Download L2 Igneous Geology 7

L2 Igneous Geology
David Brown
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
Mull
–
Hornitos (“rootless” cone)
•
fed by lava, not conduit
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
–
–
–
•
Reticulite
–
–
•
Strombolian eruptions
highly vesicular
red-brown to black
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
Lascar, Chile
Mt Pelée, Martinique
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
Redoubt, Alaska
column stops rising and
spreads out
UMBRELLA
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
XBD, Laacher See, Germany
Breccia, Tenerife
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
No, not that type!
• 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
Density Current Deposits
Coire Dubh, Rum
Wan Tsai, HK
Tejeda, Gran Canaria
Fiamme
Eutaxitic texture
Density Current Deposits
• Welding textures
– extreme welding = vitrophyre (glassy)
Non-welded
Fine-grained ash matrix
Welded
Lithic fragments
Pumice blocks and lapilli
Compacted & welded ash matrix
Vitrophyre
Fiamme
Highly compacted glassy matrix
Density Current Deposits
• Welding textures
– extreme welding = vitrophyre (glassy)
Non-welded
Fine-grained ash matrix
Welded
Lithic fragments
Pumice blocks and lapilli
Compacted & welded ash matrix
Vitrophyre
Fiamme
Highly compacted glassy matrix
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
SURGE
• Flow: high-particle concentration PDC
– fill topography
– massive, poorly sorted
• Surge: low-particle concentration PDC
– mantle topography AND topographically controlled
– sedimentary bedforms
PDC Deposition Models
• “Flow” deposits
– valley filling
• “Surge” deposits
– cross bedding
Laacher See, Germany
PDC Deposition Models
• “Surge” deposits
b
Dunes
Antidunes
Laacher See, Germany
Standard Ignimbrite Flow Unit
3b: Co-ignimbrite ash
3a: Ash-cloud Surge
(Sparks, 1976)
2b: Flow
Reverse pumice
Normal lithics
2a: Basal Flow
<1 m thick
Reverse pumice
Reverse lithics
1: Ground Surge
(Fall deposit at base)
Ash-cloud surge:
dilute top of flow
Ground surge:
in advance of flow
Not always present!
Pyroclastic flow
Standard Ignimbrite Flow Unit
“PLUG FLOW” CONCEPT
3b: Co-ignimbrite ash
(Sparks, 1976)
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)
TURBULENT
TURBULENT
Not always present!
LAMINAR “PLUG FLOW”
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 deposit
Rhyolite magma
1.
Deposition
Progressive aggradation
2) Middle part of eruption:
Low energy = fine deposit
Dacite magma
2.
1.
Deposition
Progressive aggradation
3) End part of eruption:
High energy = coarse deposit
Andesite magma
3.
2.
1.
Deposition
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 nonvesiculated blocky
fragments in ashy matrix
• Monomict
• No pumice
Montserrat, Caribbean
Tejeda,
Gran Canaria
Caldera Collapse
•
Magma rising up the fractures
–
may reach the surface forming a caldera
Caldera Collapse
• Classic caldera model of Smith & Bailey (1968)
Domes
Domes
Tumescence/
rifting
Central vent/
ring vent
Synchronous
Inward piston
• Caldera
collapse
diagram.
Resurgence
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