Gas composition monitoring with Multi-GAS during the 2018 eruption of Kīlauea, Hawai'i

For the past decade, Kīlauea Volcano maintained a lava lake in its summit crater. In late April 2018 the volcano entered a dramatic new eruptive phase with substantial changes in gas emissions: the lava lake began to drain, while more than 20 fissures opened and began erupting profuse lava and gas in residential neighborhoods in the lower East Rift Zone (LERZ) 40 km east of the summit. By late May, a single fissure (#8) became the locus of activity. As magma withdrew from the summit reservoir to feed the LERZ eruption, the summit subsided, filling the empty lava lake conduit with rubble. The H₂O, CO₂, SO₂, and H₂S gases released from both the summit and LERZ eruption sites during this episode are monitored using a sensor-based system (Multi-GAS) from the ground and by UAS.

When the lava lake vanished and the conduit was buried, summit gas emission regimes shifted dramatically. Gone were the shallow open vent degassing conditions with a robust plume and very low C/S ratios. Plummeting gas emissions at the summit and very high C/S ratios of 40-500 after conduit burial suggested the overburden and/or magma withdrawal was sufficient to prevent SO₂ exsolution. These high ratios are reminiscent of summit gas compositions prior to the open vent conditions between 2008 and May 2018. Summit plumes are spatially heterogeneous, indicating separate degassing sources for SO₂, H₂O, and CO₂. H₂S is 10-35% of total sulfur, suggesting minor cooling of the conduit may have taken place.

In the LERZ, newly opened ground cracks discharged gas rich in H₂O and SO₂ (>200 ppm), low in CO₂, and with minor amounts of H₂S. At incipient fissures, gas compositions were initially heterogeneous, with large CO₂ variations (100s to 1000s of ppm) without concomitant SO₂, suggesting the incorporation of excess CO₂ from non-volcanic sources (e.g., vegetation and burning asphalt). H₂S comprised 1-10% of total sulfur in lower temperature areas without fully developed fissures. Prior to eruption of lava at ground cracks, initial C/S ratios were 1. By late May, measurements in the plumes of erupting fissures revealed very low C/S of 0.2-0.4, consistent with visual observations of the arrival of a less-degassed magma in the LERZ. Ongoing monitoring of compositions in the Fissure 8 plume by UAS continues to indicate consistently low C/S ratios of 0.31 ± 0.01, as of this writing.