Preeruptive magma viscosity: An important measure of magma eruptibility

Abstract

[1] 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 ∼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 % 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.

Short time scales of magma-mixing processes prior to the 2011 eruption of Shinmoedake volcano, Kirishima volcanic group, Japan

“…Ascent of mainly A 1 magma began, incorporating magma A 0 (mixing II), less than 0.4 days before the sub-Plinian eruptions Shinmoedake, however, the main part (magma A 0 ) did not erupt as a large Plinian eruption. This result may indicate that the magma chamber of Shinmoedake did not have sufficient eruptibility (Takeuchi 2011) at that time and that only the mobile layer (magma A 1 ) could erupt with a minor contribution of the main part (magma A 0 ) of the magma chamber.…”

“…Therefore, Magma A 1 , the main component of the 2011 eruptive products, could have been a mobile layer (Burgisser and Bergantz 2011) that was formed long before the eruption by reheating of the mushy magma body (magma A 0 ). Eruption of mobile-layer magma is often a precursory phase of large Plinian eruptions (Takeuchi 2004(Takeuchi , 2011. In such cases, the eruptive products of the main Plinian phase are composed of phenocryst-rich silicic magma from the main part of a mushy magma chamber.…”

Intermittent and efficient outgassing by the upward propagation of film ruptures in a bubbly magma

“…Experimental conditions are summarized in Table , and notations are summarized in Table . By varying the water content of the syrup, we changed its viscosity between 12 and 4300 Pa s, which covers the typical viscosity range of basaltic melts, between 10 and 10 3 Pa s [e.g., Giordano and Dingwell , ], and most preerupted andesitic melts, <10 4 Pa s [e.g., Takeuchi , ]. The density of the syrup itself is between 1400 and 1452 kg m −3 , which is smaller and of the same order of magnitude as that for the typical density of basaltic melt, approximately 2700 kg m −3 .…”

Relationships between pre-eruptive conditions and eruptive styles of phonolite–trachyte magmas

“…In this work we use recently determined phase equilibrium results acquired on phonolitic magmas for determining their viscosity at pre-eruptive conditions and compare them to the pre-eruption melt viscosities of silicic-intermediate extrusive magmas Takeuchi, 2011). The results show that the selected phonolites have melt viscosities of 10 3.8 ± 0.4 Pa·s, being close to magma viscosities owing to their generally low crystal load.…”

“…In this work we directly compare the calculated pre-eruptive melt and magma viscosities of phonolite-trachyte magmas with those of Takeuchi (2011), which were obtained for rhyolitic-andesitic compositions (Table 3). This author used the same calculation method outlined above for estimating the pre-eruptive magma viscosities, considering also the effects of phenocrysts, bubbles, pressure, volatiles, and strain rate.…”