Sedimentary Geology 220 (2009) Contents lists available at ScienceDirect. Sedimentary Geology

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1 Sedimentary Geology 220 (2009) Contents lists available at ScienceDirect Sedimentary Geology journal homepage: Sedimentation and welding processes of dilute pyroclastic density currents and fallout during a large-scale silicic eruption, Kikai caldera, Japan Fukashi Maeno a,, Hiromitsu Taniguchi b a Volcano Research Center, Earthquake Research Institute, University of Tokyo, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, Japan b Center for Northeast Asian Studies, Tohoku University, Kawauchi, Aoba-ku, Sendai, Japan article info abstract Article history: Accepted 1 April 2009 Keywords: Dilute pyroclastic density current Surge Welding Agglutination Magma-water interaction Kikai caldera Sedimentation and welding processes of the high temperature dilute pyroclastic density currents and fallout erupted at 7.3 ka from the Kikai caldera are discussed based on the stratigraphy, texture, lithofacies characteristics, and components of the resulting deposits. The welded eruptive deposits, Unit B, were produced during the column collapse phase, following a large plinian eruption and preceding an ignimbrite eruption, and can be divided into two subunits, Units B l and B u. Unit B l is primarily deposited in topographic depressions on proximal islands, and consists of multiple thin (b1 m) flow units with stratified and crossstratified facies with various degrees of welding. Each thin unit appears as a single aggradational unit, composed of a lower lithic-rich layer or pod and an upper welded pumice-rich layer. Lithic-rich parts are fines-depleted and are composed of altered country rock, fresh andesite lava, obsidian clasts with chilled margins, and boulders. The overlying Unit B u shows densely welded stratified facies, composed of alternating lithic-rich and pumice-rich layers. The layers mantle lower units and are sometimes viscously deformed by ballistics. The sedimentary characteristics of Unit B l such as welded stratified or cross-stratified facies indicate that high temperature dilute pyroclastic density currents were repeatedly generated from limited magma-water interactions. It is thought that dense brittle particles were segregated in a turbulent current and were immediately buried by deposition of hot, lighter pumice-rich particles, and that this process repeated many times. It is also suggested that the depositional temperature of eruptive materials was high and the eruptive style changed from a normal plinian eruption, through surge-generating explosions (Unit B l ), into an agglutinate-dominated fallout eruption (Unit B u ). On the basis of field data, welded pyroclastic surge deposits could be produced only under specific conditions, such as (1) rapid accumulation of pyroclastic particles sufficiently hot to weld instantaneously upon deposition, and (2) elastic particles' interactions with substrate deformation. These physical conditions may be achieved within high temperature and highly energetic pyroclastic density currents produced by large-scale explosive eruptions Elsevier B.V. All rights reserved. 1. Introduction Sedimentation and welding processes of pyroclasts during explosive volcanic eruptions cause remarkable lithofacies variations in eruptive deposits, reflecting their transport, segregation, deposition, and additionally, their deformation mechanisms including compaction and sintering of glassy particles (Fisher and Schmincke, 1984; Cas and Wright, 1987; Branney and Kokelaar, 2002). In these mechanisms, it is generally accepted that welding is more commonly observed in primarily massive or weakly stratified ignimbrite (Ross and Smith, 1961; Riehle, 1973; Cas and Wright, 1987; Streck and Grunder, 1995; Wilson Corresponding author. Fax: address: fmaeno@eri.u-tokyo.ac.jp (F. Maeno). and Hildreth, 1997) or lava-like rheomorphic tuff (Chapin and Lowell, 1979; Branney et al., 1992; Sumner and Branney, 2002; Pioli and Rosi, 2005). These welded deposits were mostly derived from hot and dense pyroclastic density currents, rather than low density, more turbulent pyroclastic surges. Welding is complexly controlled by many physical parameters such as magmatic composition, vapor pressure, and strain rate (Riehle, 1973; Grunder and Russell, 2005), and requires the depositional temperature to be higher than minimum welding temperature (e.g., Grunder et al., 2005). Such high temperature conditions are more likely within dense pyroclastic density currents than dilute turbulent ones. In addition, surge-type bedding structures require momentum transfer during elastic particle particle or particle boundary interactions with saltation, rolling, and sliding of particles along substrate contacts (e.g., Sohn, 1997; Wohletz, 1998), and are unlikely to occur in welded deposits where particles are transported at much higher temperatures (Branney and Kokelaar, 1992, 2002; Freundt, 1998). The physical conditions that produce surge-bedding /$ see front matter 2009 Elsevier B.V. All rights reserved. doi: /j.sedgeo

2 228 F. Maeno, H. Taniguchi / Sedimentary Geology 220 (2009) structures are generally achieved in dilute turbulent pyroclastic density currents of phreatomagmatic eruption origin (e.g., Self, 1983; Allen and Cas, 1998; Houghton et al., 2003): rapid vaporization and expansion of external water promotes cooling and dilution of such currents. In the 7.3 ka eruption of Kikai caldera, inferred collapse of a eruption column produced tractionally-stratified deposits (Unit B; Maeno and Taniguchi, 2007), in spite of being densely or weakly welded, that is, welded surge deposits. In this study, the deposit is divided into two subunits, Units B l and B u, and described in detail. The lower part, Unit B l, originated from pyroclastic density currents and the upper part, Unit B u, has sedimentary characteristics similar to agglutinate or welded air fall. The characteristics and origin of agglutinated/welded pyroclastic deposits are important for understanding the emplacement mechanisms of relatively dilute pyroclastic density currents under high temperature conditions. These deposits are also important for understanding near-vent eruptive conditions and sedimentation processes during large-scale silicic eruptions. 2. Outline of 7.3 ka Kikai eruption Kikai Caldera is a Quaternary volcano located in the East China Sea, southern Kyushu (Fig. 1a). The caldera is 17 km wide and 20 km long. Most of the Kikai caldera is now beneath the sea. The subaerial parts comprise two islands on the northern caldera rim, Take-shima and Satsuma Iwo-jima (Fig. 1b, c). Iwo-dake (rhyolitic volcano) and Inamura-dake (basaltic volcano) on Satsuma Iwo-jima are the tops of submerged post-caldera stratovolcanoes (Ono et al., 1982). The7.3 ka eruption produced four main pyroclastic units derived from three eruptive phases, which can be observed on some proximal islands (notably Satsuma Iwo-jima and Take-shima islands) around the Kikai caldera and on the mainland of Kyushu. The lowermost unit consists of plinian pumice-fall deposits (Unit A, Fig. 2; Ui, 1973; Ono et al., 1982; Walker et al., 1984; Maeno and Taniguchi, 2007). These were followed by pyroclastic flows, which deposited only on proximal areas (Unit B, Fig. 2; Ono et al., 1982; Walker et al., 1984; Kobayashi and Hayakawa, 1984; Maeno and Taniguchi, 2007). The third unit is a voluminous ignimbrite (Unit C, Fig. 2; Ui, 1973; Ono et al., 1982; Fig.1. (a) Location of Kikai caldera and an isopach map of plinian fallout in phase 1 (Unit A) and the distribution of climactic voluminous ignimbrite (after Ui,1973; Walker et al.,1984; Maeno and Taniguchi, 2007). (b and c) Distribution of ignimbrite (light gray area) and underlying Unit B (dark gray area) on Satsuma Iwo-jima and Take-shima, respectively. Strikes and dips for foliation of Unit B are also shown.

3 F. Maeno, H. Taniguchi / Sedimentary Geology 220 (2009) Fig. 2. Schematic lithostratigraphic column of proximal pyroclastic deposits from the 7.3 ka Kikai eruption. Left-hand side shows the summary of lithofacies variation in Satsuma Iwojima, and right-hand side shows the variation in Take-shima. On Satsuma Iwo-jima, Unit B is subdivided into Units B l and B u. Unit B l is the lower to middle part of unit B, showing densely or weakly welded cross-stratified facies, but the upper Unit B u shows mainly densely-welded stratified facies. On Take-shima, the whole of Unit B shows weakly welded stratified to non-welded massive facies. Walker et al.,1984; Maeno and Taniguchi, 2007), and it is traceable up to 80 km from the source. The topmost unit on the mainland Kyushu is co-ignimbrite ash-fall deposit (Machida and Arai, 1978), which was dispersed over a wide area of Japan, more than 1,000 km from the Kikai caldera. 3. Characterization of deposits The proximal deposits of the 7.3 ka eruption occur on the islands of Satsuma Iwo-jima and Take-shima. The deposits comprise three major units; Unit A (pumice-fall deposits), Unit B (stratified or crossstratified pyroclastic density current deposits), and Unit C (stratified to massive voluminous ignimbrite). These represent the three eruptive phases 1, 2, and 3, reported by Maeno and Taniguchi (2007). Although Unit B is only observed on proximal islands, Unit A (pumice fallout deposits) and Unit C (climactic ignimbrite) can be observed over a wide area of southern Kyushu and its neighboring islands. Unit B mainly occurs in the topographic lows of Satsuma Iwo-jima and Take-shima (locations 1 7 in Fig. 1b, c), as illustrated in a schematic lithostratigraphic column of all the deposits in the Kikai caldera area (Fig. 2). It is much thicker on Satsuma Iwo-jima, as the main deposition is on the northwestern side of the caldera rather than the eastern side where Take-shima lies. On Satsuma Iwo-jima, Unit B is subdivided into a lower Unit B l and upper Unit B u. Unit B l is a weakly welded (partly non-welded) and cross-stratified lithofacies, whereas the upper part, Unit B u, is a mainly densely-welded and stratified lithofacies. In Take-shima, the whole of Unit B shows weakly welded stratified to non-welded massive lithofacies. The distribution of Unit B and the isopach map of Unit A (Fig. 1; Walker et al., 1984) indicate that the major vent during phases 1 and 2 (plinian eruption stage) was located near the post-eruptive volcanoes on Satsuma Iwo-jima (Maeno and Taniguchi, 2007). At locations 2 and 4, Units A and B partially lie under water. This indicates that some of the 7.3 ka deposits reclaimed land from the sea, or that subsidence occurred during or after the eruption, because the sea level has remained relatively unchanged since, only fluctuating a few times with an amplitude of 2 3 m since 7 to 6.5 ka (Zheng et al., 1994; Ōki, 2002) Unit B l Unit B l shows stratified to cross-stratified lithofacies composed of thin lapilli layers. The total thickness is topographically controlled and varies from a minimum of a few meters to a maximum of about 20 m, and each layer is only from a few centimeters to a few tens of centimeters thick. The layers occur as one or more discrete layers, as lenses, or as irregular-shaped pods. At proximal exposures in Kosakamoto and Sakamoto (locations 2 and 4), northern parts of Satsuma Iwo-jima, layers centimeters to about one meter thick displaying various degrees of welding are abundant (Figs. 3a d and 4), often including pumice fallout (b1 m thick) units. The layers are

4 230 F. Maeno, H. Taniguchi / Sedimentary Geology 220 (2009) mostly thin or absent on a top of the caldera wall. In several locations, coarser pumice-rich or lithic-rich lenses and low-angle cross stratification with minor pinch and swell layers are also observed, with stratified and cross-stratified lithofacies occurring in close proximity (Fig. 3a, b). In the lower part of Unit B l, weakly welded well-sorted and finespoor pumice-rich layers are mainly deposited (Figs. 3e g and 4b d). They resemble pyroclastic surge deposits rather than pumice fallout beds, because the thickness of layers varies from a few centimeters to a few tens of centimeters over a short distance. These lithofacies can be observed only at location 4. InthemiddleofUnitB l, multiple thin flow units are stratified and cross-stratified, and are composed of lithic-rich layers or pods (LL) and pumice-rich layers (PL) with thickness of a few centimeters to a few tens of centimeters (Fig. 3h, i). Major components of lithic-rich layers include altered country rock, fresh andesite lava, crystals, obsidian clasts, pumice lapilli (partially welded), and glass shards. Lithic size is up to 30 cm in long-axis length. At some locations, spheroidal-shaped bombs with glassy chilled rinds are included in the lower part of Unit B l. Densely welded stratified deposits, which sometimes include lithic-rich pods or lenses (a few meters long) and are mainly colored dark-red, are typical on the western and northwestern side of Satsuma Iwo-jima (locations 1, 2, and 3). In contrast, on the northeastern side of the island (location 4), the stratified deposits are weakly welded and the whole unit is represented by alternating thin lithic-rich layers or pods (LL) and pumice-rich layers (PL) (Fig. 3g, h). In these structures, Fig. 3. (a) Pumice-rich pinch and swell structures in Unit B l at location 2 (see Fig. 2) and (b) layered structures of Units B l and B u at location 4. (c) Welded cross-stratified facies and (d) stratified facies of Unit B l (magnification of parts in figure b). (e) Weakly welded pumice-rich layers with stratification. The thickness of layers varies from a few centimeters to a few tens of centimeters in the same unit within close proximity. (f) Close-up of a bedding plane of the weakly welded Unit B l deposits. (g) A lithic-rich layer (LL) is sandwiched by weakly welded pumice-rich layers (PL), which are well-sorted. (h) Close-up of welded cross-stratified facies in Unit B l. A subunit is composed of lithic-rich layers (LL) and weakly welded pumice-rich layers (PL). Small bolded arrows show erosional surfaces. (i) Close-up of densely welded stratified facies in Unit B u, which is composed of thin lithic-rich layers (LL) (arrows showing) and pumice-rich layers (PL).

5 F. Maeno, H. Taniguchi / Sedimentary Geology 220 (2009) Fig. 3 (continued).

6 232 F. Maeno, H. Taniguchi / Sedimentary Geology 220 (2009) Fig. 4. Close-up photographs of various welding textures of Units B l and B u. Degree of welding increase with contents of glassy lenses (fiammes), as well as decreasing porosity of coarse pumices and fine matrix. (a) Weakly-welded texture (Unit B l ) in distal area (TK: Take-shima). (b) Weakly-welded texture (Unit B l ), characterized by deformed pumice (SI: Satsuma Iwo-jima). (c) Moderately-welded texture (Unit B l ), with dense deformed pumices and partial fiamme (SI). (d) Moderately-welded texture (Unit B l ), characterized by fiamme and less dense matrix (SI). (e) Strongly-welded texture (Unit B u ), characterized by abundant fiamme with unclear boundaries and densely compacted matrix (SI). Lenses enclosed by dotted lines in figures (d-e) show fiamme. lithic-rich layers sometimes tractionally erode or impact underlying pumice-rich layers, and each pumice clast is well deformed (Fig. 3h). At locations 2 and 4, poorly sorted boulders are concentrated in close proximity to densely welded pumice-rich lithofacies. Some parts of the welded deposit are eroded by the boulders. Degassing pipes and segregation pods also occur in the weakly welded Unit B l (Fig. 5), and can be seen at Sakamoto on Satsuma Iwojima (location 4) and Komorikō on Take-shima (location 6). Lithic-rich pipes in Sakamoto are vertically or horizontally developing from lithic-rich pods. In this location, clear boundaries between subunits are not identified, but weakly stratified lithofacies are developed in the entire unit. On distal Take-shima, Unit B l, which shows completely stratified lithofacies, is intercalated with some pumice fall layers (less than 10 cm thick) or lenses. At location 6, pumice-rich pipes (vertical cylindrical shapes with radii of a few centimeters) are recognized in an approximately one meter thick deposit with weakly welded stratified lithofacies (Figs. 4a and 5). More distally (location 7), the unit has massive or weakly stratified lithofacies. The degree of welding is very weak as the fresh deposits can be easily shaved off with a hammer Unit B u Unit B u is mainly observed on Satsuma Iwo-jima, especially at location 4. The unit shows densely welded stratified lithofacies, composed of welded pumice-rich layers (PL) and lithic-rich layers (LL) (Fig. 3i). Cross-stratified units are absent. PL layers range from 10 to 50 cm in thickness, but LL layers are 1 to 10 cm thick. These welded stratified layers mantle Unit B l, but sometimes develop internal stratification defined by deformed pumice or fiamme (Fig. 4e). Deposits are better sorted than the lower flow/surge units of Unit B l. The deformed pumice and fiamme range from a few centimeters to a

7 F. Maeno, H. Taniguchi / Sedimentary Geology 220 (2009) few tens of centimeters in long axis length. Another notable characteristic are the sag structures produced by ballistic impacts of lithic fragments cm long. Underlying pumice-rich layers are deformed by these ballistics (Fig. 6a), indicating that Unit B u has a fallorigin. In other outcrops near location 4, deformation structures are recognized at the contact between the basal lithic-rich layer of Unit C Fig. 6. (a) Deformation structure in Unit B u formed by ballistically ejected lithic fragments from plinian stage (20 cm diameter) and a large fragment of welded tuff from Unit C that impacted underlying pumice-rich fallout layers (location 4). Arrows show foliation of layers deformed by sag. (b) A boundary between a densely welded pumicerich layer (PL), composed of the top of Unit B u and a lithic-rich layer of Unit C. Arrows indicate where the upper layer intruded into the lower layer (location 4). and the pumice-rich upper layer of Unit B u (Fig. 6b). Here, the overlying lithic-rich layer (Unit C) impacts and scratches the welded pumice-rich layer (PL of Unit B). This is similar to the flame structures which are sometimes observed at the contact between pyroclastic flow deposits and the underlying substrate. This structure indicates that the climactic pyroclastic density current which produced Unit C, generated high shear stresses on the still-hot and viscous Unit B u. These syn-depositional deformation structures on top of Unit B u show that Unit B u was produced by particle to particle agglutination. On the western side of Satsuma Iwo-jima, although the sedimentary features cannot be observed in more detail due to difficult access, weakly- to densely-welded pumice-rich stratified layers occur in the upper part of Unit B. On Take-shima, non-welded pumice fallout layers constitute a distal facies of Unit B u (Fig. 7). The grain-size is less than a few centimeters Lateral variation of density and lithofacies Fig. 5. A photograph (a) and a sketch (b) of degassing pipes and lithic-rich pods in the middle of Unit B l (location 4). The unit is weakly welded. Lithic-rich pipes are shown by arrows in Figure (b). In this location, clear boundaries between each layer are not identified, but weakly stratified facies are developed. The lower part of the figures shows a densely welded pumice fall layer. (c) A close-up photograph of a weakly welded stratified facies of Unit B l in Take-shima (location 6). Arrows in figure (c) show pumice-rich degassing pipes. Spatial relationships between exposures, representative lithofacies, and density variations of Unit B are summarized in Fig. 7. Representative lithofacies are characterized by distinctive sedimentary structures and the degree of welding, roughly defined in close-up photographs and photomicrographs in Figs. 4 and 8. In micro-scale textures, the pumice and ashy matrix in weakly welded layers of Unit B l are characterized by sintering of ash and slightly higher porosity (Fig. 8a). In contrast, densely welded matrix parts of Unit B l are characterized by eutaxitic textures and lower porosity (Fig. 8b). In Fig. 7b, D and H show distance from source, and Unit B total thickness at each location respectively. On Satsuma Iwo-jima, the lithofacies are characterized by mainly welded cross-stratified or stratified lithofacies (Figs. 4d e and 8), and densities of Units B l and B u are up to 2300 kg/m 3. Their colors range from black in weakly welded outcrops, to dark-red in densely welded equivalents. Unit B l is traceable to more distal exposures on Take-shima, but the total thickness is only a few tens of centimeters to a few meters. The deposit is mainly stratified

8 234 F. Maeno, H. Taniguchi / Sedimentary Geology 220 (2009) Fig. 7. (a) Outcrop locations and model columns (upper), and lithofacies variations (lower) of Units B l and B u, arranged by distance. Location 3 in Satsuma Iwo-jima is most proximal, and location 7 in Take-shima is distal (11-13 km from location 3). (b) Density profiles of whole Unit B at each location. D is distances from proximal area (location 3), and H is total thicknesses of whole Unit B. Sample numbers (italic) for granulometric analyses are also shown. mf: non-welded massive facies, wwmf: weakly-welded massive facies, sf: nonwelded stratified facies, wwsf: weakly-welded stratified facies, dwsf: densely-welded stratified facies, dwcsf: densely-welded cross-stratified facies. but lacks lithic-rich layers (LL). At Harbor and Komorikō (locations 5 and 6), it is weakly welded (with density kg/m 3 ) as shown in Fig. 4a. In more distal exposures (Sata-ura, location 7), lithofacies are non- or weakly welded and massive, and its density is up to c kg/m 3. A typical cooling unit is not clearly identified within individual thin welded layers or the whole of Unit B. In the proximal area, upper stratified layers have higher density than lower and middle ones. The measured density may describe the degree of welding. Dips and strikes of foliations of thin stratified flow units or fiamme of Unit B l were also measured in some locations (Fig. 1). They indicate post-depositional deformation structures in stratified flow units, i.e., along hinges on the topographic low Grain size and components Granulometry was conducted on a number of samples from the non-welded layers in Units A and B l in order to constrain sedimentation processes and pyroclast origins. Sampling locations are shown in Fig. 7. Most of the samples were split and sieved at 1/2 phi intervals with no further preparation. Representative data from the particlesize analysis are presented as histograms (Fig. 9). Samples (S01, S11, S12, S03, and S13) lack very fine sub-components and are interpreted as surge deposits or fines-depleted ignimbrite. S14 is from an ash-rich subunit in the most distal area (location 7) and has a bimodal-peak. Components of lithic-rich subunits (S01, S11, S12, S03, and S13) are mainly lithic (altered country rock or fresh andesite lava), crystals, obsidian clasts, pumice (partially welded), and glass shards (Fig. 9). Obsidian clasts are more abundant in S01, S11, and S12 from the earlier column collapse phase, compared with other lithic-rich samples (S03, S13, and S04) from the later phase. Spheroidal-shaped bombs with glassy rinds are also included in the lower part of Unit B l (Fig. 10c). Some glassy juveniles have numerous cracks on their surface and bear fragments of country rocks (Fig. 10a, b). These features indicate rapid cooling of magma, probably due to dynamic contact with water during the earlier column collapse phase. However, SEM observations (Fig. 11)

9 F. Maeno, H. Taniguchi / Sedimentary Geology 220 (2009) fused with country rock, but fine lithics show no evidence of softening or melting. The dense matrix is interpreted as resulting from pumice and ash having thoroughly mixed and compacted. 4. Chemical compositions Whole-rock SiO 2 contents of magma in the 7.3 ka Kikai eruption range from 72 to 74 wt.% for Units A and B (Maeno and Taniguchi, 2007)(Fig. 13). Magma temperature was estimated as about 960 C by Saito et al. (2003), applying two-pyroxene thermometry to intergrown pyroxene phenocrysts in pumice of the climactic stage. Compositions of glass shards are wt.% in SiO 2 and wt.% in Na 2 O+K 2 O. Water concentrations of rhyolite melt were estimated to range from 3 to 4.6 wt.% for the climactic phase, on the basis of melt inclusion analyses for plagioclase of climactic caldera-forming phase (Saito et al., 2001). 5. Discussion 5.1. Conditions of plinian column feeding and collapsing Fig. 8. Photomicrographs of juvenile materials in welded pumice-rich layers of Unit B l. (a) Weakly-welded pumice and ashy matrix, characterized by sintering of ash and slightly higher porosity. This photo corresponds with micro-scale texture of Fig. 4b. (b) Densely-welded matrix characterized by eutaxitic texture. This photo corresponds with micro-scale texture of Fig. 4d. PL: plagioclase phenocryst. of non-welded juvenile pyroclasts provide only limited evidence of magma-water interaction as the mechanism of fragmentation (a drytype phreatomagmatic eruption with a small water/magma ratio: Büttner et al., 2002), for the following reasons: (1) absence of hydration cracks or hydrated surfaces, generally considered as the result of direct contact of magma with external water; (2) rare surface alteration of glassy pyroclasts;, however, (3) for a small amount of pyroclasts, many vesicles are cut by curviplanar fractures as a result of hydrovolcanic fragmentation of the melt (Heiken and Wohletz, 1985) (Fig. 11a, b); (4) glass particles show blocky shapes with very few isolated spherical vesicles (gas bubbles) (Fig. 11a), suggesting that exsolution and expansion of magmatic gases did not play a major role in pyroclast formation; and (5) some lithic fragments from country rocks are hydrothermally altered. This indicates interaction with either an aquifer (Barbeli et al., 1988) or sea water. In the coarse-grained subunits that occur as pumice-rich lenses or non-welded flow units in Unit B l, some juvenile materials with breadcrusted surfaces can be found (Figs.10a, b and 12). The materials in the bombs are composed of irregular or rounded pumiceous domains, fragments of country rocks (altered lithic), crystals, and a matrix composed of fine pumice and ash. These clasts range from roughly spherical to roughly ellipsoid in shape, and are denser than normal pumice grains sampled from the same units. Some have a glassy rim (a few millimeters thick) and are crossed by cracks. Fragments of country rock are enclosed in a brown, dense, and apparently less vesicular rhyolite matrix (Fig. 12a, b). Irregular or rounded and vesicular domains are probably individual pumice lapilli agglomerated and The 7.3 ka Kikai eruption initiated with a plinian column-feeding stage (Phase 1), which generated Unit A fallout deposits over a wide area of southern Kyushu (Figs. 1 and 14a). The volume, column height, and discharge rate of the plinian eruption in phase 1 are estimated at c. 40 km 3 (corresponds to a DRE magma mass of kg), km, and kg/s respectively, assuming an initial magma temperature of 1200 K (Maeno and Taniguchi, 2007). It is suggested that ventopening during Phase 1 occurred subaerially because initial plinianfallout deposits do not show any evidence of magma water interaction. This may be due to the presence of a pre-eruptive volcanic edifice around the vent: the Kikai-Komorikō tephra group on the two islands representing the caldera rim, which is derived from intermittent volcanic activity between 13 to 8 ka (Okuno et al., 2000), indicates that a subaerial volcano was present in the Kikai caldera just before the 7.3 ka eruption. The initial large-scale plinian phase was progressively followed by a column-collapsing phase (Phase 2). Plinian column-collapse has been attributed to a decrease in magma water content, the break up of conduit (Sparks and Wilson, 1976; Wilson et al., 1980) or other factors. Changing of eruption style from column-feeding (Phase 1; Unit A formation) to collapsing (Phase 2; Unit B formation) during the 7.3 ka eruption is inferred to have been accompanied by magma-water interactions, as not only accidental altered lithics but also quenched bombs (although minor) and boulders are included in the lithic-rich subunits (LL). The eruption likely widened the vent toward the sea allowing access by seawater. However, the interaction between eruptive material and seawater was probably limited, so that juvenile clasts remained sufficiently hot to weld upon emplacement and entrained water was entirely heated to steam. Accretionary or armored lapilli are in fact absent from the welded layers of Unit B. The degassing pipes extending from the segregation pods into overlying welded beds indicate that gas pressure increased in the pods during or after the rapid accumulation of eruptive materials and was released before welding and cooling occurred completely. The temperature of the currents was probably higher than the minimum welding temperature, resulting in dense to weak welding of Unit B l. On the western side of Satsuma Iwo-jima, the temperature of density currents was so high that welding deformation easily occurred. The accidental lithics may have derived from the breakup of the country rock due to conduit-wall abrasion (Macedonio et al., 1994), or conduit pressure changes during magma ascent (Papale and Dobran, 1993). Heating of seawater in the pore-spaces of the wall-rock may also drive explosive expansion and wall-rock fragmentation when conduit pressures decrease as a result of reduced or negative magma rise rates (e.g., Dobran and Papale, 1993; Doubik and Hill, 1999).

10 236 F. Maeno, H. Taniguchi / Sedimentary Geology 220 (2009) Fig. 9. Grain-size distributions and components of non-welded layers or pods in Units A and B l. T1 is from the fallout unit (Unit A). S01, S11, and S12 are from the fine-grained subunits (5 to 10 cm thick) in the lower part of Unit B l, which are sandwiched by fallout deposits; S03 and S13 are from the coarse-grained subunits (a few tens of centimeter thick) in the middle of Unit B l ; S04 is from the coarse-grained subunit (about 1 m thick) in the upper part of Unit B l. S14 is from an ash-rich subunit of Unit B l in the most distal area. Sample numbers and locations are shown in Fig. 8a. Obsidian clasts and Pumice and glass shards are characterized by poorly vesicular and highly vesicular juvenile materials, respectively Transportation and sedimentation of eruptive materials The sedimentary characteristics of Unit B l indicate that multiple dilute pyroclastic density currents occurred and laterally spread from the base of the eruption column during Phase 2. Although the location of an eruptive vent is not well constrained, it may have been located near the present Iwo-dake volcano because strongly welded fallout and flow deposits are only distributed around the northern part of Satsuma Iwo-jima. Pumice-rich lenses and low-angle cross stratification with minor pinch-and-swell layers are typical of deposition from relatively low concentration, traction-dominated turbulent pyroclastic density currents. It is suggested that dense pyroclasts (including lithics and quenched materials) were effectively segregated within the current body due to highly turbulent and diluted conditions, where elutriated finer and lighter materials followed later to be progressively deposited on top and at more distal locations. The couple of a lithic-rich layer or pod and an overlying pumice-rich layer apparently represents deposition of a single aggrading unit produced by this progressive sedimentation process. In the source area, entrained seawater (even a small amount) could have vaporized, promoted an expanding pyroclasts water mixture, and imparting a high momentum to the currents, resulting in a widely spreading, high velocity turbulent current. Such pyroclastic density currents produce high basal shear rates (e.g., Branney and Kokelaar, 1992, 2002; Wilson and Houghton, 2000), resulting in tractional structures correlated with a segregation of dense lithics. Repetitive generation of dilute pyroclastic density currents may have been derived from pulsate magma supplies (fluctuation of magma discharge rate) or cyclic interactions of the magma and seawater. The presence of typical surge bedforms and sorting in a thin welded deposit, such as Unit B l, is an indication that pyroclastic density currents were generated and deposited within high temperature and highly energetic density currents, and that pyroclastic particles were sufficiently hot to weld instantaneously upon deposition. It is suggested that aggrading layers (lithic-rich layers (LL) and pumice-rich layers (PL)) were produced as a result of these processes during the column collapse phase (Fig. 14b). Although various scales of pyroclastic density currents may have been generated continuously and deposited on Satsuma Iwojima, only some large-scale high temperature currents arrived in more distal regions (i.e., Take-shima). The fate of the flows and ash-clouds beyond the present coastline remains unknown. Eruptive styles further changed towards the end of the column feeding/collapsing phases, producing the fallout deposits of Unit B u, whose deposits are characterized by well-stratified agglutinated (no cross-stratification), and densely welded facies sometimes including ballistics. These sedimentary characteristics indicate that no more highly energetic erosive pyroclastic density currents occurred. This eruption phase was thus probably less explosive than the surge-

11 F. Maeno, H. Taniguchi / Sedimentary Geology 220 (2009) Fig. 10. A photograph (a) and a sketch (b) of juvenile material from a pumice-rich lens in the lower part of Unit B l (location 4), which includes lithic fragments. Numerous cracks on the surface indicate rapid cooling. (c) Spheroidal obsidian bombs with cracked glassy rinds, which are included in lower lithic-rich layers of Unit B l. Bomb interiors are well-vesiculated. generating phase which produced Unit B l. Although fallout eruptions became dominant and eruption-intensity may have decreased toward the end of Phase 2 (Fig. 14c), it is suggested that the accumulation rate of pyroclasts was still high and that the deposition temperature was kept above the minimum welding temperature Conditions of welding deformation Pressure is generally the main driving force for compaction, expulsion, and resorption of interstitial gas in pyroclasts (e.g., Cas and Wright, 1987). In the proximal area of Satsuma Iwo-jima, the maximum thickness of Unit B is m; therefore, lithostatic pressure was less than about 0.5 MPa, and the density of the deposit is up to about 2300 kg/m 3. In the distal area at Take-shima, the thickness of the deposit is only a maximum of 4 m and compaction was not intense, but welding deformation occurred (the density of the deposit ranges from 1000 to 1500 kg/m 3 ). The 4 m thick deposit can only have experienced less than MPa lithostatic pressure, even at the bottom of the unit. The pressure ranges in both proximal and distal regions are close to atmospheric pressure; therefore, high temperature, high alkali content, or high water content are more likely to promote welding than pressure, because increases of such physical parameters can dramatically enhance welding and sintering and hence deposit density (e.g., Sparks et al., 1999; Grunder et al., 2005). Fig. 11. Scanning electron micro-images of juvenile materials in a non-welded lithic-rich layer of Unit B l : (a, b) obsidian clasts with poor vesiculation, (c, d) pumice clasts with good vesiculation. All scale bars 100 μm.

12 238 F. Maeno, H. Taniguchi / Sedimentary Geology 220 (2009) Fig. 12. Photograph (a) and sketch (b) of juvenile material in a coarse-grained lithic-rich subunit in the lower part of Unit B l (location 4), and photograph (c) and sketch (d) of juvenile material in a pumice-rich lens in the lower part of Unit B l (location 4). These bomb-like materials include irregular or rounded pumice domains, fragments of country rocks (altered lithics), crystals, and a matrix composed of fine pumice and ash. They sometimes have glassy rims (a few millimeters thick) and cracks cross the particles. Fragments of lithic are enclosed in a brown, dense, and apparently non vesicular rhyolite matrix. For example, the minimum welding temperature of rhyolite at 1 atm is estimated to be between 900 and 1000 C from experimental studies, using samples of Rattlesnake Tuff (Grunder et al., 2005). Alkali elements can reduce viscosity and promote welding deformation (e.g., Dingwell et al., 1998). Some thin well-stratified welded tuffs, such as Unit B l, are generally derived from peralkaline rhyolites (e.g., Villari, 1974), having unusually low glass viscosities; calc-alkaline rhyolite examples have also been reported (e.g., Chapin and Lowell, 1979; Bacon and Druitt, 1988; Branney et al., 1992). However, the alkali content of Kikai rhyolite is much lower than for any other example (Fig. 13). Water content is also an important factor controlling welding deformation (Sparks et al., 1999). Although the saturation limit of water in rhyolite melt is 0.1 to 0.2 wt.% at 1 atm, supersaturation (0.2 to 0.7 wt.%) may develop in the melt without full-degassing, if its internal pressure does not follow lithostatic due to a dramatic increase in viscosity associated with water loss (e.g., Navon and Lyakhovsky, 1997). Based on these physicochemical properties of erupted rhyolite, it is suggested that alkali contents did not play major roles in welding deformation at Kikai, but residual water from degassing and/or high temperature (above minimum welding temperature) was more important because it decreased the viscosity of pyroclasts. These welding conditions are also supported by experimental results (e.g., Grunder et al., 2005). Furthermore, on the basis of these considerations, some just-deposited juvenile pyroclasts may have been soft and sticky due to a high magmatic temperature above the minimum welding temperature. On the other hand, welded particles (still-hot fragmented pyroclasts) and lithic-bearing agglomerates (Figs. 10 and 12) were also deposited as Unit B l, which may be the result of a specific sequence of the events in the eruptive vent. In order to produce aggregated particles containing fragmented country rock, juvenile particles must aggregate within gas-particle mixtures moving through an open conduit (Lorentz and Zimanowski, 1984) or near-vent area, where the resulting complex aggregation of material may be almost immediately re-fragmented and re-ejected by succeeding explosions (Rosseel et al., 2006). Similar juvenile aggregates are broadly observed in the products of phreatomagmatic eruptions of low-viscosity magma of basaltic composition (e.g., Valentine and Groves, 1996; Doubik and

13 F. Maeno, H. Taniguchi / Sedimentary Geology 220 (2009) Fig. 13. Whole-rock SiO 2 variation diagram for Na 2 O+K 2 O from eruptive deposits of 7.3 Kikai eruption. Data from other thin or well-stratified welded tuff, similar to Unit B, are also plotted (Wall Mountain Tuff, Chapin and Lowell, 1979; Bad Step Tuff, Branney et al., 1992; Pantelleria ignimbrite, Villari, 1974; Wine Glass Tuff, Bacon and Druitt, 1988). Hill, 1999; Rosseel et al., 2006). In addition, Freundt and Schmincke (1995) pointed out that mantled and composite particles in ignimbrites are indications of particle coalescence during hot dilute transport. They interpreted bomb to ash-sized mantled particles to have formed by accretion of magma droplets at a range of temperature. However, we cannot observe these types of particles at Kikai, except for welded particles and lithic-bearing agglomerates (Figs. 10 and 12), and almost particles are similar to pyroclasts in normal silicic ignimbrites. Therefore, we suggest that deformation and coalescences of particles in Unit B mainly occurred instantaneously upon deposition, rather than within the current. The impacted or eroded welded layers are viscously deformed, and the deformation structures match the shapes of the brittle lithic particles. These indicate that the deposits began welding before they were deformed by impacting projectiles and overlying bed loads. Welded fallout deposits included in Unit B u are only predicted to occur for high temperature silicic and intermediate magmas with temperatures N850 C (Thomas and Sparks,1992). The welded fallout deposit can be traced within 2 km from the Kikai vent. High accumulation rate, large grain size, and long duration of deposition are the most important interdependent factors that control the degree of final agglutination or welding (Capaccioni and Cuccoli, 2005). The physicochemical data in the 7.3kaKikaimagma(whole-rockSiO 2 content72to74wt.%;magma temperature about 960 C) is in good agreement with theoretical predictions for generation of welded fallout deposits by Thomas and Sparks (1992) and Capaccioni and Cuccoli (2005) Effects of magma-water interaction on eruptive conditions Dilute (low particle concentration) pyroclastic density currents, such as the pyroclastic surges generated during Phase 2, are thought to result from contact between eruptive material and external seawater, and from the production of high pressure steam that explosively decompressed at almost atmospheric pressure. The question then is if this type of density current can keep as hot as the minimum welding temperature? To answer this, we investigated a water/magma (hot pyroclasts) mass ratio that can achieve a minimum welding temperature (we assumed C) at low particle concentrations using a simplified thermodynamic model (Wohletz, 1986, 2002). This model assumes two thermodynamic stages: the first is related to the conservation of energy in initial thermal equilibrium as an adiabatic, and the second is related to a later expansion phase where water is continuously heated by pyroclasts entrained in it and the mixture temperature decreases from the equilibrium one. A long time-scale of cooling by thermal diffusion or convection for large particles is not considered in this model. Therefore, the calculated water/magma mass ratio is a maximum. On the basis of these consideration and using KWare PHM (ver ) by Wohletz, we can calculate that if the water/magma mass ratio is less than 2 3 wt.% with an initial magma temperature of K, the mixture temperature (magma or hot pyroclasts and water) can exceed the minimum welding temperature (Nc. 800 C). The mixtures can also have low particle concentration (solid fraction), less than 0.01, which is a plausible value for pyroclastic surges (e.g., Wohletz, 1998). The results support the idea that a small amount of seawater may be vaporized and promote a dilution of currents, and that high temperature eruptive material can be transported and deposited above minimum welding temperature in near-vent areas. Assuming that the eruption rate during Phase 2 is almost the same as in Phase 1 (10 8 kg/s estimated from fallout deposits; Maeno and Taniguchi, 2007), the mixing rate of external seawater should be less than kg/s, which corresponds to a few vol.% of the erupting magma High-temperature dilute pyroclastic density currents and their deposits Surge-type pyroclastic density currents are generally transported and emplaced below the minimum welding temperatures during phreatomagmatic eruptions (e.g., Self,1983; Allen and Cas,1998; Houghton et al., 2003), because this type of density current tends to be colder than dense laminar currents from dry magmatic eruptions because rapid vaporization and expansion of external water promotes cooling and dilution of the currents. Resulting non-welded tractional structures are thought to require elastic particle particle or particle sediment interactions, such as dunes formed by traction (saltation, rolling, and sliding) of particles along the substrate (e.g., Sohn, 1997; Wohletz, 1998). On the other hand, pyroclastic density currents producing high-grade (densely welded) ignimbrites are evidently transported well above minimum welding temperatures, and some flows may have even moved above solidus temperatures (e.g., Cas and Wright, 1987; Freundt and Schmincke,1995). In that case, particles were probably plastic to partially liquid and thus sticky and able to agglomerate or coalesce, as industrial glass beads rapidly agglomerate at temperatures below their melting point (Gluckman et al., 1976). Elastic momentum transfer during particle interactions cannot occur between soft and sticky particles or between soft particles and a plastic substrate. Therefore, surge-type bedding cannot generally develop in high-grade ignimbrites (e.g., Freundt, 1998). These considerations related to the transportation and sedimentation of eruptive materials may be appropriate for a system of mono-dispersed hot juvenile particles. However, real pyroclastic density currents include brittle particles and sticky juvenile particles, that is, they are multidispersed particle systems. Pyroclastic density current deposits generated by collapsing plinian columns during the 7.3 ka eruption (Unit B l ) are characterized by tractionally-stratified lithofacies, in spite of being densely or weakly welded, that is, welded surge deposits. Generally, pyroclastic deposits are distinguished by their lithofacies, which reflect their transport and sedimentation processes (Fisher and Schmincke, 1984; Cas and Wright, 1987; Branney and Kokelaar, 2002). However, there are few literature references concerning welded pyroclastic surge deposits relative to those on welded fall and flow deposits. This could be taken to indicate that cooling in surges is so rapid as to prevent welding, and typical sedimentary traction structures and sorting characteristics may not form because agglutination or coalescence in a pyroclastic surge prevents particulate traction. Against these insights, it is considered that welded pyroclastic surge deposits can be produced only in specific conditions, such as (1) rapid accumulation of still-viscous pyroclasts sufficiently hot to weld instantaneously upon deposition, or (2) erosion of underlying beds through corrosion or elastic particles' interactions with substrate deformation.

14 240 F. Maeno, H. Taniguchi / Sedimentary Geology 220 (2009) Fig. 14. Model for plinian column feeding and collapse stages during 7.3 ka Kikai eruption. (a) plinian column-feeding phase, resulting in Unit A. (b) plinian column-collapsing phase, generating high temperature dilute currents and depositing Unit B l. This phase may have been associated with phreatomagmatic explosions. (c) High temperature deposition during fallout phase is characterized by agglutination and dense welding in proximal areas, resulting in Unit B u. The most noticeable characteristic of Unit B l is the existence of lithic-rich layers or lens-like pods (LL). The lithofacies show segregations of dense particles during the evolution of individual current pulses. In such a situation, segregated particles in the base of the current are mainly composed of brittle fragments (dense lithic pyroclasts). Underlying beds may have been composed of still-soft sticky and lighter pyroclasts at high temperature, and they could have experienced erosion through impingement of depositing dense particles (e.g., Allen, 1984; Macedonio et al., 1994). This process resulted in cross-stratified or stratified facies with erosive contacts. Conversely, if juvenile hot pyroclasts, which were plastic to partially liquid and thus sticky, are included in sufficient quantity, they are able to rapidly agglomerate or coalesce, resulting in densely or weakly welded and relatively structureless deposits (e.g., Branney and Kokelaar, 1992). These particle-particle or particle-boundary interactions between brittle particles, juvenile hot pyroclasts, and soft sticky substrates in a multi-dispersed particle system (high temperature pyroclastic density current), may have resulted in the characteristic welded surge deposit sedimentary structures. The physical conditions for such processes may be achieved within high temperature and highly energetic pyroclastic density currents produced by large-scale explosive eruptions. 6. Conclusions The collapse of a plinian column during Phase 2 of the 7.3 ka Kikai eruption produced Units B l and B u. Unit B l consists of multiple thin subunits with stratified and cross-stratified facies showing various degree of welding. Each thin subunit is composed of a lithic-rich layer or pod and a welded pumice-rich layer. Lithic-rich parts are finesdepleted, and are mostly composed of brittle-shaped dense particles, including altered lava, fresh andesite lava, obsidian clasts, and boulders. Pumice-rich parts are rich in deformed pumice and ash or fiamme. In contrast, Unit B u shows only densely welded stratified

15 F. Maeno, H. Taniguchi / Sedimentary Geology 220 (2009) facies, composed of alternating lithic-rich and pumice-rich layers. The layers mantle lower units, and are sometimes viscously deformed by ballistic impacts by lithic fragments, indicating that Unit B u is of fallorigin. This evidence indicates that high temperature dilute pyroclastic density currents were repeatedly generated from limited magmawater interactions during plinian column-collapse. Under such conditions, dense brittle particles may have been segregated in a turbulent current, immediately followed by sedimentation of hot lighter juvenile pyroclasts, resulting in multiple aggrading subunits composed of lithic-rich layers or pods and pumice-rich layers. Furthermore, it is suggested that the depositional temperature increased from Phase 1 to 2 because accumulation rates increased maintaining high temperatures, and that the eruptive style changed from a normal plinian eruption (Unit A) into an agglutinate-dominated fallout eruption (Unit B u ) through transitional surge-generating explosions (Unit B l ), because the intensity of eruption decreased toward the end of Phase 2 but the high deposition temperature was kept. On the basis of sedimentation characteristics and some theoretical considerations, it is considered that welded pyroclastic surge deposits (Unit B l ) can be only produced under specific conditions, such as: (1) the pyroclastic particles quickly accumulated and were sufficiently hot to weld instantaneously upon deposition; and (2) inelastic particleparticle or particle-substrate interactions coupled with substrate deformation or erosion by brittle particles. Underlying still-soft sticky pyroclasts could have experienced corrosion as a result of the impingement of depositing dense brittle particles at high temperature, and if juvenile hot sticky pyroclasts are included in sufficient quantity, they are able to rapidly agglomerate or coalesce, resulting in densely or weakly welded deposits. These physical conditions may be achieved within high temperature and highly energetic pyroclastic density currents produced by large-scale explosive eruptions. Acknowledgements We acknowledge K. Kano for critical comments and discussions on the early version of this paper, and also we thank T. Kobayashi for critical discussions. We appreciate the constructive reviews and comments from C. Busby and N. Geshi. We are grateful to the residence of Mishima village, Kagoshima, Japan, for help with our field survey. References Allen, J.R.L., Sedimentary structures: their character and physical basis. 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