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David Brown L2 Igneous Geology. Course 1.Dynamics 2.Classification of igneous rocks and properties of magma 3.Generation and differentiation of magma.

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Presentation on theme: "David Brown L2 Igneous Geology. Course 1.Dynamics 2.Classification of igneous rocks and properties of magma 3.Generation and differentiation of magma."— Presentation transcript:

1 David Brown L2 Igneous Geology

2 Course 1.Dynamics 2.Classification of igneous rocks and properties of magma 3.Generation and differentiation of magma 1 4.Generation and differentiation of magma 2 5.Sub-volcanic plumbing system 6.Physical volcanology 1 7.Physical volcanology 2

3 Volcanology

4 Outline Explosive basaltic eruptions ( Hawaiian, Strombolian ) Effusive intermediate/silicic eruptions –Lavas Explosive intermediate/silicic eruptions (Vulcanian, Plinian, Peléan) –Pyroclastic rocks Types and deposits Models of deposition Caldera collapse

5 EXPLOSIVE BASALTIC ERUPTIONS (Icelandic, Hawaiian, Strombolian)

6 Vent-related deposits Spatter –fluid molten lava ejected from a vent –flatten and congeal –ramparts, small cones/domes –Hornitos (rootless cone) fed by lava, not conduit Mull

7 Vent-related deposits Peles tears –after Hawaiian goddess of volcanoes –molten lava from fountains –often associated with Peles hair

8 Vent-related deposits Scoria –Strombolian eruptions –highly vesicular –red-brown to black Reticulite –burst vesicle walls –honeycomb texture Basaltic pumice

9 EFFUSIVE INTERMEDIATE/SILICIC ERUPTIONS

10 Lavas High viscosity, low T Form lava domes Small-volume flows Flow banded –mineral layers, differentiation –viscous shear Mt Pelée, Martinique Lascar, Chile Iceland

11 Lavas Rapidly cooled silicic lavas may produce flow banded obsidian Torfajökull, Iceland Teide, Tenerife

12 Lavas Some large-volume silicic lavas –controversial origin….. Obsidian Cliff, Yellowstone

13 EXPLOSIVE INTERMEDIATE/SILICIC ERUPTIONS (Vulcanian, Plinian, Peléan)

14 Pyroclastic Rocks A multitude of terms and deposits! Comprise ash, lapilli, lithic blocks, crystals and pumice Pumice similar to liquid foam produced when you open a coke bottle

15 Fragmentation and Eruption

16 Plinian Eruption Example Convective region –column entrains cold air –mixed air dilutes column, is heated –reduces density, increases buoyancy = RISE Gas thrust region –high velocity jet of gas and particles – m s -1

17 Plinian Eruption Example Umbrella region –convective column continues to build –density column = density atmosphere column stops rising and spreads out UMBRELLA Sheveluch (Kamchatka) in Russia Redoubt, Alaska

18 Plinian Eruption Example What happens next? Depends on density –ρ column vs. ρ atmosphere If ρ column < ρ atmosphere –buoyant eruption plume –pyroclastic FALL deposits If ρ column > ρ atmosphere –eruption column collapses under gravity –pyroclastic DENSITY CURRENT deposits

19 Fall Deposits Fall deposits –Ash, pumice settling from eruption column (scoria, bombs in basaltic eruptions) –Ash-fall or pumice-fall –Produce TUFF or LAPILLI-TUFF –Mantle topography

20 Fall Deposits Finely-laminated or massive Typically well sorted and graded –normal: larger clasts settle –reverse: pulsed eruptions, gas input Laacher See, Germany Santorini, Greece Arequipa, Peru

21 Fall Deposits Pyroclast dispersal

22 Fall Deposits Pyroclast dispersal

23 Density Current Deposits Pyroclastic density current –general 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)

24 Density Current Deposits Ignimbrite –May contain various massive and stratified lithofacies –TUFF, LAPILLI-TUFF, BRECCIA Breccia, Tenerife Tuff and Lapilli-Tuff, Tenerife XBD, Laacher See, Germany

25 Density Current Deposits Ignimbrite pyroclasts –Juvenile (magmatic fragments: pumice, shards, glass) –Crystals –Lithics Cognate (non-vesiculated magma fragments that have solidified) Accessory (country rock explosively ejected/fragmented during eruption) Accidental (clasts picked up by PDCs during eruption) Crystals Lithics Juvenile

26 Density Current Deposits Welding –high temperature emplacement of PDC –pumice and glass still malleable/plastic –fusing together of pumice and glass shards –compaction Fiamme –lens or flame-shaped object –typically forms from flattened pumice/shards in a welded ignimbrite Eutaxitic texture –Planar fabric of deformed shards and fiamme, typically formed by hot-state compaction in welded ignimbrites No, not that type!

27 Density Current Deposits Fiamme Eutaxitic texture Coire Dubh, Rum Tejeda, Gran Canaria Wan Tsai, HK

28 Density Current Deposits Welding textures –extreme welding = vitrophyre (glassy) Fine-grained ash matrix Pumice blocks and lapilli Lithic fragments Compacted & welded ash matrix Fiamme Highly compacted glassy matrix Non-welded WeldedVitrophyre

29 Density Current Deposits Welding textures –extreme welding = vitrophyre (glassy) Fine-grained ash matrix Pumice blocks and lapilli Lithic fragments Compacted & welded ash matrix Fiamme Highly compacted glassy matrix Non-welded WeldedVitrophyre

30 PDC Eruptions Eruption column collapse –pumice-rich ignimbrite Upwelling and overflow with no eruption column –pumice-poor ignimbrite Lava dome/flow collapse –block and ash flow Lateral blast

31 PDC Deposition Models Classic terminology: Flow vs. Surge Flow: high-particle concentration PDC –fill topography –massive, poorly sorted Surge: low-particle concentration PDC –mantle topography AND topographically controlled –sedimentary bedforms FLOW SURGE

32 PDC Deposition Models Flow deposits –valley filling Surge deposits –cross bedding Laacher See, Germany

33 PDC Deposition Models Surge deposits DunesAntidunes b Laacher See, Germany

34 Standard Ignimbrite Flow Unit 3b: Co-ignimbrite ash 3a: Ash-cloud Surge 2b: Flow Reverse pumice Normal lithics 2a: Basal Flow <1 m thick Reverse pumice Reverse lithics 1: Ground Surge (Fall deposit at base) Not always present! Ground surge: in advance of flow Pyroclastic flow Ash-cloud surge: dilute top of flow (Sparks, 1976)

35 Standard Ignimbrite Flow Unit 3b: Co-ignimbrite ash 3a: Ash-cloud Surge 2b: Flow Reverse pumice Normal lithics 2a: Basal Flow <1 m thick Reverse pumice Reverse lithics 1: Ground Surge (Fall deposit at base) Not always present! TURBULENT LAMINAR PLUG FLOW TURBULENT PLUG FLOW CONCEPT (Sparks, 1976)

36 Plug Flow (en masse) Laminar flow above basal shear layer Freezes en masse when driving stress falls (Sparks, 1976)

37 Assumptions Based on massive ignimbrite units –Absence of tractional structures = non-turbulent flow Two end member types –Turbulent low-concentration currents (surges) –Non-turbulent, laminar to plug-flow high-concentration currents (flows) Multiple units = multiple eruptions

38 Problems Surge deposits not always present Gradations between flow (massive) and surge (traction-stratified) deposits Ignimbrites show vertical chemical zoning Not considered possible through Plug Flow!

39 Progressive aggradation Deposit accumulates gradually (Branney & Kokelaar, 1992)

40 Progressive aggradation Deposited incrementally during the sustained passage of a single particulate current Deposition at denser basal part of flow Particles agglutinate, become non-particulate

41 Progressive aggradation NPF continues to aggrade –continual supply from over-riding particulate flow Changes in stratification –variations in flow steadiness and material at source

42 Progressive aggradation 1) Early part of eruption: High energy = coarse deposit Rhyolite magma 1. Deposition

43 Progressive aggradation 2) Middle part of eruption: Low energy = fine deposit Dacite magma Deposition

44 Progressive aggradation 3) End part of eruption: High energy = coarse deposit Andesite magma Deposition

45 Progressive aggradation Welding occurs during and after eruption WELDING

46 Rheomorphism Folds formed during slumping and welding of non-particulate flow Kilchrist, Skye Stob Dearg, Glencoe

47 Rheomorphism Folds formed during slumping and welding of non-particulate flow Snake River, Idaho

48 Ignimbrite or Lava?! Rheomorphic folds and columnar joints Ignimbrites may look like lavas! Tejeda, Gran Canaria

49 Block and Ash Flows Collapse of lava dome (Peléan eruption) Dense, poorly to non- vesiculated blocky fragments in ashy matrix Monomict No pumice Tejeda, Gran Canaria Montserrat, Caribbean

50 Caldera Collapse Magma rising up the fractures –may reach the surface forming a caldera

51 Caldera Collapse Classic caldera model of Smith & Bailey (1968) Caldera collapse diagram. Domes Resurgence Caldera collapse diagram. Tumescence/ rifting Central vent/ ring vent Synchronous Inward piston Domes Resurgence

52 Collapse? Piston Piecemeal Trapdoor Downsag

53 Caldera Fill Ignimbrite and collapse breccias –Megabreccia (>1 m), mesobreccia (<1 m) –Shed from caldera walls, fault scarps

54 Caldera Fill –Landslides, debris flows across caldera floor

55 Caldera Fill Sgurr nan Gillean, Rum Volcaniclastic breccia –comminuted matrix

56 Caldera Fill Ignimbrite

57 Outline Explosive basaltic eruptions ( Hawaiian, Strombolian ) Effusive intermediate/silicic eruptions –Lavas Explosive intermediate/silicic eruptions (Vulcanian, Plinian, Peléan) –Pyroclastic rocks Types and deposits Models of deposition Caldera collapse


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