Gas emissions from the resumption of eruptive activity at Kīlauea Volcano’s summit in December 2020.

Kīlauea Volcano (Hawaii, USA) had been in a state of quiescence since the end of the historic 2018 eruption on its lower East Rift Zone. Tapping the volcanic plumbing system at elevations around 300 m well below the volcano’s 1200 m summit, the 2018 eruption drained magma from the volcano’s summit reservoir and East Rift Zone, causing the drainage of a decade-old subaerial lava lake followed by widespread caldera collapse. Two years later, on the evening of 20 December 2020, the Hawaiian Volcano Observatory (HVO) once again detected a glow within the now vastly deepened Halemaʻumaʻu Crater at Kīlauea’s summit. A new eruption had begun. Observations over the next few days revealed lava flowing from three vents in the wall of the crater and into its base. A water lake, which had formed in 2019 – 2020 from groundwater infiltration, boiled off within hours and the crater began rapidly filling with lava. Over the first 3 days of the eruption, the new lava lake filled the lowermost ~150 m of the summit crater, and sulfur dioxide (SO₂) emission rates sometimes exceeded 30,000 metric tons per day (t/d) as measured by Differential Optical Absorption Spectroscopy (DOAS) traverses recorded both from the ground and by helicopter. These vigorous SO₂ emissions were also clearly detected by the Tropospheric Monitoring Instrument (TROPOMI) aboard the Sentinal-5 Precursor satellite, and comparisons of the ground-based data with those collected by TROPOMI are the topic of ongoing research. Lava effusion and gas emission rates then tailed off and, from 26 December to 2 January, DOAS measurements indicated SO₂ emissions of ~5,000 t/d, similar to the average emission rate from Kīlauea’s summit lava lake throughout most of the volcano’s 2008-2018 eruption. Data from a continuous Multiple Gas Analyzer System (MultiGAS) installed approximately 1.3 km downwind of the active vents indicate that the carbon dioxide (CO₂) to SO₂ molar ratio of the emitted gas is low (0.3 ± 0.1), consistent with a model in which the erupted lava has been previously degassed in carbon dioxide but is only now degassing the more soluble sulfur as it reaches the surface. Further MultiGAS measurements performed with an unoccupied aircraft system (UAS) show that the gas composition varies throughout the emitted plume, but that the primary constituents are water vapor (~80-90% molar), carbon dioxide (~3%), and sulfur dioxide (~7-16%), while hydrogen sulfide is below the detection limit of the instrumentation. As of 11 January 2021, lava effusion and gas emissions appear to be slowly decreasing in vigor, but it is as yet unclear whether the eruption will continue to weaken and end within the coming weeks, or whether Kīlauea Volcano will once again harbor a sustained subaerial lava lake for months or years to come.