Sedimentation and welding processes of dilute pyroclastic ...

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 andfallout 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, Japanb Center for Northeast Asian Studies, Tohoku University, Kawauchi, Aoba-ku, Sendai, Japan

⁎ Corresponding author. Fax: +81 3812 6979.E-mail address: [email protected] (F. Maeno

0037-0738/$ – see front matter © 2009 Elsevier B.V. Aldoi:10.1016/j.sedgeo.2009.04.015

a b s t r a c t
a r t i c l e i n f o
Article history:Accepted 1 April 2009

Keywords:Dilute pyroclastic density currentSurgeWeldingAgglutinationMagma-water interactionKikai caldera

Sedimentation andwelding processes of the high temperature dilute pyroclastic density currents and fallouterupted at 7.3 ka from the Kikai caldera are discussed based on the stratigraphy, texture, lithofaciescharacteristics, and components of the resulting deposits. The welded eruptive deposits, Unit B, wereproduced during the column collapse phase, following a large plinian eruption and preceding an ignimbriteeruption, and can be divided into two subunits, Units Bl and Bu. Unit Bl is primarily deposited in topographicdepressions on proximal islands, and consists of multiple thin (b1 m) flow units with stratified and cross-stratified 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 arefines-depleted and are composed of altered country rock, fresh andesite lava, obsidian clasts with chilledmargins, and boulders. The overlying Unit Bu shows densely welded stratified facies, composed of alter-nating lithic-rich and pumice-rich layers. The layers mantle lower units and are sometimes viscouslydeformed by ballistics. The sedimentary characteristics of Unit Bl such as welded stratified or cross-stratifiedfacies indicate that high temperature dilute pyroclastic density currents were repeatedly generated fromlimited magma-water interactions. It is thought that dense brittle particles were segregated in a turbulentcurrent and were immediately buried by deposition of hot, lighter pumice-rich particles, and that thisprocess repeated many times. It is also suggested that the depositional temperature of eruptive materialswas 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, weldedpyroclastic surge deposits could be produced only under specific conditions, such as (1) rapid accumulationof pyroclastic particles sufficiently hot to weld instantaneously upon deposition, and (2) elastic particles' inter-actions with substrate deformation. These physical conditions may be achieved within high temperature andhighly energetic pyroclastic density currents produced by large-scale explosive eruptions.

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1. Introduction

Sedimentation and welding processes of pyroclasts during explo-sive volcanic eruptions cause remarkable lithofacies variations ineruptive deposits, reflecting their transport, segregation, deposition,and additionally, their deformationmechanisms including compactionand sintering of glassy particles (Fisher and Schmincke, 1984; Cas andWright, 1987; Branney and Kokelaar, 2002). In these mechanisms, it isgenerally accepted that welding is more commonly observed in pri-marily massive or weakly stratified ignimbrite (Ross and Smith, 1961;Riehle, 1973; Cas and Wright, 1987; Streck and Grunder, 1995; Wilson

).

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and Hildreth, 1997) or lava-like rheomorphic tuff (Chapin and Lowell,1979; Branney et al., 1992; Sumner and Branney, 2002; Pioli and Rosi,2005). Thesewelded depositsweremostly derived fromhot and densepyroclastic density currents, rather than low density, more turbulentpyroclastic surges. Welding is complexly controlled by many physicalparameters such as magmatic composition, vapor pressure, and strainrate (Riehle, 1973; Grunder and Russell, 2005), and requires thedepositional temperature to be higher than minimum welding tem-perature (e.g., Grunder et al., 2005). Such high temperature conditionsare more likely within dense pyroclastic density currents than diluteturbulent ones. In addition, surge-type bedding structures requiremomentum transfer during elastic particle–particle or particle–boundary interactions with saltation, rolling, and sliding of particlesalong substrate contacts (e.g., Sohn, 1997; Wohletz, 1998), and areunlikely to occur inwelded deposits where particles are transported atmuch higher temperatures (Branney and Kokelaar, 1992, 2002;Freundt, 1998). The physical conditions that produce surge-bedding

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228 F. Maeno, H. Taniguchi / Sedimentary Geology 220 (2009) 227–242

structures are generally achieved in dilute turbulent pyroclasticdensity currents of phreatomagmatic eruption origin (e.g., Self, 1983;Allen and Cas, 1998; Houghton et al., 2003): rapid vaporization andexpansion of external water promotes cooling and dilution of suchcurrents.

In the 7.3 ka eruption of Kikai caldera, inferred collapse of aeruption column produced tractionally-stratified deposits (Unit B;Maeno and Taniguchi, 2007), in spite of being densely or weaklywelded, that is, ‘welded surge deposits’. In this study, the deposit isdivided into two subunits, Units Bl and Bu, and described in detail. Thelower part, Unit Bl, originated from pyroclastic density currents andthe upper part, Unit Bu, has sedimentary characteristics similar toagglutinate or welded air fall. The characteristics and origin ofagglutinated/welded pyroclastic deposits are important for under-standing the emplacement mechanisms of relatively dilute pyroclas-tic density currents under high temperature conditions. Thesedeposits are also important for understanding near-vent eruptiveconditions and sedimentation processes during large-scale siliciceruptions.

Fig.1. (a) Location of Kikai caldera and an isopachmap of plinian fallout in phase 1 (Unit A) anMaeno and Taniguchi, 2007). (b and c) Distribution of ignimbrite (light gray area) and underand dips for foliation of Unit B are also shown.

2. Outline of 7.3 ka Kikai eruption

Kikai Caldera is a Quaternary volcano located in the East ChinaSea, southern Kyushu (Fig. 1a). The caldera is 17 kmwide and 20 kmlong. Most of the Kikai caldera is now beneath the sea. The subaerialparts comprise two islands on the northern caldera rim, Take-shimaand Satsuma Iwo-jima (Fig. 1b, c). Iwo-dake (rhyolitic volcano) andInamura-dake (basaltic volcano) on Satsuma Iwo-jima are the tops ofsubmerged post-caldera stratovolcanoes (Ono et al., 1982). The 7.3 kaeruption produced four main pyroclastic units derived from threeeruptive phases, which can be observed on some proximal islands(notably Satsuma Iwo-jima and Take-shima islands) around the Kikaicaldera and on the mainland of Kyushu. The lowermost unit consistsof plinian pumice-fall deposits (Unit A, Fig. 2; Ui, 1973; Ono et al.,1982; Walker et al., 1984; Maeno and Taniguchi, 2007). These werefollowed by pyroclastic flows, which deposited only on proximalareas (Unit B, Fig. 2; Ono et al., 1982; Walker et al., 1984; Kobayashiand Hayakawa, 1984; Maeno and Taniguchi, 2007). The third unit is avoluminous ignimbrite (Unit C, Fig. 2; Ui, 1973; Ono et al., 1982;

d the distribution of climactic voluminous ignimbrite (after Ui,1973;Walker et al., 1984;lying Unit B (dark gray area) on Satsuma Iwo-jima and Take-shima, respectively. Strikes

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, showingdensely 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 weldedstratified to non-welded massive facies.

229F. Maeno, H. Taniguchi / Sedimentary Geology 220 (2009) 227–242

Walker et al.,1984;Maeno and Taniguchi, 2007), and it is traceable upto 80 km from the source. The topmost unit on the mainland Kyushuis co-ignimbrite ash-fall deposit (Machida and Arai, 1978), whichwasdispersed over a wide area of Japan, more than 1,000 km from theKikai caldera.

3. Characterization of deposits

The proximal deposits of the 7.3 ka eruption occur on the islands ofSatsuma Iwo-jima and Take-shima. The deposits comprise threemajorunits; Unit A (pumice-fall deposits), Unit B (stratified or cross-stratified pyroclastic density current deposits), and Unit C (stratifiedtomassive voluminous ignimbrite). These represent the three eruptivephases 1, 2, and 3, reported byMaeno and Taniguchi (2007). AlthoughUnit B is only observed on proximal islands, Unit A (pumice falloutdeposits) and Unit C (climactic ignimbrite) can be observed over awide area of southern Kyushu and its neighboring islands.

Unit B mainly occurs in the topographic lows of Satsuma Iwo-jimaand Take-shima (locations 1–7 in Fig. 1b, c), as illustrated in aschematic lithostratigraphic column of all the deposits in the Kikaicaldera area (Fig. 2). It is much thicker on Satsuma Iwo-jima, as themain deposition is on the northwestern side of the caldera rather thanthe eastern side where Take-shima lies. On Satsuma Iwo-jima, Unit Bis subdivided into a lower Unit Bl and upper Unit Bu. Unit Bl is a weaklywelded (partly non-welded) and cross-stratified lithofacies, whereas

the upper part, Unit Bu, is a mainly densely-welded and stratifiedlithofacies. In Take-shima, the whole of Unit B shows weakly weldedstratified 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 volcanoeson Satsuma Iwo-jima (Maeno and Taniguchi, 2007). At locations 2 and4,Units A andBpartially lie underwater. This indicates that someof the7.3 ka deposits reclaimed land from the sea, or that subsidenceoccurred during or after the eruption, because the sea level hasremained relatively unchanged since, onlyfluctuating a few timeswithan amplitude of 2–3 m since 7 to 6.5 ka (Zheng et al., 1994; Ōki, 2002).

3.1. Unit Bl

Unit Bl shows stratified to cross-stratified lithofacies composedof thin lapilli layers. The total thickness is topographically controlledand varies from a minimum of a few meters to a maximum of about20 m, and each layer is only from a few centimeters to a few tensof centimeters thick. The layers occur as one or more discrete layers,as lenses, or as irregular-shaped pods. At proximal exposures inKosakamoto and Sakamoto (locations 2 and 4), northern parts ofSatsuma Iwo-jima, layers centimeters to about one meter thick dis-playing various degrees of welding are abundant (Figs. 3a–d and 4),often including pumice fallout (b1 m thick) units. The layers are


mostly thin or absent on a top of the calderawall. In several locations,coarser pumice-rich or lithic-rich lenses and low-angle crossstratification with minor pinch and swell layers are also observed,with stratified and cross-stratified lithofacies occurring in closeproximity (Fig. 3a, b).

In the lower part of Unit Bl, weakly welded well-sorted and fines-poor pumice-rich layers are mainly deposited (Figs. 3e–g and 4b–d).They resemble pyroclastic surge deposits rather than pumice falloutbeds, because the thickness of layers varies from a few centimeters toa few tens of centimeters over a short distance. These lithofacies canbe observed only at location 4.

In the middle of Unit Bl, multiple thin flow units are stratified andcross-stratified, and are composed of lithic-rich layers or pods (LL) and

Fig. 3. (a) Pumice-rich pinch and swell structures in Unit Bl at location 2 (see Fig. 2) and (b)(d) stratified facies of Unit Bl (magnification of parts in figure b). (e) Weakly welded pumicefew tens of centimeters in the same unit within close proximity. (f) Close-up of a bedding pweakly welded pumice-rich layers (PL), which are well-sorted. (h) Close-up of welded croswelded pumice-rich layers (PL). Small bolded arrows show erosional surfaces. (i) Close-up o(LL) (arrows showing) and pumice-rich layers (PL).

pumice-rich layers (PL)with thickness of a fewcentimeters to a few tensof centimeters (Fig. 3h, i).Major components of lithic-rich layers includealtered country rock, freshandesite lava, crystals, obsidian clasts, pumicelapilli (partially welded), and glass shards. Lithic size is up to 30 cm inlong-axis length. At some locations, spheroidal-shaped bombs withglassy chilled rinds are included in the lower part of Unit Bl.

Densely welded stratified deposits, which sometimes includelithic-rich pods or lenses (a few meters long) and are mainly coloreddark-red, are typical on thewestern and northwestern side of SatsumaIwo-jima (locations 1, 2, and 3). In contrast, on the northeastern side ofthe island (location 4), the stratified deposits are weakly welded andthe whole unit is represented by alternating thin lithic-rich layers orpods (LL) and pumice-rich layers (PL) (Fig. 3g, h). In these structures,

layered structures of Units Bl and Bu at location 4. (c) Welded cross-stratified facies and-rich layers with stratification. The thickness of layers varies from a few centimeters to alane of the weakly welded Unit Bl deposits. (g) A lithic-rich layer (LL) is sandwiched bys-stratified facies in Unit Bl. A subunit is composed of lithic-rich layers (LL) and weaklyf densely welded stratified facies in Unit Bu, which is composed of thin lithic-rich layers

Fig. 3 (continued).


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 porosityof 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 byfiamme and less dense matrix (SI). (e) Strongly-welded texture (Unit Bu), characterized by abundant fiamme with unclear boundaries and densely compacted matrix (SI). Lensesenclosed by dotted lines in figures (d-e) show fiamme.


lithic-rich layers sometimes tractionally erode or impact underlyingpumice-rich layers, and each pumice clast is well deformed (Fig. 3h).At locations 2 and 4, poorly sorted boulders are concentrated in closeproximity to denselywelded pumice-rich lithofacies. Someparts of thewelded deposit are eroded by the boulders.

Degassing pipes and segregation pods also occur in the weaklywelded Unit Bl (Fig. 5), and can be seen at Sakamoto on Satsuma Iwo-jima (location 4) and Komorikō on Take-shima (location 6). Lithic-richpipes in Sakamoto are vertically or horizontally developing fromlithic-rich pods. In this location, clear boundaries between subunitsare not identified, but weakly stratified lithofacies are developed inthe entire unit.

On distal Take-shima, Unit Bl, which shows completely stratifiedlithofacies, is intercalated with some pumice fall layers (less than10 cm thick) or lenses. At location 6, pumice-rich pipes (verticalcylindrical shapes with radii of a few centimeters) are recognized in

an approximately one meter thick deposit with weakly welded strat-ified lithofacies (Figs. 4a and 5). More distally (location 7), the unit hasmassive or weakly stratified lithofacies. The degree of welding is veryweak as the fresh deposits can be easily shaved off with a hammer.

3.2. Unit Bu

Unit Bu is mainly observed on Satsuma Iwo-jima, especially atlocation 4. The unit shows densely welded stratified lithofacies, com-posed of welded pumice-rich layers (PL) and lithic-rich layers (LL)(Fig. 3i). Cross-stratified units are absent. PL layers range from 10 to50 cm in thickness, but LL layers are 1 to 10 cm thick. These weldedstratified layers mantle Unit Bl, but sometimes develop internalstratification defined by deformed pumice or fiamme (Fig. 4e).Deposits are better sorted than the lower flow/surge units of Unit Bl.The deformed pumice and fiamme range from a few centimeters to a


few tens of centimeters in long axis length. Another notablecharacteristic are the sag structures produced by ballistic impacts oflithic fragments 20–30 cm long. Underlying pumice-rich layers aredeformed by these ballistics (Fig. 6a), indicating that Unit Bu has a fall-origin.

In other outcrops near location 4, deformation structures arerecognized at the contact between the basal lithic-rich layer of Unit C

Fig. 5. A photograph (a) and a sketch (b) of degassing pipes and lithic-rich pods in themiddle of Unit Bl (location 4). The unit is weakly welded. Lithic-rich pipes are shown byarrows in Figure (b). In this location, clear boundaries between each layer are notidentified, but weakly stratified facies are developed. The lower part of the figuresshows a densely welded pumice fall layer. (c) A close-up photograph of a weaklywelded stratified facies of Unit Bl in Take-shima (location 6). Arrows in figure (c) showpumice-rich degassing pipes.

Fig. 6. (a) Deformation structure in Unit Bu formed by ballistically ejected lithic frag-ments from plinian stage (20 cm diameter) and a large fragment of welded tuff fromUnit C that impacted underlying pumice-rich fallout layers (location 4). Arrows showfoliation 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. Arrowsindicate where the upper layer intruded into the lower layer (location 4).

and the pumice-rich upper layer of Unit Bu (Fig. 6b). Here, theoverlying lithic-rich layer (Unit C) impacts and scratches the weldedpumice-rich layer (PL of Unit B). This is similar to the flame structureswhich are sometimes observed at the contact between pyroclasticflow deposits and the underlying substrate. This structure indicatesthat 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 showthat Unit Bu was produced by particle to particle agglutination.

On the western side of Satsuma Iwo-jima, although the sedimen-tary features cannot be observed in more detail due to difficult access,weakly- to densely-welded pumice-rich stratified layers occur in theupper part of Unit B. OnTake-shima, non-welded pumice fallout layersconstitute a distal facies of Unit Bu (Fig. 7). The grain-size is less than afew centimeters.

3.3. Lateral variation of density and lithofacies

Spatial relationships between exposures, representative lithofa-cies, and density variations of Unit B are summarized in Fig. 7.Representative lithofacies are characterized by distinctive sedimen-tary structures and the degree of welding, roughly defined in close-upphotographs and photomicrographs in Figs. 4 and 8. In micro-scaletextures, the pumice and ashy matrix in weakly welded layers of UnitBl are characterized by sintering of ash and slightly higher porosity(Fig. 8a). In contrast, densely welded matrix parts of Unit Bl arecharacterized by eutaxitic textures and lower porosity (Fig. 8b). InFig. 7b, D and H show distance from source, and Unit B total thicknessat each location respectively. On Satsuma Iwo-jima, the lithofacies arecharacterized by mainly welded cross-stratified or stratified lithofa-cies (Figs. 4d–e and 8), and densities of Units Bl and Bu are up to2300 kg/m3. Their colors range from black inweakly welded outcrops,to dark-red in densely welded equivalents. Unit Bl is traceable to moredistal exposures on Take-shima, but the total thickness is only a fewtens of centimeters to a few meters. The deposit is mainly stratified

Fig. 7. (a) Outcrop locations andmodel 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 totalthicknesses 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 5and 6), it is weakly welded (with density 1000–1500 kg/m3) as shownin Fig. 4a. In more distal exposures (Sata-ura, location 7), lithofaciesare non- or weakly welded and massive, and its density is up to c.1000 kg/m3. A typical cooling unit is not clearly identified withinindividual thin welded layers or the whole of Unit B. In the proximalarea, upper stratified layers have higher density than lower andmiddle ones. The measured density may describe the degree ofwelding.

Dips and strikes of foliations of thin stratified flow units or fiammeof Unit Bl were also measured in some locations (Fig. 1). They indicatepost-depositional deformation structures in stratified flow units, i.e.,along hinges on the topographic low.

3.4. Grain size and components

Granulometry was conducted on a number of samples from thenon-welded layers in Units A and Bl in order to constrain sedimenta-

tion processes and pyroclast origins. Sampling locations are shown inFig. 7. Most of the samples were split and sieved at 1/2 phi intervalswith no further preparation. Representative data from the particle-size analysis are presented as histograms (Fig. 9). Samples (S01, S11,S12, S03, and S13) lack very fine sub-components and are interpretedas surge deposits or fines-depleted ignimbrite. S14 is froman ash-richsubunit in the most distal area (location 7) and has a bimodal-peak.

Components of lithic-rich subunits (S01, S11, S12, S03, and S13) aremainly lithic (altered country rock or fresh andesite lava), crystals,obsidian clasts, pumice (partially welded), and glass shards (Fig. 9).Obsidian clasts aremore abundant in S01, S11, and S12 from the earliercolumn collapse phase, compared with other lithic-rich samples (S03,S13, and S04) from the later phase. Spheroidal-shaped bombs withglassy rinds are also included in the lower part of Unit Bl (Fig. 10c).Some glassy juveniles have numerous cracks on their surface and bearfragments of country rocks (Fig. 10a, b). These features indicate rapidcooling of magma, probably due to dynamic contact withwater duringthe earlier column collapse phase. However, SEMobservations (Fig.11)

Fig. 8. Photomicrographs of juvenile materials in welded pumice-rich layers of UnitBl. (a) Weakly-welded pumice and ashy matrix, characterized by sintering of ash andslightly higher porosity. This photo corresponds with micro-scale texture of Fig. 4b.(b) Densely-welded matrix characterized by eutaxitic texture. This photo corre-sponds with micro-scale texture of Fig. 4d. PL: plagioclase phenocryst.


of non-welded juvenile pyroclasts provide only limited evidence ofmagma-water interaction as the mechanism of fragmentation (a dry-type phreatomagmatic eruption with a small water/magma ratio:Büttner et al., 2002), for the following reasons: (1) absence ofhydration cracks or hydrated surfaces, generally considered as theresult of direct contact of magmawith external water; (2) rare surfacealteration of glassy pyroclasts;, however, (3) for a small amount ofpyroclasts, many vesicles are cut by curviplanar fractures as a result ofhydrovolcanic fragmentation of the melt (Heiken and Wohletz, 1985)(Fig. 11a, b); (4) glass particles show blocky shapes with very fewisolated spherical vesicles (gas bubbles) (Fig. 11a), suggesting thatexsolution and expansion of magmatic gases did not play a major rolein pyroclast formation; and (5) some lithic fragments from countryrocks are hydrothermally altered. This indicates interactionwith eitheran aquifer (Barbeli et al., 1988) or sea water.

In the coarse-grained subunits that occur as pumice-rich lenses ornon-welded flow units in Unit Bl, some juvenile materials with bread-crusted surfaces can be found (Figs.10a, b and 12). Thematerials in thebombs are composed of irregular or rounded pumiceous domains,fragments of country rocks (altered lithic), crystals, and a matrixcomposed of fine pumice and ash. These clasts range from roughlyspherical to roughly ellipsoid in shape, and are denser than normalpumice grains sampled from the same units. Some have a glassy rim (afewmillimeters thick) and are crossed by cracks. Fragments of countryrock are enclosed in a brown, dense, and apparently less vesicularrhyolite matrix (Fig. 12a, b). Irregular or rounded and vesiculardomains are probably individual pumice lapilli agglomerated and

fused with country rock, but fine lithics show no evidence of softeningor melting. The dense matrix is interpreted as resulting from pumiceand ash having thoroughly mixed and compacted.

4. Chemical compositions

Whole-rock SiO2 contents of magma in the 7.3 ka Kikai eruptionrange from 72 to 74 wt.% for Units A and B (Maeno and Taniguchi,2007) (Fig. 13). Magma temperaturewas estimated as about 960 °C bySaito 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.% inNa2O+K2O. Water concentrations of rhyolite melt were estimated torange from 3 to 4.6 wt.% for the climactic phase, on the basis of meltinclusion 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-feedingstage (Phase 1), which generated Unit A fallout deposits over a widearea 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 temperatureof 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 interac-tion. This may be due to the presence of a pre-eruptive volcanic edificearound the vent: the Kikai-Komorikō tephra group on the two islandsrepresenting the caldera rim, which is derived from intermittentvolcanic activity between 13 to 8 ka (Okuno et al., 2000), indicatesthat a subaerial volcano was present in the Kikai caldera just beforethe 7.3 ka eruption.

The initial large-scale plinian phase was progressively followed bya column-collapsing phase (Phase 2). Plinian column-collapse hasbeen attributed to a decrease in magmawater content, the break up ofconduit (Sparks andWilson,1976;Wilson et al., 1980) or other factors.Changing of eruption style from column-feeding (Phase 1; Unit Aformation) to collapsing (Phase 2; Unit B formation) during the 7.3 kaeruption is inferred to have been accompanied by magma-waterinteractions, as not only accidental altered lithics but also quenchedbombs (although minor) and boulders are included in the lithic-richsubunits (LL). The eruption likely widened the vent toward the seaallowing access by seawater. However, the interaction betweeneruptive material and seawater was probably limited, so that juvenileclasts remained sufficiently hot to weld upon emplacement andentrained water was entirely heated to steam. Accretionary orarmored lapilli are in fact absent from the welded layers of Unit B.

The degassing pipes extending from the segregation pods intooverlying welded beds indicate that gas pressure increased in the podsduring or after the rapid accumulation of eruptive materials and wasreleased before welding and cooling occurred completely. Thetemperature of the currents was probably higher than the minimumwelding temperature, resulting in dense to weak welding of Unit Bl.On the western side of Satsuma Iwo-jima, the temperature of densitycurrents was so high that welding deformation easily occurred.

The accidental lithics may have derived from the breakup of thecountry rock due to conduit-wall abrasion (Macedonio et al., 1994), orconduit pressure changes during magma ascent (Papale and Dobran,1993). Heating of seawater in the pore-spaces of the wall-rock mayalso drive explosive expansion and wall-rock fragmentation whenconduit pressures decrease as a result of reduced or negative magmarise rates (e.g., Dobran and Papale, 1993; Doubik and Hill, 1999).

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 themiddleof 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 andlocations 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

The sedimentary characteristics of Unit Bl indicate that multipledilute pyroclastic density currents occurred and laterally spread fromthe base of the eruption column during Phase 2. Although the locationof an eruptive vent is not well constrained, it may have been locatednear the present Iwo-dake volcano because strongly welded falloutand flow deposits are only distributed around the northern part ofSatsuma Iwo-jima. Pumice-rich lenses and low-angle cross stratifica-tion with minor pinch-and-swell layers are typical of deposition fromrelatively low concentration, traction-dominated turbulent pyroclas-tic density currents. It is suggested that dense pyroclasts (includinglithics and quenched materials) were effectively segregated withinthe current body due to highly turbulent and diluted conditions,where elutriated finer and lighter materials followed later to beprogressively deposited on top and at more distal locations. Thecouple of a lithic-rich layer or pod and an overlying pumice-rich layerapparently represents deposition of a single aggrading unit producedby 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 ahigh momentum to the currents, resulting in a widely spreading, highvelocity turbulent current. Such pyroclastic density currents producehigh basal shear rates (e.g., Branney and Kokelaar, 1992, 2002; Wilson

and Houghton, 2000), resulting in tractional structures correlatedwith a segregation of dense lithics.

Repetitive generation of dilute pyroclastic density currentsmay havebeen derived from pulsate magma supplies (fluctuation of magmadischarge rate) or cyclic interactions of the magma and seawater. Thepresence of typical surge bedforms and sorting in a thinwelded deposit,such as Unit Bl, is an indication that pyroclastic density currents weregenerated and deposited within high temperature and highly energeticdensity currents, and that pyroclastic particles were sufficiently hot toweld instantaneously upon deposition. It is suggested that aggradinglayers (lithic-rich layers (LL) and pumice-rich layers (PL)) wereproduced as a result of these processes during the column collapsephase (Fig. 14b). Although various scales of pyroclastic density currentsmay have been generated continuously and deposited on Satsuma Iwo-jima, only some large-scale high temperature currents arrived in moredistal regions (i.e., Take-shima). The fate of the flows and ash-cloudsbeyond the present coastline remains unknown.

Eruptive styles further changed towards the end of the columnfeeding/collapsing phases, producing the fallout deposits of Unit Bu,whose deposits are characterized by well-stratified agglutinated (nocross-stratification), and densely welded facies sometimes includingballistics. These sedimentary characteristics indicate that no morehighly energetic erosive pyroclastic density currents occurred. Thiseruption phase was thus probably less explosive than the surge-

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 thesurface 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 eruptionsbecame dominant and eruption-intensity may have decreased to-ward the end of Phase 2 (Fig. 14c), it is suggested that the accu-mulation rate of pyroclasts was still high and that the depositiontemperature was kept above the minimum welding temperature.

5.3. Conditions of welding deformation

Pressure is generally the main driving force for compaction, ex-pulsion, and resorption of interstitial gas in pyroclasts (e.g., Cas andWright, 1987). In the proximal area of Satsuma Iwo-jima, the max-imum thickness of Unit B is 30–40 m; therefore, lithostatic pressure

Fig. 11. Scanning electron micro-images of juvenile materials in a non-welded lithic-rich layevesiculation. All scale bars 100 μm.

was less than about 0.5 MPa, and the density of the deposit is up toabout 2300 kg/m3. In the distal area at Take-shima, the thickness ofthe deposit is only a maximum of 4 m and compaction was notintense, but welding deformation occurred (the density of the depositranges from 1000 to 1500 kg/m3). The 4 m thick deposit can only haveexperienced less than 0.04–0.06 MPa lithostatic pressure, even at thebottom of the unit. The pressure ranges in both proximal and distalregions are close to atmospheric pressure; therefore, high tempera-ture, high alkali content, or high water content are more likely topromote welding than pressure, because increases of such physicalparameters can dramatically enhance welding and sintering andhence deposit density (e.g., Sparks et al., 1999; Grunder et al., 2005).

r of Unit Bl: (a, b) obsidian clasts with poor vesiculation, (c, d) pumice clasts with good

Fig.12. Photograph (a) and sketch (b) of juvenilematerial in a coarse-grained lithic-rich subunit in the lower part of Unit Bl (location 4), and photograph (c) and sketch (d) of juvenilematerial 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 (alteredlithics), 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 areenclosed in a brown, dense, and apparently non vesicular rhyolite matrix.


For example, theminimumwelding temperature of rhyolite at 1 atm isestimated 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 weldingdeformation (e.g., Dingwell et al., 1998). Some thin well-stratifiedwelded tuffs, such as Unit Bl, are generally derived from peralkalinerhyolites (e.g., Villari, 1974), having unusually low glass viscosities;calc-alkaline rhyolite examples have also been reported (e.g., Chapinand Lowell, 1979; Bacon and Druitt, 1988; Branney et al., 1992).However, the alkali content of Kikai rhyolite is much lower than forany other example (Fig. 13).Water content is also an important factorcontrolling welding deformation (Sparks et al., 1999). Although thesaturation 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 withoutfull-degassing, if its internal pressure does not follow lithostatic dueto a dramatic increase in viscosity associated with water loss (e.g.,Navon and Lyakhovsky, 1997). Based on these physicochemicalproperties of erupted rhyolite, it is suggested that alkali contentsdid not play major roles inwelding deformation at Kikai, but residual

water from degassing and/or high temperature (above minimumwelding temperature) was more important because it decreased theviscosity of pyroclasts. These welding conditions are also supportedby experimental results (e.g., Grunder et al., 2005). Furthermore, onthe basis of these considerations, some just-deposited juvenilepyroclasts may have been soft and sticky due to a high magmatictemperature above the minimumwelding temperature.

On the other hand, welded particles (still-hot fragmented pyr-oclasts) and lithic-bearing agglomerates (Figs. 10 and 12) were alsodeposited as Unit Bl, which may be the result of a specific sequence ofthe events in the eruptive vent. In order to produce aggregated par-ticles containing fragmented country rock, juvenile particles mustaggregate within gas-particle mixtures moving through an open con-duit (Lorentz and Zimanowski, 1984) or near-vent area, where theresulting complex aggregation of material may be almost immediatelyre-fragmented and re-ejected by succeeding explosions (Rosseel et al.,2006). Similar juvenile aggregates are broadly observed in theproducts of phreatomagmatic eruptions of low-viscosity magma ofbasaltic composition (e.g., Valentine and Groves, 1996; Doubik and

Fig. 13.Whole-rock SiO2 variation diagram for Na2O+K2O from eruptive deposits of 7.3Kikai eruption. Data from other thin or well-stratified welded tuff, similar to Unit B, arealso 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 ignim-brites are indications of particle coalescence during hot dilutetransport. They interpreted bomb to ash-sized mantled particles tohave formed byaccretion ofmagmadroplets at a range of temperature.However, we cannot observe these types of particles at Kikai, except forwelded particles and lithic-bearing agglomerates (Figs.10 and 12), andalmost particles are similar to pyroclasts in normal silicic ignimbrites.Therefore, we suggest that deformation and coalescences of particlesin Unit B mainly occurred instantaneously upon deposition, ratherthan within the current. The impacted or eroded welded layers areviscously deformed, and the deformation structures match the shapesof the brittle lithic particles. These indicate that the deposits beganwelding before they were deformed by impacting projectiles andoverlying bed loads.

Welded fallout deposits included in Unit Bu are only predicted tooccur for high temperature silicic and intermediate magmas with tem-peraturesN850 °C (Thomas and Sparks,1992). Thewelded fallout depositcan be traced within 2 km from the Kikai vent. High accumulation rate,large grain size, and long duration of deposition are the most importantinterdependent factors that control the degree of final agglutination orwelding (Capaccioni and Cuccoli, 2005). The physicochemical data in the7.3 ka Kikai magma (whole-rock SiO2 content 72 to 74 wt.%; magmatemperature about 960 °C) is in good agreement with theoreticalpredictions for generation of welded fallout deposits by Thomas andSparks (1992) and Capaccioni and Cuccoli (2005).

5.4. 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 toresult from contact between eruptive material and external seawater,and from the production of high pressure steam that explosivelydecompressed at almost atmospheric pressure. The question then is ifthis type of density current can keep as hot as the minimum weldingtemperature? To answer this, we investigated a water/magma (hotpyroclasts) mass ratio that can achieve a minimum welding tem-perature (we assumed 800–900 °C) at low particle concentrationsusing a simplified thermodynamic model (Wohletz, 1986, 2002). Thismodel assumes two thermodynamic stages: the first is related to theconservation of energy in initial thermal equilibrium as an adiabatic,and the second is related to a later expansion phase where water iscontinuously heated by pyroclasts entrained in it and the mixture

temperature decreases from the equilibrium one. A long time-scale ofcooling by thermal diffusion or convection for large particles is notconsidered in this model. Therefore, the calculated water/magmamass ratio is a maximum. On the basis of these consideration andusing KWare PHM (ver. 1.03.0054) by Wohletz, we can calculate thatif the water/magma mass ratio is less than 2–3 wt.% with an initialmagma temperature of 1200–1250 K, the mixture temperature(magma or hot pyroclasts and water) can exceed the minimum wel-ding temperature (Nc. 800 °C). Themixtures can also have low particleconcentration (solid fraction), less than 0.01, which is a plausible valuefor pyroclastic surges (e.g., Wohletz, 1998). The results support theidea that a small amount of seawater may be vaporized and promote adilution of currents, and that high temperature eruptive material canbe transported and deposited above minimum welding temperaturein near-vent areas. Assuming that the eruption rate during Phase 2 isalmost the same as in Phase 1 (108 kg/s — estimated from falloutdeposits; Maeno and Taniguchi, 2007), the mixing rate of externalseawater should be less than 105–6 kg/s, which corresponds to a fewvol.% of the erupting magma.

5.5. High-temperature dilute pyroclastic density currents and their deposits

Surge-typepyroclastic density currents are generally transported andemplaced below the minimum welding temperatures during phreato-magmatic eruptions (e.g., Self,1983; Allen andCas,1998;Houghton et al.,2003), because this type of density current tends to be colder than denselaminar currents from dry magmatic eruptions because rapid vaporiza-tion and expansion of external water promotes cooling and dilutionof the currents. Resulting non-welded tractional structures are thoughtto require elastic particle–particle or particle–sediment interactions,such as dunes formed by traction (saltation, rolling, and sliding) ofparticles along the substrate (e.g., Sohn, 1997; Wohletz, 1998). On theother hand, pyroclastic density currents producing high-grade (denselywelded) ignimbrites are evidently transported well above minimumwelding temperatures, and some flows may have even moved abovesolidus temperatures (e.g., Cas and Wright, 1987; Freundt andSchmincke,1995). In that case, particleswereprobably plastic to partiallyliquid and thus sticky and able to agglomerate or coalesce, as industrialglass beads rapidly agglomerate at temperatures below their meltingpoint (Gluckman et al.,1976). Elasticmomentum transfer duringparticleinteractions cannot occur between soft and sticky particles or betweensoft particles and a plastic substrate. Therefore, surge-type beddingcannot generally develop in high-grade ignimbrites (e.g., Freundt,1998).These considerations related to the transportation and sedimentation oferuptive materials may be appropriate for a system of mono-dispersedhot juvenile particles. However, real pyroclastic density currents includebrittle particles and sticky juvenile particles, that is, they are multi-dispersed particle systems.

Pyroclastic density current deposits generated by collapsingplinian columns during the 7.3 ka eruption (Unit Bl) are characterizedby tractionally-stratified lithofacies, in spite of being densely orweakly welded, that is, ‘welded surge deposits’. Generally, pyroclasticdeposits are distinguished by their lithofacies, which reflect theirtransport and sedimentation processes (Fisher and Schmincke, 1984;Cas and Wright, 1987; Branney and Kokelaar, 2002). However, thereare few literature references concerning welded pyroclastic surgedeposits relative to those on welded fall and flow deposits. This couldbe taken to indicate that cooling in surges is so rapid as to preventwelding, and typical sedimentary traction structures and sortingcharacteristics may not form because agglutination or coalescence in apyroclastic surge prevents particulate traction. Against these insights,it is considered that welded pyroclastic surge deposits can be pro-duced only in specific conditions, such as (1) rapid accumulation ofstill-viscous pyroclasts sufficiently hot to weld instantaneously upondeposition, or (2) erosion of underlying beds through corrosion orelastic particles' interactions with substrate deformation.

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 duringfallout 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 oflithic-rich layers or lens-like pods (LL). The lithofacies show seg-regations of dense particles during the evolution of individual currentpulses. In such a situation, segregated particles in the base of thecurrent are mainly composed of brittle fragments (dense lithicpyroclasts). Underlying beds may have been composed of still-softsticky and lighter pyroclasts at high temperature, and they could haveexperienced erosion through impingement of depositing denseparticles (e.g., Allen, 1984; Macedonio et al., 1994). This processresulted in cross-stratified or stratified facies with erosive contacts.Conversely, if juvenile hot pyroclasts, which were plastic to partiallyliquid and thus sticky, are included in sufficient quantity, they are ableto rapidly agglomerate or coalesce, resulting in densely or weaklywelded and relatively structureless deposits (e.g., Branney andKokelaar, 1992). These particle-particle or particle-boundary inter-actions between brittle particles, juvenile hot pyroclasts, and softsticky substrates in a multi-dispersed particle system (high tem-

perature pyroclastic density current), may have resulted in thecharacteristic welded surge deposit sedimentary structures. Thephysical conditions for such processes may be achieved within hightemperature and highly energetic pyroclastic density currentsproduced by large-scale explosive eruptions.

6. Conclusions

The collapse of a plinian column during Phase 2 of the 7.3 ka Kikaieruption produced Units Bl and Bu. Unit Bl consists of multiple thinsubunits with stratified and cross-stratified facies showing variousdegree of welding. Each thin subunit is composed of a lithic-rich layeror pod and a welded pumice-rich layer. Lithic-rich parts are fines-depleted, and are mostly composed of brittle-shaped dense particles,including altered lava, fresh andesite lava, obsidian clasts, andboulders. Pumice-rich parts are rich in deformed pumice and ash orfiamme. In contrast, Unit Bu shows only densely welded stratified


facies, composed of alternating lithic-rich and pumice-rich layers. Thelayers mantle lower units, and are sometimes viscously deformed byballistic impacts by lithic fragments, indicating that Unit Bu is of fall-origin.

This evidence indicates that high temperature dilute pyroclasticdensity currents were repeatedly generated from limited magma-water interactions during plinian column-collapse. Under suchconditions, dense brittle particles may have been segregated in aturbulent current, immediately followed by sedimentation of hotlighter juvenile pyroclasts, resulting in multiple aggrading subunitscomposed of lithic-rich layers or pods and pumice-rich layers. Further-more, it is suggested that the depositional temperature increased fromPhase 1 to 2 because accumulation rates increased maintaining hightemperatures, and that the eruptive style changed from a normalplinian eruption (Unit A) into an agglutinate-dominated fallouteruption (Unit Bu) through transitional surge-generating explosions(Unit Bl), because the intensity of eruption decreased toward the endof Phase 2 but the high deposition temperature was kept.

On the basis of sedimentation characteristics and some theoreticalconsiderations, it is considered that welded pyroclastic surge deposits(Unit Bl) can be only produced under specific conditions, such as: (1)the pyroclastic particles quickly accumulated andwere sufficiently hotto weld instantaneously upon deposition; and (2) inelastic particle-particle or particle-substrate interactions coupled with substratedeformation or erosion by brittle particles. Underlying still-soft stickypyroclasts could have experienced corrosion as a result of theimpingement of depositing dense brittle particles at high tempera-ture, and if juvenile hot sticky pyroclasts are included in sufficientquantity, they are able to rapidly agglomerate or coalesce, resulting indensely or weakly welded deposits. These physical conditions may beachieved within high temperature and highly energetic pyroclasticdensity currents produced by large-scale explosive eruptions.

Acknowledgements

We acknowledge K. Kano for critical comments and discussions onthe early version of this paper, and also we thank T. Kobayashi forcritical discussions. We appreciate the constructive reviews andcomments from C. Busby andN. Geshi.We are grateful to the residenceof Mishima village, Kagoshima, Japan, for help with our field survey.

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Sedimentation and welding processes of dilute pyroclastic ...

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