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Having looked at outcrops that appeared in published papers with Jonathan Moles and ignimbrite expert Becky Williams (@volcanologist on Twitter) we could not reconcile the complexities that were in front of our eyes with the simple logs in the published papers. Several simple observations can be made from Figure 2: the base is not exposed; the top is exposed and is capped with sediments (which reassuringly contain clasts (i.e.
chunks) of the Thórsmörk Ignimbrite – so the sediments are younger); there are pale and dark varieties; welding is variable; there is reworking of unwelded deposits. The burning research question is – is this mess a mess related to original variable deposition of the PDCs, or is the mess due to later processes that moved separate ignimbrite domains in a way that brought them together?
Figure 3 (courtesy of Jonathan Moles) shows some of his mapping of southern Tindfjallajökull, where the Thórsmörk Ignimbrite occurs within a series of sedimentary units (tills) which – due to matrix and clast characteristics – we interpret as being glacial in origin. Figure 4 shows that only one variety of ignimbrite is found – a part-welded dark phase.
These units thin towards the margins of an ancient basin located in SE Tindfjallajökull, and in Figure 3 they can be seen thinning up to the right (towards the basin margin) with the ignimbrite sandwiched between tills. And although it looks simple from a distance, in detail it’s not as there are pods of other ignimbrite varieties around, such as the welded pale variety, and some unwelded ash. But this area did demonstrate an interesting and previously-unknown feature of the local eruptive environment at the time the ignimbrite was being deposited – that a basin was filling with diamict (glacial sediments), and that this continued after the ignimbrite was deposited.
At this exposure, reworked pale and unwelded pale ignimbrite occurs, and both are truncated by a subglacial basalt.
The subglacial basalt is connected to small dykes that have intruded reworked ignimbrite, and in places the dykes have sintered (baked) the unwelded ignimbrite.
Fortunately, pyroclastic flows are surprising good at retaining much of their heat during transport, so once they come to a halt this heat can cause sintering and welding – and thus the formation of a rock (ignimbrite) that is much more resistant to erosion.
Interesting insights into past environments that can be made from fieldwork in this area, is what happened after the ignimbrite was deposited.
So, let’s try and find some simpler exposures that might help us shed light on this variability.
This is a common survival tactic when faced with a difficult field problem – go and look for simpler examples to understand first, then come back to the complex stuff.
There are a host of other post-emplacement processes that affect pyroclastic flow deposits, but we’ll leave these as they are less relevant to this particular article.
In the field, ignimbrites have textures too variable to describe briefly here. Is there just one ignimbrite present, or are there more? What was the environment like when it was deposited? What processes affected the ignimbrite soon after it was deposited? Why are there dark and pale phases – are they the same chemically but just different physically? Why is there so much reworking, and why had previous authors not mentioned it? What do fragments of welded ignimbrite within overlying unwelded ignimbrite tell us about time gaps etc? What did the original complete PDC stratigraphy look like? When did the processes that jumbled the ignimbrite occur – during deposition, during welding, or post-welding (or some combination of the three)?
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As this is recce-level work a nice robust model can’t be provided (it would be too speculative for my liking), but I hope that you will get a bit of insight into what volcanologists do in the field, complete with uncertainties, challenges etc.