高温流紋岩質溶岩流 : 水接触型マグマ水蒸気爆発の発生機構  [in Japanese] Mechanism of a phreatomagmatic explosion due to the interaction between hot rhyolitic lava flow and external water  [in Japanese]

高温の溶岩流が浅い水域を流れるとき, 溶岩流底部で外来水と溶岩とが接触し爆発を起こすことがある。このような爆発現象はどのような機構で, また, どのような物理条件下で発生するのか, 伊豆諸島の流紋岩質火山島における地質調査結果をもとにして検討を行った。ここでは, 最も可能性の高いモデルとして, 平衡破綻型マグマ水蒸気爆発を提案した。すなわち, 溶岩流底部のブロック溶岩内に水と水蒸気とが局所平衡共存する水蒸気溜まりが形成され, そこに臨界サイズ以上の割れ目が急速に形成されることによって平衡状態が破られ, 膨張波の伝播にともなう過熱水の急激蒸発によって爆発は発生するものと考えた。このような爆発発生機構を想定すると, 同流紋岩質火山島で観察される火砕サージの発生をはじめとする多くの地質学的特徴を統一的に説明することが可能であった。

Field observations of the rhyolitic volcanic islands in the Izu-Shichito Islands indicated the presence of many geological traces of non-magmatic explosion. They are secondary-craters ranging from 100 to 200 m in diameter at the marginal part of lava flows, spiracles and related pyroclastic-surge deposits. Geological evidences suggest that they were formed by the interaction between hot rhyolitic lava flow and sea water in the bottom block lava. In the present paper, I made a brief discussion on the physical condition and mechanism of the explosion considering the geological evidences in mind, and presented a new model for the explosion; the phreatomagmatic explosion of the destruction of phase-equilibrium type. This model can explain many geological characters in the volcanic islands succesively. The present scenario for this explosion is summarized as follows; 1. Hot rhyolitic lava flow overlying external water, 2. Formation of steam reservoir with the local equilibrium between steam and water in the bottom block lava, 3. Rapid tear of massive lava overlying the steam reservoir, 4. Rapid discharge of pressurized steam, 5. Downward propagation of expansion wave and explosive vaporization of super-heated water, 6. Breakdown of lava by the attack with multi-phase flow due to the explosive vaporization, 7. Outward propagation of blast wave and the appearance of pyroclastic-surge (outburst equivalent of the multi-phase flow) on the ground.