Magma degassing during the Plinian eruption of Novarupta, Alaska, 1912

Abstract

[1] We have modeled the nucleation and isothermal growth of bubbles in dacite from the 1912 Plinian eruption of Novarupta, Alaska. Bubble growth calculations account for the exsolution of H 2 O and CO 2 , beginning with bubble nucleation and ending when bubble sizes reproduced the observed size distribution of vesicles in Novarupta pumice clasts. Assuming classical nucleation theory, bubbles nucleated with a diameter of the order of 10 À8 m and grew to sizes ranging from 10 À6 m to greater than 10 À3 m, the typical range of vesicle sizes found in Novarupta pumice. The smallest vesicles in Novarupta pumices are also the most abundant and bubbles with radii of 10 À6 m to 10 À5 m comprise almost 90% of the entire bubble population. We find that these bubbles must have nucleated and grown to their final size within a few 100 milliseconds. Despite these extremely fast growth rates, the pressures of exsolved volatiles contained within the bubbles remained high, up to about 10 7 Pa in excess of ambient pressure. Assuming a closed-system, the potential energy of these compressed volatiles was sufficient to cause magma fragmentation, even though only a fraction of the pre-eruptive volatiles had exsolved. Unless the matrix glasses of Novarupta pyroclasts retains a large fraction of pre-eruptive volatiles, the majority of magmatic volatiles (80-90%) was likely lost by open-system degassing between magma fragmentation and quenching.

Relating vesicle shapes in pyroclasts to eruption styles

“…Vesicle shapes formed during ascent within the conduit may be modified after fragmentation by bubble growth (e.g., Thomas and Sparks 1992;Kaminsky and Jaupart 1997) and shape relaxation due to capillary forces (e.g., Klug and Cashman 1996). However, for silicic and for microlite-rich basaltic magmas, post-fragmentation bubble growth should be of limited extent, due to permeable outgassing (e.g., Rust and Cashmann 2011;Gonnermann and Houghton 2012). Moreover, the characteristic time scale for shape relaxation, τ relaxation , would need to be much shorter than the characteristic quenching time, τ quenching .…”

“…For simplicity, we assume a constant value of σ = 0.05 N m −1 . The variability in σ is probably less than a factor of 2 (e.g., Gardner and Ketcham 2011 and references therein) and does not affect model results significantly (Gonnermann and Houghton 2012). The bubble growth calculation is coupled to a model for magma ascent within the conduit, from which we obtain the decompression rate, dP m /dt.…”

Discriminating secondary from magmatic water in rhyolitic matrix-glass of volcanic pyroclasts using thermogravimetric analysis

“…Recent work by Gonnermann and Houghton (2012) suggested that this may not necessarily be the case. Detailed modeling of magma degassing during Episode III of the 1912 Plinian eruption of Novarupta volcano, Alaska, indicated the possibility that the magma could have retained a significant fraction of its pre-eruptive water as dissolved water within the melt, due to disequilibrium degassing.…”

Alkali Bazalt ve Piroklastik Çökellerin Volkanolojik Özellikleri, Kula Volkanları, Batı Anadolu.

“…Lavların iç kısımlarının gözenek bakımından fakir olmasının nedeni de bu şekilde açıklanabilir. Mikrolit içeriği zengin olan bazaltik magmalarda kabarcık büyüme-si sınırlıdır (Gonnermann and Houghton 2012). Gözeneklilik miktarı en düşük olan I. Evre plato bazaltları diğer evrelerdeki bazaltlardan daha fazla mikrolit içeriğine sahiptir.…”