講演情報
[U04-05]微視的構造解析から迫るマグマ破砕メカニズム：爆発的火山噴火の発生原因の解明へ向けて★招待講演
*奥村 聡1、上杉 健太朗2、坂巻 竜也1、後藤 章夫3 (1.東北大学大学院理学研究科地学専攻地球惑星物質科学科、2. 国立研究開発法人理化学研究所高輝度光科学センター、3.東北大学東北アジア研究センター)
キーワード：
火山噴火、マグマ、破砕、X線イメージング、X線回折
Explosive volcanism is one of the most spectacular phenomena on Earth. Explosive volcanism is characterized by the vigorous release of magma fragments and volcanic gases to the surface. In such cases, the magma is the continuum in the crust but becomes fragmented during its ascent. In particular, brittle fragmentation of silicic (relatively SiO2-rich) magma results in explosive eruptions. Therefore, the cause of brittle fragmentation of magma determines whether the eruption will be explosive or effusive.
The viscoelasticity of magma and the relaxation timescale of its structure are currently thought to control fragmentation (Dingwell, 1996 Science). Under rapid deformation rates, magma exhibits elastic behaviour, while viscous magma behaviour is dominant when the deformation rate is slow. Fiber elongation experiments for magmas clearly demonstrate the transition from viscous elongation to brittle fracturing with increased elongation rates. This transition is empirically formulated and can be applied to the modelling of volcanic eruptions (Gonnermann and Manga, 2003 Nature). In contrast, the atomic- to nano-scale structure of melt across this transition is poorly understood. For example, magma exhibits shear thinning (viscosity decreases as shear rate increases) behaviour when the shear rate is close to the conditions at which magma fracturing occurs. No explanation for the relationship between shear thinning and brittle fracturing has yet been proposed. Understanding the atomic- to nano-scale structures across viscous to brittle transition will provide important information for clarifying the mechanism of magma fragmentation. We have investigated magma fracturing by using in-situ X-ray imaging and diffraction techniques. Some preliminary results of our work will be introduced in this presentation.
The viscoelasticity of magma and the relaxation timescale of its structure are currently thought to control fragmentation (Dingwell, 1996 Science). Under rapid deformation rates, magma exhibits elastic behaviour, while viscous magma behaviour is dominant when the deformation rate is slow. Fiber elongation experiments for magmas clearly demonstrate the transition from viscous elongation to brittle fracturing with increased elongation rates. This transition is empirically formulated and can be applied to the modelling of volcanic eruptions (Gonnermann and Manga, 2003 Nature). In contrast, the atomic- to nano-scale structure of melt across this transition is poorly understood. For example, magma exhibits shear thinning (viscosity decreases as shear rate increases) behaviour when the shear rate is close to the conditions at which magma fracturing occurs. No explanation for the relationship between shear thinning and brittle fracturing has yet been proposed. Understanding the atomic- to nano-scale structures across viscous to brittle transition will provide important information for clarifying the mechanism of magma fragmentation. We have investigated magma fracturing by using in-situ X-ray imaging and diffraction techniques. Some preliminary results of our work will be introduced in this presentation.