SILICIC EXPLOSIVE VOLCANISM: MODALITIES OF MAGMA ...

SILICIC EXPLOSIVE VOLCANISM:MODALITIES OF MAGMADEGASSING AND ERUPTIONDYNAMICSRaffaello CioniDip.to Scienze della Terra – Cagliarircioni@unica.itRaffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembreSILICIC VOLCANISM: THE BASICSA loose term indicating silica-rich meltsHere used as a proxy for high-viscosity/highvolatile contentNot only Rhyolites!

Raffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembreMagmatic volatiles H2O solubilitys H2O = a P n CO2 solubilityS CO2 = b PRaffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembreMagmatic volatilesPressure dependenceof solubility forH 2 O + CO 2 mixtures

Raffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembreMagmatic volatilesH 2 O – CO 2 contents of natural silicic meltsWallace, JVGR 2005Raffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembreMagmatic volatilesChlorine solubility in silicic meltsWallace, JVGR 2005

Raffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembreMagmatic volatilesSulfur solubility in silicic meltsWallace, JVGR 2005Raffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembreMagmatic volatilesMagma crystallization (@ saturation) anddegassing path for H2O + CO2 –bearing meltsWallace, JVGR 2005

Raffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembreVISCOSITYNewtonian = (d/dt) = shear stress (Pa) = viscosity (Pa s)d/dt = strain rate (sec -1 )Bingham = + b (d/dt) b = Bingham viscosity = Yield stress (strength)Pseudoplastic = o (d/dt) n o = viscosity parametern = power law coefficient(values 1 to 0.3)Raffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembreVISCOSITY

Raffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembreVISCOSITY: the presence of crystals Raffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembreVISCOSITY: the effect of vesiclesDual effect, following Capillarity NumberStrain rateMagma viscosityMean bubble radiusSurface tension

Raffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembreVISCOSITY: the effect of vesiclesDual effect, following Capillarity NumberRaffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembreDynamics of magma ascent The volcanicsystem

Raffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembreMagma degassingDecompression-induced degassingSupersaturation &Bubble nucleationBubble growth(diffusion + decompression)Raffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembreDynamics of magma ascentConservation equations: why does magmaaccelerate in the conduit?For cylindric conduits, mass conservation impliescontinuous acceleration during degassingMass

Raffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembreDynamics of magma ascentConservation equations: why does pressuregradient vary in the conduit?Variation of momentum conservation is balancedby gravitational and frictional forces, and bypressure decrease along the conduitMomentumRaffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembreDynamics of magma ascent: bubbly flowPressureGas vol.fractionVelocity

Raffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembreDynamics of magma ascentAt surface,volume fractionof gas isdominantover magmaRaffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembreMagma fragmentationVesicle-packing

Raffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembreMagma fragmentationDynamic, rigid-fragmentationTime-scale of strain < structural relaxation time of magmaRaffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembreMagma ascent: closed- vs. open-system degassing

Raffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembreDynamics of magma ascentFor uncompressible magma at low ReynoldsnumbersHagen-Poisseuille flowQ = (r 4 / 8 ) (-dP/dz – g)Q = mass flowr = conduit radius = magma density = magma viscosity-dP/dz = pressure gradientRaffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembreOpen-system degassingSyn-eruptive gas-loss

Raffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembreVulcanian eruptionsRaffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembreWATER EFFECTS ON PHASE RELATIONSHIPSIsobaric crystallization + isothermal magma ascentMagma ascentMagma chamber crystall.Cashman, AGU 2004

Raffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembreDegassing-induced crystallizationT liquidus is increased by H 2 O degassing, inducinggroundmass crystallization and viscosity increase.Competing rates of magma ascent and gas lossRaffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembreDegassing-induced crystallization

Raffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembreMagma fragmentation Dynamic StaticMagma fragmentationRaffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembreCashman KV, AGU, 2004

Raffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembreCompeting rates of processes at eruptionEruption processes are dominated by the simultaneousvariation of multiple parameters, with differentcharacteristic rates Magma ascent (mm s -1 to m s -1 ) Volatile diffusion (10 -10 m 2 s -1 ) Crystal nucleation (10 2 -10 5 mm -3 s -1 ) and growth (10 -8 mm s -1 ) Bubble ascent (mm s -1 to m s -1 ) Permeable flow through magma (10 -12 m 2 ) and conduit walls(10 -16 m 2 ) Characteristic times for Strain rate (10 -1 s) vs. viscousrelaxation of the melt (10 -3 s)Raffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembreSilicic explosive eruptions

Raffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembrePlinian eruptions Lascar, Chile, 1993Raffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembreSubplinian eruptions Chaiten, Chile, 2008

Raffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembrePlinian eruptionsCioni et alii, Encycl. of Volcanoes, 2000Raffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembrePlinian eruptionsCioni et alii, Encycl. of Volcanoes, 2000

Raffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembrePlinian eruptionsTephra falloutRaffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembrePlinian eruptionsTephra dispersal

Raffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembrePlinian eruptionsTephra accumulationPlinian eruptionsRaffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembreConditions of convection vs. collapse

Plinian eruptionsRaffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembreConvectivecolumnPyroclasticflow Reventador (Ecuador) Novembre 2002Raffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembrePlinian eruptionsPyroclastic flow deposits

Raffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembrePlinian eruptionsPyroclastic flow depositsRaffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembrePlinian eruptionsPyroclastic flow damages

Vulcanian eruptionsRaffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembre Montserrat, West Indies, 2002Vulcanian eruptionsRaffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembre

Vulcanian eruptionsRaffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembre Ngaruhoe, New Zealand, 1974Raffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembreVulcanian eruptions

Raffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembreVulcanian eruptionsRaffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembreVulcanian eruptions Santiaguito, Nicaragua, 2008

Raffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembreVulcanian eruptions Santiaguito, Nicaragua, 2008Raffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembreVulcanian eruptions

Raffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembreVulcanian eruptionsRaffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembreAsh emission activity: Rabaul (PNG) 1994

Raffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembreAsh emission activity : Rabaul (PNG) 1994Raffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembreAsh emission activity : Rabaul (PNG) 1994Low level, weakly convective plumes

Raffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembreAsh emission activity : Rabaul (PNG) 1994Ash depositsRaffaello Cioni – Silicic explosive volcanism – GEOITALIA 2009 – Rimini 7-8 settembreAsh emission activity : Rabaul (PNG) 1994Damage from the ash