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Magma fracture and lava dome collapse

Magma fracture and lava dome collapse. Hugh Tuffen. Dome collapse at Colima, Mexico in 1991. 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:

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Magma fracture and lava dome collapse

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  1. Magma fracture and lava dome collapse Hugh Tuffen Dome collapse at Colima, Mexico in 1991

  2. 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…

  3. 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)

  4. 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

  5. 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.

  6. An overview of lava dome eruptions • e.g. Montserrat 1997* e.g. MSH 2004- dome inflates from within isolated spines emplaced along shear zones

  7. 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?

  8. 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

  9. Shear fracture of magma The glass transition: flow or fracture in magma

  10. 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!

  11. Shear fracture of magma high strain rates and lower temperatures at conduit wall  fracture fracture of the magma velocity profile during Newtonian flow

  12. Shear fracture and faulting in lava a d c b

  13. Shear fracture and faulting in lava

  14. Fault-controlled silicic lava eruptions? Santiaguito, Guatemala, from Bluth & Rose, 2004

  15. 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…

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

  17. Magma fracture, seismicity…and dome collapse

  18. clustering of events prior to collapse

  19. 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

  20. Gravitational instability of lava domes • from Voight (2000) Phil Trans Roy Soc A 358, 1663-1703

  21. Gravitational instability of lava domes • from Voight (2000) Phil Trans Roy Soc A 358, 1663-1703

  22. Gravitational instability of lava domes how much of the dome will collapse?

  23. Minor collapse: MSH Dec 05

  24. Minor collapse: MSH Dec 05

  25. Minor collapse: MSH Dec 05

  26. Major collapse: Montserrat, June 1997

  27. Major collapse: Montserrat, June 1997

  28. ambient P and T DP up to 30 MPa, T=25-900°C Major collapses can lead to explosive eruptions

  29. 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?

  30. Gas pressurisation • Elsworth and Voight models

  31. Gas pressurisation • Elsworth and Voight models

  32. Gas pressurisation and weakening of the dome • Weakening of dome lava during cycles of pressurisation?

  33. Progressive damage – cracking leads to failure? • from Kilburn & Voight 1998, GRL 25, 3665-3668.

  34. STRENGTH -- weakening  shear stress time …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 31:1089-1092.

  35. 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?

  36. Rainfall-triggered collapse • Dome collapse may also occur during periods of intense rainfall • Small collapses during repose • Major collapses: >90 % 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….

  37. 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

  38. 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?

  39. THE END

  40. The following slides were prepared in response to questions from the class

  41. Magma fracture and lava dome collapse – some images for your questions Hugh Tuffen Dome collapse at Colima, Mexico in 1991

  42. 1. 1 cm

  43. Rainfall-assisted collapse • 1.

  44. Families of LF events One family different families

  45. Different types of event

  46. Factor of safety and collapse

  47. Fracture experiments

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