Preeruptive magma viscosity: An important measure of magma eruptibility

Abstract

Using a compilation of melt compositions, meltwater contents, temperatures, and phenocryst contents, the preeruptive viscosities under magma reservoir conditions are calculated for 83 erupted magmas. The basaltic-to-rhyolitic magmas have preeruptive viscosities over the range 10¹ to 10⁸ Pa s. Although bulk SiO₂ content has often been used as a qualitative measure of preeruptive magma viscosity, the bulk SiO₂ content shows a weak correlation with magma viscosity (correlation coefficient r = 0.5). Because of a wide range of phenocryst contents from 0 to ∼50 vol %, andesitic magmas have viscosities ranging from 10² to 10⁷ Pa s, which are lower or higher than those of phenocryst-poor rhyolitic magmas with 10⁵ to 10⁶ Pa s. Focusing on andesitic to rhyolitic magmas, the r between bulk SiO₂ contents and magma viscosities changes to -0.1. In contrast, the melt-only SiO₂ content from a basaltic-to-rhyolitic melt shows a good linear correlation with melt-only viscosity (r = 0.9). Although most of the calculated viscosities of erupted magmas fall below ∼10⁶ Pa s, as consistent with the previous compilation study, this paper describes 20 examples of highly viscous magmas with >10⁶ Pa s, in most cases, composed of mixtures of high-silica rhyolitic melt (75-79 wt % SiO₂) and abundant phenocrysts (30-55 vol %). In these highly viscous magmas, 9 examples have erupted following the precursory eruption of less viscous magma, suggesting that precursory dike propagation and conduit formation by the less viscous magma with <10⁶ Pa s induced the following eruption of less eruptible, highly viscous magmas.