Evidence for deep gas loss in open volcanic systems

Previous studies of Vulcanian eruptive products have shown that the respective volcanic conduits were filled for most part with low-porosity magma (i.e., < 10 vol%) prior to eruption. Comparison theoretical porosity distribution expected from closed-system degassing suggests gas loss must taken place at depth within magmatic column (between 3 and 5 km). At such high pressures 70 110 MPa), however, porosities are low enough (< 20 rule out traditional mechanisms. We tested if channelling, an outgassing mechanism based on bubble connection due crystal content (>40 proposed occur in mushy reservoirs, could also happen conduits. reanalyzed phenocryst, microlite, data recent eruptions Merapi volcano, Indonesia, Soufrière Hills Montserrat, Tungurahua Ecuador. Overall, these magmas had contents by channelling. Gases be channelled columns various levels during ascent yield mostly gas-depleted explosive behavior. Such channelling thus has capacity influence style. Depending phenocryst content, microlite growth can either foster or impede escape Considering pervasive occurrence microlites ensuing 40 vol%), likelihood implies other mechanisms might not as dominant previously envisioned.