PowerPoint PresentationSlide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29PTYS 554Evolution of Planetary SurfacesVolcanism IIIVolcanism IIIPYTS 554 – Volcanism III2Volcanism IMantle convection and partial meltingMagma migration and chambersDikes, sills, laccoliths etc…Powering a volcanic eruptionVolcanism IIMagma rheology and volatile contentSurface volcanic constructsBehavior of volcanic flowsVolcanism IIIInteraction with volatiles (Maars, Tuyas etc…)Ash columns and falls, Surges and flowsIgminbrites, tuffs, weldingPyroclastic depositsPYTS 554 – Volcanism III3Tephra/pyroclastic depositsBombs/blocks > 64mmCinders/Lapilli 2-64mmAsh < 2mmEscaping volatiles drive eruptions of different strengthsMeasured by % fragments < 1mm in sizeMeasured by area covered by tephraHigh-silica magmas drive the most explosive eruptionsVulcanianSchmincke 2004PYTS 554 – Volcanism III4Rise speed of MagmaConstant when no bubblesAccelerating when bubbles form – constant mass flux with density decrease implies velocity increase Accelerating faster - when fragmentation occurs and friction with the walls disappearsThe larger the volatile content the larger the rise speedDepth (m)Wilson and Head, 1981PYTS 554 – Volcanism III5Io – 75km high ballistic plumesPYTS 554 – Volcanism III6PlinianBubbles don’t move through the magma very muchExpand in place, limited by overburden pressureRemove the overburden and you get a downward propagating fragmentation frontBubbles expand quickly relative to viscous behavior of magmaBrittle disintegration into ash sized particlesEjection velocities of many 100s of m/sVulcanianSchmincke 2004PYTS 554 – Volcanism III7Eruption columnsGas Thrust region100-600 ms-1 in plinian eruptionsEntrains ambient airConvective zoneLarge clasts have fallen outAmbient air has been entrainedNow positively buoyantTemperature inversion at tropopauseStratosphere warms with heightPlume stops in lower stratosphereUmbrella regionLevel of neutral buoyancySome overshoot from momentumPYTS 554 – Volcanism III8Height of eruption column depends on eruption rateH α Q¼1000°C600°CPYTS 554 – Volcanism III9Turbulent flowDownward force from weight – buoyancyFalling particle is opposed by ram pressureEquating these to find the settling velocity – not very sensitive to particle sizeWhen does a particle fall out of the column?When it’s fall velocity exceeds the column’s rise velocityv ra rsdLow pressureFdown=p6d3rsgFup=12CDp4d2rav2vsettle=4rsg3CDradHigh pressurePYTS 554 – Volcanism III10Eruption height can be reconstructed from isopleth areasIsopleth: line joining location of equal maximum-clast-sizeIsopach: line joining locations of constant deposit thicknesse.g. isopleths in cm for a 30km high columnPlotted for several column heightsData for a pumice deposit in the AzoresPYTS 554 – Volcanism III11Pyroclastic events can be categorized from ancient depositsDistance over which deposits halves in thickness (bt)Depends on column height and particle sizeDistance over which maximum clast halves in size (bc)Depends on column heightPYTS 554 – Volcanism III12Pyroclastic deposits (ignimbrite) form tuffSometimes welded Ash, lapilli, country rock fragments includedFlattened lapilli when overburden was highPYTS 554 – Volcanism III13Back to the eruption columns… behavior governed by vent radius and volatile contentEntraining air makes them buoyantVolatile-rich plumes are more buoyant as they have higher ejection velocitiesBut if plume is too wide then surface area:volume is too low – air doesn’t get entrainedEruption Velocity m/sPYTS 554 – Volcanism III14Some combinations of vent radius and volatile content produce convecting eruption columns and some don’tConditions change during an eruption…Vent is enlargedVolatiles get expended earlyStable columns can become unstable and collapsePyroclastic flows…Very energetic!PYTS 554 – Volcanism III15Pyroclastic flows and fallsPYTS 554 – Volcanism III16Pyroclastic falls Usually well sortedEvenly coats pre-existing topographyAshLapilliPYTS 554 – Volcanism III17Pyroclastic flowsUsually not well sortedFills hollows in pre-existing topographyFluidized mix of hot gases and clastsGuided by pre-existing topographyOften produces welded tuffPYTS 554 – Volcanism III18Pyroclastic flow deposits can show cross-bedded stratigraphyKilbourne Hole, NMKnauth et al. 2005PYTS 554 – Volcanism III19Pyroclastic density currents Pyroclastic flowsPumice flowsClasts = vesiculated pumiceDeposits = ignimbritesNuees ArdenteesClasts = dense, unvesiculated materialDeposits = block and ash depositsPyroclastic SurgesLower density, turbulentLess momentum – restricted rangeLess affected by topographyClasts can be either vesiculated or notPYTS 554 – Volcanism III20Flows can also travel across waterDensest clasts sinkSteam cushion keeps the flow aloftChanging eruption stylesE.g. Bishop TuffLower layer is ash fallUpper layer is pyroclastic flowPYTS 554 – Volcanism III21PYTS 554 – Volcanism III22Medusae Fossae Formation Mars – a pyroclastic deposit?Watters et al.PYTS 554 – Volcanism III23Interaction with waterAt P=1 atm and T=100° C, pressure of water increases by 15 bar/1° CExplosive effects are common when magma and water come into contactPhreatic eruptionsInvolves superheated groundwaterPhreatomagmatic eruptionsProduces some volcanic rockUbehebe crater, Death ValleyCommon occurrence on IoVolatile is frozen SO2 rather than H2OMaterial ejected at close to 1 kms-1 (from plume heights)~equal parts basaltic pyroclasts and sulfur volatilesSulfur compounds refreeze as plume expandsPYTS 554 – Volcanism III24Magma to water ratio determines the feature type.PYTS 554 – Volcanism III25Maar volcanoesRepeated explosive interactions between groundwater and magmaSubterranean diatremes sometimes left as erosional remnantsPYTS 554 – Volcanism III26Layered rims and sunken floors are superficially similar to