An in-depth study of submerged volcanic sediments surrounding Japan’s Kikai caldera has shed light on both the deposition mechanisms and the scale of the event. As a result, researchers from Kobe University determined that this eruption, which occurred 7,300 years ago, was the largest volcanic eruption of the Holocene period.

In addition to lava, volcanoes also erupt large amounts of pumice, ash, and gas in a fast-moving flow known as “pyroclastic flow,” and deposits here are a valuable source of data about past eruptions. For volcanoes on land, geologists have a good understanding of the deposition mechanism of pyroclastic flows, but the sediments themselves are easily lost due to erosion.

On the other hand, for volcanoes on oceanic islands or near shores, the process of pyroclastic flow accumulation is largely uncertain, both because the interaction with water is less understood and because reliable data are difficult and therefore rare to obtain. For these reasons, it is difficult to assess the impact of many past eruptions on climate and history.

Research trip and findings

The Kobe University research team around SEAMA Nobukazu and Shimizu Satoshi went to sea aboard Kobe University’s training ship Fukae Maru (replaced by the newly built Kaijin Maru) and conducted seismic imaging as well as sediment sampling around the Kikai caldera. Extraordinary detail of seismic reflection data off the southern coast of the Japanese island of Kyushu has revealed a sedimentary structure several hundred meters below the seafloor, with a vertical resolution of 3 metres.

Shimizu explains: “Because volcanic emissions deposited in the sea are well preserved, they record a lot of information during the eruption. By using reflection seismic surveys optimized for this purpose and identifying the collected sediments, we were able to obtain important information about the distribution, extent and transport mechanisms of the eruption.”

In his article published in the magazine Journal of Volcanology and Geothermal Research Geologists report that the eruption, which occurred 7,300 years ago, expelled a large amount of volcanic products (ash, pumice, etc.), which settled on an area of ​​​​more than 4,500 square kilometers. around the explosion zone. With a volume of dense rock equivalent to 133 to 183 cubic kilometers, this event was the largest volcanic eruption known to science to have occurred in the Holocene (the last 11,700 years of Earth history since the end of the last ice age). .

Information and results

During the analysis process, the research team confirmed that the sediments on the ocean floor and the sediments deposited on nearby islands had the same origin, and from their distribution around the eruption area they could clarify the interaction between pyroclastic flow and water. They realized that the underwater part of the flow could travel very large distances even above.

Their findings provide a new perspective on the elusive dynamics of volcanic mega-events; this can be useful in detecting remnants of other events and estimating their extent.

Siama explains: “Massive volcanic eruptions, such as those that have not yet occurred in modern civilization, are due to sedimentary deposits, but the volume of eruptions has been difficult to estimate with high precision, as much of the volcanic eruption deposited on land is evaporated and lost to erosion. But the eruption of giant calderas is an important phenomenon in earth sciences.” and since we know that these have influenced global climate and thus human history in the past, understanding this phenomenon also has social implications.”

From this perspective, it is fascinating to consider that the event that created a caldera roughly the size of the modern capital was actually the largest volcanic event since humans spread across the globe.