vulcanian fountain collapse mechanisms revealed by multiphase numerical simulations l.
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  1. Vulcanian fountain collapse mechanisms revealed by multiphase numerical simulations: Influence of volatile leakage on eruptive style and particle-size segregation by A.B. Clarke, B. Voight, A. Neri & G. Macedonio

  2. Outline • Montserrat Vulcanian explosions • Here we test the effect of volatile leakage on Vulcanian explosions using a first-order leakage model to supply initial conditions for an axisymmetric, multiphase numerical model • Volatile loss can cause change in eruptive style from explosive to effusive (Jaupart and Allegre, 1991; Jaupart, 1998) • Comparison of models to real events

  3. Soufrière Hills volcano, Montserrat, BWI • Andesite dome-building eruption • Ongoing since 1995 • 1997 was a very active year, including 88 Vulcanian explosions

  4. Events preceding Vulcanian explosions on Montserrat

  5. Duration < 1 minute • Plume height 10 km (3 – 15) • Magma ejected 0.8 x 109 kg • Exit velocity 40 – 140 m s-1 • Fountain collapse height 300 – 650 m • Ash-cloud surge velocity 30 – 60 m s-1 • Pumice flow runout 3 – 6 km • Explosion interval 10 hours

  6. Numerical model • Solves Mass, Momentum and Energy for 3 particle sizes and a gas phase • Unsteady vent parameters (mass flux of each phase) calculated by model • Initial conditions and geometric parameters obtained from field data (Geometry & topography; OP = 10MPa from pumice; 3 particle sizes from deposits) • Results of pyroclastic dispersal compared to field observations

  7. q is mass flow rate of gas per unit area g & g are gas density and viscosity  is gas volume fraction Pc is gas pressure in the conduit Pl is lithostatic pressure K is permeability of country rock Radial Volatile Leakage Begin with reference simulation and apply the leakage model: 10 MPa OP; 3 particle sizes; 20 m cap; 4.3 wt.%H20; 65vol% crystals From Jaupart & Allegre, (1991)

  8. SimB (volatile loss) more energetic plume overhang style less mass to flows-68% higher & later fountain collapse ------------------------------ elutriation of fines from pyroclastic current SimC (3x SimB loss) less energetic plume boil-over style more mass to flows-82% lower & earlier fountain collapse ------------------------------------- elutriation of fines from pyroclastic current Results: effects of volatile leakage

  9. Overhang style Boil-over style

  10. Overhang style Boil-over style

  11. Elutriation of fines • Occurred for all simulations & was observed in real events • Elutriation was more dramatic for overhang-style • SimB at 80 s ~50% of fines were part of pf, but by150 s only 12% of fines remained part of the pf

  12. Conclusions • Duplication of real explosions requires some volatile leakage and/or delayed exsolution • Lateral volatile leakage plays an important role in explosion style (as well as strength) • Simulations revealed important mechanisms of fountain collapse and particle size segregation

  13. Conduit model assumptions * flow has stagnated no viscosity changes with depth * equilibrium degassing * constant crystal volume fraction with depth * constant overpressure with depth Reasonably duplicated real behavior --- however permeability (or anything that would reduce gas volume fraction, such as non-equilibrium degassing) proved to be significant in overall plume development

  14. How should we improve the conduit model? Results from Melnik 1999 suggest a few things * Still assume equilibrium degassing * Allow for viscosity changes due to crystal growth degassing * Resulting in a non-constant overpressure with depth and corresponding vesicularities How do these changes affect explosion results?

  15. Accounting for viscosity changes during ascent * had little affect on plume ascent rate * changed qualitative behavior of plume * changed pyroclastic flow runout distance

  16. How do we test which conduit model best represents reality? Pumice samples from a single event assume pumice records pre-fragmentation conditions Does pumice record pressure, temperature, and vesicularity variations with depth? If so, how do we measure these parameters?

  17. Methods Comparison against experiments on the same magma Matrix glass K2O composition (varies as the inverse of P and T) An content (increases with increasing P and T) Measure matrix glass water content In conjunction with density to better understand gas lost from system