IS THE CRITICAL CRYSTALLINITY THRESHOLD WHAT WE THINK IT IS? AN APPRAISAL OF THE OBSERVED CRYSTALLINITIES OF VOLCANIC MATERIALS

Volcanic eruptions, by definition, require the movement of magma from Earth’s interior to the surface. There is broad consensus that magma eruptibility is strongly controlled by viscosity. As a result, viscosity is one of the most important properties in understanding the likelihood of a magma to erupt and can also have a major influence on the resulting eruption style. Therefore, understanding the pre-eruptive viscosity of an erupted magma has the potential to assist in forecasting the timing and style of future volcanic events. Crystallinity has been identified as a critical parameter controlling magma viscosity, however, there is uncertainty in the crystallinities that distinguish eruptible from non-eruptible magmas, and whether highly crystalline magmas (>60 vol.%) could be erupted in some conditions. An underutilized but important source of information for understanding this relationship is the observed crystallinities in erupted volcanic materials, which by definition represent a set of eruptible magmas. Here we present a compilation of reported crystallinities for nearly 1000 volcanic samples of differing composition, tectonic setting, and eruption style, that provides valuable insight into the fundamental mechanisms which drive eruptions. Overall, the 95^th percentile crystallinity value of our full dataset is 57 vol.%, and >99% of all non-dome samples have crystallinity ≤53 vol.%. This suggests that 50–60 vol.% crystallinity represents a fundamental limit for eruptibility for most volcanic rocks. Some dome samples are clear exceptions to this and are erupted with considerably higher crystallinities. There is also a significant correlation between crystallinity and whole rock SiO₂ content as observed previously, but a shallow slope suggests whole rock and melt silica content have less impact on critical crystallinity for erupted magma than previously thought. Melt viscosity (as a function of SiO₂, temperature, and H₂O content) and crystallinity both play important roles on increasing effective viscosity, where melt viscosity plays a more important role at low crystal fractions, and crystallinity plays a more important role at crystallinities greater than ~40 vol.%.