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[SVC29-P06] Lava cave configuration and inner structure of Saiko-Fuketsu Caves in Aokigahara lava flow of Mt.Fuji
Keywords:Lava cave, Aokigahara lava flow, Saiko-Fuketsu Cave, Mt.Fuji
[Introduction]
The Aokigahara lava flow flowed out of the crater row of Nagaoyama crater-Ishizuka crater-Kudariyama crater, and flowed into the old lake ”Seno-umi” at the end and divided it in two to form the present Lake Shojiko and Lake Saiko [1]. Though almost linear lava tube caves(Zinza fuketsu No.1,etc.) formed in the high altitude area near the crater,the lava caves formed in the area where lava stagnates at the end of the lava flow have different configuration[2,3]. We will introduce the results of the investigation and examination of the configuration and characteristics of the Saiko-Fuketsu Caves formed at the end of the lava flow area.
[Location and structure of Saiko-Fuketsu caves]
The location of the Saiko-Fuketsu Caves is shown in Fig.1. The area surrounded by the dotted line shows the estimated range of the old lake ”Seno-umi” before the Aokigahara lava flows into the lake. The current elevation of the lake surface of Lake Saiko and Lake Shoji is 902m in the same altitude, the water depths are 76m and 16m, respectively, and it is clear that they communicate with each other due to cracks and cavities in the lava deposited on the bottom of the old lake. When a large amount of lava flows into the old lake, due to the pressure of the boiling and gasified lake water, the cavity of lava expands, then,the stagnated lava flow seems to create a three-dimensional and maze-like shape lava cave. As a results, it is thought to have formed a complicated cavity structure [2]. As examples for characteristic of the Saiko Fuketsu caves, the labyrinth like Saiko Bat Cave and the ring-shaped Saiko Fuketsu No. 13 are shown in Fig.2 and Fig.3. The interior structure of the cave is similar to that found in the linear lava tube cave, there have ropy lava, lava shelves, lava balls, lava bridges, lava stalactite/stalagmite, lava dome, etc.
[Formation mechanism of Saiko Fuketsu caves]
In the Saiko Fuketsu caves located near the end of the Aokihagara lava flow, a cavity is formed in the almost flat part where the lava stagnates. Although there is some influence of gavity for drain out of lava, it seems natural to think that it was also formed by the effect of cavity expansion of the inside of the lava flow due to the pressure of gasification of the lake water and drainage of inner soft lava surrounded by cooled crusted lava. It is thought that a pond-like pool is formed and the surface is solidified, the central part of the lava pond forms a dented lava column and becomes a donut-shaped soft lava shape, and the soft lava inside is drained out by gas pressure and lava static pressure. It seems that you should think about the image of push of lava with a steam pressure piston. We roughly estimated how much the steam pressure in the cavity of the Saiko Fuketsu caves was formed. The yield strength fB of lava of Aokigahara is about 5000Pa [3]. Assuming that the cavity diameter H is about 4 m and the cavity length ℓ is 20m to100m, the pressure to drain out the lava [4]: ΔP=4fBℓ/H is 1.0x105Pa to 5.0x105Pa is required, that is about 1 to 5atm. This pressure generation is thought to be possible due to the confinement and heating of the lake water by lava flow (~1000°C).
[Conclusion]
The Saiko Fuketsu caves located at the end of Aokigahara (elevation 915m-965m) seems to be formed by lava flowing into the old lake and lava drainage by water gas pressure, It shows a different shape from a simple lava tube cave(Zinza fuketsu No.1) that is formed almost linearly in the steep slope by drainage by gravity force at a high altitude near the crater (elevation ~ 1200m). Due to the fact that Lake Saiko and Lake Shoji communicate with each other, it is possible that the Saiko Fuketsu caves like lava caves also exist on the bottom of the old sea “Seno-umi”.
References:
[1]T.Chiba et al. (2007): Analysis of micro topography of the Aokigahara Lava flows,Fuji volcano,by the Light Detection and Ranging System,Fujikazan(2007),S.Aramaki, T.Fujii, S.Nakada, N.Miyaji, Yamanashi Prefectural Institute of Environmental Sciences p349 -363
[2]T.Ogawa (1996): Yamanashi Prefecture Natural Monument Emergency Survey Report, Geology/Minerals, Yamanashi Prefectural Board of Education
[3]T.Honda, et al., (2019): P-118 Investigation on lava tree molds and lava tube caves of Ishizuka lava flow in Aokigahara, Japan Volcanological Society of Japan,Proceeding of 2019 Fall Meeting
[4]S.Oka (1976): Rheology, Physical Science Selection 7 (1976 Edition), Shokabo, p53-56
The Aokigahara lava flow flowed out of the crater row of Nagaoyama crater-Ishizuka crater-Kudariyama crater, and flowed into the old lake ”Seno-umi” at the end and divided it in two to form the present Lake Shojiko and Lake Saiko [1]. Though almost linear lava tube caves(Zinza fuketsu No.1,etc.) formed in the high altitude area near the crater,the lava caves formed in the area where lava stagnates at the end of the lava flow have different configuration[2,3]. We will introduce the results of the investigation and examination of the configuration and characteristics of the Saiko-Fuketsu Caves formed at the end of the lava flow area.
[Location and structure of Saiko-Fuketsu caves]
The location of the Saiko-Fuketsu Caves is shown in Fig.1. The area surrounded by the dotted line shows the estimated range of the old lake ”Seno-umi” before the Aokigahara lava flows into the lake. The current elevation of the lake surface of Lake Saiko and Lake Shoji is 902m in the same altitude, the water depths are 76m and 16m, respectively, and it is clear that they communicate with each other due to cracks and cavities in the lava deposited on the bottom of the old lake. When a large amount of lava flows into the old lake, due to the pressure of the boiling and gasified lake water, the cavity of lava expands, then,the stagnated lava flow seems to create a three-dimensional and maze-like shape lava cave. As a results, it is thought to have formed a complicated cavity structure [2]. As examples for characteristic of the Saiko Fuketsu caves, the labyrinth like Saiko Bat Cave and the ring-shaped Saiko Fuketsu No. 13 are shown in Fig.2 and Fig.3. The interior structure of the cave is similar to that found in the linear lava tube cave, there have ropy lava, lava shelves, lava balls, lava bridges, lava stalactite/stalagmite, lava dome, etc.
[Formation mechanism of Saiko Fuketsu caves]
In the Saiko Fuketsu caves located near the end of the Aokihagara lava flow, a cavity is formed in the almost flat part where the lava stagnates. Although there is some influence of gavity for drain out of lava, it seems natural to think that it was also formed by the effect of cavity expansion of the inside of the lava flow due to the pressure of gasification of the lake water and drainage of inner soft lava surrounded by cooled crusted lava. It is thought that a pond-like pool is formed and the surface is solidified, the central part of the lava pond forms a dented lava column and becomes a donut-shaped soft lava shape, and the soft lava inside is drained out by gas pressure and lava static pressure. It seems that you should think about the image of push of lava with a steam pressure piston. We roughly estimated how much the steam pressure in the cavity of the Saiko Fuketsu caves was formed. The yield strength fB of lava of Aokigahara is about 5000Pa [3]. Assuming that the cavity diameter H is about 4 m and the cavity length ℓ is 20m to100m, the pressure to drain out the lava [4]: ΔP=4fBℓ/H is 1.0x105Pa to 5.0x105Pa is required, that is about 1 to 5atm. This pressure generation is thought to be possible due to the confinement and heating of the lake water by lava flow (~1000°C).
[Conclusion]
The Saiko Fuketsu caves located at the end of Aokigahara (elevation 915m-965m) seems to be formed by lava flowing into the old lake and lava drainage by water gas pressure, It shows a different shape from a simple lava tube cave(Zinza fuketsu No.1) that is formed almost linearly in the steep slope by drainage by gravity force at a high altitude near the crater (elevation ~ 1200m). Due to the fact that Lake Saiko and Lake Shoji communicate with each other, it is possible that the Saiko Fuketsu caves like lava caves also exist on the bottom of the old sea “Seno-umi”.
References:
[1]T.Chiba et al. (2007): Analysis of micro topography of the Aokigahara Lava flows,Fuji volcano,by the Light Detection and Ranging System,Fujikazan(2007),S.Aramaki, T.Fujii, S.Nakada, N.Miyaji, Yamanashi Prefectural Institute of Environmental Sciences p349 -363
[2]T.Ogawa (1996): Yamanashi Prefecture Natural Monument Emergency Survey Report, Geology/Minerals, Yamanashi Prefectural Board of Education
[3]T.Honda, et al., (2019): P-118 Investigation on lava tree molds and lava tube caves of Ishizuka lava flow in Aokigahara, Japan Volcanological Society of Japan,Proceeding of 2019 Fall Meeting
[4]S.Oka (1976): Rheology, Physical Science Selection 7 (1976 Edition), Shokabo, p53-56