1. L2 Igneous Geology
David Brown

2. 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

3. Volcanology

4. 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

5. EXPLOSIVE BASALTIC ERUPTIONS (Icelandic, Hawaiian, Strombolian)

6. 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

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

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

9. EFFUSIVE INTERMEDIATE/SILICIC ERUPTIONS

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

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

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

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

14. 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

15. Fragmentation and Eruption

16. 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
m s-1

17. 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

18. 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

19. 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

20. 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

21. Fall Deposits
Pyroclast dispersal

22. Fall Deposits
Pyroclast dispersal

23. 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)

24. 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

25. 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

26. 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!

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

28. 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

29. 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

30. 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

31. 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

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

33. PDC Deposition Models “Surge” deposits Dunes Antidunesb
Dunes
Antidunes
Laacher See, Germany

34. 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!

35. 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!

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

37. 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

38. 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!

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

40. 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

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

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

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

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

45. Progressive aggradation
Welding occurs during and after eruption
WELDING

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

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

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

49. 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

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

51. 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

52. Collapse?
Piston
Piecemeal
Trapdoor
Downsag

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

54. Caldera Fill
Landslides, debris flows across caldera floor

55. Caldera Fill Volcaniclastic breccia comminuted matrix
Sgurr nan Gillean, Rum

56. Caldera Fill
Ignimbrite

57. 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