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L2 Igneous Geology

L2 Igneous Geology. David Brown. Course. Dynamics Classification of igneous rocks and properties of magma Generation and differentiation of magma 1 Generation and differentiation of magma 2 Sub-volcanic plumbing system Physical volcanology 1 Physical volcanology 2. Volcanology. Outline.

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L2 Igneous Geology

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  1. L2 Igneous Geology David Brown

  2. Course • Dynamics • Classification of igneous rocks and properties of magma • Generation and differentiation of magma 1 • Generation and differentiation of magma 2 • Sub-volcanic plumbing system • Physical volcanology 1 • 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 • Pele’s tears • after Hawaiian goddess of volcanoes • molten lava from fountains • often associated with Pele’s 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 Iceland Lascar, Chile

  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 • 100-400 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 Redoubt, Alaska Sheveluch (Kamchatka) in Russia

  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 Arequipa, Peru Laacher See, Germany Santorini, Greece

  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 Tuff and Lapilli-Tuff, Tenerife Breccia, 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) Lithics Juvenile Crystals

  26. No, not that type! 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

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

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

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

  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. FLOW SURGE 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

  32. “Flow” deposits valley filling “Surge” deposits cross bedding PDC Deposition Models Laacher See, Germany

  33. “Surge” deposits PDC Deposition Models b Dunes Antidunes 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) (Sparks, 1976) Ash-cloud surge: dilute top of flow Ground surge: in advance of flow Pyroclastic flow Not always present!

  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) “PLUG FLOW” CONCEPT (Sparks, 1976) TURBULENT TURBULENT LAMINAR “PLUG FLOW” Not always present!

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

  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 Deposition 1.

  43. Progressive aggradation 2) Middle part of eruption: Low energy = fine deposit Dacite magma Deposition 2. 1.

  44. Progressive aggradation 3) End part of eruption: High energy = coarse deposit Andesite magma 3. Deposition 2. 1.

  45. WELDING Progressive aggradation Welding occurs during and after eruption

  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

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