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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. 

Silicic volcanic activity has long been framed as either violently explosive or gently effusive. However, recent ob- servations demonstrate that explosive and effusive behavior can occur simultaneously. Here, we propose that rhyolitic magma feeding subaerial eruptions generally fragments during ascent through the upper crust and that effusive eruptions result from conduit blockage and sintering of the pyroclastic products of deeper cryptic frag- mentation. Our proposal is supported by (i) rhyolitic lavas are volatile depleted; (ii) textural evidence supports a pyroclastic origin for effusive products; (iii) numerical models show that small ash particles !10−5 m can diffusive- ly degas, stick, and sinter to low porosity, in the time available between fragmentation and the surface; and (iv) inferred ascent rates from both explosive and apparently effusive eruptions can overlap. Our model reconciles previously paradoxical observations and offers a new framework in which to evaluate physical, numerical, and geochemical models of Earth’s most violent volcanic eruptions. 

The di!usion of water through silicate melts is a key process in volcanic systems. Di!usion controls the growth of the bub- bles that drive volcanic eruptions and determines the evolution of the spatial distribution of dissolved water during and after magma mingling, crystal growth, fracturing and fragmentation, and welding of pyroclasts. Accurate models for water di!u- sion are therefore essential for forward modelling of eruptive behaviour, and for inverse modelling to reconstruct eruptive and post-eruptive history from the spatial distribution of water in eruptive products. Existing models do not include the kinetics of the homogeneous species reaction that interconverts molecular (H2Om) and hydroxyl (OH) water; reaction kinetics are impor- tant because final species distribution depends on cooling history. Here we develop a flexible 1D numerical model for di!usion and speciation of water in silicate melts. We validate the model against FTIR transects of the spatial distribution of molecular, hydroxyl, and total water across di!usion-couple experiments of haplogranite composition, run at 800–1200 C and 5 kbar. We adopt a stepwise approach to analysing and modelling the data. First, we use the analytical Sauer-Freise method to deter- mine the e!ective di!usivity of total water DH2Ot as a function of dissolved water concentration CH2Ot and temperature T for each experiment and find that the dependence of DH2 Ot on CH2 Ot is linear for CH2 Ot K 1:8 wt.% and exponential for CH2 Ot J 1:8 wt.%. Second, we develop a 1D numerical forward model, using the method of lines, to determine a piece-wise function for DH2 Ot !CH2 Ot ; T " that is globally-minimized against the entire experimental dataset. Third, we extend this numerical model to account for speciation of water and determine globally-minimized functions for di!usivity of molecular water DH2 Om !CH2 Ot ; T " and the equilibrium constant K for the speciation reaction. Our approach includes three key novelties: (1) functions for dif- fusivities of H2Ot and H2Om, and the speciation reaction, are minimized simultaneously against a large experimental dataset, covering a wide range of water concentration (0:25 CH2 Ot 7 wt.%) and temperature (800  C T 1200  C), such that the resulting functions are both mutually-consistent and broadly applicable; (2) the minimization allows rigorous and robust analysis of uncertainties such that the accuracy of the functions is quantified; (3) the model can be straightforwardly used to determine functions for di!usivity and speciation for other melt compositions pending suitable di!usion-couple experiments. The modelling approach is suitable for both forward and inverse modelling of di!usion processes in silicate melts; the model is available as a MATLAB script from the electronic supplementary material. 

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.