[SVC36-P21] Investigation on Lava Tube Caves in Manno lava flow of Mt.Fuji
Keywords:Fuji, lava tube, lava flow, lava staractite
[Introduction]
Manno lava flow of Mt.Fuji is a very old lava flow and the eruption is said to be before BC 10000, the silicic acid weight fraction is as low as 49.7% 1). Manno lava flow is known to have Manno Fuketsu lava cave (another name is Dainichi Ana lava cave), Yasiki Ana lava cave,Kobou Ana lava cave, Mado Ana lava cave, Ginga Fuketsu lava cave, Kannon Ana lava cave (already buried) 2, 3). These are thought to be a series of lava tube cave systems and considered the oldest lava caves in Mt. Fuji. Among them, Mannno Fuketsu lava cave is the longest lava tube cave in Mannno lava flow with a total distance of 908 m in Mannno lava flow 2, 3). The yield strength and the surface tension of Manno lava flow were estimated by refering to the Mannno Fuketsu lava cave survey maps 2, 3) .
[Manano Fuketsu lava tube cave and lava yield strength]
A photograph inside the Manano Fuketsu lava cave (Dainichi Ana lava cave) is shown in Photo 1, and a survey map in the horizontal section and longitudinal section is shown in Fig.1. A cave slope angle α = 2.5 ° is obtained from the height difference (30 m) in the longitudinal sectional view and the cave length (700 m). The cave height is approximately 5 m. The lava yield strength fB is estimated from fB = H (ρg sin α) / 4 by the flow limit condition of the Bingham fluid flowing in the inclined circular tube 4). When the lava density is 2.5 g / cm 3, fB = 1.32 × 10 4 dyne / cm 2 can be obtained from the above conditions. It is considered to be a reasonable value for basalt lava having a silicic acid weight fraction of 49.7%.
[Lava stalactite and surface tension value]
Many lava stalactite are found in the ceiling of Manno Fuketsu lava cave(Dainichi Ana lava cave). When the lava is drained through the lava tube, the residual molten lava stagnated on the ceiling is hanging down from the ceiling by forming the lava stalactite. The surface tension of lava can be estimated from the pitch of this lava stalactite5). From the stability limit condition of the liquid film adhering to the ceiling, the specific pitch P = 2π (γ / gρ) 1/2 of the wave motion of the liquid film is obtained. Here, γ is the surface tension of the lava, g is the gravitational acceleration, and ρ is the density of the lava. Therefore, we can obtain the surface tension γ = P2 gρ / 4π2 of the lava by measuring the pitch P of the periodic position of the lava stalactite descent from the ceiling 5). The pitch measured in the Manno Fuketsu lava cave is roughly P = 3 to 4 cm (see Photo 2), and γ = 560 to 990 dyne / cm as the surface tension is obtained. The surface tension is considered to be a reasonable value5).
[Conclusion]
The yield strength and the surface tension of the Manno lava flow were estimated by the tube shape and internal structure of Manno Fuketsu lava cave(Dainichi Ana lava cave). Both values obtained are considered to be reasonable values for basalt. Yield strength and surface tension are the same as other examples of lava flows of Mt. Fuji 4, 5). We are planning to continue to investigate other lava tube cave of Mt. Fuji which has not been studied yet.
References:
1) Akira Takada, Takahiro Yamamoto, Yoshihiro Ishizuka, Shun Nakano (2016): Fuji volcano geological map (2nd edition) Instructions, National Institute of Advanced Industrial Science and Technology, Geological Survey of Japan
2) Takanori Ogawa(1980): "Geological observation of lava caverns and lava tree types in Mt. Fuji" Do-Jin Vol. 2, No. 3, Japan Cave Association, August 1980.
3) Fujinomiya City Municipal Museum (1991): "Fujinomiya volcanic cave", Fujinomiya City municipal museum survey report, No. 2, Fujinomiya City.
4) Tsutomu Honda(2018):Estimation of the yield value of Mt. Fuji and lava flow by lava tube cave and lava tree mold, 2018 Japan Geoscience Union Meeting, SVC 43-02
5) Tsutomu Honda(2015): Surface tension of lava estimated from lava stalactites and lava stalagmites seen inside lava tube cave and lava tree cavities, 2015 Japan Geoscience Union Meeting, SVC 46-07
Manno lava flow of Mt.Fuji is a very old lava flow and the eruption is said to be before BC 10000, the silicic acid weight fraction is as low as 49.7% 1). Manno lava flow is known to have Manno Fuketsu lava cave (another name is Dainichi Ana lava cave), Yasiki Ana lava cave,Kobou Ana lava cave, Mado Ana lava cave, Ginga Fuketsu lava cave, Kannon Ana lava cave (already buried) 2, 3). These are thought to be a series of lava tube cave systems and considered the oldest lava caves in Mt. Fuji. Among them, Mannno Fuketsu lava cave is the longest lava tube cave in Mannno lava flow with a total distance of 908 m in Mannno lava flow 2, 3). The yield strength and the surface tension of Manno lava flow were estimated by refering to the Mannno Fuketsu lava cave survey maps 2, 3) .
[Manano Fuketsu lava tube cave and lava yield strength]
A photograph inside the Manano Fuketsu lava cave (Dainichi Ana lava cave) is shown in Photo 1, and a survey map in the horizontal section and longitudinal section is shown in Fig.1. A cave slope angle α = 2.5 ° is obtained from the height difference (30 m) in the longitudinal sectional view and the cave length (700 m). The cave height is approximately 5 m. The lava yield strength fB is estimated from fB = H (ρg sin α) / 4 by the flow limit condition of the Bingham fluid flowing in the inclined circular tube 4). When the lava density is 2.5 g / cm 3, fB = 1.32 × 10 4 dyne / cm 2 can be obtained from the above conditions. It is considered to be a reasonable value for basalt lava having a silicic acid weight fraction of 49.7%.
[Lava stalactite and surface tension value]
Many lava stalactite are found in the ceiling of Manno Fuketsu lava cave(Dainichi Ana lava cave). When the lava is drained through the lava tube, the residual molten lava stagnated on the ceiling is hanging down from the ceiling by forming the lava stalactite. The surface tension of lava can be estimated from the pitch of this lava stalactite5). From the stability limit condition of the liquid film adhering to the ceiling, the specific pitch P = 2π (γ / gρ) 1/2 of the wave motion of the liquid film is obtained. Here, γ is the surface tension of the lava, g is the gravitational acceleration, and ρ is the density of the lava. Therefore, we can obtain the surface tension γ = P2 gρ / 4π2 of the lava by measuring the pitch P of the periodic position of the lava stalactite descent from the ceiling 5). The pitch measured in the Manno Fuketsu lava cave is roughly P = 3 to 4 cm (see Photo 2), and γ = 560 to 990 dyne / cm as the surface tension is obtained. The surface tension is considered to be a reasonable value5).
[Conclusion]
The yield strength and the surface tension of the Manno lava flow were estimated by the tube shape and internal structure of Manno Fuketsu lava cave(Dainichi Ana lava cave). Both values obtained are considered to be reasonable values for basalt. Yield strength and surface tension are the same as other examples of lava flows of Mt. Fuji 4, 5). We are planning to continue to investigate other lava tube cave of Mt. Fuji which has not been studied yet.
References:
1) Akira Takada, Takahiro Yamamoto, Yoshihiro Ishizuka, Shun Nakano (2016): Fuji volcano geological map (2nd edition) Instructions, National Institute of Advanced Industrial Science and Technology, Geological Survey of Japan
2) Takanori Ogawa(1980): "Geological observation of lava caverns and lava tree types in Mt. Fuji" Do-Jin Vol. 2, No. 3, Japan Cave Association, August 1980.
3) Fujinomiya City Municipal Museum (1991): "Fujinomiya volcanic cave", Fujinomiya City municipal museum survey report, No. 2, Fujinomiya City.
4) Tsutomu Honda(2018):Estimation of the yield value of Mt. Fuji and lava flow by lava tube cave and lava tree mold, 2018 Japan Geoscience Union Meeting, SVC 43-02
5) Tsutomu Honda(2015): Surface tension of lava estimated from lava stalactites and lava stalagmites seen inside lava tube cave and lava tree cavities, 2015 Japan Geoscience Union Meeting, SVC 46-07