Abstract

Although gas exsolution is a major driving force behind explosive volcanic eruptions, viscosity is critical in controlling the escape of bubbles and switching between explosive and effusive behavior. Temperature and composition control melt viscosity, but crystallization above a critical volume (>30 volume %) can lock up the magma, triggering an explosion. Here, we present an alternative to this well-established paradigm by showing how an unexpectedly small volume of nano-sized crystals can cause a disproportionate increase in magma viscosity. Our in situ observations on a basaltic melt, rheological measurements in an analog system, and modeling demonstrate how just a few volume % of nanolites results in a marked increase in viscosity above the critical value needed for explosive fragmentation, even for a low-viscosity melt. Images of nanolites from low-viscosity explosive eruptions and an experimentally produced basaltic pumice show syn-eruptive growth, possibly nucleating a high bubble number density.

Highlights

  • This pales into insignificance compared to the devastation that would be caused by an eruption of Vesuvius similar to 79 CE [6] in such a highly populated area or a Tambora 1815 style event, which led to an average drop in global temperature of 1.5° to 2°C and “the year without a summer” in 1816 [7]

  • We show one example where we have determined the size and time scale of nanoparticle formation and growth in a sample rapidly cooled in a single step from well above the liquidus temperature (1600°C) and held at 950°C while continuously collecting in situ small-angle x-ray scattering technique (SAXS)-wide-angle x-ray scattering (WAXS) measurements at an extremely fast acquisition rate

  • This is related to the high ratio of Fe3+/Fe2+ in the melt, the phase is still a magnetite [51] rather than the more oxidized iron oxide phase hematite

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Summary

Introduction

Helens in 1980 [1] to the near catastrophe of Pinatubo in 1990 [2] and the relatively small eruption of the Icelandic volcano Eyjafjallajökull in 2010 [3, 4], it is clear how our modern way of life can be severely disrupted by explosive, volcanic activity [5] This pales into insignificance compared to the devastation that would be caused by an eruption of Vesuvius similar to 79 CE [6] in such a highly populated area or a Tambora 1815 style event, which led to an average drop in global temperature of 1.5° to 2°C and “the year without a summer” in 1816 [7]. Such sudden energy release is caused by the subtle interplay between an exsolving volatile phase, the mechanism of gas escape, but bulk magma viscosity plays a critical role in this process [9]

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