Submarine cores record magma evolution toward a catastrophic eruption at Kikai Caldera

Magma evolution toward a caldera-forming eruption remains uncertain in many cases owing to the lack of successive volcanic records before catastrophic eruptions. Here we take an approach to this issue by analyzing a submarine core sampled near Kikai Caldera, southern Japan, which has recorded two caldera-forming eruptions at 95 and 7.3 ka and small eruptions between them. Discovery of mafic glass fragments in the submarine deposits of the 95-ka eruption, which had not been recognized in subaerial outcrops, implies the involvement of mafic magma in felsic magma-driven caldera-forming eruption. Inter-caldera volcanic activity resumed with binary mafic and felsic magma extrusions but then shifted to eruptions predominated by felsic magmas. In the final stage preceding the 7.3-ka caldera-forming eruption, the most felsic composition did not appear in glass fragments. We suggest that this period was the phase of felsic melt accumulation to grow a magma reservoir toward the next catastrophic eruption. A 9,000 year sequence of storage and build-up of a felsic magma chamber before a very large eruption at Kikai caldera, Japan, 7,300 years ago is identified through analysis of the chemical composition of volcanic deposits within marine drill cores.