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[SVC34-02] Structure and formation history of Akita-Komagatake Volcano, Northeastern Japan
Keywords:Volcano geology, Red relief image map, Correlation between tephra stratigraphy and the volcanic edifice, Akita-Komagatake volcano, Sr-Nd-Pb isotopic ratios
The structure and formation history of a volcanic edifice are fundamental information for understanding volcanic processes. However, revealing the long-term eruption history and the structure of the edifice is generally not an easy task. This study proposes diverse volcano geological research methods, applying to Akita-Komagatake Volcano, located in NE Japan. The volcano is an active volcano, with continuing activity since late Pleistocene to the present. Calderas were formed on both the northern and southern slopes of the edifice, which are referred to as the southern and northern calderas, respectively. The post-caldera activity has occurred within both calderas. Although many geological studies on the volcano have been conducted, there is still confusion regarding the development history of the edifice and the correlation between tephra and the proximal deposits.
In this study, using the red relief image map based on the LiDAR with 1-meter resolution, we examined the topography of the edifice, such as craters, distribution of eruptive deposits, their coverage relationships, and the surface topographical features. Based on the topographical analyses, a geological survey was efficiently conducted. In addition, we also carried out trench surveys and borehole drilling. These approaches enabled us to clarify the stratigraphy of the volcano and collect well-documented samples. On the other hand, tephrostratigraphy is important for constructing a chronological framework into the eruption history. The tephrostratigraphy of the volcano since the latest Pleistocene was clarified by Wachi et al. (1998), who identified 14 tephra units, labelled AK1 to AK13 in descending order. The correlation between tephra and the edifice has been widely conducted in previous studies, using lithological analysis and geochemical data, such as major element chemistry of whole-rock and volcanic glass samples. In this study, in addition to the conventional data, multi-parameter geochemical data, including trace elements, REE, and Sr-Nd-Pb isotopic ratios, were used. The tephra units can be divided into two types based on the 87Sr/86Sr ratio: 0.70395–0.70406 (low Sr type) and 0.70409–0.70433 (high Sr type). AK13-12 was classified as high Sr type, AK11-8 as low Sr type, and AK7-1 as high Sr type again. Furthermore, regarding on the Nd isotope ratio, it was found that AK13-12 could be distinguished from AK7-1 by its lower Nd isotope ratio, even though both belong to the same high Sr type. Even within the same Sr type, differences in major and trace chemical components are also recognized among the tephra units. Thus, by considering the diversity of magmatic types and their temporal variations, we could accurately correlate the tephra with the edifice.
The eruption history of the volcano, revealed by this study, is as follows. Two large explosive eruptions around 14-13 ka formed the southern caldera (AK13-12). The activity caused a sector collapse to form the northern caldera, in which post-caldera activity began: the Katakura-dake cone was formed between 12 and 9.5 ka (AK11-9), and an explosive eruption occurred at 9 ka (AK8), producing pyroclastic flows. Afterward, Onamedake cone was formed, and lava effusion continued (AK7). Then, at 7 ka, an explosive eruption occurred, forming a crater and generating pyroclastic flows (AK6). Phreatic eruptions occurred within the crater at 3.5 ka (AK5), and a pyroclastic cone was formed at 2.5 ka (AK4). Since then, activity shifted to the southern caldera; at 2 ka, phreatomagmatic eruptions occurred, forming the old Kodake pyroclastic cone (AK3). Around 1.5 ka, the Medake cone became active (AK2), followed by Kodake (AK1). In these activities, strombolian eruptions continued with lava effusion. Although activity ceased over 1000 years ago, it resumed in the 19th century, with 1932 and 1970–71 eruptions. Future research using similarly diverse methods is expected to advance further.
In this study, using the red relief image map based on the LiDAR with 1-meter resolution, we examined the topography of the edifice, such as craters, distribution of eruptive deposits, their coverage relationships, and the surface topographical features. Based on the topographical analyses, a geological survey was efficiently conducted. In addition, we also carried out trench surveys and borehole drilling. These approaches enabled us to clarify the stratigraphy of the volcano and collect well-documented samples. On the other hand, tephrostratigraphy is important for constructing a chronological framework into the eruption history. The tephrostratigraphy of the volcano since the latest Pleistocene was clarified by Wachi et al. (1998), who identified 14 tephra units, labelled AK1 to AK13 in descending order. The correlation between tephra and the edifice has been widely conducted in previous studies, using lithological analysis and geochemical data, such as major element chemistry of whole-rock and volcanic glass samples. In this study, in addition to the conventional data, multi-parameter geochemical data, including trace elements, REE, and Sr-Nd-Pb isotopic ratios, were used. The tephra units can be divided into two types based on the 87Sr/86Sr ratio: 0.70395–0.70406 (low Sr type) and 0.70409–0.70433 (high Sr type). AK13-12 was classified as high Sr type, AK11-8 as low Sr type, and AK7-1 as high Sr type again. Furthermore, regarding on the Nd isotope ratio, it was found that AK13-12 could be distinguished from AK7-1 by its lower Nd isotope ratio, even though both belong to the same high Sr type. Even within the same Sr type, differences in major and trace chemical components are also recognized among the tephra units. Thus, by considering the diversity of magmatic types and their temporal variations, we could accurately correlate the tephra with the edifice.
The eruption history of the volcano, revealed by this study, is as follows. Two large explosive eruptions around 14-13 ka formed the southern caldera (AK13-12). The activity caused a sector collapse to form the northern caldera, in which post-caldera activity began: the Katakura-dake cone was formed between 12 and 9.5 ka (AK11-9), and an explosive eruption occurred at 9 ka (AK8), producing pyroclastic flows. Afterward, Onamedake cone was formed, and lava effusion continued (AK7). Then, at 7 ka, an explosive eruption occurred, forming a crater and generating pyroclastic flows (AK6). Phreatic eruptions occurred within the crater at 3.5 ka (AK5), and a pyroclastic cone was formed at 2.5 ka (AK4). Since then, activity shifted to the southern caldera; at 2 ka, phreatomagmatic eruptions occurred, forming the old Kodake pyroclastic cone (AK3). Around 1.5 ka, the Medake cone became active (AK2), followed by Kodake (AK1). In these activities, strombolian eruptions continued with lava effusion. Although activity ceased over 1000 years ago, it resumed in the 19th century, with 1932 and 1970–71 eruptions. Future research using similarly diverse methods is expected to advance further.