Magma deformation may induce non-explosive volcanism via degassing through bubble networks

The open-system degassing of magma controls the bifurcation of explosive and non-explosive volcanism. Permeable gas flow through connected bubbles is a potentially important mechanism of open-system degassing from viscous silicic magma. However, recent experimental study revealed that the permeability of isotropically vesiculated magma is low; this suggests that effective degassing for causing effusive eruptions requires some other mechanisms that increase the magma permeability. In this study, we experimentally demonstrate that magma deformation results in an increase in the permeability of vesiculated magma. Torsional deformation experiments of vesiculated rhyolitic melts were performed at shear strain rate of <0.029 s(-1) up to a total strain of 34.6: then, the permeability of the run products was measured to quantify the degassing rate. The experimental results show that shear deformation dramatically increases the magma permeability parallel to the shear direction via the enhancement of bubble coalescence and the networking of tube-like bubbles. When shear strain is above 8, the permeability sharply increases at a vesicularity of 30 vol.%. The gas velocity along the direction of magma flow is estimated to be 10(-5) to 10(-1) m s(-1) on the basis of experimentally obtained permeabilities (10(-15) to 10(-10) m(2) at vesicularities of 30 to 80 vol.%) and Darcy's law, assuming a lithostatic pressure gradient of 0.03 MPa m(-1). The gas velocity is inferred to be large enough for gas escape to be significant during magma ascent of effusive eruption. A simple model of the magma flow along volcanic conduits indicates that magma deformation results in degassing at greater depths (a few thousand meters for rhyolite with temperatures of 700-900 degrees C and an initial water content of 5 wt.%) than in the case where the magma is isotropically vesiculated. The ratio of the radius of the volcanic conduit to its length may control the degree of magma deformation and consequently the eruption explosivity. (c) 2009 Elsevier B.V. All rights reserved.