1	Magma fracture and lava dome collapse Hugh Tuffen
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
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 à fracture
12	Shear fracture and faulting in lava
13	Shear fracture and faulting in lava
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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
17	Magma fracture, seismicity…and dome collapse
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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	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
32	Gas pressurisation and weakening of the dome
33	Progressive damage – cracking leads to failure?
34	…or strength reduction due to accelerated seismic events?
35	Other explanations?
36	Rainfall-triggered collapse
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?
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