Explosive super-eruptions can erupt up to thousands of km3 of magma with extremely high mass flow rates (MFR). The plume dynamics of these super-eruptions are still poorly understood. To understand the processes operating in these plumes we used a fluid-dynamical model to simulate what happens at a range of MFR, from values generating intense Plinian columns, as did the 1991 Pinatubo eruption, to upper end-members resulting in co-ignimbrite plumes like Toba super-eruption. Here, we show that simple extrapolations of integral models for Plinian columns to those of super-eruption plumes are not valid and their dynamics diverge from current ideas of how volcanic plumes operate. The different regimes of air entrainment lead to different shaped plumes. For the upper end-members can generate local up-lifts above the main plume (over-plumes). These over-plumes can extend up to the mesosphere. Injecting volatiles into such heights would amplify their impact on Earth climate and ecosystems.
Abstract A physics-based model to estimate source conditions for a tephra-dispersal model is developed. The source condition is generally expressed by a distribution of released particles along an eruption plume (referred to as “source magnitude distribution” SMD). The present model (NIKS-1D) calculates the SMD and the column height for given vent conditions (e.g. mass eruption rate and magma properties) on the basis of an eruption column model below the neutral buoyancy level (NBL), a downwind gravity current model around the NBL, and a particle sedimentation model. It quantitatively reproduces the following features of the SMD for typical explosive eruptions: (1) a significant amount of coarse particles are released from the rising eruption column, whereas most of the fine particles are carried to the NBL, (2) in a downwind gravity current, the coarse particles tend to decrease more rapidly with distance from the vent than the fine particles, (3) the SMD from the downwind gravity current decreases with distance more slowly in a strong ambient wind than that in a weak ambient wind, and (4) the SMD from the downwind gravity current for eruptions with large mass eruption rates decreases with distance more slowly than that for eruptions with small eruption rates. NIKS-1D includes a new parameter, $$\mu$$ μ , which represents the ratio of the volumetric flux at the source of the downwind gravity current to that of the eruption column model at the NBL. This parameter is determined by the physics of the entrainment process around the connection between the eruption column and the downwind gravity current, and depends on the intensity of eruptions. We propose an empirical formula to calculate the value of $$\mu$$ μ as a function of the mass eruption rate on the basis of the observation data from two well-studied eruption events. In a real-time tephra-dispersal forecasting system, NIKS-1D estimates the mass eruption rate from the observed plume height, and calculates the SMD from the estimated mass eruption rate as a source conditions for a tephra-dispersal forecasting immediately after an eruption.
The dynamics and evolution of the Earth's mantle are considered to be influenced by several complexities of the physical processes. In order to understand the mantle convection in the Earth, a numerical simulation is a very effective tool. Therefore, we have been improving the models which take into account of such difficult aspects as the large variation of viscosity and the existence of phase transitions. On the other hand, models of the internal structure and evolution of the mantle have been proposed based on the seismological and geological observations. The seismic tomography reveals the large scale flow of mantle convection, while the geologic data suggest that the mode and activity of the mantle convection vary episodically. Our goal is to construct the models of the mantle convection which are consistent with these observational results. In this fiscal year, our effort was focused on the improvement of the model involving the phase transition around the 660 km depth. We discuss the implication for the dynamics and evolution of the Earth's mantle.
