[MIS31-P01] Study of the lava flow under the Marius Hills Hole on the Moon by analogy on the Earth
Keywords:lava tube, viscosity, yield strength, lava flow, lunar pit
2.Examination of the earth's lava flow:Tables 1 (a), (b) and (c) show the in-situ measured values such as temperature, lava thickness, viscosity coefficient, yield strength, etc. are shown for Miharayama 1951 lava flow (SiO2: 52-53 wt%) 4), Mauna Loa 1984 lava flow (SiO2: 52 wt%) 5), Tolbatik 2013 lava flow (SiO2: 52wt%) 6) Each flow shows a low Reynolds number. In the Mauna Loa example, temperature and fluid properties including yield strength and estimated B, He are shown over long distances. As the temperature decreases along the flow, the fluid property value increases significantly. These flows are all in the laminar region from the outlet to the downstream by creating the lava tubes. The lava tube on the earth is formed in a laminar region.
3. Examination of lava flow in the lava river Rilles-A of Marius Hills on the Moon:To know B, He, and Re of the Rilles-A lava flow of Marius Hill, the lava flow thickness, viscosity coefficient, yield strength, gravity acceleration, lava density, lava flow velocity, and slope angle are required. The lava flow thickness is assumed as 17m which is tube height below Marius Hills Hole in the Rilles-A. The yield strength 131Pa7) was used as fixed value. Viscosity coefficient was assumed in the range from 5 Pa.s to 16000 Pa.s. The flow velocity was calculated by the formula 8) of free surface gravity flow and parallel plate gravity flow with an inclination angle of 0.31 ° as laminar flow. Tables 1 (d) and 1 (e) show B, Re, and He based on the obtained flow velocity. The free surface gravity flow is in the transition region with a viscosity coefficient of 100 Pa.s, and shows laminar flow at 3000 Pa.s, The parallel plate gravity flow shows laminar flow at 100 Pa.s. The temperature dependent viscosity coefficient of lunar lava is summarized in Chevrel et al (2014) 9) for lava of various chemical compositions. Although the chemical composition of Marius Hill is unknown, 100Pa.s to 3000Pa.s for high-titanium lava (sample 15555) shown in Fig. 3 of Cukierman et al (1973) 10) corresponds to lava temperature 1050-1000℃ as shown in Table 1 (d) (e). For a lava with a low titanium content (sample 68502), 100Pa.s to 3000Pa.s may correspond to a higher lava temperature of 1200-1100°C. Since the lava tube is thought not to be formed by the turbulent mixing effect avoiding the tube ceiling formation, a scenario where the laminar lava flow forms a lava tube seems to be reasonable
4. Summary:A possible scenario of lava tube formation in the Rilles-A is assumed as follows;(1) the lava is in a turbulent state in the vicinity of the eruption point when the lava is hot (low viscosity coefficient, low yield strength), (2) the temperature decreases along the flow direction (viscosity coefficient and yield value increase), and the transition from turbulent to laminar flow occurs together with formation of lava tube. The estimated lava tube formation temperature may be around 1000-1200℃ depending on the lava chemical composition. Future study needs accumulation and examination of data base with synthetic sample experiment for viscosity coefficient,yield strength based on the chemical composition of lunar lava as a function of temperature(Ishibashi11)measured both for Fuji1707 lava).
References:see abstract of Japanese version