11:00 〜 11:15
[MIS02-02] Fluid dynamics of very large plumes generated by explosive super-eruptions
キーワード：super-eruption, volcanic plume, fluid dynamics
Explosive super-eruptions releasing several hundreds to thousands of km3 of magma with extremely intense flow rates occurred in the geological past of the Earth. They impacted significantly the climate and global ecosystems. Because of lack of direct observation, plume dynamics of these eruptions are poorly understood. Simple integral models based on the Buoyant Plume Theory (Morton et al., 1956; Woods and Wohletz, 1991) have been commonly used to describe them. The validity of the assumptions behind these models (e.g., self-similarity, constant air entrainment coefficient) should be validated, because the dynamics of super-eruptions can be totally different from a simple buoyant plume. We used a three-dimensional (3D) computational fluid dynamic model (Suzuki et al., 2005) to investigate the main features of these gigantic plumes characterized by Mass Flow Rate (MFR) ranging from 109 to 1011 kg/s. The lower end of the range corresponds to the most intense Plinian columns such as the 1991 Pinatubo eruption, while the upper end to the most extreme co-ignimbrite plumes such as the Toba eruption occurred 74 ka.
We performed 3D simulations of super-eruptions and compared these results with those of the previous models. At the steady-state for low and intermediate MFR, radii of the umbrella cloud spread as function of time with the same asymptotic behavior predicted by simple box models (Woods and Kienle, 1994) and this dependence can be used to estimate MFR. Simulation results also indicate that the co-ignimbrite plume radius,, growths with MFR with the same scaling for MFR vs run-out distance predicted by previous simple models of pyroclastic flows by Bursik and Woods (1996). On the other hand, the maximum heights simulated by the 3D model showed the complex dependency on the MFR, which are significantly different from those of the simple integral model (Woods and Wohletz, 1991). This difference indicates that it is necessary to consider new scaling laws of the effective air entrainment coefficients for using the simple integral models as an extrapolation in order to reproduce the gigantic plumes. Results have large implications on the assessment of the intensity and the impact of these explosive super-eruptions on the Earth climate and past ecosystems.
References
- Bursik, MI, Woods AW (1996) The dynamics and thermodynamics of large ash flows. Bull Volcanol, 58:175–193
- Costa, A., et al (2016) Results of the eruptive column model inter-comparison study. J Volcanol Geotherm Res, 326: 2–25
- Suzuki, YJ, Koyaguchi, T, Ogawa, M, Hachisu, I (2005) A numerical study of turbulent mixing in eruption clouds using a three-dimensional fluid dynamics model. J Geophys Res, 110: B08201
- Woods, A., Kienle, J. (1994),The dynamics and thermodynamics of volcanic clouds: Theory and observations from the April 15 and April 21, 1990 eruptions of Redoubt Volcano, Alaska. J Volcanol Geotherm Res, 62: 273–299.
- Woods, AW, Wohletz, K (1991) The dimensions and dynamics of coignimbrite eruption columns. Nature, 350:225–227
We performed 3D simulations of super-eruptions and compared these results with those of the previous models. At the steady-state for low and intermediate MFR, radii of the umbrella cloud spread as function of time with the same asymptotic behavior predicted by simple box models (Woods and Kienle, 1994) and this dependence can be used to estimate MFR. Simulation results also indicate that the co-ignimbrite plume radius,, growths with MFR with the same scaling for MFR vs run-out distance predicted by previous simple models of pyroclastic flows by Bursik and Woods (1996). On the other hand, the maximum heights simulated by the 3D model showed the complex dependency on the MFR, which are significantly different from those of the simple integral model (Woods and Wohletz, 1991). This difference indicates that it is necessary to consider new scaling laws of the effective air entrainment coefficients for using the simple integral models as an extrapolation in order to reproduce the gigantic plumes. Results have large implications on the assessment of the intensity and the impact of these explosive super-eruptions on the Earth climate and past ecosystems.
References
- Bursik, MI, Woods AW (1996) The dynamics and thermodynamics of large ash flows. Bull Volcanol, 58:175–193
- Costa, A., et al (2016) Results of the eruptive column model inter-comparison study. J Volcanol Geotherm Res, 326: 2–25
- Suzuki, YJ, Koyaguchi, T, Ogawa, M, Hachisu, I (2005) A numerical study of turbulent mixing in eruption clouds using a three-dimensional fluid dynamics model. J Geophys Res, 110: B08201
- Woods, A., Kienle, J. (1994),The dynamics and thermodynamics of volcanic clouds: Theory and observations from the April 15 and April 21, 1990 eruptions of Redoubt Volcano, Alaska. J Volcanol Geotherm Res, 62: 273–299.
- Woods, AW, Wohletz, K (1991) The dimensions and dynamics of coignimbrite eruption columns. Nature, 350:225–227