Abstract
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.
Highlights
Explosive volcanic eruptions have aroused great interest in understanding the mechanisms of degassing and the transitions between explosive and effusive eruptions of silicic magmas
By focusing on long-timescale processes in decompressing rhyolite melts, we offer new insights into bubble coalescence and outgassing, which are probably the most elusive stages of the vesiculation process
The measured Φ of 80±5 vol% is in agreement within error with the gas fraction (α) of 84 vol% calculated for a closed-system equilibrium degassing (Table 4)
Summary
Explosive volcanic eruptions have aroused great interest in understanding the mechanisms of degassing and the transitions between explosive and effusive eruptions of silicic magmas. When silicic magmas rise from depth to the Earth surface, gas solubility decreases and the oversaturated melt exsolves gases as bubbles that grow by gas diffusion and vapor expansion. Either bubbles remain isolated and trapped in the melt or they coalesce to form connected gas channels that promote gas escape and eventually foam collapse (Eichelberger et al, 1986). Investigating the whole degassing process (i.e. the dynamics of bubble nucleation, growth, coalescence, and evacuation) is crucial to our understanding of the transitions in eruptive styles. Since the pioneering study of Sparks (1978), degassing and bubble dynamics in silicic melts have been extensively investigated through experiments Since the pioneering study of Sparks (1978), degassing and bubble dynamics in silicic melts have been extensively investigated through experiments (e.g. Navon et al, 1998; Gardner et al, 1999; Larsen and Gardner, 2000; Martel and Bureau, 2001; Gondé et al, 2011) and numerical models (e.g. Toramaru, 1989, 1995; Barclay et al, 1995; Proussevitch and Sahagian, 1996, 1998)
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