[SVC47-P01] Yield strength obtained from tree molds and lava tube caves of Medicine Lake Volcano
Keywords:tree mold, lava tube, yield strength, lava flow
1.Introduction: Medicine Lake Volcano in northern California is the largest volcano in the Cascade Range that has erupted nine times over the past 5200 years. On the northeastern flank of this shield volcano, many lava tree molds formed on the lava flow that flowed about 10,000 years ago and many lava caves formed 30,000 to 40,000 years ago are preserved in the Lava-Beds National Monument 1,2,3). We measured the depth of these tree molds and the height of the lava tube cave, then, estimated the yield strength of the lava flow. The slope angle was roughly estimated by reading the map contour.
2.Lava tree molds:Lava tree molds are abundant in the btm (basaltic tree mold) lava flow of SiO2: 51.9wt%, 12,330 yr B.P at an altitude of 2000m, with their diameter range from 15cm to 90cm. Since it is an old lava flow, deposits are accumulated at the bottom of most vertical tree molds, though for the btm-TM-4.2m vertical tree mold,it has been removed for C14 dating4). The position of the tree mold at N41.63854, W121.60744, with diameter: 0.79mx0.91m, depth: 4.2m as shown in Fig.1. . As the tree mold depth is the lava flow thicknessH, the critical limit thickness of lava flow fB = H (ρg sinα) was used to determine the apparent lava yield strength fB where the gravity acceleration was g = 980 cm / s2, and density was ρ = 2.5 g / cm3. As shown in Table.1, the apparent yield strength: 1.8x104 Pa is obtained from the slope angle α of 9.5 °. This apparent yield strength shows a relatively high value for a lava flow of SiO2: 51.9wt%, which may be due to lava inflation.
3.Lava tube caves:The heights H of Valentine Cave located in bvc lava flow(SiO2: 53.0 wt%, 12,260 yr B.P.) and Sentinel Cave located in bmc lava flow(SiO2: 52.3 wt%, 40Ar / 39Ar age / 36 ± 16 ka) are measured. Fig.2 and Fig.3 show photographs of inside of each cave. The lava yield strength fB was obtained from the flow critical limit condition of the gravity flow in an inclined pipe of slope angle:α,fB = H (ρg sinα) / 4. Table 2 shows the lava yield strength obtained from these lava tube caves. The obtained yield strength is in the range of 1.54x103 to 2.8x103 Pa which is a reasonable yield strength as a basaltic lava of SiO2: 52.0-53.0 wt%.
4.Conclusions:The apparent yield strength obtained from the tree mold depth (lava thickness) is 6 to 12 times higher the yield strength obtained from the lava tube cave height. Even though the btm lava flow is different from the bmc lava flow, this indicates that the lava flow caused lava inflation in general and transitioned from simple flow to inflated flow. This also shows that the lava tube cave is formed through lava inflation.
Acknowledgments: I wish deeply thank to Dr. J. Donnelly-Nolan and Dr. J. Tinsley of the US Geological Survey (USGS) for geological information and the location of lava trees and their excavation status.
References:
1)Johnston, D.A., and Donnelly–Nolan, J.M. eds(1981)., Guides to some volcanic terranes in Washington, Idaho, Oregon, and northern California: US Geological Survey Circular 838, p143
2)Katie Keller Lynn(2014): Lava Beds National Monument Geologic Resources Inventory Report Natural Resource Report NPS/NRSS/GRD/NRR—2014/804 p22-23
3)http://geotripper.blogspot.com/2010/01/other-california-if-tree-falls-in.html
4)Private communication with Donnelly–Nolan, J.M.,:Jul 16, 2018, on the locations of the carbon-14 samples collected from large tree molds in map units btm, bvc, and bbr.
5)Donnelly–Nolan, J.M (2010):Geologic Map of Medicine Lake Volcano, Northern California:Pamphlet to accompany Scientific Investigations Map 2927,USGS
2.Lava tree molds:Lava tree molds are abundant in the btm (basaltic tree mold) lava flow of SiO2: 51.9wt%, 12,330 yr B.P at an altitude of 2000m, with their diameter range from 15cm to 90cm. Since it is an old lava flow, deposits are accumulated at the bottom of most vertical tree molds, though for the btm-TM-4.2m vertical tree mold,it has been removed for C14 dating4). The position of the tree mold at N41.63854, W121.60744, with diameter: 0.79mx0.91m, depth: 4.2m as shown in Fig.1. . As the tree mold depth is the lava flow thicknessH, the critical limit thickness of lava flow fB = H (ρg sinα) was used to determine the apparent lava yield strength fB where the gravity acceleration was g = 980 cm / s2, and density was ρ = 2.5 g / cm3. As shown in Table.1, the apparent yield strength: 1.8x104 Pa is obtained from the slope angle α of 9.5 °. This apparent yield strength shows a relatively high value for a lava flow of SiO2: 51.9wt%, which may be due to lava inflation.
3.Lava tube caves:The heights H of Valentine Cave located in bvc lava flow(SiO2: 53.0 wt%, 12,260 yr B.P.) and Sentinel Cave located in bmc lava flow(SiO2: 52.3 wt%, 40Ar / 39Ar age / 36 ± 16 ka) are measured. Fig.2 and Fig.3 show photographs of inside of each cave. The lava yield strength fB was obtained from the flow critical limit condition of the gravity flow in an inclined pipe of slope angle:α,fB = H (ρg sinα) / 4. Table 2 shows the lava yield strength obtained from these lava tube caves. The obtained yield strength is in the range of 1.54x103 to 2.8x103 Pa which is a reasonable yield strength as a basaltic lava of SiO2: 52.0-53.0 wt%.
4.Conclusions:The apparent yield strength obtained from the tree mold depth (lava thickness) is 6 to 12 times higher the yield strength obtained from the lava tube cave height. Even though the btm lava flow is different from the bmc lava flow, this indicates that the lava flow caused lava inflation in general and transitioned from simple flow to inflated flow. This also shows that the lava tube cave is formed through lava inflation.
Acknowledgments: I wish deeply thank to Dr. J. Donnelly-Nolan and Dr. J. Tinsley of the US Geological Survey (USGS) for geological information and the location of lava trees and their excavation status.
References:
1)Johnston, D.A., and Donnelly–Nolan, J.M. eds(1981)., Guides to some volcanic terranes in Washington, Idaho, Oregon, and northern California: US Geological Survey Circular 838, p143
2)Katie Keller Lynn(2014): Lava Beds National Monument Geologic Resources Inventory Report Natural Resource Report NPS/NRSS/GRD/NRR—2014/804 p22-23
3)http://geotripper.blogspot.com/2010/01/other-california-if-tree-falls-in.html
4)Private communication with Donnelly–Nolan, J.M.,:Jul 16, 2018, on the locations of the carbon-14 samples collected from large tree molds in map units btm, bvc, and bbr.
5)Donnelly–Nolan, J.M (2010):Geologic Map of Medicine Lake Volcano, Northern California:Pamphlet to accompany Scientific Investigations Map 2927,USGS