Welding of hot pyroclasts in volcanic environments

During large explosive volcanic eruptions, plumes can collapse to form pyroclastic density currents that move at great speed over the landscape, devastating infrastructure in their path, and leaving behind deposits called ignimbrites. The deposition of ignimbrites from pyroclastic density currents is generally well understood. However, some ignimbrites have amalgamated – termed ‘welded’ – while hot and form welded and rheomorphic ignimbrites that can be indistinguishable from lavas. The process of welding remains poorly explained, which hampers our interpretations of welded ignimbrites in terms of the overarching volcanic process(es) that they represent.

To date, welding process investigations (dominantly experimental) have focused on welding under effectively zero additional forces, where pyroclasts weld simply because their surface-surface contacts smooth out under surface tension. By contrast, in nature, it is clear that there are substantial contributions from both shear stresses (inferred from high shear strains) and compactional vertical stresses (inferred from symmetric clast flattening features). Similarly, exiting work has focussed on rhyolite compositions that do not crystallise during welding. Again, by contrast, this is at odds with observations in nature which show that all compositions including peralkaline phonolites and trachytes exhibit welding features, often with concomitant crystallisation of the pyroclasts.

The supervisory team has previously developed (1) frameworks for the interpretation of pyroclastic emplacement processes from lithofacies and ignimbrite architecture [ref: 1]; and (2) general physical volcanology models for welding rates at the scale of pyroclasts and ash [ref: 2] (note that ‘sintering’ and ‘welding’ are synonymous in this context). These represent foundations for this project, such that a student has all the tools they need to apply these to welding in real ignimbrite deposition scenarios.

[ref 1] Brown, R.J. and Branney, M.J., 2004. Event-stratigraphy of a caldera-forming ignimbrite eruption on Tenerife: the 273 ka Poris Formation. Bulletin of Volcanology, 66(5), pp.392-416.

[ref 2] Wadsworth, F.B., Vasseur, J., Llewellin, E.W. and Dingwell, D.B., 2022. Hot sintering of melts, glasses and magmas. Reviews in Mineralogy and Geochemistry, 87(1), pp.801-840.

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In this project, fieldwork will be undertaken at type field-localities in Gran Canaria (Spain) and Pantelleria (Italy), and with possible extension fieldwork in the Snake River Plain (U.S.A.). These sites all host extensive welded ignimbrites of variable volume, deposited from eruptions of different magnitude, as well as covering the compositional range from rhyolite to phonolite. Detailed textural work on these ignimbrites will allow the student to pose hypotheses about the conditions of welding and rheomorphism in nature. Next, the student will replicate these textures using welding experiments under shear stresses, under load, and in the crystallisation window, using a range of homemade magma compositions in the hot-labs at Durham University. Finally, the student will become proficient with scaled model development and model application in order to interpret their results and to generalise/upscale to magmatic conditions. Taken together, these lines of enquiry will allow the student to make substantial steps forward in our understanding of pyroclast welding in volcanic environments and the deposition of welded ignimbrites from pyroclastic density currents.

Project Timeline

Year 1

Year 1 will involve two guided literature reviews in parallel: (1) on the dynamics of pyroclastic density currents and deposition processes associated with the formation of ignimbrites; and (2) on viscous sintering processes that underpin the physics of welding in volcanoes. Year 1 will also include the first fieldtrip to Gran Canaria to collect first samples which can be used for textural analysis by SEM, optical microscopy, and other petrophysical characterisation techniques.

Year 2

Year 2 will involve fieldwork to Pantelleria and, possibly, to the Snake River Plain to collect complementary observations of welding textures throughout the ignimbrite units. Year 2 will dominantly involve laboratory work and (1) making synthetic volcanic glass to use as an experimental material; (2) welding experiments under zero load; and (3) welding experiments under shear in new high-temperature rheometry equipment.

Year 3

Year 3 will involve the analysis of experimental results and the application of dynamic sintering theory [ref 2]. In Year 3 the student will receive extensive targeted training in dimensional analysis and the application of analytical mathematical models to the analysis of experimental results, as well as how to apply such validated models to glean insights into volcanic processes in general.

Year 3.5

In the final 6 months, the student will receive support in thesis preparation and writing of work for publication (this will also be provided throughout all 3 years as the student finishes components of the work).

& Skills

The student will be trained in: (1) field skills and the interpretation of ignimbrite lithofacies and architecture; (2) laboratory skills and how to perform scaled and well-posed experimental investigations into large natural processes; (3) dimensional analysis and mathematical modelling; (4) scientific writing, communication, and presentation skills for publications and conference contributions.

References & further reading

The following resources are recommended ahead of application:
 Branney, M.J., Kokelaar, P. and Kokelaar, B.P., 2002. Pyroclastic density currents and the sedimentation of ignimbrites. Geological Society of London.
 Wadsworth, F.B., Vasseur, J., Llewellin, E.W. and Dingwell, D.B., 2022. Hot sintering of melts, glasses and magmas. Reviews in Mineralogy and Geochemistry, 87(1), pp.801-840.
 Zanotto, E.D. and Prado, M.O., 2001. Isothermal sintering with concurrent crystallisation of monodispersed and polydispersed glass particles. Part 1. Physics and chemistry of glasses, 42(3), pp.191-198.
 Williams, R., Branney, M.J. and Barry, T.L., 2014. Temporal and spatial evolution of a waxing then waning catastrophic density current revealed by chemical mapping. Geology, 42(2), pp.107-110.

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