An explosive type volcanic eruption causes devastating disaster due to extremely high energy release rate.Numerical predictions of hazardous phenomena accompanied by explosive type volcano eruptions are carried out in order to establish a practical means of providing quantitative database for establishing better safety measures for specific volcanoes. Conservation laws of mass, momentum and energy, together with the perfect-gas equation of state are solved. In order to solve problems for specific volcanoes, three dimensional (3D) computations with corresponding ground geometry of the interested area are carried out. The 3D simulations with a computational domain large enough for collecting meaningful data are categorized as large scale computations even for today's large computer systems. Parallel computations with MPI and OpenMP together with vector processing techniques are employed in this study and some details are given. Also considered in this paper are the eruption models that describe how the energy is released. The method is applied to an imaginary eruption of Mt. Usu located in Hokkaido Japan to demonstrate how numerical simulations can be utilized for investigating volcanic eruptions.