Define stratigraphic dating
At this exposure, reworked pale and unwelded pale ignimbrite occurs, and both are truncated by a subglacial basalt.
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.Compaction of the pyroclastic flow deposit can also occur, and again this transforms the deposit into something more rock-like.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.