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

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 b c

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

a r t i c l e

i n f o

Article history: Received 30 January 2012 Accepted 11 August 2012 Available online 22 August 2012 Keywords: Non-juvenile steam eruption Phreatic eruption Eruption sequence Volcanic hazard mitigation Zao volcano Tohoku Japan

a b s t r a c t The most recent major eruption event of the Zao volcano comprised a series of phreatic eruption episodes on 15 and 19 February, 22 August, and 27–28 September 1895, with several precursory vulcanian eruptions during February–July 1894. All were generated at the Okama crater lake located inside the Umanose caldera. The eruption products consist mainly of hydrothermally altered ash with altered blocks, except for ash from 1984. The eruption deposits of 1895 are divided lithologically into six layers (1–6). Comparison of the document with the lithofacies of deposits shows that layers 1, 2, 3–4, and 5–6 were correlated respectively with eruption episodes of 15 February (episode 1), 19 February (episode 2), 22 August (episode3), and 27–28 September (episode 4). During these four episodes, ca. 0.5%, 0.5%, 1.5%, and 98% of the total mass of the products had been discharged. Based on lithologic, stratigraphic, granulometric, and component analyses and on distributional features for these layers, the following depositional mechanisms were inferred. Layers 1, 3, and 4 were formed mainly from their related small pyroclastic density currents, whereas layer 2 resulted mainly from a small pyroclastic fall. In contrast, layers 5 and 6 are larger-scale near-vent pyroclastic fall deposits from ash clouds and eruption clouds, which might have included some juvenile fragments. The three early episodes in 1985 led to the climactic episode of 27–28 September. Furthermore, the andesitic magma chamber at b 3 kb depth, which caused the 1894 vulcanian eruptions, became a hydrothermal alteration source for the 1895 erupted materials. The chamber was re-activated before 1895 eruption 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 volcanic phreatic-type eruptions at this volcano. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Non-juvenile steam explosions, which occur frequently in active Japanese volcanoes (Okuno, 1995), are a common style of eruption that has been causing severe volcanic hazards, particularly in proximal areas (e.g., Yamamoto et al., 1999; Fujinawa et al., 2006, 2008). Accumulation of related volcanological and geological data is crucial for protection against such volcanic hazards. Indeed, in 2000, during eruption of the Usu volcano, which was characterized by a series of phreatic to phreatomagmatic eruptions (Tomiya et al., 2001), nearby residents found refuge because the type and time of the eruption had been well-predicted by virtue of detailed volcanological and geological investigations (e.g., Soya et al., 1981; Ui et al., 2002). However, geologic studies of non-juvenile steam eruption are rare because non-juvenile steam eruptions usually lead to emissions of little mass. In many cases, the resultant deposits are poorly preserved in geologic time. Young, non-juvenile steam explosions such as those which occurred within the past century or so have more completely remaining ⁎ Corresponding author. Tel.: +81 23 628 4642; fax: +81 23 628 4661. E-mail address: [email protected] (M. Ban). 0377-0273/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jvolgeores.2012.08.005

thin eruption products (Fujinawa et al., 2008). Furthermore, recent eruptions have been witnessed. Several eruption accounts are available, providing extremely useful documentation for modern volcanological investigations, and supporting detailed assessment of the nature and sequence of the eruption events. For instance, regarding the 1888 phreatic eruption of the Bandai volcano, a sort of pyroclastic surge was recognized as accompanying the sector collapse and its resultant debris avalanche. The surge severely damaged several villages on the east to southeastern flank of the stratovolcano, scarring and fatally burning many residents (Sekiya and Kikuchi, 1890; Yamamoto et al., 1999; Japan Meteorological Agency, 2005; Fujinawa et al., 2008). In the 1900 steam explosion at Adatara volcano, some visitors and 82 workers at a sulfur refinery in the crater area were killed or severely injured mainly by a special sort of pyroclastic surge containing no juvenile materials (Japan Meteorological Agency, 2005; Fujinawa et al., 2006). Non-juvenile steam eruptions in Tohoku, Japan have been reported by several authors, such as Yamamoto et al. (1999) for the Bandai 1888 eruption, Ohba et al. (2007) for the Akita-Yakeyama 1997 eruption, and Fujinawa et al. (2008) for the Adatara 1900, Zao 1895, and Bandai 1888 eruptions. During the Bandai 1888 eruption event, 15–20 phreatic explosions occurred successively in one hour.

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These eruptions were accompanied by the collapse of a section of the edifice (Sekiya and Kikuchi, 1890). Several pyroclastic density currents were discharged during the eruption event. Although a preliminary description of the Zao 1895 eruption event was reported in Fujinawa et al. (2008), more detailed analyses have been necessary to reveal the nature and sequence of the eruption. In this study, we examined the sequence of the Zao 1895 eruption in greater detail by mutually correlating the complete sets of the eruption record, geological characteristics, grain size distribution, and component analyses.

2. Geological background

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

Zao volcano is located at the central part of a volcanic front of the Tohoku Japan arc (Fig. 1). This volcano, which started its activity at about 1 Ma (Takaoka et al., 1989), has remained active to the present day. Geologic, petrologic, and stratigraphic outlines of this volcano were first published by Chiba (1961), with later detailed geologic and petrologic reports presented by Oba and Konda (1989) and Sakayori (1991, 1992). Based on these studies, the Zao volcano activity is divisible 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). The newest activity (stage 4) probably commenced immediately after the collapse of the volcanic edifice, leaving a horseshoe-shaped amphitheater 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 formed in the inner part of the Umanose caldera. A crater lake, Okama, is located in the western part of Goshikidake. Eruptive products of stage 4 are classifiable geologically into four eruptive units from older to younger: Nigorigawa lavas (NGL), Komakusadaira agglutinate (KMA), Umanose agglutinate (UMA), and Goshikidake pyroclastics (GSP) (Sakayori, 1992; Ban et al., 2008). The Nigorigawa lava, which flowed along the Nigorigawa River, has mostly been eroded (Sakayori, 1992). The Komakusadaira agglutinate comprises more than 14 lithologically distinct pyroclastic layers. It mainly drapes the top of the Umanose caldera wall (Ban et al., 2008). The Umanose agglutinate, which consists of four layers, covers the Komakusadaira agglutinate in the western part of the Umanose caldera

Fig. 2. (A) Geology and stratigraphy of the Zao volcano (Sakayori, 1992; Ban et al., 2008). GSP, Goshikidake pyroclastics; UMA, Umanose agglutinate; KMA; Komakusadaira agglutinate; NGL, Nigorigawa lavas; YKL, Yokokurayama lavas; HPL, Happosawa lavas; KNP, Kumanodake pyroclastics; IML, Ichimaiishizawa lavas; SKL, Sainokawara lavas; NML, Nakamaruyama lavas; HML, Hiyamizuyama lavas; TKL, Torikabutoyama lavas; ZOL, Zaozawa lavas; SNL, Senninzawa lavas; OWL, Oiwake lavas; MYP, Mayuyamazawa pyroclastics; RO, Robanomimiiwa pyroclastics. (B) Close-up map of the summit area (Sakayori, 1992; Ban et al., 2008).

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(Ban et al., 2008). The Goshikidake pyroclastic unit is composed mainly of pyroclastic surge deposits of phreatomagmatic origin. It is divisible further into five sub-units (Ban et al., 2008). The Goshikidake pyroclastics mainly build up the Goshikidake cone. Sixteen tephra layers (Z-To 1–4, 5a, 5b and 5–14), composed mainly of scoriaceous volcanic ash, are readily observable. They have been correlated with products of stage 4 (Imura, 1999; Ban et al., 2005; Miura et al., 2008). Based mainly on 14C ages for buried leaves, wood, and reddish–dark brown paleosol that are intercalated with 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 juvenile scoriaceous tuff layers in the mountainside of the Zao edifice, suggesting that phreatic eruption has occurred frequently during stage 4. Some of these events, which occurred during historic times, are the youngest and most representative events that we are studying presently. They were widely witnessed and were described in articles published by several local newspapers and in academic reports, as described below. 3. Descriptions of the 1894 and 1895 eruptions Brief descriptions that are useful to elucidate the individual eruptive episodes that occurred during February 1894 to September 1895 have been collected from several newspaper articles published in the Tohoku Nippo, Ou-Nichinichi Nippo, Tohoku Shinbun, Yamagata-Shogyo Shinpo, Yamagata Nippo, and Yamagata-Jiyu Shinbun papers, along with flash academic reports (Kochibe, 1896a,b,c) and a research report on the Okama crater lake (Anzai, 1961). The inferred chronological order of the eruption events is presented in Table 1. A map of the Zao vicinity is depicted in Fig. 3. The volcano had been quiet from 1867–1894 (Kochibe, 1896c) before the eruption event started in February, 1894. Four discrete eruption episodes are distinguishable in a series of eruptions that occurred in 1895: 15 February of the first episode, 19 February and 22 August as the second and third episodes, and the final, climactic episode of 27–28 September.

reached the Matsukawa River at 10:00, involving many trees together with ice (Ou-Nichinichi Nippo, 22 February, 1895). The Shiroishigawa River and downstream areas of the Matsukawa River were swollen with sulfuric water carrying much driftwood for several tens of minutes 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 were witnessed 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 km northwest of the summit. Its amount was unclear. The Yamagata Nippo (24 August, 1895) reported it as a trace amount, although the Tohoku Nippo (29 August, 1895) claimed it to be “a large amount.” According Table 1 Chronologically ordered eruptive events of the 1895 eruption of Zao volcano. Year, month

Day

1867 Oct.

21st

Time

Jul.

1895 Feb.

Eruptive smoke rising up from the crater lake. Adhesion of ash to the plants was recognized after the rain in the western foot of the Zao volcano.

3rd

15th

9:30–10:30

19th

8:00 9:30 10:00

11:30–12:30

Mar.

14th

Mar. Aug. Sep.

22nd 22nd 27th

4:40–5:00

3.2. 15 February, 1895 5:05

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

5:20 6:15

3.3. 19 February, 1895 28th

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

Phenomena Rumbling. Maddy water flowed into the river. In the spa located in the eastern foot of the Zao, three people died in a flood. After the eruption, black smoke sometimes rose up.

1894 Feb.–Mar.

3.1. February–July, 1894 Several discrete emissions of eruptive smoke rising up from the crater lake were witnessed during February–March, 1894 at the Shibata 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 recognized in the villages of western foot of the Zao (Yamagata-Shogyo Shinpo, 6 July, 1894). Especially, a large amount of ash fell in Mikami Village, Minami-Murayama District located ca. 10 km west of the summit (Yamagata-Shogyo Shinpo, 6 July, 1894).

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18:00 5:00

A white smoke burst up suddenly from the crater lake with rumbling, which was witnessed from the vicinity of the crater lake and Kaminoyama Town. Ash fell around the Katta District and Sulferized water flowed into the Abukumagawa river. Rumbling. The pisolitic ash fell in the Katta District. The crater lake flooded, and the overflowed water reached into the Matsukawa river, involving many trees together with ice. Shiroishigawa river, the downstream of the Matsukawa river was swollen with sulfurized water carrying many drift woods. Fountains built up from the crater lake (ca. 3 m in height). Rumbling, lahar. Ash fall was witnessed in Yamagata City. Black and white smoke burst up with rumbling from the crater lake was witnessed from the Katta District. After the 5 min (at 5:05), the sulfurized ash fell. The smoke reached to the Sasaya Pass, Kawasaki Village and Natori District, while falling ash. The Nigorigawa river was swollen driftwood and sulfurized water. Sound similar those of canon fires in rapid succession was recognized in the Kawaoto sulfer refinery. A kind of reverberation sound such as “boom” was felt without earthquake. Then, ash fell along with the rain drops. Ash fall. Vesicleated block (30 cm in diameter) had been ejected from the crater lake and landed also near the new ore yard beyond the Haizukayama.

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to a sketch presented on 28 August (Fig. 4A) six days after the 22 August eruption, 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 been bursting up with rumbling from the Okama crater lake was witnessed at the Katta District (Ou-Nichinichi Nippo, 29 September, 1895; Tohoku Shinbun, 29 September, 1895; Tohoku Nippo, 29 September, 1895). The smoke had gradually been drawn out with the northeast wind. Five minutes later (at 5:05), the smoke reached Sasaya Pass, ca. 10 km north of the summit and the Kawasaki Village (time unclear). It reached ca. 20 km northeast of the summit as fall out ash (Tohoku Shinbun, 29 September, 1895; Tohoku Nippo, 29 September, 1895). Twenty minutes after the eruption (at 5:20), the Nigorigawa River was swollen with sulfuric water containing much driftwood (Ou-Nichinichi Nippo, 29 September, 1895; Tohoku Nippo, 29 September, 1895). The river water turned gray (Tohoku Nippo, 3 October, 1895). Most fish in the river died (Ou-Nichinichi Nippo, 29 September, 1895). At the Kawaoto sulfur refinery located ca. 7 km east from the crater lake, sounds like cannon fire were heard in rapid succession at 6:15 (Kochibe, 1896a). At the same time, according to a mine worker at the Kawaoto sulfur ore yard located ca. 200 m north of the crater lake, a kind of reverberating sound like a “boom” was felt without earthquake. Later, ash fell together with the rain (Kochibe, 1896a). The smoke of this event was reported to have extended to the Natori District, ca. 40 km east of the summit (Ou-Nichinichi Nippo, 29 September, 1895, Tohoku Nippo, 3 October, 1895). The ash from the smoke reportedly caused damage to vegetables around the Higashi-Taga Village,

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 all safe (Tohoku Nippo, 3 October, 1895). The head of the Shiroishi police documented an interesting field observation. He departed from his office at about 13:00 on 27 September and arrived at the crater lake at some time thereafter. The water level of the crater lake had decreased about 9 m from its prior, pre-eruption level. Furthermore, the crater lake water was boiling. Vertical black water fountains built up 3-m-high columns repeatedly 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 near the crater lake was inferred to be about 100 °C (Ou-Nichinichi Nippo, 3 October, 1895; Tohoku Nippo, 3 October, 1895). In addition, on the Goshikidake mountainside about 36 m northeast of the crater lake, a fissure variously reported as ca. 3 cm (Tohoku Nippo, 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 formed by the eruption dropped many larger clasts or blocks, with development of an umbrella formation at the top. In a sketch made on 6 October (Kochibe, 1896a) (Fig. 4C), a small mound was apparent at the western to 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 the new ore yard located near the Daikoku (Fig. 3B), ca. 2 km east of

Fig. 3. (A) Simplified topographic map showing geographic names around the Zao volcano and the surrounding area. (B) Close-up map of the mountain area. Old names of cities, towns and villages are shown. The curved lines show rivers. The bold lines in (B) show distribution areas of the 1895 volcanic ash (Kochibe, 1896a, partly modified).

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addition, soil and rock fragments on the surface were scattered in all directions of the pits. These are indicative of rather high-angled trajectories. In October 1895, after Kochibe (1896a) traced the fallout ash distribution, he reported that the main axis extended eastward to northeastward 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 lake became almost entirely calm except for small smoke emissions witnessed with rumbling at 0:30, 14 January, 1897 (Imperial Earthquake Investigation Committee, 1918). In addition, at Torijigoku (Fig. 3B), ca. 1.5 km northeast of the crater lake, a small eruption occurred at 10:00, 16 April, 1940. During 1939–1940, the crater lake water boiled several times (Anzai, 1961). 4. Field data Gray to whitish-pale gray colored ejecta of the 1894–1895 eruption are distributed within ca. 2 km from the crater lake center. They are well preserved within an area of 200–300 m from the center, particularly in topographic lows, on the southwestern rim of the Okama crater lake. Regarding the ejecta in the areas near the topographic high, such as near the summit of the Goshikidake and outside of the Umanose caldera, the deposits drape the paleosol or pre-existed deposits as thinner beds. We conducted detailed observations of 30 outcroppings located inside of the Umanose caldera, along with nine sites outside of the Umanose caldera (Fig. 5). Columnar sections for the representative localities are depicted in Fig. 6. 4.1. Distribution, stratigraphy, and lithofacies of the 1894 and 1895 deposits

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

the crater lake. Then, a porous block (ca. 30 cm diameter) was ejected from the crater lake, landing near the new ore yard beyond the Haizukayama (Fig. 3B) ca. 1.5 km south of the crater lake (Kochibe, 1896a). According to on-site investigations by officers of the Shibata District and 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 called Takinoyu, which had been located in the Gaga Spa area, was swept away by the flooding Nigorigawa River and a bathroom had been flooded to a depth of ca. 30 cm (Ou-Nichinichi Nippo, 29 September, 1895; Tohoku Nippo, 29 September, 1895; Tohoku Shinbun, 29 September, 1895). According to the Shibata District office, the Nigorigawa River had risen up to ca. 9 m. A bathroom and 12 huts in Gaga Spa had been carried away (Kochibe, 1896a). A field observation was performed on 6 October, 1895 (Kochibe, 1896a). Its results showed that the pits formed by deposition of blocks in the western rim of the Umanose caldera were ellipsoidal, and that soils and sand of the surface had been scattered to the westward part of the pits. Consequently, these blocks are inferred to have been ballistic. At Kumanodake (Fig. 3B), located ca. 1 km northwest of the summit, the pits formed by impact of the blocks which were thought to have been ejected during this eruption were circular. In

The 1894 deposit (designated as 1894 ash hereinafter) is composed mainly of black dense volcanic ash. However, the 1895 deposits contain tephra components of various kinds, such as altered lithic fragments and non-altered to weakly altered andesitic rock fragments, in addition to scoriae of various amounts set in the matrix, which consists of hydrothermally altered fine ash. A full set of these layers is observed restrictedly at the southwestern rim of the crater lake (SWRC) including loc. 1. At SWRC, the 1895 deposits are divisible lithologically into six layers – layers 1–6 – in ascending stratigraphy. At the type locality (loc. 1), the 1894 ash and layers 1–6 of the 1895 ash show respective thicknesses of ca. 2, b 1, 4, 5, 20, 175, and >300 cm (Fig. 6). Some layers are non-existent at several localities in eastern and northern areas (Fig. 6). At localities outside of the Umanose caldera, only the 1894 ash layer and layer 6 of 1895 are observed. Detailed descriptions are presented in Table 2. Photographs of the 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 are shown in Fig. 9. The area (m 2) and weight (kg/m 2) enclosed by each isopach are depicted on a smooth line in double logarithmic graph. The total mass was calculated by summing up the weight within the lines in each segment divided by data points (Takarada et al., 2001, 2002). Thickness (m) can be converted into the mass per square meter (kg/m 2) using the density (kg/m 3). In this study, the value of 900 kg/m 3, which is the average density of particles less than 16 mm of 1895 deposits, was used except for the portions of larger than 16 mm of layers 5 and 6. The density for each layer, except for the above portions, was determined using the weight of the product filled in a 100 ml beaker after drying in an oven at 105 °C.

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

For the portions, the value of 2.1 kg/m 3 was used as the density because about three-fourths of the materials are dense andesite. Deposits within the crater lake were omitted from calculations. Masses of layers 1 and 2 were not calculated because their isopach maps can not be defined adequately. The estimations for layers 3–5 in the distal areas were approximated by extending the isopachs linearly. The mass estimation for layer 6 in distal area was omitted from calculations because we were unable to draw proper isopachs. Consequently, the mass of layer 6 calculated from the extended line might include a larger error than those for layers 3–5. The calculated masses for layers 3–6 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 the complete set of layers is observed (loc. 1). Therefore, the mass of layers 1 and 2 might be approximated as less than 3.8× 106 kg. That result implies that the mass ejected to form each of the six layers became larger over time.

4.3. Grain size analysis Samples were collected from almost all layers at various locales except for layers 1 and 3 in some cases, where the layers are too thin to collect sufficient amounts for samples. For layer 3, samples were collected only at the southwestern rim of the crater lake (loc. 1). The method used for grain size analysis was the following. (1) The coarse fraction (>16 mm) was measured for an area of ca. 40× 40 cm for each layer using on-site photographs obtained using a digital

camera, except for layer 6 in the SWRC (loc. 1). In the exceptional case of layer 6 in loc. 1, the measured area was enlarged to ca. 100×300 cm because proportions of larger clasts are high. (2) Intermediate fractions (16–0.063 mm) were measured using ultrasonic sieving. (3) Finer fractions (b 0.063 mm) were measured using settling tube analysis (Bunan Doshitsu Shikenjo Ltd.). The pyroclastics are subdivided granulometrically into eight categories according to the definitions presented by Sohn and Chough (1989): fine ash (b 0.063 mm), medium ash (0.5–0.063 mm), coarse ash (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 from Rominger (1954): very well sorted (0b σφb 1), well sorted (1b σφb 2), normally sorted (2b σφb 3), poorly sorted (3b σφb 4), and very poorly sorted (4b σφ). Grain size distributions in the SWRC (loc. 1) and the other representative localities (Fig. 5) are portrayed in Fig. 10A and B, along with these numerical data in Table 3. The characteristics are also shown as σφ-Mdφ diagrams (Fig. 11). Overall, the larger clasts tend to be more enriched upwards. Detailed descriptions are presented in Table 2.

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

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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 inside and outside of the Umanose caldera.

through on-site observation. A binocular microscope was used for identification of smaller grains (16–0.063 mm; −3 to 4 φ). After subdivision into 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, we identified and counted altered lithic fragments, non-altered andesitic fragments, altered scoriae, pisolitic ash, and mineral and gypsum grains. Binocular microscope images for the representative grains are presented in Fig. 12. Results of the component analysis are presented in Table 4. Modal compositions for the 1894 ash and layers 1–6 are shown in Fig. 13A, B. Overall, altered lithic fragments (ca. 24–70 wt.%) and andesitic fragments (ca. 12–44 wt.%) are the dominant components through the samples 1894 ash and layers 1–6. The altered lithic fragments dominate the fraction range of 16–0.063 mm (−3 to 4 φ), whereas andesitic fragments are the most dominant type between 512–16 mm (−8 to −4 φ).

Subordinate amounts of altered scoriae were observed in all layers (ca. 2–14 wt.%). The pisolites existing in layer 2 are smaller than 2 mm (− 1 φ). A sort of parachute-type grain of the pisolite is rarely observed. Minerals are discrete plagioclase and pyroxene grains and contain up to ca. 20% in fine fractions of every layer. Gypsum is acicular or tabular, up to ca. 50% in the fine fractions, and is found only in the samples 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 eruption is distinctive in terms of its component characteristics, particularly those collected around the crater lake. Consequently, component analysis is a useful tool to identify these deposits. Detailed descriptions are presented in Table 2. 4.5. Interpretation We examined the mode of emplacement for the six layers based on field observations, estimated mass, grain size, and component analysis data. The general tendency of increasing amount of lithic clasts along

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

Grain size

1894 The 1894 ash overlies the compacted altered ash Goshikidake pyroclastics, loam or paleosol, or detritus with a sharp boundary. The 1894 ash is up to 6 cm thick in the caldera floor (loc. 26) and b10 cm thick at locations outside of the Umanose caldera (the area of the topographic high). The black 1894 ash shows a normally sorted massive facies (Fig. 7A). Layer Layer 1 overlies the 1894 ash with a sharp boundary. It is 1 observed restrictedly around the SWRC (loc. 1 and 26), corresponding to the area of topographic lows. Layer 1 is up to 1 cm thick in the western area of the crater lake (loc. 26). It shows intercalation of well-sorted whitish ash and well-sorted gray ash beds (Fig. 7A and C). Layer Generally, layer 2 directly overlies layer 1 with a sharp 2 boundary, although it is underlain by compacted altered Goshikidake pyroclastics or the 1894 ash at several locations (Fig. 7A, B). Layer 2 is more than 41 cm thick in the topographic low of the southern area of the crater lake (loc. 29), but it thins to less than 1 cm thick at areas of topographic highs, such as near the summit of Goshikidake Peak (Fig. 5; loc. 19). Layer 2 is well-sorted massive tuff consisting of pale-gray ash with small amounts of andesitic fragments, altered lithic fragments and altered subangular to subround scoriae. The bedding sags formed by clasts with ca. 5 cm diameter are also observed in places of the SWRC (loc. 1) (Fig. 7B). In addition, there is a bed consisting of pisolites of ca. 2 mm diameter, which is observed clearly at the upper part of layer 2 at loc. 26 (Fig. 7C). The pisolitic ashes are mainly gray colored solid or hollow spheres, and each pisolitic grain has a hole. Layer Layer 3 overlies layer 2 with a sharp boundary. Layer 3 is 3 up to 42 cm thick in the southern area of the crater lake (loc. 29), but less than 1 cm thick at the topographic high near the Goshikidake Peak (loc. 19). Layer 3 is a normally sorted tuff which is composed mainly of whitish-gray ash with small amounts of subangular to subrounded andesitic fragments, altered lithic fragments and altered scoriae up to 10 cm. In the SWRC (loc. 1), a concentration of lapilli of up to 2 cm thick is locally observed (Fig. 7D) in the middle portion of layer 3 (ca. 5 cm in thick). Additionally, some clasts (ca. 5 cm diameter) form bedding sags restrictedly in the SWRC, resulting in a sort of undulation at its lower boundary (Fig. 7D, E). In the southern area of the crater lake (loc. 29), layer 3 is co-mingled with layer 4 and a part of layer 3 is surrounded by mixed deposits of layers 3 and 4 (ca. 10 cm diameter) (Fig. 7H). Layer Layer 4 overlies layer 3 with a sharp boundary in most 4 areas, but the boundary is undulated locally. This layer is observed only at the topographic low (Fig. 7B, D, F, G). It is up to 30 cm thick in the southern area of the crater lake (loc. 29). Layer 4 is normally sorted tuff consisting mainly of pale-gray ash. Small amounts of subangular to subrounded andesitic fragments, altered lithic fragments and altered scoria are set in the ashy matrix. In the SWRC (loc. 1), layer 4 displays a weak lamination consisting of discontinuous lapilli trains and/or the lapilli-rich lamina (Fig. 7D, F). Layer 4 deposited away (ca. 100–150 m) from the crater lake commonly shows a facies of thin massive ash (Fig. 7G). Layer Layer 5 overlies layer 4 with a sharp boundary. It is up to 5 175 cm thick in the SWRC (loc. 1), but it thins rapidly away from the crater lake. This layer varies in thickness at the distal part from 1 cm at the southern area of the crater lake (topographic low; loc. 6) to 4 cm near the Goshikidake Peak (topographic high; loc. 4). This layer shows 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 upper part (ca. 75 cm) are several weak stratifications formed by various ratios of angular to subangular clasts of andesitic fragment, altered lithic fragments and altered scoria (Fig. 7I) of up to ca. 10 cm diameter. Near the Goshikidake

The 1894 ash shows a unimodal peak around 1 φ and The share of the andesitic fragment proportions tends to be higher in coarser grain fractions (Fig. 13A; shows a unimodal distribution. It is normally sorted Table 3). Among the analyzed samples, the 1894 ash (σφ = 2.9) with Mdφ of 1.0. shows the highest proportion of the andesite fragments. It is also richest in scoriae.

Component

Not analyzed

Not analyzed

Layer 2 shows considerable variation with respect to its granulometric characteristics from the eruption center outwards. Near the Okama crater rim, layer 2 is well-sorted and shows a bimodal granulometric distribution. The sample from loc. 1, for example, has two peaks 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 2 sample from the northern area of the crater lake (loc. 7) shows a bimodal distribution with peaks in 3 φ and 7 φ. It is also well-sorted (σφ=2.0), with Mdφ of 5.3, although the sample from the loc. 7 is more enriched in finer ash. However, the layer 2 sample from ca. 100 m west of the crater lake (loc. 26) is fine-depleted, showing a unimodal distribution with a peak around 0 φ. The Mdφ value is coarser (−0.5) and displays better sorting (σφ=1.6) at the crater lake rim.

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

Layer 3 at loc. 1 has two main peaks in 2 φ and >7 φ with a subordinate peak around −3 φ. It is normally sorted, but its Mdφ (3.9) and σφ (2.6) values are slightly higher than those for layer 2 at the same locality (loc. 1).

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

At loc. 1, we performed grain size analyses for the three representative samples collected from the upper, lower and lapilli-rich parts of layer 4. These are normally sorted (σφ=2.5–2.8) with Mdφ values of 3.6–4.7. Both the samples from lower and upper parts have two peaks of grain size around 3 φ and >7 φ, although that from the lapilli rich part has a peak of 1 φ. These features are compatible with those of the underlying layer 3, although the subordinate peak is not recognized in layer 4. The grain-size characteristics of this layer at the western and northern localities (loc. 26 and 7) are better reconciled with those of the upper and lower parts at loc. 1. In the southern area, the peak of grain size of layer 4 shifts to finer grains of 4 φ. Larger clasts, such as coarse to fine blocks, are more abundant in most samples collected from this layer than in the underlying layers 1–4. At loc. 1, we analyzed the grain size for the two representative samples of layer 5: samples collected respectively from the massive lower and weakly stratified upper parts. The lower part shows a flat pattern in the range coarser than −7 φ, with Mdφ around 0.3, although a gentle peak around 1 φ is recognized for the upper part, with Mdφ of 1.8 (Fig. 10A). Both the upper and lower parts are very poorly sorted (σφ = 4.5 and 4.8). The size distribution patterns for the samples from the northern and western areas of the crater lake (loc. 14

The most abundant component of layer 4 is also the altered lithic fragments (38–60%). It is followed by andesitic fragments (18–38%; Fig. 13A, B; Table 3). In almost all analyzed samples, the relative abundance of the andesitic fragments is highest in a size range around 0–1 φ. A small amount of pisolitic ash is contained in the southwestern rim (ca. 11 wt.% in loc. 1) and western area of the crater lake (ca. 3 wt.% in loc. 26). The lapilli rich part of loc. 1 is the sole exception. It is distinctly poor in andesitic fragments (Fig. 13A).

Similarly to the case with layers 3 and 4, the most abundant component in layer 5 is also the altered lithic fragments. The second is andesitic fragments (Fig. 13A, B; Table 3). In the histograms for the sample of layer 5, the andesitic fragments are shown to have two peaks (Fig. 13A, B). One is around 0–1 φ and the other is −6 to −4 φ. The peak of around 0–1 φ tends to be smaller than those of layer 4 samples.

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Table 2 (continued) Layer Lithology Peak, this layer is normally sorted, showing very weak lamination (Fig. 7J).

Layer Layer 6 overlies layer 5 with a sharp boundary. Layer 6 is 6 thick – up to 300 cm in the SWRC (loc. 1) – but it thins rapidly away from the crater lake. At the southern area of the crater lake, a topographic low (loc. 6), and near the Goshikidake Peak, a topographic high (loc. 4), layer 6 is as thin as ca. 20 and 16 cm, respectively. This layer shows poorly sorted gray colored matrix-supported tuff breccia at loc. 1. These features resemble those of layer 5. Another feature that resembles layer 5 is the abundant existence of large clasts up to ca. 50 cm long, with various ratios of angular to subangular clasts forming weak stratifications (Fig. 7I). Large clasts are indeed larger than the relevant of layer 5. Especially, the long axis of the large clasts, which is concentrated ca. 230 cm above the base, is aligned 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). In the distal area located >150 m from the crater lake, this layer 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 surrounding the Okama crater lake. It drapes the 1894 ash, loam, paleosol or detritus, up to 10 cm thick at the rim of the Umanose caldera (loc. 30). Layer 6, which is deposited outside of the Umanose caldera is well to poorly sorted whitish to gray lapilli tuff. Bedding sags are also found in the rim of the Umanose caldera (loc. 30) (Fig. 7L). Layer 6 tends to show distinct thinning and fining away from the crater lake.

Grain size

Component

and 26) are rather flat (Fig. 10B), which is compatible with that of the lower part at loc. 1. The flat patterned samples are poorly to very poorly sorted (σφ=3.7–4.0) with Mdφ of 2.0–2.5. In contrast, those from the southern and eastern areas of the crater lake (loc. 6 and 19) show unimodal patterns with a single peak around 0–3 φ. These are normally to poorly sorted (σφ=2.3–3.3) with Mdφ of 0.5–2.5. The σφ value tends to increase away from the crater lake, although the Mdφ value tends to decrease (Fig. 11). Layer 6 is more abundant in larger clasts, such as coarse to fine blocks, than in layer 5. At loc. 1, we collected six samples (6-a–6-f) from the lower to upper portion of layer 6 by spacing the same interval of ca. 50 cm because the lithofacies was apparently uniform within this layer by the on-site field observation. The grain size distribution pattern for sample 6-a is similar to that for the relevant lower part of layer 5: samples 6-b to 6-f show a unimodal distribution pattern with a peak around 0 and −3 φ, and contain considerable amounts of coarse to fine blocks (b−6 φ). The peak mode size tends to shift coarser upwards. Also, the amount of the large lithic fragments becomes higher upward. This is reflected by the fact that the Mdφ value decreases upwards (Mdφ: 1.5 for 6a, and −1.8 to 0.0 for 6b to 6f). In addition, the upper part 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 slightly better sorting (σφ = 3.3–4.7). Granulometrically, Layer 6 in the western area of the crater lake (loc. 26) is compatible with the relevant upper part at loc. 1, which produced a unimodal peak around − 2 φ and which contains abundant fine blocks (Fig. 10B). In other localities, layer 6 commonly shows flat patterns (Fig. 10B), which resemble those 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 the Umanose caldera) appears to be fine-depleted with a peak of −4 φ, which is similar to that of layers 6-d to 6-f at loc. 1. The peak size becomes finer and the amount of smaller particles becomes richer (Mdφ value shifts from −0.8 to 3.7) away from the crater lake. Generally, the sorting becomes better (σφ value shifts from 1.9 to 4.8) away from the eruption center (Fig. 11) in the area outside of the Umanose caldera.

with the increasing mass of ejecta from layers 1–6 suggests that the 1895 eruption became more explosive and voluminous over time. The substratum model beneath Zao volcano is presented in Fig. 14. According to geologic studies described in chapter 2, the uppermost part consists of stage 4 products (less than 150 m thick), which are mainly composed of scoria. The stage 4 products are underlain by stage 2 and 3 products (ca. 500–800 m maximum thickness) beneath the central part of the volcano. These mainly comprise andesitic lavas and the associated products. After petrologic investigation of the Akita-Yakeyama 1997 eruption, Ohba et al. (2007) inferred that the rocks surrounding the vent had suffered alteration down to ca. 1 km deep. It is reasonable to presume a similar hydrothermal alteration area in the case of the Zao 1895 eruption. The alteration might have been caused by hydrothermal fluids that originated from magma stored in a chamber. Ban et al. (2008) estimated the andesitic magma chamber depth, which caused one of the eruptions in Zao's youngest stage, as less than 3 kb. They considered that the magma chamber had been semi-solidified but that it was activated by injection of basaltic magma from the deep part before the eruption. It is probable that the andesitic magma chamber has been stored at similar depth during the youngest stage of Zao. The lithic fragments mainly comprise altered lithic fragments. These might have originated from the hydrothermal altered area

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

described above, where lithics readily underwent intense alteration. However, the non-altered large andesitic blocks presumably derive from stage 2 and 3 layers immediately below the crater lake, which was considerably resistant against alternation and which was destroyed before or during the eruption. The scoriae are from stage 4 layers formed before the 1895 eruption, however, we can not exclude 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 near the crater center. 4.5.1. 1894 ash This layer consists mainly of black andesitic ash, showing a normally sorted massive facies with unimodal character. These characteristics concur with the idea that 1894 ash was derived from vulcanian pyroclastic fall. 4.5.2. Layer 1 Layer 1 was more likely derived from rapid deposition of a near-vent pyroclastic surge (Sohn and Chough, 1989) rather than a simple air fall from the eruption column. Indeed, a well-sorted character is generally more likely to result from pyroclastic fall deposition (e.g., Cas and Wright, 1987), but layer 1 is observed restrictedly in the topographic

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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 partitions 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 undulation lower 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 indicate bedding 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 in the 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 by various 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 part of 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 near Goshikidake 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).

lows (loc. 1 and 26 in Fig. 5b). Its thinly laminated nature might well result from the pyroclastic surge. 4.5.3. Layer 2 Some observations support the interpretation that layer 2 is fundamentally deposited from a pyroclastic fall (e.g., Cas and Wright, 1987): (1) layer 2 is well sorted; (2) it is distributed not merely in the topographic lows but also in the topographic high (near the Goshikidake Peak; loc. 4 and 19); and (3) bedding sags are observed frequently at outcrops in SWRC (loc. 1). Furthermore, well-sorted pisolite grains showing a unimodal distribution are also conspicuous within this layer (e.g. loc. 26), strongly supporting its fall origin.

This layer mainly comprises altered lithic fragments. Therefore, this event is expected to occur in close association with hydrothermal activity. 4.5.4. Layer 3 A stratified and normally sorted nature without fines elutriation along with a lapillus-rich part at the southwestern rim of the crater lake (loc. 1) is similar to the “stratified tuff” described by Chough and Sohn (1990). Stratification formed by alternation of lapillus-rich and lapillus-poor layers, thin lapillus trains, color banding, and indistinct bedding planes is also observed in layer 3. Accordingly, this facies is most likely emplaced by both suspension and traction sedimentation

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

from a turbulent base surge that fluctuates in its velocity and particle concentration. In addition, the altered lithic fragments, being the most dominant component, strongly suggest that the main part of layer 3 resulted from a pyroclastic surge related to the hydrothermal system. Nevertheless, layer 3 near the Goshikidake Peak (loc. 4 and 19) at the topographic high might be of fall origin. The bedding sags in layer 3 of loc. 1 also indicate its ballistic origin. 4.5.5. Layer 4 The normally sorted nature of layer 4, without fines elutriation, mostly massive, and with weak lamination at the southwestern rim of the

crater lake (loc. 1), is compatible with those recognized in near-vent deposits of less-evolved density currents (Cas and Wright, 1987). In addition, locally developed discontinuous lapilli trains and lapilli-rich parts might result from the tractional transport of ash and lapilli (Sohn and Chough, 1989; Chough and Sohn, 1990). The main component of layer 4 is altered lithic fragments. Therefore, the main part of this layer likely originated from an explosive phreatic eruption related to the hydrothermal system. The lateral facies changes from its eruption center outwards. The facies of thin and well-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.

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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.

4.5.6. Layer 5 Lithofacies of the poorly sorted layer 5 near the crater lake is unlikely to be reconciled with the following three possibilities as the transportation and deposition modes (loc. 1, 14 and 26): hot lahar, pyroclastic flow, or pyroclastic surge. In general, hot lahar at the proximal region tends to appear as fine-depleted, clast-supported lithofacies near the vent because the fine matrix flows downstream. The pyroclastic flow and surge preferentially transport and leave the pyroclastics in the area of the topographic lows. The layer showing a lithofacies of

matrix-supported tuff breccia is distributed also in the topographic high (e.g., near the top of Goshikidake, a topographically high area), as well as in the topographic low (e.g., the southwestern rim of the crater lake). The granulometry of the layer 5 sample at the southern area of the crater lake (loc. 6) is compatible with the characteristics of a pyroclastic fall deposit (Fig. 10B). In addition, layer 5 thins rapidly and tends to be better sorted away from the crater lake. These characteristics suggest that layer 5 is a near-vent facies of a pyroclastic fall deposit. The poorly

Fig. 10. Grain size distributions of the 1894–1895 eruption deposits: (A) results obtained from samples collected at the southwestern rim of the crater lake (loc. 1), and (B) results from other representative localities (loc. 4, 6, 7, 14, 19, 26, 30, 36, and 38).

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Table 3 Granulometric 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 060831-6-5 060831-6-4 060831-6-3 060831-6-2 060831-6-1 061013-6U 061013-6L 061013-6U 061013-6L 070603-6U 070603-6M 070603-6L 070603-6 070612-6U 070612-6L 070612-6 070612-6U 070612-6M 070612-6L 070708-6U 070708-6L 070708-6U 070708-6L 070708-6U 070708-6M 070708-6L 070920-6 070920-6 070920-6 070931-6 070931-6 060831-5U 060831-5L 061013-5 070811-5 070603-5U 070603-5L 070612-5 070708-5 070708-5 071023-5 070913-5 070920-5 070931-5 070931-5 080610-4U

−1.5 −1.6 −1.8 −0.2 0.0 1.5 2.3 2.8 1.2 2.4 1.6 2.6 3.8 1.0 4.4 1.4 1.4 0.0 2.6 2.8 2. 1.9 2.7 4.2 0.4 1.2 2.2 1.2 0.2 −0.2 2.2 0.3 0.3 1.8 0.5 1.8 1.6 2.8 2.7 2. 2.5 2.2 2.5 2.0 4.2 4.2 4.7

5.3 3.0 3.0 5.1 4.8 5.7 5.7 5.7 5.5 5.6 5.2 5.3 5.4 5.6 5.5 5.0 5.6 5.5 5.7 5.7 5.2 5.2 5.7 5.7 5.6 5.0 5.7 5.0 5.4 5.4 5.6 5.5 5.5 5.7 3.0 5.2 5.6 5.6 5.6 5.0 5.7 5.6 5.5 5.6 5.7 5.8 5.8

−4.0 −3.5 −3.6 −3.2 −2.9 −3.6 −2.7 −3.4 −2.3 −1.7 −2.9 −2.9 −1.2 −3.1 −1.5 −3.0 −3.1 −3.4 −2.6 −2.4 −3.6 −4.2 −2.1 −2.6 −3.4 −3.4 −2.6 −3.6 −3.3 −3.2 −2.2 −3.2 −3.5 −3.8 −1.5 −2.2 −2. −2.8 −1.0 0.0 −1.6 −2.6 −1.0 −2.3 0.0 1.6 0.5

4.7 3.3 3.3 4.2 3.9 4.7 4.2 4.6 3.9 3.7 4.1 4.1 3.3 4.4 3.5 4.0 4.4 4.5 4.2 4.1 4.4 4.7 3.9 4.2 4.5 4.2 4.2 4.3 4.4 4.3 3.9 4.4 4.5 4.8 2.3 3.7 4.2 4.2 3.3 2.5 3.7 4.1 3.3 4.0 2.9 2.1 2.7

Loc.4 Loc.6 Loc.8

Loc.9 Loc.10 Loc.11 Loc.12

Loc.13 Loc.14 Loc.15

Layer 5

Loc.23 Loc.25 Loc.26 Loc.27 Loc.28 Loc.1 Loc.6 Loc.7 Loc.9 Loc.11 Loc.13 Loc.14 Loc.15 Loc.19 Loc.26 Loc.27 Loc.28 Loc.1

(6-a) at loc. 1. Furthermore, these show similar granulometric characteristics resembling those for layer 5 near the crater lake (loc. 1, 14 and 26). Consequently, layer 6 seems to have been deposited from a fountain-like eruption similarly to those for layer 5 near the crater lake. 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 more explosively with time, and that fine particles fell out and away from the near-vent area. Conceivably, the deposits that are expected to be correlated with the 6-b to 6-f at loc. 1 were reworked in these areas. 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 the western area of the crater lake (loc. 26) show similar granulometric characteristics to those of the relevant layer on the top of the Umanose caldera rim (loc. 30); poorly sorted and fine-depleted (Fig. 10A, B). Layer 6 (including deposits at loc. 30 and outside of the Umanose caldera) becomes thinner and finer away from the crater lake (Figs. 8 and 10B), irrespective of topography. Therefore, layer 6 is more likely to be a deposit of pyroclastic fall origin. Precipitation of the deposits by ballistic ejecta might have contributed to the poorly sorted nature at loc. 30 because loc. 30 is near the vent. Presence of the bedding sags in layer 6 support this interpretation. Therefore, the upper part of this layer at loc. 1 and the relevant deposits in the western area (loc. 26) can be interpreted as the pyroclastic fall deposit. The large clasts, of which the long axis aligns horizontally in the southwestern rim of the crater lake (loc. 1), might have resulted from the secondary transport immediately after the deposition. The upper part of layer 6 at loc. 1 contains a larger amount of angular to subrounded scoriae (Fig. 13A) than the remainder of the layer. These are altered, but one can not exclude the possibility that some angular scoriae are juvenile. The alteration might have taken place after deposition. 5. Discussion 5.1. Correlation of eruption documents to 1895 eruption deposits The first pisolitic ash fall in the 1895 eruption was recognized on 19 February at the Katta District (chapter 3; Table 1). This phenomenon is likely to be correlated with the pisolite concentrated part observed in the upper part of layer 2 (loc. 26) (Fig. 7C). Consequently, layer 2 must have been deposited by 19 February, 1895. The well-sorted whitish ash as the main component of layer 1 is inferred

sorted thick layer near the crater lake (loc. 1, 14, and 26) apparently resulted 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 fine grain size range), reflect that layer 5 was derived from hydrothermal activity. Weak stratification at the rim of the crater lake (loc. 1) and extremely weak lamination near the Goshikidake Peak (loc. 21) imply that eruptions occurred repeatedly. 4.5.7. Layer 6 Layer 6, the thickest, shows by far the largest volume among the layers deposited during 1894–1895. Therefore, this layer was unquestionably formed during the climax activity. Although the layer is massive, observations indicate repeated eruptions or pulse-like transportation in a pyroclastic density current, such as poor sorting within the inner area of 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 of the 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 of the crater are compatible with those of the basal part of this layer

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 flow and pyroclastic fall were adopted from Walker (1971). That for pyroclastic surge was quoted from Walker (1983). Layer 5 and layer 6 outside of the Umanose caldera become finer, with better sorting away from the crater lake. Black and gray arrows respectively indicate the trend of layer 5 and the trend of layer 6 outside of the Umanose caldera.

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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), pisolitic ash (D), plagioclase crystal (E), pyroxene crystal (F), gypsum crystal (acicular) (G), and gypsum crystal (tabular) (H).

deposit of the eruption on 22 August. The whitish-gray colored matrix of layer 3, which reflects high abundance of the altered lithic fragment relative to those for layers 2 and 4 (Fig. 13A), might be attributed to about half a year of duration for alteration in the intense hydrothermal activity area. Co-mingling of layers 3 and 4 in the southern area of the crater lake (loc. 29) might have resulted from reworking of the two layers

to be the deposit associated with the white smoke observed on 15 February, 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 was considerably higher than in February. In addition, the estimated mass of layer 3 was much greater than those of the underlying layers 1 and 2 (chapter 4). In this context, layer 3 is inferred to be the Table 4 Results of component analysis for samples of representative localities. Layer

Locality

Sample name

Andesite

Scoria

Altered lithic fragment

Pisolites

Crystal

Gypsum

Layer 6

Loc.1

060831-6-6 060831-6-5 060831-6-4 060831-6-3 060831-6-2 060831-6-1 061013-6U 061013-6L 061013-6U 061013-6L 070708-6U 070708-6L 070920-6 060831-5U 060831-5L 061013-5 070708-5 070913-5 070920-5 080610-4U 080617-4 (lapilli rich part) 080617-4L 080913-4 080617-4 070920-4 060831-3 060831-2 070811-2 070920-2 080610-1894

28 24 27 19 26 29 18 16 25 29 17 19 26 32 36 19 17 38 32 31 18 38 29 20 18 30 25 22 12 44

10 10 11 9 5 4 7 8 8 5 8 8 9 3 4 7 8 5 4 2 5 3 6 10 4 7 14 10 7 13

55 57 56 64 60 59 69 70 63 63 69 67 61 57 51 66 67 47 53 49 52 38 59 57 60 55 45 53 51 24

n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 11 n.d. n.d. n.d. 3 n.d. 1 2 22 n.d.

4 6 4 6 7 4 6 6 4 3 6 6 4 5 5 8 6 10 5 9 6 5 7 7 10 6 9 10 9 8

3 2 2 1 1 3 n.d n.d n.d n.d n.d n.d n.d 3 5 n.d. 2 0 6 9 8 16 n.d. 6 6 1 5 3

Loc.4 Loc.6 Loc.14

Layer 5

Loc.26 Loc.1

Layer 4

Loc.6 Loc.14 Loc.19 Loc.26 Loc.1

Layer 3 Layer 2

1984 ash n.d., not detected.

Loc.6 Loc.7 Loc.26 Loc.1 Loc.1 Loc.7 Loc.26 Loc.1

10

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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 from the other representative localities (loc. 4, 6, 7, 14, 19, and 26).

before the subsequent eruption. As described in chapter 3, the 27–28 September eruption was inferred to have been highly explosive and much larger than the 22 August eruption. Furthermore, the volume estimations for layers 5 and 6 are one order larger than those of layers 3 and 4 (chapter 4). Therefore, layers 5 and 6 are likely to correspond to the climactic eruptions that occurred on 27–28 September. As inferred from the sketches of Fig. 4, a small mound roughly 30 m high must have formed during the climactic eruptions in the western to southern rim of the crater lake. Conceivably, the weak stratifications and weak lamination in layers 5 and 6 reflect the repeated eruptions

that were documented in the reports of the eruption phenomena that occurred on 27–28 September (chapter 3). 5.2. Sequence of the 1895 eruptions The revealed eruptive sequence of the 1895 eruptions, depicted in Fig. 14, is summarized as described below (1) About one year before the 1895 activity, ashy smoke rose from the crater lake during February–March, 1894. On 3 July, 1894,

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vulcanian eruption occurred in the crater lake, ejecting black glassy ash (1894 ash). The activity ceased before long. The subsequent dormancy lasted for approximately half a year. The magma which caused the vulcanian eruption is likely to have ascended from a re-activated andesitic magma chamber by new infusion of basaltic magma from the deep part. The infusion probably ceased once after the July eruption (Fig. 14A). The reactivated magma chamber had released magmatic fluids, which reacted with the surrounding materials and transformed to the hydrothermal fluid. The hydrothermal alteration of rocks surrounding the vent is expected to have proceeded during the dormancy (Fig. 14B). (2) The activity recommenced on 15 February, 1895. White smoke burst up from the crater lake during 9:30–10:30 and the whitish ash was deposited restrictedly in the southwestern area of the crater lake to form layer 1 (Fig. 14B). The Nigorigawa River flooded with muddy water and sulfuric water flowed further into the Abukumagawa River. The ascent of the hydrothermal fluid from ca. 1 km depth is likely to have caused the eruption. Eruption of this type is expected to have been promoted by further reactivation of the andesitic magma chamber, when another injection of basaltic magma would occur. (3) The eruption on 19 February ejected ash at 9:30. Then, pisolite and ballistic clasts fell near the crater lake to form layer 2 (Fig. 14B). The crater lake flooded. The overflowing water was flushed into the Matsukawa River at 10:00. The Shiroishigawa River was swollen with sulfurized water carrying much driftwood for several tens of minutes around noon. Subsequently the volcanic activity became calm, with dormancy lasting for approximately half a year. The injection of the basaltic magma probably ceased.

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(4) On 22 August, ash fell in Yamagata City. The eruption accompanied the pyroclastic surge together with the ejection of ballistic bombs, which created layers 3 and 4. Layer 3 is observed even near the Goshikidake Peak, although layer 4 is absent in the topographic high. The dominant eruption phase gradually changed to a sort of density current (pyroclastic surge) from the pyroclastic fall during the 22 August eruption (Fig. 14C). The eruption would have been the result of new injection of the basaltic magma. The highly altered nature of the layer 3 matrix reflects about half a year of duration for the alteration (Fig. 14C). (5) On 27 September, a climactic event occurred, conceivably during 4:40–6:15. Fountain-like eruptions occurred repeatedly to form layers 5 and 6, and also caused a small mound in 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 clasts fell, with an umbrella part developing at the top. Furthermore, many large clasts fell with ballistic trajectories (Fig. 14D). The cloud extended toward the east–northeast. The fallout ash was detected as far as ca. 40 km east of the summit. The Nigorigawa River was swollen with sulfurized water containing much driftwood. Subsequently a small amount of ash fell at 18:00 on 27 September. (6) On 28 September, vesiculated blocks of ca. 30 cm in diameter were ejected from the Okama crater at 5:00. The Nigorigawa River rose ca. 9 m. The eruption deposits might be a part of clasts scattered on the surface of the crater area. This climax activity is expected to have resulted from the injection of the larger amount

Fig. 14. Schematic illustrations showing substratum models and modes of transportation and deposition of tephra in the 1895 phreatic eruptions, including the preceding 1894 eruptions. These are, respectively, for eruptions on 1894 (A), 15 and 19 February (B), 22 August (C), and 27–28 September (D).

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From the sketch drawn at ca. 20 km east of the crater (Kochibe, 1896a; Fig. 4B), the eruption cloud was estimated as about 350 m high. Based on the eruption cloud shape shown in the same sketch, the cube-root scaled explosion depth of the eruption is approximated as ca. 0.004 m/J 1/3. Based on the method of Ohba et al. (2002), the scaled cloud height was calculated as ca. 0.079 m/J 1/3. Adopting the cloud height of 350 m and the scaled cloud height of 0.079 m/J 1/3, the explosive energy was estimated as ca. 1 × 10 11 J. According to this estimate of explosive energy, the crater diameter was calculated as 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 comparable to that obtained using the calculation described above. Furthermore, using the cube-root scaled explosion depth and the explosive energy described above, the explosion depth was calculated as ca. 20 m.

6. Conclusion

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

of basaltic magma. The erupted angular scoriae might be juvenile fragments derived from the reactivated andesitic magma chamber (Fig. 14D). The estimated mass of the discharged products for the eruptions on 15 and 19 February, 22 August, and 27–28 September are, respectively, less than 7.6 × 10 6 kg, ca. 1.0 × 10 7, and more than 6.2 × 10 8 kg. It can be said that the 1895 eruption became more voluminous than prior eruptions.

The 1895 eruption of Zao occurred on 15 and 19 February, 22 August, and 27–28 September after several precursory vulcanian eruptions in February–July in 1894. The products of the 1985 eruptions are divided lithologically into six layers (layers 1–6). Each layer has distinctive features in terms of its proportion of the components and granulometric characteristics. From the correlations of the lithology with the documents reporting the relevant explosion events, the eruption episodes in February, August, and September respectively correspond to the deposition of layers 1–2, 3–4, and 5–6. The dominant transport mechanism must have been some sort of pyroclastic fall, but a low-temperature pyroclastic surge accompanied the eruptions on 22 August. The eruption became more voluminous over time. The products of eruptions on 15 and 19 February, 22 August were less than 3.8 × 10 6 kg, less than 3.8 × 10 6 kg, and ca. 1.0 × 10 7, respectively, whereas that on 27–28 September amounted to more than 6.2 × 10 8 kg. The magma which caused the vulcanian eruption of 1984 was probably from the andesitic magma chamber located shallower than 3 kb, which was probably reactivated by a new infusion of basaltic magma from some deeper area. During the dormancy of about half a year, magmatic fluids released from the andesitic chamber had altered the surrounding materials. The hydrothermally altered materials were emitted during the 1895 eruptions. Three periods of eruption in 1895 are expected to have resulted from individual infusion of basaltic magma into the andesitic magma chamber. The last one, the largest, produced the climax eruption. It is probable that some portions of the andesitic magma ascended to the surface during the climax eruption. The explosive energy discharged with the maximum explosion on 27 September was estimated as ca. 1 × 10 11 J. The crater diameter was approximated as ca. 30 m.

5.3. Explosive energy, crater size, and explosion depth of the 27 September explosion Acknowledgment We estimated the energy of the explosion and the crater diameter for the climactic explosion by combining the respective methods of Ohba et al. (2002) and Goto et al. (2001). The explosion energy is calculable using the method of Ohba et al. (2002) if one assumes the explosion cloud height correctly. Although the method must also identify cube-root scaled depth, it can be inferred from the shape and height of the eruption cloud (Ohba et al., 2002). Furthermore, the crater diameter can be approximated from the calculated explosion energy using the method of Goto et al. (2001). Goto et al. (2001) formulated the relation between the crater diameter (D) and the explosion energy (E) as the equation log D = 0.32 × log E − 2.06 based on experimentally obtained results.

We are grateful to Prof. Lionel Wilson and an anonymous reviewer for many constructive comments and suggestions on the manuscript. We acknowledge to Drs. K. Nakashima and T. Maruyama for their continual support of this research. We are grateful to Mr. K. Kontani, Mr. Y. Tachihara, and Ms. Y. Nakazawa for their field assistance and useful comments on this study. We also acknowledge to Yamagata and Miyagi Prefectural Governments for the permission of field observation. This work was financially supported in part by Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science to M. Ban (no. 22540487) and T. Ohba (no. 21510186).

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References Anzai, T., 1961. A Mysterious Crater Lake, Okama of Zao. Yamagata Geological Research Laboratory. 224 pp. (in Japanese). Ban, M., Sagawa, H., Miura, K., Tanaka, Y., 2005. Volcanic hazard map of Mt. Zao. Earth Monthly 27, 317–320 (in Japanese). Ban, M., Sagawa, H., Miura, K., Hirotani, S., 2008. Evidence for short-lived stratified magma chamber: petrology of Z-To tephra layer (~5.8 ka) at Zao volcano, NE Japan. In: Zellmer, G., Annen, C. (Eds.), Dynamics of Crustal Magma Transfer, Storage, and Differentiation Integrating Geochemical and Geophysical Constraints: Geol. Soc. London, Special Publication, 304, pp. 149–168. Cas, R.F.A., Wright, J.V., 1987. Volcanic Successions: Modern and Ancient. Allen and Unwin, London. 528 pp. Chiba, T., 1961. Petrology of Zao volcano. Journal of the Japanese Association of Mineralogists, Petrologists and Economic. Geologists 46, 74–81 (in Japanese, with English Abstr.). Chough, S.K., Sohn, Y.K., 1990. Depositional mechanics and sequences of base surges, Songaksan tuff ring, Cheju Island, Korea. Sedimentology 37, 1115–1135. Fujinawa, A., Kamoshida, T., Tanase, A., Tanimoto, K., Nakamura, Y., Kontani, K., 2006. Reconsideration of the 1900 explosive eruption at Adatara volcano, northeastern Japan. Bulletin of the Volcanological Society of Japan 51, 311–325 (in Japanese, with English Abstr.). Fujinawa, A., Ban, M., Ohba, T., Kontani, K., Miura, K., 2008. Characterization of lowtemperature pyroclastic surges that occurred in the northeastern Japan arc during the late 19th century. Journal of Volcanology and Geothermal Research 178, 113–130. Goto, A., Taniguchi, H., Yoshida, M., Ohba, T., Oshima, H., 2001. Effects of explosion energy and depth to the formation of blast wave and crater: field explosion experiment for the understanding of volcanic explosion. Geophysical Research Letters 28, 4287–4290. Imperial Earthquake Investigation Committee, 1918. Eruption of the Zao volcano (Kattadake or Kumanodake). Bulletin of Imperial Earthquake Investigation Committee 86, 150–153 (in Japanese). Imura, R., 1999. Zao volcano. In: Takahashi, M., Kobayashi, T. (Eds.), Field Guide of Volcanoes in Japan, 4, Volcanoes in Tohoku Districts. Tsukiji Press, Tokyo, pp. 70–88 (in Japanese). Japan Meteorological Agency, 2005. National Catalogue of the Active Volcanoes in Japan, Third edition. Japan Meteorological Business Support Center, Tokyo, 635 pp. (in Japanese). Kochibe, T., 1896a. An outline of research for the Zao explosion (sequel). Journal of Geography 88, 183–189 (in Japanese). Kochibe, T., 1896b. An outline of research for the Zao explosion (sequel). Journal of Geography 89, 239–244 (in Japanese). Kochibe, T., 1896c. An outline of research for the Zao explosion. Journal of Geography 90, 283–288 (in Japanese). Miura, K., Ban, M., Yagi, H., 2008. The tephra layers distributed around the Eastern Foot of the Zao volcano — ages and volumes of the Za-To 1 to 4 tephras. Bulletin of the Volcanological Society of Japan 53, 151–157.

157

Oba, Y., Konda, T., 1989. Geology and petrology of central Zao volcano, Yamagata prefecture. Bulletin of Yamagata University 12, 199–210 (in Japanese, with English Abstr.). Ohba, T., Taniguchi, H., Oshima, H., Yoshida, M., Goto, A., 2002. Research group on volcanic explosions. Effect of explosion energy and depth on the nature of explosion cloud—a field experimental study. Journal of Volcanology and Geothermal Research 115, 33–42. Ohba, T., Taniguchi, H., Miyamoto, T., Hayashi, S., Hasenaka, T., 2007. Mud plumbing system of an isolated phreatic eruption at Akita Yakeyama volcano, northern Honshu, Japan. Journal of Volcanology and Geothermal Research 161, 35–46. Okuno, M., 1995. Magnitude and frequency of phreatic eruptions, inferred from tephrastratigraphy. The Geographical Reports of Kanazawa University, 17, pp. 1–46. Rominger, J.F., 1954. Relationships of plasticity and grain size of Lake Agassiz sediments. Journal of Geology 62, 537–571. Sakayori, A., 1991. Magmatic evolution of Zao volcano, Northeast Japan. Bulletin of the Volcanological Society of Japan 36, 79–92 (in Japanese, with English Abstr.). Sakayori, A., 1992. Geology and petrology of Zao volcano. Journal of Mineralogy, Petrology and Economic Geology 87, 433–444 (in Japanese, with English Abstr.). Sekiya, S., Kikuchi, Y., 1890. The eruption of Bandai-san. Tokyo Imperial University College of Science Journal 3, 91–172. Sohn, Y.K., Chough, S.K., 1989. Depositional processes of the Suwolbong tuff ring, Cheju Island (Korea). Sedimentology 36, 837–855. Soya, T., Katsui, Y., Niida, K., Sakai, I., 1981. Geological map of Usu volcano (1: 25,000). Geological Survey of Japan. 10p (in Japanese). Takaoka, N., Konno, K., Oba, Y., Konda, T., 1989. K–Ar dating of lavas from Zao volcano, north-eastern Japan. Journal of Geological Society of Japan 95, 157–170 (in Japanese, with English Abstr.). Takarada, S., Yoshimoto, M., Kitagawa, J., Hiraga, N., Yamamoto, T., Kawanabe, Y., Takada, A., Nakano, S., Hoshizumi, H., Miyagi, I., Nishimura, Y., Miura, D., Hirose, W., Ishimaru, S., Kakihara, Y., Endo, Y., Yahata, M., Norota, S., Niida, K., Ishizuka, Y., Kudo, T., Aizawa, K., Honma, H., Egusa, M., Ishii, E., Takahashi, R., 2001. Volcanic ash falls from the Usu 2000 eruption and situation at the source area. Bulletin. Geological Survey of Japan 52, 167–179 (in Japanese, with English Abstr.). Takarada, S., Hoshizumi, H., Miyagi, I., Nishimura, Y., Miyabuchi, Y., Miura, D., Kawanabe, Y., 2002. Proximal deposits of the Usu 2000 eruption. Bulletin of the Volcanological Society of Japan 47, 645–661 (in Japanese, with English Abstr.). Tomiya, A., Miyagi, I., Hoshizumi, H., Yamamoto, T., Kawanabe, Y., Sato, H., 2001. Essential material of the March 31, 2000 eruption of Usu Volcano: Implication for the mechanism of the phreatomagmatic eruption. Bulletin. Geological Survey of Japan 52, 215–229 (in Japanese, with English Abstr.). Ui, T., Nakagawa, M., Inaba, C., Yoshimoto, M., Geological Party, Joint Research Group for the Usu 2000 Eruption, 2002. Sequence of the 2000 eruption, Usu Volcano. Bulletin of the Volcanological Society of Japan 47, 105–117 (in Japanese, with English Abstr.). Walker, G.P.L., 1971. Grain-size characteristics of pyroclastic deposits. Journal of Geology 79, 696–714. Walker, G.P.L., 1983. Ignimbrite types and ignimbrite problems. Journal of Volcanology and Geothermal Research 17, 65–88. Yamamoto, T., Nakamura, Y., Glicken, H., 1999. Pyroclastic density current from the 1888 phreatic eruption of Bandai volcano, NE Japan. Journal of Volcanology and Geothermal Research 90, 191–207.