LAUSR

Abstract From June to August 2015, Mt. Raung located at the east end of Java, Indonesia, erupted effusively with Strombolian activity at the intra-caldera cinder cone. The effused lava from the base of the cone entirely buried the caldera floor with a lava bed and ultimately formed a temporary lava lake. In this study, details of the eruption sequence and formation process of the temporary lava lake are analyzed based on a series of high-resolution images combined with the high-frequency thermal observation results of Himawari-8. The volume of effused lava was also estimated using a digital elevation model (ASTER GDEM) based on the lava bed bathymetry in the caldera and lava bed distribution at several points over the course of the eruption. The total effusion volume was estimated to be 7.5 × 107 m3, and the average effusion rate was 1.4 × 106 m3/day. The activity consisted of four stages. The Precursory Stage was characterized by low-level thermal anomalies, which are thought to result from high-temperature spots at the bottom of several pits formed in the buried vent, probably due to magma head partial exposure or gas emission. Pulses 1 and 2 were effusive stages that formed the lava bed. The pattern recognized in the time-series variation of thermal anomalies was confirmed to reflect the lava effusion rate using the volumes estimated in this study. The lava bed is divided into the Lower and Upper lava groups, which were generated in Pulses 1 and 2, respectively, except for the fine lobes formed around the margin of the Lower lava group. The Terminal Stage also showed low-level thermal anomalies, most likely caused by gas emissions. The intra-caldera cone grew to approximately 100 m higher than its initial height by mid-August and then collapsed into the conduit at the end of the Terminal Stage. The lava bed increased in thickness through lobe multi-layering and later through the addition of lava directly from the vent into the lava bed interior. Over the course of the lava bed growth, the bed outline nearly matched the caldera contours. Uplift of the lava bed surface caused lateral fluidization of the bed, and molten lava was extruded at the bed margins, forming fine lobes. This process enlarged the zone of fine lobes around the lava bed. Thus, the lava bed growth resulted in the appearance of a lava lake. In the middle of Pulse 1, a small amount of hot material was thrown onto the slope beyond the caldera rim. In the future, the caldera floor uplift caused by this activity (approximately 49 m) will increase the risk of erupted material ejection outside the caldera. In a comparison with the 2013–15 Nishinoshima eruption, which was a similar effusive eruption, Mt. Raung was shown to have predominantly large lobes over most of the lava bed, while in Nishinoshima, narrow medium-scale lobes occupied the majority of the surface. This may be attributed to the high effusion rate of lava for Mt. Raung, which was approximately seven times higher on average than the lava effusion rate for Nishinoshima.