5:15 PM - 6:45 PM
[SVC27-P06] Formation of hummocks distributed at the southwestern foot of Bandai Volcano
Keywords:Bandai Volcano, debris avalanche, hummocks, landform interpretation, petrology
The geology and landform of the southern foot of Bandai Volcano is characterized by the dominance of mass movement deposits fed by the Bandai Volcano and Nekoma Volcano, and by widely distributed hummocks. Most of these hummocks are thought to have been formed by the Okinashima Debris Avalanche, the largest debris avalanche in this area, in 46 ka (Yamamoto and Suto, 1996). On the other hand, there are small hummocks near the source (Mimura, 1988; Yoshida, 2013), and that several debris avalanche deposits are distributed above the Okinashima Debris Avalanche deposit (lower order: Iwane Debris Avalanche deposit, Furukannon Debris Avalanche deposit; Chiba and Kimura, 2001). This suggests that the hummocks in this area were formed by multiple events. In this study, hummocks are classified according to the longitudinal distribution characteristics of their sizes, and their formation history is clarified by comparing them with debris avalanche deposits using petrographic techniques. The Furukannon Debris Avalanches, whose source is not agreed upon, were compared with the mountain body rocks using petrological techniques.
A CS map was created using the 5m DEM published by GSI, and the hummocks was deciphered. The relationship between the size and the distance from the source was also investigated. As a result, the longitudinal distribution characteristics of the hummocks at the southern foot of Bandai volcano are mostly uniform and can be explained by the Okinashima Debris Avalanche, but the hummocks within 7000m from the source are smaller and can be further classified into smaller hummocks group (S group) and relatively large hummocks group (M group).
The rocks in the debris avalanche deposits were compared with those of the hummocks by mineral compositional analysis and chemical compositional analysis. As a result, three debris avalanche deposits distributed in this area showed different compositional regions in the SiO2-TiO diagram and SiO2-Fe*O/MgO diagram. In particular, the compositional domains of the Furukannon Debris Avalanche deposit are consistent with those plotted in the calc-alkali series among the rocks constituting the Nekoma Volcano in the SiO2-Fe*O/MgO diagram when compared with the data of the rocks that make up the volcanic edifice by Koarai and Shimizu (1995). The rocks of Nekoma Volcano, those plotted in the calc-alkaline series are distributed around Nekomagatake (Kimura et al., 2001), suggesting that the Furukannon Debris Avalanche is fed from the southern slope of Nekomagatake. Compared with each hummocks group, the compositional domains of the M group and the Iwane Debris Avalanche deposit coincide. Therefore, M group is considered to have been formed by the Iwane Debris Avalanche. On the other hand, the S group is even smaller than the M group, but its petrological features do not match those of the Iwane Debris Avalanche deposit and the Furukannon Debris Avalanche deposit, and it is included in the compositional range of the Okinashima Debris Avalanche deposit (hummocks beyond 7000m from the source). Chiba and Kimura (2001) reported a small debris avalanche deposit resulting from the recollapse of an amphitheater formed by the Okinashima Debris Avalanche. The locations where these deposits are found roughly coincide with the distribution area of the S group, suggesting that the recollapse of the amphitheater and the associated small debris avalanche were the factors in the formation of the S group.
References
Kimura, J.-I., Tanji, T., Yoshida, T. and Izumi,S. 2001, Island arc, 10, 116-134.
Koarai, M. and Shimizu, J. 1995, Proceedings of the 5th Symposium on Geo Environments and Geo-technics, 353-358.
Chiba, S. and Kimura, J.-I. 2001, Journal of Mineralogical and Petrological Science, 30, 126-156.
Mimura, K. 1988, Journal of Geography, 97, 37-42.
Yamamoto, T., Suto, S. 1996, Bull Geol Surv, 47, 335-359.
Yoshida, H. 2013, Trans Jpn Geomorphol Union, 34, 1-19.
A CS map was created using the 5m DEM published by GSI, and the hummocks was deciphered. The relationship between the size and the distance from the source was also investigated. As a result, the longitudinal distribution characteristics of the hummocks at the southern foot of Bandai volcano are mostly uniform and can be explained by the Okinashima Debris Avalanche, but the hummocks within 7000m from the source are smaller and can be further classified into smaller hummocks group (S group) and relatively large hummocks group (M group).
The rocks in the debris avalanche deposits were compared with those of the hummocks by mineral compositional analysis and chemical compositional analysis. As a result, three debris avalanche deposits distributed in this area showed different compositional regions in the SiO2-TiO diagram and SiO2-Fe*O/MgO diagram. In particular, the compositional domains of the Furukannon Debris Avalanche deposit are consistent with those plotted in the calc-alkali series among the rocks constituting the Nekoma Volcano in the SiO2-Fe*O/MgO diagram when compared with the data of the rocks that make up the volcanic edifice by Koarai and Shimizu (1995). The rocks of Nekoma Volcano, those plotted in the calc-alkaline series are distributed around Nekomagatake (Kimura et al., 2001), suggesting that the Furukannon Debris Avalanche is fed from the southern slope of Nekomagatake. Compared with each hummocks group, the compositional domains of the M group and the Iwane Debris Avalanche deposit coincide. Therefore, M group is considered to have been formed by the Iwane Debris Avalanche. On the other hand, the S group is even smaller than the M group, but its petrological features do not match those of the Iwane Debris Avalanche deposit and the Furukannon Debris Avalanche deposit, and it is included in the compositional range of the Okinashima Debris Avalanche deposit (hummocks beyond 7000m from the source). Chiba and Kimura (2001) reported a small debris avalanche deposit resulting from the recollapse of an amphitheater formed by the Okinashima Debris Avalanche. The locations where these deposits are found roughly coincide with the distribution area of the S group, suggesting that the recollapse of the amphitheater and the associated small debris avalanche were the factors in the formation of the S group.
References
Kimura, J.-I., Tanji, T., Yoshida, T. and Izumi,S. 2001, Island arc, 10, 116-134.
Koarai, M. and Shimizu, J. 1995, Proceedings of the 5th Symposium on Geo Environments and Geo-technics, 353-358.
Chiba, S. and Kimura, J.-I. 2001, Journal of Mineralogical and Petrological Science, 30, 126-156.
Mimura, K. 1988, Journal of Geography, 97, 37-42.
Yamamoto, T., Suto, S. 1996, Bull Geol Surv, 47, 335-359.
Yoshida, H. 2013, Trans Jpn Geomorphol Union, 34, 1-19.