Post on 10-Jul-2015
WHEN IS A LAVA FLOW NOT A LAVA FLOW? UNRAVELING ANCIENT SUPER-ERUPTIONS USING FIELD
OBSERVATIONS AND STRUCTURAL MAPPING
Graham Andrews – Geological Survey of Canada (Vancouver) April 6th 2010
Grey‟s Landing ignimbrite, ID
OUTLINE
1. About me
2. Introduction to supervolcanoes
3. High-grade ignimbrites
4. Structural analysis
5. Model of syn-depositional flow
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-eruptionsfolded Silurian turbidites, N.I.
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
experimentsHolocene subglacial lava flow,
B.C.
• Whitsunday
(Cretaceous)
•Campi Flegri• Yellowstone, WY
• Chon Aike
(Jurassic)
• BTIP
(Paleocene)
• Sierra Madre Occidental
(Oligocene)
associated with mature extensional continental arcs,
continental rifts, and hot-spots.
• Long Valley, CA• Valles, NM • Tenerife
• Taupo
• Toba
•Santorini
• Altiplano-Puna
(Miocene)
• Taal
• Aso
• SRP (Miocene) • Mid-Continent Rift
(mid-Proterozoic)
• Yardea Dacite
(mid-Proterozoic)
• Karoo
(Jurassic)
• Etendeka
(Cretaceous)
• Bijli (Proterozoic)
modern ancient
SUPERVOLCANOES
SUPER-ERUPTIONS - INTRO
Devastating rhyolite
eruptions >10 km3
Every 100 – 1000
years
Global impact
Nearly made Homo
sapiens extinct (Toba,
Indonesia 70 ka)
Largest volcanic
features and depositsMontserrat, January 2010
Superb, but not super…
NOVA Special impression of a super-eruption at
Yellowstone
SUPER-ERUPTIONS – ASH DISPERSAL
Miocene SRP distal ash fall deposits
are >5 m thick in Nebraska (1600 km
E)
Ashfall Fossil
Beds State Park,
NE
Teleoceras, U-
HaulSRP Nebraska
F&M
SUPER-ERUPTIONS - PRODUCTS
Mazama Tuff, Crater Lake, OR
Big Obsidian Flow, Newberry, OR
rhyolite lava flowsignimbrite
• Effusive eruptions – lavas ooze out and
flow
• Explosive eruptions – hot ash and pumice
avalanches away from volcano deposit
ignimbrites
SO WHAT ARE IGNIMBRITES?
An ignimbrite is the deposit of a pyroclastic
flow.
Manam, Papua New Guinea,
1996
SO WHAT ARE IGNIMBRITES?
Ignimbrites are typically composed of „juvenile‟ ash and pumice lapilli, plus variable amounts of „accidental‟ lithiclapilli.
lithic
lapillus
pumice
lapilli
Fasnia ignimbrite –
Tenerife
IGNIMBRITE WELDING GRADE
non-welded
„typical‟ igs
moderate high-grade coalescence
Temp
> 1000 ºC< 750 ºC
SRP igs
“fluffy”
pumices
Fasnia ignimbrite –
Tenerife
IGNIMBRITE WELDING GRADE
non-welded
„typical‟ igs
moderate high-grade coalescence
Temp
> 1000 ºC< 750 ºC
fiamme -
flattened
pumices
un-named ignimbrite – South
Korea
SRP igs
plastic compaction <40%
“pure shear”
IGNIMBRITE WELDING GRADE
non-welded
„typical‟ igs
moderate high-grade coalescence
Temp
> 1000 ºC< 750 ºC
Facies:
rheomorphic
rheomorphism “eye
structure” - sheath fold
very flattened and
stretched fiamme –
no pumice
Mogan D
ignimbrite – Gran
Canaria
SRP igs
>10:1 stretching ratio
“simple shear”
IGNIMBRITE WELDING GRADE
non-welded
„typical‟ igs
moderate high-grade coalescence
Temp
> 1000 ºC< 750 ºC
Facies:
‘lava-like’ &
rheomorphic
„flow-banding‟ –
indistinguishabl
e from a lava
rheomorphism
isoclinal flow fold
Grey‟s Landing
ignimbrite – Idaho
SRP igs
>100:1 stretching ratio
“simple shear”
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
Mogan „D‟ - Gran
Canaria
stretched vesicle
‘rodding’ lineation – L
fabric
transport
direction
L1 & F1
L1 & F1
L = stretching lineation
F = fold hinge
Understanding rheomorphism is
all about structural geology
rheomorphic ignimbrites are
ductile shear zones!!!
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.
The SCALE of folding is dictated by the SCALE of the layer being deformed.
[Andrews & Branney,
in press. - GSA Bull. ]
CASE STUDY – GREY’S LANDING IG., IDAHO
Yellowstone hot-
spot track
[Andrews et al., 2008
- Bull. Volc.]
[Andrews & Branney,
in press. - GSA Bull. ]
Grey‟s
Landing
Ignimbrite
[Andrews et al., 2008
- Bull. Volc.]
[Andrews & Branney,
in press. - GSA Bull. ]
contorted
domain
flat
domain
flat domain
folds
[Andrews & Branney, in press. - GSA Bull. ]
flat domain
folds
[Andrews & Branney, in press. - GSA Bull. ]
[Andrews & Branney, in press. - GSA Bull. ]
[Andrews & Branney, in press. - GSA Bull. ]
simple
shear
flow
[Andrews et al., 2008
- Bull. Volc.]
[Andrews & Branney,
in press. - GSA Bull. ]
contorted
domain
flat
domain
contorted domain folds - small
refolded
F1 “flat”
folds
contorted domain folds - large
pure
shear
flow
[Andrews & Branney, in press. - GSA Bull. ]
syn-depositional rheomorphism
[Andrews & Branney,
in press. - GSA Bull. ]
syn-depositional rheomorphism
[Andrews & Branney,
in press. - GSA Bull. ]
aerosol can analogy
aerosol of
paint particles
aerosol can analogy
coalesced flow of paint
aerosol of
paint particles
T1
[Andrews PhD]
T2
[Andrews PhD]
T3
[Andrews PhD]
T4
[Andrews PhD]
T5 – deposition ceased
[Andrews PhD]
horizontal shortening –
gravity-driven
contorted domain
flat domain
[Andrews PhD]
contorted domain
flat domain
post-depositional rheomorphism
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.
Thank you –
questions?
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
D2 folding of the upper ‘free’ surface
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 thereforeD2 is slope dependent.
D2 is probably gravity-driven.
[Andrews & Branney, in press. - GSA Bull. ]
super-eruptions
produced huge ‘flood
rhyolites’
Jarbidge Canyon, ID-NV
super-eruptions
produced huge ‘flood
rhyolites’
Q - were the eruptions
explosive (ignimbrites)
or effusive (lavas)?
Jarbidge Canyon, ID-NV
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