Magma fracture and lava dome collapse

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Magma fracture and lava dome collapse

• Hugh Tuffen

Dome collapse at Colima, Mexico in 1991
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Outline

• Lava dome eruptions - overview
• When does dome lava break?
• Shear fracture in conduits and domes
• Fracture, seismicity and degassing
• Mechanisms and consequences of collapse
• -gravitational collapse
• -collapse due to gas pressurisation
• -rainfall-triggered collapse
• Some unresolved issues

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An overview of lava dome eruptions

• Domes of silica-rich lava (andesite-rhyolite),
  typically tens-hundreds of metres high
• Domes grow over months-years, punctuated by
  collapse events and explosive eruptions
• Highly hazardous can generate pyroclastic flows
  that destroy settlements on volcano flanks
• High viscosity magma 106 to 1014 Pa s (due to
  high SiO2, plus degassing, crystallisation and
  cooling)
• Examples include Unzen, Montserrat, Colima, Popo,
  Merapi MSH
• Key problem how to predict dome collapse
• and explosions (e.g. Sparks 2003 EPSL Frontiers)

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An overview of lava dome eruptions
Unzen lava dome in Japan, surrounded by
pyroclastic deposits
The lava dome at Colima, Mexico
pyroclastic flows generated by dome collapse
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An overview of lava dome eruptions

• Two styles of dome growth endogenous and
  exogenous
• There may be several or dozens of phases of dome
  growth and collapse some endogenous, some
  exogenous.

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An overview of lava dome eruptions

• e.g. Montserrat 1997 e.g. MSH 2004-
• dome inflates from within isolated spines
  emplaced along shear zones

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An overview of lava dome eruptions

• Will shear zones develop in the dome? If so, an
  exogenous eruption will take place.
• But how do these shear zones form?

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Shear fracture of magma

• Deforming magma may either flow or fracture
• High temperatures, low strain rates flow
• Lower temperatures, high strain rates fracture
• This is due to the viscoelasticity of the melt,
  as described in papers by Don Dingwell. The
  transition from liquid-like to solid-like
  behaviour in silicate melts is known as the
  glass transition

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Shear fracture of magma

• The glass transition flow or fracture in magma

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Shear fracture of magma

• From deformation experiments by Dingwell and
  Webb silicate melts will fracture if strain rate
  ? viscosity is greater than a certain value (108
  Pa).
• This is because viscous flow of the melt is too
  slow to allow the stresses to relax so the
  stresses rise until the melt fractures.
• We can now think about whether and where magma
  will fracture!

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Shear fracture of magma

• high strain rates and lower temperatures at
  conduit wall ? fracture

fracture of the magma
velocity profile during Newtonian flow
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Shear fracture and faulting in lava
a
d
c
b
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Shear fracture and faulting in lava
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Fault-controlled silicic lava eruptions?
Santiaguito, Guatemala, from Bluth Rose, 2004
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Magma fracture and exogenous dome growth

• When will exogenous dome growth occur? when
  shear zones propagate from the conduit wall into
  the dome (Hale 2005 PhD thesis and 2004 IAVCEI
  abstract)
• Shear fracture of magma in the conduit generates
  these shear zones.
• There is much discussion around how factors such
  as magma discharge rate influence this exo/endo-
  transition

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Magma fracture and seismicity
EQs were occurring within a small volume (/- 40
m) and need a repeated trigger mechanism

• rgrg

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Magma fracture, seismicityand dome collapse
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clustering of events prior to collapse
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How and why do domes collapse?

• Due to gravitational instability.
• which may or may not be assisted by
• 1) Gas pressurisation of the dome
• or
• 2) Rainfall on the dome surface

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Gravitational instability of lava domes

• from Voight (2000) Phil Trans Roy Soc A 358,
  1663-1703

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Gravitational instability of lava domes

• from Voight (2000) Phil Trans Roy Soc A 358,
  1663-1703

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Gravitational instability of lava domes

• how much of the dome will collapse?

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Minor collapse MSH Dec 05
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Minor collapse MSH Dec 05
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Minor collapse MSH Dec 05
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Major collapse Montserrat, June 1997
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Major collapse Montserrat, June 1997
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Major collapses can lead to explosive eruptions
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Gas pressurisation

• Dome and shallow conduit pressurised by volcanic
  gases (volatile-rich magma)
• Tilt cycles cyclic pressurisation of dome (e.g.
  Voight et al. 1999 Science)
• At Montserrat, collapse events related to
  pressurisation
• But, how did pressurisation lead to collapse?

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Gas pressurisation

• Elsworth and Voight models

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Gas pressurisation

• Elsworth and Voight models

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Gas pressurisation and weakening of the dome

• Weakening of dome lava during cycles of
  pressurisation?

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Progressive damage cracking leads to failure?

• from Kilburn Voight 1998, GRL 25, 3665-3668.

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or strength reduction due to accelerated seismic
events?

• Alternative explanation fault strength increases
  between events (healing of magma).
• Therefore, accelerate the slip events ?weaken the
  fault!
• Reference Tuffen et al. (2003), Geology
  311089-1092.

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Other explanations?

• Another possible explanation is that volcanic
  gases repeatedly injected into the dome are
  chemically attacking the lava and weakening it.
• Maybe there are other explanations too?

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Rainfall-triggered collapse

• Dome collapse may also occur during periods of
  intense rainfall
• Small collapses during repose
• Major collapses gt90 of the dome
• Hypothesis saturation of the dome carapace with
  rainwater impedes gas loss from the dome interior
• ?gas pressure builds up inside dome,
  destabilised.

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Magma fracture and lava dome collapses

• We have seen that magma fracture has a huge
  influence on how domes behave and collapse
• 1) The formation of shear fractures controls the
  exogenous-endogenous transition
• 2)Gravitational failure requires the dome lava to
  fracture
• 3) If domes do collapse, unloading may trigger
  explosive fragmentation of magma in the conduit
• 4) Fracturing also allows gas to escape and
  triggers shallow volcanic earthquakes

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Magma fracture and lava dome collapses

• Plenty of unresolved issues that are currently
  being addressed (work in progress).such as
• when is dome growth exogenous/endogenous?
• what is the strength of hot dome lava?
• how is lava strength affected by alteration and
  cracking?
• how does seismicity link in with dome collapse?
• do earthquakes trigger collapses or vice versa?

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THE END
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• The following slides were prepared in response to
  questions from the class

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Magma fracture and lava dome collapse some
images for your questions

• Hugh Tuffen

Dome collapse at Colima, Mexico in 1991
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• 1.

1 cm
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Rainfall-assisted collapse

• 1.

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Families of LF events
different families

• One family

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Different types of event
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Factor of safety and collapse
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Fracture experiments