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-jimaeruption, Kikai caldera, south of Kyushu, Japan

Received: 17 May 2004 / Accepted: 5 October 2005C© Springer-Verlag 2005

Abstract The 1934–1935 Showa Iwo-jima eruptionstarted with a silicic lava extrusion onto the floor of thesubmarine Kikai caldera and ceased with the emergence ofa lava dome. The central part of the emergent dome con-sists of lower microcrystalline rhyolite, grading upwardinto finely vesicular lava, overlain by coarsely vesicularlava with pumice breccia at the top. The lava surface isfolded, and folds become tighter toward the marginal partof the dome. The dome margin is characterized by twozones: a fracture zone and a breccia zone. The fracturezone is composed of alternating layers of massive lava andwelded oxidized breccia. The breccia zone is the outermostpart of the dome, and consists of glassy breccia interpretedto be hyaloclastite. The lava dome contains lava with twoslightly different chemical compositions; the marginal partbeing more dacitic and the central part more rhyolitic. Thefold geometry and chemical compositions indicate that themarginal dacite had a slightly higher temperature, lowerviscosity, and lower yield stress than the central rhyolite.The high-temperature dacite lava began to effuse in theearlier stage from the central crater. The front of the domecame in contact with seawater and formed hyaloclastite.During the later stage, low-temperature rhyolite lava ef-fused subaerially. As lava was injected into the growingdome, the fracture zone was produced by successive frac-turing, ramping, and brecciation of the moving dome front.In the marginal part, hyaloclastite was ramped above the seasurface by progressive increments of the new lava. The cen-tral 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, Japane-mail: fmaeno@cneas.tohoku.ac.jpTel.: +81-22-795-7552Fax: +81-22-795-6272

H. TaniguchiCenter for Northeast Asian Studies, Tohoku University,Kawauchi, Aoba-ku, Sendai 980-8576, Japan

Subaerial emplacement of lava was the dominant processduring 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 lavaflows or domes are well recognized, based on geological(Pichler 1965; De Rosen-Spence et al. 1980; Yamagishi1987; Cas et al. 1990; Kano et al. 1991; Goto and McPhie1998; DeRita et al. 2001; Kano 2003), theoretical, andexperimental studies (Griffiths and Fink 1992; Gregg andFink 1995). Almost all of this knowledge is limited tocases where the entire process took place under the sea.If a submarine dome continues to grow and emerge abovethe sea surface, the cooling dynamics and the mode ofemplacement of the lava, in governing the surface andinternal structures of the dome, will reflect the combinationof submarine and subaerial settings. The detailed processof emergent dome growth is, however, rarely describedin modern oceans (the 1953–1957 eruption of Tulumanvolcano, Reynolds et al. 1980; the 1952–1953 eruptionof Myojinsho volcano, Fiske et al. 1998) and in ancientvolcanic terrains (De Rosen-Spence et al. 1980; Cas et al.1990; DeRita et al. 2001).

This paper describes the partly emergent Showa Iwo-jimalava dome produced by a submarine eruption in 1934–1935of the Kikai caldera, Kyushu, Japan. The eruption wasobserved directly by Tanakadate (1935a,b), and the surfaceand internal structure of this silicic dome are well exposedand preserved, providing a good opportunity for examininglava dome growth.

Geological setting

Showa Iwo-jima lava dome exists on the northern rim ofKikai caldera, 40 km southwest of Cape Sata, southern

Fig. 1 a The location of Kikaicaldera, south of Kyushu, Japan.b The location of ShowaIwo-jima dome. c Submarinetopography around the ShowaIwo-jima dome

Kyushu, Japan (Fig. 1a). Kikai caldera is 17 km wide and20 km long, and almost completely submerged. It is locatedat the southern end of a volcano-tectonic depression alongthe volcanic front of southwestern Japan. This caldera wasproduced at 6.5 ka by the catastrophic eruption of the Koyaignimbrite, which covered southern Kyushu Island. ShowaIwo-jima and the adjacent Satsuma Iwo-jima were formedby post-caldera eruptions at the caldera rim. SatsumaIwo-jima comprises Iwo-dake (rhyolitic volcano) andInamura-dake (basaltic volcano) (Fig. 1b). Inamura-dakewas produced at 3.5–2.8 ka, and Iwo-dake has been activesince 5.6 ka (Ono et al. 1982; Okuno et al. 2000; Kawanabe

and Saito 2002; Maeno and Taniguchi 2005). Showa Iwo-jima was erupted in 1934–1935 from a vent on the calderafloor, 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 describedby Tanakadate (1935a,b) and Matumoto (1936). Theeruption is divided into the following four stages.

The first stage was characterized by submarine activ-ity. Floating pumices (Kano 2003) were first noticed in

Fig. 2 a Submarine eruption ofShowa Iwo-jima with a plumeof steam, viewed from thesummit crater of Iwo-dakevolcano (September 1935). Thediameter of the plume was notdescribed, but was probably lessthan 1 km. b The blocks ofpumice (arrows) floating on thesea (September 1935). The sizeof the largest block reachedabout 10 m in length. c Newlava islet, viewed from the topof Iwo-dake volcano (January1935). d Sketches of the ShowaIwo-jima dome (January 21 andMarch 31, 1935) based on directobservation by Tanakadate andmodified from Tanakadate(1935a,b). The cone was madeof pyroclastic deposits. e Mapof the Showa Iwo-jima domebefore erosion by wave action(July 1935; Matumoto 1936)and at present. Photos byTanakadate in Matumoto (1943)

September 1934 and were accompanied by earthquakes(Fig. 2a, b). The second stage started around December8 when a pyroclastic cone first became visible above thesea level and emitted ‘white smoke’ from its crater. Duringthis stage, there were numerous explosive eruptions re-peated at intervals of 1–2 min. Each eruption ejected enor-mous cauliflower-shaped ‘dark smoke’ through the middleof the ‘white smoke’. The pyroclastic cone was destroyedby a strong explosion on December 30. The third stagewas characterized by lava effusion, accompanied by somephreatomagmatic eruptions which generated cock’s tail jetsrepeated at intervals of less than a few minutes. In earlyJanuary 1935, new lava emerged on the western side of theislet. On January 8, a new pyroclastic cone was visible onthe lava (Fig. 2c). On January 21, the height of the newcone exceeded 12 m above high tide level. The volcanicislet had a maximum length of 300 m in the NE directionand was about 150 m across (Fig. 2d). In the fourth stagefrom late January to March, new silicic lava effused and adome grew. On February 10, a new small islet, composedof lava, appeared 50 m northwest from the main islet. Inearly March, small explosions were sometimes observed.The central crater of the main islet widened and the craterrim collapsed. Later, effusion of a large amount of lavaburied the entire crater. The former pyroclastic cone wascovered with the lava. On March 26, a new main islet,the present Showa Iwo-jima lava dome, was observed withlittle ‘smoke’. All activity seemed to decline at this time.The dome was about 300 m in length in the NS directionand 530 m across, and its height was 55 m above the sealevel (Fig. 2d, e). Part of the other new islet near the maindome disappeared in 1936 as a result of erosion by waveaction.

By assuming an elliptical plate shape (height, longand short axis lengths) for the dome, the total volume ofeffused lava from 21 January (12 m × 250 m × 150 m)to 26 March in 1935 (55 m × 530 m × 270 m) canbe estimated at about 6.2×106 m3, and the averageeffusion rate during this period to be about 1×105 m3/day,comparable with 3×105 m3/day in the first 6 months ofactivity (May 1991–November 1991) of the 1991–1996Mt. 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 itsheight is 20 m above the sea level (Fig. 3a, b). The dome ison top of the submarine edifice which rises 300 m from theseafloor (Fig. 1b, c) and which was produced by the firststage of the eruption from September to December 1934.The Showa Iwo-jima lava dome produced between Januaryand March 1935 subsided about 30 m in 3 months just afterthe eruption (from April to July in 1935), and has beenreduced in area by wave erosion (Fig. 2e).

The dome consists of two main parts: a central part anda marginal part. The marginal part is also characterizedby 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 zoneconsists of hyaloclastite. Figure 4 shows a cross-section ofthe southwestern dome (X–Y line in Fig. 3b).

Structure of the central part

The central part of the Showa Iwo-jima lava dome containsa central crater about 50 m across and an eastern craterabout 20 m across. Lava in both craters has multiple creasestructures (Anderson and Fink 1992; Fig. 3a, b) and is finelyvesicular with curviplanar surfaces. Microcrystalline rhy-olite (MRHY) occurs at depths of 3–5 m in deep fractures(Fig. 5c). The rock faces exposed by crease structures arestriated, perhaps due to the scraping of lava on lava duringemplacement.

Around the central crater, in the western sector, the domesurface is wrinkled, and the dome consists of lower mi-crocrystalline 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 andupper coarsely vesicular lava (Fig. 5a; CVL) with a den-sity of 500–600 kg/m3. These parts are coherent. The sur-face consists of a mixture of finely vesicular (FVPb) andcoarsely vesicular (CVPb) pumice breccia. Coarsely vesic-ular pumice breccia is dominant on the surface of the west-ern sector (Fig. 5d). Pumice clasts of the surface brecciaare blocky and a few tens of centimeters to a few metersin length. The densities of pumice clasts in the FVPb andCVPb are 1,000–1,200 kg/m3 and 500–600 kg/m3, respec-tively. The eastern sector of the dome consists of finelyvesicular lava (FVL) and pumice breccia (FVPb) (Fig. 5e),and minor coarsely vesicular lava (CVL) and pumice brec-cia (CVPb). Although the densities are variable, almost allof the lava in the central part has 16–18 vol.% phenocrystcontents. The interior of the dome (MRHY) is exposedon the southern and northern coasts at Locations A andD (Fig. 3), and shows an onion-like structure defined byflow-banding (Fig. 6a, b). Lava wrinkles are increasinglytight toward the marginal part (Fig. 6c, d).

From the wavelength (L) and amplitude (H) of lava wrin-kles determined at 20 and 6 locations along the western andeastern cross-sections, respectively (Fig. 7a, b), the H/L ra-tio increases from 0.2 (4 m/18 m) to 1.3 (4 m/3 m) towardthe margin of the lava dome (Table 1, Fig. 8). Here, we use anormalized distance (D/D0) from one of the craters to eachlava wrinkle, because the dome shape is not a perfect circlebut a distorted ellipse (Fig. 7c). D0 is the distance fromthe 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, D0is the distance from the center of central crater (C) to themargin (Q7). For the eastern lava wrinkles, D/D0 was mea-sured from the eastern crater. Any relationship between theorientation of flow-banding in the surface wrinkles and inthe interior is obscure, due to vesiculation and brecciationof the lava.

Fig. 3 a Aerial photograph ofShowa Iwo-jima dome taken bythe Geographical SurveyInstitute of Japan in 1977. bGeological map of ShowaIwo-jima dome. The outermargin is well-exposed due tothe erosion of wave action.Locations A–H arerepresentative outcropsinvestigated. Solid and dashedlines show the crests andtroughs of wrinkles, and thicksolid lines show the axes ofcrease structures. Arrows showthe strike of subverticalflow-banding. A cross-sectionalong X–Y is shown in Fig. 4.Closed squares are samplinglocalities. Modal analyses andwhole-rock compositions ofnumbered samples are listed inTables 2 and 3. FVL, finelyvesicular lava; FVPb, finelyvesicular pumice breccia;CVPb, coarsely vesicularpumice breccia

Structure of the marginal part

Fracture zone

The fracture zone is exposed on an erosion surface imme-diately 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 oxidizedbreccias (WOB, Fig. 9a). Welded oxidized breccias oc-cur in layers concordant with the subvertical flow-bandingin the massive lava, and some breccia layers are lentic-ular. Both ML and WOB are a few tens of centimetersto about 1 m thick. The massive lava (Fig. 9b) is poorlyvesicular, grayish, glassy dacite with abundant phenocrysts

Fig. 4 X–Y cross section (Fig. 3b) of the southwestern part ofShowa Iwo-jima dome. The dome has two main parts: a central partwith the original surface preserved and a marginal part with an ero-sion surface. The marginal part is characterized by a fracture zoneand a breccia zone. The fracture zone is composed of massive lava(ML) and welded oxidized breccias (WOB). The central part consists

of lower microcrystalline rhyolite (MRHY), middle finely vesicularlava (FVL), and upper coarsely vesicular lava (CVL); finely vesic-ular pumice breccia (FVPb) and coarsely vesicular pumice breccia(CVPb) cover the surface. Dashed lines show the approximate bound-ary of lithofacies in the stratigraphic section (left) and on the crosssection

Fig. 5 a Coarsely vesicularlava, b finely vesicular lava, andc microcrystalline rhyolite inthe central part of ShowaIwo-jima dome. The surfaceconsists of finely and coarselyvesicular pumice breccias. dCoarsely vesicular pumicebreccia (CVPb) is dominant onthe western dome surface. eFinely vesicular pumice breccia(FVPb) is dominant on theeastern dome surface

Fig. 6 Photograph a andsketch b show an onion-likestructure in the central part ofShowa Iwo-jima dome. Theforeground in the photo is thecentral part near Location A(Fig. 3), and the background isthe marginal part (fracture zoneand breccia zone) at Location B(Fig. 3). Photograph c andsketch d show folded lava in thewestern side of the central part(at Location E; Fig. 3). Arrowsshow the orientation of flowbanding. MRHY,microcrystalline rhyolite; FVL,fine vesicular lava; FVPb, finevesicular pumice breccia

Fig. 7 a A photo of a crosssection of Showa Iwo-jimadome at Location A (Fig. 3) isshown on the left. Definitions ofthe amplitude (H) andwavelength (L) of a lava wrinkleare given on the right. Arrowspoint to crests of examples ofmeasured wrinkles. bDistribution of measured lavawrinkles along the western(circles) and eastern (squares)parts of the dome. c Definitionof normalized distance (D/D0)from one of the craters to eachlava wrinkle. D0 is the distancefrom the center of one crater tothe margin before erosion,crossing each lava wrinkle. Forexample, for the wrinkle 7 (W7),D is the distance from thecentral crater (C) to P7, D0 isthe distance from the center ofcentral crater (C) to the margin(Q7). For the eastern lavawrinkles, D/D0 was measuredfrom the center of the easterncrater

Fig. 8 Results of wavelength analysis for lava wrinkles in the west-ern sector (closed circles) and the eastern sector (closed squares)of Showa Iwo-jima dome. H/L is the ratio of the amplitude (H) towavelength (L) of a lava wrinkle. D/D0 is the normalized distance

(>20 vol.%) and sparse spherulites 2–5 mm across. Theflow bands are a few to 30 mm thick, brown and light gray-ish bands. The strike of the flow bands is shown as arrows inFig. 3.

Welded oxidized breccias (Fig. 9c) comprise reddish,coarsely vesicular pumice clasts and nonvesicular (glassyor crystalline) lava fragments that are a few centimeters to1 m in length. These breccias are welded to various degrees.The matrix of the WOB comprises fragments of vesicular ornonvesicular 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 brokenphenocrysts have various shapes, and most of them aresubhedral (Fig. 10b). In contrast, the phenocrysts in thenear-vent massive lava are euhedral (Fig. 10a). The spacesbetween the broken phenocrysts are filled with a clear tobrownish-tan glassy material that appears isotropic undercrossed polars, and most likely has a hyalopilitic texture.Large, elongate crystals in the tuffisite are oriented parallelto the ML layers.

In the southeastern part (Location B), the degree of weld-ing compaction in the oxidized breccias is higher than atother breccias, and fragments of coarsely vesicular pumiceare lens-like in shape. Tongues of massive lava that ex-tend into the WOB (arrows in Fig. 11a) have striated sur-faces. Lenticular-shaped cavities (a few centimeters to a fewtens of centimeters in length) with vesicular surfaces occurat 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 onthe surfaces of cavities, similar to lithophysae (e.g. McPhieet al. 1993).

Table 1 Wavelength, amplitude, and normalized distance fromsources for wrinkles in the central part of Showa Iwo-jima lava dome

Distance Normalized Wavelength Amplitudedistance

D (m) D/D0 L (m) H (m) H/L

WestW1 36 0.16 18 4 0.22W2 69 0.30 14 4 0.29W3 84 0.37 11 3 0.27W4 86 0.60 9 3 0.33W5 90 0.40 7 2 0.29W6 109 0.48 10 2 0.20W7 129 0.57 7 3 0.43W8 95 0.51 9 3 0.33W9 112 0.60 8 3 0.38W10 123 0.66 7 3 0.43W11 131 0.58 7 3 0.43W12 142 0.63 6 2 0.33W13 156 0.69 7 2 0.29W14 190 0.84 4 3 0.75W15 206 0.91 3 4 1.33W16 210 0.97 2 2 1.00W17 41 0.14 18 4 0.22W18 66 0.70 5 3 0.60W19 75 0.81 3 3 1.00W20 64 0.57 4 3 0.75EastE1 72 0.64 4 2 0.50E2 73 0.55 7 3 0.43E3 101 0.70 6 3 0.50E4 112 0.78 4 2 0.50E5 134 0.59 4 2 0.50E6 180 0.73 5 5 1.00

Breccia zone

A breccia zone forms the seaward edge of the dome (Figs. 3and 4), and is characterized by breccia and tuffisite, whichpartially overlie the massive glassy or microcrystallinedacite lava (Fig. 12a). In the southeastern part (Location B)and southwestern part (Location F), this zone is distributedfrom the sea level to a few meters in height at themargin. The breccia in this zone is made up of polyhedralfragments, ranging from a few centimeters to 1 m in length,of poorly vesicular, glassy to microcrystalline dacite, and asmall amount of glassy pumiceous dacite. Most clasts areblack 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 brecciasuggest that it is insitu hyaloclastite (Pichler 1965).Tuffisite occurs in the fractures in the massive lava and thecoarse breccia of this zone (Fig. 12c). The components areglass particles (less than a few mm) and broken phenocrysts(less than a few mm in length), and the tuffisite is grayishin color.

Fig. 9 a Fracture zone atLocation E (Fig. 3) in ShowaIwo-jima dome. Massive lava(ML) is interlayered withwelded oxidized breccias(WOB). b Massive lava ispoorly vesicular, glassy dacitewith brown and light grayishbands. The orientation of theflow banding is parallel to thecontacts of the ML and WOBlayers. c Welded oxidizedbreccias comprise reddish,coarsely vesicular pumiceclasts, nonvesicular (glassy orcrystalline) lava fragments, andtuffisite

Fig. 10 Photomicrographs ofvolcanic rocks of ShowaIwo-jima. Phenocrysts areplagioclase (Pl), hypersthene(Hyp), augite (Aug), and Fe-Tioxide (Ox). Scale bar is 1 mm. aMassive lava in the central part,characterized by euhedralphenocrysts. The groundmassshows a hyalopilitic texture. bTuffisite in the fracture zone,characterized by brokenphenocrysts. c Brokenphenocrysts in tuffisite (planepolarized light). The tuffisitehas a grainy, clear tobrownish-tan glassy matrix thatappears isotropic under crossedpolars, and is most likely ahyalopilitic texture (arrowsshow fragmented crystals)

Fig. 11 a Sketch of fracturezone, comprising glassymassive lava (ML) and weldedoxidized breccias (WOB) atLocation B (Fig. 3) in ShowaIwo-jima dome. Arrows point totongues of lava extending fromthe main massive lava. Dashedlines show the flow-banding,with vertical foliation. bLenticular-shaped cavities (afew tens of centimeters to about1 m length) with vesicularsurfaces at the hinges of foldedmassive lava. c Spherulites (Sp),composed of albite-richplagioclase, commonly occurnear the cavities. d SEM imageof cristobalite (Cr) on thesurface of a cavity

Petrography and chemical composition

The modal proportions of phenocrysts were measured forseven representative samples by point counting (6,000points per sample, Table 2). Sampling localities are shownin Fig. 3. Phenocrysts (plagioclase, hypersthene, augite,and Fe-Ti oxide) are more than 200 µm (Fig. 12a) in size,including free crystals and microphenocrysts aggregatingas fine-grained mafic inclusions. For the massive lava inthe dome center (SiW1r, SiW2s, SiW5Q, and SiW7Ob),the total phenocryst contents are 16–18 vol.%, excludingthe vesicles (<10 vol.%), and for the massive lava in thedome margin (SiE18E, SiW9L, and SiE16F), they are 23–25 vol.%, excluding the vesicles (<10 vol.%). The ground-mass of both parts shows a hyalopilitic texture comprisingplagioclase microlites. Microlites are less abundant in themarginal part than the central part. The higher total phe-nocryst content of the dome margin is due to the presenceof a large number of fine-grained mafic inclusions. Theinclusions are also present in the central part, but are lessabundant than the marginal part.

Whole-rock and glass major element compositions of thesame samples are listed in Table 3. The whole-rock SiO2contents of dome samples range from 67 to 73 wt.%, andthe 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 fromthe dome center (SiW1r, SiW2s, SiW5Q, and SiW7Ob) arerich in SiO2 (Fig. 13b).

Saito et al. (2002) suggested that a magma chamber ispresent beneath Showa Iwo-jima and that the magma isstratified upward from a lower basaltic layer through a thinmiddle layer of andesite to an upper rhyolitic layer. Theyconcluded that multiple injections of very similar basalticmagma have occurred since the growth of the neighboring

Iwo-dake dome, based on the chemical variation of theIwo-dake and Showa Iwo-jima mafic inclusions. Althoughthe linear chemical trend (Fig. 13a) and petrography of theShowa Iwo-jima dome probably reflect the heterogeneity ofthe magma before emplacement, there is little difference inthe physical properties (e.g. viscosity) of the end-memberscompositions, as discussed below.

Physical properties of lava

The morphology of the silicic lava domes both in air andunder the water is controlled mainly by temperature, viscos-ity, yield stress and effusion rate, according to theoretical,experimental and geological studies (subaerial: Huppertet al. 1982; Blake 1990; Griffiths and Fink 1993; Fink andGriffiths 1998; Nakada et al. 1999; submarine dome:Griffiths and Fink 1992; Gregg and Fink 1995).

The viscosity of the central and marginal parts of thedome can be estimated by using the surface-folding modelof Fink and Fletcher (1978) and Fink (1980). We considerhere the folding of a planar flow, subjected to a uniformcompressive strain rate. For the central rhyolitic part, giventhe parameters 20 m for the dominant wavelength of wrin-kles, 5 m for the approximate thickness of the brittle crust(the depth of fractures), and 1011 Pa S for the viscosity of thelava surface at the glass-transition temperature, the calcu-lated viscosity of the dome interior is 108–109 Pa S using themodel of Hess and Dingwell (1996). The glass-transitiontemperature (Tg) for the Showa Iwo-jima lava is estimatedat 710–760◦C using the equation Tg (◦C) = 778−223 WH2O(Taniguchi 1981 ), where WH2O is the water content. A wa-ter content of 0.1–0.3 wt.% was used, based on the resultsof FTIR analyses of the groundmass glasses for 11 sam-ples from the central to marginal part (Maeno, unpublished

Fig. 12 a Hyaloclastite in thebreccia zone, ramped to about5 m in height above the seasurface and overlying massivelava at Location B (Fig. 3) inShowa Iwo-jima dome. b In thebreccia zone, glassy clasts arecharacterized by contractioncracks. c Many fracturesdeveloped on the surface of alava fragment are filled withtuffisite (arrows)

data). For the outer part, given the parameters 10 m for thedominant wavelength of wrinkles, 3 m for the approximatethickness of the brittle crust, and 1011 Pa S for the viscosityof the lava surface at the glass-transition temperature, thecalculated viscosity of the dome interior is 107–108 Pa S.

The dome contains lava with two slightly different chem-ical compositions: the less silicic lava makes up the outermargins and the more silicic lava forms the more proximalportions (Fig. 13). However, composition has little influ-ence on the viscosity (less than one order of magnitude)at temperatures higher than the glass-transition tempera-ture, as calculated by the model of Shaw (1972) usingthe groundmass glass composition. The effect of the phe-nocryst content on viscosity was also calculated by theEinstein–Roscoe equation η = η0(1 − Rϕ)−2.5, where ηand η0 are the bulk and liquid phase viscosities, respec-tively, R is constant set to be 1.67 (Marsh 1981 ), and ϕ is

the volume fraction of crystals. The viscosity of the centralpart with a phenocryst content of 16–18 vol.% is estimatedat only 0.2-0.3 orders in Pa S higher than that of themarginal 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 haslittle influence on the viscosity.

We can estimate the temperature of the dome interiorusing the calculated viscosity from the surface-foldingmodel (Fink and Fletcher 1978; Fink 1980). The inner lavain the central part with a viscosity of 108–109 Pa S shouldhave a temperature of 830–900◦C for a water content of0.3 wt.%, and one of 910–990◦C for a water content of0.1 wt.%, when calculated according to the method ofHess and Dingwell (1996). Therefore, we suggest thatthe temperature of the inner lava in the central part was830–990◦C. For the outer lava for which the viscosity is

Table 2 Modal abundance ofrepresentative samples ofShowa Iwo-jima lava

1 2 3 4 5 6 7SiW1r SiW2s SiW5Q SiW7Ob SiE18E SiW9L SiE16F

Plagioclase 14.4 14.5 12.6 14.9 17.6 21.4 18.4Hypersthene 0.9 0.3 0.7 1.2 1.5 1.8 2.7Augite 1.0 1.2 1.7 1.1 1.8 1.1 1.2Fe-Ti oxide 1.3 0.9 1.3 0.7 1.2 1.1 1.4Total 17.6 16.9 16.2 18.0 22.0 25.5 23.7Groundmass 82.4 83.1 83.8 82.0 78.0 74.5 76.3

Sampling localities are shown inFig. 3

Table 3 Whole-rock and glass major element composition (wt.%) of Showa Iwo-jima lava

Whole-rock major element composition Groundmass glass composition1 2 3 4 5 6 7 1 7SiW1r 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 1.67TiO2 0.55 0.55 0.57 0.59 0.63 0.65 0.65 0.41 0.05 0.61 0.08Al2O3 13.52 13.73 13.69 13.86 14.39 14.69 14.52 11.73 0.08 12.17 0.20Fe2O3 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 0.26MnO 0.09 0.09 0.10 0.10 0.11 0.11 0.12 0.06 0.04 0.07 0.05MgO 0.72 0.77 0.84 0.96 1.11 1.18 1.15 0.08 0.01 0.10 0.06CaO 2.53 2.66 2.73 3.02 3.38 3.74 3.70 0.84 0.05 0.80 0.20Na2O 4.22 4.21 4.20 4.11 4.10 4.24 4.23 3.41 0.07 2.65 0.18K2O 2.49 2.44 2.40 2.35 2.27 2.10 2.08 3.31 0.04 5.60 0.17P2O5 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 compositionAnalytical 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 byelectron microprobe (JEOL JSM-5410 with wavelength dispersive solid state detector of Oxford Link ISIS) using defocused electron beamof 10–20 m in diameter, accelerating voltage of 15 keV, and selecting random points (number of parentheses; n) in each thin sectionSD standard deviation of electron probe micro-analyses

estimated to be 107–108 Pa S, the temperature is estimatedat 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 atemperature of 970◦C using pyroxene geothermometry forpre-eruption Showa Iwo-jima magma.

Another parameter, yield stress (σ ), strongly dependson temperature. We use the equation for yield stress,σ = σl × eb(Tl−T ) (Doragoni et al. 1992), where T and Tlare the temperature of lava and the liquidus temperature,respectively, σl is the yield stress at T=Tl and b is theconstant. A 50–100◦C temperature difference results in adifference of 2–4 orders of magnitude in the yield stress.Hence, the high-temperature lava of the outer part probablyhad a lower yield stress, and the low-temperature lava ofthe 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 phys-ical properties of the actively moving lava: temperature,viscosity, yield stress, and effusion rate. These physical pa-rameters directly control the thickness and aspect ratio ofthe dome (Huppert et al. 1982; Blake 1990; Griffiths andFink 1993; Fink and Griffiths 1998; Nakada et al. 1999).Some silicic lava domes exhibit a spiny surface morphol-ogy, due to high viscosity (e.g. Unzen, Montserrat andRedoubt; Fink and Griffiths 1998). Spiny domes tend to behigh with very steep sides, and their tops are commonlypunctured by one or more subvertical spines with smooth

and curving sides (Fink and Griffiths 1998). Other siliciclava domes, such as the Showa Iwo-jima lava dome, havean axisymmetric shape (Fig. 3). Axisymmetric domes haveregular outlines, relatively low relief, and surfaces that tendto be covered by abundant small (a few tens of centimetersto a few meters in length) fragments (Fink and Griffiths1998). Although the average effusion rate (1×105 m3/day)for Showa Iwo-jima is almost the same as that of the earlierstage of Mount Unzen (3×105 m3/day; Nakada and Fujii1993), the dome shape is very different. The volume ofthe dome of Showa Iwo-jima is 6.2×106 m3 in the total 2months 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 isderived from the deformation behavior that is transitionalbetween Bingham plastics (spiny shape) and viscous fluids(axisymmetric shape). The viscosity of the Mount Unzenlava was estimated to be over 1011.5 Pa S by brittle fail-ure condition analysis (Goto 1999), or 2–4×1010 Pa S,based on the analysis using moving lobe conditions (Sutoet al. 1993), and the temperature was 780–880◦C (Nakadaand Motomura 1999). On the other hand, the lava for theShowa Iwo-jima dome had a lower viscosity (107–9 Pa S)and lower yield stress than that of Mount Unzen, due toits inferred high temperature (830–990◦C), resulting in itsaxisymmetric shape.

Subaerial growth of Showa Iwo-jima dome

The aerial photographs of Showa Iwo-jima show surfacefolds that have continuous, arcuate fold axes, roughly par-allel to the outer part. The surfaces of some noneroded

subaerial silicic lava flows or domes are commonly com-posed of vesicular pumice fragments that were jostled andground against each other while the active flow advancedand its surface was alternately extended and compressed(Fink 1980, 1983; Castro et al. 2002). The decrease indensity from lessvesicular lava in the deeper part, gradingup into morevesicular lava on the surface of dome, sug-gests that magmatic volatiles diffused outward from thedome interior. It is suggested that upon final decompres-sion at the dome surface, rhyolite lava inflated to vesicularpumice, and a cooling contraction during movement frac-tured the pumice (e.g. Fink 1980, 1983; Anderson et al.1998). Compression and extension during lava emplace-ment should further disrupt the dome carapace, resultingin pumice breccia. Since the upper surface of the domewas unconfined, folds grew vertically as the dome progres-sively enlarged. Wrinkles in the outer part have shorterwavelengths than wrinkles near the central crater (Figs. 4and 8). Outer lava may have had a lower viscosity becauseof its higher temperature and it also experienced the most

Fig. 13 a SiO2-K2O diagram for Showa Iwo-jima dome samples.Sample locations are shown in Fig. 3. b Compositional variationof Showa Iwo-jima lava dome for normalized distance (D/D0) fromthe 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.%)

Fig. 14 Model for the formation of the fracture zone in ShowaIwo-jima dome (cross sections of the marginal part). a As lava wasrepeatedly injected into the growing dome, fractures and ramp struc-tures moved outwards. With time, each fraction of fractured andramped lava became progressively attenuated, due to stretching andshearing. b When the active dome surface was alternately extendedand compressed, the fractures propagated into the interior of theviscous lava, resulting in thin layers of massive lava (ML layers).The surface of each ML layer would have vesiculated and brecciatedinsitu, due to a temperature gradient as shown in the right figure.c Newly produced breccias were embedded in the lava, and werereheated and welded when the fractures closed, resulting in mas-sive lavas (ML) interlayered with welded oxidized breccias (WOB).Closed arrows show the direction of stress

strain for the longest time. The evidence from the sur-face and internal structure suggests that the dome surfaceof the central part, immediately inside the marginal part,was more folded than the near-crater part, as the leadingedge almost ceased flowing due to the formation of a rigidmargin.

Fracturing and brecciation in the marginal partof Showa Iwo-jima dome

Formation of the fracture zone

The structures and components of the dome margin re-flect the conditions of lava emplacement. The fracture zonein the Showa Iwo-jima dome is interpreted as the prod-uct of the actively moving lava in the dome margin. Oncethe surface of the dome margin was cold, it would havebehaved in a brittle manner, and fracturing and rampingcould then be important (Macdonald 1972). As lava wasrepeatedly injected into the growing dome, the fracturesand ramp structures moved outwards, and the orientationof the flow foliation in the marginal part became predomi-nantly subvertical. With time, each fraction of the fracturedand ramped lava became progressively attenuated, due tostretching and shearing (Fig. 14a). While the active domesurface was alternately extended and compressed, the frac-tures along the foliation propagated into the viscous interiorlava, resulting in thin layers of massive lava, that is, MLlayers (Fig. 14b). The surface of each ML layer wouldhave undergone insitu vesiculation and brecciation, due toa temperature gradient between the hot interior lava andthe surface of the open fractures (right figure in Fig. 14b).When the fractures closed, newly produced breccias wereembedded in the lava, reheated and welded, resulting inmassive lava (ML) interlayered with welded oxidized brec-cia (WOB) (Fig. 14c). Tongues of massive lava extendedinto the WOB (ML with striated surface) and lenticular-shaped cavities formed at the hinges of the folded massivelava, 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 tomovement of the active lava. Manley (1996) suggested thatpumice at the surface of domes is comminuted, producingloose shards, bits of pumice, chips of dense glass, and frag-ments of phenocrysts. This debris sifts down around looseblocks and into open fractures deeper in the flow, whereit can be reheated, compressed, and annealed to varyingdegrees. Fine matrix may have been locally remobilizedby the escaping gas shortly after emplacement. Secondaryminerals crystallized on the surfaces of cavities, showingthat gas continued to escape from the lava after the em-placement.

Formation of the breccia zone

Breccia in the outer margin of the dome is interpreted tobe hyaloclastite generated by brittle spalling of hot lavathat was rapidly cooled by seawater during lava emplace-ment (e.g. Pichler 1965; De Rosen-Spence et al. 1980;Yamagishi 1987; Kano et al. 1991). While the breccia wasbeing generated, a large amount of steam was producedand rose up from the margin, as observed by Tanakadate(1935a,b). Tuffisite in the breccia zone may have been pro-duced by quenching of the lava under the sea. Hyaloclastite

breccia, observed in the subaerial parts of the dome atpresent, may have been uplifted during the lava emplace-ment. If the dome front became rigid and almost stoppedadvancing, further output of lava from the vent could havesqueezed earlier-erupted lava and breccia outwards, result-ing in ramping of breccia above the sea surface. The brecciazone is limited to the outer margin near Locations B and F,implying that there was very little uplifted breccia, or elseit has been almost entirely eroded.

Formation of Showa Iwo-jima Island

The Showa Iwo-jima eruption in 1934–1935 is dividedinto four stages; (1) the formation of a submarine edificewith floating pumice, from September to December in1934, (2) phreatomagmatic explosions and pyroclasticcone-formation in December, (3) lava effusion andformation of a new pyroclastic cone on the western sideof the lava, in early January 1935 (Figs. 2d and 15a) andfollowed by (4) effusion of the new silicic lava and domegrowth from January to March (Figs. 2d and 15b–d).

The growth of the dome from late January is summarizedin the following scenario: The high-temperature dacite lavabegan to effuse from the central crater using two separatevents (Fig. 15b). Judging from some photographs (exam-ple: Fig. 2c), during the earlier dome growth stage, a largeamount of steam was produced and rose up from the mar-gin. During the later stage, low-temperature rhyolite lavaeffused subaerially, and flowed out to the shallow seaflooron the eastern side and onto formerly emplaced lava on thewestern side (Fig. 15c). Although Tanakadate (1935a,b) re-ported only one central crater, our observation indicates thatanother vent also exists in the center of the eastern sector.The breccia zone was formed by rapid cooling when lavaentered the sea. In March 1935, the central part of the dome,composed of rhyolite lava, grew up subaerially, resulting inan axisymmetric shape (Fig. 15d). In the marginal part, thefracture zone was produced by lava being successively frac-tured, ramped, and brecciated. Newly produced brecciaswere embedded in the lava and reheated when the fracturesclosed, resulting in the formation of welded oxidizedbreccias (WOB). During this stage, hyaloclastite brecciawas also ramped above the sea surface by progressive em-placement of increments of new lava. The central part wasfolded, and pumice breccia and wrinkles developed. Thesmall area of breccia shows that subaerial emplacementwas the dominant process during the growth of the exposedparts of the Showa Iwo-jima dome. Post-emplacementsubsidence and wave erosion of the marginal part resultedin the present Showa Iwo-jima dome (Fig. 15e).

Conclusion

The Showa Iwo-jima dome consists of two main parts; acentral part and a marginal part. The marginal part is alsocharacterized by two zones; a fracture zone and a brec-cia zone. The lava dome contains lava with two slightly

Fig. 15 Model for the emplacement of Showa Iwo-jima silicic lavadome (cross sections; directions are southwest (SW) and east (E), asshown in the insert). a Pyroclastic cone formation and minor lava ef-fusion with phreatomagmatic eruptions in early January 1935. b Thedacitic lava effused from the crater of the pyroclastic cone. c Rhyolitelava effused subaerially using two separate vents, and flowed out tothe shallow seafloor on the eastern side and onto formerly emplacedlava on the southwestern side, from late January, 1935. d The fracture

zone, characterized by the ML-WOB layers, was produced by frac-turing and ramping during movement of the lava. Hyaloclastite at thedome margin was ramped above the sea surface. The central part ofthe dome remained subaerial, resulting in the Showa Iwo-jima lavadome in late March, 1935. e Post-emplacement subsidence and waveerosion of the marginal part produced the present Showa Iwo-jimalava dome. Dashed lines show the approximate boundary of denselava (MRHY) and vesicular lava (FVL, CVL)

different chemical compositions, one of them being moredacitic in the marginal part and the other being more rhy-olitic in the central part. The high-temperature dacite lavabegan to effuse from the central craters and flowed onto theshallow seafloor. The surface of the dome came into contact

with seawater and brecciated, forming hyaloclastite. Dur-ing the later stage, low-temperature rhyolite lava effusedsubaerially, and the fracture zone was produced by suc-cessive fracturing, ramping, and brecciation of the activelymoving dome front. At the southern margin, hyaloclastite

breccia was ramped above the sea surface by progressive in-crements of new lava. The central part was folded, forminga surface layer of pumice breccia. Subaerial emplacementof lava was a dominant process during the growth of theShowa Iwo-jima dome. The process of dome growth in ashallow sea can be elucidated by this evidence.

Acknowledgements We acknowledge A Goto, T Miyamoto, MIchihara, and A Yokoo for their logistical advice. We also thank HFujimaki for the XRF analyses. We are grateful to GeographicalSurvey Institute of Japan for permission to use the aerial photographof Showa Iwo-jima, and to Mishima village, Kagoshima, Japan, forhelp with our field survey. We thank J McPhie, K Kano, and K Daddfor constructive reviews of the manuscript. This research was partlysupported by a Grant-in-aid for Scientific Research from the Ministryof Education, Culture, Sports, Science and Technology, Japan (No.14080203).

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