Volcanology

1
Pyroclastic eruptions and their deposits
Based on power point lectures by Wendy Bohrson
2
Introduction

• Explosive volcanism involves transfer of
  fragmented volcanic material (gases and
  lithics)from depth onto Earths 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.

3
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.

4
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.

5
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

6
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).

7
Parts of the Eruption Column

• Gas thrust region
• Convective ascent region
• Umbrella region

8
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 pyroclastsgas 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.

9
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.

10
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).

11
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 umbrella-shaped region.

12
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.

13
Eruption Columns and Plumes
14
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.

15
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.
  

16
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.

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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.

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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.

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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.

20
Pyroclastic Flow and Co-Ignimbrite Plume,
Pinatubo, 1991
21
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.

22
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.

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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.

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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 Earths
  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.

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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.

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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.

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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 tran
sporting material across large horizontal
distances. For example, turbidity currents on the
seafloor may carry material thousands of
kilometres. Gravity currents occur at a variety o
f scales throughout nature. Examples include
oceanic fronts, avalanches, seafloor turbidity
currents, lahars, pyroclastic flows, and lava
flows.
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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.
30
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.
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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.

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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 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.
33
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.

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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.

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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.

36
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.

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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.

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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
  

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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)
  

40
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).

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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.

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Spectra between Deposition Mechanisms

• Surge to flow controls not fully understood,
  but primary control is particle concentration.
  

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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.

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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.

45
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.

46
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)

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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.

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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.

49
Layer 3

• Layer 3 is ash-cloud layer, which is layer
  deposited from secondary or co-ignimbrite cloud.
  
  

50
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.

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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.

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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.

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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

54
Volcanic Sinter

• Geysers rising from pools bounded by sinter
  terraces are among the spectacular thermal
  features of El Tatio in the northern Andes.

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Unwelded Ignimbrite in Outcrop
Unwelded Note fluffy (inflated) pumice
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Unwelded Ignimbrite in Thin Section
Unwelded Note cuspate forms are clearly
evident delicate structures preserved
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Moderately Welded Ignimbrite in Thin Section
Moderately welded Ash (glass particles) appear
more collapsed
58
Densely Welded Ignimbrite in Outcrop
Densely welded Note fiamme. Eutaxitic texture
(question in lab)
59
Densely Welded in Thin Section
Densely welded Ash (glass particles) collapsed
and stretched
60
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.

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Devitrification
Incipient devitrification
Highly devitrified
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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.

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Spherulites
Spherulite
Radial crystals within
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Hand Sample Characteristics Vapor-Phase
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.

65
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

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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.

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Outcrop Characteristics Compositional Zoning at
Crater Lake

• Mazama ignimbrite This pyroclastic flow was
  generated by the caldera-forming 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.

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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.

69
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.

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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.

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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 ash-cloud 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).

72
Nuee Ardente vs. Pumice Flow

• Nuée ardente deposits are composed of dense,
  non-vesiculated, 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 block-and-ash deposits.
  They are denser than ignimbrites, and typically
  are less extensive.

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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.