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