[S-VC50_2PM1] 火山噴火のダイナミクスと素過程

[SVC50-15] 火道内を上昇する珪長質マグマの流動様式：粘性流動ｖｓ摩擦すべり

*奥村 聡¹、上杉 健太朗² (1.東北大学大学院理学研究科地学専攻、2.公益財団法人高輝度光科学研究センター)

キーワード：珪長質マグマ, 火山噴火, 粘性流動, 摩擦すべり, 放射光X線

Outgassing rate and bulk magma viscosity that control the style of volcanic eruptions depend on flow type of magma ascending in a volcanic conduit. When magma behaves as a Newtonian fluid, magma in the conduit experiences shear strain large enough to cause effective outgassing. On the other hand, once shear starts to localize, bulk magma viscosity may decrease due to slip deformation and outgassing rate also decreases in parts other than shear-localized region (Okumura et al., 2013 EPSL). Silicic magma experiences shear-induced brittle fracturing and subsequent frictional sliding along the fracturing zone during its ascent (e.g, Gonnermann and Manga, 2003 Nature; Tuffen et al., 2003 Geology). Therefore, outgassing rate and bulk magma viscosity during the ascent are expected to change dramatically. Previous studies (Tuffen et al., 2003 Geology; Gonnermann and Manga, 2005 EPSL) also proposed that fractured magma can heal during magma ascent and that fracturing and healing processes may control the dynamics of magma ascent. In contrast to this model, some experimental studies (e.g. Okumura et al., 2010) indicated that fractured magma cannot heal as long as the deformation continues. In this study, we perform deformation experiments for fractured magma to investigate flow type of magma in the conduit, i.e. viscous flow or frictional sliding, and controlling factors of the transition from viscous flow to frictional sliding.The deformation experiments were carried out using a custom-made torsional deformation apparatus which was installed in synchrotron radiation X-ray imaging system (BL20B2) of SPring-8. To simulate fractured silicic magma, we crushed rhyolite obsidian and sorted them into fragments of 75-250 μm in size. The powdered sample was sandwiched by two obsidian discs and they were twisted by rotating a piston attached with a rotational motor. The torsional deformation experiments were performed at temperatures of 800 and 900℃ under 1-10 MPa pressures. The rotational rate was set to be 0.1 to 10 rpm, corresponding to strain rates of 10^-2 to 1 s^-1 if the sample deforms homogeneously. The deformed samples were observed in situ using an X-ray radiography.At a temperature of 900℃ and rotational rates of 0.1-1 rpm, homogeneous deformation through a sample was observed under a pressure of 10 MPa, which indicates viscous deformation. In contrast, the sliding at the interface between powdered obsidian and the disc was observed under 1 and 5 MPa pressures. At a temperature of 800℃, the sliding was found under 1-10 MPa pressures. These results indicate that frictional sliding along fractured zone is flow type of magma in shallow parts of the conduit (<10 MPa). We assume that flow type is determined by competition of shear stress necessary for viscous flow and frictional sliding. If magma has high viscosity and shear stress to deform a sample viscously is large, the flow type becomes frictional sliding. At a temperature of 900℃, viscous flow and frictional sliding were found at 10 and 1-5 MPa pressures, respectively. At this condition, magma viscosity is approximately 10⁷ Pa s (Hess and Dingwell., 1996) and shear stress necessary for viscous deformation is 1 MPa at a strain rate of 0.1 s^-1. Because the frictional sliding was observed at pressures of 1-5 MPa, the frictional coefficient is estimated to be ca. 0.1. When we use this value and the criterion for shear-induced brittle fracturing proposed by Okumura et al. (2010), the dynamics of magma ascent is controlled by frictional sliding at shallow parts of the conduit. In addition, silicic magma can ascend quickly due to low frictional coefficient of fractured magma.