PYROCLASTIC ERUPTIONS:

1
PYROCLASTIC ERUPTIONS

• One of most awe-inspiring natural phenomena on
  Earth!

Augustine volcano, Alaska
Nuclear weapons test
2
PYROCLASTIC ERUPTIONS

• Vesiculation of magmatic volatiles driving force
• Other factors magma viscosity (changes on
  ascent)
• mass eruption rate
• crater geometry
• In last decade, major leap in progress in
  mathematical modeling
• Link between observations understanding
  critical processes
• Important for volcanic hazard assessment
  (AVIATION)

3
PLUME BASICS

• Eruption columns 3 parts
• 1/ Gas Thrust Region (immediately above vent)
• 2/ Convective Zone (hot gases carry fine ash)
• 3/ Umbrella Cloud

4
ERUPTION COLUMNS
Buoyancy carries plume to height HB. Momentum to
height HT.
5

• GAS THRUST ZONE
• As magma ascends it rises up conduit
• vesiculates fragments
• accelerates as mix of gas
  particles
• released at vent (high speed)
• Highest ejection velocities Plinian eruptions
• Silicic magma (5 volatiles)
• On bursting into atmosphere v. different
  chemical/physical environment
• Air is heated by pyroclasts expands
• If heat exchange v. efficient particles move
  into Convective Zone

6
CONVECTIVE REGION

• Convecting eruption column super-buoyant

Column rises 10s of kms for long period
(possibly crossing into stratosphere)
Buoyancy due to heating of air by ejecta
As more air entrained plume widens in
area temperature decreases On rising,
atmosphere less dense density contrast lowers
until plume is equal to air
NOTE Small changes in vent radius mass
eruption rate will produce big changes to plume
7
UMBRELLA REGION

• Above neutral density layer horizontal spreading
  predominates
• Plume intrudes as gravity layer (in all
  directions incl. upwind)
• Characteristic mushroom-cloud develops
• NOTE The base of Umbrella Cloud is where zone
  of sedimentation of tephra occurs.
• Blankets of ash larger tephra can spread over
  entire countries
• EXAMPLE Mount Pinatubo, Phillipines 230, 000
  km2 land covered

8
UMBRELLA CLOUD OF MT PINATUBO, June 15 1991
1331
1431
1531
1631
Max. dia. of plume 550 km
9
ERUPTION COLUMN HEIGHT

• ERUPTION INTENSITY (or discharge rate) is
  critical factor controls measure of thermal
  energy
• Satellite imagery allows estimation of both
  eruption height or intensity (look at length of
  shadows cast by plume)
• Use equation HT 1.67 Q 0.259

From observations of eruption columns
10
COLUMN HEIGHT OF PINATUBO ERUPTION
From imagery
From equation
Integrating data - volume of 3.5 km3 for
climactic phase
11
FALLOUT DISPERSAL

• Pyroclastic deposits provide several parameters
  for field geologists to measure
• Eg Maximum clast size
• Large clasts higher density, fall closer to
  vent
• As eruption column increases, clast dispersal
  also increases
• ISOPLETHS - lines joining points of maximum clast
  size
• Isopleths drawn on map clast distribution

12
CLAST SIZE DISTRIBUTION
Isopleths more widely spaced in higher columns
13
CLAST FALLOUT

• WIND - In strong winds, v. large eruption columns
  still show near-circular isopleths punch into
  stratosphere
• In weak winds, greater influence on small
  eruptions
• ISOPLETH MAPS From field measurements of clast
  sizes, critical parameters of the eruption can be
  inferred
• Eg. Distance of isopleth from vent measured along
  wind direction is function of column height
  wind speed (useful for prehistoric eruptions)

14

• A. Isopleth map for powerful eruption. Column
  43km

B. Isopleth maps for small eruption, column
14km (at different wind speeds)
NOTE Downwind extension influences smaller
columns
15
APPLICATION

• Understanding eruption column development
• Important for potential effects of tephra fallout
  on communities at risk

Eg. Modeling effects of tephra fall at Mt
Vesuvius, Italy
AVIATION Volcanic ash melts onto engines thus
disabling them Eg 747 jet flying over Java had
all 4 engines stall when flying through plume
from Galunggung volcano (1982) Eg 2 747 jet lost
all engines flying through plume of Mt Redoubt
(1990). NO MAJOR DISASTER YET.!
16
MT REDOUBT plume (1990)
17
PYROCLASTIC DENSITY CURRENTS

• What goes up MUST come down!
• IF particles from explosion are DENSER than air
• column collapse (gravitationally)
• If v. energetic hot continue down flanks of
  volcano as a PDC
• PDCs also form LAVA DOME COLLAPSE
  VOLCANIC BLASTS

18
COLUMN COLLAPSE pyroclastic flows Montserrat
(Sept 1997)
Texture - pumiceous lava blocks
19
PYROCLASTIC FLOWS - from DOME COLLAPSE
Pyroclastic flows (PFs) composed of dense basal
avalanche (800º C blocks) and turbulent ash-cloud
surges (at 350ºC)
PFs can travel 70 mph carry enormous blocks
5 km
Texture of blocks - dense, crystalline lava
20
Deposits from dome collapse Block Ash Flows
Angular dense lava blocks in ash matrix
21
DEPOSITS FROM COLUMN COLLAPSE (Plinian scale)

• Thick sheets of pumiceous IGNIMBRITE containing
  pumice, lithics, ash crystals

Large PFs collapsing from Plinian-style eruption
column
Unwelded ignimbrite