The observation of breakout formation during the 2011-2012 Puyehue-Cordón Caulle lava emplacement is unique, and breakouts are not generally considered to occur within rhyolite lavas. Instead, breakout formation is typically associated with the emplacement of mafic lava flows (e.g. Pinkerton, 1987; Cashman et al., 1994; Blake and Bruno, 2000). Thus, despite the large differences in composition and viscosity, observations of rhyolitic breakout formation suggests underlying fundamental processes in the emplacement of lava flows (Tuffen et al., 2013). The thesis aims to draw comparisons between the emplacement of silicic lava flows and mafic lava flows. To this end, I focus on the 2011-2012 Cordón Caulle eruption as an exceptional example of a cooling-limited rhyolite lava flow, a type of lava flow for which there has previously been little observational data. Specifically, the project addresses four main questions:
1. What was the rheological control on the development of the Cordón Caulle lava flow, and how does it compare to other cooling-limited lava flows?
2. What processes contributed to the formation and evolution of breakouts from the Cordón Caulle lava flow?
3. What conditions favour core upwelling to the surface of a silicic lava flow, instead of breakout formation?
4. What commonalities exist in the emplacement of mafic and silicic lava flows? To address these aims, the work uses a multiscale approach, utilising a mixture of remote sensing, field observations, microstructural characterisation, and analytical models. The three papers within this thesis provide an overview of the methods used and a detailed methodology, and associated data, can be found in the appendices (Appendix 8A – 8D).
The results of this project are given in three papers:
1. Paper 1 aims to provide a better understanding of the rheological controls on the emplacement of two cooling-limited lava flows by incorporating observations of lava emplacement to constrain straightforward quantitative models of lava lengthening. The controls on a basaltic lava flow from Mt Etna were examined first and then the same models were applied to understand the rheological control on the emplacement of the Cordón Caulle rhyolitic lava flow. 2. Paper 2 addresses the processes that led to the formation of breakouts at
Cordón Caulle. The paper uses detailed observations of breakout formation and morphology from satellite imagery and a field campaign to characterise breakout morphology. I then incorporate microstructural observations to determine the processes that contributed to breakout formation.
3. Paper 3 determines the conditions that favour the development of coarsely vesicular pumice diapirs instead of breakouts in some rhyolite lava flows. This paper uses a combination of field studies of rhyolite lava flows in the USA, satellite imagery and microstructural characterisation.
Finally, Section 6 draws the findings from the three papers together, and lava flow emplacement is discussed more broadly, incorporating evidence from observations of basaltic and silicic compositions to suggest common processes inherent in lava flow emplacement.
3
Emplacing a cooling-limited rhyolite lava flow:
similarities with basaltic lava flows
N. Magnall1, M.R. James1, H. Tuffen1, C. Vye-Brown2
1Lancaster Environment Centre, Lancaster University, Lancaster, UK 2British Geological Survey, The Lyell Centre, Edinburgh, UK
The following paper originally appeared in Frontiers in Earth Science, subsection Volcanology, published 8 June 2017 (DOI: 10.3389/feart.2017.00044). It was first submitted to the journal 27 February 2017 and was accepted for publication 22 May 2017 after revisions suggested by Luis Lara (editor), Jon Major, and Colin Wilson (reviewers). The modelling within the paper, the remote sensing work, fieldwork at Cordón Caulle and Mt Etna, as well as the bulk of the writing, was conducted by myself. The main role of the co-authors was with the initial conception of the project, field assistance, and the writing of the manuscript. The detailed methodology for this paper can be found in the appendices: Appendix 8A details the remote sensing techniques used, Appendix 8B expands on the data gathered in the field and Appendix 8D outlines details of the quantitative modelling.
The paper focusses on the rheological controls on the emplacement of the 2001 Mt Etna basaltic lava flow and the 2011-2012 Cordón Caulle rhyolitic lava flow. To do this we used simple quantitative models (Kerr and Lyman, 2007; Castruccio et al., 2013), which assumed that the lengthening of the lava flow was controlled by either its Newtonian viscosity, non-Newtonian yield strength, or by the strength of a surface crust. The predicted lengthening was then compared to the observed flow lengthening. In order to improve the applicability of such models (Kerr and Lyman, 2007; Castruccio et al., 2013) we first identify the changes in rheological control through observations from satellite, field, and published observations. For both lava flows the formation of
breakouts indicated a strong crustal control during emplacement, suggesting a crustal control model is the most applicable for the latter stages of emplacement. This is typical of basaltic lava flows but had not been inferred for the emplacement of high-silica content lava flows, which Castruccio et al. (2013) suggested were primarily controlled by a non-Newtonian yield strength.
The combination of techniques used within this study allowed for an improved understanding of the rheological controls on rhyolite lava flow emplacement:
• Observations of the development of breakouts and large surface fractures during lava flow emplacement at both Cordón Caulle and Mt Etna, suggested the influence of a surface crust on the latter phases of lava flow emplacement. • Quantitative modelling suggested that for both the 2001 Mt Etna basaltic lava flow and the Cordón Caulle rhyolite lava flow, the flows were initially controlled by a Newtonian viscosity, and later by the yield strength of the surface crust. Suggesting similarities in their emplacement.
• The initial Newtonian viscosity control on the Cordón Caulle rhyolite lava flow suggests a fundamental difference between the emplacement of crystal-poor (e.g. rhyolite) and crystal-rich (e.g. dacite) siliceous lava flows, which are likely controlled by their yield strength.