報告書番号

N09017

タイトル（和文）

火山噴出物解析によるマグマの粘性評価 -大規模火砕噴火の発生可能性評価に向けて-

タイトル（英文）

Magma viscosity estimation based on analysis of erupted products - Potential assessment for large-scale pyroclastic eruptions -

概要 （図表や脚注は「報告書全文」に掲載しております）

電力重要構造物への影響が懸念される大規模火砕噴火の発生可能性を考える上で，粘性の観点からマグマの噴火能力を評価することは重要な方法の一つである．本報告では，国内火山で噴火実績のあるマグマについて既存文献の噴出物解析データに基づき粘性計算を実施し，マグマ粘性を定量的に明らかにした．マグマ粘性評価可能事例は9火山16噴火にとどまるものの，噴火実績のあるマグマのマグマ溜まり条件での粘性は10^2-10^7 Pa sの5桁の範囲をとることがわかった．噴火実績のあるマグマ粘性の範囲は，力学モデルから推定される岩脈伝播の粘性限界（約10^6 Pa s）をおおむね下回っており，モデルと噴火実績が調和的であった．岩脈伝播の粘性限界を越える粘性を持つ高粘性マグマ（10^7 Pa s程度）は自身で噴火を開始する能力が非常に低いと考えられる．しかし，より低粘性のマグマが先駆噴火し，マグマ溜まりから地表への通り道が形成されれば，高粘性マグマが噴火能力を獲得できる場合がある．

概要 （英文）

After the formulation of guidelines for volcanic hazards in site evaluation for nuclear installations (e.g. JEAG4625-2009), it is required to establish appropriate methods to assess potential of large-scale pyroclastic eruptions at long-dormant volcanoes, which is one of the most hazardous volcanic phenomena on the safety of the installations. In considering the volcanic dormancy, magma eruptability is an important concept. The magma eruptability is dominantly controlled by magma viscosity, which can be estimated from petrological analysis of erupted materials. Therefore, viscosity estimate of magmas erupted in past eruptions should provide important information to assess future activities at hazardous volcanoes. In order to show the importance of magma viscosity in the concept of magma eruptability, this report overviews dike propagation processes from a magma chamber and nature of magma viscosity. Magma viscosity at pre-eruptive conditions of magma chambers were compiled from previous petrological studies on past eruptions in Japan. There are only 16 examples of eruptions at 9 volcanoes satisfying data requirement for magma viscosity estimate. Estimated magma viscosities range from 10^2 to 10^7 Pa s for basaltic to rhyolitic magmas. Most of examples fall below dike propagation limit of magma viscosity (ca. 10^6 Pa s) estimated based on a dike propagation model. Highly viscous magmas (ca. 10^7 Pa s) than the dike propagation limit are considered to lose eruptability which is the ability to form dikes and initiate eruptions. However, in some cases, small precursory eruptions of less viscous magmas commonly occur just before climactic eruptions of the highly viscous magmas, suggesting that the precursory dike propagation by the less viscous magmas induced the following eruptions of highly viscous magmas (ca. 10^7 Pa s).

報告者

キーワード