2CourseDynamicsClassification of igneous rocks and properties of magmaGeneration and differentiation of magma 1Generation and differentiation of magma 2Sub-volcanic plumbing systemPhysical volcanology 1Physical volcanology 2
16Plinian Eruption Example Convective regioncolumn entrains cold airmixed air dilutes column, is heatedreduces density, increases buoyancy= RISEGas thrust regionhigh velocity jet of gas and particlesm s-1
17Plinian Eruption Example Umbrella regionconvective column continues to builddensity column =density atmospherecolumn stops rising and spreads outUMBRELLARedoubt, AlaskaSheveluch(Kamchatka)in Russia
18Plinian Eruption Example What happens next?Depends on densityρ column vs. ρ atmosphereIf ρ column < ρ atmospherebuoyant eruption plumepyroclastic FALL depositsIf ρ column > ρ atmosphereeruption column collapses under gravitypyroclastic DENSITY CURRENT deposits
19Fall Deposits Fall deposits Ash, pumice settling from eruption column (scoria, bombs in basaltic eruptions)Ash-fall or pumice-fallProduce TUFF or LAPILLI-TUFFMantle topography
20Fall Deposits Finely-laminated or massive Typically well sorted and gradednormal: larger clasts settlereverse: pulsed eruptions, gas inputArequipa, PeruLaacher See, GermanySantorini,Greece
23Density Current Deposits Pyroclastic density currentgeneral term for a “ground-hugging” current of pyroclasts and gas (including air)moves because denser than surrounding atmosphere (or water)Ignimbrite (“ash flow tuff”)deposit of a PDC, rich in pumice or pumiceous ash shards (gas bubble wall, cuspate)
24Density Current Deposits IgnimbriteMay contain various massive and stratified lithofaciesTUFF, LAPILLI-TUFF, BRECCIATuff and Lapilli-Tuff, TenerifeBreccia, TenerifeXBD, Laacher See, Germany
25Density Current Deposits Ignimbrite pyroclastsJuvenile (magmatic fragments: pumice, shards, glass)CrystalsLithicsCognate (non-vesiculated magma fragments that have solidified)Accessory (country rock explosively ejected/fragmented during eruption)Accidental (clasts picked up by PDCs during eruption)LithicsJuvenileCrystals
26Density Current Deposits Weldinghigh temperature emplacement of PDCpumice and glass still malleable/plasticfusing together of pumice and glass shardscompactionFiammelens or “flame-shaped object”typically forms from flattened pumice/shards in a welded ignimbriteEutaxitic texturePlanar fabric of deformed shards and fiamme, typically formed by hot-state compaction in welded ignimbritesNo, not that type!
27Density Current Deposits FiammeEutaxitic textureCoire Dubh, RumTejeda, Gran CanariaWan Tsai, HK
32PDC Deposition Models “Flow” deposits “Surge” deposits valley filling cross beddingLaacher See, Germany
33PDC Deposition Models “Surge” deposits Dunes Antidunes bDunesAntidunesLaacher See, Germany
34Standard Ignimbrite Flow Unit 3b: Co-ignimbrite ash3a: Ash-cloud Surge2b: FlowReverse pumice Normal lithics2a: Basal Flow<1 m thickReverse pumiceReverse lithics1: Ground Surge(Fall deposit at base)(Sparks, 1976)Ash-cloud surge:dilute top of flowGround surge:in advance of flowPyroclastic flowNot always present!
35Standard Ignimbrite Flow Unit “PLUG FLOW” CONCEPT3b: Co-ignimbrite ash3a: Ash-cloud Surge2b: FlowReverse pumice Normal lithics2a: Basal Flow<1 m thickReverse pumiceReverse lithics1: Ground Surge(Fall deposit at base)(Sparks, 1976)TURBULENTTURBULENTLAMINAR “PLUG FLOW”Not always present!
36Plug Flow (en masse) Laminar flow above basal shear layer “Freezes” en masse when driving stress falls(Sparks, 1976)
37Assumptions Based on massive ignimbrite units Two end member types Absence of tractional structures= non-turbulent flowTwo end member typesTurbulent low-concentration currents (surges)Non-turbulent, laminar to plug-flow high-concentration currents (flows)Multiple units = multiple eruptions
38Problems Surge deposits not always present Gradations between “flow” (massive) and “surge” (traction-stratified) depositsIgnimbrites show vertical chemical zoningNot considered possible through Plug Flow!