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