SPATIOTEMPORAL EVOLUTION OF A MARINE CALDERA-FORMING ERUPTION, GENERATING A LOW-ASPECT RATIO PYROCLASTIC FLOW, 7.3 KA, KIKAI CALDERA, JAPAN: IMPLICATION FROM NEAR-VENT ERUPTIVE DEPOSITS

Abstract

The VEI 7, 7.3 ka caldera-forming eruption of the Kikai caldera occurred in a shallow sea, and caused devastating damage to the prehistoric human settlements of southern Kyushu, Japan. This Holocene activity at this volcano records eruptions with compositional range of 56 to 74 wt.% in SiO2 spanning with the climactic eruption. In this study, stratigraphy, component, and lithology of the pyroclastic deposits were analyzed at near-vent islands (Satsuma Iwo-jima and Take-shima) in order to reconstruct this eruption. Stratigraphic sections are characterized by plinian pumice-fall deposits (Unit A), intraplinian flow deposits (Unit B), climactic pyroclastic flow deposits (Units C1–C3), and co-ignimbrite ash-fall deposits (Unit D). In total the estimated magma volume in the system was 70–80 km3 (DRE) and the eruption therefore represents the evacuation of a major silicic magmatic system. The plinian stage (Phase 1) is subdivided into an initial small phase and a second large one. The column height in the second phase was estimated to be 40–43 km. The total tephra volume of this stage was estimated to be 40 km3. The magma discharge rate has been calculated from the column height data to be 2 × 108 kg/s. The eruption duration is also estimated to be a minimum of approximately 28 h. Collapse of the column (Phase 2) produced Unit B, which consists of multiple thin lithic-rich or pumice-rich layers or pods, including welded pumice-fall layers. The deposits are characterized by stratified or cross-stratified facies and display various degree of welding. These sedimentary characteristics indicate that, during the plinian column collapse, high temperature turbulent density currents were generated where dense pyroclasts were well segregated, resulting in the lithic-rich layers or pods.