In situ observation of nanolite growth in volcanic melt : a driving force for explosive eruptions

Although gas exsolution is a major driving force behind explosive volcanic eruptions, viscosity is critical in controlling the escape of bubbles and switching between explosive and effusive behaviour. Temperature and composition control melt viscosity, but crystallization above a critical volume (>30 vol.%) can lock up the magma, triggering an explosion. Here, we present an alternative to this well-established paradigm by showing how a surprisingly small volume of nano-sized crystals can cause a disproportionate increase in magma viscosity. Our in situ observations on a basaltic melt, rheological measurements in an analogue system and modelling demonstrate how just a few vol.% of nanolites results in a dramatic increase in viscosity above the critical value needed for explosive fragmentation, even for a low viscosity melt. Images of nanolites from low viscosity explosive eruptions as well as an experimentally produced basaltic pumice show syn-eruptive growth, possibly nucleating a high bubble number density