Undersea survey uncovers largest Holocene volcanic eruption

A detailed study of volcanic underwater sediments around the Kigai caldera in Japan elucidated the depositional mechanisms and scale of occurrence. As a result, the Cobb University research team found that the 7,300-year-old event was the largest volcanic eruption in the Holocene.

In addition to lava, volcanoes expel large amounts of pumice, ash, and gases into a fast-moving flow called a „pyroclastic flow,” and its deposits are a valuable source of data about 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 near-shore volcanoes, the pyroclastic flow deposition process is often unclear because the interaction with water is poorly understood and reliable data are difficult to obtain, and therefore limited. For these reasons, it is difficult to assess the impact of past eruptions on climate and history.

A Kobe University research team around Shima Nobukasu and Shimizu Satoshi went out to sea aboard the Kobe University-owned training vessel Fuke Maru (replaced by the newly built Kaijin Maru) to conduct seismic imaging and sediment sampling around the Kikai caldera. South coast of Kyushu Island, Japan. Fine detail of the seismic reflection data revealed sedimentary structures with a vertical resolution of 3 meters and depths of several hundreds of meters below the seafloor. Shimizu explains: „Because volcanic eruptions deposited in the sea are well preserved, they record a lot of information during eruptions. By using seismic reflection probes optimized for this purpose and locating the collected sediments, we were able to obtain important information about the distribution, volume and transport mechanisms of the eruption.”

In their paper published in the Journal of Volcanology and Geothermal Research, the geologists report that an eruption 7,300 years ago ejected large amounts of volcanic material (ash, pumice, etc.) that settled over an area of ​​more than 4,500 square kilometers. around the blast site. With an equivalent volume of 133 to 183 cubic kilometers of dense rock, the event was the largest volcanic eruption in the Holocene (the most recent 11,700 years of Earth's history following the end of the last Ice Age). .

During the study, the research team confirmed that the sediments on the sea floor and those deposited on the nearby islands are of the same origin and from their distribution around the eruption site they clarify the connection between the pyroclastic flow and the water. They observed that the underwater part of the flow can travel even farther upstream.

Their findings provide new insights into the elusive dynamics of volcanic megaevents that could be useful for identifying remnants of other events and estimating their size. Seema explains, „Major volcanic eruptions not yet experienced by modern civilization rely on sedimentary records, but it is difficult to estimate eruption size with great accuracy because many volcanic eruptions deposited on land have been lost to erosion. But giant caldera eruptions are an important event in geoscience, and they are important for global climate and the past. We know that time has had an impact on human history, and understanding this phenomenon has social significance.” In this light, it is tempting to think that the event that created a caldera about the size of a modern capital city was actually the largest volcanic event since humans spread across the globe.

This research was funded by the Ministry of Education, Culture, Sports, Science and Technology of Japan under the Second Earthquake and Volcanic Hazards Monitoring and Research Program (Earthquake and Volcanic Hazard Reduction Research) and the Japan Society for the Advancement of Science (Grant 20H00199). .

Kobe University is a national university with roots in the Kobe Business School established in 1902. It is now one of Japan's leading comprehensive research universities, with nearly 16,000 students and nearly 1,700 faculty in 10 faculties and schools and 15 graduate schools. Combining the social and natural sciences to develop leaders with an interdisciplinary perspective, Cobb University creates knowledge and fosters innovation to address society's challenges.