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