Quaternary Volcanoes > Active Volcanoes > Zao

4: Eruptive history of Zao Volcano

   The activities of Zao Volcano were divided into six categories based on age, rock type, and activity style ( Figure 2 ). These include Stage I (ca. 1Ma), with the formation of the Robanomimiiwa volcanic edifice, which are aqueous eruption products consisting of tholeiitic basalts; Stage II (ca. 500 ka), with the formation of the andesitic Torikabutoyama–Yokokurayama volcanic edifice in the north region; Stage III (ca. 350–250 ka), with the formation of the Old Kumanodake volcanic edifice, which is a group of stratovolcanoes consisting of andesite to dacite, and the basaltic andesite Nakamaruyama volcanic edifice; Stage IV (ca. 250– 200 ka), with the formation of the Kattadake volcanic edifice, an andesite stratovolcano; Stage V (ca. 130–40 ka): with the formation of the Kumanodake–Jizosan volcanic edifice including andesite stratovolcanoes; and Stage VI (ca. 35 ka to the present), the most recent activity stage, with the formation of the Okama-Goshikidake volcanic edifice consisting of basaltic andesite pyroclastics. The estimated total volume of eruption products is about 25 km³ (Committee for Catalog of Quaternary Volcanoes in Japan, 1999). The volume was converted to DRE and we will use the newly estimated volumes for each edifice and each unit hereafter. In addition, the name of each unit was determined as follows: “Lava and Pyroclastics” indicate those formed through a continuous eruption without a geologically dormant period, whereas “Lavas and Pyroclastics” indicate those that formed through multiple eruptions.

4.1 Stage I: Formation of Robanomimiiwa Volcanic Edifice (ca. 1 Ma)
   The eruption products of Stage I are widely distributed around Robanomimiiwa in the central area and around the source area of Maruyamasawa and Goshikizawa and consist of basaltic pyroclastic rocks, dikes, and sills. These eruption products were previously divided into Robanomimiiwa pyroclastic rocks and Maruyamasawa pyroclastic rocks (Sakayori, 1992).In this map, they are grouped together here as Robanomimiiwa Pyroclastics and Dikes. The thickness of these pyroclastic rocks reach up to 200 m. Most of this unit show hyaloclastite facies which consists of lapilli and volcanic blocks with low vesicularity filled with fine matrix of the same composition. Nearly horizontal beddings are identified by changes in grain size of rock fragments. Elongated volcanic bombs with chilled margins up to meters in scale are sometimes found. Dikes that intruded the pyroclastic rocks extend north–northwest to south–southeast and intrude the pyroclastic rocks almost vertically. The width ranges from dozens of centimeters to several meters. In the vertical sections of some dikes near the Robanomimiiwa, dikes diverge upward with partially autobrecciated and gradually transformed into hyaloclastite-like pyroclastic breccia. At the source of Goshikizawa, sills and dike complex is observed. The pyroclastic rocks were formed through the aqueous eruptions of basaltic magma, and the dikes show the fossils of the vents. There is no evidence of seawater invading this area at that time; it is likely that a lake was present prior to the activity of Zao Volcano. Erosion has advanced, and the area is covered by subsequent eruption products; therefore, accurate reconstruction of the volcanic edifice and old landforms at the time of formation is difficult. Ages obtained from the rocks of this volcanic edifice indicate approximately 1.8 Ma to 700 ka (Takaoka et al., 1989; Yamasaki et al., 2014). These age data show a large scatter and may be overestimated because these samples are from dikes. Thus, the accurate formation age is unknown. In this geological map, we consider the formation of the Robanomimiiwa Volcanic Edifice as around 1 Ma. The total volume of the existing eruption products is estimated to be about 3 km³.

4.2 Stage II: Formation of Torikabutoyama–Yokokurayama Volcanic Edifice (ca. 500 ka)
   The eruption products of this stage is distributed in the areas of Torikabutoyama, Gorodake, Sanpokojinsan, and Yokokurayama in the northwest region of the map. It consists of Yokokurayama, Torikabutoyama, Gorodake, and Sanpokojinsan lavas; the rock types of the first three lavas are similar. Owing to subsequent collapses and erosions, the original shape of the volcanic edifice has been lost. The age is about 500 ka, and the total volume is estimated to be approximately 3 km³.

4.3 Stage III: Formation of Old Kumanodake and Nakamaruyama Volcanic Edifice (ca. 350–250 ka)
   The Old Kumanodake Volcanic Edifice, which is exposed mainly in the eroded area east and west of Kumanodake, and the Nakamaruyama Volcanic Edifice, which forms Nakamaruyama were formed in this stage.

4.3.1 Old Kumanodake Volcanic Edifice
   The age is approximately 350–250 ka, and the total volume is estimated to be approximately 12 km³. Eruption products are andesite–dacite, which are divided into Zaosawa Lavas, Senninzawa Lavas and Kaerazunotaki Lavas. Other than the block-and-ash-flow deposit in the lower Senninzawa Lavas, all units consist of lavas. Each layer of the Zaosawa Lavas is thick, exceeding 100 m at Zaosawa; Senninzawa Lavas are about 10 m thick, and Kaerazunotaki Lavas are about 10–30 m. The distribution area of Zaosawa Lavas is wide, and the trend in the whole-rock composition differs slightly among areas. The eruption center likely differed for each layer of lava, and the timing of the eruption likely varied slightly.

4.3.2 Nakamaruyama Volcanic Edifice
This volcanic edifice consists of basaltic andesite Nakamaruyama Lavas. The age of activity is approximately 270 ka and the volume is estimated to be approximately 0.12 km³. The thickness of most of lava is limited to several meters but some reaches about 10 m. The vent is assumed to be near the present Nakamaruyama summit.

4.4 Stage IV: Formation of Kattadake Volcanic Edifice (ca. 250–200 ka)
   The Kattadake edifice was formed in the south part, composed of many lava flows. The age is approximately 250–200 ka and the total volume is estimated to be approximately 3.6 km³. To the west of Kattadake, the following three units occur from the bottom to the top: Bodaira Lava, Ichimaiishizawa Lavas, and Odanokami Lava. To the east, Sainokawara Lavas, Seizandaira Lavas, Kanabukisawa Lavas, and Zao Echo Line Lavas occur from the bottom upward. Kattadake Lava at the center covers Odanokami Lava and Seizandaira Lavas, and Zao Echo Line Lavas lies above Ichimaiishizawa Lavas based on distribution. The thickness of individual lava is usually thick; for example, Bodaira Lava is more than 100 m thick. All lavas are assumed to have erupted from the present-day Kattadake or slightly to the north. The flow distances of the lavas in latter stage are shorter than that of the lavas in early stage. The duration of the Stage IV was short as about 50 kyrs, and the eruption rate is relatively high at approximately 0.1 km³/kyrs ( Figure 3).

4.5 Stage V: Formation of Kumanodake–Jizosan Volcanic Edifice (ca. 130–40 ka)
   About 130 ka, activity began with a crater near Kumanodake-Jizodake, forming a stratovolcano containing a large amount of pyroclastics. From the bottom upward, it consists of Kumanodake–seiho Lavas and Pyroclastics; Kanshodaira Lavas, Jizosan Lava, and Jizosan–higashi Lava; Kumanodake Main Edifice Lavas and Pyroclastics; Umanose-kabu Lavas and Pyroclastics; and Kumanodake–sancho Lavas and Umanose Lava. The volume is estimated to be approximately 2.2 km³.

   The Kumanodake–seiho Lavas and Pyroclastics are composed mainly of tuff breccia or agglomerates in multiple layers approximately several meters thick, as distinguished by graded structures. At least three layers of lavas of 10 m or less can be observed in the upper part. Kanshodaira Lavas in the upper part and Jizosan Lava above these lavas are composed mainly of lavas. Jizosan Lava is covered by Jizosan–higashi Lava; Jizosan–higashi Lava is composed of agglomerates near the eruption center and lavas in the distance. The ages are approximately 130–90 ka for these units. The Kumanodake Main Volcanic Edifice Lavas and Pyroclastics are exposed in the lower part of the Umanose Caldera wall and consist of andesitic lavas with pyroclastic rocks. Lavas in the lower caldera wall yield an age of approximately 90 ka. The Umanose-kabu Lavas and Pyroclastics overlie the Kumanodake Main Volcanic Edifice Lavas and Pyroclastics with an angular unconformity. These consist of tuff breccia, agglomerates, and lava with an age of approximately 70 ka. The Kumanodake-sancho Lava consists of agglomerates and agglutinates that contains volcanic bombs about 2 m in size near the vent. In the distance, it consists solely of lava. Umanose Lava mainly consists of lava, although agglomerates appear in some parts at higher elevations; the age is about 40 ka. The duration of Stage V is about 90 kyrs, and the eruption rate is about 0.02 km³/kyrs, which is low compared to that for Stage IV.

4.6 Stage VI: Formation of Okama‒Goshikidake Volcanic Edifice (ca. 35 ka to the present)
   Stage VI began about 35 ka (Miura et al., 2008; Ban et al., 2008; Takebe and Ban, 2011), and the centers of the eruption would be formed in several locations near Okama–Goshikidake inside of the Umanose Caldera. Pyroclastics are dominated with subordinate lavas. Pyroclastics of this stage divided into Kumanodake, Komakusadaira, and Kattadake Pyroclastics; Umanose Agglutinate; and Goshikidake Pyroclastics (Ban et al., 2008) from lower to upper. Lavas are Goshikidake–toho Lava, Kattadake–hoppo Lava, Nigorikawa Lava, Furikotaki Lava, and Goshikidake–nannpo Lava. The first three lavas form the lower part of the Kattadake Pyroclastics, whereas the latter two form the lower part of the Goshikidake Pyroclastics. The total volume of this stage is 0.74 m³, which is less than 10% of the total volume of Zao Volcano.

   Kumanodake Pyroclastics consists the eastern half of the Kumanodake summit area. This unit is composed of agglutinates, agglomerates, lapilli tuff, and tuff breccia. The total thickness of the layers is more than 20 m, although the thicknesses of the individual layers are mostly 20 cm or less. These rocks overlie the Kumanodake–sancho Lava of Stage III and are covered by the Komakusadaira Pyroclastics. The volume is estimated to be 1.9 × 10-3 km³. The Komakusadaira Pyroclastics consist of multiple pyroclastic layers composed of agglutinates, volcanic breccia, tuff breccia, lapilli tuff, and tuff. The total thickness is more than 30 m, and the volume is estimated to be about 0.03 km³.

   The Komakusadaira Pyroclastics is truncated by the wall of the Umanose Caldera; these rocks are not found inside the caldera. On the other contrary, pyroclastics younger than the Kattadake Pyroclastics are observed from the edge to the inside. Therefore, the formation of the Umanose Caldera was after that of the Komakusadaira Pyroclastics and prior to the Kattadake pyroclastics activity. The Umanose Caldera would be formed through a single large-scale collapse (e.g., Konda and Oba, 1985), however, debris avallanche deposits corresponding to such event have not been observed. The amount of eruption products emitted during the activities of the Komakusadaira Pyroclastics is about 0.3 km³ including the tephra, which is insufficient for creating a collapsed type caldera. Most likely, the Umanose Caldera is an erosion caldera that was originally eruption craters at the Komakusadaira Pyroclastics eruption, and expanded through repeated small-scale collapses and erosion.

   The Nigorikawa Lava is locally present on the opposite side of the Gaga Hot Spring with a maximum thickness of about 20 m and directly covers a gravel layer. The Goshikidake–toho Lava is found mainly in an area about 400–700 m east of Goshikidake. The thickness reaches ca. 50 m and is overlain by the Goshikidake Pyroclastics, Umanose Agglutinate, and Kattadake Pyroclastics. The Kattadake–hoppo Lava is observed to the southwest part of the Umanose Caldera wall at an elevation of approximately 1,500 m. Based on the elevation at which this unit destributes, it likely erupted before the caldera became deep. The maximum thickness is ca. 5 m and becomes thinner toward the north. Directly below the lava, a scoria fall deposit, 1 m in thickness consisting of scoria (~5 cm) is present. This scoria deposit is also included in this unit. This lava is overlain by the Kattadake Pyroclastics. The rock types of these three lavas are similar, and mafic inclusions are not present in any of them. The combined volume of these three lavas is about 6 × 10-3 km³.

   The Kattadake Pyroclastics consist of agglomerates, volcanic breccia, tuff breccia, lapilli tuff, and tuff with a total thickness of at least 30 m and an estimated volume of 1.5 × 10-3 km³. The Umanose Agglutinate is found mainly near Umanose but also occurs locally in east part of Goshikidake. This unit is at least 10 m thick and consists of agglutinates, agglomerates, and scoria fall layers. The volume is estimated to be about 1 × 10-3 km³.

   The Furikotaki Lava is found continuously from the base of the Goshikidake Volcanic Edifice, about 400 m north of the present-day Goshikidake summit, to Furikotaki, and also occurs in a small scale in eastward of Furikotaki. The Goshikidake–nanpo Lava is found on the opposite side of southern Goshikidake across Nigorisawa. The lower part consists of hyaloclastite-like tuff breccia. The upper part consists of aqueous autobrecciated lava. The vertical coarse joints from which extend a large number of fine joints in the vertical direction constitute false pillow lava (Watanabe and Katsui, 1976). The rock types of Furikotaki and Goshikidake–south lavas are quite similar. Both are covered by the lower part of the Goshikidake Pyroclastics; however their stratigraphic relationships with the pyroclastic rocks of Stage VI below the Umanose Agglutinate are unknown. The combined volume of these two lavas is estimated to be about 0.75 × 10-3 km³.

   The Goshikidake Pyroclastics formed a pyroclastic cone to create Goshikidake. These rocks consist mainly of pyroclastic rocks such lapilli tuff, pyroclastic breccia, tuff breccia, and agglutinates with an estimated volume of 0.015 km³. During the Goshikidake Pyroclastics activity, the eastern part of Goshikidake collapsed to form the Nigorikawa Debris Avalanche Deposit.

   According to previous studies on the volcanic stratigraphy (Imura, 1994; Geological Survey, 2000; Ban et al., 2005; Miura et al., 2008; Kawano et al., 2014), 21 scoria lapilli–volcanic ash layers are recognized corresponding to the most recent activity stage. Layers 1 to 16 include Zao–Togatta tephra (Za–To), and Za–To 5 is subdivided into six layers ( Figure 4). It has been estimated that Za–To 1–4 (ca. 33–13 ka) were formed during the formation of the Kumanodake, Komakusadaira, and Kattadake Pyroclastics, whereas Za–To 5–8 (ca. 9–4.1 ka) formed during the Umanose Agglutinate activity, and Za–To 9–16 (ca. 2 ka) was formed during the Goshikidake Pyroclastics activity (Ban et al., 2005; Kawano et al., 2014). The tephra layers occurring widely in the foot of the mountain far from the volcanic edifice contain mostly Za–To 1–4; their isopachs are shown in Figure 5. The volume of these tephra layer are 0.3–0.03 km³ (Miura et al., 2008), and volcanic explosivity index (VEI; Newhall and Self, 1982) of the eruption is 4–3. The largest eruption during Stage VI was Za–To 2, which is comparable to the Komakusadaira Pyroclastics; the total volume of the eruption products reachs 0.3 km³. Za–To 2 is observed on the foot of the mountain as a scoria fall layer with coarse-grained volcanic ash layers. The main axis of distribution of this scoria layer, known as Zao Kawasaki Scoria (Itagaki et al., 1981), is east–northeast. In Sendai city about 40 km from the vent, this layer has a maximum thickness of 2 cm (Itagaki et al., 1981). Based on the scale, this layer is assumed to be by quasi-Plinian eruption. The age is about 30 ka (Nagatomo et al., 2005; Miura et al., 2008).

   The vent location moved from south of the present-day Goshikidake summit to Okama since the activity of Za–To 11. By the radiocarbon ages, stratigraphic relationships, and petrologic compositions of the eruption products, Za–To 11 is likely to equivalent to the 1230 eruption. Also the ages and the other evidence show that Za–To 14 is from a 17th century eruption, Za–To 15 is from eruptions at the end of the 18th century to the first half of the 19th century, and Za–To 16 is from an eruption during the mid-19th century. Grey–white tephra likely from the 1895 eruption is found above Za–To 16 with paleosoil in between.

   The volume of Stage-VI products shown in Figure 3 includes the tephra. The average eruption rate during Stage VI is about 0.03 km³/kyrs, which is similar to that of Stage V. Regarding the ages of Kumanodake, Komakusadaira, and Kattadake Pyroclastics, the time interval between each eruption was long compared with those of following two pyroclastic eruptions, although the scale of each eruption is large. Comparing the Goshikidake Pyroclastics and Umanose Agglutinate activities, the former had shorter interval between eruptions and the scale of each eruption is smaller than the latter; the eruption rate is slightly higher in the former.

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