Using Pyroclastic Deposits to Infer Eruption Parameters

1
Using Pyroclastic Deposits to Infer Eruption
Parameters
GEO3975 (Pyroclastics) Fall 2006
2
Outline of Talk

• What do I mean by eruption parameters?
• Will discuss theory (briefly), then discuss
  empirical evidence from deposits

3
What do I mean by Eruption Parameters?

• Quantitative and qualitative aspects (of volcanic
  explosions)
• Limitations with both deposits and researchers,
  hence qualitative descriptions
• are common, and conceptual and numerical modeling
  done
• This class focuses on what data we can get on
  explosive eruptions (not transport or deposition)
  from
• study of the deposits and their pyroclasts. Look
  at transport and deposition in next class
• Important eruption parameters conduit
  processes, exit velocity (and velocity profile),
  mass flux,
• role of gas thrust versus convection,vent width,
  column height, column bulk density,
• vent width, column collapse,magmatic or
  phreatomagmatic,eruption style (eg Strombolian
  etc),
• temperature and rheology of pyroclasts on
  eruption.
• All of these parameters can change with time

4
Conduit Processes Basics

• Large number of considerations, including
  exsolution, fragmentation, interaction with walls
  etc etc
• Principle factors which need to be considered
  include buoyancy of magma, magma (over)pressure,
• magma viscosity, many aspects of rock mechanics
  (eg tensile strength),confining P
• Buoyancy and/or magma pressure at source are
  driving forces. Prevention of upward movement
• is due mostly to rock strength, confining P, heat
  loss, magma viscous forces (including shear
• against conduit walls), drops in magma pressure,
  degassing etc
• Magmas are about 10-15 less dense than their
  solidified equivalents. Felsic magmas are less
• dense than crust, hence buoyant rise but mafic
  magmas have a closer hydrostatic equilibrium with
• the crust
• All conduits fed from below by dikes that contain
  coherent magma
• At some depth dikes become pipe-like structures
• Conduits can open to vents at the surface when
  magmatic or phreatomagmatic explosion occurs
• (ie when rapidly accelerating dispersed flow
  begins) or of course without this (ie
  effusively).

5
Lithic Pyroclasts and Conduit Processes (1)

• One source of lithic clasts are the walls (of
  magma chamber, conduit or vent)
• Walls are commonly pre-fractured prior to
  fragmentation and incorporation into
• magma or magmagas dispersion
• Pre-fracturing occurs when strength of rock is
  exceeded (typically tensile strength,
• but may be compressive or shear strength)
• Fracturing may be due to hydraulic fracturing or
  passage of shock waves
• Hydraulic fracturing is very efficient mechanism
  of rock fracture, hence lithics are
• very common in phreatomagmatic deposits
• Lithics derived from upper few hundred metres are
  most common (this is because
• magma or magmagas dispersion has greatest
  acceleration at near-surface, vent-walls
• commonly collapse into open space and
  near-surface rocks are more likely to be poorly
• consolidated)
• Lithics more common in proximal and early
  deposits

6
Lithic Pyroclasts and Conduit Processes (2)

• Inverse sequence of lithic clasts (cf to a known
  stratigraphy) may indicate increased
• depth of explosion with time. Postulated for
  maar volcanoes, but only rarely
• demonstrated. Inverse sequences also in
  syncaldera ignimbrites
• Proportion of lithic clast types typically wont
  closely reflect sub-surface proportions
• or stratigraphy (due to difference in rock
  strength and acceleration of melt or
• meltgas near-surface)
• High proportion of lithic clasts (gt30vol?)
  suggests hydraulic fracturing and possibility of
• phreatomagmatic explosion
• Lithic clast stratigraphy (and lateral variation)
  can be particularly useful in understanding
• caldera collapse and associated large-volume
  ignimbrites and in modeling changes in
• conduit size or morphology
• Degree of disintegration of lithic clasts into
  crystals/grains can be used to infer
• degree of consolidation of conduit/vent walls and
  hence wall porosity/permeability
• (and inferences about degassing)

7
Conduits and Rock Strength

• Rock strength has profound influence of conduit
  and vent growth and morphologies
• Strength of any rock can be tensile, compressive,
  shear, abrasive etc
• These are all dependent on rock type and several
  other factors..
• Within each rock type there is considerable
  variation depending on grain size, grain shape
• grain mineralogy, grain alignments (eg foliation
  in metamorphic rocks, bedding/lamination in
• sed rocks, flow-aligned crystals in igneous
  rocks), porosity (sedimentary rocks, vesicularity
  in
• volcanic rocks), degree of alteration etc
• Sedimentary rocks typically have lower rock
  strengths than igneous or metamorphic rocks
• Fine-grained rocks generally have higher rock
  strengths than the coarser equivalents
• (eg basalt versus gabbro)
• Metamorphic rocks generally have lower rock
  strengths than igneous rocks (due to
• development of preferred orientations)

8
Magmatic Fragmentation Reminder

• Magmatic fragmentation is explosive fragmentation
  of a magma as a consequence
• of the release of energy from excess gas P in
  exsolved bubbles of juvenile gas
• Flow changes from continuous liquid phase to
  continuous gas phase after fragmentation
• Depth and explosivity of magmatic fragmentation
  dependent (mostly) on magma
• rheology, gas fraction, bubble pressure, magma
  permeability, wall rock permeability
• and magma acceleration in the conduit

9
Primary Magma Fragmentation and Later Processes
(Dobran, 2001)

• Primary magma fragmentation (ductile/brittle) in
  the conduit is followed by ductile/brittle
• fragmentation/modification during transport (in
  conduit, air/water and on surface), syn and
• post-deposition
• This post-magmatic fragmentation and textural
  modfication in the conduit, vent
• and column (ie eruption) due to ongoing
  vesiculation and bubble expansion and collapse,
• impacts, quench contraction, shearing in flow and
  against margins etc.
• To use pyroclasts to infer primary magmatic
  fragmentation processes, need to extract
• effects of all these other processes on pyroclast
  characteristics!

10
Spontaneous Collapse of Foams

• When vapor fraction (vesicularity) reaches 0.7
  (actually 0.74) then any bubble suspension
• becomes a foam (max packing of spherical bubbles
  of same size)
• Foams with non-spherical bubbles and mixtures of
  bubble sizes can have vapor fractions
• greater than 0.7
• Magma (and many other) foams will
  disintegrate/collapse (ductile) at vapor fraction
  of 0.7
• Hence this value is often included in models as a
  fragmentation limit
• This is collapse in a static (not moving, no
  bubble expansion) foam under
• constant P and T. May not be explosive (eg foam
  on a pint of Theakstons Old Peculiar)
• Collapse under these conditions is by melt
  drainage (thinning of bubble walls) by
• gravity and capillary forces. Can be explosive
  if bubble fluid Ps are high enough
• Collapse reduces surface energy
• Foams may not collapse completely, just enough to
  reduce surface energy

11
Can we use Vesicles in Pyroclasts to Infer how
Magma Foam Fragments?

• Pumice vesicularity is commonly around 70-75 but
  varies greatly
• Houghton and Wilson (1989) recorded void
  fractions of 0.6 to 0.89 in pumice
• Reticulite can have void fractions of gt0.95
• Many pumice has more than one size range (eg
  bimodal) of vesicles, which allows for
• higher void fractions (packing of different sized
  spheres allows this)
• Important to note therefore that gas fraction is
  not the sole
• control on magmatic fragmentation
• What are the other controls?

12
Control of Gas Permeability on Magmatic
Fragmentation

• Magma foams may fragment at much lower vapor
  fraction than 0.7
• One of the reasons for this is gas permeability
  in the foam
• Foam permeability is mainly controlled by of
  ruptured bubble walls/bubble apertures
• formed by bubble coalescence
• Bubble walls can be liquid or solid but rupturing
  must be (initially) a non-explosive process
• Factors controlling bubble coalescence include
  melt viscosity, bubble size distribution,
• bubble gas pressures, bubble separation, time etc
  (Klug and Cashman, 1996)
• Rapid gas flow through this network can cause
  explosive magma fragmentation by
• rupturing of bubble walls (due to friction and P
  differences) or if very high permeabilty
• and especially if conduit walls are highly
  permeable may just escape (degas)
• Hence there is a critical value of both magma and
  wallrock gas permeability that is
• important for generating magmatic explosions

13
Porosities of Pumice

• Easy to study porosity of pumice in lab
• Often use He-pycnometry, which measures grain
  density relative to bulk density (ie
• with porosity) by forcing He gas into sample
• The apparent density of the material is
  calculated by determining the volume of the
  sample by
• introducing a known quantity of He into the
  sample chamber at two different pressures.
• The relationship P1V1P2V2 is used to calculate
  the volume, and the density is determined by
• dividing this into the mass of the sample.
• Can also use point counting in thin-section or
  liquid immersion techniques
• Porosity of pumicevesicularityfracture space
• Porosities of pumice also important for working
  out DREs. Volumes of pyroclastic deposits
• often quoted as Dense Rock Equivalents (DRE)

14
Identifying Coalesced Vesicles

• Recognising coalesced bubbles is important for
  models of gas permeability in magma
• Vesicles commonly preserve a remnant of
  interbubble walls
• We know that coalescence can occur during buoyant
  rise (bigger bubbles rise faster)
• or during shearing within the melt or against
  conduit walls
• Coalescence can also be post-conduit or column
  (ie impact or loading)
• Coalescence of many tens of bubbles in mafic
  melts may occur, but typically
• cm-sized bubbles in silicic pumice consist of a
  few to 15 merged bubbles
• In absence of obvious remnant walls (septa),
  coalesced bubbles can be identified by
• the fact that they are more fractal than single
  bubbles

15
Ductile vs Brittle Fragmentation and Glass
Transition

• 1/temperature is proportional to viscosity
• Shorter timescale correlates with faster
  acceleration up conduit
• Note glass transition is not a fixed temp, but a
  variable temp range
• Rapid strain/acceleration and higher viscosities
  (rapid cooling) favor brittle fragmentation

16
Fracture of Silicate Glass

• Initial (pre-fragmentation) material in
  volcanology is dominantly a viscous liquid or
  glass,
• with other solids and vapor bubbles
• Fracture of a glass is not related to an ordered
  microstructure
• Glass cracks extremely easily with numerous
  cracks (many nucleation points)
• Sub-microscopic cracks produced even when a dust
  particle settles on glass surface
• River pattern steps are typically very closely
  spaced (less than 2 microns)
• Striations, conchoidal chipping and
  mirror-mist-hackle fracturing are common
• Cracks propagate more easily along interfaces
  between glass and bubbles or solids,
• i.e both act as weak areas in the glass
• Glass with high of microlites fractures more
  easily than one with few microlites

17
Control of Viscosity on Magmatic Fragmentation

• Very important control
• Higher melt viscosities increase likelihood of
  magma fragmentation at vapor fractions lower
• than 0.7
• Higher bubble vapor pressures are developed in
  melts of higher viscosity

18
Control of Shearing on Magmatic Fragmentation

• Shearing of a (pressurized) bubbly foam would
  clearly favor fragmentation (at
• void fractions less than 0.7)
• Shear strain is highest during rapid acceleration
  (uppermost parts of conduit)
• (and at margins of conduit)
• Tube vesicles can be used to calculate shear
  rates (and flow acceleration rates)
• Elongation/deformation of one vesicle population
  (but not another) can be used
• to infer when most shearing took place relative
  to cooling and vesiculation history

19
Pyroclast Morphologies

• Pyroclasts vary widely in shape. Morphologies
  can be quantified
• Shape of fluidal pyroclasts is is determined by
  nature of flow in conduit (strain rate,
• shearing against margins, impacts etc), ongoing
  vesiculation, surface tension vs viscous
• forces, aero (or hydro) dynamic moulding, impact
  with ground, welding and loading etc
• Shape of brittle pyroclasts determined by
  presence, shape and
• distribution of bubbles and crystals in
  pre-fragmentation state, nature of flow in
  conduit,
• (as above), impact with ground and loading

3 types of pyroclast shapes common in
Plinian deposits. These are all ash-sized.
Lapilli and block-sized pumice is same as
micropumice shown here, just bigger

• Cuspate shards represent plateau borders, platy
  shards
• are from (usually) larger bubble walls (films)
• and pumice (or micropumice in this case) from
• fragmentation (across bubbles) into bubbly clots

20
Pyroclast Size Distribution

• Different size ranges of pyroclasts in a deposit
  reflect different sorting
• and/or fragmentation processes (in conduit,
  column, surface transport and deposition)
• For example, Plinian deposits commonly comprise
  large lapilli and finer ash (bimodal)
• Ash derived from walls of bubbles and pumice
  represents clots of melt with
• smaller vesicles (ie ash from burst bubbles and
  pumice lapilli preserve unburst
• bubbles)

Heiken, 1987 (bubble wall ash shard)
21
Vesicle (Bubble) Number Densities

• Vesicle (bubble) number densities depend on
  competition between bubble nucleation
• and coalescence rates and time between nucleation
  and glass transition
• High BND typically indicate rapid nucleation
  and/or fast ascent rates
• Its complex. For example, low viscosity static
  magma favors higher coalescence
• and lower BND, but low viscosity magma also
  usually favor higher ascent rates
• lowering the time for coalescence and favoring
  higher BND
• Post initial fragmentation processes in ductile
  clasts can reduce BND by collapsing and
• annealing vesicles. Important to look at vesicle
  deformation textures when assessing
• inferences from BND

22
Vesicle Size Distribution (VSD)

• Vesicles may be same size, serial in size
  distribution or have two or more modes
• Bimodal distribution is common in pumice (left)
  and
• clearly indicates two episodes of nucleation
• Bimodal distribution in pumice commonly
  interpreted to
• represent nucleation of early bubbles on
  microlites and then later (including
  post-fragmentation)
• nucleation of smaller bubbles
• Accelerating flow near surface commonly shears
  smaller still ductile bubbles but not older
  larger bubbles which may have had more rigid walls

Heiken, 1987
23
Vesicle Morphologies

• Vesicle morphologies can vary from near-perfect
  spheres (common as small young bubbles
• in low viscosity magmas), highly elongate bubbles
  or annealed bubble remnants
• (metres in length, common in high viscosity lava
  domes) to highly irregular
• coalesced bubble groups with numerous bubble wall
  remnants (common in pumice and
• aa lava flows)
• Vesicle morphologies record (often complex)
  history of nucleation, bubble growth and
• coalescence, deformation and annealing. Two or
  more distinct populations is common

Heiken and Wohletz, 1985
24
Examples of SEM images of vesicular pyroclasts
Typical Plinian pumice 80 vesicles
Plinian pumice vesicle coalescence etc
Reticulite (gt98 vesicles) polyhedral network
Tube pumice stretched vesicles
Microlites (small xls) inhibiting bubble growth
Collapsed/compressed vesicles
(Cashman et al. 2000)
25
Surtseyan (basaltic) pyroclast. Note how
smallest bubbles are near spherical, and
deformation of some bubbles but not larger
(older) bubbles (more rigid walls or higher gas
P?)
26
Pyroclast Abrasion Conchoidal and V-Shaped Pits

• Conchoidal (chip) fracture is a type of fracture
  that propagates along a 3-D curved surface,
• and is characteristic of point-of-contact (ie
  impact in volcanology) fractures in glass.
• Conchoidal fractures display surfaces with radial
  striations in a dish-like cavity (below)
• By studying such chips on pyroclasts it is
  theoretically possible to get control
• on the amount of impact abrasion, impact
  velocity, direction of impact and
• (possibly) nature of impactor during
  post-magmatic modification
• Need (with curved pits) to be sure you are not
  looking
• at a broken vesicle or a pit caused
• by dissolution (quite common)
• Abrasion pits on crystal pyroclasts may also be
  V-shaped
• (below)

Hull, 1999
Conchoidal fracture (cavity) in epoxy resin
(glass)
27
Crystals in Pyroclastic Deposits

• Phenocrysts
• Microlites
• Quench crystallites
• Vapour-phase crystals
• Hydrothermal mineralization
• Xenocrysts (plucked from walls or from
  disintegration of lithic clasts)
• Surface deposits
• Phenocrysts and xenocrysts typically make up most
  crystal pyroclasts
• In mafic rocks most crystal pyroclasts will be
  plagioclase, olivine or pyroxene
• In intermediate rocks they will mostly be
  plagioclase, alkali feldspar, pyroxene or
• amphibole
• In silicic rocks they will mostly be quartz and
  alkali feldspar
• Quartz and olivine resist abrasive disintegration
  (no cleavage) so quartz-rich and olivine
• -rich concentrations are quite common

28
What can crystal pyroclasts tell us about
volcanic explosions?

• Crystals to understand conduit processes, and
  eruptive transport and depositional
• processes
• Broken (brittle fractured) crystals may be more
  common in PDC rather than fall
• deposits (due to greater abrasion during
  transport)
• Systematic changes in crystal composition in
  pyroclastic deposit can be used to infer
• magma chamber processes (eg zoned chambers etc)
• Very high percentages of crystals (gt60?) in
  lithic-rich pyroclastic deposits
• generally indicates a lava dome source
• Fracture directions in crystals relative to their
  flow alignment can be used to
• indicate implosion of conduit (see paper handed
  out today)

29
Discussion of Vulcanian Mechanisms Paper as An
Example of Using Pyroclasts to Infer Conduit
processes
30
From here 13th Oct
Conduit Processes Empirical Evidence from
Conduit-Fill Deposits

• Could study conduit processes by looking at
  exposures of
• conduit-fills (and overlying vent-fills)
• Descriptions of conduit-fills not common in the
  literature
• Most descriptions are of diatreme fills from
  basaltic maar volcanoes and kimberlites
• Morphology of kimberlite diatremes is well known
  from diamond mining. They are only
• broadly pipe-shaped but with many irregular
  offshoots and margins
• Conduit-fills from basaltic maars dominantly
  comprise slumped wallrock, slumped
• Primary volcaniclastic rock, peperite and
  irregular intrusions. Much of this is
  late-stgae and
• as its maars, this is phreatomagmatic.
  Conduit-fills from magmatic explosions are much
  less
• known

31
Eruption columns Reminder
Umbrella
Convection
Gas thrust
Wind velocity
Gas thrust energy
(Schmincke, 2000)

• Plinian eruptions are sustained explosions up to
  hours long
• Eruption columns can be up to 30-40km high
• Rapid decompression of large volumes of gas-rich
  magma

32
Vent Width

• Vent is surface expression of conduit
• Explosive excavation and gravitational collapse
  widen top of vent with time
• Gravitational collapse of walls, fallback and
  intrusion can narrow vent with time
• Vent widening can lead to column collapse
• Vent constriction can lead to re-establishment of
  buoyant columns
• Vent difficult to correlate any deposit
  characteristics with changes in vent width
• (as other factors involved)

33
Exit Velocities

• Exit velocity is velocity of magmagas dispersion
  when it appears at surface (vent)
• Exit velocities are important because they
  influence column heights, dispersal,
• impact fragmentation, amount of cooler air or
  water sucked in, cooling rates etc
• Exit velocities dependent mostly on magma
  temperature, gas (steam) content and vent
• Width (narrower the vent the greater the
  velocity)
• Exit velocities in most explosive eruptions are
  supersonic but subsonic
• velocities also occur.
• Exit velocities vary from few 10s m/s to few
  hundred m/s

34
Convection of Pyroclasts

• Above the gas thrust zone of a column, low
  density and fine pyroclasts
• are usually moved upwards in mixture of
  convecting air or water and expanding
• (hot, decompressing) juvenile gas
• Bulk density of pyroclasts hot air (or water)
  vs density of surrounding cooler air or water
• is most important consideration
• Size of clasts, vesicularity, pyroclast/air
  ratio, expansion rate of air (dependent on
• magma temperature, air temperature, rate of
  incorporation of cool air into eruption column
• and efficiency of mixing of air magma, pyroclast
  size) determine convection velocities
• (and hence buoyancy versus collapse)
• Influence of water in/on ground or in atmosphere
  on convection?
• Higher F, water expands faster than air (for same
  heat input), but also cools magma more
• efficiently, and may also cause ash to clump
  (more likely to fallout)
• Some water will promote greater convection and
  higher column heights, too much

35
Column Bulk Density

• Most models of eruption columns average out the
  density of solid particles and gas (ie bulk
  density)
• Bulk density of eruption column is arguably most
  important parameter
• Bulk density is affected by many factors density
  of clasts, temperature of clasts,
• air or water as surrounding fluid relative roles
  of gas thrust versus convection.
• At simplest level , bulk density is a battle
  between loading the column with solids and liquid
  
• (water) and expansion of water and air
• Even very small of solids and liquid water in
  column has very significant effect on density as
  solids
• (about 2500kgm-3) are about 2000 times more dense
  than air (about 1.2kgm-3 at sea level for
• dry air). Liquid water (1000kgm-3) is about 800
  times more dense than air

36
Effect of mass discharge rate and external water
on eruption columns
Sigurdsson et al.
37
Column Collapse

• Columns collapse (or partially collapse) when
  part or all of its bulk density of
• pyroclastsair exceeds surrounding air or water
  density.
• Collapse usually generates PDCs (but not if
  collapse is confined)
• There are many reasons for collapse including
• Increased vent radius (increased mass discharge
  rate)
• Reduction in juvenile gas content
• Larger pyroclasts
• External water increased cooling (less
  convection), loading of column
• Increased incorporation of lithic clasts (eg
  from vent/conduit erosion)
• Change in wind velocities
• Other weather influence (clouds, humidity)
• Vent shape (eg funnel-shape vents decompress
  columns faster than cylindrical vents,
• favoring boil-over eruptions for example)
• IMPORTANT Correlating an unobserved PDC with
  one particular collapse mechanism can be tricky,
  unless there is clear evidence for one of these
  collapse mechanisms

38
Column Collapse in Water Some Considerations

• Eruption columns collapse very commonly
• Increasing bulk density gt surrounding fluid (air
  or water) causes collapse
• Increase due to vent widening, increasing mass
  flux

• Column collapse underwater less well understood
  than in air
• Density contrast with surrounding water is less
  than with air, so maybe columns in water less
  likely
• to collapse (given similar initial conditions)
  than in air
• But this is balanced by fact that water is much
  more efficient at cooling magma and convection
• velocities are slower than air so this would
  favor collapse

39
Temperature and Rheology of Pyroclasts

• Data on this best collected from deposits, eg by
  thermoremnant magnetization, pyroclast
• morphologies (fluidal or angular, ie ductile or
  brittle fragmentation), degree of welding etc
• T of pyroclasts influence the following eruption
  parameters
• bulk density (eg melt is less dense than glass
  by convection of entrained air and water etc)
• convection velocities in plume or current (and
  hence dispersal)
• degree of post-magmatic fragmentation (ie ongoing
  vesiculation) and modification such as welding

40
Evidence from Deposits for Magmatic vs.
Phreatomagmatic Explosion?

• Involvement of external water has profound
  influence on many eruption parameters, but
• how do we recognise its signature in the
  pyroclasts and deposits?
• Require a combination of clues, eg for
  phreatomagmatic explosion High F/D ratio, low
  density
• PDCs, palagonite, bomb sags, poorly vesicular
  vitric pyroclasts, etc. Most of evidence is at
  deposit
• not pyroclast scale
• May be difficult to be sure if interaction with
  external water was really the explosion trigger,
  eg
• water in column could be juvenile, from air or
  from surrounding water (but not an explosion
  trigger)
• Phreatomagmatic signature most difficult to
  recognise in high explosivity eruptions, eg
• Plinian/Phreatoplinian where significant
  contribution from magmatic volatile bubble
  expansion
• Magmatic vs phreatomagmatic can change with time
  (even during course of a single depositional
• event)

41
Phreatomagmatic Pyroclasts

• Pyroclasts produced by magma-water interaction
  may be distinctive
• Distinctions include smaller mean size, dominance
  of blocky (brittle fracture)
• morphology, adhered finer glass and/or crystals,
  and potential for low (or no)
• vesicularity

• Left is SEM image of typical low vesicularity,
  blocky
• (with adhered finer particles) phreatomagmatic
  ash
• Adhered particles mostly due to syn-eruptive (or
  shortly after)
• adhesion (And later cementation) on water films
  condensed on
• pyroclast surfaces
• Origin of blocky morphology is probably due to
  higher crack
• nucleation rates on contact with water compared
  to air, more
• rapid cooling promotes brittle fracturing.
  Blocky
• cracking due to amorphous nature of glass
• Low vesicularity pyroclasts indicate that
  external water expansion
• has driven fragmentation
• Medium to high vesicularity pyroclasts in
  phreatomagmatic deposits
• are commonly not blocky

Heiken, 1987