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Abstract High-threat explosive silicic eruptions commonly contain banded pumice, reflecting magma mingling in the conduit prior to or during eruption. Heterogeneities in tuffs have been attributed to the draw-up of compositionally distinct magmas, in which low-viscosity magmas ascend more quickly than high-viscosity magmas. The Rattlesnake Tuff of the High Lava Plains in Oregon (northwestern United States) represents a zoned magma reservoir where at least five different rhyolite compositions are preserved in banded pumice samples in variable mingled combinations. Geochemical gradients recorded across band boundaries in pumice were modeled using a Monte Carlo least-square minimization procedure to find the complementary error function that best fit observed Si and Ba diffusion profiles by iteratively varying the concentration of each plateau (i.e., the concentration on either side of the band boundary), the center and spacing of the diffusion profile, diffusion length scale, and temperature. Modeling indicates maximum time scales between mingling and conduit ascent from minutes to hours. Viscosity calculations for each rhyolite composition confirm that highly viscous rhyolites have longer ascent times than low-viscosity magmas, strongly supporting a model of sequential tapping of a zoned chamber controlled by viscosity. 

There is a broad consensus that magma eruptibility—the ability of magma stored in the subsurface to erupt onto Earth's surface—is strongly controlled by viscosity. Related to this, a critical parameter that controls viscosity is crystallinity. 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, which provides valuable insight into the fundamental mechanisms which drive eruptions. Overall, the 95th 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.%.