Reconstructing Explosive Magma Vesiculation

William Marsh Rice University, Houston TX

Investigators

Abstract

The 1912 Plinian eruption of Novarupta volcano, Alaska is one the largest historical explosive eruptions and the subject of this study. Plinian eruptions are of very high explosive intensity and destructiveness. They can last for days with individual Plinian phases lasting for hours and producing tens of kilometers tall, sustained columns of hot gases and volcanic particles. These volcanic plumes can become dispersed on a hemispherical or global scale with significant societal impact. Key mechanisms to producing a Plinian eruption are the 'explosive' release of magmatic volatiles, such as water, carbon dioxide and sulfur, in conjunction with the 'explosive' fragmentation of the erupting magma into pyroclasts. The latter are variably sized particles of quenched magma, including volcanic ash. The interrelation between this explosive release of magmatic volatiles and magma fragmentation to result in a Plinian eruption is the focus of this project. This project will involve the numerical modeling of vesicle size distributions measured in pycroclasts from the 1912 Novarupta eruption. Vesicle size distributions provide a direct record of magma ascent conditions that can be investigated through modeling of bubble nucleation and growth in the ascending magma, both a consequence of volatile exsolution from the magma during ascent-driven decompression. The time-pressure history of an ascending parcel of magma, from which a given pyroclast is derived, is the key parameter determining the fit of modeled to observed vesicle size distributions. This investigation will test the hypothesis that decompression rates are surprisingly large during Plinian eruptions, as well as the question of how bubble nucleation and magma fragmentation are related. The large decompression rates apparently required to nucleate the large numbers of bubbles preserved as vesicles in pyroclasts can only be sustained for very short times, presumably immediately prior to or perhaps during magma fragmentation. It is thought that fragmentation is the consequence of accumulated bubble overpressure in excess of the magma's tensile strength. This project will integrate existing observational analyses of eruptive products with theoretical and empirical work on bubble nucleation and magma fragmentation through a comprehensive quantitative analysis. The resultant reconstruction of magma decompression, bubble nucleation, bubble growth and ultimately magma fragmentation, as recorded by the well-documented 1912 Novarupta pyroclast samples, is expected to lead to new advances in our understanding of explosive volcanism in general and Plinian style volcanic eruptions in particular.