FIELD TRIP Viernes, Oct 12th Andesitic sequence breccias, falls

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FIELD TRIP - Viernes, Oct 12thAndesitic sequence
(breccias, falls eroded lava dome) Meet in
Dept car-park at 0830am
Culebra - subaerial basaltic flows
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PYROCLASTIC FALL DEPOSITS

• In some cases, the volume of pyroclastics erupted
  is far greater than the amount of lava erupted
• DEFINITION - material falling to Earth from
  eruption column
• Fresh air-fall mantles landscape (even
  thickness)
• Often thin film BUT covers 1000s of kms high
  volume
• Deposits lt 2cm rarely preserved (rainfall!)
• Construct Isopach Map contour lines of equal ash
  thickness

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FALL DEPOSITS

• Important parameter Grain size
• NOTE Best general term for fall material Tephra
• Wide range of clasts present
• Important parameter Maximum clast size
• Collect 10 largest visible clasts in field
• For finer deposits Sieving (degree of
  sorting)
• Fall deposits generally well-sorted
• From granulometry construct Isopleth map
• lines of equal clast size

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• Typical fall deposit - narrow size range, strong
  central peak (1mm) - v. well sorted
• Typical PDC (pf) deposit - v.poorly sorted
• Samples from Bandelier Tuff, Valles Caldera

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Bandelier Tuff (aka ignimbrite)
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PYROCLASTIC FALLS

• 3 other useful parameters measured
• 1/ Vesicularity of clasts volume fraction of
  vesicles
• Dry magmatic eruptions vesicularity 70 to 80
• Magma-water eruptions vesicularity highly
  variable
• 2/ Crystal concentrations distinctive
  distribution patterns in some ignimbrites where
  crystals blasted out of glassy froth. NOTE
  Crystals are denser than glass.
• 3/ Lithic content most pyroclastic deposits
  contain lithic fragments ripped off wall of vent
  conduit
• Note the content size variation (important for
  interpretation!)

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PYROCLASTIC FALLS

• Distinguish eruptive activity sieving deposits
• Technique of Pyle plot dec. in thickness from
  vent decrease in fragment size
• These 2 parameters measure of column height
• eruptive intensity
• Parameters can be calculated from Isopleth Maps

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• Pylogram for classifying pyroclastic fall
  deposits
• Bc is the clast half-distance HT is eruption
  column height

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Hawaiian Deposits

• Hawaiian eruptions small volume of tephra
• Typical event gas-rich magma spray liquid lava
  into air as fire-fountains
• Clasts ejected at high speed fall to ground as
  liquid
• Globs of lava weld together spatter cones
  (10s of meters high)
• Large globs thrown further splat onto ground
  as cow-pat bombs
• Molten basaltic lava v. low viscosity (ie 100 Pa
  s)
• Ejected lava shaped by surface tension Pelees
  tears (teardrop shaped) Pelees Hair (fine
  strands) stretched out by the wind

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Mauna Ulu, 1969
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• Reticulite - extremely vesiculated
  basaltic froth

IMPORTANT Plasticity of basalt magma minor
volume of ash !
Pelees Hair - strands of basaltic glass
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Strombolian Eruptions

• Also basaltic different flow regime
• Typical products scoria cinders
• Clasts are vesiculated v. angular
  grey-black
• Lava fragments easily not much ash!
• Erupted volumes small (lt0.01 km3)
• Eruption column rarely exceeds few 100 meters
  above vent
• Scoria cones develop large blocks bombs
• regular layering formed by pulsations in
  eruption

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Spatter around scoria cone, Mt Etna
Cinder cone along rift on Mount Cameroon
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Strombolian Eruptions

• Volcanic Bombs globs of viscous lava
  aerodynamically shaped during fallout
  (spindle bombs)

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Strombolian Eruptions
Rhythmic layering in scoria cone, Costa Rica
Roadcut through scoria cone, note large bombs
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Isopach
Isopleth
Circular tephra maps typical of scoria cone
eruptions
Scoria inundating house in Heimaey, Iceland 1973
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VULCANIAN DEPOSITS

• Vulcanian events variable, small cannon-like
  blasts to sustained explosions (many hours)
• Eruption columns lt 10 km high
• Large ballistics near-vent (lt 1 km)
  fine-grained further away
• Generally small volume deposit with limited
  dispersal
• Famous vulcanian events isle of Vulcano, Italy
• 1880s 2 meters of fine ash meter size lava
  bombs
• Characteristic Breadcrust bombs (cooled exterior
  with molten, expanding interior)

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VULCANIAN DEPOSITS
Crater rim of Vulcano Dark layers from 1888
eruption
Breadcrust bomb
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VULCANIAN DEPOSITS
Vulcanian plume from Lascar volcano, 1986 c.15 km
high
Vulcanian explosion, Monty 1997
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VULCANIAN DEPOSITS

• Vulcanian events typically andesite dacite
  magmas

common in Pacific Ring of Fire
Eg. Soufrière Hills Volcano - 88 Vulcanian
explosions in 2 months (1997), cyclic
activity (8 hours )
Deposits of finger-shaped pumice flows (pumice
levèes) lapilli ash
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PLINIAN DEPOSITS

• Eruption column gt 30 km high
• Plinian eruptions sheet-forming deposits

Extensive mantle landscape evenly
Well sorted angular
Plinian fall - angular pumices
Roadcut near Arequipa, Peru
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PLINIAN DEPOSITS

• Plinian deposits thicken towards vent
• Vent may be negative topographic feature caldera
• Caldera location not always obvious (erosion of
  walls)

Pastos Grandes caldera, SW Bolivia
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PLINIAN ERUPTIONS

• Type example - AD 79 Vesuvius eruption
• Witnessed first described by Pliny the Younger
• Several distinct horizons in stratigraphy (on
  Roman soil)
• Initially, thin layer of fine ash small
  explosions
• Overlain by main tephra fall thick Plinian
  pumice fall

grey pumice
Pumice color changes from white to
grey Compostional zonation in magma chamber
white pumice
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ISOPLETH maps Eg. 15 cm isopleth White pumice
much closer to vent indicates column height
increased as eruption progressed Calculations
suggest column height was 27 km in white pumice
phase increased to 33 km during grey pumice
phase
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Bay of Naples
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VESUVIUS ERUPTION

• Increasing size of lithics as eruption continued
• further evidence for increase in intensity
• All isopleths elongate to SE Strong NW wind

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VESUVIUS ERUPTION

• In Pompeii, 1.3 m of white pumice fell -
  phonolitic magma
• Overlain by 1.2 m of grey pumice layer (more
  mafic)
• Researchers measured variations in Max. pumice
  lithics in exposures around the volcano
• Magma volumes
• White pumice 2.5 km3 (1km3 DRE)
• Grey pumice 6.4 km3 (2.6 km3)

Total 3.6 km3 magma
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VESUVIUS ERUPTION

• CHRONOLOGY
• Phonolitic magma chamber at 3 to 5 km depth
• Chamber was compositionally stratified
  volatile-rich overlying more mafic magma (less
  volatiles)
• Early Stages - magma interacted with groundwater
  (phreatics)
• Conduit opening explosive decompression
  Plinian activity, eruption plume as high as 27
  km, white pumice fall
• Widening of vent higher mass eruption rates of
  more mafic magma (grey pumice). Eruption column
  33 km
• Column collapse pyroclastic flows major
  devastation
• eruption waned stopped