THE VOLCANIC EXPLOSIVE-EFFUSIVE TRANSITION: INSIGHTS FROM A CRYSTALLINITY-BASED PERMEABILITY MODEL

Lava dome-forming arc volcanoes commonly switch between effusive and explosive styles of eruption. Eruptions are driven by gas overpressure, which can be released through development of permeability, yet surprisingly the least permeable magmas often erupt effusively. In order to generate a permeable pore network and test the effects of crystallinity on permeability development, we studied gas exsolution and outgassing through experimental vesiculation of hydrous dacite samples containing crystal fractions between 0 and 0.8. We explored the evolution of the maximum unconnected gas volume or percolation threshold as a function of crystallinity. Outgassing occurs rapidly via fracturing at crystal fractions ≥ 0.7, implying that crystal-rich effusive lavas are likely healed remains of fractured, initially more hydrous magmas. Fracturing and bubble coalescence are both inefficient at crystal fractions of 0.5–0.6, implying eruptive behavior of lava domes can fluctuate in response to minor changes in crystallinity or ascent rate. We apply these interpretations to natural systems whose erupted products are well-characterized. Using a new compilation of published data from natural volcanic samples, we provide parameters for a permeability model as a function of crystallinity that delineates percolation curves representing effusive and explosive behavior, respectively, linked by a transition zone. Delays in gas escape and sustained vesiculation increases porosity beyond anticipated values, shifting permeability development to higher porosities, and crossing into the explosive-effusive transition zone, introducing explosive potential. Our model implies that the onset of permeable outgassing, or percolation threshold, occurs at low porosities < 0.1 for crystal fractions ≥ 0.37. Percolation thresholds systematically increase to higher porosities up to 0.75 for crystal fractions < 0.37. For a given crystal content, a percolation curve represents a threshold. Magmas with higher porosity or lower permeability than this threshold have greater explosive potential. Lower crystal contents exhibit a wider transitional zone where either style is possible, whereas higher crystal content magmas rapidly transition from dominantly effusive to dominantly explosive behavior.