Advancement of magma fragmentation by inhomogeneous bubble distribution

M Kameda,K Uesugi,S Maruyama,M Ichihara,S Okumura,Y Aoki,N Kurokawa

doi:10.1038/s41598-017-16941-x

Abstract

Decompression times reported in previous studies suggest that thoroughly brittle fragmentation is unlikely in actual explosive volcanic eruptions. What occurs in practice is brittle-like fragmentation, which is defined as the solid-like fracture of a material whose bulk rheological properties are close to those of a fluid. Through laboratory experiments and numerical simulation, the link between the inhomogeneous structure of bubbles and the development of cracks that may lead to brittle-like fragmentation was clearly demonstrated here. A rapid decompression test was conducted to simulate the fragmentation of a specimen whose pore morphology was revealed by X-ray microtomography. The dynamic response during decompression was observed by high-speed photography. Large variation was observed in the responses of the specimens even among specimens with equal bulk rheological properties. The stress fields of the specimens under decompression computed by finite element analysis shows that the presence of satellite bubbles beneath a large bubble induced the stress concentration. On the basis of the obtained results, a new mechanism for brittle-like fragmentation is proposed. In the proposed scenario, the second nucleation of bubbles near the fragmentation surface is an essential process for the advancement of fragmentation in an upward magma flow in a volcanic conduit.

Highlights

The rapid decompression of vesicular magma is an important mechanism in brittle fragmentation[1,2,3], which leads to explosive volcanic eruptions[4,5]
The fragmentation criterion indicates the conditions under which fragmentation occurs and defines the fragmentation speed
Fragmentation experiments have revealed that the fragmentation front proceeds into the sample at a certain speed depending on the overpressure and void fraction[6,17]

Summary

Introduction

The rapid decompression of vesicular magma is an important mechanism in brittle fragmentation[1,2,3], which leads to explosive volcanic eruptions[4,5]. Kameda et al.[12] found that when τr < tdec < τv, solid-like fragmentation occurs but with a significant time delay They defined this type of fragmentation behavior as brittle-like fragmentation. From their detailed observation, Kameda et al.[13] found that the onset of brittle-like fragmentation is triggered by the sudden release of a considerable amount of gas from a crack on the specimen surface. Kameda et al.[13] found that the onset of brittle-like fragmentation is triggered by the sudden release of a considerable amount of gas from a crack on the specimen surface They suggested that the crack initiates in the interior of the specimen by the ductile rupturing of the continuous phase or connection of the bubbles as a result of the complicated morphology of the bubble distribution. Based on the obtained results, the effects of the size and spatial distributions on the development of cracks and the triggering of brittle-like fragmentation are discussed

Methods

Results

Conclusion