[SVC50-P02] Investigation on Funatsu Tainai lava tree molds in Kenmarubi-I lava flow
Keywords:Lava tree mold, Kennmarubi-I lava flow, Funatsu tainai
Funatsu Tainai lava tree molds are a national natural monument located in the midstream area of Kenmarubi-I lava flow, and their measurement and investigation were carried out by K.Ogawa of Speleological Society and published as Yamanashi-ken natural monument urgent investigation report1). The investigation is continued and new additional lava tree molds are found and named as Yamanashi-ken monument important material by NPO Vulcano-Speleological Society. The volcanologic knowledge obtained from the current state of those investigations will be reported.
[The lava flow thickness and yield strength]
Funatsu Tainai lava tree molds count many vertical lava tree molds among which main natural monument and monument important material are listed in Table.1. The depth of the vertical tree mold are between 2.1m and 5.4m. Most has 4 m-5 m of depth as shown in Table.2. A diameter of a tree in this area was at most 1.9 m. The depth of the vertical lava tree molds shows a lava flow thickness. H, and gradient angle α in this area is approximately 8 degree, so the Bingham yield strength:fB can be estimated as fB=6.9x104~1.9x105dyne/cm2 from lava flow critical condition :H=fB/ (ρ g sin α) of simple lava flow where ρ =2.5g/cm3 and g=980cm/sec3. This fB is regarded as the proper value as basaltic lava of SiO2 50.88wt% (Tsuya 4)) or 51.1% (Takada 5)), though it seems a little bit high because of the temperature fall at this area. This value agree with estimated value 5.0x104~1.5x105dyne/cm2 by Yamashita 6). Therefore Kenmarubi-I lava flow can be regarded as a simple lava flow defined by Walker 7,8).
[Lava rib structure and surface tension estimated in Tainai]
Funatsu Tainai shows a complex lava tree molds which include lava stalactite from the ceiling and the ribbed wall formed by re-melting inside it9). It's possible to estimate the surface tension of the lava from the pitch of the lava stalactite and ribbed wall10). From instability onset conditions of melted liquid thin film, pitch(wave length) is shown as P=2 π (γ/g ρ)1/2 ,where γ is the surface tension of the lava and g is the gravity, and ρ is the density of the lava. Therefore it's possible to estimate a surface tension γ = P2 g ρ/4 π2 by measuring P of lava stalactite which hangs down from the ceiling inside the Tainai or from ribbed structure of side wall. The pitch obtained from Funatsu Tainai indicated in Fig.1 is P=3~4cm approximately, then, γ =560-990 dyne/cm is obtained as the surface tension. It's the reasonable value as surface tension of basaltic lava.
Similar results of yield strength and surface tension are also obtained for the Yoshida Tainai lava tree molds in the lava flow of Kenmarubi-II(SiO2 51.2wt%5)). Kenmarubi-I and the Kenmarubi-II are regarded as a simple lava flow. These lava flows has so low thickness that could not make a lava tube cave. Without being buried, much of lava tree molds are left. So, the biggest complex lava tree mold in the world exists in this area3). On the other hand, Aokigahara lava flow which has thick lava flow and high flow rate making a lot of lava tube caves indicates a compound lava flow. Further researches and investigations are under going for both Tainai lava tree molds.
1) Yamanashi-ken natural monument urgent investigating committee and lava cave/tree mold are an investigation group of Ogawa:Yamanashi-ken natural monument urgent investigation report (1996), Yamanashi-ken Education Board , p182-344.
2) H.Tachihara,T.Makita (1998): 1997 1998 year lava tree mold report,NPO Vulcano-Speleological Society.
3) H.Tachihara (2011): Volcano cave and lava tree mold. Caving journal No.43,p15-17
4) H.Tsuya(1971): The geographical feature of Mt. Fuji and the geological feature. Mt. Fuji overall investigation report, Fuji Kyuko p71
5) A.Takada et al (2016): Fuji volcanic geologic map (2nd edition), National Institute of Advanced Industrial Science and Technology ,Geological Survey of Japan.
6) S.Yamashita et al (2002): Reproduction of the Kenmarubi Lava Flow by a Numerical Simulation :. Japan Geoscience Union Meeting V032-P022
7)G.P.L.Walker(1971):Compound and simple lava flows and flood basalts. Bull.Volcanol.35,p579-590
8) S.Umino (2007): Characteristics of Lava flows of Fuji volcano, Fuji Volcano, Yamanashi Institute of Environmental Sciences p269-283
9) T.Honda (1998): Physico-chemical Explanation for Remelting Process of Inner Surface Wall of Tainai Tree Molds Located on the Flank of Mt. Fuji. Journal of the Speleological Society of Japan,vol 23,p29-38
10) T.Honda (2015): Estimation of surface tension of lava from lava stalactite and lava stalagmite appeared in lava tube cave and tree mold. Japan Geoscience Union Meeting SVC46-07