Magmas may flow or break depending on their deformation rate. The transition between such viscous and brittle behavior controls the style of volcanic eruptions. While the brittle failure of silicate melts is reasonably well characterized, the effect of crystals on the viscous-brittle transition has not yet been constrained. Here we examine the effect of suspended crystals on the mechanical failure of magmas using torsion experiments performed at temperatures (600–900 °C), strain rates (10−4–10−1 s−1), and confining pressures (200–300 MPa) relevant for volcanic systems. We present a relationship that predicts the critical stress and associated strain rate at which magmas fail as a function of crystal fraction. Furthermore, the results demonstrate that the viscous to brittle transition occurs at lower stresses and strain rates when crystals are present. The fractures formed during brittle failure of crystal-bearing magma originate in the melt phase, which enables gas to escape, and hence to reduce gas overpressure. These degassing pathways heal on relatively short time scales owing to the high confining pressure at depth, highlighting the possibility that coherent lavas may actually be the healed remains of partially degassed magma parcels that have undergone many cycles of fracturing and healing.

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