Preeruptive magma viscosity: An important measure of magma eruptibility

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 10 1 to 10 8 Pa s. Although bulk SiO 2 content has often been used as a qualitative measure of preeruptive magma viscosity, the bulk SiO 2 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 10 2 to 10 7 Pa s, which are lower or higher than those of phenocryst‐poor rhyolitic magmas with 10 5 to 10 6 Pa s. Focusing on andesitic to rhyolitic magmas, the r between bulk SiO 2 contents and magma viscosities changes to −0.1. In contrast, the melt‐only SiO 2 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 ∼10 6 Pa s, as consistent with the previous compilation study, this paper describes 20 examples of highly viscous magmas with >10 6 Pa s, in most cases, composed of mixtures of high‐silica rhyolitic melt (75–79 wt % SiO 2 ) 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 <10 6 Pa s induced the following eruption of less eruptible, highly viscous magmas.