Brittleness of fracture in flowing magma

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 β 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 β, 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 β exhibits a strong transition that correlates well with the observed transition between ductile expansion to brittle fragmentation. Moreover, β 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, 2008) may be attributed to small fluctuations with sufficiently high frequencies to satisfy the conditions for brittle fracture.