Pyroclastic Flows- Subaerial

Introduction

• Pyroclastic Flows or Ignimbrites- historical-

Tufflavas

• Four historic eruptions provided examples of

pyfs and the processes that resulted in

pyroclastic flow deposits:

– Mt. Pelee- 1902- related to Dome

– Valley of 10,000 Smokes- 1912-composite volcano

and valley ponding

– Soufriere-1902- composite volcano

– Krakatoa- 1883-caldera collapse

• One recent eruption- Mt. St. Helens-

lateral blast from a composite volcano

with associated debris avalanche

History

• What has not been witnessed are

pyroclastic flows formed from caldera

collapse and ring fracture eruptions-

Yellowstone, Long Valley, Crater Lake

• Actual deposits formed from these various

eruptions differed but all were laid down by

hot, glowing avalanches of pyroclastic

material which moved rapidly over the

ground.

• Pelean and St. Vincient eruptions- Nuee

Ardentes

Definition

• Terms- ash flow/ash-flow tuff, ignimbrite, pyroclastic flow:

• Volcanically produced surface flows of pyroclastic debris which travel as high particle concentration gas-solid dispersions

• They are thus gravity controlled, hot, and in some instances partly fluidized

• Ash flow-> 50% ash-size material

• Ignimbrite- welded or unwelded, pumiceous ash-rich deposit

• Pyroclastic flow deposit: massive, poorly sorted, ash-rich laid down by a particulate gaseous flow

• Pyf’s associated with domes- small, few exceed 1km3

• Those associated with the crater of a composite volcano are larger, range from 1km3 to 15km3

• Those erupted from ring fissures are the largest ranging from 15km3 to more than 3000km3

• Regardless of size all are emplaced by a range of

similar gravity-driven mechanisms.

• All are topo. controlled-pond in valleys, disappear

or thin on hills, voluminous enough can bury topo.

• Chief difference in the deposits arise from:

– Initial temperature differences of the erupting material

– Ratio of gases/solids in the eruption and/or

steam/magma ratio

– Presence or absence of an eruption column

– Rate of eruption

– Volume of material erupted

Formation

• Collapse of a growing dome

• Explosion from beneath a growing dome

or the spine of a dome

• Gravitational collapse of an overloaded

eruption column

• Frothing or boiling over the vent of a gas-

charged magma

• Debris avalanche followed by a lateral

blast due to decompression of the magma

Collapse of a Growing Dome

Merapi-Type

• Mild explosions-Non-explosive, gas-poor,

hot

• Avalanches triggered by:

– Earthquakes

– Rapid internal expansion of the dome-

gravitational over steepened

– Heavy rains on hot dome

– Over pressure due to gas build up inside

cooled exterior-mild explosions

Deposits from Collapsed Domes

• Called Block and Ash Flows

• Composed of unsorted, angular, moderately vesicular to dense blocks up to several meters in diameter and lapilli set in an ash-size matrix which forms less than 50% of the deposit

• Fragments come from solidified outer part of the dome and still hot, more gaseous interior.

Deposits from Dome Collapse

• Deposits are massive, poorly sorted, non-

welded, and may exhibit a poorly defined

reverse grading. Can be matrix or clast

supported

• Deposits are small volume traveling

anywhere from <1/4 to 10 from source.

• Generally 1-10 m thick grading to 1-2 m at

end- topographically controlled

Block and Ash Flow- Cooling Joint from Dome

Merapi Block and Ash

Ash Cloud Surge

• Fast moving, gaseous flow of low particle

concentration

• Hot, knocks over trees, destroys buildings

• Not topographically controlled

Deposits

• < 1 m thick

• Sand-size to fine ash, bedded

• Beds wavy, dune forms

• Occur at margins, above and below block

and ash flow

Explosions from a Dome

Pelean Type

• Beneath the base of an emerging dome or

growing spine

• Similar to Merapi-type but differ by:

– Being more explosive- larger

– More juvenile material; more pumice and less

lithic fragments

– Form block and ash flows with a higher % of

juvenile material

Gravitational Collapse of an

Eruption Column

• Eruption Columns

– Lower, denser part-gas thrust portion. In this part

material is accelerated by the rapid expansion of gas

as it leaves the vent or fissure- then decelerates as it

interacts with atmosphere

– Upper, lighter part called the convective phase. It

rises and expands because it has lower density then

the atmosphere and turbulently mixes with the

atmosphere- forms thunderheads-anvil shapes

Gas-Thrust

• Since it has a density greater than that of the atmosphere (ie does not mix with air) it is subject to gravitational collapse

• Gas and solids will fall back towards the vent and spread out as flows of hot, pyroclastic material.

• Collapse is episodically and progressive so can get a series of pyroclastic flows from one eruption column

Gravitational Collapse

• Three major types

– Summit eruption of composite volcano- no

collapse of edifice-Soufiere, Augustino

– Summit eruption of composite volcano-

collapse of edifice-Krakatoa, Crater Lake

– Accurate or linear fissure eruptions not

necessarily related to a positive volcanic

landform-Toba, Tambora, Yellowstone

Type 1: St. Vincient Type

• Named for eruptions from Soufiere volcano in

1901-1902

• Pyroclastic flows formed from gas-rich magma.

• Deposits are rich in pumice and crystals and

poor in lithic debris (5%). More than 85% of the

deposit is composed of sand-size material

(pumice, pumice dust, crystals)

• Deposits are poorly sorted and massive

Krakatoan-Type

• Named for the colossal Krakatoan eruption of 1883.

• The pyroclastic flows were associated with the collapse of the summit of the volcano and subsequent collapse of the volcanic structure

• Collapse was due to the eruption of the pyroclastic material which was so rapid it left part of the magma chamber vacant

• No support- collapse into the chamber-forms a volcanic depression- caldera (small)

• Deposits similar to St. Vincient but much more voluminous and widespread, more pumice

Valles-Type –No volcanic landform

• Eruptions from arcuate fissures formed by regional arching of the crust by large bodies of rising magma

• Volumes of ejected material are so great that the roof over the magma chamber collapses along the arcuate fissures

• This produces a large volcanic depression called a caldera (large)

• Deposits can be of vast dimensions and vary from sheet-like (outside) to thick (>2500feet) inside.

• Deposits may be welded or non-welded. They are pumice-rich with abundant vitric ash (pumice dust). Lithics < 5%, crystals variable.

Two New Types

• Boiling over eruptions-Subaqueous

• Lateral Blast- Mt. St. Helens

Boiling Over Eruptions

• Boiling or frothing of a gas-charged magma out of vent or fissures

• No eruption column

• Most common in subaqueous environments

• Magma froths and eruption continuous as long as lots of gas in magma to drive it

• Deposits similar to those of Valles- lots of inflated pumice

Mt. St. Helens Type

• Lateral blast due to debris avalanche

• Associated with composite volcanoes

• Forms due to rapid decompression of a high level

magma

• Decompression due to landslide

• Once decompressed magma explodes out landslide scar

• Deposits- huge debris flows. Lateral blast material is

thin, extends for a few km’s to 30 km from source,

mixture of pumice and lithics. Will overlay debris flow

deposits

Mobility

• Pyf’s travel at incredible speeds- 90 to

more than 600 km/hour

• Distances traveled range from about 1km

to more than 200 km

• They can climb barriers up to 800 m high

• They also walk on water and can do so for

km’s- 150 km from Tambora, 75 from

Krakatoa

Mobility

• Explained by:

– Exsolution of gas from juvenile particles which bouys up the particles and reduces friction between them

– The heating and expansion of air engulfed at the leading edge nof the flow- provides a cushion for the flow to ride on

– Expanded state-escaping gas, increased inflation

– All three are probably operative

Heat Conserving

• Estimated lose about 500C in 10 hours

• Can flow over a 100 km’s in that time

• This reason for welding phenomena in

some of these flows

• Mixing with air is restricted to a thin

surface layer

Heat Conserving

• Variables that determine emplacement

temperature are:

– Mixing with air or water in eruption column

– Initial temperature of erupted material

– Volume of the flow

– Based on this pyf’s range from non-welded

through poorly welded with vapor phase

crystallization to densely welded

Components of Pyroclastic Flow

deposits

• Composed dominantly of ash-size particles forming a matrix into which are imbedded varying amounts of pumice lapilli and/or lithic fragments (picked up from conduit or ground.)

• Ash-size material is composed of glass shards, small pumice particles, crystals

• Crystals are commonly angular and broken

Ground Surge

• Not always present

• Small % of pumice lapilli

• In total % of particles is < 30%

• >70% gas-air-steam

• Newtonian and turbulent

• Bedded, variety of bed forms (x-bedding, dunes), beds thin and thicken, lense-like

• Generally only a meter or 2 thick

• Origin uncertain

• Forms due to interaction with air and ground surface at front

Main Body

• Thickest and coarsest grained

• Poorly sorted and massive

• Particles > 40%, non-

newtonian flow, laminar

• Highly variable as to thickness,

length, % of crystals, lithics and

pumice

• Matrix is composed of shards

and vitric ash (pumice dust)

• Lithics minor

Main Body

• May get:

– Normal grading of lithics-

denser and sink

– Inverse grading of pumice-

lighter and floats

– Pumice may be 3 times

larger at top than bottom

Co-Ignimbrite Ash

• Ash cloud deposits

• Derived from fall out and

from elutriation of head

of flow- back streaming

• Well bedded, graded

bedding, poorly sorted

• Ash-size material

Lithic Lag Breccias

• Fragment supported beds of lithic blocks in an

ash matrix

• Blocks rounded in some deposits, in others

delicate blocks, like chunks of palesols and

lake seds. Preserved

• Proximal deposits

• Merge laterally into ground surge or lithic

concentration zones in main body

• Load loss due to change from collapse to flow

Lithic-Rich Areas

• Can get local lithic-rich areas in main body

due to:

– Dramatic change in topo-steep to shallow

– Sharp bends in valleys-inside of bend

Flow Units and Cooling Units

• Basic stratigraphic and field distinction

• No problem where flows have not been welded

• Flow Units- single depositional units that represent a single pyroclastic flow

• Thickness of a flow unit can vary from a few cm’s to many 10’s of meters

• One flow may follow another within minutes, hours, days

• Boundary between flow units marked by changes in composition, % and size of pumice, surge beds, ash beds, crystal content, composition, nature of matrix, etc.

Cooling Units

• Several hot flow units pile up rapidly, one on top of the other, they may cool together as a single unit-little or no cooling between eruptive events.

• They do this because cooling from emplacement temperature (>700 degrees C) to surface temperature takes several 10’s of years depending on flow thicknesses

• These are referred to as simple cooling units because thy have a simple arrangement of welding zones

Welding• Welding- the cohesion, plastic deformation

and eventual coalescence of pumice and

shards at high temperatures under load

stress

• Welding exhibited by:

– Sticking together of shards-pumice

– Flattening of pumice under load stress

– Flow as a coherent liquid (rheomorphism)

Nonwelded

Welded

Nonwelded

Welding

• Emplaced at temps.high enough to weld

generally cool slowly enough for the glass

to devitrify leading to:

– Microscopic crystalls which preserve

pyroclastic texture

– Total over printing by a coarse granophyric

texture.

– Everything in between

• Welding and devitrification release

volatiles from glass:

– Vapor phase crystals-quartz, alkali feldspar,

hydrous minerals

– Lithophysae

Welded Zone divided into:

Vapor Phase-poorly

welded

Devitrified or glassy

(Moderately-Densely)

> With thickness and temp.

Vapor Phase: flow lithified by high temperature vapor crystals-

Sanidine, Tridimite etc.

Welding

• Nonwelded tuffs-

– Pumice popcorn

shaped

– In thin section lots of

visible pumice, shards,

see bubble walls,

convex and concave

shapes

– This depends of

alteration,

recrystallization

Welding

• Welded Tuffs

– Eutaxitic foliation-partial elongation and

wrapping of pumice and glassy lenticles

around lithics and crystals

– Larger particles deform first

– Fabric to tuff- swirling and bedding parallel

– Pumice deformed, lithics and crystals are not

– Shards flatten out to become streaks

Welding

• First Criteria- look for

deformation of

pumice lapilli

• Pumice will be

deformed:

– Against lithics and

crystals

– Sticks to them

Fiami- flame like- ends of pumice

Can reach length to width ratios of 100

To 1

Moderate WeldingStreaky shards

Dense Welding

Pumice still

Glassy-

Will devitrify

Devitrification (glass to mineral[s]

• Starts in welded zone

• In shards get quartz and/or feldspar

growing in a radiating pattern refered to as

axiolitic

• In pumice get formation of spherulites and

vapor phase crystals in vesicles

Recrystallization steps during

devitrification

• Shards

– Axiolite formation

– Axiolites recrystalize to coarse grains

– Grains replaced by plates of quartz and/or

feldspar

– End up with a single quartz or feldspar grain-

never know it was a shard

Quartz/albite

Quartz/albite

Coarse

qtz

4 grains

Pumice

• Spherulites

• Spherulites to radiating qtz and/or feldspar

• Radiating to 4 coarse grains

• Grains to 2 then to a single grain

• This but one spherulite in a pumice, gets

hard to tell from amygdule

Older (Paleozoic-Precambrian)

Pyroclastic Flows

• Devitrification occurs to the same extent in

groundmass and pumice

• Becomes hard to differentiate pumice from

matrix

• Get only pumice outline (alteration or other

minerals-trace outline

• Different alteration- more permeable-alters

immediately-syndepositional

Rheomorphism

• On sloping ground can flow as a coherent

viscous liquid once dense welding has

eliminated intergranular friction

• Leads to

– Stretching and boudinaging of fiami

– Deformation of welding fabrics in flow folds

– Wholesale flow to produce features similar to

those of lavas

Lack of Pumice in SRS

SRS

Interesting-Special Features of

Pyroclastic Flows

• Columnar Joints-tend

to be rectangular-

cooling of densely

welded tuff

• Swiss Cheese effect-

weathering of

nonwelded tuffs-forms

large caverns

• Fumaroles and degassing pipes- volcanic gases within pyf’s may be liberated during devitrification and escape to the surface along permeable zones which evolve into fumarolic pipes

• Groundwater moving through permeable flows becomes heated-bouyant-rises to surface- 10,000 smokes

• Carbon from incorporated trees