1. Graham Andrews – Geological Survey of Canada (Vancouver) April 6th 2010
Grey‟s Landing ignimbrite, ID
WHEN IS A LAVA FLOW NOT A LAVA FLOW?
UNRAVELING ANCIENT SUPER-ERUPTIONS USING FIELD
OBSERVATIONS AND STRUCTURAL MAPPING

2. OUTLINE
1. About me
2. Introduction to supervolcanoes
3. High-grade ignimbrites
4. Structural analysis
5. Model of syn-depositional flow

3. A LITTLE ABOUT ME…
 Geology at High
School
 Uni. of Leicester, UK
 U/G – specialized in
structure, tectonics, ig.
pet. & volcanology
 PhD –
 ductile deformation of
rhyolite tuffs & lavas
 SRP super-eruptions
folded Silurian turbidites, N.I.

4. A LITTLE ABOUT ME…
 PDF at UBC & GSC
 BC Neogene regional
geology and tectonics
 Calderas, extension,
and mineralization in
NV and BC
 Eocene MCCs
 Volcanic dams
 Pseudotachylites
 Rheology
Holocene subglacial lava flow, experiments

5. SUPERVOLCANOES
• BTIP
(Paleocene)
• Yellowstone, WY
• SRP (Miocene) • Mid-Continent Rift •Campi Flegri
(mid-Proterozoic)
• Long Valley, CA • Aso
• Valles, NM • Tenerife •Santorini • Bijli (Proterozoic)
• Sierra Madre Occidental • Taal
(Oligocene)
• Karoo • Toba • Whitsunday
(Jurassic) (Cretaceous)
• Altiplano-Puna
(Miocene)
• Etendeka • Yardea Dacite
• Chon Aike (Cretaceous) (mid-Proterozoic) • Taupo
(Jurassic)
modern ancient
associated with mature extensional continental arcs,
continental rifts, and hot-spots.

6. SUPER-ERUPTIONS - INTRO
Superb, but not super…
 Devastating rhyolite
eruptions >10 km3
 Every 100 – 1000
years
 Global impact
 Nearly made Homo
sapiens extinct (Toba,
Indonesia 70 ka)
 Largest volcanic
features and deposits
Montserrat, January 2010

7. NOVA Special impression of a super-eruption at
Yellowstone

8. SUPER-ERUPTIONS – ASH DISPERSAL
Ashfall Fossil
Beds State Park,
NE
Teleoceras, U-
Haul SRP Nebraska
Miocene SRP distal ash fall deposits
F&M
are >5 m thick in Nebraska (1600 km

9. SUPER-ERUPTIONS - PRODUCTS
ignimbrite
rhyolite lava flows
Big Obsidian Flow, Newberry, OR
• Effusive eruptions – lavas ooze out and
flow
• Explosive eruptions – hot ash and pumice
avalanches away from volcano deposit
Mazama Tuff, Crater Lake, OR

10. SO WHAT ARE IGNIMBRITES?
 An ignimbrite is the deposit of a pyroclastic
flow.
Manam, Papua New Guinea,
1996

11. SO WHAT ARE IGNIMBRITES?
 Ignimbrites are typically composed of „juvenile‟ ash and
pumice lapilli, plus variable amounts of „accidental‟ lithic
lapilli.
pumice
lapilli
lithic
lapillus
Fasnia ignimbrite –

12. IGNIMBRITE WELDING GRADE
non-welded moderate high-grade coalescence
Temp
< 750 ºC > 1000 ºC
„typical‟ igs
SRP igs
“fluffy”
pumices
Fasnia ignimbrite –

13. IGNIMBRITE WELDING GRADE
non-welded moderate high-grade coalescence
Temp
< 750 ºC > 1000 ºC
„typical‟ igs
SRP igs
fiamme -
flattened
plastic compaction <40%
pumices
“pure shear”
un-named ignimbrite – South

14. IGNIMBRITE WELDING GRADE
non-welded moderate high-grade coalescence
Temp
< 750 ºC > 1000 ºC
„typical‟ igs
SRP igs
very flattened and >10:1 stretching ratio
stretched fiamme – “simple shear”
no pumice
Facies:
rheomorphic
Mogan D
rheomorphism  “eye ignimbrite – Gran
structure” - sheath fold Canaria

15. IGNIMBRITE WELDING GRADE
non-welded moderate high-grade coalescence
Temp
< 750 ºC > 1000 ºC
„typical‟ igs
SRP igs
„flow-banding‟ – >100:1 stretching ratio
indistinguishabl “simple shear”
e from a lava
Facies:
‘lava-like’ &
rheomorphic
Grey‟s Landing
rheomorphism 
ignimbrite – Idaho
isoclinal flow fold

16. WHAT IS RHEOMORPHISM?
 Rheomorphism is the plastic deformation
of a welded ignimbrite during
emplacement, as a result of ductile flow
 Rheomorphism requires low viscosity tuff,
therefore:
 high temperature (>900 °C)
 high dissolved H2O, F or Cl
 high Al or Na + K composition
 a combination of the 3

17. Mogan „D‟ - Gran Understanding rheomorphism is
Canaria
all about structural geology 
rheomorphic ignimbrites are
ductile shear zones!!!
L1 & F 1
transport
direction
L1 & F 1
stretched vesicle L = stretching lineation
‘rodding’ lineation – L F = fold hinge

18. WHAT IS RHEOMORPHISM?
 Rheomorphic flow:
 may be syn- and / or post-depositional,
 syn-depositional rheomorphism is strongly
simple-shear, producing lineations and sheath
folds  like a ductile shear zone in the
crust,
 post-depositional rheomorphism is moderately
pure-shear, producing buckle-style folds 
like a lava flow or a glacier.
 TheSCALE of folding is dictated by the
SCALE of the layer being deformed.

19. CASE STUDY – GREY’S LANDING IG., IDAHO
Yellowstone hot-
spot track
[Andrews & Branney,
in press. - GSA Bull. ]

20. Grey‟s
Landing
Ignimbrite
[Andrews et al., 2008 [Andrews & Branney,
- Bull. Volc.] in press. - GSA Bull. ]

21. contorted
domain
flat
domain
[Andrews et al., 2008 [Andrews & Branney,
- Bull. Volc.] in press. - GSA Bull. ]

22. [Andrews & Branney, in press. - GSA Bull. ]
flat domain
folds

23. [Andrews & Branney, in press. - GSA Bull. ]
flat domain
folds

24. [Andrews & Branney, in press. - GSA Bull. ]

25. simple
shear
flow
[Andrews & Branney, in press. - GSA Bull. ]

26. contorted
domain
flat
domain
[Andrews et al., 2008 [Andrews & Branney,
- Bull. Volc.] in press. - GSA Bull. ]

27. refolded
F1 “flat”
folds
contorted domain folds - small

28. contorted domain folds - large

29. pure
shear
flow
[Andrews & Branney, in press. - GSA Bull. ]

30. syn-depositional rheomorphism
[Andrews & Branney,
in press. - GSA Bull. ]

31. syn-depositional rheomorphism
[Andrews & Branney,
in press. - GSA Bull. ]

32. aerosol can analogy
aerosol of
paint particles

33. aerosol can analogy
aerosol of
paint particles
coalesced flow of paint

34. T1
[Andrews PhD]

35. T2
[Andrews PhD]

36. T3
[Andrews PhD]

37. T4
[Andrews PhD]

38. T5 – deposition ceased
[Andrews PhD]

39. post-depositional rheomorphism
horizontal
shortening –
gravity-
driven
contorted
domain
contorted
domain flat
domain
flat
domain
[Andrews PhD]

40. SUMMARY
 Supervolcanoes are huge, complex systems
requiring study by volcanologists, structural
geologists, petrologists, geochemists,
geophysicists, etc.  lots of research
opportunities.
 Small & medium-scale features (volcanic and
structural) reveal how rheomorphic ignimbrites
form  kinematics recreate the flow,
 The same approach works for lavas, glaciers,
plutons, mudflows, mylonite zones, etc.

41. Thank you –
questions?

42. FUTURE RESEARCH & PROJECTS
 Remote-sensing mapping of the SRP & Boise
areas
  GIS, satellite images, airphotos, existing geology
data
 Mapping, textural description, and measurements
of welded ignimbrites and lavas (ID, NV, OR,
Spain)
  SEM, petrography, fieldwork, XRD
 Paleo-elevations in the SRP (very long term!)
  fieldwork, stratigraphy, thermochronology, O,
palynology, geophysics,
 Pleistocene basaltic volcanism in BC
  Ar/Ar dating, fieldwork, stratigraphy
 Origin of multi-rimmed basalt pillows (BC & Idaho)
  SEM, XRD, petrography

43. D2 folding of the upper ‘free’ surface

44. L1 is independent of
the underlying slope
 therefore D1 is not
gravity controlled.
F2 is perpendicular to
the dip direction of
the slope, L2 is
parallel  therefore
D2 is slope
dependent.
D2 is probably
gravity-driven.

46. [Andrews & Branney, in press. - GSA Bull. ]

47. super-eruptions
produced huge ‘flood
rhyolites’
Jarbidge Canyon, ID-NV

48. super-eruptions
produced huge ‘flood
rhyolites’
Q - were the eruptions
explosive (ignimbrites)
or effusive (lavas)?
Jarbidge Canyon, ID-NV

49. super-eruptions
produced huge ‘flood
rhyolites’
Q - were the eruptions
explosive (ignimbrites)
or effusive (lavas)? A – both. But how
to tell them apart?
Jarbidge Canyon, ID-NV