5:15 PM - 6:45 PM
[SVC26-P02] Detailed description and eruption sequence of the Rishiri Wanko-no-sawa tephra and the Rishiri Hotoku tephra, Hokkaido.
Keywords:Rishiri Volcannic Island, Rishiri Wanko-no-sawa tephra, Rishiri Hotoku tephra
Detailed description of tephra associated with a single eruptive event is useful for establishing eruption sequence and for distinguishing it from deposits by other eruptive events. Two tephras from the Rishiri Volcannic Island (RVI), in the northernmost part of Hokkaido, are distributed in an east-west direction from the Teshio Plain to the Tombetsu Plain on the main island of Hokkaido (Miura 1995). Although fall units of these tephras have been described for each tephra, no estimation of eruption sequence has been made.
In this study, we focus on the Rishiri Wanko-no-sawa tephra (Rs-Wn) and the Rishiri Hotoku tephra (Rs-Ho), derived from RVI. We conducted field surveys for the purpose of lithologies description and collected sample in Toyotomi, Hokkaido. Then, we analyzed every unit of Rs-Wn and discussed its eruption sequence in terms of grain size change and tephra characteristic change. At an outcrop in the southern part of RVI, we collected Numaura Maar Pyroclastic deposits (NUP), in order to detect the source of the tephra by comparing NUP with two tephras.
Rs-Wn is composed of nine fall units (Rs-Wn1 to Rs-Wn9 from bottom to top). Dominant units of Rs-Wn2 to 5 are mainly composed of pumice lapilli. Rs-Ho, immediately above Rs-Wn is composed of scoria lapilli and scoriaceous ash. We estimated eruption sequence of the Rs-Wn eruption using of properties pumiceous unit, that is, the grain-size and color distributions of pumice lapilli analyses. The former shows that Rs-Wn2 to 5 were formed by Plinian eruption. Sparks (1986) concluded that the maximum dispersal distance of particles is significantly greater with the height of the plume column in a simulation of a Plinian eruption under the windless condition. Therefore, during the Rs-Wn eruption, the eruption intensity had gradually increased from Rs-Wn2 to 4 and decreased into Rs-Wn5. The color distribution analysis indicates that white pumice lapilli increased from Rs-Wn2 to 4, and banded pumice lapilli predominated. On the other hand, intermediate-colored and gray pumice lapilli predominated and banded pumice lapilli were fewer in Rs-Wn5. The eruption of Rs-Wn might have followed a similar root to Hoei eruption of Mt.Fuji in 1707. The presence of banded pumice in the Rs-Wn eruption reinforces the results of Kawahara (2023) suggesting that Rs-Wn eruption was accompanied by magmatic mixing.
Using SEM-EDS, we measured major chemical compositions of volcanic glass shards within Rs-Wn2 to 5, and discussed possibility to identify each unit. Comparison of the major chemical compositions among units within the same color pumice lapilli revealed that Rs-Wn5 has a wide compositional range on the mafic side for white and gray pumice lapilli, and Rs-Wn4 has a wide compositional range on the siliciclastic side for gray pumice lapilli. In comparison of the major chemical compositions among colors within the same unit, Rs-Wn3 was characterized by significantly lower FeO content in intermediate-color pumice lapilli, and Rs-Wn4 was characterized by two trends in gray pumice lappili. each unit of Rs-Wn3 to 5 has distinctive compositions, so each unit of Rs-Wn2 to 5 can be distinguished by their major chemical compositions.
Since the NUP is correlated with neither Rs-Wn nor Rs-Ho based on their major chemical compositions, the source vent of the Rs-Wn and Rs-Ho is not Numaura Maar. Therefore, we are able to delete one of the potential source of these eruptions. In addition, the trends of the major chemical compositions of Rs-Wn and Rs-Ho are prominently different, so they have different eruption sources. The weight percent ratios of SiO2 and Na2O+K2O for the Rs-Ho are 54.4%, 6.1%, and those for NUP are 53.6%,5.25%, respectively, suggesting that the Rs-Ho has a composition of basaltic-trachy-andesite.
In this study, we focus on the Rishiri Wanko-no-sawa tephra (Rs-Wn) and the Rishiri Hotoku tephra (Rs-Ho), derived from RVI. We conducted field surveys for the purpose of lithologies description and collected sample in Toyotomi, Hokkaido. Then, we analyzed every unit of Rs-Wn and discussed its eruption sequence in terms of grain size change and tephra characteristic change. At an outcrop in the southern part of RVI, we collected Numaura Maar Pyroclastic deposits (NUP), in order to detect the source of the tephra by comparing NUP with two tephras.
Rs-Wn is composed of nine fall units (Rs-Wn1 to Rs-Wn9 from bottom to top). Dominant units of Rs-Wn2 to 5 are mainly composed of pumice lapilli. Rs-Ho, immediately above Rs-Wn is composed of scoria lapilli and scoriaceous ash. We estimated eruption sequence of the Rs-Wn eruption using of properties pumiceous unit, that is, the grain-size and color distributions of pumice lapilli analyses. The former shows that Rs-Wn2 to 5 were formed by Plinian eruption. Sparks (1986) concluded that the maximum dispersal distance of particles is significantly greater with the height of the plume column in a simulation of a Plinian eruption under the windless condition. Therefore, during the Rs-Wn eruption, the eruption intensity had gradually increased from Rs-Wn2 to 4 and decreased into Rs-Wn5. The color distribution analysis indicates that white pumice lapilli increased from Rs-Wn2 to 4, and banded pumice lapilli predominated. On the other hand, intermediate-colored and gray pumice lapilli predominated and banded pumice lapilli were fewer in Rs-Wn5. The eruption of Rs-Wn might have followed a similar root to Hoei eruption of Mt.Fuji in 1707. The presence of banded pumice in the Rs-Wn eruption reinforces the results of Kawahara (2023) suggesting that Rs-Wn eruption was accompanied by magmatic mixing.
Using SEM-EDS, we measured major chemical compositions of volcanic glass shards within Rs-Wn2 to 5, and discussed possibility to identify each unit. Comparison of the major chemical compositions among units within the same color pumice lapilli revealed that Rs-Wn5 has a wide compositional range on the mafic side for white and gray pumice lapilli, and Rs-Wn4 has a wide compositional range on the siliciclastic side for gray pumice lapilli. In comparison of the major chemical compositions among colors within the same unit, Rs-Wn3 was characterized by significantly lower FeO content in intermediate-color pumice lapilli, and Rs-Wn4 was characterized by two trends in gray pumice lappili. each unit of Rs-Wn3 to 5 has distinctive compositions, so each unit of Rs-Wn2 to 5 can be distinguished by their major chemical compositions.
Since the NUP is correlated with neither Rs-Wn nor Rs-Ho based on their major chemical compositions, the source vent of the Rs-Wn and Rs-Ho is not Numaura Maar. Therefore, we are able to delete one of the potential source of these eruptions. In addition, the trends of the major chemical compositions of Rs-Wn and Rs-Ho are prominently different, so they have different eruption sources. The weight percent ratios of SiO2 and Na2O+K2O for the Rs-Ho are 54.4%, 6.1%, and those for NUP are 53.6%,5.25%, respectively, suggesting that the Rs-Ho has a composition of basaltic-trachy-andesite.