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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