[U-02_28AM1] Particle Geophysics

★ [U02-02] 火道内マグマ対流：Muon観測からの制約

*篠原 宏志¹、田中 宏幸² (1.産業技術総合研究所、2.東京大学、地震研究所)

キーワード：火道内マグマ対流, ミュオン, 火山体浅部構造, 密度構造

Muographic imaging is a powerful tool to radiographically reveal density structure of a shallow volcanic edifice with high energy muons and was applied to the rhyolitic dome of Iwodake, Satsuma-Iwojima in order to understand the conduit magma convection in this volcano. In this paper, we will discuss the constraints obtained by the muographic measurements performed in 2008 and their implication to the conduit magma convection model. Conduit magma convection is a model to explain persistent degassing, that is continuous emission of large amount of volcanic gases without eruption and is driven by the density contrast between the ascending non-degassed magma and the descending degassed magma that is created by outgassing at the top of a magma column (Kazahaya et al., 1994). This model is commonly applied to less viscous basaltic magma systems but the application to andesitic or rhyolitic magma system is a matter of debate, because the large viscosity of these magmas can slow down the magma flows in the conduit. Although theoretical evaluation indicated that a larger diameter of a conduit can compensate the larger magma viscosity and can cause the rapid magma flows in the conduit, it is difficult to prove its occurrence under the ground, as the conduit magma convection is a steady state process with few seismic signals nor deformation. In contrast, the conduit magma convection suggests that intensive degassing occurs at top of a magma column, which is likely detectable as a low density zone in a shallow magma conduit system. Therefore the density structure survey the muon-radiography is an ideal method to reveal the size, shape and magnitude of density anomaly at the shallow volcanic edifice. Quantitative re-evaluation of the muon radiography data at the Iwodake rhyolitic cone obtained by Tanaka et al. (2009) confirms the existence of a low-density body of 300 m in diameter and with 0.9-1.0 g cm^-3 at depths of 135-190 m from the summit crater floor. The low-density material is interpreted as rhyolitic magma with 60% vesicularity on average, and existence of this unstable highly vesiculated magma at shallow depth without any recent eruptive or intrusive activity is considered evidence of conduit magma convection. The structure of the convecting magma column top was modeled based on density calculations of vesiculated ascending and outgassed descending magmas, compared with the observed density anomaly. The existence of the low-density anomaly was confirmed by comparison with published gravity measurements, and the predicted degassing at the shallow magma conduit top agrees with observed heat discharge anomaly distribution localized at the summit area. This study confirms that high viscosity of silicic magmas can be compensated by a large size conduit to cause the conduit magma convection phenomena. The rare occurrence of conduit magma convection in a rhyolitic magma system at Iwodake is suggested to be due to its specific magma features of low H₂O content and high temperature.