Spatiotemporal evolution of pyroclastic density currents and eruption dynamics during the 6.5 ka caldera-forming eruption, Kikai volcano, Japan

The 6.5 ka caldera-forming eruption from the Kikai volcano devastatingly damaged to prehistoric human activities of southern Kyushu, Japan. Stratigraphy, component, and lithology of the pyroclastic deposits were analyzed, with respect to a precise temporal framework to determine how large pyroclastic density currents can evolve in time and space and how large explosive eruptions evolve during caldera collapse. Stratigraphical sections are characterized by plinian pumice fall deposits (Unit A), intraplinian flow deposits (Unit B), voluminous ignimbrite (Unit C), and co-ignimbrite ashfall deposits (Unit D). A plinian stage is subdivided into a first small phase and a second large one. A column height in the second phase was estimated at 40-43 km, and collapse of the column produced Unit B, which consists of multiple thin subunits with stratified or cross-stratified facies in various degree of welding. Each thin subunit is a few to a few tens of centimeters in thickness, and is composed of a basal lithic-rich layer and an upper welded pumice-rich layer. It is just one flow unit of a small-scale pyroclastic density current. Lithic-rich layers are composed of mostly altered lithic, bearing obsidian clasts with chilled cracks and submarine boulders, and are fine-depleted. These evidences indicate that, during the plinian column collapse, high temperature dilute currents were generated repeatedly from phreatomagmatic explosions and segregations of dense pyroclasts in a turbulent condition resulted in producing lithic-rich layers. Unit C is subdivided into three units (C1-C3). Unit C1 shows non-welded stratified facies, which consists of lithic and crystals, minorly including quenched juvenile materials. Unit C2 shows welded stratified facies, which consists of lithic-rich layers and pumice-rich layers. These two subunits only occur in topographic depressions and can gradually change into Unit C3, which is thickest and poor-sorted with non-welded massive facies and sometimes includes fragments of welded tuff of Unit B. These facts indicate that a climactic voluminous current was deposited after a welding of Unit B at some locations, and it has had a turbulent basal part producing Unit C1 and C2 and a main sustained body producing Unit C3. Erupted magma has gradually changed from rhyolite into andesite-bearing one and pyroclasts in the current should have aggraded progressively because andesitic juvenile clasts are only included in the Unit C3. Main caldera subsidence may have started before a Unit C3 deposition, from a finding in the form of a fault overlaid by Unit C3 on the caldera rim.