著者

奥村 聡 三輪 学央

出版者

特定非営利活動法人 日本火山学会

雑誌

火山 (ISSN:04534360)

巻号頁・発行日

vol.66, no.1, pp.35-43, 2021-03-31 (Released:2021-03-25)

参考文献数

65

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A phreatomagmatic explosion is a type of eruption observed on the Earth’s surface. This explosion is common because Earth is a water planet and its surface is extensively covered with water. The mechanism of this explosion can be explained as follows: magma and water mix, following which efficient heat conduction occurs and the water evaporates, finally causing the explosion. However, the mechanism of mixing of the high viscosity magma with water during its ascent remains elusive, although it often causes a phreatomagmatic explosion. In this paper, we review the previously proposed mechanisms of phreatomagmatic explosion and then, based on the petrological and geophysical observations for the 2015 eruption of Kuchinoerabujima Volcano, evaluate whether the recognized mechanisms can explain the mixing of high viscosity magma with water. To explain such an explosion, we consider a new model based on the rheological view of the magma. The laboratory experiments have revealed that the high viscosity magma exhibits shear-induced brittle fracturing, resulting in dilatancy and increased permeability. In addition, the fracturing is a common process observed in high viscosity silicic magmas intruded into shallow parts of the upper crust. Based on these observations, we propose that the shear-induced brittle failure of the high viscosity magma in a volcanic conduit causes the decompression of fluid in the magma and water in the crust diffuses into the magma, resulting in heating and pressurisation of water and additional fracturing (magma fracturing and water diffusion model). The feedback between pressurisation, fracturing, and additional thermal interactions results in an explosion. This hypothesis is attractive because of the efficient mixing of high viscosity magma and water, thus facilitating a spontaneous interaction during magma ascent. To strengthen this hypothesis, additional laboratory experiments and field-based observations will be necessary in future studies.

著者

櫻井 亮輔 中村 美千彦 奥村 聡 無盡 真弓 中谷 貴之

出版者

特定非営利活動法人 日本火山学会

雑誌

日本火山学会講演予稿集 2018 (ISSN:24335320)

巻号頁・発行日

pp.200, 2018 (Released:2019-03-06)

著者

奥村 聡

出版者

一般社団法人 日本鉱物科学会

雑誌

岩石鉱物科学 (ISSN:1345630X)

巻号頁・発行日

vol.52, no.1, pp.230724, 2023 (Released:2023-09-26)

参考文献数

17

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Magma rheology is a key factor in understanding and modelling volcanic eruptions. Until now, macroscopic rheology experiments reveal the viscosity of the magma and conditions at which shear thinning and brittle failure occur. However, it remains unclear what mechanisms control complex magma rheology from the atomic and molecular-scale structure perspective. More specifically, no experimental data on molecular-scale structure have been obtained for deforming magma in the non-Newtonian regime. To resolve this situation, we have developed an experimental system for time-resolved X-ray diffraction and scattering at SPring-8, Japan. Based on the experiments on this system, we found that intermediate-range ordering (IRO), which is related to the size of the ring formed by SiO4 tetrahedra, expands under tensional deformation. In particular, the IRO shows elastic and anisotropic deformation in the non-Newtonian regime. On the other hand, the short-range ordering such as T-O and T-T distances, where T and O represent Si and Al in the T-site and oxygen, respectively, shows no clear change during the deformation. These results imply that shear thinning and brittle failure may originate from the expansion of the ring size because the large ring is relatively weak and its formation results in cavitation. According to this model, the magma fails when the stress is large enough, rather than the strain rate, because the IRO deforms according to the stress applied to the structure. Recent experiments also observed that small and anisotropic rings form under compression. Previous rheology experiments did not confirm the difference between the conditions, at which shear thinning and brittle failure occur, under tension and compression, but the experimentally-determined molecular-scale structure clearly shows different behavior. To fully understand the mechanism of magma rheology from the view of the molecular-scale structure, we need to perform additional studies including the experiments and theoretical approaches.

著者

奥村 聡

出版者

一般社団法人 日本鉱物科学会

雑誌

岩石鉱物科学 (ISSN:1345630X)

巻号頁・発行日

pp.230724, (Released:2023-09-07)

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Magma rheology is a key factor in understanding and modelling volcanic eruptions. Until now, macroscopic rheology experiments reveal the viscosity of the magma and conditions at which shear thinning and brittle failure occur. However, it remains unclear what mechanisms control complex magma rheology from the atomic and molecular-scale structure perspective. More specifically, no experimental data on molecular-scale structure have been obtained for deforming magma in the non-Newtonian regime. To resolve this situation, we have developed an experimental system for time-resolved X-ray diffraction and scattering at SPring-8, Japan. Based on the experiments on this system, we found that intermediate-range ordering (IRO), which is related to the size of the ring formed by SiO4 tetrahedra, expands under tensional deformation. In particular, the IRO shows elastic and anisotropic deformation in the non-Newtonian regime. On the other hand, the short-range ordering such as T-O and T-T distances, where T and O represent Si and Al in the T-site and oxygen, respectively, shows no clear change during the deformation. These results imply that shear thinning and brittle failure may originate from the expansion of the ring size because the large ring is relatively weak and its formation results in cavitation. According to this model, the magma fails when the stress is large enough, rather than the strain rate, because the IRO deforms according to the stress applied to the structure. Recent experiments also observed that small and anisotropic rings form under compression. Previous rheology experiments did not confirm the difference between the conditions, at which shear thinning and brittle failure occur, under tension and compression, but the experimentally-determined molecular-scale structure clearly shows different behavior. To fully understand the mechanism of magma rheology from the view of the molecular-scale structure, we need to perform additional studies including the experiments and theoretical approaches.

著者

櫻井 亮輔 中村 美千彦 奥村 聡 無盡 真弓 中谷 貴之

出版者

日本地球惑星科学連合

雑誌

日本地球惑星科学連合2019年大会

巻号頁・発行日

2019-03-14