3:00 PM - 3:15 PM
[SVC32-05] Numerical study on the quasi-periodic behavior of effusive eruption and its application to the 1991-1995 eruption of Mt. Unzen
Keywords:Volcanic conduit flow model, Magma plumbing system, Mt. Unzen
Geophysical observations have detected the periodic or damped oscillations of the chamber pressure and discharge rate in effusive eruption. It has been proposed that the cause of the oscillatory phenomena in such eruptions is the instability of the volcanic conduit flow due to the changes in the physical properties of magma and the deformation of the conduit. Maeda (2000) constructed the numerical model assuming that the magma supply rate varies with time in a soliton-like manner due to viscoelastic deformation of the deep conduit and the shallow conduit radius varies with time depending on the pressure of the magma chamber. This model can explain the pulsation of the eruption rate and chamber pressure. Maeda (2000) also simulated the damped oscillatory behavior of the discharge rate with two peaks observed in the 1991-1995 eruption of Mt. Unzen. Although it is known that changes in the physical properties of magma during ascent in the conduit affect the eruptive transition, Maeda (2000) assumed that the viscosity and density of magma are uniform in the vertical direction with the properties of lava dome observed at the surface. As a result, the conduit deformation with a radius of tens to hundreds of meters occurred, which is an overestimation compared with the result of Noguchi et al. (2008) using the size distribution and nuclear density of plagioclase microlites in the eruption products.
This study aims to clarify the effects of conduit deformation and changes in physical properties of magma during ascent on quasi-periodic variations in chamber pressure and eruption rate in effusive eruptions. For this purpose, we developed a numerical model that combines a one-dimensional steady volcanic conduit flow model that incorporates changes in physical properties and a model of a magma plumbing system that considers viscoelastic deformation of the deep and shallow conduit introduced by Maeda (2000). In the conduit flow model, We considered the volatiles exsolution, the bubbles nucleation, the vertical degassing, and the change in the effective viscosity of magma depending on the amount of dissolved volatile components and crystals. The general behavior of the model was investigated by varying parameters of the constructed model that affect the physical properties of the magma. In addition, numerical calculations using physical properties and geological conditions obtained from material studies and geodetic observations as model parameters were applied to the 1991-1995 eruption of Mt. Unzen.
The results of the parameter study showed that the density of magma in the shallow conduit affects the eruption rate, the amplitude of the pulsation of the excess pressure in the magma reservoir, the radius of the conduit, and the scale of its variation. The density of magma is affected by the initial water content of the magma and the magnitude of permeability, which controls the efficiency of degassing. On the other hand, the change in phenocryst contents, which affects the effective viscosity of the magma, only affects the conduit radius.
The application of the model to the 1991-1995 eruption found that the model can explain the observed data even if the radius of the volcano and its variation are about 50 m smaller than in previous studies. The results are reasonable with petrological studies (Noguchi et al., 2008). Using the estimated parameters, the viscosity of the crust and the volume of the magma chamber, which are generally difficult to estimate, were estimated to be 1 × 1013 Pa s and 9 × 108 m3 , respectively. The viscosities of the crust are smaller than the general values, which may be due to the effect of assuming a cylindrical conduit, which is more difficult to deform than a dike. In the future, it will be necessary to develop a more realistic model, such as one that includes a dyke shape conduit or one in which the conduit radius changes in the depth direction.
This study aims to clarify the effects of conduit deformation and changes in physical properties of magma during ascent on quasi-periodic variations in chamber pressure and eruption rate in effusive eruptions. For this purpose, we developed a numerical model that combines a one-dimensional steady volcanic conduit flow model that incorporates changes in physical properties and a model of a magma plumbing system that considers viscoelastic deformation of the deep and shallow conduit introduced by Maeda (2000). In the conduit flow model, We considered the volatiles exsolution, the bubbles nucleation, the vertical degassing, and the change in the effective viscosity of magma depending on the amount of dissolved volatile components and crystals. The general behavior of the model was investigated by varying parameters of the constructed model that affect the physical properties of the magma. In addition, numerical calculations using physical properties and geological conditions obtained from material studies and geodetic observations as model parameters were applied to the 1991-1995 eruption of Mt. Unzen.
The results of the parameter study showed that the density of magma in the shallow conduit affects the eruption rate, the amplitude of the pulsation of the excess pressure in the magma reservoir, the radius of the conduit, and the scale of its variation. The density of magma is affected by the initial water content of the magma and the magnitude of permeability, which controls the efficiency of degassing. On the other hand, the change in phenocryst contents, which affects the effective viscosity of the magma, only affects the conduit radius.
The application of the model to the 1991-1995 eruption found that the model can explain the observed data even if the radius of the volcano and its variation are about 50 m smaller than in previous studies. The results are reasonable with petrological studies (Noguchi et al., 2008). Using the estimated parameters, the viscosity of the crust and the volume of the magma chamber, which are generally difficult to estimate, were estimated to be 1 × 1013 Pa s and 9 × 108 m3 , respectively. The viscosities of the crust are smaller than the general values, which may be due to the effect of assuming a cylindrical conduit, which is more difficult to deform than a dike. In the future, it will be necessary to develop a more realistic model, such as one that includes a dyke shape conduit or one in which the conduit radius changes in the depth direction.