|
Discussion |
1 Instituto de Ciencias de la Tierra ‘Jaume Almera’, CSIC, Lluís Solé i Sabarís s/n, 08028 Barcelona, Spain (e-mail: flegros{at}ija.csic.es)
2 Department Earth Sciences, University of Bristol, Bristol, BS8 1RJ, UK (e-mail: steve.sparks{at}bristol.ac.uk)
3 T. H. Huxley School of Environment, Earth Science and Enginering, Imperial College, Exhibition Road, London SW7, UK
4 Geological Institute, Bulgarian Academy of Sciences, Sofia, 1113, Bulgaria
F. Legros writes: 
Sparks et al. (1999) propose a new mechanism for welding of pyroclastic deposits by gas resorption rather than gas expulsion. Based on a scaling analysis, they suggest two possible regimes for gas behaviour following emplacement of hot pyroclastic deposits: a gas retention regime and a gas escape regime. In their novel gas retention regime, soluble gases are resorbed back into the glass rather than expulsed out of the compacting deposit. This occurs when the time-scale of compaction is less than the time-scale of gas escape. Sparks et al. (1999) propose that in the gas retention regime, resorption of water greatly speeds up welding by reducing the viscosity of the glass. They suggest that welding is more likely to occur in the gas retention regime for thick deposits and present some geological evidence for this mechanism in intra-caldera and intrusive welded tuffs. They also discuss several geological situations where gas resorption might explain rapid welding in thinner tuffs. Here, I discuss one of their assumptions, namely that water dissolved in the glass is at equilibrium with the local pore pressure. My analysis suggests that gas retention and resorption may actually be the dominant mechanism of welding, even in thin deposits, but, in contrast with Sparks et al., I conclude that this mechanism cannot significantly decrease the viscosity of the glass and hence the time of welding. I then discuss some geological implications.
The key point of the present discussion is the quantity of gas effectively present in the compacting deposit. Sparks et al. assume that there is always enough gas available so that the amount of volatiles dissolved in the glass is always equal to the solubility given by the Henry’s law. This situation corresponds to the experiments by Friedman et al. (1963) in which the vapour pressure is held constant during the course of an experiment. The theoretical models of Riehle (1973) and Riehle et al. (1995) use an empirical compaction coefficient derived from these experiments and so implicitly make the same assumption.
In a real pyroclastic deposit, the situation is likely to be different. Consider the example of the top layer of a deposit with a gas volume fraction of 50% initially at atmospheric pressure. Assuming that glass has equilibrated to ambient pressure (Sparks et al. 1999), its initial dissolved water content is about 0.13%. If the gas filling the pores of the deposit is composed of pure water vapour at magmatic temperature, the mass of vapour per cubic metre of deposit is only about 0.1 kg, while the mass of glass per cubic metre is about 1000 kg. If the pressure increase due to later burial is sufficient to cause all the water to resorb back into the glass, the dissolved water content would increase by only 0.01%, and the decrease in glass viscosity (Dingwell et al. 1996) and hence in compaction time-scale would be low. The resorption of all the interstitial vapour would occur at a pressure of only 0.12 MPa, i.e. at a depth of a few metres within the deposit.
The important point is that, unless there is an external gas source or the glass is still degassing, the possible increase in pore pressure due to burial occurs at constant gas mass, not constant gas volume. The amount of gas available is determined by the initial pore pressure in the uncompacted deposit, which is unlikely to be in excess of 0.2 MPa. This corresponds to the pressure at the base of a 1 km thick expanded pyroclastic flow with a 10 kg m–3 density or at the base of a deflated,
 |
References |
|---|
 TOP References |
|---|
Cole, P., Calder, E.S, Druitt, T.H, Hoblitt, R, Robertson, R.E.A, Sparks, R.S.J. & Young, S.R 000. Pyroclastic flows generated by the eruption of the Soufriere Hills Volcano, Montserrat. Geophysical Research Letters 25, 3425-3428.
Dade, B. & Huppert, H.E. 1996. Emplacement of the Taupo ignimbrite by a dilute turbulent flow. Nature 381, 509-512.[CrossRef][GeoRef]
Dingwell, D.B., Romano, C. & Hess, K.U. 1996. The effect of water on the viscosity of a haplogranite melt under P-T-X conditions relevant to silicic volcanism. Contributions to Mineralogy and Petrology 124, 19-28.[CrossRef][ISI][GeoRef]
Dobran, F., Neri, A. & Macedonio, G. 1993. Numerical simulations of collapsing volcanic columns. Journal of Geophysical Research 98, 4231-4259.[GeoRef]
Druitt, T.H. 1998. Pyroclastic density currents. In: Gilbert, J.S, Sparks, & R.S.J. (eds) . The Physics of Explosive Volcanic Eruptions. Geological Society, London. Special Publications, 145, 145-182.
Friedman, I., Long, W. & Smith, R.L. 1963. Viscosity and water content of rhyolitic glass. Journal of Geophysical Research 68, 6523-6535.[GeoRef]
Ragan, D.H. & Sheridan, M.F. 1972. Compaction of the Bishop Tuff, California. Geological Society of America Bulletin 83, 95-106.[Abstract][ISI][GeoRef]
Riehle, J.R. 1973. Calculated compaction profiles of rhyolitic ash-flow tuffs. Geological Society of America Bulletin 84, 2194-2216.
Riehle, J.R., Miller, T.F. & Bailey, R.A. 1995. Cooling, degassing and compaction of rhyolitic ash-flow tuffs: computational model. Bulletin of Volcanology 57, 319-336.[ISI][GeoRef]
Sparks, R.S.J., Tait, S.R. & Yanev, Y. 1999. Dense welding caused by volatile resorption. Journal of the Geological Society, London 156, 217-225.
Thomas, N., Jaupart, C. & Vergniolle, S. 1994. On the vesicularity of pumice. Journal of Geophysical Research 99, 15 633-15 644.[CrossRef]
Wilson, C.J.N. 1985. The Taupo eruption, New Zealand II. The Taupo ignimbrite. Philosophical Transactions of the Royal Society 314, 229-310.[CrossRef]
Wilson, L., Sparks, R.S.J. & Walker, G.P.L. 1980. Explosive volcanic eruptions IV—control of magma properties andconduit geometry on eruption column behaviour. Geophysical Journal of the Royal Astronomical Society 63, 117-148.[ISI][GeoRef]
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||
