Week 7 – lava flows and domes

LEC424 Physical Volcanology

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)

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

Origin of lava flows

Fragmented at vent, then welded

Basaltic explosion crater Ljotipollur, Torfajökull, Iceland

Lava flows

More limited fragmentation at vent

Origin of lava flows

Lava morphology and structural features

Lava flow field: not just a single simple channel

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

Breakouts (basalt) Applegarth et al 2010

Breakouts (rhyolite)

Levee

Tuffen et al. in prep.

Lava tubes

Solid crust formationParfitt and Wilson chapter 9: calculate crustal thickness

What are implications of tube formation for lava dynamics?

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?)

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

Lava surface texture

pahoehoe

`a`a

Pahoehoe to `a`a transition

pahoehoe

`a`a

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)

Fink 1980

Rhyolitic obsidian flows

Rhyolitic obsidian flows

Lava flows that interact with water

Pillow lava

Columnar jointing

How might external water affect the dynamics of lava emplacement?

Rheology of lava flows

Newtonian viscosityYield strength: Bingham fluid (important for lavas!)

h

Some values for rhyolitic lava

Flow advance rate: Jeffrey’s equation

Flow advance rate: Jeffrey’s equation

Flow advance rate: Jeffrey’s equation

Nyiragongo, Republic of Congo100 km/h lavas (nephelinite, low SiO2, exceptionally low viscosity)

Flow thickness depends on yield strength

See lava spreadsheet to play with parametersSee also Lyman et al 2004 yield strength

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)

Lengths of lava flows

Length increases with effusion rate (but many complications!)

GPL Walker 1973

Effusion rate trends

•Determined using satellite and ground techniques

•Thermal budgets and measurement of lava velocity

•Models include dyke feeding from magma chamber

Evolving viscosity of lavas

Viscosity of lavas change from vent to flow frontThis profoundly affects lava dynamics

What influences lava viscosity?

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?)

Forecasting of lava flow advancehttp://www.agu.org/journals/gl/gl1113/2011GL047545/Vicari et al 2011

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

Experimental lava flows

Real lava: Lev et al 2012 (out this week!)http://www.sciencedirect.com/science/article/pii/S0377027312002405

Experimental lava flows

Analogues – wax, Griffiths 2000

Coffee!

Next – lava domes