ANALYSIS OF PYROCLAST SIZE DISTRIBUTIONS DURING SPATTERING ACTIVITY AT HALEMA‘UMA‘U IN 2015

High-speed cameras enable improved quantification and descriptions of low-intensity basaltic eruptions. Explosive activity in April and December, 2015 at the free surface of the Halema‘uma‘u lava lake at Kīlauea is differentiated by duration into 3 styles: isolated events, clusters of events, and prolonged episodes. Isolated events are discrete, single, bubble bursts that persist for a few tenths of seconds to seconds and are separated by repose periods of similar or longer time scales. Cluster of events are closely spaced, repeated events grouped around a narrow point source, which persist for seconds to minutes. Prolonged episodes are groupings of numerous events overlapping in space and time that persist for tens of minutes to hours. All three are associated with the ejection of pyroclasts (spatter) to heights of meters to tens of meters above the free surface.

We compare pyroclast population grain size from low and high intensity spattering events. The relatively high intensity events were better fragmented, erupted greater amounts of mass, and were more poorly sorted than the low intensity events. Evidence for this is shown by how the median diameter and the total erupted mass decrease and increase, respectively, with the mass eruption rate (which serves as a measure of intensity). The pyroclast populations at Halema‘uma‘u are generally well sorted, which is a feature of magmatic or ‘dry’ fall deposits (Houghton and Carey, 2015)

We also compare these events to total grainsize distributions for the 2001 basaltic subplinian eruption of Etna (Scollo et al., 2007) and the May 2008 silicic Plinian eruption of Chaiten volcano (Alfano et al., 2016). The weaker activity at the lava lake produces markedly coarser pyroclasts. We hypothesize that this reflects far lower thermal efficiency in terms of translating magmatic heat into mechanical energy to fragment the magma.

Fragmentation during bubble-bursting followed trends previously established for explosive volcanism. Globally, the intensity of this activity fits on the lowest end of basaltic explosive behavior, which Houghton et al., 2015 showed was distinguished by duration. In style, this activity sits between high Hawaiian fountaining events and isolated Strombolian explosions. Classifying this activity helps the gap between these two classical eruption styles.