Exploration of the accumulation of magma in Kikai Caldera from the Chikyu submarine cores

1. Key Points

• The Kikai caldera has repeatedly experienced caldera-forming eruptions ^(*1). The evolution of magma from one catastrophic eruption to the next catastrophic eruption was studied using submarine cores collected by the D/V Chikyu near Kikai Caldera.

• The age and chemical composition of the core samples indicate that the magmas that caused the catastrophic eruption 95,000 years ago all erupted from the magma reservoir and that another magma was recharged from at least 43,000 years ago until the next catastrophic eruption 7,300 years ago.

• Most felsic magmas, which are similar to those of the catastrophic eruption 7,300 years ago in terms of chemical composition, have not occurred since 16,000 years ago. This indicates that the felsic magmas were stored in the magma reservoir for over 9,000 years before the next catastrophic eruption.

Overview

Caldera-forming eruptions, despite being rare, have hazardous impacts on the environment of Earth. The growth of magma reservoirs beneath calderas has been the focus of interest, however, remains uncertain in many cases due to the lack of successive volcanic records prior to the catastrophic eruption. Researchers from the Japan Agency for Marine-Earth Science and Technology, in collaboration with researchers from Kobe University, have conducted a project to understand magma evolution from the catastrophic eruption 95,000 years ago to the eruption 7,300 years ago at Kikai Caldera, southern Japan, using submarine cores collected during the Chikyu SCORE ^(*2) cruise. The oldest cores that surfaced after the catastrophic eruption 95,000 years ago showed a 14C age of 43,000 years, indicating that inter-caldera volcanism commenced no later than 43,000 years ago. The inter-caldera magmas differ from those of the catastrophic eruption 95,000 years ago in terms of geochemical composition, indicating a transition of the magma supply to the shallow magma chamber after the catastrophic eruption. Although both felsic and mafic magmas occurred 43,000 years ago, the proportion of felsic magmas increased until 16,000 years ago. However, most of the felsic magmas, with composition corresponding to the composition of the magmas that erupted later during the catastrophic eruption 7,300 years ago, have not appeared since 16,000 years ago. This indicated that the felsic melt was stored under the caldera to form a magma reservoir for over 9,000 years before the next catastrophic eruption.

This research will be published in Communications Earth ＆ Environment on August 22 (JST). This work was supported by the Japan Society for the Promotion of Science under the KAKENHI program (grant numbers 20H02011, 23H01281, 19H00718, and 20H00199) and the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan under its The Second Earthquake and Volcano Hazards Observation and Research Program (Earthquake and Volcano Hazard Reduction Research).