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[SVC30-02] Correlation of proximal and distal deposits of a pyroclastic eruption based on the variation of juvenile materials
Keywords:Caldera-forming eruption, Juvenile materials, Correlation of tephra, Co-ignimbrite ash, Toya volcano
In the case of a caldera-forming pyroclastic eruption, voluminous eruptive deposits are widely distributed. Therefore, it would be usually difficult to correlate deposits of each eruption phase from proximal to distal areas. However, eruption volume of each phase is important to discuss mechanism of the eruption. For this reason, correlation of eruptive deposits must be essential based on the confident evidence. We present the case study on Toya volcano and correlate eruptive deposits, by focusing on the temporal changes in petrological diversity of juveniles. During 106 ka Toya caldera-forming eruption, two large-scale PDC (pyroclastic density current) occurred, and the co-ignimbrite ash associated with these PDC reached more than 300 km in the distance from the caldera. In previous studies, there is no consensus regarding the comparison of the scale of these PDC, and the correlation between the co-ignimbrite ash and these PDC has not been discussed.
At first, we investigated the stratigraphy of the eruptive deposits at sites within 8 km from the caldera rim and divide the deposits into six units, from Unit 1 to 6 in ascending order. Of these, Unit 2 and Unit 5 and 6 are large-scale PDC. Although this classification is the same as that by Goto et al. (2018), we found a relatively long interval between Unit 3 and 4. In addition, we newly divide juveniles into three types, CP type pumice (CP), CR type pumice (CR), and gray banded pumice (GRY). These can be distinguished not only by lithofacies but also by phenocryst assemblages, mineral compositions, and whole-rock and matrix glass chemistry. Although Unit 1 to 3 only contain CP, small amounts of CR and GRY are also recognized in Unit 4. Then, the proportion of CP decreases to about 90% in Unit 5, and that of CR becomes particularly high. Moreover, CP decreases to about 50% in Unit 6, and GRY increases in addition to CR. Next, we compare proportions of types of juveniles of PDC deposits from proximal to distal areas. When juvenile materials become fine grained, the classification is confirmed by the matrix glass chemistry. It is suggested that Unit 2 and 6 are not distributed beyond 30 km from the caldera. On the other hand, Unit 5 has reached more than 50 km far from the caldera, indicating that it is the most voluminous. Next, the co-ignimbrite ash is correlated with each proximal unit based on the matrix glass chemistry. Considering the time interval, the units are divided into two units for the correlation, Unit 2 and 5 to 6. The ash samples were investigated at four locations more than 80 km far from the caldera in northeast and south-southwest directions. The ash sample was collected from the upper and lower parts of the layer at each site. At areas up to about 120 km far from the caldera, the lower part of the ash layer is correlated to Unit 2, and the upper part to Unit 5 and 6. However, at areas further than 100 – 150 km, the ash layers are composed only of Unit 5 to 6. Thus, it is concluded that the co-ignimbrite ash distributed more than about 120 km far from the caldera was derived from Unit 5 to 6.
The correlation of eruptive deposits of a caldera-forming eruption of Toya volcano has been successfully completed from proximal to distal areas. In addition, in terms of the time interval and the temporal change of eruptive magma types, the eruption can be divided into two main stages: Stage I (Unit 1 to 3) and II (Unit 4 to 6). The estimated eruptive volumes are approximately 39 km3 DRE in Stage 1, 133 km3 DRE in Stage 2, and the maximum in Unit 5 with 106 km3 DRE. Stage II is the main stage of the eruption and occurred with a time interval after Stage I. It also erupted various types of magmas, such as CR and GRY magmas in addition to CP one. The catastrophic eruption phases were Unit 5 and 6, in which obvious lag breccia layers occurred, suggesting that collapse and enlargement of the caldera had occurred during these units.
(Ref.) Goto et al., 2018, J. Geography.
At first, we investigated the stratigraphy of the eruptive deposits at sites within 8 km from the caldera rim and divide the deposits into six units, from Unit 1 to 6 in ascending order. Of these, Unit 2 and Unit 5 and 6 are large-scale PDC. Although this classification is the same as that by Goto et al. (2018), we found a relatively long interval between Unit 3 and 4. In addition, we newly divide juveniles into three types, CP type pumice (CP), CR type pumice (CR), and gray banded pumice (GRY). These can be distinguished not only by lithofacies but also by phenocryst assemblages, mineral compositions, and whole-rock and matrix glass chemistry. Although Unit 1 to 3 only contain CP, small amounts of CR and GRY are also recognized in Unit 4. Then, the proportion of CP decreases to about 90% in Unit 5, and that of CR becomes particularly high. Moreover, CP decreases to about 50% in Unit 6, and GRY increases in addition to CR. Next, we compare proportions of types of juveniles of PDC deposits from proximal to distal areas. When juvenile materials become fine grained, the classification is confirmed by the matrix glass chemistry. It is suggested that Unit 2 and 6 are not distributed beyond 30 km from the caldera. On the other hand, Unit 5 has reached more than 50 km far from the caldera, indicating that it is the most voluminous. Next, the co-ignimbrite ash is correlated with each proximal unit based on the matrix glass chemistry. Considering the time interval, the units are divided into two units for the correlation, Unit 2 and 5 to 6. The ash samples were investigated at four locations more than 80 km far from the caldera in northeast and south-southwest directions. The ash sample was collected from the upper and lower parts of the layer at each site. At areas up to about 120 km far from the caldera, the lower part of the ash layer is correlated to Unit 2, and the upper part to Unit 5 and 6. However, at areas further than 100 – 150 km, the ash layers are composed only of Unit 5 to 6. Thus, it is concluded that the co-ignimbrite ash distributed more than about 120 km far from the caldera was derived from Unit 5 to 6.
The correlation of eruptive deposits of a caldera-forming eruption of Toya volcano has been successfully completed from proximal to distal areas. In addition, in terms of the time interval and the temporal change of eruptive magma types, the eruption can be divided into two main stages: Stage I (Unit 1 to 3) and II (Unit 4 to 6). The estimated eruptive volumes are approximately 39 km3 DRE in Stage 1, 133 km3 DRE in Stage 2, and the maximum in Unit 5 with 106 km3 DRE. Stage II is the main stage of the eruption and occurred with a time interval after Stage I. It also erupted various types of magmas, such as CR and GRY magmas in addition to CP one. The catastrophic eruption phases were Unit 5 and 6, in which obvious lag breccia layers occurred, suggesting that collapse and enlargement of the caldera had occurred during these units.
(Ref.) Goto et al., 2018, J. Geography.