EGU2020-14016
https://doi.org/10.5194/egusphere-egu2020-14016
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Evolution of mechanical properties of lava dome rocks across the Soufrière Hills eruption, and application in discrete element models

Claire Harnett1, Jackie Kendrick2, Anthony Lamur2, Mark Thomas3, Adam Stinton4, Paul Wallace2, and Yan Lavallee2
Claire Harnett et al.
  • 1UCD School of Earth Sciences, University College Dublin, Dublin, Ireland.
  • 2Department of Earth, Ocean and Ecological Sciences, University of Liverpool, Liverpool, UK.
  • 3School of Earth and Environment, University of Leeds, Leeds, UK.
  • 4Montserrat Volcano Observatory, Flemmings, Montserrat.

Lava dome collapses pose a hazard to surrounding populations, but equally represent important processes for deciphering the eruptive history of a volcano. Models examining lava dome instability rely on accurate physical and mechanical properties of volcanic rocks. Here we focus on determining the physical and mechanical properties of a suite of temporally-constrained rocks from different phases of the 1995–2010 eruption at Soufrière Hills volcano in Montserrat. We determine the uniaxial compressive strength, tensile strength, density, porosity, permeability, and Young’s modulus using laboratory measurements, complemented by Schmidt hammer testing in the field.

By viewing a snapshot of each phase, we find the highest tensile and compressive strengths in the samples attributed to Phase 4, corresponding to a lower permeability and an increasing proportion of isolated porosity. Samples from Phase 5 show lower compressive and tensile strengths, corresponding to the highest permeability and porosity of the tested materials. Overall, this demonstrates a reliance of mechanical properties primarily on porosity, however, a shift toward increasing prevalence of pore connectivity in weaker samples identified by microtextural analysis demonstrates that here pore connectivity also contributes to the strength and Young’s Modulus, as well as controlling permeability. The range in UCS strengths are supported using Schmidt hammer field testing. We determine a narrow range in mineralogy across the sample suite, but identify a correlation between increasing crystallinity and increasing strength. We correlate these changes to residency-time in the growing lava dome during the eruption, where stronger rocks have undergone more crystallization. In addition, subsequent recrystallization of silica polymorphs from the glass phase may further strengthen the material.

We incorporate the variation in physical and mechanical rock properties shown within the Soufrière Hills eruptive into structural stability models of the remaining over-steepened dome on Montserrat, considering also the possible effect of upscaling on the edifice-scale rock properties, and the resultant dome stability.

How to cite: Harnett, C., Kendrick, J., Lamur, A., Thomas, M., Stinton, A., Wallace, P., and Lavallee, Y.: Evolution of mechanical properties of lava dome rocks across the Soufrière Hills eruption, and application in discrete element models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14016, https://doi.org/10.5194/egusphere-egu2020-14016, 2020

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Presentation version 1 – uploaded on 01 May 2020
  • CC1: Questions and answers from the live chat during EGU2020, Michael Heap, 11 May 2020

    Q: Can you explain how you upscaled your rock physical properties to the “dome lengthscale”?

    A: For the modelling, we used an 80% upscaling factor, which we obtained from the Colima modelling that I presented earlier in this session

    Q: How many samples did you collect? To what extent are the sample suites from any given eruptive phase representative of the material properties of the dome at that tiem?

    A: We had 2 blocks from each phase - we were limited in Montserrat to the samples that can be temporally constrained, but I certainly agree that we cannot make any robust arguments about temporal variation from a limited sample set. For this reason, we choose to focus on the co-variance of the physical and mechanical properties, and simply use the variation as endmembers in the modelling.

    Q: Very large differences in rock strength and interesting discrete element model results, do you see this in the field? I.e. that the weaker material domes have already collapsed more?

    A: Good question - this is very hard to know because we don't have a good idea of the spatial variation in rock properties across the dome. I think it intuitively makes sense, but it is hard to obtain good evidence for this

    Q: What is the range of rock strengths that you find at Souffriere Hills

    A: The range in average compressive strengths for each phase is ~6-50MPa, but the absolute range is a little higher

    Q: If higher rock strength = longer repose time, and the most recent rocks are weaker, how do you explain the current repose time, which may be the longest (if you assume the eruption isn't over)

    A: If I've understood your question correctly... the most recent rocks came from phase 5, which means they will relate to the repose between phase 4 and phase 5. I would imagine that the rocks currnetly emplaced in the dome are strong!

    Q: Isnt permeabiltiy and rock strength changing with time? how would you consider that?

    A: Yes, certainly. The models in the presentation are "static" in some way, and just show the absolute effect of strength on stability. This modelling technique is not good at investigating permeability in detail, but it could certainly be used to investigate rock strength changing over time (i.e. by having a gradual strengthening/weakening as the dome ages) - this is a work in progress!