13 June 2011

Volcanic ash: not always a nuisance

When typing the title of this post I noticed what associations "volcanic ash" stir up in at least my brain nowadays. This morning there was something on the radio about the Icelandic ash having impeded some travel, and that's the first thing many of us will think about upon hearing that phrase. But my working life these days is much more influenced by it than merely through diminished travel options.

I blogged about it before; ash layers can be used for dating. There might be a specific ash layer in my sediments, the chemical composition of which is known. So if I now gather ash from every layer of sediment I have it might be found in, and then go to a specialised lab, they can analyse my ash shards and tell me if they found this specific layer in any of my samples. We knew it fell in our field area; there's eye witness accounts from that time. But we also know it was extremely high tide when that cloud came down, so we can only hope it wasn't flushed away in its entirety. We'll hopefully soon find out!

But before I can have my stuff analysed I have to extract sufficient shards from my samples. That involves burning everything that will burn, and dissolving most that will dissolve, in order to end up with a sample with not much more than silt and ash in it. And then I have to manually pick 50 shards from each sample. Very fine work! The particles are between 25 and 63 micrometers in size (or between 0.025 and 0.063 mm; maybe that is easier to imagine) and that's small. I do hope we'll find that one specific layer! Otherwise I've done some very fidgety faffing for very little reward...

This is what they look like under the microscope. Number for scale.

And here's the microslide with the numbers seen with the naked eye, also for scale.

It's not always that difficult, though; this is where we got the sediment from, and at the level of my camera bag you can see a thick red layer. That's a more conspicuous marker; it's the so-called medieval layer, deposited by some big eruption in the winter of 1226/1227. If only they were all so easy to spot! Pic by Roland.

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