A Kobe University team has discovered that the 7,300-year-old volcanic eruption is the largest one in the last 11,700 years.
As per the high-resolution marine seismic reflection surveys, the uppermost unit exhibiting prominent seismic facies thins radially away from the caldera and covers over 4500 square meters of the seafloor. The estimated volume of the submarine deposits is over 71 km3 based on the isopach map.
Volcanoes throw out vast quantities of ash, gases, and pumice. The flow, known as a pyroclastic flow, is dangerous for everything in their path as it flows with a speed that is up to 700 km/h and a temperature that is over 1000 degrees Celsius.
The study
On land, the mechanism of transport and sedimentation processes of pyroclastic material is known. For example, it is known that they are lost in case of erosion. However, when it comes to the oceans, it is unclear due to the interaction that occurs with the water.
“It is necessary to investigate the distribution of pyroclastic deposits originating from a modern large eruption in sufficiently deep water to better understand the dynamics,” the scientists wrote in a new study.
The Kikai Caldera is situated south of Kyusyu Island in southwest Japan. That is the place where the Philippine tectonic plate slides beneath the Eurasian plate. It was formed in a basin with a water depth of approximately 200 meters.
Led by geologist Satoshi Shimizu, the Kobe University team took the wheel of the Fukae Maru training ship and examined the deposits from the depths. Seismic reflection surveys were conducted between 2016 and 2019. The ship Fukae Maru, equipped with many devices, is an advanced intelligent ship.
The largest volcanic eruption
Thanks to the high-resolution marine seismic reflection surveys chemical analyses of volcanic glasses and a remotely operated vehicle they got the results. “The total bulk volume of the K-Ah eruption is 332–457 km3 which is probably the largest Holocene eruption in the world,” they stated.
“Due to the fact that volcanic ejecta deposited in the sea preserve well, they record a lot of information at the time of eruption,” said Shimizu in the statement.
By using seismic reflection surveys optimized for this target and by identifying the collected sediments, they were able to obtain important information on the distribution, volume, and transport mechanisms of the ejecta.
For example, the sediments at the bottom of the ocean and the volcanic rocks found on the nearby islands have the same origin.
Shimizu stated that large volcanic eruptions are yet to be experienced by modern civilization and as a result they have to rely on sedimentary records. However, it has been difficult to estimate eruptive volumes with high precision. Many of the ejecta deposited on land have been lost due to erosion. “Giant caldera eruptions are an important phenomenon in geoscience, and because we also know that they influenced the global climate and thus human history in the past. Understanding this phenomenon has also social significance,” adds Shimizu.
Tim Druitt, a professor of volcanology, said to that the eruption is known, but the authors have enhanced their understanding of it.
In a study published six years ago, researchers detected a giant lava dome that exists below the Kikai Caldera. That holds a volume of more than 32 cubic kilometers of trapped magma. The most serious problem that they are worrying about is not an eruption of this lava dome. “But the occurrence of the next super-eruption,” said volcanologist Yoshiyuki Tatsumi to The New York Times.
