The latest volcanism in Nari Caldera, Ulleung Island: Transition from explosive to effusive eruption

Nari Scoria Deposits and Albong Trachyandesite (lavas), distributed in Nari caldera, Ulleung Island, suggest the transition from explosive to effusive eruption in the latest trachyandesite magma, which is a shift in eruption regime linked to a change in the type of degassing of the erupted magma. According to vesicularity, the Nari Scoria Deposits have been classified into 4 main lithologies: highly vesicular scoria (HVS), moderately vesicular scoria (MVS), lowly vesicular scoria (LVS), and dense blocks, and the Albong Trachyandesite: incipiently vesicular and dense lavas. Their textural observation include significantly coarse bubbles, thick glass walls, and irregularly shaped vesicles, with features confirming bubble coalescence and collapse. These textural analyses show portions of the melt underwent highly variable degrees of outgassing. From the textural data, the decrease in and widening range of vesicularity, and the significant increase in bubble collapse shows the transition from the closed to open system degassing. Free escape of the gas caused a cessation of magma fragmentation and conduit blockage. However, exsolution of volatiles continued in parts of the melt ascending along the conduit system, and promoted extrusion of a dome of partially outgassed magma. In conclusion, the Nari scoria deposits and the Albong lavas reveal that their volcanism began explosively with a series of Strombolian eruption through the degasing pattern and have a transition to effusive eruption.