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 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

EXPLOSIVE BASALTIC ERUPTIONS (Icelandic, Hawaiian, Strombolian)

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

Vent-related deposits Pele’s tears after Hawaiian goddess of volcanoes molten lava from fountains often associated with Pele’s hair

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

EFFUSIVE INTERMEDIATE/SILICIC ERUPTIONS

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

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

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

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

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

Fragmentation and Eruption

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

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

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

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

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

Fall Deposits Pyroclast dispersal

Fall Deposits Pyroclast dispersal

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)

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

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

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!

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

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 Welded Vitrophyre

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 Welded Vitrophyre

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

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

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

PDC Deposition Models “Surge” deposits Dunes Antidunes b Dunes Antidunes Laacher See, Germany

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!

Standard Ignimbrite Flow Unit “PLUG FLOW” CONCEPT 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) TURBULENT TURBULENT LAMINAR “PLUG FLOW” Not always present!

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

Assumptions Based on massive ignimbrite units Two end member types 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

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!

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

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

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

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

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

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

Progressive aggradation Welding occurs during and after eruption WELDING

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

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

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

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

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

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

Collapse? Piston Piecemeal Trapdoor Downsag

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

Caldera Fill Landslides, debris flows across caldera floor

Caldera Fill Volcaniclastic breccia comminuted matrix Sgurr nan Gillean, Rum

Caldera Fill Ignimbrite

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