Week 7 – lava flows and domes LEC424 Physical Volcanology.
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Transcript of Week 7 – lava flows and domes LEC424 Physical Volcanology.
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Week 7 – lava flows and domes
LEC424 Physical Volcanology
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LEC424 2012-2013
wk1. Physical properties of magma I: flow or break? (HT) wk2. Physical properties of magma II: crystals and bubbles (HT) wk3. Fieldtrip: Borrowdale Volcanics (JSG, HT) wk4. Practical: magma in the lab (HT)wk5. Practical: volcanic rocks in the hand wk6. Plumes and ash (JSG)wk7. Lavas and domes (HT)wk8. Practical: assessed (HT)wk9. Planetary volcanism I (LW)wk10. Planetary volcanism II (LW)
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Lava flowsSource texts:
Parfitt & Wilson Chapter 9 (Lava flows)
Encyclopaedia 291-306 Lava flows and flow fields (Kilburn)957-971 Lava flow hazards (Peterson and Tilling)
Applegarth 2010 evolution of a basaltic lava flow fieldFink 1980 rhyolitic lava flowsGriffiths 2000 review of lava flow dynamics, analogue experimentsHarris 2002 observations of a dacite lava flowLev et al 2012 lava flow experiments
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Origin of lava flows
Fragmented at vent, then welded
Basaltic explosion crater Ljotipollur, Torfajökull, Iceland
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Lava flows
More limited fragmentation at vent
Origin of lava flows
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Lava morphology and structural features
Lava flow field: not just a single simple channel
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Levees and channels
Lascar, Chile, andesite (left)
Kilauea, basalt (right)
Levee cross section from Tarquini et al 2012 JVGR http://dx.doi.org/10.1016/j.jvolgeores.2012.04.026
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Breakouts (basalt) Applegarth et al 2010
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Breakouts (rhyolite)
Levee
Tuffen et al. in prep.
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Lava tubes
Solid crust formationParfitt and Wilson chapter 9: calculate crustal thickness
What are implications of tube formation for lava dynamics?
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Forming a crust
Solid crust formationThickness ~ 2.3(t)0.5
Parfitt and Wilson chapter 9: calculate crustal thickness(Conductive cooling into air...what if additional coolant?)
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Tumuli
Anderson et al 2012 Bull VolcDOI 10.1007/s00445-012-0576-2
•Requires bending of solidified crust
•Crust too thick, can’t be bent
•Only found in basaltic lavas
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Lava surface texture
pahoehoe
`a`a
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Pahoehoe to `a`a transition
pahoehoe
`a`a
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Internal structure of basaltic lava flows
Internal architecture reflects
Crystallisation, vesiculation, deformation (brittle or ductile), cooling
Complexities: multiple tubes and flow
units in typical flow field (Harris and
Rowland 2009)
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Fink 1980
Rhyolitic obsidian flows
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Rhyolitic obsidian flows
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Lava flows that interact with water
Pillow lava
Columnar jointing
How might external water affect the dynamics of lava emplacement?
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Rheology of lava flows
Newtonian viscosityYield strength: Bingham fluid (important for lavas!)
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h
Some values for rhyolitic lava
Flow advance rate: Jeffrey’s equation
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Flow advance rate: Jeffrey’s equation
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Flow advance rate: Jeffrey’s equation
Nyiragongo, Republic of Congo100 km/h lavas (nephelinite, low SiO2, exceptionally low viscosity)
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Flow thickness depends on yield strength
See lava spreadsheet to play with parametersSee also Lyman et al 2004 yield strength
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Lengths of lava flows
Limited by cooling or volume?If limited by cooling length limited by crust formation
Graz number: equivalent diameter of flow/diffusivity*timeGz = d2/t
Essentially time for upper and lower cooling boundaries to converge
Compilation of global data: flows stop when Gz<320 (Pinkerton)
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Lengths of lava flows
Length increases with effusion rate (but many complications!)
GPL Walker 1973
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Effusion rate trends
•Determined using satellite and ground techniques
•Thermal budgets and measurement of lava velocity
•Models include dyke feeding from magma chamber
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Evolving viscosity of lavas
Viscosity of lavas change from vent to flow frontThis profoundly affects lava dynamics
What influences lava viscosity?
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Forecasting of lava flow advance
•Flow models (e.g. Magflow) essentially modified version of Jeffrey’s equation +
•Use topography and possible vent positions
•Input parameters such as effusion rate
•Forecast extent of lava flows
•(Important factors: Compound flow field? Tubes? Breakouts? Degassing?)
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Forecasting of lava flow advancehttp://www.agu.org/journals/gl/gl1113/2011GL047545/Vicari et al 2011
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Current tools for investigating lava flow dynamics
•Experiments (real magma, analogues): rheology and lava flow evolution
•Experiments (real magma): crystallisation and degassing
•Imaging of active lava flows (terrestrial and remote sensing)
•Geological investigation of ancient lava flow structures and textures
All feed into: improved models of lava flow advance, better forecasting
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Experimental lava flows
Real lava: Lev et al 2012 (out this week!)http://www.sciencedirect.com/science/article/pii/S0377027312002405
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Experimental lava flows
Analogues – wax, Griffiths 2000
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Coffee!
Next – lava domes