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The rise of the Halemaʻumaʻu lava lake in 2013–2018 to depths commonly 40 meters or less below the rim of the vent was an excellent opportunity to study outgassing and the link to associated eruptive activity. We use videography to investigate the rise and bursting of bubbles through the free surface of the lake in 2015. We focus on low-energy explosive activity (spattering) in which the ascent and bursting of meter-sized, mechanically decoupled bubbles trigger the ejection of fluidal bombs to tens of meters above the free surface. A decay in initial pyroclast velocity with time follows the same functional form as that observed for ejecta at Stromboli (Italy), suggesting a similar bubble-burst mechanism. 

Abstract Most basaltic explosive eruptions intensify abruptly, allowing little time to document processes at the start of eruption. One opportunity came with the initiation of activity from fissure 8 (F8) during the 2018 eruption on the lower East Rift Zone of Kīlauea, Hawaii. F8 erupted in four episodes. We recorded 28 min of high‐definition video during a 51‐min period, capturing the onset of the second episode on 5 May. From the videos, we were able to analyze the following in‐flight parameters: frequency and duration of explosions; ejecta heights; pyroclast exit velocities; in‐flight total mass and estimated mass eruption rates; and the in‐flight total grain size distributions. The videos record a transition from initial pulsating outgassing, via spaced, but increasingly rapid, discrete explosions, to quasisustained, unsteady fountaining. This transition accompanied waxing intensity (mass flux) of the F8 eruption. We infer that all activity was driven by a combination of the ascent of a coupled mixture of small bubbles and melt, and the buoyant rise of decoupled gas slugs and/or pockets. The balance between these two types of concurrent flow determined the exact form of the eruptive activity at any point in time, and changes to their relative contributions drove the transition we observed at early F8. Qualitative observations of other Hawaiian fountains at Kīlauea suggest that this physical model may apply more generally. This study demonstrates the value of in‐flight parameters derived from high‐resolution videos, which offer a rapid and highly time‐sensitive alternative to measurements based on sampling of deposits posteruption.