Timescales of bubble coalescence, outgassing, and foam collapse in decompressed rhyolitic melts

- Decompression-induced degassing timescales in hydrous rhyolitic melts.
- Degassing and outgassing timescales are cast in viscosity of the bubbly melt.
- Extensive bubble coalescence occurs after ∼1 h for bulk viscosity ∼105.5 Pas.
- Outgassing occurs after ∼5 h for bulk viscosity ∼104 Pas and ∼300 h for 105.5 Pas.
- Implications for pumice to dense pyroclasts transition at Mt Pelée and Mt Pinatubo.

The timescale of degassing and outgassing in hydrous rhyolitic melts is investigated in a wide range of conditions by means of decompression experiments. The evolution of vesicularity, bubble diameter, and number density is characterized as a function of time either of decompression or spent at final pressure, in order to determine the effect of final pressure, temperature, syn- versus post-decompression degassing, melt composition, and microlites, on the timescale of bubble growth, coalescence, and outgassing.The result suggests that different bubble evolution and degassing-outgassing timescale corresponding to explosive and effusive eruption regimes can be cast in bulk viscosity (melt + bubbles; ηbulk) versus decompression time (rather than path) space. The ηbulk-time relationship defines three domains of (i) bubble nucleation and growth, restricted to short durations and high ηbulk (<∼0.03 h for ηbulk∼105-6 Pas), (ii) equilibrium degassing with coalescence increasing from negligible (permeability > 10−13 m2) to extensive (permeability ∼10−11-12 m2), and (iii) outgassing, restricted to long durations and low ηbulk (>∼10 h for ηbulk<106 Pas; permeability >10−10 m2) that eventually leads to foam collapse.These findings are applied to the case studies of Mt Pelée and Mt Pinatubo to infer the transition from pumice to dense pyroclasts in volcanic eruptions and the possibility of evolving from an explosive Plinian eruption to an effusive dome-growth event by giving the vesicular magma enough time to outgas and collapse (i.e. hundreds to tens of hours for ηbulk∼105 to 104 Pas, respectively). We also show the drastic effect of microlites on re-arranging preexistent bubbles and potentially triggering a late nucleation event.