A detailed survey of the volcanic underwater deposits around the Kikai caldera in Japan clarified the deposition mechanisms as well as the event’s magnitude.
A detailed survey of the volcanic underwater deposits around the Kikai caldera in Japan clarified the deposition mechanisms as well as the event’s magnitude. As a result, the Kobe University research team found that the event 7,300 years ago was the largest volcanic eruption in the Holocene by far.
In addition to lava, volcanos eject large amounts of pumice, ashes and gases as a fast-moving flow, known as “pyroclastic flow,” and its sediments are a valuable data source on past eruptions. For volcanoes on land, geologists understand the sedimentation mechanism of pyroclastic flows well, but the sediments themselves get lost easily due to erosion. On the other hand, for volcanoes on oceanic islands or near the coast, the pyroclastic flow deposition process is largely unclear, both because the interaction with water is less well understood and because reliable data is difficult to obtain and therefore sparse. For these reasons, it is difficult to estimate the impact of many past eruptions on the climate and on history.
A Kobe University research team around SEAMA Nobukazu and SHIMIZU Satoshi took to the seas on the Kobe University-owned training vessel Fukae Maru (since replaced by the newly built Kaijin Maru) and conducted seismic imaging as well as sediment sampling around the Kikai caldera, off the south coast of Japan's Kyūshū island. The outstanding detail of the seismic reflection data revealed the sedimentary structure with a vertical resolution of 3 meters and down to a depth of several hundred meters below the seafloor. Shimizu explains: “Due to the fact that volcanic ejecta deposited in the sea preserve well, they record a lot of information at the time of eruption. By using seismic reflection surveys optimized for this target and by identifying the collected sediments, we were able to obtain important information on the distribution, volume, and transport mechanisms of the ejecta.”
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