Sedimentation and Welding Processes of Dilute Pyroclastic Density Currents and Fallout During a Large-Scale Silicic Eruption, Kikai Caldera, Japan

Sedimentary Geology 220 (2009) 227–242

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Sedimentary Geology

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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: Sedimentation and welding processes of the high temperature dilute pyroclastic density currents and fallout Accepted 1 April 2009 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 Keywords: produced during the column collapse phase, following a large plinian eruption and preceding an ignimbrite Dilute pyroclastic density current eruption, and can be divided into two subunits, Units B and B . Unit B is primarily deposited in topographic Surge l u l depressions on proximal islands, and consists of multiple thin (b1m)flow units with stratified and cross- Welding fi Agglutination strati ed facies with various degrees of welding. Each thin unit appears as a single aggradational unit, Magma-water interaction composed of a lower lithic-rich layer or pod and an upper welded pumice-rich layer. Lithic-rich parts are Kikai caldera fines-depleted and are composed of altered country rock, fresh andesite lava, obsidian clasts with chilled

margins, and boulders. The overlying Unit Bu 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 Bl 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 ex-

plosions (Unit Bl), into an agglutinate-dominated fallout eruption (Unit Bu). 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. © 2009 Elsevier B.V. All rights reserved.

1. Introduction and Hildreth, 1997) or lava-like rheomorphic tuff (Chapin and Lowell, 1979; Branney et al., 1992; Sumner and Branney, 2002; Pioli and Rosi, Sedimentation and welding processes of pyroclasts during explo- 2005). These welded deposits were mostly derived from hot and dense sive volcanic eruptions cause remarkable lithofacies variations in pyroclastic density currents, rather than low density, more turbulent eruptive deposits, reﬂecting their transport, segregation, deposition, pyroclastic surges. Welding is complexly controlled by many physical and additionally, their deformation mechanisms including compaction parameters such as magmatic composition, vapor pressure, and strain and sintering of glassy particles (Fisher and Schmincke, 1984; Cas and rate (Riehle, 1973; Grunder and Russell, 2005), and requires the Wright, 1987; Branney and Kokelaar, 2002). In these mechanisms, it is depositional temperature to be higher than minimum welding tem- generally accepted that welding is more commonly observed in pri- perature (e.g., Grunder et al., 2005). Such high temperature conditions marily massive or weakly stratiﬁed ignimbrite (Ross and Smith, 1961; are more likely within dense pyroclastic density currents than dilute Riehle, 1973; Cas and Wright, 1987; Streck and Grunder, 1995; Wilson 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 ⁎ Corresponding author. Fax: +81 3812 6979. much higher temperatures (Branney and Kokelaar, 1992, 2002; E-mail address: [email protected] (F. Maeno). Freundt, 1998). The physical conditions that produce surge-bedding

0037-0738/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2009.04.015 228 F. Maeno, H. Taniguchi / Sedimentary Geology 220 (2009) 227–242 structures are generally achieved in dilute turbulent pyroclastic 2. Outline of 7.3 ka Kikai eruption density currents of phreatomagmatic eruption origin (e.g., Self, 1983; Allen and Cas, 1998; Houghton et al., 2003): rapid vaporization and Kikai Caldera is a Quaternary volcano located in the East China expansion of external water promotes cooling and dilution of such Sea, southern Kyushu (Fig. 1a). The caldera is 17 km wide and 20 km currents. long. Most of the Kikai caldera is now beneath the sea. The subaerial In the 7.3 ka eruption of Kikai caldera, inferred collapse of a parts comprise two islands on the northern caldera rim, Take-shima eruption column produced tractionally-stratiﬁed deposits (Unit B; and Satsuma Iwo-jima (Fig. 1b, c). Iwo-dake (rhyolitic volcano) and Maeno and Taniguchi, 2007), in spite of being densely or weakly Inamura-dake (basaltic volcano) on Satsuma Iwo-jima are the tops of welded, that is, ‘welded surge deposits’. In this study, the deposit is submerged post-caldera stratovolcanoes (Ono et al., 1982). The 7.3 ka divided into two subunits, Units Bl and Bu, and described in detail. The eruption produced four main pyroclastic units derived from three lower part, Unit Bl, originated from pyroclastic density currents and eruptive phases, which can be observed on some proximal islands the upper part, Unit Bu, has sedimentary characteristics similar to (notably Satsuma Iwo-jima and Take-shima islands) around the Kikai agglutinate or welded air fall. The characteristics and origin of caldera and on the mainland of Kyushu. The lowermost unit consists agglutinated/welded pyroclastic deposits are important for under- of plinian pumice-fall deposits (Unit A, Fig. 2; Ui, 1973; Ono et al., standing the emplacement mechanisms of relatively dilute pyroclas- 1982; Walker et al., 1984; Maeno and Taniguchi, 2007). These were tic density currents under high temperature conditions. These followed by pyroclastic ﬂows, which deposited only on proximal deposits are also important for understanding near-vent eruptive areas (Unit B, Fig. 2; Ono et al., 1982; Walker et al., 1984; Kobayashi conditions and sedimentation processes during large-scale silicic and Hayakawa, 1984; Maeno and Taniguchi, 2007). The third unit is a eruptions. 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. F. Maeno, H. Taniguchi / Sedimentary Geology 220 (2009) 227–242 229

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 Iwo- jima, and right-hand side shows the variation in Take-shima. On Satsuma Iwo-jima, Unit B is subdivided into Units Bl and Bu. Unit Bl is the lower to middle part of unit B, showing densely or weakly welded cross-stratified facies, but the upper Unit Bu 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 the upper part, Unit Bu, is a mainly densely-welded and stratified to 80 km from the source. The topmost unit on the mainland Kyushu lithofacies. In Take-shima, the whole of Unit B shows weakly welded is co-ignimbrite ash-fall deposit (Machida and Arai, 1978), which was stratified to non-welded massive lithofacies. dispersed over a wide area of Japan, more than 1,000 km from the The distribution of Unit B and the isopach map of Unit A (Fig. 1; Kikai caldera. Walker et al., 1984) indicate that the major vent during phases 1 and 2 (plinian eruption stage) was located near the post-eruptive volcanoes 3. Characterization of deposits 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 The proximal deposits of the 7.3 ka eruption occur on the islands of 7.3 ka deposits reclaimed land from the sea, or that subsidence Satsuma Iwo-jima and Take-shima. The deposits comprise three major occurred during or after the eruption, because the sea level has units; Unit A (pumice-fall deposits), Unit B (stratified or cross- remained relatively unchanged since, only fluctuating a few times with stratified pyroclastic density current deposits), and Unit C (stratified an amplitude of 2–3 m since 7 to 6.5 ka (Zheng et al., 1994; Ōki, 2002). to massive voluminous ignimbrite). These represent the three eruptive phases 1, 2, and 3, reported by Maeno and Taniguchi (2007). Although 3.1. Unit Bl Unit B is only observed on proximal islands, Unit A (pumice fallout deposits) and Unit C (climactic ignimbrite) can be observed over a Unit Bl shows stratified to cross-stratified lithofacies composed wide area of southern Kyushu and its neighboring islands. of thin lapilli layers. The total thickness is topographically controlled Unit B mainly occurs in the topographic lows of Satsuma Iwo-jima and varies from a minimum of a few meters to a maximum of about and Take-shima (locations 1–7inFig. 1b, c), as illustrated in a 20 m, and each layer is only from a few centimeters to a few tens schematic lithostratigraphic column of all the deposits in the Kikai of centimeters thick. The layers occur as one or more discrete layers, caldera area (Fig. 2). It is much thicker on Satsuma Iwo-jima, as the as lenses, or as irregular-shaped pods. At proximal exposures in main deposition is on the northwestern side of the caldera rather than Kosakamoto and Sakamoto (locations 2 and 4), northern parts of the eastern side where Take-shima lies. On Satsuma Iwo-jima, Unit B Satsuma Iwo-jima, layers centimeters to about one meter thick dis- is subdivided into a lower Unit Bl and upper Unit Bu. Unit Bl is a weakly playing various degrees of welding are abundant (Figs. 3a–dand4), welded (partly non-welded) and cross-stratified lithofacies, whereas often including pumice fallout (b1 m thick) units. The layers are 230 F. Maeno, H. Taniguchi / Sedimentary Geology 220 (2009) 227–242 mostly thin or absent on a top of the caldera wall. In several locations, pumice-rich layers (PL) with thickness of a few centimeters to a few tens coarser pumice-rich or lithic-rich lenses and low-angle cross of centimeters (Fig. 3h, i). Major components of lithic-rich layers include stratification with minor pinch and swell layers are also observed, altered country rock, fresh andesite lava, crystals, obsidian clasts, pumice with stratified and cross-stratified lithofacies occurring in close lapilli (partially welded), and glass shards. Lithic size is up to 30 cm in proximity (Fig. 3a, b). long-axis length. At some locations, spheroidal-shaped bombs with

In the lower part of Unit Bl, weakly welded well-sorted and fines- glassy chilled rinds are included in the lower part of Unit Bl. poor pumice-rich layers are mainly deposited (Figs. 3e–g and 4b–d). Densely welded stratified deposits, which sometimes include They resemble pyroclastic surge deposits rather than pumice fallout lithic-rich pods or lenses (a few meters long) and are mainly colored beds, because the thickness of layers varies from a few centimeters to dark-red, are typical on the western and northwestern side of Satsuma a few tens of centimeters over a short distance. These lithofacies can Iwo-jima (locations 1, 2, and 3). In contrast, on the northeastern side of be observed only at location 4. the island (location 4), the stratified deposits are weakly welded and

InthemiddleofUnitBl, multiple thin flow units are stratified and the whole unit is represented by alternating thin lithic-rich layers or cross-stratified, and are composed of lithic-rich layers or pods (LL) and pods (LL) and pumice-rich layers (PL) (Fig. 3g, h). In these structures,

Fig. 3. (a) Pumice-rich pinch and swell structures in Unit Bl at location 2 (see Fig. 2) and (b) layered structures of Units Bl and Bu at location 4. (c) Welded cross-stratiﬁed facies and

(d) stratified facies of Unit Bl (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 Bl 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 Bl. 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 Bu, which is composed of thin lithic-rich layers (LL) (arrows showing) and pumice-rich layers (PL). F. Maeno, H. Taniguchi / Sedimentary Geology 220 (2009) 227–242 231

Fig. 3 (continued). 232 F. Maeno, H. Taniguchi / Sedimentary Geology 220 (2009) 227–242

Fig. 4. Close-up photographs of various welding textures of Units Bl and Bu. 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 Bl) in distal area (TK: Take-shima). (b) Weakly-welded texture (Unit Bl), characterized by deformed pumice

(SI: Satsuma Iwo-jima). (c) Moderately-welded texture (Unit Bl), with dense deformed pumices and partial ﬁamme (SI). (d) Moderately-welded texture (Unit Bl), characterized by

fiamme and less dense matrix (SI). (e) Strongly-welded texture (Unit Bu), 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 an approximately one meter thick deposit with weakly welded strat- pumice-rich layers, and each pumice clast is well deformed (Fig. 3h). iﬁed lithofacies (Figs. 4a and 5). More distally (location 7), the unit has At locations 2 and 4, poorly sorted boulders are concentrated in close massive or weakly stratiﬁed lithofacies. The degree of welding is very proximity to densely welded pumice-rich lithofacies. Some parts of the weak as the fresh deposits can be easily shaved off with a hammer. welded deposit are eroded by the boulders.

Degassing pipes and segregation pods also occur in the weakly 3.2. Unit Bu welded Unit Bl (Fig. 5), and can be seen at Sakamoto on Satsuma Iwo- jima (location 4) and Komorikō on Take-shima (location 6). Lithic-rich Unit Bu is mainly observed on Satsuma Iwo-jima, especially at pipes in Sakamoto are vertically or horizontally developing from location 4. The unit shows densely welded stratified lithofacies, com- lithic-rich pods. In this location, clear boundaries between subunits posed of welded pumice-rich layers (PL) and lithic-rich layers (LL) are not identified, but weakly stratified lithofacies are developed in (Fig. 3i). Cross-stratified units are absent. PL layers range from 10 to the entire unit. 50 cm in thickness, but LL layers are 1 to 10 cm thick. These welded

On distal Take-shima, Unit Bl, which shows completely stratified stratified layers mantle Unit Bl, but sometimes develop internal lithofacies, is intercalated with some pumice fall layers (less than stratification defined by deformed pumice or fiamme (Fig. 4e).

10 cm thick) or lenses. At location 6, pumice-rich pipes (vertical Deposits are better sorted than the lower ﬂow/surge units of Unit Bl. cylindrical shapes with radii of a few centimeters) are recognized in The deformed pumice and ﬁamme range from a few centimeters to a F. Maeno, H. Taniguchi / Sedimentary Geology 220 (2009) 227–242 233 few tens of centimeters in long axis length. Another notable characteristic are the sag structures produced by ballistic impacts of lithic fragments 20–30 cm long. Underlying pumice-rich layers are deformed by these ballistics (Fig. 6a), indicating that Unit Bu has a fall- origin. 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 Bu 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 pumice-

rich layer (PL), composed of the top of Unit Bu 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 Bu (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 Bu. These syn-depositional deformation structures on top of Unit Bu show that Unit Bu 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 Bu (Fig. 7). The grain-size is less than a few centimeters.

3.3. Lateral variation of density and lithofacies

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

Bl are characterized by sintering of ash and slightly higher porosity (Fig. 8a). In contrast, densely welded matrix parts of Unit Bl 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 Fig. 5. A photograph (a) and a sketch (b) of degassing pipes and lithic-rich pods in the characterized by mainly welded cross-stratified or stratified lithofa- middle of Unit Bl (location 4). The unit is weakly welded. Lithic-rich pipes are shown by cies (Figs. 4d–e and 8), and densities of Units Bl and Bu are up to arrows in Figure (b). In this location, clear boundaries between each layer are not 2300 kg/m3. Their colors range from black in weakly welded outcrops, identified, but weakly stratified facies are developed. The lower part of the figures to dark-red in densely welded equivalents. Unit B is traceable to more shows a densely welded pumice fall layer. (c) A close-up photograph of a weakly l distal exposures on Take-shima, but the total thickness is only a few welded stratified facies of Unit Bl in Take-shima (location 6). Arrows in figure (c) show pumice-rich degassing pipes. tens of centimeters to a few meters. The deposit is mainly stratified 234 F. Maeno, H. Taniguchi / Sedimentary Geology 220 (2009) 227–242

Fig. 7. (a) Outcrop locations and model columns (upper), and lithofacies variations (lower) of Units Bl and Bu, 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: non- welded 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 tion processes and pyroclast origins. Sampling locations are shown in and 6), it is weakly welded (with density 1000–1500 kg/m3) as shown Fig. 7. Most of the samples were split and sieved at 1/2 phi intervals in Fig. 4a. In more distal exposures (Sata-ura, location 7), lithofacies with no further preparation. Representative data from the particle- are non- or weakly welded and massive, and its density is up to c. size analysis are presented as histograms (Fig. 9). Samples (S01, S11, 1000 kg/m3. A typical cooling unit is not clearly identified within S12, S03, and S13) lack very fine sub-components and are interpreted individual thin welded layers or the whole of Unit B. In the proximal as surge deposits or fines-depleted ignimbrite. S14 is from an ash-rich area, upper stratified layers have higher density than lower and subunit in the most distal area (location 7) and has a bimodal-peak. middle ones. The measured density may describe the degree of Components of lithic-rich subunits (S01, S11, S12, S03, and S13) are welding. mainly lithic (altered country rock or fresh andesite lava), crystals, Dips and strikes of foliations of thin stratified flow units or fiamme obsidian clasts, pumice (partially welded), and glass shards (Fig. 9). of Unit Bl were also measured in some locations (Fig. 1). They indicate Obsidian clasts are more abundant in S01, S11, and S12 from the earlier post-depositional deformation structures in stratified flow units, i.e., column collapse phase, compared with other lithic-rich samples (S03, along hinges on the topographic low. S13, and S04) from the later phase. Spheroidal-shaped bombs with

glassy rinds are also included in the lower part of Unit Bl (Fig. 10c). 3.4. Grain size and components Some glassy juveniles have numerous cracks on their surface and bear fragments of country rocks (Fig. 10a, b). These features indicate rapid Granulometry was conducted on a number of samples from the cooling of magma, probably due to dynamic contact with water during non-welded layers in Units A and Bl in order to constrain sedimenta- the earlier column collapse phase. However, SEM observations (Fig.11) F. Maeno, H. Taniguchi / Sedimentary Geology 220 (2009) 227–242 235

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 SiO2 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 inter- grown pyroxene phenocrysts in pumice of the climactic stage. Com-

positions of glass shards are 74–75 wt.% in SiO2 and 6.4–6.7 wt.% in Na2O+K2O. 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

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 km3 (corresponds to a DRE magma mass of 2×1013 kg), 40–43 km, and 2×108 kg/s respectively, assuming an initial magma temperature of 1200 K (Maeno and Taniguchi, 2007). It is suggested that vent- opening during Phase 1 occurred subaerially because initial plinian- fallout 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 Fig. 8. Photomicrographs of juvenile materials in welded pumice-rich layers of Unit that a subaerial volcano was present in the Kikai caldera just before Bl. (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. the 7.3 ka eruption. (b) Densely-welded matrix characterized by eutaxitic texture. This photo corre- The initial large-scale plinian phase was progressively followed by sponds with micro-scale texture of Fig. 4d. PL: plagioclase phenocryst. a column-collapsing phase (Phase 2). Plinian column-collapse has been attributed to a decrease in magma water content, the break up of of non-welded juvenile pyroclasts provide only limited evidence of conduit (Sparks and Wilson, 1976; Wilson et al., 1980) or other factors. magma-water interaction as the mechanism of fragmentation (a dry- Changing of eruption style from column-feeding (Phase 1; Unit A type phreatomagmatic eruption with a small water/magma ratio: formation) to collapsing (Phase 2; Unit B formation) during the 7.3 ka Büttner et al., 2002), for the following reasons: (1) absence of eruption is inferred to have been accompanied by magma-water hydration cracks or hydrated surfaces, generally considered as the interactions, as not only accidental altered lithics but also quenched result of direct contact of magma with external water; (2) rare surface bombs (although minor) and boulders are included in the lithic-rich alteration of glassy pyroclasts;, however, (3) for a small amount of subunits (LL). The eruption likely widened the vent toward the sea pyroclasts, many vesicles are cut by curviplanar fractures as a result of allowing access by seawater. However, the interaction between hydrovolcanic fragmentation of the melt (Heiken and Wohletz, 1985) eruptive material and seawater was probably limited, so that juvenile (Fig. 11a, b); (4) glass particles show blocky shapes with very few clasts remained sufficiently hot to weld upon emplacement and isolated spherical vesicles (gas bubbles) (Fig. 11a), suggesting that entrained water was entirely heated to steam. Accretionary or exsolution and expansion of magmatic gases did not play a major role armored lapilli are in fact absent from the welded layers of Unit B. in pyroclast formation; and (5) some lithic fragments from country The degassing pipes extending from the segregation pods into rocks are hydrothermally altered. This indicates interaction with either overlying welded beds indicate that gas pressure increased in the pods an aquifer (Barbeli et al., 1988) or sea water. during or after the rapid accumulation of eruptive materials and was In the coarse-grained subunits that occur as pumice-rich lenses or released before welding and cooling occurred completely. The non-welded flow units in Unit Bl, some juvenile materials with bread- temperature of the currents was probably higher than the minimum crusted surfaces can be found (Figs.10a, b and 12). The materials in the welding temperature, resulting in dense to weak welding of Unit Bl. bombs are composed of irregular or rounded pumiceous domains, On the western side of Satsuma Iwo-jima, the temperature of density fragments of country rocks (altered lithic), crystals, and a matrix currents was so high that welding deformation easily occurred. composed of fine pumice and ash. These clasts range from roughly The accidental lithics may have derived from the breakup of the spherical to roughly ellipsoid in shape, and are denser than normal country rock due to conduit-wall abrasion (Macedonio et al., 1994), or pumice grains sampled from the same units. Some have a glassy rim (a conduit pressure changes during magma ascent (Papale and Dobran, few millimeters thick) and are crossed by cracks. Fragments of country 1993). Heating of seawater in the pore-spaces of the wall-rock may rock are enclosed in a brown, dense, and apparently less vesicular also drive explosive expansion and wall-rock fragmentation when rhyolite matrix (Fig. 12a, b). Irregular or rounded and vesicular conduit pressures decrease as a result of reduced or negative magma domains are probably individual pumice lapilli agglomerated and rise rates (e.g., Dobran and Papale, 1993; Doubik and Hill, 1999). 236 F. Maeno, H. Taniguchi / Sedimentary Geology 220 (2009) 227–242

Fig. 9. Grain-size distributions and components of non-welded layers or pods in Units A and Bl. 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 Bl, 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 Bl; S04 is from the coarse-grained subunit (about 1 m thick) in the upper part of Unit Bl. S14 is from an ash-rich subunit of Unit Bl 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.

5.2. Transportation and sedimentation of eruptive materials and Houghton, 2000), resulting in tractional structures correlated with a segregation of dense lithics.

The sedimentary characteristics of Unit Bl indicate that multiple Repetitive generation of dilute pyroclastic density currents may have dilute pyroclastic density currents occurred and laterally spread from been derived from pulsate magma supplies (fluctuation of magma the base of the eruption column during Phase 2. Although the location discharge rate) or cyclic interactions of the magma and seawater. The of an eruptive vent is not well constrained, it may have been located presence of typical surge bedforms and sorting in a thin welded deposit, near the present Iwo-dake volcano because strongly welded fallout such as Unit Bl, is an indication that pyroclastic density currents were and flow deposits are only distributed around the northern part of generated and deposited within high temperature and highly energetic Satsuma Iwo-jima. Pumice-rich lenses and low-angle cross stratifica- density currents, and that pyroclastic particles were sufficiently hot to tion with minor pinch-and-swell layers are typical of deposition from weld instantaneously upon deposition. It is suggested that aggrading relatively low concentration, traction-dominated turbulent pyroclas- layers (lithic-rich layers (LL) and pumice-rich layers (PL)) were tic density currents. It is suggested that dense pyroclasts (including produced as a result of these processes during the column collapse lithics and quenched materials) were effectively segregated within phase (Fig. 14b). Although various scales of pyroclastic density currents the current body due to highly turbulent and diluted conditions, may have been generated continuously and deposited on Satsuma Iwo- where elutriated finer and lighter materials followed later to be jima, only some large-scale high temperature currents arrived in more progressively deposited on top and at more distal locations. The distal regions (i.e., Take-shima). The fate of the flows and ash-clouds couple of a lithic-rich layer or pod and an overlying pumice-rich layer beyond the present coastline remains unknown. apparently represents deposition of a single aggrading unit produced Eruptive styles further changed towards the end of the column by this progressive sedimentation process. In the source area, feeding/collapsing phases, producing the fallout deposits of Unit Bu, entrained seawater (even a small amount) could have vaporized, whose deposits are characterized by well-stratified agglutinated (no promoted an expanding pyroclasts–water mixture, and imparting a cross-stratification), and densely welded facies sometimes including high momentum to the currents, resulting in a widely spreading, high ballistics. These sedimentary characteristics indicate that no more velocity turbulent current. Such pyroclastic density currents produce highly energetic erosive pyroclastic density currents occurred. This high basal shear rates (e.g., Branney and Kokelaar, 1992, 2002; Wilson eruption phase was thus probably less explosive than the surge- F. Maeno, H. Taniguchi / Sedimentary Geology 220 (2009) 227–242 237

Fig. 10. A photograph (a) and a sketch (b) of juvenile material from a pumice-rich lens in the lower part of Unit Bl (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 Bl. Bomb interiors are well-vesiculated.

generating phase which produced Unit Bl. Although fallout eruptions was less than about 0.5 MPa, and the density of the deposit is up to became dominant and eruption-intensity may have decreased to- about 2300 kg/m3. In the distal area at Take-shima, the thickness of ward the end of Phase 2 (Fig. 14c), it is suggested that the accu- the deposit is only a maximum of 4 m and compaction was not mulation rate of pyroclasts was still high and that the deposition intense, but welding deformation occurred (the density of the deposit temperature was kept above the minimum welding temperature. ranges from 1000 to 1500 kg/m3). The 4 m thick deposit can only have experienced less than 0.04–0.06 MPa lithostatic pressure, even at the 5.3. Conditions of welding deformation bottom of the unit. The pressure ranges in both proximal and distal regions are close to atmospheric pressure; therefore, high tempera- Pressure is generally the main driving force for compaction, ex- ture, high alkali content, or high water content are more likely to pulsion, and resorption of interstitial gas in pyroclasts (e.g., Cas and promote welding than pressure, because increases of such physical Wright, 1987). In the proximal area of Satsuma Iwo-jima, the max- parameters can dramatically enhance welding and sintering and imum thickness of Unit B is 30–40 m; therefore, lithostatic pressure 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 Bl: (a, b) obsidian clasts with poor vesiculation, (c, d) pumice clasts with good vesiculation. All scale bars 100 μm. 238 F. Maeno, H. Taniguchi / Sedimentary Geology 220 (2009) 227–242

Fig. 12. Photograph (a) and sketch (b) of juvenile material in a coarse-grained lithic-rich subunit in the lower part of Unit Bl (location 4), and photograph (c) and sketch (d) of juvenile material in a pumice-rich lens in the lower part of Unit Bl (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 water from degassing and/or high temperature (above minimum estimated to be between 900 and 1000 °C from experimental studies, welding temperature) was more important because it decreased the using samples of Rattlesnake Tuff (Grunder et al., 2005). viscosity of pyroclasts. These welding conditions are also supported Alkali elements can reduce viscosity and promote welding by experimental results (e.g., Grunder et al., 2005). Furthermore, on deformation (e.g., Dingwell et al., 1998). Some thin well-stratiﬁed the basis of these considerations, some just-deposited juvenile welded tuffs, such as Unit Bl, are generally derived from peralkaline pyroclasts may have been soft and sticky due to a high magmatic rhyolites (e.g., Villari, 1974), having unusually low glass viscosities; temperature above the minimum welding temperature. calc-alkaline rhyolite examples have also been reported (e.g., Chapin On the other hand, welded particles (still-hot fragmented pyr- and Lowell, 1979; Bacon and Druitt, 1988; Branney et al., 1992). oclasts) and lithic-bearing agglomerates (Figs. 10 and 12) were also

However, the alkali content of Kikai rhyolite is much lower than for deposited as Unit Bl, which may be the result of a speciﬁc sequence of any other example (Fig. 13). Water content is also an important factor the events in the eruptive vent. In order to produce aggregated par- controlling welding deformation (Sparks et al., 1999). Although the ticles containing fragmented country rock, juvenile particles must saturation limit of water in rhyolite melt is 0.1 to 0.2 wt.% at 1 atm, aggregate within gas-particle mixtures moving through an open con- supersaturation (0.2 to 0.7 wt.%) may develop in the melt without duit (Lorentz and Zimanowski, 1984) or near-vent area, where the full-degassing, if its internal pressure does not follow lithostatic due resulting complex aggregation of material may be almost immediately to a dramatic increase in viscosity associated with water loss (e.g., re-fragmented and re-ejected by succeeding explosions (Rosseel et al., Navon and Lyakhovsky, 1997). Based on these physicochemical 2006). Similar juvenile aggregates are broadly observed in the properties of erupted rhyolite, it is suggested that alkali contents products of phreatomagmatic eruptions of low-viscosity magma of did not play major roles in welding deformation at Kikai, but residual basaltic composition (e.g., Valentine and Groves, 1996; Doubik and F. Maeno, H. Taniguchi / Sedimentary Geology 220 (2009) 227–242 239

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. 1.03.0054) by Wohletz, we can calculate that if the water/magma mass ratio is less than 2–3 wt.% with an initial magma temperature of 1200–1250 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 (108 kg/s — estimated from fallout deposits; Maeno and Taniguchi, 2007), the mixing rate of external seawater should be less than 105–6 kg/s, which corresponds to a few vol.% of the erupting magma. Fig. 13. Whole-rock SiO2 variation diagram for Na2O+K2O 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., 5.5. High-temperature dilute pyroclastic density currents and their deposits 1992; Pantelleria ignimbrite, Villari, 1974; Wine Glass Tuff, Bacon and Druitt, 1988). Surge-type pyroclastic density currents are generally transported and Hill, 1999; Rosseel et al., 2006). In addition, Freundt and Schmincke emplaced below the minimum welding temperatures during phreato- (1995) pointed out that mantled and composite particles in ignim- magmatic eruptions (e.g., Self,1983; Allen and Cas,1998; Houghton et al., brites are indications of particle coalescence during hot dilute 2003), because this type of density current tends to be colder than dense transport. They interpreted bomb to ash-sized mantled particles to laminar currents from dry magmatic eruptions because rapid vaporiza- have formed by accretion of magma droplets at a range of temperature. tion and expansion of external water promotes cooling and dilution However, we cannot observe these types of particles at Kikai, except for of the currents. Resulting non-welded tractional structures are thought welded particles and lithic-bearing agglomerates (Figs.10 and 12), and to require elastic particle–particle or particle–sediment interactions, almost particles are similar to pyroclasts in normal silicic ignimbrites. such as dunes formed by traction (saltation, rolling, and sliding) of Therefore, we suggest that deformation and coalescences of particles particles along the substrate (e.g., Sohn, 1997; Wohletz, 1998). On the in Unit B mainly occurred instantaneously upon deposition, rather other hand, pyroclastic density currents producing high-grade (densely than within the current. The impacted or eroded welded layers are welded) ignimbrites are evidently transported well above minimum viscously deformed, and the deformation structures match the shapes welding temperatures, and some ﬂows may have even moved above of the brittle lithic particles. These indicate that the deposits began solidus temperatures (e.g., Cas and Wright, 1987; Freundt and welding before they were deformed by impacting projectiles and Schmincke,1995). In that case, particles were probably plastic to partially overlying bed loads. liquid and thus sticky and able to agglomerate or coalesce, as industrial

Welded fallout deposits included in Unit Bu are only predicted to glass beads rapidly agglomerate at temperatures below their melting occur for high temperature silicic and intermediate magmas with tem- point (Gluckman et al.,1976). Elastic momentum transfer during particle peratures N850 °C (Thomas and Sparks,1992). The welded fallout deposit interactions cannot occur between soft and sticky particles or between can be traced within 2 km from the Kikai vent. High accumulation rate, soft particles and a plastic substrate. Therefore, surge-type bedding large grain size, and long duration of deposition are the most important cannot generally develop in high-grade ignimbrites (e.g., Freundt, 1998). interdependent factors that control the degree of ﬁnal agglutination or These considerations related to the transportation and sedimentation of welding (Capaccioni and Cuccoli, 2005). The physicochemical data in the eruptive materials may be appropriate for a system of mono-dispersed

7.3kaKikaimagma(whole-rockSiO2 content72to74wt.%;magma hot juvenile particles. However, real pyroclastic density currents include temperature about 960 °C) is in good agreement with theoretical brittle particles and sticky juvenile particles, that is, they are multi- predictions for generation of welded fallout deposits by Thomas and dispersed particle systems. Sparks (1992) and Capaccioni and Cuccoli (2005). Pyroclastic density current deposits generated by collapsing

plinian columns during the 7.3 ka eruption (Unit Bl) are characterized 5.4. Effects of magma-water interaction on eruptive conditions by tractionally-stratified lithofacies, in spite of being densely or weakly welded, that is, ‘welded surge deposits’. Generally, pyroclastic Dilute (low particle concentration) pyroclastic density currents, deposits are distinguished by their lithofacies, which reflect their such as the pyroclastic surges generated during Phase 2, are thought to transport and sedimentation processes (Fisher and Schmincke, 1984; result from contact between eruptive material and external seawater, Cas and Wright, 1987; Branney and Kokelaar, 2002). However, there and from the production of high pressure steam that explosively are few literature references concerning welded pyroclastic surge decompressed at almost atmospheric pressure. The question then is if deposits relative to those on welded fall and flow deposits. This could this type of density current can keep as hot as the minimum welding be taken to indicate that cooling in surges is so rapid as to prevent temperature? To answer this, we investigated a water/magma (hot welding, and typical sedimentary traction structures and sorting pyroclasts) mass ratio that can achieve a minimum welding tem- characteristics may not form because agglutination or coalescence in a perature (we assumed 800–900 °C) at low particle concentrations pyroclastic surge prevents particulate traction. Against these insights, using a simplified thermodynamic model (Wohletz, 1986, 2002). This it is considered that welded pyroclastic surge deposits can be pro- model assumes two thermodynamic stages: the first is related to the duced only in specific conditions, such as (1) rapid accumulation of conservation of energy in initial thermal equilibrium as an adiabatic, still-viscous pyroclasts sufficiently hot to weld instantaneously upon and the second is related to a later expansion phase where water is deposition, or (2) erosion of underlying beds through corrosion or continuously heated by pyroclasts entrained in it and the mixture elastic particles' interactions with substrate deformation. 240 F. Maeno, H. Taniguchi / Sedimentary Geology 220 (2009) 227–242

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 Bl. 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 Bu.

The most noticeable characteristic of Unit Bl is the existence of perature pyroclastic density current), may have resulted in the lithic-rich layers or lens-like pods (LL). The lithofacies show seg- characteristic welded surge deposit sedimentary structures. The regations of dense particles during the evolution of individual current physical conditions for such processes may be achieved within high pulses. In such a situation, segregated particles in the base of the temperature and highly energetic pyroclastic density currents current are mainly composed of brittle fragments (dense lithic produced by large-scale explosive eruptions. pyroclasts). Underlying beds may have been composed of still-soft sticky and lighter pyroclasts at high temperature, and they could have 6. Conclusions experienced erosion through impingement of depositing dense particles (e.g., Allen, 1984; Macedonio et al., 1994). This process The collapse of a plinian column during Phase 2 of the 7.3 ka Kikai resulted in cross-stratified or stratified facies with erosive contacts. eruption produced Units Bl and Bu. Unit Bl consists of multiple thin Conversely, if juvenile hot pyroclasts, which were plastic to partially subunits with stratified and cross-stratified facies showing various liquid and thus sticky, are included in sufficient quantity, they are able degree of welding. Each thin subunit is composed of a lithic-rich layer to rapidly agglomerate or coalesce, resulting in densely or weakly or pod and a welded pumice-rich layer. Lithic-rich parts are fines- welded and relatively structureless deposits (e.g., Branney and depleted, and are mostly composed of brittle-shaped dense particles, Kokelaar, 1992). These particle-particle or particle-boundary inter- including altered lava, fresh andesite lava, obsidian clasts, and actions between brittle particles, juvenile hot pyroclasts, and soft boulders. Pumice-rich parts are rich in deformed pumice and ash or sticky substrates in a multi-dispersed particle system (high tem- fiamme. In contrast, Unit Bu shows only densely welded stratified F. Maeno, H. Taniguchi / Sedimentary Geology 220 (2009) 227–242 241 facies, composed of alternating lithic-rich and pumice-rich layers. The Chapin, C.E., Lowell, G.R., 1979. Primary and secondary flow structures in ash-flow tuffs of the Gribbles Run paleovalley, central Colorado. In: Chapin, C.E., Elston, W.E. (Eds.), layers mantle lower units, and are sometimes viscously deformed by Ash-flow tuffs: Geological Society of America Special Paper, vol. 180, pp. 137–154. ballistic impacts by lithic fragments, indicating that Unit Bu is of fall- Dingwell, D.B., Hess, K.-U., Romano, C., 1998. Extremely fluid behavior of hydrous origin. peralkaline rhyolites: experimental viscosity data and a model. Earth and Planetary Science Letters 158, 31–38. This evidence indicates that high temperature dilute pyroclastic Dobran, F., Papale, P., 1993. Magma-water interaction in closed systems and application density currents were repeatedly generated from limited magma- to lava tunnels and volcanic conduits. Journal of Geophysical Research 98 (B8), water interactions during plinian column-collapse. Under such 14041–14058. conditions, dense brittle particles may have been segregated in a Doubik, P., Hill, B.E., 1999. Magmatic and hydromagmatic conduit development during the 1975 Tolbachik Eruption, Kamchatka, with implications for hazards assessment turbulent current, immediately followed by sedimentation of hot at Yuca Mountain, NV. Journal of Volcanology and Geothermal Research 91, 43–64. lighter juvenile pyroclasts, resulting in multiple aggrading subunits Fisher, R.V., Schmincke, H.-U., 1984. Pyroclastic rocks. Springer-Verlag, Berlin, 472 pp. composed of lithic-rich layers or pods and pumice-rich layers. Further- Freundt, A., 1998. 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