Development of numerical models on explosive volcanic eruptions

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Numerical models of eruption cloud and conduit flow were developed in order to link the dynamics of explosive eruption and field observations in the pyroclastic deposits. The following 3 results were obtained.(1)Development of a 3-D numerical code of eruption cloud3-dimensional turbulence plays an essential role in mixing between eruption cloud and surrounding air, and hence, the dynamics of eruption cloud. It has been shown that such turbulent mixing can be correctly reproduced by our numerical code where 3-D coordinate and the third order accuracy with high spatial resolution are employed.(2)Development of a coupled model of bubble nucleation-growth and conduit dynamicsA coupled model of microscopic bubble nucleation-growth and macroscopic conduit dynamics has been developed, using the cell model (a model where bubbly flow is approximated as an aggregate of bubbles with shell of incompressible viscous or viscoelasitc materials). The model enables us to predict the dependence of viscosity on magma fragmentation and timing of bubble nucleation and bubble size-distribution in magma during explosive eruptions.(3)Reconstruction of detailed eruption sequence for actual explosive eruptions.Eruption sequence of the Asama 1783 eruption was reconstructed on the basis of detailed geological survey and petrological investigation for the ejecta as well as analyses of old documents. The results show that the Asama 1783 eruption is characterized by simultaneous generation of explosive eruption cloud and clastogenic lava flow, and that such an eruption style is explained by relatively low degree of magma fragmentation (coarse grain-size of the ejecta) during this eruption.