Quaternary Volcanoes > Active Volcanoes > Hachijojima

3: History of subaerial portion of Hachijojima Volcano

   Hachijojima consists of two volcanic edifices, the Higashiyama Volcano and Nishiyama Volcano, as discussed above. The Higashiyama Volcano was active first, and the end of its active period overlaps with the early stage of the active period for the Nishiyama Volcano. The active period of the Kojima volcano was not well-known before. However, in the present geological map, based on dating results, it was discovered that the eruptive activities of this volcano had ceased by the middle of its active period. We summarize the history of these two volcanoes in this section.

3.1 Higashiyama Volcano
   The formation of the Higashiyama Volcano has been summarized by Tsukui et al. (1991), Suga (1994, 1998), and Sugihara (1998). The oldest volcanoes that constitute the subaerial part of the Higashiyama Volcano are heavily eroded edifices, such as Kurosaki volcanic rocks, Shioma volcanic rocks, Koiwato Volcano, Yokomagaura Volcano, and Mishoutai Volcano, which are found on the outer rim of the Higashiyama Volcano. In the present geological map, we refer to all of these as the Older Higashiyama Volcano. Over these volcanoes, a stratovolcano—the Nishihakuunzan Volcano—has developed at the center of the Higashiyama Volcano. Kaneoka et al. (1970) reported a K-Ar age of < 0.14 Ma for lava obtained from the Older Higashiyama Volcano.

   The Nishihakuunzan Volcano is a stratovolcano that forms the center of the present Higashiyama Volcano. At the summit, there is a semicircular basin open to the northwest, which is known as the Nishihakuunzan Caldera. The Nishihakuunzan Volcano in this geological map includes the main stratovolcano of Suga (1994) and includes products from the Noboryo Toge and Sueyoshi Stages. It consists of volcanic products from the Higashiyama Volcano before the formation of the Nishihakuunzan Caldera. The Noboryo Toge Stage consists mainly of the basaltic scoria fall, where coarse-grained volcanic ash is dominant at the top. The Ishizumigahana lava exposed at Ishizumigahana belongs this stage. The Sumiyoshi Stage, which is equivalent to the final stage of the Nishihakuunzan Volcano, is characterized by large-scale andesitic–dacitic pumice flow and pumice fall, where 29-ka Aira Tn volcanic ash is interbedded (Sugihara and Oda, 1989). As such, the formation of the Nishihakuunzan Volcano is assumed to be approximately 30,000 yr BP. These pyroclastic deposits have the largest volume among those of the Nishihakuunzan Volcano, and this deposit may be associated with the formation of the Nishihakuunzan Caldera.

   Mihara Volcano (Suga, 1994) is a stratovolcano that developed from inside the Nishihakuunzan Caldera to the southern foot of the Higashiyama volcano. It mainly consists of basaltic lava flows and pyroclastic materials. In this geological map, it includes products from the Nakanogo and Mitsune Stages of Suga (1994). The Nakanogo Stage is characterized by the eruption of basaltic lava flows and scoria from the summit or flank. The Mitsune Stage consists of pumice fall, scoria fall and lava flow associated with scoria cone formation, and products of phreatic eruption, all from flank eruption.

   Much of the Kashidate and Nakanogo areas at the southern foot of the Higashiyama Volcano are on the gentle slope consisting of the Nakanogo Stage products from the Mihara Volcano (Suga, 1993). Carbon-14 dating of this ejecta indicated that the formation age is approximately 10–15 ka (Sugihara, 1998). The sea cliffs around Nakanogo have outcrops of Nakanogo lava (Suga, 1994) and Nakanogo pyroclastic deposits overlying the lava (Suga, 1994) from the Nakanogo Stage. The Nakanogo lava flows consist of multiple flow units, and its thickness exceeds 20 m. The Nakanogo pyroclastic deposit is a block and ash flow deposit that contains a large amount of poorly-vesiculated essential basaltic clasts. The Mihara lava flow from the summit crater occurs in the east of the Nakanogo settlement to the Aigae Port (Tsukui et al., 1991). To the west of the Kashidate settlement, Nakonohana Scoria erupted from the shore of the western foot of the Higashiyama Volcano (Tsukui et al., 1991). At the summit of the Mihara Volcano, there is a crater with a diameter of 1.3 × 0.9 km with small cinder cones inside. In this geological map, we refer to them as pyroclastic deposits inside the Mihara Volcano crater. The activities of these small cinder cones correspond to the final stage of activities of the summit crater, and are linked to the Mihara Red Ash (Suga, 1993) found all over the foot of the volcano. The Mihara Red Ash erupted at approximately 10.5 ka (Sugihara, 1998).

   After the eruption of the Mihara Red Ash, there were multiple flank eruptions. The major flank eruptions in the final stage of the Mihara Volcano activities include the Higashihakuunzan Pumice Fall (Tsukui et al., 1991), with the likely source located on the northern foot of the Higashiyama Volcano; Hachimanyama scoria fall (Suga, 1993) and Hachimanyama lava (Isshiki, 1959) from the Hachimanyama Scoria Cone at the center of the Kashidate settlement; Miharadaki Volcanic Ash Fall and tuff breccia from the southern flank of the Higashiyama Volcano (Suga, 1993); Myohoji Pumice Fall from the north of Kashidate settlement (Tsukui et al., 1991; Suga, 1993); and Mizuumiyama Pumice Fall from Higashihakuunzan (Suga, 1993). In this geological map, we show distribution of Hachimanyama Scoria Cone Deposits, Hachimanyama lava, and Miharadaki Volcanic Breccia. Among the final stage of the Mihara Volcano, the Higashihakuunzan Pumice Fall to Mizuumiyama Pumice Fall are interbedded with the ashes of the Mitsune Stage in the early phase of activity of the Nishiyama Volcano, which is described later ( Figure 3). Therefore, the final stage of the Higashiyama Volcano activity and the early stage of the Nishiyama Volcano activity overlapped in time.

3.2 Nishiyama Volcano
   Eruption history of the Nishiyama Volcano had been partially reported. However, its entire history remained unclear. In this geological map, we classified the activities of the Nishiyama Volcano into the Mitsune Stage, Senjojiki Stage, Okoshigahana Stage, and Fujitozando Stage based on their mode of activities. Small volcanoes on the seafloor around the Nishiyama Volcano are most likely satellite cones of the Nishiyama Volcano. However, as these are difficult to correlate with subaerial activities, we will describe them together.

Mitsune Stage
   The earliest activity of the Nishiyama Volcano, based on volcanic deposits on land, is the Mitsune Stage characterized by phreatomagmatic eruption. The Mitsune Stage mainly consist of poorly-vesiculated scoriaceous volcanic ash interbedded in the uppermost unit of the Higashiyama Volcano ( Figure 3), and seven units of Mitsune Ash (1 to 7) have been identified (Suga, 1993). The eruption center during the Mitsune Stage are most likely located northwest of the Higashiyama Volcano, i.e., the present Nishiyama Volcano, based on the distribution of the ashes (Suga, 1993). There were also eruptions from Kandoyama on the eastern foot of the Nishiyama Volcano, from near Yokomagaura on the flank of the Higashiyama Volcano and from near Yaene Port on the southeastern foot. These eruptions erupted Kandoyama lava, Kandoyama Volcanic Breccia, Yokomagaura Volcanic Breccia, Yaene Volcanic Breccia, and Yaene Ash.

   The activities of the Mitsune Stage are supposed to be mainly phreatomagmatic eruptions based on components of the products. Based on the relative age of the ejecta sequence from the Higashiyama Volcano, the eruption age of the Higashiyama Volcano tephra (Mihara Red Ash) immediately below Mitsune Ash 1, which is first (oldest) ash of the Mitsune Stage, is assumed to be approximately 10,000 yr BP (Sugihara, 1998). In this mapping survey, shells contained in the base of volcanic breccia constituting Kandoyama on the eastern foot of the Nishiyama Volcano provided a carbon-14 age of approximately 10,000 yr BP ( Table 1）. These ages indicate that the phreatomagmatic eruption activities in the Nishiyama Volcano area began at approximately 10,000 yr BP. Carbon-14 age of approximately 4400 yr BP has been obtained for the soil immediately below the Mizuumiyama Pumice, which underlies the Mitsune Ash 7 (MA7), the youngest ash from a series of phreatomagmatic eruptions in the earliest stage of the Nishiyama, and also a carbon-14 age of 3150 ± 30 yr BP from the base of the Yaene stage on the southern foot of the Nishiyama Volcano has been obtained ( Table 1). Since products after the Mitsune stage are supposed to have erupted subaerially, the Nishiyama Volcano is estimated to have emerged above sealevel by 4000–3000 yr BP, which corresponds to the Senjojiki Stage.

Senjojiki Stage
   In the Senjojiki Stage, a large volume of basalts erupted from the summit and satellite vents of the Nishiyama Volcano between 3000 and 1000 yr BP, forming the main part of the stratovolcano of the Nishiyama Volcano. The urban areas of Hachijo-machi are found on the gentle slope created by the multiple lava flows of this stage. These lava flows are widely exposed from the Yaene Port to Minamihara Senjojiki on the west coast and from Sokodo to Kaminato ports on the east coast. These lava flows well preserve original morpohology such as levee. Based on the distribution, these lava flows erupted from the mid-flank or higher on the Nishiyama Volcano. However, as the top of the lava flows is overlain by younger ejecta, it is unknown whether these lava flows erupted from the summit crater or from the lateral fissure near the summit. Soil found between the lava flows of Minamihara Senjojiki and soil under the lava flow in the south of Hachijo Botanical Park yielded ages between 3000 and 2000 yr BP ( Table 1).

   On the flank of the Nishiyama Volcano, there are multiple cinder cones that formed because of fissure eruption. These cinder cones have a radial distribution, with the summit crater of Hachijo-Fuji at the center. The carbon-14 age of charred plant fragments found at the base of the ejecta on a cinder cone located by the road heading from the base to Noboryo Toge was 930 ± 20 yr BP ( Table 1). On the southern foot of the cinder cone known as Tokusato Tonbu in Hachijo Botanical Park, charred plant fragments found immediately below a spatter layer yielded a carbon-14 age of 1650 ± 20 yr BP ( Table 1).

   In the area of the Higashiyama Volcano, several layers of scoria falls that erupted from the Nishiyama Volcano were found overlying the Mitsune Stage products. On the sea cliff near Funatsuk ihana, a scoria flow deposit of this stage can be found. These observations indicate that in this stage, some large-scale scoria eruptions took place in addition to lava flows.

Okoshigahana Stage
   The Okoshigahana Stage is characterized by large-volume lava flow from the summit of the Nishiyama Volcano that began at approximately 700 yr BP followed by scoria eruptions. Lava from the summit of the Nishiyama Volcano flowed in almost every direction away from the volcano, reaching Okoshigahana and Imazaki Beach (Okoshigahana Lava). These lava flows were followed immediately by a relatively large-scale pyroclastic eruption at the summit of the Nishiyama Volcano, and the scoria fall deposited on the edge of the crater of the Nishiyama Volcano and the northwestern part of the Island (Okoshigahana Scoria fall). The scoria fall on the summit was strongly welded, and its thickness exceeds 20 m. This was followed by an eruption of Akasari lava from the summit crater, which reached Akasari Beach in the area northeast of the Nishiyama Volcano. Charcoal fragments found under these lava flows and scoria falls yielded a carbon-14 age of 700 yr BP ( Table 1 ). At the end of the Okoshigahana Stage, the crater observed at the summit of the Nishiyama Volcano at present is presumed to have formed.

Fujitozando Stage
   After the large-scale eruption from the summit of the Nishiyama Volcano, eruptions continued around the summit crater and southeast flank until approximately 400 yr BP. This period is named as the Fujitozando Stage. In this stage, multiple lava flows were emplaced from the summit crater toward the southeast, and multiple eruption fissures formed on the flank.

   Debana Lava is aa lava that erupted from a fissure that formed on the northwestern flank of the Nishiyama Volcano. It overlies the Okoshigahana lava group. The stratigraphic relationship with other Fujitozando Stage prouducts is unknown.

   Osarigahana Lava is pahoehoe lava that erupted from the summit of the Nishiyama Volcano. It can be found on the shore north of Osarigahana and along the Fuji Forest Road. A lava tree mold at the base of this lava flow yielded a carbon-14 age of 360 ± 20 yr BP ( Table 1).

   The Osarigahana Pyroclastic Flow deposit is distributed from the summit of Hachijo-Fuji to Osarigahana Beach on the eastern flank, containing a large amount of poorly-vesiculated scoria and clasts. Most clasts are lava fragments, but characterically accompanied by abundant gabbroic clasts. This deposit contains poorly-vesiculated cauliflower scoria without any charred material within deposits, and did not receive high-temperature oxidization. Therefore, this deposit is most likely low-temperature pyroclastic flow deposit caused by phreatomagmatic eruption. Charred wood fragments caught in lava flows directly underlying pyroclastic deposits along the Fuji Kanjyo forestry road yielded a carbon-14 age of 360 ± 20 yr BP ( Table 1).

   Idesarigahana lava is pahoehoe lava found on the north side of Kandoyama on the eastern flank of the Nishiyama Volcano up to Idesarigahana Beach to the north. It erupted from the summit of the Nishiyama Volcano. Charred wood found in the soil overlain by this lava yielded a carbon-14 age of 250 ± 20 yr BP. However, this age is inconsistent with the stratigraphy ( Table 1). This lava overlies the above-described Osarigahana pyroclastic deposit and is equivalent to a part of Ny3L described by Sugihara and Shimada (1998).

   The Idesarigahanaue lava is a lava flow from the eastern rim of the summit crater on the Nishiyama Volcano, reaching the western foot of Kandoyama. It erupted from the summit of the Nishiyama Volcano. This lava overlies the above-mentioned Idesarigahana lava and is also equivalent to a part of Ny3L described by Sugihara and Shimada (1998).

   Frijiaen Lava is pahoehoe lava erupted from southeastern flank at an elevation of approximately 266 m, on the upper slope from the Hachijojima Airport. The eruption age of this lava is unknown but it overlies Okoshigahana lava and is overlain by Fujitozando Scoria.

   The Funatsukihana Scoria Cone deposit and Funatsukihana lava are a scoria cone deposit and small-scale lava flow found near an eruption fissure with total length of 1.5 km, stretching from the flank of the Nishiyama Volcano to near Funatsukihana. Near Funatsukihana, there is a welded scoria with a thickness of approximately 15 m. Soil overlain by the scoria yielded a carbon-14 age of 310 ± 20 yr BP ( Table 1).

   The Fujitozando Scoria Cone deposit, scoria fall, and lava are the youngest product on the flank of the Nishiyama Volcano based on stratigraphic sequence. This deposit overlies the Idesarigahanaue lava and Frijiaen lava. Charred wood found at the base of the scoria fall yielded a carbon-14 age of 400 ± 30 yr BP ( Table 1). This deposit is equivalent to Ny4, which is from 1605AD eruption, according to Sugihara and Shimada (1998). This deposit erupted from scoria cones found along Fujitozando from the airport to Hachijo-Fuji. This scoria fall is found to the east of the scoria cones and can be traced to the Kaminato Port. Lava erupted from the middle of the eruption fissure flowed to the south, and reached areas of the airport and the Hachijo Botanical Garden. Inside the Nishiyama Volcano summit crater, a lava dome with a flat top with a diameter of approximately 400 m occurs. The formation age of this lava dome is unknown; however, as it is not overlain by Fujitozando Scoria, it was most likely formed at the end of the Fujitozando Stage.

   Submarine secondary cones around Nishiyama Volcano Volcanic chains extend from the Nishiyama Volcano to the NNW on the seafloor north of the Hachijojima are chains of satellite cones of the Nishiyama Volcano based on their distribution, petrography, and chemical compositions (Ishizuka et al., 2008). These satellite cones that constitute the volcanic chain are covered by pumice, which was erupted from the Kurose-Nishi Hole over a wide area. However, in the upper edifice of some of the cones, basaltic scoria overlies the pumice layer. This indicates that activities of the satellite cones took place before and after the event in which the Kurose-Nishi Hole erupted a large amount of pumice. The formation age of these satellite cones is unknown. However, since the seafloor observation did not confirm any sediments above pumice or basaltic scoria, these must be quite recent.

   Near the summit of these satellite cones, agglutinate and volcanic bombs accumulate to form a steep slope, and some volcanic bombs exceed a diameter of 1 m. The survey of a satellite cone relatively close to the Nishiyama Volcano confirmed multiple pillow basalts that flowed down the slope. Both volcanic bombs and lava consist of aphyric basalt, and are supposed have resulted from a single event of eruptive activity.

   A profile from a seismic reflection survey indicated that satellite cones that constitute the volcanic chains are all at the top of the sediments in this area. This implies that these cones formed by very recent activities ( Figure 4a). A series of normal faults with small displacement is found in the area around the volcanic chains. This indicates that the area where the volcanic chains occur is an extensional field with large horizontal differential stress. Magma transported from the Nishiyama Volcano in a direction perpendicular to the direction of the minimum compressional stress is supposed have erupted and formed the volcanic chains (Ishizuka et al., 2008).

   Small volcanoes found on the seafloor to the northeast side is also considered a secondary cone of the Nishiyama Volcano, where several large volcanic bombs have been confirmed near the crater. This volcano consists of basalt rich in plagioclase phenocrysts extremely similar to ejecta found on the subaerial part of the Nishiyama Volcano. Although the eruption age is unknown, it was most likely active at the same time as the Nishiyama Volcano.

3.3 Kojima Volcano
   The Kojima Volcano is a part of the Hachijojima Volcano and is an independent stratovolcano located to the west of Hachijojima. The formation age of Kojima is unknown, but the presence of multiple scoria falls erupting from the Nishiyama Volcano interbedded in the soil that overlies the Kojima Volcano products and the carbon-14 age of the base of the soil indicate that the main part of the volcano formed by 3000 yr BP ( Table 1). The Kojima Volcano mainly consists of alternating layers of lava and pyroclastic deposits, and agglutinate dominates in the central part of the edifice exposed on the cliffs in the northeast and southwest. Multiple, relatively thick lava erupted from the summit onto the beaches located in the northwest and southeast.

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