Blurring the boundaries between explosive and effusive eruption styles: New Evidence for Sintering-Driven Eruption Transitions

Understanding the controls on the explosive-effusive volcanic eruption transitions is central to hazard management and fundamental to building a more complete picture of eruption dynamics. While traditional models suggest that explosive and effusive eruptions represent very distinct behaviours and timescales, most silicic eruptions are complex, often exhibiting hybrid explosive-effusive behaviour and/or explosive ash-venting through effusive lavas. Dominantly explosive and dominantly effusive phases of eruptions can occur simultaneously or be separated in time by years. A recent conceptual eruption model proposes that the fragmentation and subsequent sintering of material within the conduit may provide a framework that explains both explosive and effusive eruptions, thereby blurring the boundaries of eruptive styles previously established. Here, we investigate this by measuring the material properties – porosity and permeability – of 150 samples from more than ten iconic eruptions representing both explosive (ignimbrites) and effusive (domes lavas) events. We find that across all porosities, the properties of dome lavas are strikingly similar to those of ignimbrites, suggesting that dome lavas may be a lava-like, thoroughly sintered product of pyroclasts. We also present macro- and micro-textures, with a focus on pore-network textures, to support the possibility that the lava samples are sintered products. Textural similarities include: (1) broken phenocrysts; (2) convolute pore networks typical of sintering; (3) cuspate vesicles at low porosity, indicative of the end-stage of sintering; (4) juxtaposed textures with very different groundmass crystallinity and mineralogy; and (5) direct textural evidence that some lavas are clastic. Our results provide visual and empirical evidence of sintering dynamics in dome rocks, supported by a large database revealing shared characteristics between effusive and explosive magma samples. We identify that, as for the blurred boundary between eruption styles, there is a significant overlap in physical properties across both types of samples and the presence of sintering relic textures in dome rocks, features previously associated primarily with explosive products. We provide evidence here that sintering is a key process governing eruptive transitions. By quantifying the similarities between effusive and explosive volcanic rocks, these discoveries and new datasets contribute to our understanding of eruption styles, paving the way for a new interpretation of conduit processes operating before, during, and after volcanic eruption.