vulcanian fountain collapse mechanisms revealed by multiphase numerical simulations: influence of...
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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
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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
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•Andesite dome-building eruption
•Ongoing since 1995
• 1997 was a very active year, including 88 Vulcanian explosions
Soufrière Hills volcano, Montserrat, BWI
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Events preceding Vulcanian explosions on Montserrat
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•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
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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
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Radial Volatile Leakage
• 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
From Jaupart & Allegre, (1991)
lPcPkgq
arg
g rPK
q
Begin with reference simulation and apply the leakage model:
10 MPa OP; 3 particle sizes; 20 m cap; 4.3 wt.%H20; 65vol% crystals
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Results: effects of volatile leakage
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
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Overhang style
Boil-over style
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Overhang style
Boil-over style
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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
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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
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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
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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 growthdegassing
* Resulting in a non-constant overpressure with depth and corresponding vesicularities
How do these changes affect explosion results?
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Accounting for viscosity changesduring ascent
* had little affect on plume ascent rate
* changed qualitative behavior of plume
* changed pyroclastic flow runout distance
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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, andvesicularity variations with depth?
If so, how do we measure these parameters?
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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