11:00 〜 13:00
[SVC28-P03] 伊豆小笠原弧福徳岡ノ場火山2021年8月噴火の緊急海底調査（予察的報告）
キーワード：海底噴火、福徳岡の場、緊急調査、海洋地質、噴火ダイナミクス
Fukutoku-Oka-no-Ba (FOB) is an active submarine volcano located in the southernmost Izu-Bonin Arc, which erupted on August 13, 2021 for the first time in 11 years, according to observations by meteorological satellite HIMAWARI-8 and Japan Coast Guard aircraft. The eruption reached a plume altitude of 16 km at the tropopause (JMA Volcanic Activity Report, August 13, 2021, 13:30), and satellite images showed that discolored water appeared below the sea surface at the onset of the eruption, followed by pumice floating above the sea surface. Furthermore, aircraft observation by the Coast Guard found a new emergent island with a diameter of about 1 km was born and pumice raft on the sea surface started to drift northwestward (JMA Volcanic Activity Report, August 16, 2021, 14:00). Formation of this new island at FOB volcano was the first time in 35 years since the 1986 eruption for such an event to occur. The pumice raft has been spreading with the ocean currents since then, and transported west by currents and winds. They started to arrive at the Daito and Ryukyu Islands from early October, 2021 (Yoshida et al., 2022, Island Arc), washing up on local beaches and coasts, causing hazards to local fisheries and marine transportations.
The volume of ejecta from this eruption is estimated to be more than 0.1 km3, and considering the altitude of the plume, it is highly likely that this was a Plinian-type eruption with a Volcanic Explosion Index (VEI) of 4. This is the largest eruption to have occurred in Japan since the beginning of the 21st century, including land-based volcanoes. This 2021 eruption of FOB volcano provides crucial opportunity to understand the eruption dynamics and tephra dispersal mechanism during the explosive submarine eruption. Recent efforts to combine high-resolution satellite observation and extensive seafloor surveys have provided more detailed knowledge of the dynamics of submarine volcanism as shown by the 2012 eruption of Havre volcano in the Kermadec arc (e.g. Carey et al., 2018, Sci. Adv.). Furthermore, comparing the 2021 eruption of FOB to the 2012 eruption of Havre may provide quantitative constraints on how water depth controls vesiculation and fragmentation of the magma, as Havre 2021 eruption occurred at a depth of 500 m or more, while the 2021 eruption of FOB occurred in shallow water with a pre-eruption summit depth of about 30 m (Ito et al., 2011, Rep. Hydro. Ocean Res.).
Soon after the August 2021 eruption, we proposed a rapid response cruise to conduct seafloor survey and rock/sediment sampling at the FOB. This is to observe and collect samples before the tephra from the 2021 eruption becomes disturbed by secondary resedimentation processes. The proposal is accepted and two research cruises are scheduled in April and August, 2022 using R/V SHINSEIMARU of JAMSTEC. We will conduct sediment sampling by gravity and box corers and rock sampling by dredge in April, and more detailed seafloor observation and sampling using a remotely operated vehicle (ROV) in August, with detailed seafloor mapping on both cruises. We expect to understand the detailed time sequence of the 2021 eruption from the sediment cores and the spatial extent and variety of the ejecta from ROV observations. By integrating these observations with satellite and airborne observations, we hope to decipher the August 2021 eruption in detail, including the distribution and deposition mechanisms of the ejecta beneath the sea surface. In this presentation, we will introduce our survey plans and preliminary results from the SHINSEIMARU KS-22-5 cruise in April.
The volume of ejecta from this eruption is estimated to be more than 0.1 km3, and considering the altitude of the plume, it is highly likely that this was a Plinian-type eruption with a Volcanic Explosion Index (VEI) of 4. This is the largest eruption to have occurred in Japan since the beginning of the 21st century, including land-based volcanoes. This 2021 eruption of FOB volcano provides crucial opportunity to understand the eruption dynamics and tephra dispersal mechanism during the explosive submarine eruption. Recent efforts to combine high-resolution satellite observation and extensive seafloor surveys have provided more detailed knowledge of the dynamics of submarine volcanism as shown by the 2012 eruption of Havre volcano in the Kermadec arc (e.g. Carey et al., 2018, Sci. Adv.). Furthermore, comparing the 2021 eruption of FOB to the 2012 eruption of Havre may provide quantitative constraints on how water depth controls vesiculation and fragmentation of the magma, as Havre 2021 eruption occurred at a depth of 500 m or more, while the 2021 eruption of FOB occurred in shallow water with a pre-eruption summit depth of about 30 m (Ito et al., 2011, Rep. Hydro. Ocean Res.).
Soon after the August 2021 eruption, we proposed a rapid response cruise to conduct seafloor survey and rock/sediment sampling at the FOB. This is to observe and collect samples before the tephra from the 2021 eruption becomes disturbed by secondary resedimentation processes. The proposal is accepted and two research cruises are scheduled in April and August, 2022 using R/V SHINSEIMARU of JAMSTEC. We will conduct sediment sampling by gravity and box corers and rock sampling by dredge in April, and more detailed seafloor observation and sampling using a remotely operated vehicle (ROV) in August, with detailed seafloor mapping on both cruises. We expect to understand the detailed time sequence of the 2021 eruption from the sediment cores and the spatial extent and variety of the ejecta from ROV observations. By integrating these observations with satellite and airborne observations, we hope to decipher the August 2021 eruption in detail, including the distribution and deposition mechanisms of the ejecta beneath the sea surface. In this presentation, we will introduce our survey plans and preliminary results from the SHINSEIMARU KS-22-5 cruise in April.