Reworking of Pyroclastic DepositsRocks

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GEO3975 Pyroclastics Fall 2006
Reworking of Pyroclastic Deposits/Rocks
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Summary of Class

• This class could be a sedimentology course (!),
  but we will instead focus on selected reworking
• processes debris-flow-HFF-stream flow spectrum
  study of Pinatubo lahars and briefly discuss the
  entrance
• of PDCs into the ocean
• Stress importance of facies analysis/architecture
  and paleoenvironment reconstruction,
  understanding of
• geomorphology, run-off/infiltration and rainfall,
  wind on transport and deposition mechanisms for
  subaerial settings
• geomorphology, water depth and water flow for
  subaqueous settings
• Read handout of chapter 10 in Volcanic
  Successions and Chapter 3-5 in Sedimentary
  Rocks in the Field Atlas
• (both for background reading, not testable) and
  Carey chapter on Volcaniclastic Sedimentation
  Around Island Arcs
• in Encyclopedia (testable for next test)

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Reworking General Comments (1)

• Reworkingresedimentation (syn and post-eruptive)
  and epiclastic redeposition
• Essential to recognize reworked deposits. Many
  volcanoes spend more time in repose than eruption
• Resedimented and epiclastic deposits can dominate
  deposits/rocks at many volcanoes
• Erosion rates are highly variable, but typically
  about 0.1-1.0m per thousand years (could erode to
  ground a 3km high
• volcano in a few Ma)
• Understanding geomorphology, drainage, rainfall,
  winds etc is essential for subaerial settings
  geomorphology,
• ice flow, ice depth and water drainage for
  glacial settings geomorphology, water depth and
  flow for submarine
• settings
• Generally see increase in sorting, bedding, clast
  heterogeneity, roundness and decrease in grain
  size with distance
• from vent
• Explosive volcanoes commonly radically disrupt
  drainage over wide areas and lead to rapid
  bulk-up of clast
• concentrations in streams and rivers

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Reworking General Comments (2)

• Given high rates of reworking and large sediment
  volumes on many volcanoes then poor sorting and
• massive deposits/rocks dominate
• Distinguishing primary high-concentration PDC
  deposits from such reworked deposits is a common
  problem
• Metastable nature of glass and many igneous
  minerals and abundance of hot water
• mean that weathering and generation of can
  proceed at high rate, but chemical weathering on
  flanks may be
• bypassed due to high rates of sedimentation
• Clay, high rates of primary deposition,
  volcanic/tectonic uplift, locally high rainfall
  over edifice and
• widely variable mechanical properties of layers
  (eg pumice fall and lavas) can all
• significantly lower slope stabilities giving rise
  to various mass flows, including DADs, debris
  flows etc

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Thermoremnant Magnetization

• TRM very useful for distinguishing PDC from
  debris flow and HFF deposits in particular
• How does TRM work? Use magnetic minerals in
  coherent lava/intrusion or in pumice/glass clasts
• (most useful in latter)
• At temps below their Curie Point minerals
  (typically 500-600C) this field is fixed (ie
  remnant) on cooling
• Magnetites CT is 580C
• If all minerals in pumice/glass record same
  orientation and inclination, then deposit was
  emplaced above
• 500-600C
• Problems Many debris flows and PDCs have
  heterogeneous spatial and temporal T profiles

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Debris Flow-HFF-Stream Flow Spectrum

• With distance from vent clast concentration
  (during transport) generally decreases. Hence
  commonly
• see spectrum of debris flow-HFF-stream flow
  deposits with distance
• This spectrum of deposits forms alluvial fans or
  ring plains around majority of subaerial silicic
  volcanoes
• Stream flow on the fans is dominated by braided
  streams
• Understanding alluvial fans and braided rivers is
  therefore vital for understanding the lower
  elevations of many
• intermediate to silicic vvolcanoes

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Debris Flow-HFF-Stream Flow Spectrum
Smith and Lowe, 1991
Lahars

• Laharshyperconcentrated (flood) flow (HFF) and
  debris flow
• Note ideal HFF deposit has normal grading,
  parallel bedding, better sorting
• Ideal debris flow deposit is poorly sorted,
  reverse graded base

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What is a lahar?

• A lahar is a clastic deposit derived from
  volcaniclastic deposits that is
• emplaced by a debris flow or hyperconcentrated
• flood flow (HFF) mechanism
• May also include significant non-volcanic debris
  (soil, talus, till etc)
• Grain size populations vary enormously
• Lahars are just part of a large spectrum of
  secondary (reworked)
• processes that operate in volcanic areas (eg
  fluvial, aeolian, marine, glacial
• etc)
• Lahars by definition have water as their
  interstitial fluid
• They may be cohesive or non-cohesive (cohesive if
  gt3 clay/ash)
• Water may be hot (up to 100ºC) or cold (even ice)
• Clasts may be very much hotter (up to several
  hundred ºC)

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Triggering of lahars

• Post-eruptive lahars can be triggered by
• rain (especially heavy rain)
• snow/ice meltwater
• hydrothermal fluids
• river or lake dam burst
• bursts of subglacial water vaults (not directly
  related to an eruption)
• earthquake disturbance of wet slopes
• gravitational failure of wet slopes
• Syn-eruptive lahars can be triggered by
• pyroclastic deposits emplaced and mechanically
  mixed with snow or ice
• downslope mixing with water (and/or dewatering)
  of debris avalanche or PDC flows
• bursts of subglacial water vaults (during
  eruptive episodes)
• Surtseyan eruption (may be directly erupted or
  eruption-fed)
• Lahars are more likely to form on steep slopes
  (ie volcanoes) but can
• form on slopes as low as a few degrees

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Lahars transport processes

• Debris flows are laminar flows, HFFs have
  significant fluid turbulence
• Cohesion typically provides little support in
  most volcanogenic debris flows (as often not much
  clay-sized material)
• Debris flow (gt80 sediment by weight)
• HFF (40-80 sediment by weight)
• Streamflow (lt40 sediment by weight)
• Flow transformation is common (by bulking and
  dilution)

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Lahars downstream bulking and dilution

• Lahars erode their banks and bulk-up (mostly) by
  undercutting banks
• and to a much lower extent by picking up clasts
  from surface. Large-volume
• lahars that spillover the river valleys can
  incorporate large amount of
• vegetation (with attached debris)
• Bulking-up may cause an original stream flow to
  change downstream
• into a HFF or debris flow (ie lahar)
• Dilution can occur due to sedimentation and by
  incorporation of water

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Lahars depositional processes

• Deposition from ideal debris flows is mostly by
  en-masse freezing of a plug
• Deposition from ideal HFFs is mostly by
  gradual accretion from a
• density-stratified flow
• Note the similarity with the two ignimbrite
  depositional models!
• It is often not easy to be certain about debris
  flow vs HFF identification of a
• particular deposit

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Lahars Debris Flow deposits

• Poorly sorted, massive, matrix-supported, no
  traction structures, boulders common
• May have reverse-graded base (or be entirely
  reverse-graded)
• Dewatering structures (eg dish structures, pipes)
  may be present
• May have vesicles (mostly cold air) (often
  overlooked)
• May show some clast imbrication
• Clasts are typically angular to sub-angular
• May contain a wide variety of clasts
  (heterolithic)

Massive nature, poor sorting, vesicles and
dewatering structures all indicate rapid
deposition from high particle concentration flow
Human bone in lahar (Mexico)
Adobe mud bricks in Mexican lahar
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Debris flow deposit (lahar) Mehrten Formation, CA

• Note
• heterolithic
• angular clasts
• matrix-support
• massive
• poor sorting
• no grading

10cm
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Lahars HFF deposits

• HFFs are more dilute than debris flows
• They are laminar to turbulent flows. Lower clast
  concentrations
• allows for better bedding, sorting and grading
  than debris flow deposits
• HFF deposits are distinguished from debris flow
  deposits on basis of parallel
• bedding, better sorting, normal grading (above a
  possible reverse
• graded base) and better clast imbrication.
  Boulders not as common in HFF
• No traction current structures (as in
  streamflows)
• Debris flows traveling down water-filled
  stream/river valleys initially push
• the water ahead of them but eventually may
  incorporate some water, and
• dilute to HFFs or even normal streamflows (with
  traction current deposits)
• Dewatering structures may be present
• Vesicles may be present

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Typical Lahar and Associated Facies Sequence
HFF?/ Streamflow?
Debris Flow
Finer base of debris flow
Streamflow
Debris Flows
Merhten Fmn, CA
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Lahars hummocky topography
Lahars, non-volcanic debris flows, debris
avalanches and other types of high sediment
concentration flows commonly preserve hummocks on
their surface (as above)
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Pinatubo lahars

• The most intensive research on lahars
• has been focused on those of Mount
• Pinatubo in the Philippines

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Reworking of Pinatubo 1991-92 Eruption Products

• Study of reworking of Pinatubo products is
  applicable to many stratovolcanoes
• Fire and Mud (Newhall, ed) is very detailed
  study of this eruption. Best account yet of
  volcaniclastic
• reworking following a major pyroclastic eruption.
  This volume is available online at
• http//pubs.usgs.gov/pinatubo/contents.html
• About 6km3 of PDC and 0.5km3 of tephra fall
  deposited on flanks on June 15th 1991
• Rates of deposition and volumes of lahars
  exceeded predictions
• Sediment yields order of magnitude higher than St
  Helens in 1980
• Rain-induced. Typically linear relationship
  between rainfall and lahar volumes
• But heavy rain not only factor. Fine ash
  deposition and erosive stripping and burial of
  vegetation
• increased run-off enormously. Reduced
  infiltration by order of magnitude
• Old drainages exhumed within months and in 3
  months 0.8km3 of pyroclastics were reworked into
  alluvial fans.
• After 3 years about 2km3 was reworked.
  Deposition of large volume lahars still ongoing
  14 years later

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June 15th 1991 Pinatubo Plinian eruption. 2.5km
diameter caldera formed
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Most primary pyroclastics to north. Lahars
radially channeled down six or seven main river
valleys
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Pierson et al., 1996 figure
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Hot lahars (about 50ºC), Pinatubo (3 months after
primary deposition). Some plumes from collapse of
banks, others are from phreatic explosion pits
etc
USGS Open File image
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USGS Open File images
Initial rilling on June 25th 1991
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Dissection of pyroclastic deposits at Pinatubo (3
years after eruption). Note dominance of parallel
ribbed gullies on steep slopes, developed from
initial rilling. Main drainages are exhumed
older drainages
USGS Open File image
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Watershed disturbance led to damming of streams
and formation of several lakes (0.4km long
example on right)
Largest of these (gt8km2) developed on Mapanuepe
River (before and after images on right June 23
1991 and Sept 10 1991). This lake burst its dam
and partially drained in Sept 1991
USGS Open File images (Umbal and Rodolfo, 1996)
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Nature of Lahar Deposits at Pinatubo

• Debris flows were both cohesive (rich in ash) and
  non-cohesive (rich in pumice lapilli)
• (debris flows dont have to be cohesive!)
• Non-cohesive flows derived dominantly pumice
  fallout and cohesive flows from PF deposits and
• hydrothermallyaltered regions
• Debris flows transformed by water dilution and
  deposition of coarse lithics into HFFs distally
• and in the waning stages of lahar deposition
• Association with valley ponded lake/pond deposits
  was common.
• Lahar deposits interbedded with normal streamflow
  deposits, secondary PDC deposits, talus,
• channel/slope mass flows (eg slumps), phreatic
  explosion debris etc

Valley-ponded lakes recorded following many large
volume oyroclastic eruptins, eg after Santiaguito
1902 eruption (left, from Cas and Wright,
1987) and after St Helens, 1980 eruption etc
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USGS Open File image
Interbedded debris flow (massive) and HFF
deposits (stratified). Best illustrated in
upper part of RHS photo
Massive pumice-rich cohesionless debris flow
deposit, Pinatubo (August 20-21 1991 lahars)
USGS Open File images
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http//www.smate.wwu.edu/teched/geology/vo-Mt-Pin-
lahars.html
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What Happens When Pyroclastic Density Currents
Enter the Sea?

• Several studies of ancient uplifted deposits and
  most recently of recent nearshore cores from
  Montserrat
• (eg. Trofimov et al., 2006, Geology, 34, 549-552
  image below from this paper)
• Entrance into sea typically promotes efficient
  flow separation into denser base and
  water-supported finer top
• Fines-rich turbidite plumes develop from top
  parts of flows. Formed about 60 vol of
  offshore sediment from
• PDCs at Montserrat
• Efficient separation of fines means subaqueous
  PDC deposits are skewed to coarser mean grain
  sizes
• in proximal areas
• At Montserrat PDCs that reached coast are
  dominantly BAFs. Denser valley-confined bases of
  these
• retained steep-ridge like morphology on entering
  water
• At Montserrat hydrovolcanic (FCI) explosions
  occurred when PDCs reached shore. Low density
  PDC
• currents traveled inland from these eruptions