5:15 PM - 7:15 PM
[SVC34-P11] Sequence of Zao Volcano 17th century eruptions based on geologic and petrologic features of volcanic ashes.
Keywords:Zao Volcano, eruption sequence, tephra layer, scoria, bubbling glass
1. Introduction
Zao Volcano is an active volcano located in the central part of volcanic front of the northeastern Japan arc. The summit crater, Okama, has been active since around 1200 AD, and 11~17 of the Zao-Togatta tephra (Za-To) have been formed by this activity. In this study, we examined the eruption sequence based on the volcanic ash layer of Za-To15. Za-To15 is estimated to have been formed by an eruption in the 17th century.
2. Results
Za-To15 is composed of white, gray, and black layers in the ascending order. The gray and black layers are fairy laminated. The white layer is poorly sorted, on the other hand the gray layer and the black layer are well sorted. The sorting of the black layer is better than the gray layer. The fine-grained particles are included in the white and gray layers, and the amount is larger in the former than the latter. The main components are the altered rock fragment and the bubbling glass in the white layer, the scoria, bubbling glass and lithic fragment in the gray layer, and the scoria and lithic fragment in the black layer. The scoria and the bubbling glass, which are the juvenile materials, the former generally has a higher crystallinity than the latter. In addition, only in the white layer, a small amount of scoria having extremely low crystallinity was observed.
3. Discussion
The white layer is characterized by including high amount of white altered rock fragments and subordinate scoria and bubbling glass. The sorting index is relatively worse. These characteristics indicate the white layer was formed by hydrothermal phreatomagmatic eruptions. On the other hand, the black layer contains scoria and bubbling glass and few altered rock fragments. Therefore, the black layer was formed by the magmatic eruption. The gray layer has granulometric and component characteristics similar to the black layer but has fine grain particles and white altered rock fragments in some extent. Therefore, the gray layer is thought to have been formed by eruptions during the transition period from phreatic eruptions to magmatic eruptions. Considering the stratigraphic relation, the eruption began with the hydrothermal phreatomagmatic eruption, followed by a transitional period, led to a magmatic eruption, and then ceased.
The lamina has developed in the gray and black layers, which were formed by multiple eruptions. Therefore, both the gray and black layers were formed by repeated small eruptions. The higher proportion of scoria to lithic fragments suggests that each eruption would be much explosive than the typical vulcanian eruption. Generally, the scoria has a higher crystallinity than the bubbling glass. Therefore, the cooling rate of the scoria was slower than that of the bubbling glass. The scoria would be semi-solidified when the magma ascended to near surface. The bubbling glass was formed by rapid cooling and solidification of magma that ascended just before the eruption from the depth. In addition, the low crystallinity of scoria in the white layer was probably formed by specific formation process involving the hydrothermal system.
Zao Volcano is an active volcano located in the central part of volcanic front of the northeastern Japan arc. The summit crater, Okama, has been active since around 1200 AD, and 11~17 of the Zao-Togatta tephra (Za-To) have been formed by this activity. In this study, we examined the eruption sequence based on the volcanic ash layer of Za-To15. Za-To15 is estimated to have been formed by an eruption in the 17th century.
2. Results
Za-To15 is composed of white, gray, and black layers in the ascending order. The gray and black layers are fairy laminated. The white layer is poorly sorted, on the other hand the gray layer and the black layer are well sorted. The sorting of the black layer is better than the gray layer. The fine-grained particles are included in the white and gray layers, and the amount is larger in the former than the latter. The main components are the altered rock fragment and the bubbling glass in the white layer, the scoria, bubbling glass and lithic fragment in the gray layer, and the scoria and lithic fragment in the black layer. The scoria and the bubbling glass, which are the juvenile materials, the former generally has a higher crystallinity than the latter. In addition, only in the white layer, a small amount of scoria having extremely low crystallinity was observed.
3. Discussion
The white layer is characterized by including high amount of white altered rock fragments and subordinate scoria and bubbling glass. The sorting index is relatively worse. These characteristics indicate the white layer was formed by hydrothermal phreatomagmatic eruptions. On the other hand, the black layer contains scoria and bubbling glass and few altered rock fragments. Therefore, the black layer was formed by the magmatic eruption. The gray layer has granulometric and component characteristics similar to the black layer but has fine grain particles and white altered rock fragments in some extent. Therefore, the gray layer is thought to have been formed by eruptions during the transition period from phreatic eruptions to magmatic eruptions. Considering the stratigraphic relation, the eruption began with the hydrothermal phreatomagmatic eruption, followed by a transitional period, led to a magmatic eruption, and then ceased.
The lamina has developed in the gray and black layers, which were formed by multiple eruptions. Therefore, both the gray and black layers were formed by repeated small eruptions. The higher proportion of scoria to lithic fragments suggests that each eruption would be much explosive than the typical vulcanian eruption. Generally, the scoria has a higher crystallinity than the bubbling glass. Therefore, the cooling rate of the scoria was slower than that of the bubbling glass. The scoria would be semi-solidified when the magma ascended to near surface. The bubbling glass was formed by rapid cooling and solidification of magma that ascended just before the eruption from the depth. In addition, the low crystallinity of scoria in the white layer was probably formed by specific formation process involving the hydrothermal system.