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Lava Dome Collapses Part II
When and why lava domes collapse
Eliza Calder, SUNY, Buffalo
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Islands old capital - Plymouth
SHV lava dome
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Damage since 1995

• Half of island rendered barren and uninhabitable
• 2/3 population left
• 19 lives lost
• 92 people were evacuated, many suffered multiple
  evacuations and displacement
• Economy severely affected by loss of
  infrastructure, farmland, and adverse impacts on
  tourism
• Former capital, Plymouth, buried under metres of
  debris and several villages swept away
  pyroclastic density currents.

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First, a bit more about Lava Dome Growth

• Andesite
• Average extrusion rates 2-3 m3/s, but unsteady
  extrusion
• 3 dome growing phases, with 2 year pauses in
  between

Eruption began 18 July 1995-ongoing 1st Phase
dome growth mid-November 1995 to mid March
1998 (20-month interval of degradation) 2nd Phase
of dome growth mid-November 1999 to July
2003 (26-month interval of degradation) 3rd Phase
of dome growth Aug 2005 to ongoing
Dome growth /intermission periods
Phase I 28
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Phase II 44
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III 18
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and shear lobes
Crater rim
shear lobe
collapse scar
Shear lobe extrusion, determines
location/direction and timing of collapses
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1 What are the various Collapse
Phenomena (terminology) Rockfalls -
quasi-continuous during extrusion Discrete
pyroclastic flows - i.e. individual pyroclastic
flows, of a modest sizefew min durations Major
Collapse events - retrogressive failures 1-210 x
106 m3 Remove significant portion of dome - are
sustained for several hours. Debris Avalanches
- collapse of section of old edifice (crater
wall) and new dome (these last two types can
generated major submarine avalanches)
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Rockfalls
Note also
Solid lava
talus

• Montserrat lava dome 1997, extruding 3 m3/s
• Rockfall from actively growing face
• continuum into small pyroclastic flows

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Deposits from rockfalls small pyroclastic flows
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Discrete pyroclastic flows

• Modest pyroclastic flow - this one is valley
  confined
• Upper buoyant ash plumes 100s m -gt 14 km

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• Nose of pyroclastic flow ( 2 m high), in White
  River valley
• Concentration of coarse material in basal
  avalanche
• elutriation to produce overriding ash cloud
  surge/lofting ash plumes

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Components of these flows
Denser than air - continues as gravity current
with horizontal component of motion
Buoyant plumes
Overriding turbulent ash cloud
Fragmentation Fluidization Elutriation
Dense basal avalanche
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The internal structure of the basal
avalanches (Inferred from vertical sequence of
zones in experimental grain flows)
Turbulence created by grain saltation facilitates
entrainment of fine particles into the overriding
ash cloud.
Velocity and bulk density gradients
Flow rheology and resistant stresses determined
by short-lived inter-granular collisions
Smeared out during deposition as friction
consumes momentum at base while high portions
continue to move forward
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Granular avalanche
Energetics of the flows
Locally fluidised (by topography) -gt elutriation
High velocity flow Strong turbulent mixing - but
maintains sharp interface with ash cloud
High velocity pervasive turbulence vertical
concentration gradient
-gt Whole range observed in Montserrat
flows Depends on volume of collapsed materiel,
source depth within dome and degree of gas
pressurization of collapsed material
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Major collapse events
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4 energy peaks
20 May Collapse
LPRF
RSAM - 1 min Measurement of average seismic
amplitude
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Major collapse event - Deposits
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Major collapse event - Deposits
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Major collapse event - Deposits A 2.4 km2, V 9
million m3
Pumice flow deposits
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Mapping deposits of the same flow21 September
1997 Collapse Deposits
Dome
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Dome up here somewhere
Debris Avalanche - 26 Dec 1997 90 degree sector
of the island devastated
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Map of Debris Avalanche deposit
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• Lava dome collapse flows - summary
• Emplaced as highly concentrated avalanches with
  overriding dilute ash clouds.
• Basal avalanche follows valleys (unless filled
  in)
• Ash cloud surge, less topographically constrained
  /can decouple at bends etc.
• Ash clouds surge component varies considerably in
  flows of different volumes. Small volume flows,
  tend to have minor surge component generated by
  elutriation from the underlying avalanches.
  Larger volume flows develop violent surges,
  derived at collapse onset by gas decompression of
  internal parts of the dome as well as during the
  propagation of the flow.

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2 Flow Mobility Different flow types -
different mobility
We also have..column collapse pumice flows from
Vulcanian explosions
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Pumice flow deposits
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Flow mobility and runout relationships Runout
can be related by simple frictional arguments to
height dropped by L H/tan a Where
H/L the coefficient of friction or Heim
coefficient and a internal angle of friction,
where flow only occurs where angle is gt a .
H
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• Surges and column collapse flows are
  systematically more mobile (they spread further
  for a given volume) than dome collapse flows and
  avalanches.
• Order of magnitude difference in their mobility
  ratios
• These are expanded currents where fluidization
  and/or turbulence play dominant role in particle
  support mechanism.

Geometric Mobility parameter
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Plan-shape parameter
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Summary of observations of pyroclastic density
currents on Montserrat
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• Mobility Depends on
• Starting conditions
• Height of fall/volume
• Degree of gas pressure
• 2. Ingredients
• Large dense blocks/ash
• Small light pumices/ash
• 3. Flow mechanisms
• Disintegrating solid -gtflow
• Dilute cloud which condenses with time -gtflow

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3 Frequency-Magnitude Relationships Characterize
distribution Controls on distribution -
understand how it might change Forcing by
extrusion rate fluctuations Upper limit defined
by size of dome Morphology controls
direction Buttressing - important Use of
distributions Probabilistic hazard
mapping Recurrence probabilities
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I am interested in the history of collapses of
different sizes through the eruption ie the
continuous mass-wasting process as the dome
grows. We might think of this in terms of a
sandpile growing as, sand is added on the top.
The frequency and size of granular avalanches
that cascade down the surface relate to the rate
at which material is added to the pile.
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The relationship between rockfalls and extrusion
rate
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• Frequency- Magnitude Relationships of collapses
• Dome Collapse Volume data
• -Volumes only calculated for largest events gt 1 x
  106 m3, 34 or so events for 12 year eruption
  period.
• Small events very frequent - no volumes but do
  have, seismic energy, duration, - can be used as
  a proxy for magnitude.

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Rockfall events that occurred 1996-1999. Seismic
data does not distinguish between rockfalls and
pyroclastic flows
Observational data i.e. measured volumes for
larger ( gt 106 m) that occurred 1996-1999. n
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Volume x 106 m3
Repose days
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Frequency distribution affected by extrusion rate
Also..Freq of events in 1yr, 5yrs, 20 yrs etc -
will be determined by the extrusion rate
variation during those time periods.
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Collapse direction and Buttressing
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Morphological controls on collapse
direction Horseshoe crater Buttress Most
activity (dome growth and collapse) focused to
East. When dome is large/high, directions are
more variable
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Spatial Distribution of Collapses From
observations of collapses with pfs gt 2km during
1st and 2nd dome growth episodes.-gt dome needs to
achieve a summit altitude 950 m asl before
material can be shed to N, W and S. With no
height barrier for collapse to E.(Wadge et al,
2006) Directional weighting derived by Time
spent where dome growth in quadrangle -----------
------------------------------------- Freq of
Obs flows/day in quadrangle
N
E
S
W
(Wadge et al, September 2006)
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Tracking directions of the rockfalls in real-time
time
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-gt Hazard mapping, PYROFLOW in Monte Carlo mode
using directional weighting (Wadge et al,
September 2006) -gt Note probability of inundation
in Plymouth falls
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4 Prediction and collapse precursors
Despite the efforts towards understanding lava
dome collapses, the propensity for lava domes to
collapse with little or no warning remains a
serious hazard that monitoring bodies have to
deal with. gt Prediction - start by identifying
precursory activity
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4 Prediction and collapse precursors Characteri
ze distribution Controls on distribution -
understand how it might change Forcing by
extrusion rate fluctuations Upper limit defined
by size of dome Morphology controls
direction Buttressing - important Use of
distributions Probabilistic hazard
mapping Recurrence probabilities
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Precursory Signatures
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4 to 18 h cycles
10-25 micro radian 140 - 320 k m3 magma
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Collapses Volume Accounting
time
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Collapses Volume Accounting
time
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• Lastly, Back to - Collapse Mechanisms
• Oversteepening
• Gas pressurization
• Rainfall
• But also important
• Talus erosion and undermining especially
  associate with rainfall (next 2 slides)
• Thrust forces associated with intrusion into base
  of dome
• (as seen during cycles)
• Seismic acceleration during earthquake swarms
• (only generates rockfalls/small flows)

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..Destabilization of the talus slope two types
of avalanches down sandpiles (from experiments)
Daerr Douady, Nature, 399, 241 (1999)
Triangular (downhill)
Balloon (uphill)
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..also lets recall whats under a big dome -
The exact character of the internal contacts is
this area Is not clear and probably does not
resemble this ! BUT what must be the case, is
that large domes are at least in part constructed
over an unconsolidated base.
..so undermining and weak base important too
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So, now weve got through a few more of these
topics -

• and, Im looking forward to talking on Thursday !