Explosive or effusive style of volcanic eruption determined by magma storage conditions

Most volcanoes erupt both effusively and explosively, with explosive behaviour being responsible for most human fatalities. Eruption style is thought to be strongly controlled by fast conduit processes, limiting our ability for prediction. Here we address a critical question in the quest to develop timely forecasting of eruptive behaviour: are there conditions in which the outcome of an eruption is predetermined by the state of the magma in the subvolcanic reservoir? We analyse the pre-eruptive storage conditions of 245 units from volcanoes around the world. We show that pre-eruptive crystallinity, dissolved water content and the presence of exsolved volatiles in the chamber exert a primary control on eruptive styles. Magmas erupt explosively over a well-defined range in dissolved water content (~4–5.5 wt%) and crystallinity (less than 30 vol%). All other conditions, namely higher crystallinity, dissolved water contents below 3.5 wt% and, counterintuitively, in excess of 5.5 wt%, favour effusive activity. Between these ranges, there is a narrow field of transitional storage properties that do not discriminate between eruptive styles, and where the conduit exerts the main control on eruptive behaviour. Our findings suggest that better estimates of crystallinity and water content in subvolcanic chambers are key to forecasting eruptive style.