Pyroclastic eruptions and their
deposits
Based on power point lectures by Wendy Bohrson
Introduction

Explosive volcanism involves transfer of fragmented
volcanic material (+gases and lithics)from depth onto
Earth’s surface.

Systems of transport and deposition distinguished for
three majors types of pyroclastic deposits: fall, flow,
surge.

Transport and deposition function of characteristics:
particle trajectory, solids concentration, extent to
which concentration fluctuates with time,
presence/absence of cohesion.
Review of fragmentation

Rising magma can begin to fragment when
bubble volume reaches 65-70 volume percent.

Fragmentation can occur because bubbles
become over-pressured and burst

Can also occur because melt film between
bubbles is so thin that they act as brittle
materials. Thin films burst when stress
exceeds their strength.
Review of fragmentation

When bubbles burst, the material changes from a mixture of
bubbles in a continuous stream of melt to droplets of melt in a
continuous stream of gas.

Changes drastically the viscosity and density of mixture.

Mixture accelerates up the conduit (can reach supersonic speeds)

Mixture of gas and particles that exits eruption conduit is called
an eruption column.
Eruption Column
Eruption column defined as:

Droplets of melt (molten) and quenched melt (glass
particles)

Crystals

Country rock/Wallrock (lithic fragments)

All dispersed in a continuous gas phase
Eruption Column: General Overview

Mixture erupted out of conduit/vent/crater vertically or laterally
(sub-vertically) at velocities up to several hundred m/s.

Initially, density of mixture is greater than surrounding atmosphere.

As material is thrust upward, incorporates (mixes with) cooler,
surrounding air into column.

Atmosphere heats up and the density of mixture becomes lower than
surrounding atmosphere.

Eventually, mixture has same density as surrounding
atmosphere/air.

Friction at outer boundaries (between air and column causes some
gravitational fallout of particles).
Parts of the Eruption Column



Gas thrust region
Convective ascent region
Umbrella region
Parts of the Eruption Column: More Detail
Gas Thrust/Jet Region

Mixture of pyroclasts and gas jetted
102-103 of meters into atmosphere by
initial acceleration

Nozzle velocity defined as maximum
velocity to which pyroclasts+gas can be
accelerated by expansion of magmatic
gas.

100 m/s for Strombolian/Hawaiian to
>600 m/s for Plinian

Nozzle velocity controlled by mainly by
volatile content in magma, which
controls explosive pressure ni
fragmentation zone.

Jet/Gas thrust phase typically extends
up to several km above vent; column
width narrow.
Parts of the Eruption Column: More Detail
Convective Ascent Region

Because gas thrust region is highly
turbulent, cool surrounding air mixed
into column.

Air is heated and resulting expansion
decreases bulk density of mixture.

Transition occurs when bulk density less
than that of surrounding atmosphere.

Forces driving motion dominated by
buoyancy and mixture rises (hot air
balloon).

Mixture rises as an convective eruption
column or plume.

Width of column increases.
Parts of the Eruption Column: More Detail
Convective Ascent Region

Convective part can rise 10s of km
upward.

Vertical velocities of plume vary from
10-100 m/s.

Velocity function of source conditions.

Velocity maxima reached in core of
plume.

At edges, particles encounter velocities
that are insufficient to keep particles
aloft.

Some fall back to surface (more in a
minute on this topic).
Parts of the Eruption Column: More Detail
Umbrella Region

Density of atmosphere decrease with
height.

Thus convective part of plume will
eventually reach a level of neutral
buoyancy.

Buoyancy no longer the driving
force: plume will start to move
laterally at a level Hb.

Excess momentum will carry some
particles higher. Top of plume is Ht.

Lateral movement forms the
distinctive mushroom or umbrellashaped region.
Transformation to Tephra Fountain

When jet does not incorporate
enough air into the mixture to
maintain buoyancy, rising jet
will decelerate until height
where velocity reaches zero.

Plume density in all (or part of)
the column greater than
atmosphere, particles will fall
back to surface.

Reflects column collapse.

Jet transforms into tephra
fountain.

Leads to formation of
pyroclastic density currents.
Eruption Columns
and Plumes
Rabaul, 1994

On the morning of September 19, 1994, two volcanic cones on the opposite sides of the
3.8 mile (6 km) Rabaul caldera begun erupting with little warning.

This photo shows the large white billowing eruption plume is carried in a westerly
direction by the weak prevailing winds.

At the base of the eruption column is a layer of yellow-brown ash being distributed by
lower level winds.
Tongariro, 1975

A vulcanian explosion from
Ngauruhoe (Tongariro) volcano in
New Zealand on February 19,
1975, ejects a dark, ash-laden
cloud.

Large, meter-scale ejected blocks
trailing streamers of ash can be
seen in the eruption column.

Blocks up to 20 m across were
projected hundreds of meters
above the vent.
Another type of volcanic plume

Another type of volcanic plume forms in association with
pyroclastic flows and surges, which are mixtures of hot
particles and hot gases that are denser than surrounding
atmosphere.

As flows travel away from source, sedimentation of
particles from base of flow and heating of entrained air
decreases bulk density.

These secondary or co-ignimbrite plumes generated from
tops of flows by buoyant rise.

Allows plumes to have much larger areal distribution.
Formation of a co-ignimbrite plume

1980 Mt. St Helens good example of formation of
a co-ignimbrite plume.

Pyroclastic flow moving at 100 m/s covered an
area of 600 km2.

When flow decelerated, finer particles became
buoyant because of heating of entrained air.

Secondary or co-ignimbrite plume ascended to 25
km above surface of Earth.
Formation of Co-Ignimbrite Plume
Pyroclastic flow heats up
entrained air.
In addition, sedimentation
occurs. Larger, denser
particles deposited at base
of flow.
Thus, because of both of these
processes, concentration of
particles and thus density
of material decreases.
Eventually, density less than
that of surrounding
atmosphere.
Buoyant cloud/plume
develops.
Formation of Co-Ignimbrite Plume
Co-ignimbrite plume lacks
gas thrust/jet region.
Begins ascent with
relatively low velocity.
Second, source area and
radius tend to be much
larger than those of the
primary plume.
Will also develop an
umbrella region.
Pyroclastic Flow and Co-Ignimbrite
Plume, Pinatubo, 1991
Makian, Indonesia, 1988

A vigorous eruption column rises
above Indonesia's Makian volcano in
this July 31, 1988, view from
neighboring Moti Island.

The six-day eruption began on July
29, producing eruption columns that
reached 8-10 km altitude.

Pyroclastic flows on the 30th reached
the coast of the island, whose 15,000
residents had been evacuated.

A flat-topped lava dome was extruded
in the summit crater at the conclusion
of the eruption.
Another type of volcanic plume

Another type of volcanic plume forms in association with
pyroclastic flows and surges, which are mixtures of hot
particles and hot gases that are denser than surrounding
atmosphere.

As flows travel away from source, sedimentation of
particles from base of flow and heating of entrained air
decreases bulk density.

These secondary or co-ignimbrite plumes generated from
tops of flows by buoyant rise.

Allows plumes to have much larger areal distribution.
Transport vs.
Depositional Systems

Transport system: responsible for movement
of the assemblage of fragmented material
(including gas)

Depositional system: controls on the way in
which the material comes to rest to form a
deposit.
Transport Systems
Two major classes identified in explosive eruptions.

Vertical plumes: dominant trajectory of motion is initially
upward. These generate fall deposits via deposition from
wind-driven clouds at elevations of several to 10s of km
above Earth’s surface.

Laterally moving systems: dominant trajectory of motion
is initially sideways. Generate surge and flow deposits
from gravity-controlled, ground-hugging density currents
(i.e., pyroclastic density currents).

Note that there are complications to this simple division-For example, secondary/co-ignimbrite plumes.
Transport Systems
Leads to three types of transport
systems

Fall: high buoyant plume carries all but
densest(largest) particles up to 10s of
km high; particles are sedimented from
plume. Dispersal controlled by wind
direction.

Surge: ground-hugging relatively dilute
density current with gradual downward
increase in density. Not influenced by
wind, but can gnerate a secondary
plume.

Flow: ground-hugging concentrated
(relatively dense) density current, often
with accompanying secondary cloud.
Pyroclastic Density Currents
For laterally moving systems, two end-member types of
transport systems have been identified:

Dilute: referred to as pyroclastic surge.

Concentrated: referred to as pyroclastic flow.

Note that these represent a spectrum, with gradations
between.
Gravity current
In fluid dynamics, a gravity current is a primarily horizontal flow in a
gravitational field that is driven by a density difference. Typically, the density
difference is small enough for the Boussinesq approximation to be valid.
Gravity currents are typically of very low aspect ratio (that is, height over
typical horizontal lengthscale). The pressure distribution is thus approximately
hydrostatic, apart from near the leading edge (this may be seen using
dimensional analysis). Thus gravity currents may be simulated by the shallow
water equations, with special dispensation for the leading edge which behaves
as a discontinuity.The leading edge of a gravity current is a region in which
relatively large volumes of ambient fluid are displaced. Mixing is intense and
head is lost. According to one paradigm, the leading edge of a gravity current
'controls' the flow behind it: it provides a boundary condition for the flow.The
leading edge moves at a Froude number of about unity; estimates of the exact
value vary between about 0.7 and 1.4.
Gravity currents are capable of transporting material across large horizontal
distances. For example, turbidity currents on the seafloor may carry material
thousands of kilometres.
Gravity currents occur at a variety of scales throughout nature. Examples
include oceanic fronts, avalanches, seafloor turbidity currents, lahars, pyroclastic
flows, and lava flows.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Pyroclastic flows are a common and devastating result of
some volcanic eruptions. They are fast moving fluidized bodies of
hot gas, ash and rock (collectively known as tephra) which can
travel away from the vent at up to 150 km/h. The gas is usually
at a temperature of 100-800 degrees Celsius. The flows
normally hug the ground and travel downhill under gravity, their
speed depending upon the gradient of the slope and the size of
the flow.
Pyroclastic Flow:
High-speed avalanches of hot
ash, rock fragments, and gas
move down the sides of a
volcano during explosive
eruptions or when the steep
edge of a dome breaks apart
and collapses. These
pyroclastic flows, which can
reach 1500 degrees F and
move at 100-150 miles per
hour, are capable of knocking
down and burning everything
in their paths.
Pyroclastic Density Currents:
Concentrated Currents or Flows

Has solids in concentrations of 10s of volume percent.
Thus are higher density than surges.

Have a free surface, above which solids concentration
decreases sharply.

Transport material by fluidization. Most flows considered
laminar.

Velocities vary, buy typically 10s of m/s. Can be much
faster. Velocities of up to several hundred m/s inferred
based on heights of obstacles overcome by flows.
Pyroclastic Flow: High-speed avalanches of hot
ash, rock fragments, and gas move down the sides of a
volcano during explosive eruptions or when the steep edge
of a dome breaks apart and collapses. These pyroclastic
flows, which can reach 1500 degrees F and move at 100150 miles per hour, are capable of knocking down and
burning everything in their paths.
Pyroclastic Surge: A more energetic and dilute
mixture of searing gas and rock fragments is called a
pyroclastic surge. Surges move easily up and over ridges;
flows tend to follow valleys.
Pyroclastic Density Currents: Dilute
Currents or Surges

Contain less than 0.1-1.0% by volume of solids, even near
ground surface. Thus are low density.

Are density-stratified, with highest particle concentration
near ground surface.

Transport material primarily by turbulent suspension.

Transport systems modeled as one that loses particles by
sedimentation. Depletes the system of mass. Eventually,
system may become buoyant, in which case becomes a plume.

Velocities vary, buy typically 10s of m/s. Can be much faster.
Structural Differences between Surge and Flow
In basal m to 10s of m, surges show
increase in density due to
sedimentation.
Also show decrease in mean
velocity due to increased ground
friction (drag).
Deposits generated by
sedimentation through basal
zone.
Lower solids concentrations than
flows.
Structural Differences between Surge and Flow
Much higher solids concentration
than surge.
Particles concentrated in basal
deposit m to 10s of meters.
Highest velocity in this region.
Rapid transition between high
velocity, high concentration
region and overriding cloud.
Deposition occurs both because of
ground friction and also because
the flow eventually comes to
rest.
Depositional Systems

Clasts in explosive eruptions have a period of transport,
and yet, all particles eventually come to rest.

Deposition system concept that allows investigation of
processes operating in final stages of movement; essentially
the transition from mobile to immobile particles.
Controls on Depositional Characteristics
There are four fundamental controls on how deposition occurs.

Clast trajectory: vertical to horizontal--> controls
whether deposit mantles surface, or has evidence of
lateral depositional characteristics.

Concentration of particles: from low to high-->
determines degree of sorting, scale of bedforms.

Presence/absence of cohesion. Cohesion will result in
rapid and irreversible deposition. Increases slope
angle of deposition as well.

Presence/absence of fluctuation of particle
concentration with time: steady vs. unsteady--> single
deposit or succession of deposits.
Four Major Controls on Depositional Processes
Particle trajectory: vertical yields mantling
of topography; horizontal may lead to
bedding
Particle concentration: low concentration
can lead to good sorting (fall); high can
lead to poor sorting (flow)
Particle cohesion: cohesive particles will
preclude slumping, also allow deposit to
be placed on steeper slopes.
Fluctuation in particle concentration:
sustained yield uniformly graded
deposit; non-sustained yield succession
beds
Effect of Particle Concentration vs. Angle of
Trajectory
Fall: vertical trajectory, low
concentration
Surge: lateral (horizontal) trajectory,
low concentration (thus leads to
lower density than flow)
Flow: lateral (horizontal) trajectory,
high concentration (thus leads to
higher density than surge)
More on Fall vs. Surge vs. Flow
Fall: drape landscape, no cross beds or
wave bedforms, well sorted, bedded,
evidence for high temperatures (welding)
absent
Surge: pinch and swell, basal scouring, cross
bedding, (i.e., features that express
lateral transport), good to poor sorting,
sustained high temperatures rare.
Flow: thicken into or are confined in valleys
because flow is gravity driven, show
basal scouring but lack internal
bedforms, poor sorting. Sustained high
temperatures (welding) typical. High T
indicative of efficient transport (little
mixing with ambient air).
Spectra between Deposition Mechanisms
Surge to fall: gradation between the
two, depending on wind.
Fall to flow: distinction between
these two function of ability of
material to trap gas. Falls
accumulate too slowly to keep gas
trapped.
Gas required to fluidize pyroclastic
material. That is, trapped gas
(which expands because it is hot)
will support the weight of the
particles. Behaves like a liquid.
Flows require sedimentation rates of
> 1 m/s.
Spectra between Deposition Mechanisms
Surge to flow: controls not fully
understood, but primary control
is particle concentration.
More on Particle Cohesion

Important in the low T environment: preferentially
affect fines. Clumping inferred to occur in wet
conditions (e.g., accretionary lapilli). Causes premature
deposition of fines, which in turn causes deposits to be
more poorly sorted.

Presence of water in low concentrations also increases
cohesion, allowing fall and surge deposits to be
deposited on surfaces with angles greater than dry angle
of repose.

Water in high concentrations will promote soft-sediment
deformation and slumping.
More on Particle Cohesion

At high temperatures, near source, cohesion of hot
clasts can results in formation of over-steepened
features such as spatter cones and ramparts.

At a distance from source in pyroclastic flows, when
material coalesces, deposit can retain momentum from
transport. If deposited on slope, can flow back downhill
under influence of gravity. Produces fountain fed lava
flows and rheomorphic flow deposits.
More on Role of Fluctuation in Particle
Concentration

Fluctuations in particle concentration, particularly in
fall deposits yield different types of fall deposits (topic
for the future).

Differences also evident in surge vs. flow. Surges are
modeled to be more variable in transport and
deposition systems, whereas flows are interpreted to be
more steady-state.

Reflects differences in momentum and length scale of
deposition: momentum in flows greater and beds are
typically thicker.
Review of Ignimbrites
Standard ignimbrite flow unit comprises 3 layers:

Layer 1: deposit laid down at flow front
during strong interaction with ambient
air and ground surface

Layer 2: main deposit

Layer 3: deposit from overriding dilute
cloud (co-ignimbrite cloud)
Layer 1

Highly variable in character;
suggests that this layer strongly
influenced by local topography,
etc.

Most common type is ground layer
or lithic-rich layer, which is a layer
enriched in heavy components like
lithic fragments.

Interpretation is that lithics
sedimented out of head of
pyroclastic flow.

Can also sometimes find a basal
surge layer. Interpreted to be the
result of surge advancing at the
head of the flow.
Layer 2

Layer 2a: variably developed ash
layer interpreted to form because of
interaction with ground surface.

Layer 2b: normal grading of
density particles, such as lithics.
Larger lithics concentrated at
bottom.

Reverse-grading at top because
pumice are less dense than medium.

Also common are lapilli pipes,
which are vertical pipes depleted in
fines. They are gas escape
structures. Fine particles escape
with gas.
Layer 3

Layer 3 is ash-cloud layer,
which is layer deposited from
secondary or co-ignimbrite
cloud.
Flow unit vs. Cooling unit

Flow unit--individual units that represent
distinct depositional events; may follow
within minutes, hours, days, or longer

Cooling unit--a package of rock that cooled as
a unit.

So an ignimbrite may be composed of a
number of flow units, and one or more
cooling units.
Welded Ignimbrites

Because ignimbrites contain lots of gases, and
are at high T when deposited, they develop a
number of textures/structures.

Include welding, devitrification, vapor-phase
alteration.

Collectively called welded ignimbrites.
Welded Ignimbrites

Welding is cohesion, deformation, eventual
coalescence of pyroclasts at high T under load
stress.

Degree of welding determined by
composition, post-emplacement T, cooling
rate, load stresses.
Hand Sample Characteristics:
Sintering, Compaction, Rheomorphism

Sintering: cohesion of clasts across points of
contact where load stresses are focused.

Compaction: flattening of pyroclasts, which
leads to development of fiamme, eutaxitic
texture.

Rheomorphism: flow as coherent liquid, post
emplacement
Volcanic Sinter

Geysers rising from pools bounded by sinter terraces are
among the spectacular thermal features of El Tatio in
the northern Andes.
Unwelded Ignimbrite in Outcrop
Unwelded: Note fluffy (inflated) pumice
Unwelded Ignimbrite in Thin
Section
Unwelded: Note cuspate forms are clearly evident;
delicate structures preserved
Moderately Welded Ignimbrite in
Thin Section
Moderately welded: Ash (glass particles) appear
more collapsed
Densely Welded Ignimbrite in
Outcrop
Densely welded: Note fiamme. Eutaxitic texture (question in
lab)
Densely Welded
in Thin Section
Densely welded: Ash
(glass particles)
collapsed and stretched
Hand Sample Characteristics:
Devitrification

Devitrification: occurs when deposits cool slowly;
represents process where glassy, amorphous structure
replaced by fine to coarser grained minerals.

Results in the crystallization of microlites along the
boundaries of the glass shards or within glass mass.

The mineral compositions produced are mainly
cristobalite (a high-temperature form of quartz) and
alkali feldspar.
Devitrification
Incipient devitrification
Highly devitrified
Hand Sample Characteristics:
Devitrification

Devitrification may occur around scattered
nuclei to form spherulites.

Spherulites delineated by radiating crystals of
acicular cristobalite and feldpar.

These spherical aggregates are common features
in both rhyolitic lavas and felsic ignimbrites.
Spherulites
Spherulite
Radial crystals within
Hand Sample Characteristics: VaporPhase Alteration

Vapor-phase alteration -- post-depositional process;
Crystallization takes place in open spaces, under the
influence of a vapor phase.

Hot vapors, derived from magmatic gas-exsolution and
from heated groundwater, are generally enriched in H2O,
CO2, and SO2. They also have the ability to scavenge
numerous additional elements from the volcanic debris,
such as Si, Al, Na, and K.

Cooling of these element-rich phases may result in the
crystallization of a variety of minerals into open cavities
as the gases ascend upward through the flow.
Hand Sample Characteristics:
Vapor-Phase Alteration

The main phases of vapor-phase crystallization are
tridymite, cristobalite, and alkali feldspar.

Lithophysae is a hollow, bubble-like structure composed
of concentric shells vapor-phase minerals found within
the cavities of pyroclastic flows.

The advanced product of vapor-phase crystallization is
sillar, a whitish, well-cemented, coherent rock with little
pore space. Sillar zones are often found in association
with abundant fumarole pipes in degassed ignimbrites
Outcrop Characteristics: Fumarole Pipes

These dark, lithic-rich pipes are gas
segregation structures that provide
direct routes for the degassing of the
ignimbrite.

The escaping gases cause fragments
of different sizes and densities to
jostle apart from one another. The
largest fragments in the pipes are
~20 cm in diameter.

Most of the finer material, however,
has been blown out of the pipes
(elutriated) by the escaping gas.

The ignimbrite was derived from an
eruption 4.6 million years ago,
associated with the Cerro Galan
caldera.
Outcrop Characteristics: Compositional
Zoning at Crater Lake

Mazama ignimbrite: This pyroclastic
flow was generated by the calderaforming eruption of Mt. Mazama
about 6,845 years ago.

The ignimbrite shows magnificent
compositional zonation. The pale
(felsic) lower part has a rhyodacitic
composition and the darker (mafic)
upper part is andesitic.

This vertical zonation is inverse of the
zonation in the magma chamber
before eruption. The upper part of
the chamber (which erupted first)
was rhyodacitic and the lower part of
the chamber (which erupted last) was
andesitic.
Outcrop Characteristics: Fumarole Pipes at
Crater Lake

The splendid pinnacles have
been described as fossil
fumarole pipes that are more
resistant to erosion than the rest
of the ignimbrite.
Review of Types of Pyroclastic
Flows

Terminology of pyroclastic flows and pyroclastic flow
deposits can be complex and confusing. In general, there
are two end-member types of flows:
(1) PUMICE FLOWS -- these contain vesiculated,
low-density pumice derived from the collapse of an
eruption column; produces unwelded to welded
ignimbrite.
(2) NUÉE ARDENTES -- these contain dense lava
fragments derived from the collapse of a growing
lava dome or flow; produces a block and ash
flow.
Nuee Ardente and Block and
Ash Flow

The French geologist Alfred Lacroix attached the name
nuée ardente (glowing cloud) to the pyroclastic flow
from Mt. Pelée that destroyed the city of St. Pierre in
1902.

The flow was generated from the explosive collapse of a
growing lava dome at the summit of the volcano, which
then swept down on the city.

Thus, nuée ardente eruptions are often called Peléen
eruptions.
Sequence of Events

Mt. Unzen nuée ardentes -- the
sequence of events associated with the
1991-95 nuée ardente eruptions from
Mt. Unzen, Japan.

Collapse of a growing lava dome
generates the nuée ardente.

Within seconds a faster-moving cloud of
smaller ash-sized fragments (the ashcloud surge) forms above and in front of
the nuée ardente.

In some cases, dome collapse is
attributed to explosive eruption at the
summit crater. Explosive collapse may
clear the throat of the volcano, thus
generating vertical eruption columns

Eruption can also be initiated by dome
collapse (gravitational).
Nuee Ardente vs. Pumice Flow

Nuée ardente deposits are composed of dense, nonvesiculated, blocky fragments derived from the
collapsed lava dome.

They therefore differ significantly from the highly
vesiculated ignimbrites which are derived from eruption
column collapse.

Nuée ardente deposits contain blocks in a fine-grained
matrix of ash. The deposits, therefore, are called blockand-ash deposits. They are denser than ignimbrites, and
typically are less extensive.
1902 Mt. Pelee, Martinique

The village of St. Pierre on the island of
Martinique was destroyed by a
pyroclastic flow similar to the one shown
here.

This photo was taken a few months after
the destruction of St. Pierre. Pyroclastic
flows had not been previously described
by volcanologists.

This type of pyroclastic flow is called a
nuée ardente, composed of hot,
incandescent solid particles derived from
the collapse of a lava dome.

Other types of pyroclastic flows, derived
from collapse of the eruptive column, are
pumice bearing, and their deposits are
called ignimbrites
.
Photo by Lacroix, 1902.
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