A detailed study of the volcanic sediments found off the coast of Japan elucidated the depositional mechanisms and the magnitude of the volcanic eruption event that created a large underwater caldera. As a result, the Kobe University research team found that the 7,300-year-old event was the largest volcanic eruption in the Holocene (the current geological era began approximately 11,700 years ago).
In addition to lava, volcanoes eject large amounts of pumice, ash, and gases as a fast-moving stream.Pyroclastic flow,” and its sediments are a valuable source of data on past eruptions.
For volcanoes on land, geologists understand the sedimentation mechanism of pyroclastic flows well, but sediment is easily lost due to erosion.
On the other hand, for oceanic islands or nearshore volcanoes, the pyroclastic flow deposition process is often unclear because the interaction with water is poorly understood and reliable data on deposits hidden beneath water is difficult to obtain. So rarely.
For these reasons, it is difficult to assess the impact of past eruptions on climate and history.
A Kobe University research team around geologist Satoshi Shimizu went out to sea aboard the Kobe University-owned training vessel Fuke Maru to conduct seismic imaging and sediment sampling around the Kikai Caldera, a large submarine off the southern coast of Kyushu Island, Japan.
Map showing the location of the Kikai Caldera.
Shimizu et al. 2024/Journal of Volcanology and Geothermal Research
„Because the volcanic emissions deposited in the sea are well preserved, they record a lot of information at the time of the eruption. By using these targeted seismic reflection surveys and identifying the collected sediments, we were able to obtain important information on the distribution, volume and transport mechanisms of the ejecta,” explains Shimizu.
In their study, geologists found that an eruption 7,300 years ago ejected large amounts of volcanic material such as ash and pumice, causing the collapse of the volcano and the formation of the Kikai caldera. The A thick layer of ejected volcanic material is continuously monitored due to its ability to reflect seismic waves. They confirmed that the sediments on the sea floor and the volcanic rocks found on nearby islands are of the same origin, covering more than 4,500 square kilometers around the eruption site.
A rock-equivalent volume between 133 and 183 cubic-kilometers (Mount St. Helens erupted a cubic kilometer of rock in 1980. and 7,700 years ago forming the slope of Mt Mazama Crater Lake(Involving „only” 100 cubic-kilometers of erupted material), the event was the largest volcanic eruption in the last 11,700 years.
The research team's findings provide new insights into the elusive dynamics of volcanic mega-events that could be useful for identifying remnants of other events and estimating their size.
„Major volcanic eruptions not yet experienced by modern civilization rely on sedimentary records, but it is difficult to estimate eruption volumes with great precision because many of the volcanic eruptions deposited on the ground have been lost to erosion. But giant caldera eruptions are an important event in geoscience, and they play a significant role in global climate and past human history. We know the impact, and understanding this phenomenon has social significance,” concludes study co-author Nobukazu Shima.
Full Research Paper”Submarine pyroclastic sediments from the Kikai-Agahoya eruption forming a 7.3 ka caldera.” published in Journal of Volcanology and Geothermal Research and can be found online here.
Additional material and interviews provided Gobi University.