Abstract
Bagana, arguably the most active volcano in Papua New Guinea, has been in a state of near-continuous eruption for over 150 years, with activity dominated by sluggish extrusion of thick blocky lava flows. If current extrusion rates are representative, the entire edifice may have been constructed in only 300–500 years. Bagana exhibits a remarkably high gas flux to the atmosphere, with persistent sulfur dioxide (SO2) emissions of several thousand tons per day. This combination of apparent youth and high outgassing fluxes is considered unusual among persistently active volcanoes worldwide. We have used satellite observations of SO2 emissions and thermal infrared radiant flux to explore the coupling of lava extrusion and gas emission at Bagana. The highest gas emissions (up to 10 kt/day) occur during co-extrusive intervals, suggesting a degree of coupling between lava and gas, but gas emissions remain relatively high (~2,500 t/d) during inter-eruptive pauses. These passive emissions, which clearly persist for decades if not centuries, require a large volume of degassing but non-erupting magma beneath the volcano with a substantial exsolved volatile phase to feed the remarkable SO2 outgassing: an additional ~1.7–2 km3 basaltic andesite would be required to supply the excess SO2 emissions we observe in our study interval (2005 to present). That this volatile phase can ascend freely to the surface under most conditions is likely to be key to Bagana's largely effusive style of activity, in contrast with other persistently active silicic volcanoes where explosive and effusive eruptive styles alternate.
Highlights
Long-lived eruptions from silicic volcanoes are a common and hazardous mode of volcanism
Our multi-year satellite observations of SO2 mass loading and thermal infrared radiant flux expand our previous work on this volcano (McCormick et al, 2012; Wadge et al, 2012, 2018) and demonstrate that lava extrusion and gas emissions are persistent, they are highly variable on interannual and sub-annual timescales
We contend that a firstorder coupling exists between lava extrusion and gas emissions, with peak SO2 mass loadings identified during episodes or “phases” of active extrusion lasting several months
Summary
Long-lived eruptions from silicic volcanoes are a common and hazardous mode of volcanism. The characteristics of persistent silicic volcanism include: the emplacement of lava flows and domes. Many persistently active silicic volcanoes are major sources of volcanic gas, with outgassing fluxes remaining high even during pauses between eruptive phases (Delgado-Granados et al, 2001; Edmonds et al, 2003; Arellano et al, 2008; Lopez et al, 2013b). Few volcanoes worldwide are so persistently active as Bagana, Papua New Guinea, which has many of the traits outlined above (long-lived eruptions, crystal-rich magmas, slow extrusion rate, substantial outgassing) yet seems to lack the classic episodic extrusive/explosive eruption cycle so characteristic of dome-building volcanoes (Barmin et al, 2002; Sheldrake et al, 2016). Many aspects of Bagana’s activity remain enigmatic, including the mechanisms of its prodigious gas output and the key processes controlling the timing and intensity of lava extrusion
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