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Fortunately, the Thorsmörk Ignimbrite is a bit simpler and has an ash matrix (light or dark coloured) with a scattering of pale-coloured pumices and rock fragments. The answers to 1 and 2 are being worked on as I write, by Ph D student Jonathan Moles ( The provisional and exciting answer to 2 appears to be Torfajökull to the NNE, but that’s a story for another time….These reflect the components of the original pyroclastic flow – ash and pumice being fragmented magma, and rock fragments being chunks of older rocks that were fractured and pulverised during the eruption (e.g. For simplicity, I’ll refer to the ignimbrite as either welded, part-welded, or unwelded. Figure 1 shows the location of the various volcanoes in southern Iceland along with exposures of the Thórsmörk Ignimbrite (in green).*Note that I have chosen to use the term ‘pyroclastic flow’ instead of the jargon term ‘Pyroclastic Density Current’ (PDC for short), as I consider PDC a cumbersome and partly-misleading term. Nobody is currently doing serious and comprehensive studies on the Thórsmörk Ignimbrite, so many of these questions will remain unanswered for the foreseeable future.I first looked at the Thórsmörk Ignimbrite in September 2013, and after a couple of days in the field followed by some literature reading, I realised that there were still many unanswered research questions about the Thórsmörk Ignimbrite. So, the aim of this blog article is just to give you some insight into what I’ve gleaned from a few days of fieldwork, and I’ll mainly address points 4-6 above.Let’s look at a great exposure in Steinsholtsdalur, to the NE of Eyjafjallajökull.In this location, there is the most spectacular and largest exposure of reworked Thórsmörk Ignimbrite that I have seen – but as I’ve not been everywhere, bigger and better ones may exist.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.
Finally, another reason for this field-based article with lots of images is that this area is very accessible, and so if you – dear reader – wished to have a look for yourself, it is straightforward to do so.He can be found on Twitter under the username @subglacial.One of the wonderful aspects of working as a volcanologist is Iceland is that fascinating new puzzles and their bigger cousins (enigmas) keep appearing.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)?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.