SciEnTific REPORtS | 7: 16755 | DOI:10.1038/s41598-017-16941-x
Advancement of magma
fragmentation by inhomogeneous
, M. Ichihara
, S. Maruyama
, N. Kurokawa
, Y. Aoki
, S. Okumura
& K. Uesugi
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 dened as the solid-like fracture of a material whose bulk rheological properties are close
to those of a uid. 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 elds of the specimens under decompression computed by nite 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 ow in a volcanic conduit.
e rapid decompression of vesicular magma is an important mechanism in brittle fragmentation
, which leads
to explosive volcanic eruptions
. Factors controlling the fragmentation process have been investigated through
laboratory experiments using shock tube apparatuses with natural volcanic rocks
, porous solids
, and viscoelastic or viscous uids
. e fragmentation of solid samples in such experiments occurs
when the decompression amplitude is large, the void fraction is large, and the permeability is small
results have led to fragmentation criteria based on the critical stress fracture criteria for brittle failure. Dierent
models use dierent stresses to dene the critical stress, including the bulk stress of the porous material
the stress around bubbles with overpressure
. Recently, Heap et al.
demonstrated by numerical calculation the
importance of a small external dierential stress in addition to the bubble overpressure and the inhomogeneous
distribution of bubbles on the initiation and progression of fragmentation.
e fragmentation criterion not only indicates the conditions under which fragmentation occurs but also
denes 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
formula for the fragmentation speed by combining the fragmentation criterion with the equations for the con-
servation of mass and energy across the fragmentation surface. In his derivation, the conservation of momentum
equation is replaced by an assumed power-law relationship between the pressure and the density, considering
that rate of momentum transfer depends on the process of gas liberation from disrupted pores and is not known.
Koyaguchi & Mitani
eliminated this problem by assuming fragmentation is instantaneous aer the critical stress
has been reached. Koyaguchi et al.
have demonstrated that their model can t the experimentally observed
if the stress at the midpoint of the bubble wall is used as the measure of critical stress. More
recent theoretical models
have revealed that the experimentally observed speed and layer-by-layer behavior
of fragmentation is generated by the combined eect of the sample adhering to the shock tube and the escape of
Department of Mechanical Systems Engineering, Tokyo University of Agriculture and Technology, Koganei,
Tokyo, 184-8588, Japan.
Earthquake Research Institute, University of Tokyo, Bunkyo-ku, Tokyo, 113-0032, Japan.
Department of Earth Science, Tohoku University, Sendai, Miyagi, 980-8578, Japan.
Japan Synchrotron Radiation
Research Institute, Sayo-cho, Hyogo, 679-5198, Japan. Correspondence and requests for materials should be
addressed to M.K. (email: email@example.com)
Received: 14 August 2017
Accepted: 17 November 2017
Published: xx xx xxxx