Pre-eruptive volatile content, degassing paths and depressurisation explaining the transition in style at the subglacial rhyolitic eruption of Dalakvísl, South Iceland

•We investigate a mixed effusive–explosive subglacial rhyolitic eruption.•Explosive samples show high initial H2O, closed system degassing and rapid ascent.•Effusive samples show low initial H2O, open system degassing and slow ascent.•Bubble distributions suggest rapid decompression e.g. syn-eruptive jökulhlaup.•Provides new insights into the complexity of rhyolitic effusive-explosive transitions.

Many volcanoes express a variety of contrasting eruptive styles with behavioural transitions between explosive and effusive activities. Understanding eruptive mechanisms is of key importance but we currently have little insight into the behavioural controls for rhyolitic eruptions beneath ice. This paper addresses a subglacial rhyolitic eruption at Dalakvísl in South Iceland that involved both explosive and effusive activities. We present measurements of pre- and post-eruptive magmatic volatile contents that we use to track magma degassing from the chamber to the surface and reconstruct pressure conditions during the eruption. Explosive and effusive phases involve significant differences in: (1) pre-eruptive volatile contents; (2) the ‘openness’ of magma degassing; and (3) inferred confining pressure. This final factor is potentially associated with a jökulhlaup-induced decompression. Due to the combination of these factors Dalakvísl experienced a change from effusive to explosive behaviour, although we cannot currently resolve cause from effect. Our work points towards complex interactions between the dynamics of magma ascent and the response of overlying ice and meltwater during rhyolitic eruptions beneath ice.