MidTerm Exam 23rd October' 25%' Based on all online

1

• Mid-Term Exam (23rd October). 25. Based on all
  online
• notes up to and including Fri 13th notes
• 2. Research Seminars
• Students are required to present a 15-minute
  (plus 5 minutes of questions)
• research seminar using Powerpoint on October
  27th. A properly written (not just notes)
• version of the seminar should be handed in on
  this
• day also.
• The topics have been assigned as follows
• Kristen Vesicles and their interpretation
• Elizabeth Upper flow regime structures and
  their interpretation
• Rachel Discussion of the glass transition
  temperature (Tg) in magmas
• Mary Alteration of volcanic glass and resultant
  textures
• Veronica Autoclastic Fragmentation Processes

2
Describing Pyroclastic Fallout and PDC Deposits
GEO3975 Topics in Volcanology (Pyroclastics) Fall
2006
3
Outline of Lecture

• Discuss only description today, and
  interpretation in a later class
• Fallout and PDC covers vast majority of primary
  pyroclastic rocks
• /deposits. Primary pyroclastic processes not
  included are mass flows
• without gas support (eg DAD, hot rock avalanche
  etc)
• Will discuss checklists, facies concept, logging
  and mapping pyroclastic
• deposits,

4
Pyroclastic Deposits in Field Description
Checklist

• GPS Location (and note location in notebook and
  on topo map)
• Give location a in notebook
• Photo of general location (this is VITAL)
• Record all photo s in notebook
• Notes on overall first impression
• Photos of closer views
• Nature of contacts (upper, basal and marginal)
  sharp, diffuse, irregular, gradational,
  channeled/scoured, faulted etc
• Photos of contacts
• Description of beds grading (normal, reverse,
  coarse-tail etc), thickness, bedding/laminations,
  lateral and vertical
• variations in all of these characteristics
• Photos and descriptions of structures. These
  could include ripples, dunes, bomb sags,
  desiccation cracks, convoluted
• lamination, slumps, rheomorphic folds, nature of
  jointing, imbrication, matrix-supported or
  clast-supported etc, banding
• (eg flow banding) pay particular attention to
  describing and measuring traction current
  structures
• Clasts nature (what are they, proportions of
  different types, ie G, C, L), size ranges,
  distribution of sizes, mean
• grain size max clast size shapes and
  distribution of shapes, sorting (size and density
  sorting), mineralogy, phenocrysts,
• vesicles in clasts, alteration
• Matrix nature of matrix clasts (if discernible)
  grain size, vesicles, alteration, cement, lateral
  and vertical variations
• Photos of clasts and matrix
• Samples of clasts and matrix (important to get
  matrix!)

5
Isopachs and Isopleths
Sieving is necessary to construct isopleths for
fine-grained deposits
Isopachcontour of same deposit
thickness Isoplethcontour of same max clast size
6
Facies Concept for Pyroclastic Deposits

• Facies originally defined in Gressly (1838) to
  refer to sum total of palaeontological and
• lithological characteristics of a stratigraphic
  unit. Used in many different ways since.
• Most useful definition below
• Facies concept developed mostly since 1970s and
  very widely used in sedimentology and volcanology
• This is a very powerful method for describing and
  interpreting all clastic rocks, so please use it!
• A facies is just a fancy usually non-genetic name
  for a group of rocks/deposits grouped together
• by several shared characteristics (usually at
  least two, including grain size, bedding/massive,
  fossils,
• chemistry etc). Lithofacies is most commonly
  used in volcanology group with shared
  lithological
• characteristics. Can be at any scale, but
  usually at bed scale
• Facies concept is all of these a usually
  non-genetic shorthand (facies codes), a way of
  looking for
• spatial association patterns (vertically or
  laterally, ie facies associations, eg certain
  type of PDC
• deposit always occurs below a certain fall
  deposit term facies successions refers to
  repeated
• patterns of facies in vertical sense), and a way
  of seeing 3-D spatial patterns (facies
  architecture)
• Facies Codes usually initially assigned in the
  field. Can be just a number (eg facies 1), but
  more useful

7
Allostratigraphy and Bounding Surfaces

• These concepts and terms more familiar to
  sedimentologists rather than volcanologists
• Can be used usefully with facies for all clastic
  rock sequences
• Allostratigraphic unit is unit (single facies or
  gt1 facies) bounded by a bounding surface
• Bounding surface is any discontinuity between
  (see Fig 2 in Roger Walker handout) units
• can be unconformity, onlap, downlap etc

8
Mapping Pyroclastic Rocks/Deposits

• Best to use (non-genetic) facies as map units
• Mapping on ground can be very difficult and/or
  time-consuming. Pick your areas well!!
• Ground-truthing airphoto interpretations is
  easier
• Mapping on large scale ground photographs (eg
  cliff faces etc) is particularly useful technique

9
Logging Pyroclastic Deposits

• Read p12-14 in McPhie et al.
• Pyroclastic deposits should be logged vertically
  and several logs fenced together
• Use standard symbols (see p12-14 in McPhie et
  al.) for fills or invent your own consistent
  symbols
• Record on the log thicknesses, grain sizes
  (mean), nature of bed contacts, structures such
• as ripples/dunes etc, and anything else you see
  as significant. Logs should have notes by side
  of them,
• dont just rely on the graphics!

10
Pyroclastic fallout basics

• Fallout is transport and depositional processes
  following eruption of
• pyroclasts that involves transport through
  atmosphere and/or water and deposition by
  suspension or ballistically onto land or water
• Suspension of pyroclasts arises from support from
  gas thrust, hot convecting air, juvenile steam,
  wind and water then loss of this support
• All eruption styles include both suspension and
  ballistic deposits
• Ballistic deposits decrease away from vent to
  some ballistic limit
• Suspension deposits can be globally-distributed
  if plume reaches
• tropopause
• All eruption styles (except Hawaiian and
  Strombolian) commonly
• generate associated PDC deposits
• Suspension deposits are better graded and usually
  finer-bedded if water-lain
• Ballistic pyroclasts may preserve bomb sags if
  impacting material was cohesive

11
Plinian fallout deposits
12
Pyroclastic falls sedimentary structures

• Fall deposits display sedimentary structures that
  are typically of
• grain-by-grain deposition (good sorting, grading,
  well-bedded, etc)
• Degree of sorting and grading typically increase
  with distance from vent

Reverse grading in Strombolian scoria
13
Grading

• Grading can
• be of several types
• Grading is typically
• better in fallout
• compared to flow
• deposits
• Grading in reworked
• deposits can be
• well-developed also

14
Ballistic emplacement
(Schmincke, 2000)
Proximal Plinian fall deposit with large
ballistically emplaced lithics
15
Ballistic emplacement
Bomb pit (plan view) (Crater Peak 1992 eruption)

• Bomb sags (x-section) are most common within the
  ballistic limit of
• phreatomagmatic centers (tuff cones, rings etc)
• Cohesive (wet) nature of high F deposits
  preserves evidence of impact better
• than drier coarser deposits

16
Structures in PDC deposits

• Huge variety of structures
• Erosion of underlying material (especially
  proximally and with high concn PDCs)
• Traction current structures (in low concentration
  PDC deposits)
• Fluid escape structures
• Size and density concentration zones (base, top,
  lee sides of obstacles or ripples/dunes)
• PCZs and fines-depleted areas are most common
• All types of grading
• All degrees of welding
• Bomb sags in proximal areas
• Thermal oxidation
• Vesicles in matrix (if welded or cohesive), can
  be large blisters in welded deposits
• U-shaped channels
• Plastering structures
• Etc

17
Reminder of PDC Terminology and Definitions

• Density currentgravity current
• Pyroclastic density current (PDC) spectrum of
  currents with variable gas/particle ratios
• High gas/particlepyroclastic surge
• Low gas/particlepyroclastic flow
• Ignimbritepumice-rich pyroclastic flow deposit
• Pyroclastic flows are often subdivided into
  block-and-ash
• flows, pumice flows (ignimbrites) and scoria
  flows, depending on nature of dominant clast type
• Nuées Ardentesblock-and-ash flows
• Pyroclastic surges can be subdivided into wet
  and dry

18
Reminder of Ignimbrite Terminology and Definitions

• Layer-concept subdivision of ideal
  ignimbrite into layers with
• different depositional regimes. Now applied
  to other ignimbrites
• PCZpumice-concentration zone (PCZ)
• Fines-depletedIgnimbrite deposit (or part of)
  where loss of fine
• ash has occurred
• Crystal concentration zone self-explanatory
• Ground-layer general term for any distinctive
  layer at base of an
• ignimbrite sequence (with a variety of
  origins). Deposited from head?
• Co-ignimbrite fallout Fallout emplaced during
  (or shortly after)
• flow (ignimbrite) emplacement from same
  density current
• Co-ignimbrite lag fallout Coarse fallout that is
  emplaced in proximal areas and was to
  coarse/dense to be transported in the flow
• Co-ignimbrite surge Surge deposits emplaced
  during (or shortly after) flow (ignimbrite)
  emplacement (esp if from same density current)
• Valley-Ponded Ignimbrite (VPI) Topo-ponded
  facies of highly energetic ignimbrites (eg
  Ultraplinian)
• Ignimbrite Veneer Deposit (IVD) Thin
  topo-mantling facies of highly energetic
  ignimbrites (eg Ultraplinian)

19
Ignimbrites terminology and definitions (2)

• LARIs and HARIs low and high aspect-ratio
  ignimbrites. LARIs
• are typically more hazardous
• Valley of Ten Thousand Smokes ignimbriteHARI
• Taupo Ignimbrite LARI
• Ash-flow tuff (common in US, but now getting
  obsolete)ignimbrite

20
Ignimbrite deposits (basics)

• Ignimbrite deposits are (by definition)
  pumice-rich. Bulk of this is lapilli-sized
• Deposits also include variable proportions of
  lithics, crystals and glass
• (some of which may be micropumice)

21
Ignimbrite deposits (bedding)
Bedding/laminations are common but usually poorly
developed in ignimbrites. Many ignimbrite
sections are massive Parallel beds/laminae are
most common. (particularly at base and top and
distally) Rheomorphic folding common in
welded ignimbrites
22
Ignimbrites flow units and cooling units

• Jointing or welding may indicate that several
  flow units
• cooled together (cooling unit)
• Flow unit represents individual density currents
  or lobes
• of same current. May also be from surging flow

23
Ignimbrites welding

• Some ignimbrites are highly welded (high grade
  ignimbrites) and
• look like lava flows with columnar jointing etc
• Some are unwelded
• Degree of welding varies spatially within deposit
• Welding is favoured by high T and thicker
  deposits and lower
• glass viscosities

Photomicrograph of flattened and welded glassy
pyroclasts in ignimbrite (Gran Canaria) Severe
stretching of fiamme indicates post-depositional m
ovement (ie rheomorphism)
Schmincke, 2000
24
Fines-depleted pipes in ignimbrite
Francis, 1993
25
Reminder on Pyroclastic surges terminology and
origins

• Terminology ash cloud surges, ground surges and
  base surges

Origins of pyroclastic surges

• 1. Ash cloud surges and some ground surges are
  co-emplaced with pyroclastic
• flow deposits
• Ash cloud surges are associated with underlying
  denser PDC flows
• Ground surge deposits underlie denser PDC
  deposits (pyroclastic flows)
• Ground surges may be produced by jetting in head
  region of pyroclastic flows
• or may just be precursor surges

• Surges may also be produced by column collapse or
  boil-over. Term
• base surge is used if a ring is produced at base
  of column. Most surges
• that are not associated with pyroclastic flows
  (eg base surges)
• involve magma-water interaction

26
Structures in Surge Deposits

• Bedded or massive
• Plane beds and laminae (lower and upper flow
  regime)
• TCS
• Antidunes
• Chute-and-pool structures
• Accretionary and armoured lapilli
• Vesiculated Tuffs
• U-shaped channels
• Plastering

27
Typical surge deposits at Hanauma Bay, Oahu
28
Examples of TCS in Surge Deposits
Surge dunes (Taal volcano)
Climbing ripples (ripple drift), Hanauma Bay, Oahu
29
Proximal to Distal Surge Bedforms
Note that some tuff cones/rings display
different sequences of lateral changes from
those illustrated here
Examples of lateral bedform changes in surge
deposits at Suwolbong tuff ring (South Korea).
Dark is lapilli and white is ash. Note
development of massive beds near crater, planar
beds more distally and dune beds in distal
regions. Note also reduction in wavelength of
dunes with distance
30
Describing Traction Current Structures in
Volcaniclastic Deposits

• Very important that TCS are described in great
  detail, as non-volcanicTCS are very common!
• TCS only found in sand-sized (medium ash to
  lapilli-sized) deposits
• Crest height
• Crest separation
• Sharp or rounded crests
• Morphology in plan view (planform)
• Lee and stoss-side (if preserved) angles of
  cross-beds/laminae. Very important as
• angles of lt15-20 degrees often taken to indicate
  antidune structures (more common in
• surge deposits than fluvial deposits)
• Scouring on lee side?
• Angle of climb in climbing ripples
• Associated concentration zones (crystals, lithics
  etc)
• Grain size changes down lee and stoss sides?
• Confusing TCS patterns can arise from
  interference of contemporaneous PDC currents or
• modification of earlier TCS
• Superimposition of smaller TCS on larger ones is
  common (eg ripples on stoss side of
• dunes)
• Gross morphology of TCS bed (planar, lenticular
  etc)

31
Ripple and Dune plan profiles (planforms)
(Collinson and Thompson, 1982)
32
Measurements of Ripples and Dunes
Collinson and Thompson, 1982
Hcrest height or amplitude
33
Bedform Phase Diagrams
To illustrate in 2D water depth is fixed
Pye (ed), 1992
34
Ripples, Dunes and Sandwaves Nomenclature and
Comments

• Ripples smaller than dunes, but otherwise same,
  though in sedimentary rocks there seems to be
  size
• gap in range 3-10cm height (not clear if this is
  true for surge deposits).
• Both ripples and dunes migrate by stoss side
  erosion and lee side deposition. Ripples only in
  
• sediment with mean grain size of lt0.6mm, above
  this size for same flow velocities lower regime
  plane
• beds form. Wavelength of ripples usually lt50cm.
  Wavelength of ripples and dunes declines with
• flow velocity
• At higher flow velocities ripples get bigger
  (amplitude), more sinuous crested (eventually
• linguoid) with scour pits, then become dunes.
  Coarser sediment also generates larger ripples
• Sandwave term sometimes applied to large simple
  straignt-crested TCS (i.e straight-crested
• dune) with amplitudes of several meters and
  wavelengths of hundreds of metres. Term dune
  usually
• applied to TCS (gt10cm) with more limited crestal
  continuity. Term sandwave has been used by
  some
• volcanologists to include all ripples and dunes!
• Straight-crested dunes produce tabular x-beds in
  section and sinuous-crested dunes produce tabular
• x-beds
• Bar is sometimes confusingly used to refer to
  bedforms transitional between ripples and dunes,

• Ripples are smaller than dunes but otherwise
  same. Ripples lt10cm crest height (or lt3cm)

35
Antidune and Chute-and-Pool Structures in Surge
Deposits

• These are upper flow regime (high velocity)
  structures that are probably more common
• in surge deposits than in fluvial deposits
• Can be very difficult to recognise and interpret
  (at least for me)
• Antidunes and chute-and-pool structures develop
  at highest flow velocities when standing waves
• (in-phase waves) erode plane beds (bedform and
  waveform in phase). Both are very unstable
• and rarely preserved
• Antidunes usually have low-angled cross laminae
  (less than 15 degrees) produced by periodic
• break-up and upstream migration of standing waves
• Antidunes should be initimately associated with
  upper regime plane beds
• Chute-and-pool structures develop at still higher
  velcities and have steep backset
• (upstream-dipping) laminae in scoured pools at
  downstream end of steep inclined eroded surfaces
• (formed in a hydraulic jump). Can be very
  difficult to recognise. More on this in the next
  class!
• Chute-and-pool structures described from flume
  experiments, fluvial rocks and recorded from

36
Sinuous-crested out-of-phase ripples in
pyroclastic surge beds near Koko Crater, Hawaii.
Crest separation is about 12cm
37
Accretionary and armoured lapilli

• These types of lapilli form
• in wet (steam-rich) plumes
• and involve a nucleus
• accreting ash as it moves up and
• down in the plume.
• Electrostatic forces and
• cementation by salts and/or ice
• during fall are important.
• They do not require rain, but
• abundant water/steam in column is
• important
• Largest examples are formed
• by sustained recycled support in
• column (convection, blasts) like
• hailstones

Accretionary lapilli (above) are lapilli-sized
subspherical clasts comprising concentric
layer(s) of ash around a small nucleus.
Armoured lapilli are typically larger, have a
larger nucleus and fewer (usually only one)
concentric layers (distinction is vague
though). Both occur in fall and flow
deposits Both can be deformed on impact or
compaction Both can have vesicles
38
Plastering and Vesiculated Tuffs
Vesiculated tuff
Many vesiculated tuff arise from compaction of
accretionary lapilli layers. Vesiculation can
make the tuffs mobile and erosive
Plastering of vertical surface by Surtseyan
fallout (ash-lapilli)
39
U-shaped channels in primary pyroclastic deposits
are commonly interpreted as evidence for surge
deposition. Based on a single paper by Fisher
(1977). Re-examination of type locality (Koko
Crater) demonstrates that they were eroded by hot
syn-eruptive debris flows and as such are only
evidence of very steam/water-rich plumes or
currents. Very similar Channels could of course
just be generated by rainwater run-off
40
Hammer handle rests on massive syn-eruptive
debris flow that eroded underlying surge and fall
deposits at Koko Crater, Hawaii
41
Syn-eruptive debris flows are vesiculated
(hot) and were derived from mobilized
accretionary lapilli beds upslope