Brittleness of fracture in flowing magma

Abstract

[1] Understanding the transition to brittle fracture of flowing magma is essential for estimating the explosiveness of a volcanic eruption. In order to quantify brittleness of flowing magma, a new parameter b is introduced, which is a function of the ratio of the rate of change of the strain energy due to elastic distortional deformation to the mechanical power due to total distortional deformation rate. From the perspective of b, dependence of brittle fracture of magma on stress, decompression rate, strain rate, and shear-thinning effects are reviewed. The experimental data of Kameda et al. (2008) for rapid decompression of simulants of bubbly magma have also been reexamined. The results show that b exhibits a strong transition that correlates well with the observed transition between ductile expansion to brittle fragmentation. Moreover, b is useful in considering the effects of shear-thinning and steady-creep conditions on brittle fracture. Statements in the literature that suggest that materials behave in a brittle manner at sufficiently high strain rate are correct. However, these statements do not precisely quantify the notion of high strain rate so they are easy to misinterpret when considering the effect of shear thinning. Although shear thinning tends to increase the absolute magnitude of the total strain rate, it does not necessarily increase brittleness. Also, in principle, it is unlikely for brittle fracture to occur at steady creep. Cracking observed in steady-creep experiments of magma (Lavallee et al., 2007 may be attributed to small fluctuations with sufficiently high frequencies to satisfy the conditions for brittle fracture.

Slidequake Generation versus Viscous Creep at Softrock-landslides: Synopsis of Three Different Scenarios at Slumgullion Landslide, Heumoes Slope, and Super-Sauze Mudslide

“…The rates and manifestations of deformation are indicative of the underlying processes, e.g., at Super-Sauze aseismic viscous creep likely occurs in areas of lower displacement rates and brittle failure manifests as slidequakes where rates are higher. This dependence of deformation mode on loading rate may be similar to that inferred for ascending magma, in which fast-rising magma deformed brittle and generated earthquakes, while slow-rising magma is characterized by ductile, aseismic deformations (e.g., Tuffen et al, 2003;Ichihara and Rubin, 2010). The observations at Super-Sauze support this hypothesis of a relationship between loading rate and slidequake generation.…”

Strain‐induced outgassing of three‐phase magmas during simple shear

“…The shear-induced brittle fracturing at conduit margins described here is thought to be responsible for the earthquakes that often accompany extrusion of highly viscous magma plugs, which many authors have observed both texturally and seismically, and which can be an effective precursor for dome eruptions [Stasiuk et al, 1996;Tuffen et al, 2003;Tuffen and Dingwell, 2005;Iverson et al, 2006;Cashman et al, 2008;Lavallee et al, 2008;Tuffen et al, 2008;Pallister et al, 2012]. Furthermore, this behavior facilitates mobilization of silicic magmas that are naturally resistant to flow.…”

“…Crystal rearrangement during shear, clearly observed in our experiments, has been shown to induce the localization of deformation necessary for brittle failure of silicate melts [ Ichihara and Rubin , ; Laumonier et al , ]. Okumura et al [] also demonstrated strain localization occurring in crystal‐free vesicular rhyolite, similar to sample b36×0Ω2.…”

On the feasibility of magma fracture within volcanic conduits: Constraints from earthquake data and empirical modelling of magma viscosity

“…the stress‐strain rate relationship becomes non‐linear). Silicate melts can thus transition from fluid to solid‐like response [ Ichihara and Rubin , 2010], and undergo brittle fracture depending on the state of stress, strain rate, and temperature conditions. Maxwell's theory of viscoelastic deformation provides a theoretical framework for the study of magma fracture in silicic magma.…”