David Brown
L2 Igneous Geology
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
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
Lascar, Chile
Iceland
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
Sheveluch(Kamchatka)
in Russia
Redoubt, Alaska
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
Laacher See, Germany
Santorini,Greece
Arequipa, Peru
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
Breccia, TenerifeTuff and Lapilli-Tuff, 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)
Crystals
Lithics
Juvenile
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– valley filling
• “Surge” deposits– cross bedding
Laacher See, Germany
PDC Deposition Models
• “Surge” deposits
Dunes Antidunes
b
Laacher See, Germany
Standard Ignimbrite Flow Unit
3b: Co-ignimbrite ash
3a: Ash-cloud Surge
2b: FlowReverse pumice Normal lithics
2a: Basal Flow<1 m thickReverse pumiceReverse 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)
Standard Ignimbrite Flow Unit
3b: Co-ignimbrite ash
3a: Ash-cloud Surge
2b: FlowReverse pumice Normal lithics
2a: Basal Flow<1 m thickReverse pumiceReverse lithics
1: Ground Surge
(Fall deposit at base)
Not always present!
TURBULENT
LAMINAR “PLUG FLOW”
TURBULENT
“PLUG FLOW” CONCEPT
(Sparks, 1976)
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– 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 depositRhyolite magma
1.Deposition
Progressive aggradation
2) Middle part of eruption:Low energy = fine depositDacite magma
1.
2. Deposition
Progressive aggradation
3) End part of eruption:High energy = coarse depositAndesite magma
1.
2.
3. Deposition
Progressive aggradation
Welding occurs during and after eruption
WELDING
Rheomorphism
• Folds formed during slumping and welding of non-particulate flow
Kilchrist, SkyeStob 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 CanariaMontserrat, Caribbean
Caldera Collapse
• Magma rising up the fractures
– may reach the surface forming a caldera
Caldera Collapse
• Classic caldera model of Smith & Bailey (1968)
• Caldera collapse diagram.
Domes
Resurgence
• Caldera collapse diagram.
Tumescence/rifting
Central vent/ring vent
SynchronousInward piston
Domes
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
Sgurr nan Gillean, Rum
• Volcaniclastic breccia– comminuted matrix
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
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