Interpreting The Geometry And Multistage Evolution Of The Nari Caldera, Ulleung Island, Korea

We reconstruct the evolution of the Nari caldera, Ulleung Island, based on geomorphological observations, detailed stratigraphy, and the distribution and faces changes of the related clastic deposits that are produced by the volcanism during the past 40 ka. Currently, the caldera shows the overlapped structure of multicycles related to explosive eruptions, from which we recognize two caldera-forming events. Each collapse is a special "collapse-marker deposit", defined by "Bongrae welded tuff (40 ka)" and "pre-Maljandeung landslide breccia", and "N-4C tuff breccia in the Maljandeung Tuff (11 ka)", respectively. The Bongrae welded tuff was produced by hot pyroclastic flows derived from boiling-over eruptions, whereas the landslide and tuff breccias were formed by debris flows from landsliding on the caldera wall, and by dense pyroclastic flows from the collapse of eruptive columns, respectively. The older Seongin caldera was formed as a result of emptying magma chamber by boiling-over eruption from magma arising from deep chamber, and the younger Nari caldera by pyroclastic-forming eruption. However, it is currently impossible for us to recognize clear vertical displacement in the older caldera because of losing its stepped shape by subsequent collapses and ring faults. The Nari Caldera is the result of piston block subsidence, which is 3.4 km wide in topographical rim and 2 km wide in structural ring faults. The caldera block has a vertical displacement of about 750 m, all of which have been accumulated along the ring fault. Thus we currently conclude that the outer boundary of the caldera is the result of the morphological evolution of the multistage caldera rim formed from repeated collapses by the voluminous drainage of magma chamber during the Plinian eruption.