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Vulcanian fountain collapse mechanisms revealed by multiphase numerical simulations:

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##### Vulcanian fountain collapse mechanisms revealed by multiphase numerical simulations:

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**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**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**Soufrière Hills volcano, Montserrat, BWI**• Andesite dome-building eruption • Ongoing since 1995 • 1997 was a very active year, including 88 Vulcanian explosions**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**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**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)**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**Overhang style**Boil-over style**Overhang style**Boil-over style**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**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**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**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?**Accounting for viscosity changes**during ascent * had little affect on plume ascent rate * changed qualitative behavior of plume * changed pyroclastic flow runout distance**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?**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