How does magma reach the surface? - Indico...
Transcript of How does magma reach the surface? - Indico...
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How does magma reach the surface?
1980, explosive
2004-2008, effusive
Michael Manga
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Why do volcanoes (only sometimes) erupt explosively?
1980, explosive
2004-2008, effusive
Michael Manga Gonnermann and Manga, Magma ascent in the volcanic conduit,Cambridge Univ Press, 2013
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Why do volcanoes erupt explosively? (textbook version)
Effusive eruption: No fragmentation
Water, CO2, SO2
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Why do volcanoes erupt explosively?
Open questions: • When, where and how does fragmentation occur? • Why so much diversity in eruption style?
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Three key processes1. Bubble nucleation, exsolution and
bubble growth
vesicular basalt (from the moon)
Mt Etna, Italy 2005 (R. Caniel)
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Volatile exsolution and bubble growth
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Three key processes 2. Loss of gases, called outgassing,
supresses eruption
outgassing
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Vesicular magma is permeable
Klug et al. (2002)
Connections between bubbles allow gases to escape from magma Permeability depends on vesicularity and bubble size
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If stresses in film surrounding bubbles too large
Pin If Pin - Pout > critical value then film ruptures
Pout
melt film
bubble
Three key processes 3. Fragmentation
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A second way to break magmas . . .
Def
orm
atio
n ra
te
Condition: strain rate > CG/ mr with C ~ 0.01
Relaxationtimescale
mr/G
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Are deformation rates high enough to fragment ascending magma?
we will refer to this brecciation
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Three key processes
1) Nucleation (forming new) and growth of bubbles 2) Outgassing (loss of gas from the magma) 3) Fragmentation and brecciation (breaking magma into
pieces)
Approach 1. Lab experiments and theoretical models to study individual processes and properties 2. Computer simulations 3. Test models with measurements made on rocks
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Numerical model
magma (bubbles+ melt) is locally homogeneous
Solve for growth of bubbles, determine rheology
Feedbacks between scales through temperature, pressure
Solve equations for conservation of mass, momentum, energy at two scales
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Conduit flow • conservation of mass, momentum, energy (include viscous dissipation; density, rheology from subgrid model) • non-turbulent, no fragmentation, • “single” phase magma (melt + bubbles) • cylindrical conduit , radial velocity is zero • steady flow
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Conduit flow • conservation of mass, momentum, energy (include viscous dissipation; density, rheology from subgrid model) • non-turbulent, no fragmentation, cylindrical conduit • “single” phase magma (melt + bubbles) • radial velocity is zero • steady flow
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Subgrid model: Volatile exsolution and bubble growth
Solubility of H20, CO2 from Liu et al. (2005)
Diffusivity of H20, CO2 from Zhang and Behrens (2000)
Proussevitch and Sahagian (1998)
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Subgrid model: Volatile exsolution and bubble growth
Bird et al. (1960)
Conservation of mass, momentum and energy, coupled with solubility model and modified Redlich-Kwong equation of state for water-CO2 mixtures
Lensky et al. (2001)
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Equilibrium (solubility-limited)
Growth is governed by changes in solubilityDecompression time scale
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Growth is by diffusion-limited when
S-R determined by number density of bubbles Nd
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Growth is by viscosity-limited when
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• Melt viscosity depends on amount of dissolved water and temperature (and composition)
Hess and Dingwell (1996)
• Melt viscosity depends on deformation rate • Magma viscosity affected by presence and properties
of bubbles and crystals
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Strain-rate dependent viscosity of melt phase
from Simmons et al. 1982
Silicic magmas are similar (Webb and Dingwell)
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Strain-rate dependent viscosity of bubbly suspension
increasing shear rate
Pal (2003) fit to data from Rust and Manga (2002)
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Vesicular magma is permeable
Klug et al. (2002)
Connections between bubbles allow gases to escape from magma Permeability depends on vesicularity and bubble size Outgassing efficient when - exceeds rate of gas exsolution
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Fragmentation criteria: thresholds determined experimentally
Condition: strain rate > CG/ mr with C ~ 0.01 e.g., Webb and Dingwell (1990), Webb (1997), Papale (1998)
If Pin - Pout > critical value then film ruptures
Pin
Pout
melt film
bubble
FragmentationBrecciation
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Experiments with real magma
If Pin - Pout > 1 Mpa/ then film ruptures Pin
Pout
melt film
bubble
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viscosity limits expansion
fragmentation
Example: Mount St Helens 1980 conditions
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Why do volcanoes erupt explosively?
Open questions: • When, where and how does fragmentation occur?
• Why so much diversity in eruption style?
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Change in eruption style with changing ascent rate
outgassing possible
asce
ndin
g m
agm
a St Helens, 2005
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Change in eruption style with changing ascent rate
brecciation outgassing
Little Glass Mountain, CA, 500 AD
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We predict that flow-induced fragmentation (brecciation) occurs at the sides of conduits Is there any evidence that this occurs?
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Obsidian is banded at all scales
Do these bands (in some cases) record fragmentation?
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Power spectrum: Scale invariant banding
Band widths are scale invarient over 4 orders of magnitude
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Brecciation, rewelding and deformation
10 cm
10 cm
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Simple shear . . . .
. . . . rotation and stretching
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A representative model Cantor model
reorient, reweld
stretch
fragment, change color
Bands consistent with repeated brecciation, reorientation of fragments, welding (stick back together) and stretching (reproduce power law and multifractal characteristic of bands)
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Change in eruption style with changing ascent rate
fragmentation brecciation
outgassing
Mono craters, CA 1350 AD
pumice + obsidian
effusive
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Mono Crater, CA
Test models using the measured concentration of water and CO2
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Water diffuses faster than CO2
Concentration of gases in bubbles is not necessarilyin equilibrium with that in the melt (diffusion limited growth)
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Water diffuses faster
Ascent rate to match data similar to other estimates
model (non equilibrium)
closed open
Data from Neuman et al. (1989)
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Does brecciation always happen? Not if the magma rises fast enough
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Does brecciation always happen? Not if the magma rises fast enough
when Brinkman number(viscous dissipation/heat diffusion)
becomes large
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no brecciation, blunt
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Change in eruption style with changing ascent rate
heating
fragmentation brecciation
outgassing St Helens 1980
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Change in eruption style with changing ascent rate
heating
fragmentation brecciation
outgassing
Pinatubo 1991
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Basaltic (low viscosity) eruptions
Increasing bubble/melt speed and volume fraction of bubbles
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Basaltic eruption styles
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Basaltic eruption styles
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Basaltic eruption styles
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Basaltic eruption styles
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Basaltic eruption styles
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Pumice clasts can break if collisions are energetic enough
Will large pumice clasts breakup before exiting volcanic conduits?
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Analytical model
• Assumptions: choked flow (exit velocity is the speed of sound in a dusty gas)
• Dissipation of granular energy balanced by production owing to shear
Details in Nature Geoscience, 2012
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Numerical simulations
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Equation of motion of particle
Detailed expressions in Dufek, Wexler and Manga, J Geophys Res (2009)
drag from gas effects of collisions buoyancy force
Lagrangian particles
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Lagrangian analysis
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Most large clasts are disrupted for fragmentation > few 100 m
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Dimensionless number Process
Reynolds number Bubble growth << 1(inertia/viscous forces) Magma ascent <103
Peclet number Diffusive growth >> 1 for low Nd; supersaturation,(diffusion/decompression timescale) nucleation new bubbles
Peclet number Bubble expansion >> 1 is viscosity high enough; (viscous/decompression timescale) overpressure, fragmentation
Brinkman number if large enough, lowers viscous and(viscous dissipation/diffusion of heat) conduit walls prevents shear brecciation
Dimensionless shear rates Magma ascent if large enough, shear thinning and blunt (shear stress/surface tension or shear rate x relaxation time of melt)
Ascent rate bubbles/magma Bubble separation
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• Interplay between bubble growth, brecciation, outgassing, and fragmentation governs eruption style
Why do volcanoes (only sometimes) erupt explosively?