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The impact of pre-existing gas on the ascent of explosively erupted magma

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Most, if not all, magmas contain gas bubbles at depth before they erupt. Those bubbles play a crucial role in eruption dynamics, by allowing magma to degas, which causes the magma to accelerate as it ascends towards the surface. There must be a limit to that acceleration, however, because gas bubbles cannot grow infinitely fast. To explore that limit, a series of experiments was undertaken to determine the maximum rate at which bubbly high-silica rhyolite can decompress. Rhyolite melt that was hydrated at 150 MPa with ~5.3 wt.% dissolved water and contained 7 to 18 vol.% bubbles can degas in equilibrium at 875°C when decompressed at rates up to 1.2 MPa s−1 from 150 to 78 MPa, and up to 1.8 MPa s−1 when decompressed further to 42 MPa. In contrast, that same rhyolite cannot degas in equilibrium at 750°C if decompressed faster than 0.015–0.025 MPa s−1. When combined with other published experiments, the maximum rate of decompression for equilibrium degassing is found to increase by a factor of ten for every 50–75°C increase in temperature. When compared to predictions from conduit flow models that assume equilibrium degassing, it is found that such models greatly over-estimate the rate at which relatively cold rhyolite can decompress, whereas that assumption is largely correct for hot rhyolite, and thus for most other magmas, all of which are less viscous than rhyolite. In addition, most bubbles that were 20–30 µm in size at high pressure were lost from the population at low pressure. That absence suggests that only relatively large vesicles seen in volcanic pumice may be relics of pre-eruptive bubbles, even if small bubbles were originally present at depth.

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  1. Burgisser A, Gardner JE (2005) Experimental constraints on degassing and permeability in volcanic conduit flow. Bull Volcanol 67:42–56

  2. Cashman KV, Mangan MT (1994) Physical aspects of magmatic degassing II. constraints on vesiculation processes from textural studies of eruptive products. Rev Miner 30:447–478

  3. Gardner JE (2007) Bubble coalescence in rhyolitic melts during decompression from high pressure. J Volcanol Geotherm Res 166:161–176

  4. Gardner JE, Denis MH (2004) Rates of heterogeneous bubble nucleation in silicate melts. Geochim Cosmochim Acta 68:3587–3597

  5. Gardner JE, Hilton M, Carroll MR (1999) Experimental constraints on degassing of magma: isothermal bubble growth during continuous decompression from high pressure. Earth Planet Sci Lett 168:201–218

  6. Gardner JE, Rutherford M, Carey S, Sigurdsson H (1995) Experimental constraints on pre-eruptive water contents and changing magma storage prior to explosive eruptions of Mount St. Helens volcano. Bull Volcanol 57:1–17

  7. Gardner JE, Thomas RME, Jaupart C, Tait S (1996) Fragmentation of magma during Plinian volcanic eruptions. Bull Volcanol 58:144–162

  8. Gardner JE, Hilton M, Carroll MR (2000) Bubble growth in highly viscous silicate melts during continuous decompression from high pressure. Geochim Cosmochim Acta 64:1473–1483

  9. Gerlach TM, McGee KA (1994) Total sulfur dioxide emissions and pre-eruption vapor-saturated magma at Mount St. Helens. Geophys Res Lett 21:2833–2836

  10. Gerlach TM, Westrich HR, Casadevall TJ, Finnegan DL (1994) Vapor saturation and accumulation in magmas of the 1989–1990 eruption of Redoubt volcano, Alaska. J Volcanol Geotherm Res 62:317–337

  11. Gerlach TM, Westrich HR, Symonds RB (1996) Preeruption vapor of the climactic Mount Pinatubo eruption: source of giant stratospheric sulfur dioxide cloud. In: Newhall C, Punongbayan RS (eds) Fire and mud: eruptions and lahars of Mount Pinatubo, Philippines. University of Washington Press, Seattle, pp 415–433

  12. Geschwind C–H, Rutherford MJ (1992) Cummingtonite and the evolution of the Mount St Helens magma system: an experimental study. Geology 20:1011–1014

  13. Giordano G, Russell JK, Dingwell DB (2008) Viscosity of magmatic liquids: a model. Earth Planet Sci Lett 271:123–134

  14. Gualda G, Anderson AT Jr (2007) Magnetite scavenging and the buoyancy of bubbles in magmas. Part 1: discovery of a pre-eruptive bubble in Bishop rhyolite. Contrib Mineral Petrol 153:733–742

  15. Humphreys MCS, Menand T, Blundy JD, Klimm K (2008) Magma ascent rates in explosive eruptions: constraints from H2O diffusion in melt inclusions. Earth Planet Sci Lett 270:25–40

  16. Hurwitz S, Navon O (1994) Bubble nucleation in rhyolite melts: experiments at high pressure, temperature, and water content. Earth Planet Sci Lett 122:267–280

  17. Jaupart C (2000) Magma ascent at shallow levels. In: Sigurdsson H (ed) Encyclopedia of volcanoes. Academic, San Diego, pp 237–248

  18. Klug C, Cashman KV (1994) Vesiculation of May 18, 1980. Mount St. Helens magma. Geology 22:468–472

  19. Larsen JF, Gardner JE (2000) Experimental constraints on bubble interactions in rhyolitic melts: implications for vesicle size distributions. Earth Planet Sci Lett 180:201–214

  20. Liu Y, Anderson AT, Wilson CJN, Davis AM, Steele IM (2006) Mixing and differentiation in the Oruanui rhyolitic magma, Taupo, New Zealand. Contrib Mineral Petrol 144:397–415

  21. Liu Y, Anderson AT, Wilson CJN (2007) Melt pockets in phenocrysts and decompression rates of silicic magmas before fragmentation. J Geophys Res 112:B06204. doi:10.1029/2006JB004500

  22. Mangan M, Sisson T (2000) Delayed, disequilibrium degassing in rhyolite magma: decompression experiments and implications for explosive volcanism. Earth Planet Sci Lett 183:441–455

  23. Massol H, Koyaguchi T (2005) The effect of magma flow on nucleation of gas bubbles in a volcanic conduit. J Volcanol Geotherm Res 143:69–88

  24. Mastin LG, Ghiorso MS (2000) A numerical program for steady state flow of magma-gas mixtures through vertical eruptive conduits. USGS Open-File Rep 00-209

  25. Moore G, Vennemann T, Charmichael ISE (1998) An empirical model for the solubility of H2O in magmas to 3 kilobars. Am Mineral 85:36–42

  26. Mourtada-Bonnefoi CC, Laporte D (1999) Experimental study of homogeneous bubble nucleation in rhyolitic magmas. Geophys Res Lett 26:3505–3508

  27. Sparks RSJ, Brazier S (1982) New evidence of degassing processes during explosive eruptions. Nature 295:218–220

  28. Sparks RSJ, Barclay J, Jaupart C, Mader HM, Phillips JC (1994) Physical aspects of magma degassing I. Experimental and theoretical constraints on vesiculation. Rev Miner 30:413–445

  29. Wallace P (2001) Volcanic SO2 emissions and the abundance and distribution of exsolved gas in magma bodies. J Volcanol Geotherm Res 108:85–106

  30. Wallace PJ, Gerlach TM (1994) Magmatic vapor source for sulfur dioxide released during volcanic eruptions: evidence from Mount Pinatubo. Science 265:497–499

  31. Wallace P, Anderson AT Jr, Davis AM (1999) Gradients in H2O, CO2, and exsolved gas in a large-volume silicic magma system: interpreting the record preserved in melt inclusions from the Bishop Tuff. J Geophys Res 104:20097–20122

  32. Zhang Y, Behrens H (2000) H2O diffusion in rhyolite melts and glasses. Chem Geol 169:243–262

  33. Zhang Y, Belcher R, Ihinger PD, Wang L, Xu Z, Newman S (1997) New calibration of infrared measurement of dissolved water in rhyolitic glasses. Geochim Cosmochim Acta 61:3089–3100

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We thank Giovanni Sosa for analyzing the rhyolite obsidian used in this study. This project was partially funded by a grant from the U.S. National Science Foundation (EAR-0738664).

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Correspondence to James E. Gardner.

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Editorial responsibility: D. Dingwell.

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Gardner, J.E. The impact of pre-existing gas on the ascent of explosively erupted magma. Bull Volcanol 71, 835–844 (2009).

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  • Bubbles
  • Viscosity
  • Magma degassing
  • Explosive eruptions