9:45 AM - 10:00 AM
[SVC43-02] Estimation of the yield strength of lava flows of Mt.Fuji by lava tube caves and lava tree molds
Keywords:Fuji, lava flow, yield strength, lava tube cave, tree mold
[Introduction]
Many lava tube caves and lava tree molds exist in lava flows of Mt. Fuji. Lava tube cave and lava tree mold coexists in Suyama Tainai lava flow and Ganno-Ana lava flow. Only lava tube cave exists in Subashiri-guchi lava flow located in the hight altitude. Only lava tree mold exists in relatively thin lava flow such as Takamarubi lava flow, Higashisuzuka-South lava flow, Kenmarubi I and Kenmarubi II lava flows. The yield strength of the Mt. Fuji lava flows was estimated from the hollow height of the lava tube cave and the depth of the lava tree mold and compared each other by using Bingham flow model. The lava flow structure of these Mt.Fuji is also discussed based on the difference of these yield strength.
[Considered model]
The lava flow is modeled by Bingham fluid flowing on the inclined plane or in the inclined cylindrical pipe with gravity potential. For the lava flow of density ρ, and yield strength fB, with slope angle α, under the gravity g, the lava flow stop condition is H=nfB/ (ρ g sin α) where H is the lava thickness. The case of lava which flows on the incline plane with a free surface is n=1, and the case of lava which flows through an inclined circular tube is n=4. The yield strength is obtained from fB =H (ρ g sin α)/4,for n=4, where H is the lava tube cave height(see Fig.1)and from fB=H (ρ g sin α) for n=1, where H is lava thickness(depth of tree mold)(see Fig.2).
[Estimation of the yield strength by the lava tube cave]
When the lava tube cave hollow height is Hc, the lava yield strength will be fBc= Hc (ρ g sin α)/4. The height of lava tube caves and slope angles for Suyama Tainai Cave 2), for Subashiri Tainai Caves 3,4) and for Ganno-Ana Cave 5) are indicated in Table.1. The estimated lava yield strength are also shown in Table 1. The yield strength shows relatively low value between 0.8x103 N/m2 and 3.2x103N/m2.
[Estimation of the apparent yield strength by the lava flow thickness (the tree mold depth)]
The yield strength fBt= Ht (ρ g sin α) is estimated from the lava flow stop condition of the free surface of lava flow of lava depth Ht which is equivalent of the depth of lava tree mold. Slope angle α is estimated from a contour line of the map. The results of lava flows for Suyama Tainai 2), Ganno-Ana3,4), Takamarubi6), Higashisuzuka South7,8), Kenmarubi I9,10), Kenmarubi II9),are shown in Table.1. The apparent yield strength obtained from the lava depth shows higher value than those obtained from the lava tube cave hollow height.The highest value is fBt=10.1x103N/m2 for Kenmarubi II. It seems that the lava flow caused a deviation from simple flow due to inflation and repeated accumulation of lava, consequently, indicates higher value of apparent yield strength.
[Summary]
(1) The yield strength obtained from the lava flow thickness (the depth of the tree mold) is an apparent yield strength, because the lava flow has caused inflation and repeated accumulation of lava. (2) The minimum yield strength can be obtained from the thickness of the toe or the lobe in the front edge of lava flow. (3) The true yield strength of lava can be obtained from the hollow height of the lava tube cave. (4) The lava tube cave can be formed when a lava flow caused an increase of the thickness more than 4 times due to inflation of lava.
Reference:
1)G.Hulme(1974):Geophys.J.R.Astr.Soc.vol39,p361
2)T.Honda et al(2017): B3-06,Fall meeting2017 of The Vulcanological Society of Japan
3)H.Ishihara (1929): Historical site and natural monuments survey report volume 4 , Yamanashi-Prefecture,p96
4)T.Honda at al(2014): P2-14,Fall meeting2014 of The Volcanological Society of Japan p166
5)T.Honda (2017): 26th Meeting of Kanda-machi of the Speleological Society of Japan,p64
6)T.Ogawa(1997): Yamanashi-Prefecture the north of Mr.Fuji Kashiwara lava tree mold group observation report, the volcanic cave department of the Japanese speleological society, and Mt. Fuji volcanic cave research society, Sakae Print Company
7)K.Ogawa (1986):Fuji-city natural monuments survey report of the geological and the geographical feature of Fuji-city area edited by Fuji-city
8)T.Honda (2002): V033-006,JPGU meeting 2002
9) Yamanashi-Prefecture natural monument urgent survey committee,lava caves and tree molds investigation group, T.Ogawa(ed):Yamanashi-Prefecture natural monument urgent survey report (1996), Yamanashi-Prefecture Education Committee, p182-344.
10) T.Honda et al(2017): SVC50-P02, JPGU-AGU Joint Meeting 2017
Many lava tube caves and lava tree molds exist in lava flows of Mt. Fuji. Lava tube cave and lava tree mold coexists in Suyama Tainai lava flow and Ganno-Ana lava flow. Only lava tube cave exists in Subashiri-guchi lava flow located in the hight altitude. Only lava tree mold exists in relatively thin lava flow such as Takamarubi lava flow, Higashisuzuka-South lava flow, Kenmarubi I and Kenmarubi II lava flows. The yield strength of the Mt. Fuji lava flows was estimated from the hollow height of the lava tube cave and the depth of the lava tree mold and compared each other by using Bingham flow model. The lava flow structure of these Mt.Fuji is also discussed based on the difference of these yield strength.
[Considered model]
The lava flow is modeled by Bingham fluid flowing on the inclined plane or in the inclined cylindrical pipe with gravity potential. For the lava flow of density ρ, and yield strength fB, with slope angle α, under the gravity g, the lava flow stop condition is H=nfB/ (ρ g sin α) where H is the lava thickness. The case of lava which flows on the incline plane with a free surface is n=1, and the case of lava which flows through an inclined circular tube is n=4. The yield strength is obtained from fB =H (ρ g sin α)/4,for n=4, where H is the lava tube cave height(see Fig.1)and from fB=H (ρ g sin α) for n=1, where H is lava thickness(depth of tree mold)(see Fig.2).
[Estimation of the yield strength by the lava tube cave]
When the lava tube cave hollow height is Hc, the lava yield strength will be fBc= Hc (ρ g sin α)/4. The height of lava tube caves and slope angles for Suyama Tainai Cave 2), for Subashiri Tainai Caves 3,4) and for Ganno-Ana Cave 5) are indicated in Table.1. The estimated lava yield strength are also shown in Table 1. The yield strength shows relatively low value between 0.8x103 N/m2 and 3.2x103N/m2.
[Estimation of the apparent yield strength by the lava flow thickness (the tree mold depth)]
The yield strength fBt= Ht (ρ g sin α) is estimated from the lava flow stop condition of the free surface of lava flow of lava depth Ht which is equivalent of the depth of lava tree mold. Slope angle α is estimated from a contour line of the map. The results of lava flows for Suyama Tainai 2), Ganno-Ana3,4), Takamarubi6), Higashisuzuka South7,8), Kenmarubi I9,10), Kenmarubi II9),are shown in Table.1. The apparent yield strength obtained from the lava depth shows higher value than those obtained from the lava tube cave hollow height.The highest value is fBt=10.1x103N/m2 for Kenmarubi II. It seems that the lava flow caused a deviation from simple flow due to inflation and repeated accumulation of lava, consequently, indicates higher value of apparent yield strength.
[Summary]
(1) The yield strength obtained from the lava flow thickness (the depth of the tree mold) is an apparent yield strength, because the lava flow has caused inflation and repeated accumulation of lava. (2) The minimum yield strength can be obtained from the thickness of the toe or the lobe in the front edge of lava flow. (3) The true yield strength of lava can be obtained from the hollow height of the lava tube cave. (4) The lava tube cave can be formed when a lava flow caused an increase of the thickness more than 4 times due to inflation of lava.
Reference:
1)G.Hulme(1974):Geophys.J.R.Astr.Soc.vol39,p361
2)T.Honda et al(2017): B3-06,Fall meeting2017 of The Vulcanological Society of Japan
3)H.Ishihara (1929): Historical site and natural monuments survey report volume 4 , Yamanashi-Prefecture,p96
4)T.Honda at al(2014): P2-14,Fall meeting2014 of The Volcanological Society of Japan p166
5)T.Honda (2017): 26th Meeting of Kanda-machi of the Speleological Society of Japan,p64
6)T.Ogawa(1997): Yamanashi-Prefecture the north of Mr.Fuji Kashiwara lava tree mold group observation report, the volcanic cave department of the Japanese speleological society, and Mt. Fuji volcanic cave research society, Sakae Print Company
7)K.Ogawa (1986):Fuji-city natural monuments survey report of the geological and the geographical feature of Fuji-city area edited by Fuji-city
8)T.Honda (2002): V033-006,JPGU meeting 2002
9) Yamanashi-Prefecture natural monument urgent survey committee,lava caves and tree molds investigation group, T.Ogawa(ed):Yamanashi-Prefecture natural monument urgent survey report (1996), Yamanashi-Prefecture Education Committee, p182-344.
10) T.Honda et al(2017): SVC50-P02, JPGU-AGU Joint Meeting 2017