9:30 AM - 9:45 AM
[SVC28-21] Comparison of subaerial and submarine deposits from the Kikai Akahoya eruption
The 7.3 ka Kikai Akahoya (K-Ah) eruption is a large-scale eruption of the submarine Kikai caldera volcano. It started with a Plinian Funakura pumice fall followed by an intra-Plinian Funakura pyroclastic flow (PF). This was followed by large-scale Koya ignimbrite accompanied by the Akahoya co-ignimbrite ash. On land, Funakura PF is deposited only on proximal islands, which have caldera walls. Koya ignimbrite is thickly deposited on the caldera walls and thinly deposited ~1 m in distal areas tens of kilometers across the sea. The Multi-Channel seismic survey (MCS) around the caldera showed that the Unit S-1, which is estimated to be the K-Ah products, was deposited thickly over a wide area (Shimizu et al., 2020JpGU). Based on their distribution and internal structure, it is assumed that it was deposited by flowing over the seafloor rather than as a fall deposit. The purpose of this study is to clarify the flow and depositional process of pyroclastic materials by comparing the components, depositional structures, and chemical compositions of the volcanic glass of the K-Ah ejecta on both land and seafloor.
On-land investigation and sampling were conducted in the proximal and distal areas. Marine deposits were drilled by the CHIKYU SCORE program to a depth of about 100 m at a location 4.3 km northeast of Takeshima. Also, piston cores (lengths of 1 to 4 m) were conducted by the Shinseimaru and Kairei at 7 sites in the proximal area, including near the SCORE drilling site, and at 14 sites in the distal area. SCORE drilled intermittently from 10 - 100 m below the seafloor, and Unit S-1 is identified by MCS as two different units Lower S-1 (depth 17-22 m) and Upper S-1 (depth <17 m, but not sampled below 10 m). Lower S-1 is a poorly sorted, reddish-brown pumice lapilli tuff often with lamina; Upper S-1 is a poorly sorted massive lapilli tuff with pumice and lithic fragments. These boundaries are distinct because the components are different. In the piston core near the SCORE site, 2 m of sediment corresponding to the uppermost part of Upper S-1 was collected. The submarine deposit consists of pumice lapilli, volcanic ash, lithic fragments, and crystals, and the sedimentary structure is often massive, but at some sites, there are normal grading of the pumice and weak lamination.
The chemical composition of the volcanic ash glass in each deposit was measured by EPMA. The Funakura PF contains only high SiO2 glass (SiO2 = ~75 wt.%). In the lower part of the Koya ignimbrite, there is only high SiO2 glass as in the Funakura PF, but low SiO2 (SiO2= ~65 wt.%) is mixed into the upper part (Fujihara and Suzuki-Kamata, 2013). The SCORE samples, Lower S-1 and Upper S-1, both contain only high SiO2 glass consistent with that of the on-land K-Ah products. The piston core samples, both near and far, including the area around the SCORE site, contain both high and low SiO2 glasses consistent with the K-Ah eruption.
On land, the Funakura PF composed of high SiO2 magma was deposited, and then the Koya ignimbrite is presumed to have erupted only high SiO2 magma in the early stage and gradually deposited from the lower levels with the mixing of low SiO2 magma as the eruption continued (Fujihara and Suzuki-Kamata, 2013). The submarine K-Ah products also contain only high SiO2 glass in the Lower S-1 and lower part of Upper S-1, suggesting that the deposits were initially deposited by the Funakura PF and later by the early Koya ignimbrite. The uppermost Unit S-1 near the seafloor by piston cores contains both high and low SiO2 glasses, suggesting that they were deposited during the late K-Ah eruption stage. On the other hand, the sedimentary structure and grain size of marine deposits are different from those of on-land deposits, and they often show sedimentary structures suggestive of underwater sedimentation, suggesting that they were deposited in a different flow regime than large-scale pyroclastic flows on land.
On-land investigation and sampling were conducted in the proximal and distal areas. Marine deposits were drilled by the CHIKYU SCORE program to a depth of about 100 m at a location 4.3 km northeast of Takeshima. Also, piston cores (lengths of 1 to 4 m) were conducted by the Shinseimaru and Kairei at 7 sites in the proximal area, including near the SCORE drilling site, and at 14 sites in the distal area. SCORE drilled intermittently from 10 - 100 m below the seafloor, and Unit S-1 is identified by MCS as two different units Lower S-1 (depth 17-22 m) and Upper S-1 (depth <17 m, but not sampled below 10 m). Lower S-1 is a poorly sorted, reddish-brown pumice lapilli tuff often with lamina; Upper S-1 is a poorly sorted massive lapilli tuff with pumice and lithic fragments. These boundaries are distinct because the components are different. In the piston core near the SCORE site, 2 m of sediment corresponding to the uppermost part of Upper S-1 was collected. The submarine deposit consists of pumice lapilli, volcanic ash, lithic fragments, and crystals, and the sedimentary structure is often massive, but at some sites, there are normal grading of the pumice and weak lamination.
The chemical composition of the volcanic ash glass in each deposit was measured by EPMA. The Funakura PF contains only high SiO2 glass (SiO2 = ~75 wt.%). In the lower part of the Koya ignimbrite, there is only high SiO2 glass as in the Funakura PF, but low SiO2 (SiO2= ~65 wt.%) is mixed into the upper part (Fujihara and Suzuki-Kamata, 2013). The SCORE samples, Lower S-1 and Upper S-1, both contain only high SiO2 glass consistent with that of the on-land K-Ah products. The piston core samples, both near and far, including the area around the SCORE site, contain both high and low SiO2 glasses consistent with the K-Ah eruption.
On land, the Funakura PF composed of high SiO2 magma was deposited, and then the Koya ignimbrite is presumed to have erupted only high SiO2 magma in the early stage and gradually deposited from the lower levels with the mixing of low SiO2 magma as the eruption continued (Fujihara and Suzuki-Kamata, 2013). The submarine K-Ah products also contain only high SiO2 glass in the Lower S-1 and lower part of Upper S-1, suggesting that the deposits were initially deposited by the Funakura PF and later by the early Koya ignimbrite. The uppermost Unit S-1 near the seafloor by piston cores contains both high and low SiO2 glasses, suggesting that they were deposited during the late K-Ah eruption stage. On the other hand, the sedimentary structure and grain size of marine deposits are different from those of on-land deposits, and they often show sedimentary structures suggestive of underwater sedimentation, suggesting that they were deposited in a different flow regime than large-scale pyroclastic flows on land.