Dating tephra layers

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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.

Something extra needs to happen to convert the loose aggregation of pyroclasts and rock fragments that comprise a pyroclastic flow deposit into a rock (ignimbrite).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.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?

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