Journal of Volcanology and Geothermal Research 247–248 (2012) 139–157

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Sequence of the 1895 eruption of the Zao volcano, Tohoku Japan

Kotaro Miura a, Masao Ban a,⁎, Tsukasa Ohba b, Akihiko Fujinawa c

a Department of Earth and Environmental Sciences, Yamagata University, 4-12 Kojirakawa-machi 1-chome, Yamagata 990-8560, Japanb Faculty of Engineering and Resource Science, Akita University, 1-1 Tegata gakuen-machi, Akita-shi, Akita 010-8502, Japanc Department of Earth Sciences, Ibaraki University, 1-1 Bunkyo 2-chome, Mito 310-8512, Japan

⁎ Corresponding author. Tel.: +81 23 628 4642; fax:E-mail address: ban@sci.kj.yamagata-u.ac.jp (M. Ban

0377-0273/$ – see front matter © 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.jvolgeores.2012.08.005

a b s t r a c t

a r t i c l e i n f o

Article history:Received 30 January 2012Accepted 11 August 2012Available online 22 August 2012

Keywords:Non-juvenile steam eruptionPhreatic eruptionEruption sequenceVolcanic hazard mitigationZao volcanoTohoku Japan

The most recent major eruption event of the Zao volcano comprised a series of phreatic eruption episodes on 15and 19 February, 22 August, and 27–28 September 1895, with several precursory vulcanian eruptions duringFebruary–July 1894. All were generated at the Okama crater lake located inside the Umanose caldera. The erup-tion products consist mainly of hydrothermally altered ash with altered blocks, except for ash from 1984. Theeruption deposits of 1895 are divided lithologically into six layers (1–6). Comparison of the document withthe lithofacies of deposits shows that layers 1, 2, 3–4, and 5–6were correlated respectively with eruption episodesof 15 February (episode 1), 19 February (episode 2), 22August (episode3), and 27–28 September (episode 4). Dur-ing these four episodes, ca. 0.5%, 0.5%, 1.5%, and 98%of the totalmass of the products had beendischarged. Based onlithologic, stratigraphic, granulometric, and component analyses and on distributional features for these layers, thefollowing depositional mechanisms were inferred. Layers 1, 3, and 4 were formed mainly from their related smallpyroclastic density currents,whereas layer 2 resultedmainly froma small pyroclastic fall. In contrast, layers 5 and 6are larger-scale near-vent pyroclastic fall deposits fromash clouds and eruption clouds,whichmight have includedsome juvenile fragments. The three early episodes in 1985 led to the climactic episode of 27–28 September. Fur-thermore, the andesitic magma chamber at b3 kb depth, which caused the 1894 vulcanian eruptions, became ahydrothermal alteration source for the 1895 erupted materials. The chamber was re-activated before 1895 erup-tion by injection of basaltic magmas from greater depth. The injection reached maximum at the climactic event.The inferred course of that series of eruption episodes provides useful information to predict future volcanicphreatic-type eruptions at this volcano.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Non-juvenile steam explosions, which occur frequently in activeJapanese volcanoes (Okuno, 1995), are a common style of eruptionthat has been causing severe volcanic hazards, particularly in proxi-mal areas (e.g., Yamamoto et al., 1999; Fujinawa et al., 2006, 2008).Accumulation of related volcanological and geological data is crucialfor protection against such volcanic hazards. Indeed, in 2000, duringeruption of the Usu volcano, which was characterized by a series ofphreatic to phreatomagmatic eruptions (Tomiya et al., 2001), nearbyresidents found refuge because the type and time of the eruption hadbeen well-predicted by virtue of detailed volcanological and geologi-cal investigations (e.g., Soya et al., 1981; Ui et al., 2002). However,geologic studies of non-juvenile steam eruption are rare becausenon-juvenile steam eruptions usually lead to emissions of littlemass. In many cases, the resultant deposits are poorly preserved ingeologic time.

Young, non-juvenile steam explosions such as those which oc-curred within the past century or so have more completely remaining

+81 23 628 4661.).

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thin eruption products (Fujinawa et al., 2008). Furthermore, recenteruptions have been witnessed. Several eruption accounts are avail-able, providing extremely useful documentation for modern volcano-logical investigations, and supporting detailed assessment of thenature and sequence of the eruption events.

For instance, regarding the 1888 phreatic eruption of the Bandai vol-cano, a sort of pyroclastic surge was recognized as accompanying thesector collapse and its resultant debris avalanche. The surge severelydamaged several villages on the east to southeastern flank of the strato-volcano, scarring and fatally burning many residents (Sekiya andKikuchi, 1890; Yamamoto et al., 1999; Japan Meteorological Agency,2005; Fujinawa et al., 2008). In the 1900 steam explosion at Adataravolcano, some visitors and 82 workers at a sulfur refinery in the craterareawere killed or severely injuredmainly by a special sort of pyroclas-tic surge containing no juvenile materials (Japan MeteorologicalAgency, 2005; Fujinawa et al., 2006).

Non-juvenile steam eruptions in Tohoku, Japan have beenreported by several authors, such as Yamamoto et al. (1999) for theBandai 1888 eruption, Ohba et al. (2007) for the Akita-Yakeyama1997 eruption, and Fujinawa et al. (2008) for the Adatara 1900, Zao1895, and Bandai 1888 eruptions. During the Bandai 1888 eruptionevent, 15–20 phreatic explosions occurred successively in one hour.

Fig. 1. Locality map of the active volcanoes in Tohoku, Japan. Circles show the volcanic cen-ters. Stars show volcanoes at which non-juvenile steam eruption has occurred during thepast century.

Fig. 2. (A) Geology and stratigraphy of the Zao volcano (Sakayori, 1992; Ban et al., 2008). Gtinate; NGL, Nigorigawa lavas; YKL, Yokokurayama lavas; HPL, Happosawa lavas; KNP, KNakamaruyama lavas; HML, Hiyamizuyama lavas; TKL, Torikabutoyama lavas; ZOL, ZaozawaRO, Robanomimiiwa pyroclastics. (B) Close-up map of the summit area (Sakayori, 1992; Ba

140 K. Miura et al. / Journal of Volcanology and Geothermal Research 247–248 (2012) 139–157

These eruptions were accompanied by the collapse of a section of theedifice (Sekiya and Kikuchi, 1890). Several pyroclastic density cur-rents were discharged during the eruption event.

Although a preliminary description of the Zao 1895 eruption eventwas reported in Fujinawa et al. (2008), more detailed analyses havebeen necessary to reveal the nature and sequence of the eruption.In this study, we examined the sequence of the Zao 1895 eruptionin greater detail by mutually correlating the complete sets of theeruption record, geological characteristics, grain size distribution,and component analyses.

2. Geological background

Zao volcano is located at the central part of a volcanic front of theTohoku Japan arc (Fig. 1). This volcano, which started its activity atabout 1 Ma (Takaoka et al., 1989), has remained active to the presentday. Geologic, petrologic, and stratigraphic outlines of this volcanowere first published by Chiba (1961), with later detailed geologic andpetrologic reports presented by Oba and Konda (1989) and Sakayori(1991, 1992). Based on these studies, the Zao volcano activity is divisi-ble into four stages: stage 1 of around 1 Ma, stage 2 of around 300 ka,stage 3 of 300–100 ka, and stage 4 of 30 ka to the present (Fig. 2). Thenewest activity (stage 4) probably commenced immediately after thecollapse of the volcanic edifice, leaving a horseshoe-shaped amphithe-ater in the summit area: the Umanose caldera of ca. 1.7 km diameter(Sakayori, 1992). In stage 4, Goshikidake cone (ca. 0.1 km3)was formedin the inner part of theUmanose caldera. A crater lake, Okama, is locatedin the western part of Goshikidake.

Eruptive products of stage 4 are classifiable geologically into foureruptive units from older to younger: Nigorigawa lavas (NGL),Komakusadaira agglutinate (KMA), Umanose agglutinate (UMA), andGoshikidake pyroclastics (GSP) (Sakayori, 1992; Ban et al., 2008). TheNigorigawa lava, which flowed along the Nigorigawa River, has mostlybeen eroded (Sakayori, 1992). The Komakusadaira agglutinate com-prises more than 14 lithologically distinct pyroclastic layers. It mainlydrapes the top of the Umanose caldera wall (Ban et al., 2008). TheUmanose agglutinate, which consists of four layers, covers theKomakusadaira agglutinate in the western part of the Umanose caldera

SP, Goshikidake pyroclastics; UMA, Umanose agglutinate; KMA; Komakusadaira agglu-umanodake pyroclastics; IML, Ichimaiishizawa lavas; SKL, Sainokawara lavas; NML,lavas; SNL, Senninzawa lavas; OWL, Oiwake lavas; MYP, Mayuyamazawa pyroclastics;n et al., 2008).

Table 1Chronologically ordered eruptive events of the 1895 eruption of Zao volcano.

Year, month Day Time Phenomena

1867Oct. 21st Rumbling. Maddy water flowed into the

river. In the spa located in the easternfoot of the Zao, three people died in aflood. After the eruption, black smokesometimes rose up.

1894Feb.–Mar. Eruptive smoke rising up from the crater

lake.Jul. 3rd Adhesion of ash to the plants was

recognized after the rain in the westernfoot of the Zao volcano.

1895Feb. 15th 9:30–10:30 Awhite smoke burst up suddenly from the

crater lake with rumbling, which waswitnessed from the vicinity of the craterlake and Kaminoyama Town. Ash fellaround the Katta District and Sulferizedwater flowed into the Abukumagawariver.

19th 8:00 Rumbling.9:30 The pisolitic ash fell in the Katta District.10:00 The crater lake flooded, and the

overflowed water reached into theMatsukawa river, involving many treestogether with ice.

11:30–12:30 Shiroishigawa river, the downstream of theMatsukawa river was swollen withsulfurizedwater carryingmany drift woods.

Mar. 14th Fountains built up from the crater lake (ca.3 m in height).

Mar. 22nd Rumbling, lahar.Aug. 22nd Ash fall was witnessed in Yamagata City.Sep. 27th 4:40–5:00 Black and white smoke burst up with

rumbling from the crater lake waswitnessed from the Katta District. Afterthe 5 min (at 5:05), the sulfurized ashfell.

5:05 The smoke reached to the Sasaya Pass,Kawasaki Village and Natori District,while falling ash.

5:20 The Nigorigawa river was swollendriftwood and sulfurized water.

6:15 Sound similar those of canon fires in rapidsuccession was recognized in the Kawaotosulfer refinery. A kind ofreverberation sound such as “boom”wasfelt without earthquake. Then, ash fellalong with the rain drops.

18:00 Ash fall.28th 5:00 Vesicleated block (30 cm in diameter)

had been ejected from the crater lake andlanded also near the new ore yardbeyond the Haizukayama.

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(Ban et al., 2008). The Goshikidake pyroclastic unit is composed mainlyof pyroclastic surge deposits of phreatomagmatic origin. It is divisiblefurther into five sub-units (Ban et al., 2008). The Goshikidake pyroclas-tics mainly build up the Goshikidake cone.

Sixteen tephra layers (Z-To 1–4, 5a, 5b and 5–14), composedmainly of scoriaceous volcanic ash, are readily observable. Theyhave been correlated with products of stage 4 (Imura, 1999; Ban etal., 2005; Miura et al., 2008). Based mainly on 14C ages for buriedleaves, wood, and reddish–dark brown paleosol that are intercalatedwith the relevant tephra layers (Ban et al., 2005; Miura et al., 2008),the ages for KMA, UMA, and GSP were estimated, respectively, as ca.33–12.9 ka, 7.5–4.1 ka, and 2.0 ka–present.

Phreatic eruption products are often intercalated with the juvenilescoriaceous tuff layers in the mountainside of the Zao edifice,suggesting that phreatic eruption has occurred frequently duringstage 4. Some of these events, which occurred during historic times,are the youngest andmost representative events that we are studyingpresently. They were widely witnessed and were described in articlespublished by several local newspapers and in academic reports, as de-scribed below.

3. Descriptions of the 1894 and 1895 eruptions

Brief descriptions that are useful to elucidate the individual eruptiveepisodes that occurred during February 1894 to September 1895 havebeen collected from several newspaper articles published in the TohokuNippo, Ou-Nichinichi Nippo, Tohoku Shinbun, Yamagata-ShogyoShinpo, Yamagata Nippo, and Yamagata-Jiyu Shinbun papers, alongwith flash academic reports (Kochibe, 1896a,b,c) and a research reporton the Okama crater lake (Anzai, 1961).

The inferred chronological order of the eruption events ispresented in Table 1. A map of the Zao vicinity is depicted in Fig. 3.The volcano had been quiet from 1867–1894 (Kochibe, 1896c) beforethe eruption event started in February, 1894. Four discrete eruptionepisodes are distinguishable in a series of eruptions that occurred in1895: 15 February of the first episode, 19 February and 22 August asthe second and third episodes, and the final, climactic episode of27–28 September.

3.1. February–July, 1894

Several discrete emissions of eruptive smoke rising up from thecrater lake were witnessed during February–March, 1894 at theShibata District office, situated ca. 10 km northeast of the summit(Kochibe, 1896c).

On 3 July, fallout ash that adhered to plants after a rainfall was rec-ognized in the villages of western foot of the Zao (Yamagata-ShogyoShinpo, 6 July, 1894). Especially, a large amount of ash fell in MikamiVillage, Minami-Murayama District located ca. 10 km west of thesummit (Yamagata-Shogyo Shinpo, 6 July, 1894).

3.2. 15 February, 1895

According to the report by Kochibe (1896c) and local newspapers,the eruption on 15 February, 1985, is divisible into the following threephases: (1) smoke on a thermal burst up around 9:30–10:30, (2) wetash fell within a local area covering ca. 5 km diameter at the south-eastern part of the summit and non-cohesive lahar flowed upstreamin the Nigorigawa River, and (3) sulfuric water poured into theShiroishigawa and Abukuma rivers, damaging nearby areas severely.

3.3. 19 February, 1895

On 19 February, the summit area of Zao rumbled again at 8:00. Pi-solitic ash fell on the Katta District at 9:30 (Ou-Nichinichi Nippo, 22February, 1895). The crater lake flooded. Its overflowing waters

reached the Matsukawa River at 10:00, involving many trees togetherwith ice (Ou-Nichinichi Nippo, 22 February, 1895). The ShiroishigawaRiver and downstream areas of the Matsukawa River were swollenwith sulfuric water carrying much driftwood for several tens of mi-nutes around noon (Ou-Nichinichi Nippo, 22 February, 1895).

After this event, no eruption was recorded for almost six months,although fountains (ca. 3 m high) rising on the crater lake werewitnessed by a mountain climber on 14 March (Kochibe, 1896a).

3.4. 22 August, 1895

An ash fall phenomenon was witnessed in Yamagata City, ca. 15 kmnorthwest of the summit. Its amount was unclear. The Yamagata Nippo(24 August, 1895) reported it as a trace amount, although the TohokuNippo (29 August, 1895) claimed it to be “a large amount.” According

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to a sketch presented on 28 August (Fig. 4A) six days after the 22 Augusteruption, the western to southern rim of the Okama crater lake was flat,as it appeared in the Yamagata-Jiyu Shinbun, 4 September, 1895.

3.5. 27 September, 1895

At about 5:00, black and white eruptive smoke that had beenbursting up with rumbling from the Okama crater lake waswitnessed at the Katta District (Ou-Nichinichi Nippo, 29 Septem-ber, 1895; Tohoku Shinbun, 29 September, 1895; Tohoku Nippo,29 September, 1895). The smoke had gradually been drawn outwith the northeast wind. Five minutes later (at 5:05), the smokereached Sasaya Pass, ca. 10 km north of the summit and the KawasakiVillage (time unclear). It reached ca. 20 km northeast of the summitas fall out ash (Tohoku Shinbun, 29 September, 1895; Tohoku Nippo,29 September, 1895). Twenty minutes after the eruption (at 5:20), theNigorigawa River was swollen with sulfuric water containing muchdriftwood (Ou-Nichinichi Nippo, 29 September, 1895; Tohoku Nippo,29 September, 1895). The riverwater turned gray (TohokuNippo, 3 Oc-tober, 1895). Most fish in the river died (Ou-Nichinichi Nippo, 29 Sep-tember, 1895). At the Kawaoto sulfur refinery located ca. 7 km eastfrom the crater lake, sounds like cannon firewere heard in rapid succes-sion at 6:15 (Kochibe, 1896a). At the same time, according to a mineworker at the Kawaoto sulfur ore yard located ca. 200 m north of thecrater lake, a kind of reverberating sound like a “boom”was feltwithoutearthquake. Later, ash fell together with the rain (Kochibe, 1896a). Thesmoke of this event was reported to have extended to the Natori Dis-trict, ca. 40 km east of the summit (Ou-Nichinichi Nippo, 29 September,1895, Tohoku Nippo, 3 October, 1895). The ash from the smoke report-edly caused damage to vegetables around the Higashi-Taga Village,

Fig. 3. (A) Simplified topographic map showing geographic names around the Zao volcanotowns and villages are shown. The curved lines show rivers. The bold lines in (B) show dis

Natori District (Ou-Nichinichi Nippo, 29 September, 1895). At 18:00,similar fallout ash was observed again (Kochibe, 1896a). Fortunately,approximately 40 climbers at the volcano summit on that day were allsafe (Tohoku Nippo, 3 October, 1895).

The head of the Shiroishi police documented an interesting fieldobservation. He departed from his office at about 13:00 on 27 Sep-tember and arrived at the crater lake at some time thereafter. Thewater level of the crater lake had decreased about 9 m from itsprior, pre-eruption level. Furthermore, the crater lake water was boil-ing. Vertical black water fountains built up 3-m-high columns repeat-edly at the northwestern part in the crater lake with rumbling(Ou-Nichinichi Nippo, 3 October, 1895; Tohoku Nippo, 3 October,1895). From this account, the temperature of rock fragments nearthe crater lake was inferred to be about 100 °C (Ou-NichinichiNippo, 3 October, 1895; Tohoku Nippo, 3 October, 1895).

In addition, on the Goshikidakemountainside about 36 m northeastof the crater lake, a fissure variously reported as ca. 3 cm (TohokuNippo, 3 October, 1895) and 3 m (Ou-Nichinichi Nippo, 3 October,1895) wide and 3 m deep had developed.

According to another sketch made on 27 September by a Mr. Izumi,chief of the Kawaoto sulfur refinery (Fig. 4B), the eruption cloud formedby the eruption dropped many larger clasts or blocks, with develop-ment of an umbrella formation at the top. In a sketchmade on 6October(Kochibe, 1896a) (Fig. 4C), a small moundwas apparent at the westernto southern rim of the crater lake.

3.6. 28 September, 1895

According to Mr. Izumi, a small amount of ash fell at 5:00 on thenew ore yard located near the Daikoku (Fig. 3B), ca. 2 km east of

and the surrounding area. (B) Close-up map of the mountain area. Old names of cities,tribution areas of the 1895 volcanic ash (Kochibe, 1896a, partly modified).

Fig. 4. Sketches of the summit area of Zao volcano after Kochibe (1896a): (A) 28August, 1895; (B) 27 September, 1895; and (C) 6 October, 1895.

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the crater lake. Then, a porous block (ca. 30 cm diameter) was ejectedfrom the crater lake, landing near the new ore yard beyond theHaizukayama (Fig. 3B) ca. 1.5 km south of the crater lake (Kochibe,1896a).

According to on-site investigations by officers of the Shibata Districtand two policemen, the source of the hot spring used by the Gaga Spa(Fig. 3B), ca. 5 km east of the crater lake, was safe from the damage.Moreover, the people and animals were all safe. However, a hut calledTakinoyu, which had been located in the Gaga Spa area, was sweptaway by the flooding Nigorigawa River and a bathroom had beenflooded to a depth of ca. 30 cm (Ou-Nichinichi Nippo, 29 September,1895; TohokuNippo, 29 September, 1895; Tohoku Shinbun, 29 Septem-ber, 1895). According to the Shibata District office, the Nigorigawa Riverhad risen up to ca. 9 m. A bathroom and 12 huts in Gaga Spa had beencarried away (Kochibe, 1896a).

A field observation was performed on 6 October, 1895 (Kochibe,1896a). Its results showed that the pits formed by deposition ofblocks in the western rim of the Umanose caldera were ellipsoidal,and that soils and sand of the surface had been scattered to the west-ward part of the pits. Consequently, these blocks are inferred to havebeen ballistic. At Kumanodake (Fig. 3B), located ca. 1 km northwest ofthe summit, the pits formed by impact of the blocks which werethought to have been ejected during this eruption were circular. In

addition, soil and rock fragments on the surface were scattered inall directions of the pits. These are indicative of rather high-angledtrajectories. In October 1895, after Kochibe (1896a) traced the falloutash distribution, he reported that the main axis extended eastward tonortheastward up to ca. 20 km from the crater lake (Fig. 3).

3.7. After 1895

After the climactic eruption of the 28 September, the crater lakebecame almost entirely calm except for small smoke emissionswitnessed with rumbling at 0:30, 14 January, 1897 (ImperialEarthquake Investigation Committee, 1918). In addition, at Torijigoku(Fig. 3B), ca. 1.5 km northeast of the crater lake, a small eruption oc-curred at 10:00, 16 April, 1940. During 1939–1940, the crater lakewater boiled several times (Anzai, 1961).

4. Field data

Gray to whitish-pale gray colored ejecta of the 1894–1895 eruptionare distributed within ca. 2 km from the crater lake center. They arewell preserved within an area of 200–300 m from the center, particu-larly in topographic lows, on the southwestern rim of the Okama craterlake. Regarding the ejecta in the areas near the topographic high, suchas near the summit of the Goshikidake and outside of the Umanose cal-dera, the deposits drape the paleosol or pre-existed deposits as thinnerbeds. We conducted detailed observations of 30 outcroppings locatedinside of the Umanose caldera, along with nine sites outside of theUmanose caldera (Fig. 5). Columnar sections for the representative lo-calities are depicted in Fig. 6.

4.1. Distribution, stratigraphy, and lithofacies of the 1894 and 1895deposits

The 1894 deposit (designated as 1894 ash hereinafter) is com-posed mainly of black dense volcanic ash. However, the 1895 depositscontain tephra components of various kinds, such as altered lithicfragments and non-altered to weakly altered andesitic rock frag-ments, in addition to scoriae of various amounts set in the matrix,which consists of hydrothermally altered fine ash. A full set of theselayers is observed restrictedly at the southwestern rim of the craterlake (SWRC) including loc. 1. At SWRC, the 1895 deposits are divisiblelithologically into six layers – layers 1–6 – in ascending stratigraphy.At the type locality (loc. 1), the 1894 ash and layers 1–6 of the 1895ash show respective thicknesses of ca. 2, b1, 4, 5, 20, 175, and>300 cm (Fig. 6). Some layers are non-existent at several localitiesin eastern and northern areas (Fig. 6). At localities outside of theUmanose caldera, only the 1894 ash layer and layer 6 of 1895 are ob-served. Detailed descriptions are presented in Table 2. Photographs ofthe representative outcrops of the deposits are shown in Fig. 7.

4.2. Discharged mass of the 1895 deposits

The discharged mass was estimated based on data from isopachs(Fig. 8) using the method described by Takarada et al. (2001, 2002).The estimated discharge masses for each of the 1895 layers areshown in Fig. 9. The area (m2) and weight (kg/m2) enclosed byeach isopach are depicted on a smooth line in double logarithmicgraph. The total mass was calculated by summing up the weightwithin the lines in each segment divided by data points (Takaradaet al., 2001, 2002). Thickness (m) can be converted into the massper square meter (kg/m2) using the density (kg/m3). In this study,the value of 900 kg/m3, which is the average density of particlesless than 16 mm of 1895 deposits, was used except for the portionsof larger than 16 mm of layers 5 and 6. The density for each layer, ex-cept for the above portions, was determined using the weight of theproduct filled in a 100 ml beaker after drying in an oven at 105 °C.

Fig. 5. Topographic map of the Zao volcano. Black circles represent localities of the surveyed outcrops. Numbered circles represent the representative localities.

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For the portions, the value of 2.1 kg/m3 was used as the density be-cause about three-fourths of the materials are dense andesite. De-posits within the crater lake were omitted from calculations.

Masses of layers 1 and 2 were not calculated because their isopachmaps can not be defined adequately. The estimations for layers 3–5 inthe distal areas were approximated by extending the isopachs linearly.The mass estimation for layer 6 in distal area was omitted from calcula-tions because we were unable to draw proper isopachs. Consequently,the mass of layer 6 calculated from the extended line might include alarger error than those for layers 3–5. The calculated masses for layers3–6 were ca. 3.8×106, 5.7×106, 7.0×107, and more than 5.5×108 kg.Layers 1 and 2 are thinner than layer 3 at the type locality where thecomplete set of layers is observed (loc. 1). Therefore, the mass of layers1 and 2might be approximated as less than 3.8×106 kg. That result im-plies that the mass ejected to form each of the six layers became largerover time.

4.3. Grain size analysis

Samples were collected from almost all layers at various localesexcept for layers 1 and 3 in some cases, where the layers are toothin to collect sufficient amounts for samples. For layer 3, sampleswere collected only at the southwestern rim of the crater lake (loc. 1).

The method used for grain size analysis was the following. (1) Thecoarse fraction (>16 mm) was measured for an area of ca. 40×40 cmfor each layer using on-site photographs obtained using a digital

camera, except for layer 6 in the SWRC (loc. 1). In the exceptional caseof layer 6 in loc. 1, the measured area was enlarged to ca. 100×300 cmbecause proportions of larger clasts are high. (2) Intermediate fractions(16–0.063 mm) were measured using ultrasonic sieving. (3) Finer frac-tions (b0.063 mm) were measured using settling tube analysis (BunanDoshitsu Shikenjo Ltd.).

The pyroclastics are subdivided granulometrically into eight catego-ries according to the definitions presented by Sohn and Chough(1989): fine ash (b0.063 mm), medium ash (0.5–0.063 mm), coarseash (2–0.5 mm), fine lapillus (4–2 mm), medium lapillus (16–4 mm),coarse lapillus (64–16 mm), fine block (256–64 mm), and coarse block(>256 mm). The grain diameter is shown in φ scale as φ=−log2d,where d represents the diameter. The definition of the sorting is fromRominger (1954): very well sorted (0bσφb1), well sorted (1bσφb2),normally sorted (2bσφb3), poorly sorted (3bσφb4), and very poorlysorted (4bσφ). Grain size distributions in the SWRC (loc. 1) and theother representative localities (Fig. 5) are portrayed in Fig. 10A and B,along with these numerical data in Table 3. The characteristics are alsoshown as σφ-Mdφ diagrams (Fig. 11). Overall, the larger clasts tend tobe more enriched upwards. Detailed descriptions are presented inTable 2.

4.4. Component analysis

For larger grains of 512–16 mm(−8 to−4φ) diameter,we identifiedrock types using the naked eye and analyzed rock type proportions

Fig. 6. Representative columnar sections for the 1895 products in Zao volcano. Locations of respective sections are presented in Fig. 5. The scales differ for sections obtained insideand outside of the Umanose caldera.

145K. Miura et al. / Journal of Volcanology and Geothermal Research 247–248 (2012) 139–157

through on-site observation. A binocular microscope was used for identi-fication of smaller grains (16–0.063 mm; −3 to 4 φ). After subdivisioninto 13 fractions, which were 1) 512–256 mm (−8 φ), 2) 256–128 mm(−7 φ), 3) 128–64 mm (−6 φ), 4) 64–32 mm (−5 φ), 5) 32–16 mm(−4 φ), 6) 16–8 mm (−3 φ), 7) 8–4 mm (−2 φ), 8) 4–2 mm (−1 φ),9) 2–1 mm (0 φ), 10) 1–0.5 mm (1 φ), 11) 0.5–0.25 mm (2 φ), 12)0.25–0.125 mm (3 φ), and 13) 0.125–0.063 mm (4 φ), respectively, weidentified and counted altered lithic fragments, non-altered andesiticfragments, altered scoriae, pisolitic ash, and mineral and gypsum grains.Binocular microscope images for the representative grains are presentedin Fig. 12. Results of the component analysis are presented in Table 4.Modal compositions for the 1894 ash and layers 1–6 are shown inFig. 13A, B.

Overall, altered lithic fragments (ca. 24–70 wt.%) and andesitic frag-ments (ca. 12–44 wt.%) are the dominant components through thesamples 1894 ash and layers 1–6. The altered lithic fragments dominatethe fraction range of 16–0.063 mm (−3 to 4 φ), whereas andesitic frag-ments are themost dominant type between 512–16 mm (−8 to−4φ).

Subordinate amounts of altered scoriae were observed in all layers(ca. 2–14 wt.%). The pisolites existing in layer 2 are smaller than2 mm (−1 φ). A sort of parachute-type grain of the pisolite is rarelyobserved. Minerals are discrete plagioclase and pyroxene grains andcontain up to ca. 20% in fine fractions of every layer. Gypsum is acicularor tabular, up to ca. 50% in the fine fractions, and is found only in thesamples collected near the crater lake (Fig. 13A, B; loc. 1, 7, 14 and 26).

Each of the 1894 ash layers and the six layers of the 1895 eruptionis distinctive in terms of its component characteristics, particularlythose collected around the crater lake. Consequently, componentanalysis is a useful tool to identify these deposits. Detailed descrip-tions are presented in Table 2.

4.5. Interpretation

We examined the mode of emplacement for the six layers based onfield observations, estimated mass, grain size, and component analysisdata. The general tendency of increasing amount of lithic clasts along

Table 2Detailed features of lithology, grain size, and components of the deposits of the 1895 eruption of Zao volcano.

Layer Lithology Grain size Component

1894ash

The 1894 ash overlies the compacted alteredGoshikidake pyroclastics, loam or paleosol, or detrituswith a sharp boundary. The 1894 ash is up to 6 cmthick in the caldera floor (loc. 26) and b10 cm thick atlocations outside of the Umanose caldera (the area ofthe topographic high). The black 1894 ash shows anormally sorted massive facies (Fig. 7A).

The 1894 ash shows a unimodal peak around 1 φ andshows a unimodal distribution. It is normally sorted(σφ=2.9) with Mdφ of 1.0.

The share of the andesitic fragment proportions tendsto be higher in coarser grain fractions (Fig. 13A;Table 3). Among the analyzed samples, the 1894 ashshows the highest proportion of the andesitefragments. It is also richest in scoriae.

Layer1

Layer 1 overlies the 1894 ashwith a sharp boundary. It isobserved restrictedly around the SWRC (loc. 1 and 26),corresponding to the area of topographic lows. Layer 1 isup to 1 cm thick in the western area of the crater lake(loc. 26). It shows intercalation of well-sorted whitishash and well-sorted gray ash beds (Fig. 7A and C).

Not analyzed Not analyzed

Layer2

Generally, layer 2 directly overlies layer 1 with a sharpboundary, although it is underlain by compacted alteredGoshikidake pyroclastics or the 1894 ash at severallocations (Fig. 7A, B). Layer 2 is more than 41 cm thickin the topographic low of the southern area of the craterlake (loc. 29), but it thins to less than 1 cm thick at areasof topographic highs, such as near the summit ofGoshikidake Peak (Fig. 5; loc. 19). Layer 2 is well-sortedmassive tuff consisting of pale-gray ash with smallamounts of andesitic fragments, altered lithic fragmentsand altered subangular to subround scoriae. The bed-ding sags formed by clasts with ca. 5 cm diameter arealso observed in places of the SWRC (loc. 1) (Fig. 7B). Inaddition, there is a bed consisting of pisolites of ca. 2 mmdiameter, which is observed clearly at the upper part oflayer 2 at loc. 26 (Fig. 7C). The pisolitic ashes are mainlygray colored solid or hollow spheres, and each pisoliticgrain has a hole.

Layer 2 shows considerable variation with respect to itsgranulometric characteristics from the eruption centeroutwards. Near the Okama crater rim, layer 2 iswell-sorted and shows a bimodal granulometricdistribution. The sample from loc. 1, for example, has twopeaks at 3–4 φ and >7 φ, and is well-sorted (σφ=2.0),with Mdφ of 4.4. Similarly to layer 2 of loc. 1, the layer 2sample from the northern area of the crater lake (loc. 7)shows a bimodal distributionwith peaks in 3φ and 7φ. Itis also well-sorted (σφ=2.0), with Mdφ of 5.3, althoughthe sample from the loc. 7 is more enriched in finer ash.However, the layer 2 sample from ca. 100 m west of thecrater lake (loc. 26) is fine-depleted, showing a unimodaldistribution with a peak around 0 φ. The Mdφ value iscoarser (−0.5) and displays better sorting (σφ=1.6) atthe crater lake rim.

The most abundant component in layer 2 is the alteredlithic fragment, and altered scoriae and the andesiticfragments are similarly subordinated in abundance(Fig. 13A, B; Table 3). Pisolitic ashes are conspicuousin the western area of the crater lake (Fig. 13B; Loc.26). The andesitic fragments tend to be moreabundant in larger size fractions.

Layer3

Layer 3 overlies layer 2 with a sharp boundary. Layer 3 isup to 42 cm thick in the southern area of the crater lake(loc. 29), but less than 1 cm thick at the topographic highnear the Goshikidake Peak (loc. 19). Layer 3 is a normallysorted tuff which is composed mainly of whitish-gray ashwith small amounts of subangular to subroundedandesitic fragments, altered lithic fragments and alteredscoriae up to 10 cm. In the SWRC (loc. 1), a concentrationof lapilli of up to 2 cm thick is locally observed (Fig. 7D) inthe middle portion of layer 3 (ca. 5 cm in thick).Additionally, some clasts (ca. 5 cm diameter) formbedding sags restrictedly in the SWRC, resulting in a sort ofundulation at its lower boundary (Fig. 7D, E). In thesouthern area of the crater lake (loc. 29), layer 3 isco-mingledwith layer 4 and apart of layer 3 is surroundedby mixed deposits of layers 3 and 4 (ca. 10 cm diameter)(Fig. 7H).

Layer 3 at loc. 1 has two main peaks in 2 φ and >7 φwith a subordinate peak around −3 φ. It is normallysorted, but its Mdφ (3.9) and σφ (2.6) values areslightly higher than those for layer 2 at the samelocality (loc. 1).

The most abundant component is the altered lithicfragments (ca. 55%). The second most abundant is theandesitic fragments (ca. 30%; Table 4). The proportionof the andesitic fragments remains almost constant,around 30%, in each of the grain size fractions(Fig. 13).

Layer4

Layer 4 overlies layer 3 with a sharp boundary in mostareas, but the boundary is undulated locally. This layeris observed only at the topographic low (Fig. 7B, D, F,G). It is up to 30 cm thick in the southern area of thecrater lake (loc. 29). Layer 4 is normally sorted tuffconsisting mainly of pale-gray ash. Small amounts ofsubangular to subrounded andesitic fragments, alteredlithic fragments and altered scoria are set in the ashymatrix. In the SWRC (loc. 1), layer 4 displays a weaklamination consisting of discontinuous lapilli trainsand/or the lapilli-rich lamina (Fig. 7D, F). Layer 4deposited away (ca. 100–150 m) from the crater lakecommonly shows a facies of thin massive ash(Fig. 7G).

At loc. 1, we performed grain size analyses for the threerepresentative samples collected from the upper, lowerand lapilli-rich parts of layer 4. These are normally sorted(σφ=2.5–2.8) with Mdφ values of 3.6–4.7. Both thesamples from lower and upper parts have two peaks ofgrain size around 3 φ and >7 φ, although that from thelapilli rich part has a peak of 1 φ. These features arecompatible with those of the underlying layer 3, althoughthe subordinate peak is not recognized in layer 4.The grain-size characteristics of this layer at the westernand northern localities (loc. 26 and 7) are betterreconciled with those of the upper and lower parts at loc.1. In the southern area, the peak of grain size of layer 4shifts to finer grains of 4 φ.

The most abundant component of layer 4 is also thealtered lithic fragments (38–60%). It is followed byandesitic fragments (18–38%; Fig. 13A, B; Table 3). Inalmost all analyzed samples, the relative abundance ofthe andesitic fragments is highest in a size range around0–1 φ. A small amount of pisolitic ash is contained in thesouthwestern rim (ca. 11 wt.% in loc. 1) and westernarea of the crater lake (ca. 3 wt.% in loc. 26). The lapillirich part of loc. 1 is the sole exception. It is distinctlypoor in andesitic fragments (Fig. 13A).

Layer5

Layer 5 overlies layer 4 with a sharp boundary. It is up to175 cm thick in the SWRC (loc. 1), but it thins rapidlyaway from the crater lake. This layer varies in thickness atthe distal part from1 cm at the southern area of the craterlake (topographic low; loc. 6) to 4 cm near theGoshikidake Peak (topographic high; loc. 4). This layershows a lithofacies of pale-gray, poorly sorted,matrix-supported tuff breccia at loc. 1. Its lower part (ca.1 m thick) shows a massive facies, although in the upperpart (ca. 75 cm) are severalweak stratifications formedbyvarious ratios of angular to subangular clasts of andesiticfragment, altered lithic fragments and altered scoria(Fig. 7I) of up to ca. 10 cmdiameter. Near theGoshikidake

Larger clasts, such as coarse to fine blocks, are moreabundant in most samples collected from this layerthan in the underlying layers 1–4. At loc. 1, weanalyzed the grain size for the two representativesamples of layer 5: samples collected respectivelyfrom the massive lower and weakly stratified upperparts. The lower part shows a flat pattern in the rangecoarser than −7 φ, with Mdφ around 0.3, although agentle peak around 1 φ is recognized for the upperpart, with Mdφ of 1.8 (Fig. 10A). Both the upper andlower parts are very poorly sorted (σφ=4.5 and 4.8).The size distribution patterns for the samples from thenorthern and western areas of the crater lake (loc. 14

Similarly to the case with layers 3 and 4, the mostabundant component in layer 5 is also the alteredlithic fragments. The second is andesitic fragments(Fig. 13A, B; Table 3). In the histograms for the sampleof layer 5, the andesitic fragments are shown to havetwo peaks (Fig. 13A, B). One is around 0–1 φ and theother is −6 to −4 φ. The peak of around 0–1 φ tendsto be smaller than those of layer 4 samples.

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Table 2 (continued)

Layer Lithology Grain size Component

Peak, this layer is normally sorted, showing very weaklamination (Fig. 7J).

and 26) are rather flat (Fig. 10B), which is compatiblewith that of the lower part at loc. 1. The flat patternedsamples are poorly to very poorly sorted (σφ=3.7–4.0)with Mdφ of 2.0–2.5. In contrast, those from thesouthern and eastern areas of the crater lake (loc. 6 and19) show unimodal patterns with a single peak around0–3φ. These are normally to poorly sorted (σφ=2.3–3.3)withMdφ of 0.5–2.5. Theσφ value tends to increase awayfrom the crater lake, although the Mdφ value tends todecrease (Fig. 11).

Layer6

Layer 6 overlies layer 5 with a sharp boundary. Layer 6 isthick – up to 300 cm in the SWRC (loc. 1) – but it thinsrapidly away from the crater lake. At the southern area ofthe crater lake, a topographic low (loc. 6), and near theGoshikidake Peak, a topographic high (loc. 4), layer 6 is asthin as ca. 20 and 16 cm, respectively. This layer showspoorly sorted gray colored matrix-supported tuff brecciaat loc. 1. These features resemble those of layer 5. Anotherfeature that resembles layer 5 is the abundant existenceof large clasts up to ca. 50 cm long, with various ratios ofangular to subangular clasts forming weak stratifications(Fig. 7I). Large clasts are indeed larger than the relevantof layer 5. Especially, the long axis of the large clasts,which is concentrated ca. 230 cm above the base, isaligned horizontally (Fig. 7I). In the topographic high,some clasts (ca. 1 cm diameter) form bedding sags(Fig. 7J), with angular to subrounded clasts (up to ca.40 cm diameter) distributed on the surface (Fig. 7K). Inthe distal area located >150 m from the crater lake, thislayer tends to show very weak lamination (Fig. 7J).Layer 6 is deposited also outside of the Umanose caldera(Fig. 7L), and covers a ca. 2 km diameter area surround-ing the Okama crater lake. It drapes the 1894 ash, loam,paleosol or detritus, up to 10 cm thick at the rim of theUmanose caldera (loc. 30). Layer 6, which is depositedoutside of the Umanose caldera is well to poorly sortedwhitish to gray lapilli tuff. Bedding sags are also found inthe rim of theUmanose caldera (loc. 30) (Fig. 7L). Layer 6tends to show distinct thinning and fining away from thecrater lake.

Layer 6 ismore abundant in larger clasts, such as coarse tofine blocks, than in layer 5. At loc. 1, we collected sixsamples (6-a–6-f) from the lower to upper portion oflayer 6 by spacing the same interval of ca. 50 cm becausethe lithofacies was apparently uniform within this layerby the on-site field observation. The grain sizedistribution pattern for sample 6-a is similar to that forthe relevant lower part of layer 5: samples 6-b to 6-fshow a unimodal distribution patternwith a peak around0 and−3 φ, and contain considerable amounts of coarseto fine blocks (b−6 φ). The peakmode size tends to shiftcoarser upwards. Also, the amount of the large lithicfragments becomes higher upward. This is reflected bythe fact that theMdφ value decreases upwards (Mdφ: 1.5for 6a, and−1.8 to 0.0 for 6b to 6f). In addition, the upperpart gets a more fines-depleted character (Fig. 10A).Because of this, sample 6-a is very poorly sorted(σφ=4.7), although samples 6-b to 6-f show slightlybetter sorting (σφ=3.3–4.7).Granulometrically, Layer 6 in the western area of thecrater lake (loc. 26) is compatible with the relevant upperpart at loc. 1, which produced a unimodal peakaround −2 φ and which contains abundant fineblocks (Fig. 10B). In other localities, layer 6 com-monly shows flat patterns (Fig. 10B), which resemblethose of layer 6-a at loc. 1.Layer 6, deposited outside of the Umanose caldera,shows a unimodal grain size distribution pattern(Fig. 10B). Only one sample at loc. 30 (rim of theUmanose caldera) appears to be fine-depleted with apeak of−4φ, which is similar to that of layers 6-d to 6-fat loc. 1. The peak size becomes finer and the amount ofsmaller particles becomes richer (Mdφ value shifts from−0.8 to 3.7) away from the crater lake. Generally, thesorting becomes better (σφ value shifts from 1.9 to 4.8)away from the eruption center (Fig. 11) in the areaoutside of the Umanose caldera.

The most abundant component in layer 6 is also thealtered lithic fragments (Fig. 13A, B; Table 3). Theandesitic fragments are the second dominantcomponent, but the andesitic fragment proportion inthe fractions of 16–0.063 mm (−3–4 φ) is the poorestamong the analyzed six layers (Fig. 13A, B). Especiallyin samples 6-c–6-f, the andesitic fragment proportionis extremely low, so that scoriae are more abundantthan the andesitic fragments there. In contrast,regarding coarser fractions (−8 to −4 φ), theandesitic block is consistently the most dominantcomponent in all analyzed samples of layers 1–6(Fig. 13A, B).

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with the increasing mass of ejecta from layers 1–6 suggests that the1895 eruption became more explosive and voluminous over time.

The substratummodel beneath Zao volcano is presented in Fig. 14.According to geologic studies described in chapter 2, the uppermostpart consists of stage 4 products (less than 150 m thick), which aremainly composed of scoria. The stage 4 products are underlain bystage 2 and 3 products (ca. 500–800 m maximum thickness) beneaththe central part of the volcano. These mainly comprise andesitic lavasand the associated products. After petrologic investigation of theAkita-Yakeyama 1997 eruption, Ohba et al. (2007) inferred that therocks surrounding the vent had suffered alteration down to ca. 1 kmdeep. It is reasonable to presume a similar hydrothermal alterationarea in the case of the Zao 1895 eruption. The alteration might havebeen caused by hydrothermal fluids that originated from magmastored in a chamber. Ban et al. (2008) estimated the andesiticmagma chamber depth, which caused one of the eruptions in Zao'syoungest stage, as less than 3 kb. They considered that the magmachamber had been semi-solidified but that it was activated by injec-tion of basaltic magma from the deep part before the eruption. It isprobable that the andesitic magma chamber has been stored at simi-lar depth during the youngest stage of Zao.

The lithic fragments mainly comprise altered lithic fragments.These might have originated from the hydrothermal altered area

described above, where lithics readily underwent intense alteration.However, the non-altered large andesitic blocks presumably derivefrom stage 2 and 3 layers immediately below the crater lake, whichwas considerably resistant against alternation and which wasdestroyed before or during the eruption. The scoriae are from stage4 layers formed before the 1895 eruption, however, we can not ex-clude the possibility that some angular scoriae in layer 6 are juvenile.Gypsum is found near the crater lake (only in loc. 1, 7, 14 and 26),suggesting that this was formed secondarily by the fumarole nearthe crater center.

4.5.1. 1894 ashThis layer consists mainly of black andesitic ash, showing a normally

sorted massive facies with unimodal character. These characteristicsconcur with the idea that 1894 ash was derived from vulcanian pyro-clastic fall.

4.5.2. Layer 1Layer 1wasmore likely derived from rapid deposition of a near-vent

pyroclastic surge (Sohn and Chough, 1989) rather than a simple air fallfrom the eruption column. Indeed, a well-sorted character is generallymore likely to result from pyroclastic fall deposition (e.g., Cas andWright, 1987), but layer 1 is observed restrictedly in the topographic

Fig. 7. Photographs showing the representative facies for the 1894 ash and layers 1–6. Boundaries between the layers are shown as red dotted lines. White dotted lines show par-titions of the lapillus-rich part or, upper and lower parts of layer 5. (A) The 1894 ash and layers 1–3 (loc. 1). (B) The bedding sag (arrow) of the bottom of layer 2, and the undulationlower boundary of layer 3 (loc. 1). (C) Pisolitic ash in upper part of layer 2 (loc. 26). (D) Local concentration of lapilli (lapilli-rich part) in layers 3 and 4 (loc. 1). Black arrows indicatebedding sags. (E) Bedding sag in layer 3 (black arrow) (loc. 1). (F) Weak laminations with discontinuous lapilli trains (arrow) in layer 4 (loc. 1). (G) The 1894 ash and layers 2–6 inthe southern area of the crater lake (loc. 6). (H) Layer 3 co-mingled with layer 4 (loc. 29). Disrupted blocks of layer 3 are distributed in layer 4. (I) Weak stratification formed byvarious ratios of large lithic fragments in upper part of layer 5 and layer 6 (loc. 1). The long axis of large clasts aligns horizontally in the upper part of layer 6 (arrows). The lower partof layer 5 is mostly massive. (J) Layers 5 and 6 in the topographic high (loc. 21). Some clasts in layer 6 form bedding sags (arrow). (K) Clasts distributed on the surface nearGoshikidake Peak (loc. 21). (L) Bedding sags observed in the lower part of layer 6 outside of the Umanose caldera (loc. 30).

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Fig. 7 (continued).

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lows (loc. 1 and 26 in Fig. 5b). Its thinly laminated naturemightwell re-sult from the pyroclastic surge.

4.5.3. Layer 2Some observations support the interpretation that layer 2 is fun-

damentally deposited from a pyroclastic fall (e.g., Cas and Wright,1987): (1) layer 2 is well sorted; (2) it is distributed not merely inthe topographic lows but also in the topographic high (near theGoshikidake Peak; loc. 4 and 19); and (3) bedding sags are observedfrequently at outcrops in SWRC (loc. 1). Furthermore, well-sorted pi-solite grains showing a unimodal distribution are also conspicuouswithin this layer (e.g. loc. 26), strongly supporting its fall origin.

This layer mainly comprises altered lithic fragments. Therefore, thisevent is expected to occur in close association with hydrothermalactivity.

4.5.4. Layer 3A stratified and normally sorted nature without fines elutriation

along with a lapillus-rich part at the southwestern rim of the craterlake (loc. 1) is similar to the “stratified tuff” described by Chough andSohn (1990). Stratification formed by alternation of lapillus-rich andlapillus-poor layers, thin lapillus trains, color banding, and indistinctbedding planes is also observed in layer 3. Accordingly, this facies ismost likely emplaced by both suspension and traction sedimentation

Fig. 8. Thickness and isopachs of the 1894 ash and layers 1–6.

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from a turbulent base surge that fluctuates in its velocity and particleconcentration.

In addition, the altered lithic fragments, being the most dominantcomponent, strongly suggest that the main part of layer 3 resultedfrom a pyroclastic surge related to the hydrothermal system. Never-theless, layer 3 near the Goshikidake Peak (loc. 4 and 19) at the topo-graphic high might be of fall origin. The bedding sags in layer 3 of loc.1 also indicate its ballistic origin.

4.5.5. Layer 4The normally sorted nature of layer 4,without fines elutriation,most-

ly massive, and with weak lamination at the southwestern rim of the

crater lake (loc. 1), is compatible with those recognized in near-vent de-posits of less-evolved density currents (Cas and Wright, 1987). In addi-tion, locally developed discontinuous lapilli trains and lapilli-rich partsmight result from the tractional transport of ash and lapilli (Sohn andChough, 1989; Chough and Sohn, 1990).

The main component of layer 4 is altered lithic fragments. There-fore, the main part of this layer likely originated from an explosivephreatic eruption related to the hydrothermal system. The lateral fa-cies changes from its eruption center outwards. The facies of thin andwell-sorted massive ash in the southern area of the crater lake (loc. 6),are interpreted as deposition of fallout ash from the density current,reflecting the distance from the eruption center.

Fig. 9. Relation between the covered area (m2) and mass (kg/m2) of the deposits for layers 3–6. The total mass was calculated by integrating the lines in each segment.

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4.5.6. Layer 5Lithofacies of the poorly sorted layer 5 near the crater lake is unlikely

to be reconciledwith the following three possibilities as the transporta-tion and deposition modes (loc. 1, 14 and 26): hot lahar, pyroclasticflow, or pyroclastic surge. In general, hot lahar at the proximal regiontends to appear as fine-depleted, clast-supported lithofacies near thevent because the fine matrix flows downstream. The pyroclastic flowand surge preferentially transport and leave the pyroclastics in thearea of the topographic lows. The layer showing a lithofacies of

Fig. 10. Grain size distributions of the 1894–1895 eruption deposits: (A) results obtained fromother representative localities (loc. 4, 6, 7, 14, 19, 26, 30, 36, and 38).

matrix-supported tuff breccia is distributed also in the topographichigh (e.g., near the top of Goshikidake, a topographically high area), aswell as in the topographic low (e.g., the southwestern rim of the craterlake).

The granulometry of the layer 5 sample at the southern area of thecrater lake (loc. 6) is compatible with the characteristics of a pyroclasticfall deposit (Fig. 10B). In addition, layer 5 thins rapidly and tends to bebetter sorted away from the crater lake. These characteristics suggestthat layer 5 is a near-vent facies of a pyroclastic fall deposit. The poorly

samples collected at the southwestern rim of the crater lake (loc. 1), and (B) results from

Table 3Granulometric characteristics of analyzed samples from the 1895 eruption.

Layer Locality Sample name Mdφ 84 φ 16 φ σφ

Layer 6 Loc.1 060831-6-6 −1.5 5.3 −4.0 4.7060831-6-5 −1.6 3.0 −3.5 3.3060831-6-4 −1.8 3.0 −3.6 3.3060831-6-3 −0.2 5.1 −3.2 4.2060831-6-2 0.0 4.8 −2.9 3.9060831-6-1 1.5 5.7 −3.6 4.7

Loc.4 061013-6U 2.3 5.7 −2.7 4.2061013-6L 2.8 5.7 −3.4 4.6

Loc.6 061013-6U 1.2 5.5 −2.3 3.9061013-6L 2.4 5.6 −1.7 3.7

Loc.8 070603-6U 1.6 5.2 −2.9 4.1070603-6M 2.6 5.3 −2.9 4.1070603-6L 3.8 5.4 −1.2 3.3

Loc.9 070603-6 1.0 5.6 −3.1 4.4Loc.10 070612-6U 4.4 5.5 −1.5 3.5

070612-6L 1.4 5.0 −3.0 4.0Loc.11 070612-6 1.4 5.6 −3.1 4.4Loc.12 070612-6U 0.0 5.5 −3.4 4.5

070612-6M 2.6 5.7 −2.6 4.2070612-6L 2.8 5.7 −2.4 4.1

Loc.13 070708-6U 2. 5.2 −3.6 4.4070708-6L 1.9 5.2 −4.2 4.7

Loc.14 070708-6U 2.7 5.7 −2.1 3.9070708-6L 4.2 5.7 −2.6 4.2

Loc.15 070708-6U 0.4 5.6 −3.4 4.5070708-6M 1.2 5.0 −3.4 4.2070708-6L 2.2 5.7 −2.6 4.2

Loc.23 070920-6 1.2 5.0 −3.6 4.3Loc.25 070920-6 0.2 5.4 −3.3 4.4Loc.26 070920-6 −0.2 5.4 −3.2 4.3Loc.27 070931-6 2.2 5.6 −2.2 3.9Loc.28 070931-6 0.3 5.5 −3.2 4.4

Layer 5 Loc.1 060831-5U 0.3 5.5 −3.5 4.5060831-5L 1.8 5.7 −3.8 4.8

Loc.6 061013-5 0.5 3.0 −1.5 2.3Loc.7 070811-5 1.8 5.2 −2.2 3.7Loc.9 070603-5U 1.6 5.6 −2. 4.2

070603-5L 2.8 5.6 −2.8 4.2Loc.11 070612-5 2.7 5.6 −1.0 3.3Loc.13 070708-5 2. 5.0 0.0 2.5Loc.14 070708-5 2.5 5.7 −1.6 3.7Loc.15 071023-5 2.2 5.6 −2.6 4.1Loc.19 070913-5 2.5 5.5 −1.0 3.3Loc.26 070920-5 2.0 5.6 −2.3 4.0Loc.27 070931-5 4.2 5.7 0.0 2.9Loc.28 070931-5 4.2 5.8 1.6 2.1Loc.1 080610-4U 4.7 5.8 0.5 2.7

Fig. 11. Median diameter vs. sorting plots of 1894 ash and 1895 eruption deposits.Grain size was used for −5 φ to 5 φ. Envelopes showing ranges for pyroclastic flowand pyroclastic fall were adopted from Walker (1971). That for pyroclastic surge wasquoted from Walker (1983). Layer 5 and layer 6 outside of the Umanose caldera be-come finer, with better sorting away from the crater lake. Black and gray arrows re-spectively indicate the trend of layer 5 and the trend of layer 6 outside of theUmanose caldera.

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sorted thick layer near the crater lake (loc. 1, 14, and 26) apparentlyresulted from the fountain-like eruption, phreatic pyroclastic fall,where no extensive sorting had occurred. The altered lithic fragments,as the most dominant component (especially in the middle to finegrain size range), reflect that layer 5was derived fromhydrothermal ac-tivity. Weak stratification at the rim of the crater lake (loc. 1) and ex-tremely weak lamination near the Goshikidake Peak (loc. 21) implythat eruptions occurred repeatedly.

4.5.7. Layer 6Layer 6, the thickest, shows by far the largest volume among the

layers deposited during 1894–1895. Therefore, this layer was unques-tionably formedduring the climax activity. Although the layer ismassive,observations indicate repeated eruptions or pulse-like transportation ina pyroclastic density current, such as poor sorting within the inner areaof the caldera, weak stratification in the rim of the crater lake (loc. 1),and extremely weak lamination in distal areas.

All of layer 6 deposited in eastern, southern, and northern areas ofthe crater (loc. 4, 6 and 14) must be correlated to layer 6-a at loc. 1.Lithofacies of layer 6 in the eastern, southern and northern areas ofthe crater are compatible with those of the basal part of this layer

(6-a) at loc. 1. Furthermore, these show similar granulometric charac-teristics resembling those for layer 5 near the crater lake (loc. 1, 14and 26). Consequently, layer 6 seems to have been deposited from afountain-like eruption similarly to those for layer 5 near the craterlake. Bedding sags in the eastern area of the crater lake (loc. 4)show their ballistic transportation, supporting this interpretation.

Samples of 6-b and 6-c are less sorted and fine-depleted than 6-a.They seem to show that the fountain-like eruption developed moreexplosively with time, and that fine particles fell out and away fromthe near-vent area. Conceivably, the deposits that are expected tobe correlated with the 6-b to 6-f at loc. 1 were reworked in theseareas. By this point, they had re-formed the detritus on the surface.

The upper part (6-d–6-f) of layer 6 at loc. 1 and layer 1 in thewestern area of the crater lake (loc. 26) show similar granulometriccharacteristics to those of the relevant layer on the top of theUmanose caldera rim (loc. 30); poorly sorted and fine-depleted(Fig. 10A, B). Layer 6 (including deposits at loc. 30 and outside ofthe Umanose caldera) becomes thinner and finer away from the cra-ter lake (Figs. 8 and 10B), irrespective of topography. Therefore, layer6 is more likely to be a deposit of pyroclastic fall origin. Precipitationof the deposits by ballistic ejecta might have contributed to the poorlysorted nature at loc. 30 because loc. 30 is near the vent. Presence ofthe bedding sags in layer 6 support this interpretation. Therefore,the upper part of this layer at loc. 1 and the relevant deposits in thewestern area (loc. 26) can be interpreted as the pyroclastic fall depos-it. The large clasts, of which the long axis aligns horizontally in thesouthwestern rim of the crater lake (loc. 1), might have resultedfrom the secondary transport immediately after the deposition. Theupper part of layer 6 at loc. 1 contains a larger amount of angular tosubrounded scoriae (Fig. 13A) than the remainder of the layer.These are altered, but one can not exclude the possibility that someangular scoriae are juvenile. The alteration might have taken placeafter deposition.

5. Discussion

5.1. Correlation of eruption documents to 1895 eruption deposits

The first pisolitic ash fall in the 1895 eruption was recognized on19 February at the Katta District (chapter 3; Table 1). This phenome-non is likely to be correlated with the pisolite concentrated part ob-served in the upper part of layer 2 (loc. 26) (Fig. 7C). Consequently,layer 2 must have been deposited by 19 February, 1895. Thewell-sorted whitish ash as the main component of layer 1 is inferred

Fig. 12. Photographs showing examples of major components for 512 to 0.063 mm (−8 to 4 φ). Grains are andesitic fragments (A), altered lithic fragments (B), scoriae (C), pisoliticash (D), plagioclase crystal (E), pyroxene crystal (F), gypsum crystal (acicular) (G), and gypsum crystal (tabular) (H).

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to be the deposit associated with the white smoke observed on 15February, 1895 (Table 1) (Fig. 7A).

As described in chapter 3, the ash cloud of 22 August reached ca.15 km northwest of the summit, meaning that the explosivity wasconsiderably higher than in February. In addition, the estimatedmass of layer 3 was much greater than those of the underlying layers1 and 2 (chapter 4). In this context, layer 3 is inferred to be the

Table 4Results of component analysis for samples of representative localities.

Layer Locality Sample name Andesite Sc

Layer 6 Loc.1 060831-6-6 28 10060831-6-5 24 10060831-6-4 27 11060831-6-3 19 9060831-6-2 26 5060831-6-1 29 4

Loc.4 061013-6U 18 7061013-6L 16 8

Loc.6 061013-6U 25 8061013-6L 29 5

Loc.14 070708-6U 17 8070708-6L 19 8

Loc.26 070920-6 26 9Layer 5 Loc.1 060831-5U 32 3

060831-5L 36 4Loc.6 061013-5 19 7Loc.14 070708-5 17 8Loc.19 070913-5 38 5Loc.26 070920-5 32 4

Layer 4 Loc.1 080610-4U 31 2080617-4 (lapilli rich part) 18 5080617-4L 38 3

Loc.6 080913-4 29 6Loc.7 080617-4 20 10Loc.26 070920-4 18 4

Layer 3 Loc.1 060831-3 30 7Layer 2 Loc.1 060831-2 25 14

Loc.7 070811-2 22 10Loc.26 070920-2 12 7

1984 ash Loc.1 080610-1894 44 13

n.d., not detected.

deposit of the eruption on 22 August. The whitish-gray colored ma-trix of layer 3, which reflects high abundance of the altered lithic frag-ment relative to those for layers 2 and 4 (Fig. 13A), might beattributed to about half a year of duration for alteration in the intensehydrothermal activity area.

Co-mingling of layers 3 and 4 in the southern area of the craterlake (loc. 29) might have resulted from reworking of the two layers

oria Altered lithic fragment Pisolites Crystal Gypsum

55 n.d. 4 357 n.d. 6 256 n.d. 4 264 n.d. 6 160 n.d. 7 159 n.d. 4 369 n.d. 6 n.d70 n.d. 6 n.d63 n.d. 4 n.d63 n.d. 3 n.d69 n.d. 6 n.d67 n.d. 6 n.d61 n.d. 4 n.d57 n.d. 5 351 n.d. 5 566 n.d. 8 n.d.67 n.d. 6 247 n.d. 10 053 n.d. 5 649 n.d. 9 952 11 6 838 n.d. 5 1659 n.d. 7 n.d.57 n.d. 7 660 3 10 655 n.d. 6 145 1 9 553 2 10 351 22 924 n.d. 8 10

Fig. 13.Modal compositions of the 1894 ash and layers 2–6. Data in (A) are results obtained for samples from the southern rim of the crater lake (loc. 1). Those in (B) are those fromthe other representative localities (loc. 4, 6, 7, 14, 19, and 26).

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before the subsequent eruption. As described in chapter 3, the 27–28September eruption was inferred to have been highly explosive andmuch larger than the 22 August eruption. Furthermore, the volumeestimations for layers 5 and 6 are one order larger than those of layers3 and 4 (chapter 4). Therefore, layers 5 and 6 are likely to correspondto the climactic eruptions that occurred on 27–28 September. As in-ferred from the sketches of Fig. 4, a small mound roughly 30 m highmust have formed during the climactic eruptions in the western tosouthern rim of the crater lake. Conceivably, the weak stratificationsand weak lamination in layers 5 and 6 reflect the repeated eruptions

that were documented in the reports of the eruption phenomena thatoccurred on 27–28 September (chapter 3).

5.2. Sequence of the 1895 eruptions

The revealed eruptive sequence of the 1895 eruptions, depicted inFig. 14, is summarized as described below

(1) About one year before the 1895 activity, ashy smoke rose from thecrater lake during February–March, 1894. On 3 July, 1894,

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vulcanian eruption occurred in the crater lake, ejecting black glassyash (1894 ash). The activity ceased before long. The subsequentdormancy lasted for approximately half a year. The magmawhich caused the vulcanian eruption is likely to have ascendedfrom a re-activated andesitic magma chamber by new infusion ofbasaltic magma from the deep part. The infusion probably ceasedonce after the July eruption (Fig. 14A). The reactivated magmachamber had released magmatic fluids, which reacted with thesurrounding materials and transformed to the hydrothermalfluid. The hydrothermal alteration of rocks surrounding the ventis expected to have proceeded during the dormancy (Fig. 14B).

(2) The activity recommenced on 15 February, 1895. White smokeburst up from the crater lake during 9:30–10:30 and the whitishash was deposited restrictedly in the southwestern area of thecrater lake to form layer 1 (Fig. 14B). The Nigorigawa Riverflooded with muddy water and sulfuric water flowed furtherinto the Abukumagawa River. The ascent of the hydrothermalfluid from ca. 1 km depth is likely to have caused the eruption.Eruption of this type is expected to have been promoted by fur-ther reactivation of the andesitic magma chamber, when anotherinjection of basaltic magma would occur.

(3) The eruption on 19 February ejected ash at 9:30. Then, pisoliteand ballistic clasts fell near the crater lake to form layer 2(Fig. 14B). The crater lake flooded. The overflowing water wasflushed into the Matsukawa River at 10:00. The ShiroishigawaRiver was swollen with sulfurized water carrying much driftwoodfor several tens of minutes around noon. Subsequently the volca-nic activity became calm, with dormancy lasting for approximate-ly half a year. The injection of the basalticmagma probably ceased.

Fig. 14. Schematic illustrations showing substratum models and modes of transportation aeruptions. These are, respectively, for eruptions on 1894 (A), 15 and 19 February (B), 22 Au

(4) On 22 August, ash fell in Yamagata City. The eruption accom-panied the pyroclastic surge together with the ejection of bal-listic bombs, which created layers 3 and 4. Layer 3 is observedeven near the Goshikidake Peak, although layer 4 is absent inthe topographic high. The dominant eruption phase graduallychanged to a sort of density current (pyroclastic surge) fromthe pyroclastic fall during the 22 August eruption (Fig. 14C).The eruption would have been the result of new injection ofthe basaltic magma. The highly altered nature of the layer 3matrix reflects about half a year of duration for the alteration(Fig. 14C).

(5) On 27 September, a climactic event occurred, conceivablyduring 4:40–6:15. Fountain-like eruptions occurred repeat-edly to form layers 5 and 6, and also caused a small moundin the western–southern rim of the crater lake. Gradually,the eruption intensified. Eventually an eruption cloud (cf.Fujinawa et al., 2008) formed, from which many large clastsfell, with an umbrella part developing at the top. Furthermore,many large clasts fell with ballistic trajectories (Fig. 14D). Thecloud extended toward the east–northeast. The fallout ash wasdetected as far as ca. 40 km east of the summit. The NigorigawaRiver was swollen with sulfurized water containing much drift-wood. Subsequently a small amount of ash fell at 18:00 on 27September.

(6) On 28 September, vesiculated blocks of ca. 30 cm in diameterwere ejected from the Okama crater at 5:00. The NigorigawaRiver rose ca. 9 m. The eruption depositsmight be a part of clastsscattered on the surface of the crater area. This climax activity isexpected to have resulted from the injection of the larger amount

nd deposition of tephra in the 1895 phreatic eruptions, including the preceding 1894gust (C), and 27–28 September (D).

Fig. 15. Sub-lake topographic (a) cross section and (b) contour map at 1933 after Anzai(1961).

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of basalticmagma. The erupted angular scoriaemight be juvenilefragments derived from the reactivated andesitic magma cham-ber (Fig. 14D).

The estimated mass of the discharged products for the eruptionson 15 and 19 February, 22 August, and 27–28 September are, respec-tively, less than 7.6×106 kg, ca. 1.0×107, and more than 6.2×108 kg.It can be said that the 1895 eruption became more voluminous thanprior eruptions.

5.3. Explosive energy, crater size, and explosion depth of the 27 Septemberexplosion

We estimated the energy of the explosion and the crater diameterfor the climactic explosion by combining the respective methods ofOhba et al. (2002) and Goto et al. (2001). The explosion energy is cal-culable using the method of Ohba et al. (2002) if one assumes the ex-plosion cloud height correctly. Although the method must alsoidentify cube-root scaled depth, it can be inferred from the shapeand height of the eruption cloud (Ohba et al., 2002). Furthermore,the crater diameter can be approximated from the calculated explo-sion energy using the method of Goto et al. (2001). Goto et al.(2001) formulated the relation between the crater diameter (D) andthe explosion energy (E) as the equation log D=0.32×log E−2.06based on experimentally obtained results.

From the sketch drawn at ca. 20 km east of the crater (Kochibe,1896a; Fig. 4B), the eruption cloud was estimated as about 350 mhigh. Based on the eruption cloud shape shown in the same sketch,the cube-root scaled explosion depth of the eruption is approximatedas ca. 0.004 m/J1/3. Based on the method of Ohba et al. (2002), thescaled cloud height was calculated as ca. 0.079 m/J1/3. Adopting thecloud height of 350 m and the scaled cloud height of 0.079 m/J1/3,the explosive energy was estimated as ca. 1×1011 J. According tothis estimate of explosive energy, the crater diameter was calculatedas ca. 30 m. According to the sub-lake topographic map of 1933(Anzai, 1961), the bottom of the lake showed a funnel shape(Fig. 15), with its neck of ca. 20–30 m diameter. This value is compa-rable to that obtained using the calculation described above. Further-more, using the cube-root scaled explosion depth and the explosiveenergy described above, the explosion depth was calculated as ca.20 m.

6. Conclusion

The 1895 eruption of Zao occurred on 15 and 19 February, 22August, and 27–28 September after several precursory vulcanianeruptions in February–July in 1894. The products of the 1985 eruptionsare divided lithologically into six layers (layers 1–6). Each layer has dis-tinctive features in terms of its proportion of the components andgranulometric characteristics. From the correlations of the lithologywith the documents reporting the relevant explosion events, the erup-tion episodes in February, August, and September respectively corre-spond to the deposition of layers 1–2, 3–4, and 5–6.

The dominant transport mechanism must have been some sort ofpyroclastic fall, but a low-temperature pyroclastic surge accompaniedthe eruptions on 22 August. The eruption became more voluminousover time. The products of eruptions on 15 and 19 February, 22 Au-gust were less than 3.8×106 kg, less than 3.8×106 kg, and ca.1.0×107, respectively, whereas that on 27–28 September amountedto more than 6.2×108 kg.

The magma which caused the vulcanian eruption of 1984 wasprobably from the andesitic magma chamber located shallower than3 kb, which was probably reactivated by a new infusion of basalticmagma from some deeper area. During the dormancy of about halfa year, magmatic fluids released from the andesitic chamber had al-tered the surrounding materials. The hydrothermally altered mate-rials were emitted during the 1895 eruptions. Three periods oferuption in 1895 are expected to have resulted from individual infu-sion of basaltic magma into the andesitic magma chamber. The lastone, the largest, produced the climax eruption. It is probable thatsome portions of the andesitic magma ascended to the surface duringthe climax eruption.

The explosive energy discharged with the maximum explosion on27 September was estimated as ca. 1×1011 J. The crater diameter wasapproximated as ca. 30 m.

Acknowledgment

We are grateful to Prof. Lionel Wilson and an anonymous re-viewer for many constructive comments and suggestions on themanuscript. We acknowledge to Drs. K. Nakashima and T. Maruyamafor their continual support of this research. We are grateful to Mr. K.Kontani, Mr. Y. Tachihara, and Ms. Y. Nakazawa for their field assis-tance and useful comments on this study. We also acknowledge toYamagata and Miyagi Prefectural Governments for the permissionof field observation. This work was financially supported in part byGrant-in-Aid for Scientific Research from the Japan Society for thePromotion of Science to M. Ban (no. 22540487) and T. Ohba (no.21510186).

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