LAUSR

In this companion study to Simons et al. (Bull Volcanol 82, 2020a), we examine Strombolian style explosive activity at Yasur (Vanuatu) over 11 weeks via seismic, thermal-infrared and visual observations. In part 1 of this study (Simons et al., Bull Volcanol 82, 2020a), we investigated the link between variations in the surface expression and style of volcanism at Yasur in relation to vent, shallow conduit and shallow magma reservoir processes. Now, in part 2, based on the database of 4998 explosions, we describe, define and classify Yasur’s Strombolian eruption styles and magnitudes, and compare and contrast to Stromboli and other similar centres. The styles of most of Yasur’s explosions fit with other known Strombolian eruptions, but they are 2–3 times more frequent and with higher average eruption heights than at Stromboli. Observed activity at Yasur also produced more ash than at Stromboli. The most powerful eruptive periods are dominated by type 2a explosions (ballistics and ash) and type 2b (ash-dominated) explosions. A new type of Strombolian style explosion was observed at Yasur and is here termed a type 3 explosion. They occur when vents are buried by a thick cap of loose, poorly sorted pyroclastic breccia. Type 3 explosions begin as a series of emergent ash-rich jets that rupture and fountain from the breccia cover, generating prolonged jetting of ash to form vigorous low plumes. The loose breccia cover apparently dissipates the violent early phase of the explosion. The greatest hazard of type comes from their occurrence with little warning from a smooth crater floor without an apparent vent and after longer than typical intervals between eruptions. The findings extend the observed range of Strombolian explosion styles and imply that surficial vent cover and burial may influence gas slug stability and contribute to variations in ash and ballistic output.