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Gigantic lava dome found under sea off Kagoshima Pref.

KOBE -- The Kobe Ocean-Bottom Exploration Center (KOBEC) announced on Feb. 9 that one of the world's largest lava domes was discovered at Kikai Caldera, an undersea volcano about 50 kilometers south of the Satsuma Peninsula in Kagoshima Prefecture.

    The lava dome, measuring 600 meters high, 10 kilometers in diameter and more than 32 cubic kilometers in volume, may be a sign that a large magma chamber capable of triggering an enormous caldera eruption is growing, KOBEC officials said.

    Kikai Caldera is a 20-kilometer diameter hollow created by a massive eruption about 7,300 years ago that is said to have wiped out the Jomon culture across southern Kyushu. KOBEC explored the caldera's bottom three times during the two-year period up to October 2017, using the Fukae Maru, a training ship owned by Kobe University graduate school, and confirmed a dome within the caldera.

    After analyzing rocks and stones collected from the dome, researchers found that their chemical property was different from that of materials ejected when the Kikai Caldera was formed. A further survey of the surrounding faults found a distortion caused by a bulge from the inside. These findings led researchers to conclude that the dome is made of lava and was created by an eruption sometime after the formation of Kikai Caldera.

    Furthermore, a hydrothermal plume was confirmed billowing above the lava dome, and researchers speculate that there is volcanic activity immediately beneath the dome.

    According to KOBEC, the Japanese archipelago has seen a total of 10 massive caldera eruptions that spewed magma of at least 40 cubic kilometers each over the past 120,000 years, the last of which was apparently the eruption giving birth to Kikai Caldera.

    Professor Yoshiyuki Tatsumi, head of KOBEC and a magma specialist, commented, "Although the probability of a gigantic caldera eruption hitting the Japanese archipelago is 1 percent in the next 100 years, it is estimated that the death toll could rise to approximately 100 million in the worst case scenario."

    Papers on the discovery of the giant lava dome were published in the electronic edition of the British scientific journal "Scientific Reports" on Feb. 9. Among the 17 authors of the papers is Hideaki Takizawa, 35, a popular Japanese actor and singer affectionately called Tackey, who collected the rocks and stones that were used in the analysis of the dome.

    During the shooting of a TV program for public broadcaster NHK last fall, Takizawa dived to the top of the lava dome -- which according to KOBEC lies about 20 meters below the surface of the sea -- to collect the rocks and stones. "He played an extremely crucial role," professor Tatsumi commented.

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