2022 年 67 巻 2 号 p. 149-169
We studied the 40 ka Kp I eruption deposits of Kutcharo volcano to unravel its eruption sequence and generation mechanisms. Previous studies have suggested that Kp I is the youngest caldera-forming eruption in this volcano and is characterized by large-scale phreatomagmatic activity. We divided Kp I eruption deposits into 7 units (Units 1～7, in ascending order). Units 1～6 consist of alternating thin pumice and thick fine ash layers. Units 1, 3, and 5 are pumice falls (totaling 1.6 km3), while Units 2, 4, and 6 are ash falls (totaling 52.2 km3) with abundant accretionary lapilli. Stratigraphically higher ash fall units are larger in volume, finer in grain size, and more widely distributed (e.g., Units 2, 4, and 6 are 0.2 km3, 13 km3, 39 km3 respectively). Unit 7 is a climactic ignimbrite (76 km3) that subdivides into lower (Unit 7-L), and upper (Unit 7-U) parts based on the pumice size and the existence of a lithic concentration zone (LCZ).
Considering its wide dispersion, high fragmentation, and existence of abundant accretionary lapilli, Unit 6 can be considered to have been deposited by a “phreatoplinian style” eruption. Even though the ejected magma volume increased during the eruption of Unit 1 to 6, interaction between ascending magma and ground water caused maximum explosivity during the eruption that deposited Unit 6. Highly fragmentated magmas might have promoted vaporization and mixing with surface (lake) water to form the buoyant eruption column of Unit 6 eruption phase. Unit 7 is the most voluminous and the richest in lithic fragments at the LCZ, suggesting caldera collapse that generated a climactic pyroclastic flow.
In addition to glass shards of bubble wall and pumiceous types, Kp I eruption deposits also commonly contain flake-, and blocky-shaped glass shards produced by phreatomagmatic (quenching) fragmentation. For both types of glass shards to have been generated, part of the ascending magma would have interacted with ground water before and/or during the magmatic fragmentation (vesiculation) that generally occurs below a depth of approximately 1,000 m in felsic H2O-saturated magma systems. In conclusion, a large and deep (～1,000 m) aquifer in the former caldera basin was sustainably supplied with ground water through the conduit system. Generation of the phreatoplinian eruption seems to have been controlled by a plumbing where conduits penetrated the huge aquifer of a pre-existing caldera structure that preserved/hosted a large amount of external water.