David Brown L2 Igneous Geology. Course 1.Dynamics 2.Classification of igneous rocks and properties...
-
Upload
kyle-obrien -
Category
Documents
-
view
240 -
download
0
Transcript of David Brown L2 Igneous Geology. Course 1.Dynamics 2.Classification of igneous rocks and properties...
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