IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

0P1_3D-O4

Room B1

Date/Time: July 20 15:15-15:30

Numerical simulations of magma intrusion into crust near the surface by means ofdiscrete element method
Shigekazu Kusumoto
Graduate School of Science and Engineering for Research, University of Toyama, Japan
E-mail: kusu@sci.u-toyama.ac.jp

We employed the software PFC2D (Particle Flow Code in 2 Dimensions) based on the discrete element method
and attempted to simulate magma intrusion into crust near the surface. Although behaviors (e.g., initiation of ashallow magma chamber and formation of a sill) of the intruded magma depend on various factors, here we paid
attention to the intrusion magma velocity, which reflects overpressure conditions.
In discrete element modeling, all materials are approximated as assemblies of many particles, and each particle
is connected by two elastic springs (having normal and shear stiffness). In addition, "contact bond" parameters
for determining the strength of material (rock) must be set between all adjacent particles. By trial and error, theseparameters are determined by biaxial test in a computer. A Young modulus of 16 GPa and a Poisson ratio of0.21, as elastic constants of the surface crust, and viscosities in range of 107 - 1010 Pa s, for middle-high viscousmagma, were assumed in this study. The normal and shear contact bonds for intruded magma were not set inorder to maintain fluidity.
Before applying the above-mentioned magma model to simulations of magma intrusion into the crust, extrusions
to the surface of these magma were simulated in order to confirm whether the magma model behaves realistically.
As a result, it was shown that the shape of extruded magma (lava) depends on the viscosity of the magma and its
flow velocity in the conduit. The high viscous magma makes a lava dome at the surface and its interior structure is
distorted concentric circle patterns. These structures due to the high viscous magma have been simulated by someanalogue experiments. From these pre-simulation results, it was concluded that the developed magma model wassuitable for simulations of magma intrusion.
As a preliminary simulation model for magma intrusion, we modeled the surface crust by a rectangle of widthand depth 5 and 1.6 km, respectively. The radius and density of the particles constituting the model crust wereassumed to be 4.8 - 6.4 m and 2500 kg/m3 , respectively. The radius and density of the particles constituting themodel magma were assumed to be 0.8 - 0.96 m and 2200 kg/m3 , respectively, and a probable intrusion velocity
was given to the particles. The volume of the intruded magma was set to 2 105 m3 in all simulations. As a result,it was found that the shape of the intruded magma was circular at an initial stage of intrusion, and that this initial
shape did not depend on the intrusion velocity. However, the subsequent change in the shape over time did dependon the velocity. A low velocity made the intruded magma become elliptical, elongating upward and deforming thesurface greatly over a wide area. In contrast, a high velocity made the magma intrude into a circular shape, thelower parts of which elongated sideways, or in a rounded triangular shape.

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