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On December 22, 2018 the south-western flank of Anak Krakatau collapsed into the sea, removing 93.8 × 106 m3 of subaerial lavas, and generated a tsunami. Synchronously with the collapse, a large volcanic plume of SO2 and ash (14–15 km in height) has formed, marking the onset of a paroxysmal eruption lasting from December 22, 2018 to January 06, 2019. From remote sensing analysis, we show that the eruption can be divided into three main phases. Phase I and II show both tephra and gas emissions while phase III is mostly degassing. The total amount of SO2 injected in the atmosphere is 173±52 kt, while the minimum bulk magma volume emplaced, estimated from a topographic reconstruction, is ∼45 × 106 m3. This value compares well with a petrologic-based estimate of 56.4 × 106 m3, making the existence of external sulphur sources and sinks quite unlikely. The ice-rich ash plume formation shows that a strong sea-water/magma interaction was responsible for the phreatomagmatic activity throughout the eruption. However, we distinguish a first Vulcanian blast-derived eruption (lasting 40 min) just after the collapse having a Mass Eruption Rate (MER) of 9 × 105 kgs−1, followed by a sustained lower-intensity eruption resulting in ash emissions over hours (MER = 5 × 105 kgs−1). From December 23, daytime photos show typical Surtseyan activity.
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