First measurement of the volcanic gas output from Anak Krakatau, Indonesia

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Anak Krakatau is the active cone that has built up in the caldera of Krakatau volcano after the 1883 cataclysmic eruption, in the Sunda Strait. Initially submarine, this new cone definitely emerged from the sea in 1930 and since then has progressively grown up through both explosive and effusive eruptions (~ one eruption every 3years). Here we report on the first quantification of volcanic gas output from Anak Krakatau, based on airborn UV measurements of the SO2 flux in 2014, and then discuss its implication in terms of magma degassing budget since 1930. We find that during non-eruptive activity Anak Krakatau passively emits 190±40 tons per day of SO2, which is comparable to the emission rate during lava dome extrusion at Merapi, central Java, but substantially more than those measured on few other Indonesian volcanoes (Tangkubanparahu, Slamet, Bromo and Papandayan). Anak Krakatau thus appears to be an important persistent emitter of volcanic volatiles in the Indonesian arc, even though this very active region still remains weakly documented on that aspect. Combining with available data for the composition of its high-temperature (~700°C) crater gases, Anak Krakatau may release annually 0.07Tg of SO2, 3Tg of H2O, and 0.13moles of 3He. Using published data for the sulfur content of its feeding magma, we estimate that about 1.3km3 of magma may have been degassed during its subaerial growth over the past 85years. The subaerial cone represents only 14% of this volume. Thus, a substantial fraction (1.1km3) of the degassed magma did not extrude and may have accumulated in the plumbing system. This inference is consistent with geophysical and petrologic evidence of the presence of dense magma bodies in the shallow crustal basement of Krakatau volcano.