Episode 4 (2019–2020) Nishinoshima activity: abrupt transitions in the eruptive style observed by image datasets from multiple satellites

In December 2019, a new activity started at Nishinoshima volcano in the southern part of the Izu–Ogasawara arc, Japan. This is now referred to as Episode 4 of a series of activities that began in 2013. We analyzed the eruption sequence, including erupted volume and effusion rate, based on combined observations of thermal anomalies by Himawari-8 and topographic changes by ALOS-2. The total eruption volume during Episode 4 was ~ 132 × 10 6 m 3 , and the average effusion rate over the entire period was 0.51 × 10 6 m 3 day −1 (5.9 m 3 s −1 ), which was two to three times higher than that of Episode 1. Episode 4 had three stages. In Stage 1, effusive activity was dominant, and most of the lava erupted from a northeast vent at the foot of the pyroclastic cone to cover the northern half of the island. The average effusion rate was estimated to be 0.46 × 10 6 m 3 day −1 (5.3 m 3 s −1 ). In Stage 2, an intensive lava fountain with a high discharge rate developed, and it increased the size of the pyroclastic cone rapidly. The effusion rate temporarily reached 2.6 × 10 6 m 3 day −1 (30 m 3 s −1 ). Pyroclastic rocks accounted for 45–88% of the total erupted volume in this stage. Lava flows with rafted cone material were generated, and those possibly caused by intensive spatter falls on the slope were also formed. These lavas flowed down the southern half of the island. In Stage 3, continuous phreatomagmatic eruptions released ash and spread it over a wide area. The high effusion rate and the drastic change in the activity style in Episode 4 can be explained by deep volatile-rich magma being supplied to a shallower magma chamber prior to Episode 4. When the volatile-rich magma reached a shallow part of the conduit in Stage 2, fragmentation occurred due to rapid volume expansion to eject large amounts of magma and form the intensive lava fountain. Observations by satellite-borne ultraviolet–visible image sensors detected a rapid increase in SO 2 emissions in response to the intensive lava-fountain activity. The less-differentiated nature of the ash fragments collected during Stage 2 may reflect the composition of the volatile-rich magma. Large-scale discolored-seawater areas appeared during the late period of Stage 1, which may have been caused by ascent of the volatile-rich magma. Graphical Abstract