Rheologic controls on the magnitude and mechanisms of outgassing in silicic lavas

The volume of gas trapped in bubbles in silicic lavas dictates the hazard they pose. Trapped gas is released from the lava (outgassing) if pathways form to connect bubbles (regions of high gas pressure) to the atmosphere (region of low gas pressure). We conducted deformation experiments on synthetic bubbly lavas to (1) quantify syn-deformation outgassing when pathways form by different mechanisms (coalescence, shear-induced connectivity, melt fracturing), and (2) establish the relationship among flow conditions, parameterized by the Weissenberg number (Wi), and outgassing mechanism and magnitude. At low Wi (≤10−7), coalescence causes extensive (>75%) outgassing. At intermediate Wi (10−6−10−4), almost no outgassing occurs until high strain, when shear-induced connectivity allows some (>50%) outgassing. At high Wi (≥10−2), sample-scale pathways form by melt fracturing but result in almost no (<25%) outgassing. We fit the results with an empirical model that predicts outgassing extent as a function of strain and Wi. Our results show extensive outgassing requires coalescence, which means a lava must cool very slowly and/or deform at a low strain rate to inhibit explosive behavior.