諏訪之瀬島火山1813年噴火(文化噴火)の噴火様式とマグマの脱水過程

抄録

The sequence and eruption styles of the 1813 eruption of Suwanose-jima Volcano, SW Japan, are reconstructed on the basis of detailed stratigraphical and petrographic features of juvenile materials. The mode of generation of the proximal deposits is discussed in the light of their distribution, surface morphology and also inner structure. It is suggested that a large pyroclastic cone was generated by successive fountaining from fissure and some craters, and that lava flows are generated in two ways; by effusion directly from vents and by secondary flowage of the pyroclastic cone. The juvenile materials of the 1813 eruption are divided into three types in terms of water content and apparent density; low porosity scoria (LPS), high porosity scoria (HPS) and dense spatters and lava flows (DSL). Direct measurements of water content for fresh samples show that the water content of LPS is systematically higher (0.2-4 wt%) than HPS and DSL (0.1-0.2 wt%), whereas mineralogical data of phenocrysts indicate that all these types had similar water content (-3 wt%) just before the eruption. It is suggested that a part of magma quenched and stopped exsolving gas due to interaction with ground water at the depth of〜100 m to form LPS, while the rest continued exsolving gas up to the surface to form HPS and DSL. The difference in apparent density between HPS and DSL is accounted for by different degrees of bubble separation at the surface.