lava dome contains lava with two 1998; DeRita et al. 2001; Kano 2003), theoretical, and slightly different chemical compositions; the marginal part experimental studies (Griffiths and Fink 1992; Gregg and being more dacitic and the central part more rhyolitic. The Fink 1995). Almost all of this knowledge is limited to fold geometry and chemical compositions indicate that the cases where the entire process took place under the sea. marginal dacite had a slightly higher temperature, lower If a submarine dome continues to grow and emerge above viscosity, and lower yield stress than the central rhyolite. the sea surface, the cooling dynamics and the mode of The high-temperature dacite lava began to effuse in the emplacement of the lava, in governing the surface and earlier stage from the central crater. The front of the dome internal structures of the dome, will reflect the combination came in contact with seawater and formed hyaloclastite. of submarine and subaerial settings. The detailed process During the later stage, low-temperature rhyolite lava ef- of emergent dome growth is, however, rarely described fused subaerially. As lava was injected into the growing in modern oceans (the 1953–1957 eruption of Tuluman dome, the fracture zone was produced by successive frac- volcano, Reynolds et al. 1980; the 1952–1953 eruption turing, ramping, and brecciation of the moving dome front. of Myojinsho volcano, Fiske et al. 1998) and in ancient In the marginal part, hyaloclastite was ramped above the sea volcanic terrains (De Rosen-Spence et al. 1980; Cas et al. surface by progressive increments of the new lava. The cen- 1990; DeRita et al. 2001). tral part was folded, forming pumice breccia and wrinkles. This paper describes the partly emergent Showa Iwo-jima lava dome produced by a submarine eruption in 1934–1935 Editorial Responsibility J. McPhie of the Kikai caldera, Kyushu, Japan. The eruption was observed directly by Tanakadate (1935a,b), and the surface F. Maeno () and internal structure of this silicic dome are well exposed Institute of Mineralogy, Petrology, and Economic Geology, Graduate School of Science, Tohoku University, and preserved, providing a good opportunity for examining Aramaki-Aza-Aoba, Aoba-ku, Sendai 980-8578, Japan lava dome growth. e-mail: [email protected] Tel.: +81-22-795-7552 Fax: +81-22-795-6272 Geological setting

H. Taniguchi Center for Northeast Asian Studies, Tohoku University, Showa Iwo-jima lava dome exists on the northern rim of Kawauchi, Aoba-ku, Sendai 980-8576, Japan 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 and Saito 2002; Maeno and Taniguchi 2005). Showa Iwo- 20 km long, and almost completely submerged. It is located jima was erupted in 1934–1935 from a vent on the caldera at the southern end of a volcano-tectonic depression along floor, 300 m deep and 2 km east of Satsuma Iwo-jima. 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 Showa Iwo-jima eruption in 1934–1935 Iwo-jima and the adjacent Satsuma Iwo-jima were formed by post-caldera eruptions at the caldera rim. Satsuma The 1934–1935 Showa Iwo-jima eruption was described Iwo-jima comprises Iwo-dake (rhyolitic volcano) and by Tanakadate (1935a,b) and Matumoto (1936). The Inamura-dake (basaltic volcano) (Fig. 1b). Inamura-dake eruption is divided into the following four stages. was produced at 3.5–2.8 ka, and Iwo-dake has been active The first stage was characterized by submarine activ- since 5.6 ka (Ono et al. 1982; Okuno et al. 2000; Kawanabe ity. Floating pumices (Kano 2003) were first noticed in

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) September 1934 and were accompanied by earthquakes fracture zone consists of alternating massive lava (ML) (Fig. 2a, b). The second stage started around December and welded oxidized breccias (WOB). The breccia zone 8 when a pyroclastic cone first became visible above the consists of hyaloclastite. Figure 4 shows a cross-section of sea level and emitted ‘white smoke’ from its crater. During the southwestern dome (X–Y line in Fig. 3b). this stage, there were numerous explosive eruptions repeated at intervals of 1–2 min. Each eruption ejected enor- mous cauliflower-shaped ‘dark smoke’ through the middle of the ‘white smoke’. The pyroclastic cone was destroyed Structure of the central part by a strong explosion on December 30. The third stage was characterized by lava effusion, accompanied by some The central part of the Showa Iwo-jima lava dome contains phreatomagmatic eruptions which generated cock’s tail jets a central crater about 50 m across and an eastern crater repeated at intervals of less than a few minutes. In early about 20 m across. Lava in both craters has multiple crease January 1935, new lava emerged on the western side of the structures (Anderson and Fink 1992; Fig. 3a, b) and is finely islet. On January 8, a new pyroclastic cone was visible on vesicular with curviplanar surfaces. Microcrystalline rhy- the lava (Fig. 2c). On January 21, the height of the new olite (MRHY) occurs at depths of 3–5 m in deep fractures cone exceeded 12 m above high tide level. The volcanic (Fig. 5c). The rock faces exposed by crease structures are islet had a maximum length of 300 m in the NE direction striated, perhaps due to the scraping of lava on lava during and was about 150 m across (Fig. 2d). In the fourth stage emplacement. from late January to March, new silicic lava effused and a Around the central crater, in the western sector, the dome dome grew. On February 10, a new small islet, composed surface is wrinkled, and the dome consists of lower mi- of lava, appeared 50 m northwest from the main islet. In crocrystalline rhyolite (MRHY) with a density of 2,200– early March, small explosions were sometimes observed. 2,400 kg/m3, grading upward into finely vesicular lava The central crater of the main islet widened and the crater (Fig. 5b; FVL) with a density of 1,100–1,400 kg/m3 and rim collapsed. Later, effusion of a large amount of lava upper coarsely vesicular lava (Fig. 5a; CVL) with a den- buried the entire crater. The former pyroclastic cone was sity of 500–600 kg/m3. These parts are coherent. The sur- covered with the lava. On March 26, a new main islet, face consists of a mixture of finely vesicular (FVPb) and the present Showa Iwo-jima lava dome, was observed with coarsely vesicular (CVPb) pumice breccia. Coarsely vesic- little ‘smoke’. All activity seemed to decline at this time. ular pumice breccia is dominant on the surface of the west- The dome was about 300 m in length in the NS direction ern sector (Fig. 5d). Pumice clasts of the surface breccia and 530 m across, and its height was 55 m above the sea are blocky and a few tens of centimeters to a few meters level (Fig. 2d, e). Part of the other new islet near the main in length. The densities of pumice clasts in the FVPb and dome disappeared in 1936 as a result of erosion by wave CVPb are 1,000–1,200 kg/m3 and 500–600 kg/m3, respec- action. tively. The eastern sector of the dome consists of finely By assuming an elliptical plate shape (height, long vesicular lava (FVL) and pumice breccia (FVPb) (Fig. 5e), and short axis lengths) for the dome, the total volume of and minor coarsely vesicular lava (CVL) and pumice brec- effused lava from 21 January (12 m × 250 m × 150 m) cia (CVPb). Although the densities are variable, almost all to 26 March in 1935 (55 m × 530 m × 270 m) can of the lava in the central part has 16–18 vol.% phenocryst be estimated at about 6.2×106 m3, and the average contents. The interior of the dome (MRHY) is exposed effusion rate during this period to be about 1×105 m3/day, on the southern and northern coasts at Locations A and comparable with 3×105 m3/day in the first 6 months of D (Fig. 3), and shows an onion-like structure defined by activity (May 1991–November 1991) of the 1991–1996 flow-banding (Fig. 6a, b). Lava wrinkles are increasingly Mt. Unzen eruption (Nakada and Fujii 1993). tight toward the marginal part (Fig. 6c, d). From the wavelength (L) and amplitude (H)oflavawrin- kles determined at 20 and 6 locations along the western and Structure of Showa Iwo-jima dome 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 Showa Iwo-jima dome is presently about 270 m wide the margin of the lava dome (Table 1, Fig. 8). Here, we use a (NS direction) and 500 m long (EW direction), and its normalized distance (D/D0) from one of the craters to each height is 20 m above the sea level (Fig. 3a, b). The dome is lava wrinkle, because the dome shape is not a perfect circle on top of the submarine edifice which rises 300 m from the but a distorted ellipse (Fig. 7c). D0 is the distance from seafloor (Fig. 1b, c) and which was produced by the first the center of one of the craters to a margin before erosion, stage of the eruption from September to December 1934. crossing each lava wrinkle. For example, for the wrinkle 7 The Showa Iwo-jima lava dome produced between January (W7), D is the distance from the central crater (C) to P7, D0 and March 1935 subsided about 30 m in 3 months just after is the distance from the center of central crater (C) to the the eruption (from April to July in 1935), and has been margin (Q7). For the eastern lava wrinkles, D/D0 was mea- reduced in area by wave erosion (Fig. 2e). sured from the eastern crater. Any relationship between the The dome consists of two main parts: a central part and orientation of flow-banding in the surface wrinkles and in a marginal part. The marginal part is also characterized the interior is obscure, due to vesiculation and brecciation by two zones: a fracture zone and a breccia zone. The 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 massive lava (ML) is interlayered with welded oxidized breccias (WOB, Fig. 9a). Welded oxidized breccias oc- Fracture zone cur in layers concordant with the subvertical flow-banding in the massive lava, and some breccia layers are lentic- The fracture zone is exposed on an erosion surface imme- ular. Both ML and WOB are a few tens of centimeters diately above the sea level (Locations B, C, D, E and F), to about 1 m thick. The massive lava (Fig. 9b) is poorly grading outward into the breccia zone (Fig. 3). In this zone, vesicular, grayish, glassy dacite with abundant phenocrysts Fig. 4 X–Y cross section (Fig. 3b) of the southwestern part of of lower microcrystalline rhyolite (MRHY), middle finely vesicular Showa Iwo-jima dome. The dome has two main parts: a central part lava (FVL), and upper coarsely vesicular lava (CVL); finely vesic- with the original surface preserved and a marginal part with an ero- ular pumice breccia (FVPb) and coarsely vesicular pumice breccia sion surface. The marginal part is characterized by a fracture zone (CVPb) cover the surface. Dashed lines show the approximate bound- and a breccia zone. The fracture zone is composed of massive lava ary of lithofacies in the stratigraphic section (left) and on the cross (ML) and welded oxidized breccias (WOB). The central part consists section

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 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)andeastern(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 Normalized Wavelength Amplitude distance

D (m) D/D0 L (m) H (m) H/L West W1 36 0.16 18 4 0.22 W2 69 0.30 14 4 0.29 W3 84 0.37 11 3 0.27 W4 86 0.60 9 3 0.33 W5 90 0.40 7 2 0.29 W6 109 0.48 10 2 0.20 W7 129 0.57 7 3 0.43 W8 95 0.51 9 3 0.33 W9 112 0.60 8 3 0.38 W10 123 0.66 7 3 0.43 W11 131 0.58 7 3 0.43 W12 142 0.63 6 2 0.33 W13 156 0.69 7 2 0.29 Fig. 8 Results of wavelength analysis for lava wrinkles in the west- W14 190 0.84 4 3 0.75 ern sector (closed circles) and the eastern sector (closed squares) W15 206 0.91 3 4 1.33 of Showa Iwo-jima dome. H/L is the ratio of the amplitude (H)to W16 210 0.97 2 2 1.00 wavelength (L) of a lava wrinkle. D/D0 is the normalized distance W17 41 0.14 18 4 0.22 > W18 66 0.70 5 3 0.60 ( 20 vol.%) and sparse spherulites 2–5 mm across. The W19 75 0.81 3 3 1.00 flow bands are a few to 30 mm thick, brown and light gray- W20 64 0.57 4 3 0.75 ish bands. The strike of the flow bands is shown as arrows in East Fig. 3. E1 72 0.64 4 2 0.50 Welded oxidized breccias (Fig. 9c) comprise reddish, coarsely vesicular pumice clasts and nonvesicular (glassy E2 73 0.55 7 3 0.43 or crystalline) lava fragments that are a few centimeters to E3 101 0.70 6 3 0.50 1 m in length. These breccias are welded to various degrees. E4 112 0.78 4 2 0.50 The matrix of the WOB comprises fragments of vesicular or E5 134 0.59 4 2 0.50 nonvesicular lava (>1 cm) and reddish to grayish “tuffisite” E6 180 0.73 5 5 1.00 (Macdonald 1972), which is composed of glass particles (less than a few mm) and broken phenocrysts. The broken Breccia zone phenocrysts have various shapes, and most of them are subhedral (Fig. 10b). In contrast, the phenocrysts in the A breccia zone forms the seaward edge of the dome (Figs. 3 near-vent massive lava are euhedral (Fig. 10a). The spaces and 4), and is characterized by breccia and tuffisite, which between the broken phenocrysts are filled with a clear to partially overlie the massive glassy or microcrystalline brownish-tan glassy material that appears isotropic under dacite lava (Fig. 12a). In the southeastern part (Location B) crossed polars, and most likely has a hyalopilitic texture. and southwestern part (Location F), this zone is distributed Large, elongate crystals in the tuffisite are oriented parallel from the sea level to a few meters in height at the to the ML layers. margin. The breccia in this zone is made up of polyhedral In the southeastern part (Location B), the degree of weld- fragments, ranging from a few centimeters to 1 m in length, ing compaction in the oxidized breccias is higher than at of poorly vesicular, glassy to microcrystalline dacite, and a other breccias, and fragments of coarsely vesicular pumice small amount of glassy pumiceous dacite. Most clasts are are lens-like in shape. Tongues of massive lava that ex- black in color and have many fractures on their surfaces; tend into the WOB (arrows in Fig. 11a) have striated sur- some have contraction cracks (Fig. 12b; Yamagishi 1994). faces. Lenticular-shaped cavities (a few centimeters to a few The clast shapes and the grain size of the glassy breccia tens of centimeters in length) with vesicular surfaces occur suggest that it is insitu hyaloclastite (Pichler 1965). at the hinges of the folds in the massive lava (Fig. 11b). Tuffisite occurs in the fractures in the massive lava and the Spherulites (Fig. 11c) composed of albite-rich plagioclase, coarse breccia of this zone (Fig. 12c). The components are cristobalite (Fig. 11d), and Opal-CT, commonly occur on glass particles (less than a few mm) and broken phenocrysts the surfaces of cavities, similar to lithophysae (e.g. McPhie (less than a few mm in length), and the tuffisite is grayish et al. 1993). in color. Fig. 9 a Fracture zone at Location E (Fig. 3)inShowa 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)inShowa 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 Iwo-dake dome, based on the chemical variation of the Iwo-dake and Showa Iwo-jima mafic inclusions. Although The modal proportions of phenocrysts were measured for the linear chemical trend (Fig. 13a) and petrography of the seven representative samples by point counting (6,000 Showa Iwo-jima dome probably reflect the heterogeneity of points per sample, Table 2). Sampling localities are shown the magma before emplacement, there is little difference in in Fig. 3. Phenocrysts (plagioclase, hypersthene, augite, the physical properties (e.g. viscosity) of the end-members and Fe-Ti oxide) are more than 200 µm (Fig. 12a) in size, compositions, as discussed below. including free crystals and microphenocrysts aggregating as fine-grained mafic inclusions. For the massive lava in the dome center (SiW1r, SiW2s, SiW5Q, and SiW7Ob), Physical properties of lava the total phenocryst contents are 16–18 vol.%, excluding the vesicles (<10 vol.%), and for the massive lava in the The morphology of the silicic lava domes both in air and dome margin (SiE18E, SiW9L, and SiE16F), they are 23– under the water is controlled mainly by temperature, viscos- 25 vol.%, excluding the vesicles (<10 vol.%). The ground- ity, yield stress and effusion rate, according to theoretical, mass of both parts shows a hyalopilitic texture comprising experimental and geological studies (subaerial: Huppert plagioclase microlites. Microlites are less abundant in the et al. 1982; Blake 1990; Griffiths and Fink 1993; Fink and marginal part than the central part. The higher total phe- Griffiths 1998; Nakada et al. 1999; submarine dome: nocryst content of the dome margin is due to the presence Griffiths and Fink 1992; Gregg and Fink 1995). of a large number of fine-grained mafic inclusions. The The viscosity of the central and marginal parts of the inclusions are also present in the central part, but are less dome can be estimated by using the surface-folding model abundant than the marginal part. of Fink and Fletcher (1978) and Fink (1980). We consider Whole-rock and glass major element compositions of the here the folding of a planar flow, subjected to a uniform same samples are listed in Table 3. The whole-rock SiO2 compressive strain rate. For the central rhyolitic part, given contents of dome samples range from 67 to 73 wt.%, and the parameters 20 m for the dominant wavelength of wrin- the glass SiO2 contents range from 76–79 wt.% (Fig. 13a, kles, 5 m for the approximate thickness of the brittle crust Table 3). The samples from the marginal part (SiE18E, (the depth of fractures), and 1011 Pa S for the viscosity of the SiW9L, and SiE16F) are poor in SiO2, whereas those from lava surface at the glass-transition temperature, the calcu- the dome center (SiW1r, SiW2s, SiW5Q, and SiW7Ob) are lated viscosity of the dome interior is 108–109 Pa S using the rich in SiO2 (Fig. 13b). model of Hess and Dingwell (1996). The glass-transition Saito et al. (2002) suggested that a magma chamber is temperature (Tg) for the Showa Iwo-jima lava is estimated ◦ ◦ = − present beneath Showa Iwo-jima and that the magma is at 710–760 C using the equation Tg ( C) 778 223 WH2O stratified upward from a lower basaltic layer through a thin (Taniguchi 1981 ), where WH2O is the water content. A wa- middle layer of andesite to an upper rhyolitic layer. They ter content of 0.1–0.3 wt.% was used, based on the results concluded that multiple injections of very similar basaltic of FTIR analyses of the groundmass glasses for 11 sam- magma have occurred since the growth of the neighboring ples from the central to marginal part (Maeno, unpublished Fig. 12 a Hyaloclastite in the breccia zone, ramped to about 5minheightabovethesea 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 the volume fraction of crystals. The viscosity of the central dominant wavelength of wrinkles, 3 m for the approximate part with a phenocryst content of 16–18 vol.% is estimated thickness of the brittle crust, and 1011 Pa S for the viscosity at only 0.2-0.3 orders in Pa S higher than that of the of the lava surface at the glass-transition temperature, the marginal lava with a phenocryst content of 22–25 vol.%, calculated viscosity of the dome interior is 107–108 Pa S. when the temperature and the water content are the same. The dome contains lava with two slightly different chem- The result is that the difference of phenocryst contents has ical compositions: the less silicic lava makes up the outer little inﬂuence on the viscosity. margins and the more silicic lava forms the more proximal We can estimate the temperature of the dome interior portions (Fig. 13). However, composition has little inﬂu- using the calculated viscosity from the surface-folding ence on the viscosity (less than one order of magnitude) model (Fink and Fletcher 1978; Fink 1980). The inner lava at temperatures higher than the glass-transition tempera- in the central part with a viscosity of 108–109 Pa S should ture, as calculated by the model of Shaw (1972) using have a temperature of 830–900◦C for a water content of the groundmass glass composition. The effect of the phe- 0.3 wt.%, and one of 910–990◦C for a water content of nocryst content on viscosity was also calculated by the 0.1 wt.%, when calculated according to the method of −2.5 Einstein–Roscoe equation η = η0(1 − Rϕ) , where η Hess and Dingwell (1996). Therefore, we suggest that and η0 are the bulk and liquid phase viscosities, respec- the temperature of the inner lava in the central part was tively, R is constant set to be 1.67 (Marsh 1981 ), and ϕ is 830–990◦C. For the outer lava for which the viscosity is

Table 2 Modal abundance of 1234567 representative samples of Showa Iwo-jima lava SiW1r SiW2s SiW5Q SiW7Ob SiE18E SiW9L SiE16F Plagioclase 14.4 14.5 12.6 14.9 17.6 21.4 18.4 Hypersthene 0.9 0.3 0.7 1.2 1.5 1.8 2.7 Augite 1.0 1.2 1.7 1.1 1.8 1.1 1.2 Fe-Ti oxide 1.3 0.9 1.3 0.7 1.2 1.1 1.4 Total 17.6 16.9 16.2 18.0 22.0 25.5 23.7 Sampling localities are shown in Fig. 3 Groundmass 82.4 83.1 83.8 82.0 78.0 74.5 76.3 Table 3 Whole-rock and glass major element composition (wt.%) of Showa Iwo-jima lava Whole-rock major element composition Groundmass glass composition 123 4 5 6 7 1 7 SiW1r SiW2s SiW5Q SiW7Ob SiE18E SiW9L SiE16F SiW1r SD SiE18E SD

SiO2 72.00 71.87 71.63 70.70 69.80 68.99 68.12 79.05 0.27 75.30 1.67 TiO2 0.55 0.55 0.57 0.59 0.63 0.65 0.65 0.41 0.05 0.61 0.08 Al2O3 13.52 13.73 13.69 13.86 14.39 14.69 14.52 11.73 0.08 12.17 0.20 Fe2O3 1.25 1.29 1.38 1.58 1.98 2.00 2.08 – – – – FeO 1.83 1.94 2.13 2.15 2.13 2.27 2.26 1.21 0.04 1.58 0.26 MnO 0.09 0.09 0.10 0.10 0.11 0.11 0.12 0.06 0.04 0.07 0.05 MgO 0.72 0.77 0.84 0.96 1.11 1.18 1.15 0.08 0.01 0.10 0.06 CaO 2.53 2.66 2.73 3.02 3.38 3.74 3.70 0.84 0.05 0.80 0.20

Na2O 4.22 4.21 4.20 4.11 4.10 4.24 4.23 3.41 0.07 2.65 0.18 K2O 2.49 2.44 2.40 2.35 2.27 2.10 2.08 3.31 0.04 5.60 0.17 P2O5 0.11 0.11 0.12 0.13 0.14 0.16 0.16 – – – – + H2O 0.30 0.25 0.24 0.38 0.17 0.21 0.19 – – – – − H2O 0.03 0.08 0.14 0.10 0.11 0.12 0.26 – – – – Total 99.65 99.98 100.15 100.05 100.32 100.46 99.52 100.08 (n=11) 98.87 (n=10) 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 and curving sides (Fink and Griffiths 1998). Other silicic at 50–100◦C higher than that of the inner part. Saito et al. lava domes, such as the Showa Iwo-jima lava dome, have (2001, 2002) estimated a water content of less than 1 wt.% an axisymmetric shape (Fig. 3). Axisymmetric domes have by FTIR analysis of melt inclusions in plagiolclase and a regular outlines, relatively low relief, and surfaces that tend temperature of 970◦C using pyroxene geothermometry for to be covered by abundant small (a few tens of centimeters pre-eruption Showa Iwo-jima magma. to a few meters in length) fragments (Fink and Griffiths Another parameter, yield stress (σ ), strongly depends 1998). Although the average effusion rate (1×105 m3/day) on temperature. We use the equation for yield stress, for Showa Iwo-jima is almost the same as that of the earlier b(Tl−T ) 5 3 σ = σl × e (Doragoni et al. 1992), where T and Tl stage of Mount Unzen (3×10 m /day; Nakada and Fujii are the temperature of lava and the liquidus temperature, 1993), the dome shape is very different. The volume of 6 3 respectively, σl is the yield stress at T=Tl and b is the the dome of Showa Iwo-jima is 6.2×10 m in the total 2 constant. A 50–100◦C temperature difference results in a months of activity, and the volume of Unzen is 23×106 m3 difference of 2–4 orders of magnitude in the yield stress. in the first 6 months of activity (Nakada and Fujii 1993), Hence, the high-temperature lava of the outer part probably respectively. The difference between their morphologies is had a lower yield stress, and the low-temperature lava of derived from the deformation behavior that is transitional the central part probably had a higher yield stress. 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 fail- Discussion ure condition analysis (Goto 1999), or 2–4×1010 Pa S, based on the analysis using moving lobe conditions (Suto Morphology of Showa Iwo-jima dome et al. 1993), and the temperature was 780–880◦C (Nakada and Motomura 1999). On the other hand, the lava for the The morphology of a lava dome is controlled by the phys- Showa Iwo-jima dome had a lower viscosity (107–9 Pa S) ical properties of the actively moving lava: temperature, and lower yield stress than that of Mount Unzen, due to viscosity, yield stress, and effusion rate. These physical pa- its inferred high temperature (830–990◦C), resulting in its rameters directly control the thickness and aspect ratio of axisymmetric shape. 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 morphol- Subaerial growth of Showa Iwo-jima dome ogy, due to high viscosity (e.g. Unzen, Montserrat and Redoubt; Fink and Griffiths 1998). Spiny domes tend to be The aerial photographs of Showa Iwo-jima show surface high with very steep sides, and their tops are commonly folds that have continuous, arcuate fold axes, roughly par- punctured by one or more subvertical spines with smooth allel 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 decompres- sion 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

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, Fig. 13 a SiO2-K2O diagram for Showa Iwo-jima dome samples. Sample locations are shown in Fig. 3. b Compositional variation was more folded than the near-crater part, as the leading of Showa Iwo-jima lava dome for normalized distance (D/D0) from edge almost ceased flowing due to the formation of a rigid the vents. The samples from the marginal part are SiO2-poor (67– margin. 70 wt.%), whereas those from the dome center are SiO2-rich (70– 73 wt.%) Fracturing and brecciation in the marginal part breccia, observed in the subaerial parts of the dome at of Showa Iwo-jima dome present, may have been uplifted during the lava emplacement. If the dome front became rigid and almost stopped Formation of the fracture zone advancing, further output of lava from the vent could have squeezed earlier-erupted lava and breccia outwards, result- The structures and components of the dome margin re- ing in ramping of breccia above the sea surface. The breccia flect the conditions of lava emplacement. The fracture zone zone is limited to the outer margin near Locations B and F, in the Showa Iwo-jima dome is interpreted as the prod- implying that there was very little uplifted breccia, or else uct of the actively moving lava in the dome margin. Once it has been almost entirely eroded. 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 Formation of Showa Iwo-jima Island repeatedly injected into the growing dome, the fractures and ramp structures moved outwards, and the orientation The Showa Iwo-jima eruption in 1934–1935 is divided of the flow foliation in the marginal part became predomi- into four stages; (1) the formation of a submarine edifice nantly subvertical. With time, each fraction of the fractured with floating pumice, from September to December in and ramped lava became progressively attenuated, due to 1934, (2) phreatomagmatic explosions and pyroclastic stretching and shearing (Fig. 14a). While the active dome cone-formation in December, (3) lava effusion and surface was alternately extended and compressed, the frac- formation of a new pyroclastic cone on the western side tures along the foliation propagated into the viscous interior of the lava, in early January 1935 (Figs. 2d and 15a) and lava, resulting in thin layers of massive lava, that is, ML followed by (4) effusion of the new silicic lava and dome layers (Fig. 14b). The surface of each ML layer would growth from January to March (Figs. 2d and 15b–d). have undergone insitu vesiculation and brecciation, due to The growth of the dome from late January is summarized a temperature gradient between the hot interior lava and in the following scenario: The high-temperature dacite lava the surface of the open fractures (right figure in Fig. 14b). began to effuse from the central crater using two separate When the fractures closed, newly produced breccias were vents (Fig. 15b). Judging from some photographs (exam- embedded in the lava, reheated and welded, resulting in ple: Fig. 2c), during the earlier dome growth stage, a large massive lava (ML) interlayered with welded oxidized brec- amount of steam was produced and rose up from the mar- cia (WOB) (Fig. 14c). Tongues of massive lava extended gin. During the later stage, low-temperature rhyolite lava into the WOB (ML with striated surface) and lenticular- effused subaerially, and flowed out to the shallow seafloor shaped cavities formed at the hinges of the folded massive on the eastern side and onto formerly emplaced lava on the lava, indicating pull-apart of the lava by internal shear (e.g. western side (Fig. 15c). Although Tanakadate (1935a,b)re- Castro et al. 2002). ported only one central crater, our observation indicates that Formation of a fine matrix in the WOB was also related to another vent also exists in the center of the eastern sector. movement of the active lava. Manley (1996) suggested that The breccia zone was formed by rapid cooling when lava pumice at the surface of domes is comminuted, producing entered the sea. In March 1935, the central part of the dome, loose shards, bits of pumice, chips of dense glass, and frag- composed of rhyolite lava, grew up subaerially, resulting in ments of phenocrysts. This debris sifts down around loose an axisymmetric shape (Fig. 15d). In the marginal part, the blocks and into open fractures deeper in the flow, where fracture zone was produced by lava being successively frac- it can be reheated, compressed, and annealed to varying tured, ramped, and brecciated. Newly produced breccias degrees. Fine matrix may have been locally remobilized were embedded in the lava and reheated when the fractures by the escaping gas shortly after emplacement. Secondary closed, resulting in the formation of welded oxidized minerals crystallized on the surfaces of cavities, showing breccias (WOB). During this stage, hyaloclastite breccia that gas continued to escape from the lava after the em- was also ramped above the sea surface by progressive emplacement. placement 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 Formation of the breccia zone parts of the Showa Iwo-jima dome. Post-emplacement subsidence and wave erosion of the marginal part resulted Breccia in the outer margin of the dome is interpreted to in the present Showa Iwo-jima dome (Fig. 15e). 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; Conclusion Yamagishi 1987; Kano et al. 1991). While the breccia was being generated, a large amount of steam was produced The Showa Iwo-jima dome consists of two main parts; a and rose up from the margin, as observed by Tanakadate central part and a marginal part. The marginal part is also (1935a,b). Tuffisite in the breccia zone may have been pro- characterized by two zones; a fracture zone and a brec- duced by quenching of the lava under the sea. Hyaloclastite cia zone. The lava dome contains lava with two slightly Fig. 15 Model for the emplacement of Showa Iwo-jima silicic lava zone, characterized by the ML-WOB layers, was produced by frac- dome (cross sections; directions are southwest (SW) and east (E), as turing and ramping during movement of the lava. Hyaloclastite at the shown in the insert). a Pyroclastic cone formation and minor lava ef- dome margin was ramped above the sea surface. The central part of fusion with phreatomagmatic eruptions in early January 1935. b The the dome remained subaerial, resulting in the Showa Iwo-jima lava dacitic lava effused from the crater of the pyroclastic cone. c Rhyolite dome in late March, 1935. e Post-emplacement subsidence and wave lava effused subaerially using two separate vents, and flowed out to erosion of the marginal part produced the present Showa Iwo-jima the shallow seafloor on the eastern side and onto formerly emplaced lava dome. Dashed lines show the approximate boundary of dense lava on the southwestern side, from late January, 1935. d The fracture lava (MRHY) and vesicular lava (FVL, CVL) different chemical compositions, one of them being more with seawater and brecciated, forming hyaloclastite. Dur- dacitic in the marginal part and the other being more rhy- ing the later stage, low-temperature rhyolite lava effused olitic in the central part. The high-temperature dacite lava subaerially, and the fracture zone was produced by suc- began to effuse from the central craters and flowed onto the cessive fracturing, ramping, and brecciation of the actively shallow seafloor. The surface of the dome came into contact moving dome front. At the southern margin, hyaloclastite breccia was ramped above the sea surface by progressive in- Hess KU, Dingwell DB (1996) Viscosities of hydrous leucogranitic crements of new lava. The central part was folded, forming melts: a non-Arrhenian model. Am Miner 81:1297–1330 a surface layer of pumice breccia. Subaerial emplacement Huppert HE, Shepherd JB, Sigurdsson H, Sparks RSJ (1982) On lava dome growth, with application to the 1979 lava extrusion of the of lava was a dominant process during the growth of the Soufriere of St. Vincent. J Volcanol Geotherm Res 14:199–222 Showa Iwo-jima dome. The process of dome growth in a Kano K (2003) Subaqueous pumice eruptions and their products: shallow sea can be elucidated by this evidence. a review. In: White DL, Smellie JL, Clague DA (eds), Explo- sive subaqueous volcanism. Geophysical Monograph, vol 140. Acknowledgements We acknowledge A Goto, T Miyamoto, M American Geophysical Union, pp 213–229 Ichihara, and A Yokoo for their logistical advice. We also thank H Kano K, Takeuchi K, Yamamoto T, Hoshizumi H (1991) Subaqueous Fujimaki for the XRF analyses. We are grateful to Geographical rhyolite block lavas in the Miocene Ushikiri Formation, Shimane Survey Institute of Japan for permission to use the aerial photograph Peninsula, SW Japan. J Volcanol Geotherm Res 46:241–253 of Showa Iwo-jima, and to Mishima village, Kagoshima, Japan, for Kawanabe Y, Saito G (2002) Volcanic activity of the Satsuma- help with our field survey. We thank J McPhie, K Kano, and K Dadd Iwojima area during the past 6,500 years. Earth Planets Space for constructive reviews of the manuscript. This research was partly 54:295–301 supported by a Grant-in-aid for Scientific Research from the Ministry Macdonald GA (1972) Volcanoes. Prentice Hall, Englewood Cliffs, of Education, Culture, Sports, Science and Technology, Japan (No. New Jersey 14080203). Maeno F, Taniguchi H (2005) Eruptive history of Satsuma Iwo-jima Island, Kikai caldera, after a 6.5 ka caldera-forming eruption. Bull Volcanol Soc Jpn 2nd Ser 50:71–85 (in Japanese with En- References glish abstract) Manley CR (1996) In situ formation of welded tuff-like textures in Anderson SW, Fink JH (1992) Crease structures as indicators of the carapace of a voluminous silicic lava flow, Owyhee Country, emplacement rates and surface stress regimes of lava flows. SW, Idaho. Bull Volcanol 57:672–686 Geol Soc Am Bull 104:615–626 Matumoto T (1936) Submarine eruption off the Iwo-jima. Jpn Acc Anderson SW, Stofan ER, Plaut JJ, Crown DA (1998) Block size Soc Rep 11:468–470 (in Japanese) distributions on silicic lava flow surfaces: implications for em- Matumoto T (1943) The four gigantic caldera volcanoes of Kyushu. placement conditions. Geol Soc Am Bull 110:1258–1267 Jpn J Geol Geogr 19:1–57 Blake S (1990) Viscoplastic models of lava domes. In: Fink JH (ed) Marsh BD (1981) On the crystallinity, probability of occurrence, and Lava flows and domes. IAVCEI Proceedings, vol 2, pp 88–128 rheology of lava and magma. Contrib Mineral Petrol 78:85–98 Cas RAF, Allen RL, Bull SW, Clifford BA, Wright JV (1990) Sub- McPhie J, Doyle M, Allen R (1993) Volcanic textures: a guide to the aqueous, rhyolitic dome-top tuff cones: a model based on the interpretation of textures in volcanic rocks. Center for Ore De- Devonian Bunga Beds, southeastern Australia and a modern posit and Exploration Studies, University of Tasmania, Hobart, analogue. Bull Volcanol 52:159–174 pp 198 Castro J, Cashman KV, Joslin N, Olmsted B (2002) Structural ori- Nakada S, Fujii T (1993) Preliminary report on volcanic activity at gin of large gas cavities in the Big Obsidian Flow, Newberry Unzen Volcano, November 1990–November 1991: Dacite lava Volcano. J Volcanol Geotherm Res 114:313–330 domes and pyroclastic flows. J Volcanol Geotherm Res 54:319– DeRita D, Giordano G, Cecili A (2001) A model of submarine 333 rhyolite dome growth: Ponza Island (central Italy). J Volcanol Nakada S, Motomura Y (1999) Petrology of the 1991–1995 eruption Geotherm Res 107:221–239 at Unzen: effusion pulsation and groundmass crystallization. J De Rosen-Spence AF, Provost G, Dimroth E, Gochnauer K, Owen Volcanol Geotherm Res 89:173–196 V (1980) Archean subaqueous felsic flows, Rouyn-Noranda, Nakada S, Shimizu H, Ohta K (1999) Overview of the 1990–1995 Quebec, Canada, and their Quaternary equivalents. Precambrian eruption at Unzen. J Volcanol Geotherm Res 89:1–22 Res 12:43–77 Okuno M, Fukushima D, Kobayashi T (2000) Tephrochronology in Doragoni M, Pondrelli S, Tallarico A (1992) Longitudinal deforma- southern Kyusyu, SW, Japan: tephra layers for the past 10,000 tion of a lava flow: the influence of Bingham rheology. J Volcanol years. J Soc Human History 12:9–23 (in Japanese with English Geothem Res 52:247–254 abstract) Fink JH (1980) Surface folding and viscosity of rhyolite flows. Ge- Ono K, Soya T, Hosono T (1982) Geology of the Satsuma-Io-Jima ology 8:250–254 District. Quadrangle Series, Scale 1:50,000. Geol Surv Japan pp Fink JH (1983) Structure and emplacement of a rhyolitic obsidian 1–80 (in Japanese with English Abstract) flow: Little Glass Mountain, Medicine Lake Highland, northern Pichler H (1965) Acid hyaloclastite. Bull Volcanol 28:293–310 California. Geol Soc Am Bull 94:362–380 Reynolds MA, Best JG, Johnson RW (1980) 1953–1957 eruption Fink JH, Fletcher RC (1978) Ropy pahoehoe: surface folding of a of Tuluman volcano: rhyolitic volcanic activity in the northern viscous fluid. J Volcanol Geotherm Res 4:151–170 Bismarck Sea. Geol Surv Papua New Guinea Mem 7 Fink JH, Griffiths RW (1998) Morphology, eruption rates, and rheol- Saito G, Kazahaya K, Shinohara H, Stimac J, Kawanabe Y (2001) ogy of lava domes: insight from laboratory models. J Geophys Variation of volatile concentration in a magma system of Res 103:527–745 Satsuma-Iwojima volcano deduced from melt inclusion anal- Fiske RS, Cashman KV, Shibata A, Watanabe K (1998) Tephra dis- yses. J Volcanol Geotherm Res 108:11–31 persal from Myojinsho, Japan, during its shallow submarine Saito G, Stimac J, Kawanabe Y, Goff F (2002) Mafic inclusions eruption of 1952–1953. Bull Volcanol 59:262–275 in rhyolites of Satsuma-Iwojima volcano: evidence for mafic- Goto A (1999) A new model for volcanic earthquake at Unzen vol- silicic magma interaction. Earth Planets Space 54:303–325 cano: melt rupture model. Geophys Res Lett 26:2541–2544 Shaw HR (1972) Viscosities of magmatic silicate liquid: an empirical Goto Y, McPhie J (1998) Endogenous growth of Miocene submarine method of prediction. Am J Sci 272:870–893 dacite cryptodome, Rebun Island, Hokkaido, Japan. J Volcanol Suto S, Sakaguchi K, Watanabe K, Saito E, Kawanabe Y, Kazahaya Geotherm Res 54:19–32 K, Takarada S, Soya T (1993) Dynamics of flowage and viscosity Gregg TKP, Fink JH (1995) Quantification of submarine lava-flow of the 1991 lava of Unzen Volcano, Kyushu. Bull Geol Surv Jpn morphology though analog experiments. Geology 23:73–76 44:609–629 (in Japanese with English abstract) Griffiths RW, Fink JH (1992) Solidification and morphology of sub- Tanakadate H (1935a) Preliminary report of the eruption of Iwojima, marine lavas: a dependence on extrusion rate. J Geophys Res Kagoshima Prefecture. Bull Volcanol Soc Jpn Ser 1 2:188–209 97:19729–19737 (in Japanese) Griffiths RW, Fink JH (1993) Effects of surface cooling on the spread- Tanakadate H (1935b) Evolution of a new volcanic islet near Io-zima ing of lava flows and domes. J Fluid Mech 252:667–702 (Satsuma Prov.). Proc Imp Acad Jpn 11:152–154 Taniguchi H (1981) Effects of water on the glass transformation Yamagishi H (1987) Studies on the Neogene subaqueous lavas and temperature of rhyolitic rock melt. J Jpn Assoc Min Petrol Econ hyaloclastites in southwest Hokkaido. Rep Geol Surv Hokkaido Geol 76:49–57 59:55–117 Tiba T (1970) JB-1 and JG-1-Geological Survey of Japan silicate Yamagishi H (1994) Subaqueous volcanic rocks: atlas and glossary. rock standard. J Geol Soc Jpn 76 9:441–447 Hokkaido University Press Yajima K, Ono M, Fujimaki H (2001) Analytical accuracy and pre- cision of major and trace elements for bulk rocks using a 1:5 dilution glass bead by XRF. Jpn Mag Mineral Petrol Sci 30:28– 32 (in Japanese with English Abstract)