Persistent growth of a young andesite lava cone: Bagana volcano, Papua New Guinea

Bagana, an andesite lava cone on Bougainville Island, Papua New Guinea, is thought to be a very young central volcano. We have tested this idea by estimating the volumes of lava extruded over different time intervals (1-, 2-, 3-, 9-, 15-, 70-years) using digital elevation models (DEMs), mainly created from satellite data. Our results show that the long-term extrusion rate at Bagana, measured over years to decades, has remained at about 1.0 m3 s−1. We present models of the total edifice volume, and show that, if our measured extrusion rates are representative, the volcano could have been built in only ~300 years. It could also possibly have been built at a slower rate during a longer, earlier period of growth. Six kilometres NNW of Bagana, an andesite-dacite volcano, Billy Mitchell, had a large, caldera-forming plinian eruption 437 years ago. We consider the possibility that, as a result of this eruption, the magma supply was diverted from Billy Mitchell to Bagana. It seems that Bagana is a rare example of a very youthful, polygenetic, andesite volcano. The characteristics of such a volcano, based on the example of Bagana, are: a preponderance of lava products over pyroclastic products, a high rate of lava extrusion maintained for decades, a very high rate of SO2 emission, evidence of magma batch fractionation and location in a trans-tensional setting at the end of an arc segment above a very steeply dipping and rapidly converging subduction zone

Volcanic activity and gas emissions along the South Sandwich Arc

The South Sandwich Volcanic Arc is one of the most remote and enigmatic arcs on Earth. Sporadic observations from rare cloud-free satellite images—and even rarer in situ reports—provide glimpses into a dynamic arc system characterised by persistent gas emissions and frequent eruptive activity. However, our understanding of the state of active volcanic activity along this arc is incomplete compared to arcs globally. Here, we present detailed geological and volcanological observations made during an expedition to the South Sandwich Islands in January 2020. We report in situ measurements of gas chemistry, emission rate, and carbon isotope composition from along the arc. We show that Mt Michael on Saunders Island is a persistent source of gas emissions, releasing 145 ± 59 t d-1 SO2 in a plume characterised by a CO2/SO2 molar ratio of 1.8 ± 0.2. Combining this CO2/SO2 ratio with our independent SO2 emission rate measured near-simultaneously, we derive a CO2 flux of 179 ± 76 t d-1. Outgassing from low temperature (90–100 °C) fumaroles is pervasive in the active centres of Candlemas and Bellingshausen, with measured gas compositions indicative of interaction between magmatic fluids and hydrothermal systems. Carbon isotope measurements of dilute plume and fumarole gases from along the arc indicate a magmatic δ13C of -4.5 ± 2.0 ‰. Interpreted most simply, this result suggests a carbon source dominated by mantle-derived carbon; however, based on a carbon mass balance from sediment core ODP 701, we show that mixing between depleted upper mantle and a subduction component composed of sediment and altered crust is also permissible. The South Sandwich Volcanic Arc is an ideal tectonic setting in which to explore geochemical processes in a young, developing arc