Science Interviews


Tue, 15th May 2012

Unearthing our History with Tephra Ash - Planet Earth Online

Dr Christine Lane and Victoria Cullen, University of Oxford

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When a volcano explodes it ejects material ranging from rocks the size of cars to the smallest particles of ash. This ash can travel thousands of miles forming an invisible layer on the landscape, and by studying these microscopic grains scientists can date archaeological sites and this can help to clarify the effects of environmental and climatic change or even determine the movement of the human population within the last 100,000 years.

Well, Sue Nelson met up with Dr Christine Lane and Victoria Cullen at the University of Oxford's Research Laboratory for Archaeology to Christine began by showing some of the variety of material collected from a volcano...

Christine Lane - These are some samples that I collected in the field from various places which are from close to volcanic sources so a bit different to what we look at normally. One of them is a pumice, a bit like you would use in the bath, a very light rock full of air.

This rock is what we call an obsidian and it's actually made of exactly the same material as this pumice, it's glass, but it's got no air bubbles in it so it'sErta Ale Volcano really heavy and it's much denser. What we're looking at in the lab here is the same material but it's travelled maybe thousands up to maybe three to five thousand kilometres from the source. So what we're looking at now is volcanic ash and you can see it looks much more like dust, just particles of dust or very small grains like sand size grains.

Sue Nelson - And do you have a specific name for this scale, this size of ash?

Christine Lane - All material erupted from a volcano when it has erupted explosively we call tephra and tephra is actually the Greek word  for ash.

Sue Nelson - Victoria, you're going to show me aren't you exactly what this looks like.

Victoria Cullen - As Christine says it is actually glass and if you can imagine when you break a glass in your house it fractures in very distinct ways, very sharp edges and in some locations you get these bubbles as well which also kept all the glass shards, so if I get some slides out for you know we can actually look at what it looks like down a microscope. If I get you to look down there and you can see some very pinky purply looking shards of glass...

Sue Nelson - Do you know it reminds me of a child's kaleidoscope.

Victoria Cullen - If you were to find this in an archaeological environmental site you wouldn't actually be able to see if physically looking at the site, it's invisible to the naked eye. So this is why it's called cryptotephra or hidden secret tephra also known as microtephra. So when we look at certain sites because it's travelled so far, it's so fine, it's so small, when it is deposited it is just invisible to the naked eye. And then they come under the microscope and actually look if tephra is there in the first place.

Sue Nelson - So now that we know, obviously, that we've got tephra where do you go from here?

Victoria Cullen - We take them down to the microprobe and we analyse them for their chemistry. Every glass from a single eruption  has one composition, one chemical composition, which is frozen at the time of the explosion and that composition acts like a fingerprint so we can identify from a tephra, the composition of a tephra shard, which eruption it came from.

Sue Nelson - This looks like a giant microscope effectively - more than a metre high but with computer screens on either side and a console.

Victoria Cullen - Yes, this is our electron microprobe. It is like a giant microscope but it works at much higher resolution, but it also has four different, what we call spectrometers, and these are effectively the detectors that record the  composition of the material that we put in there.

If I choose a little grain to focus in on we can now on the computer console we can have a look at that here - and it looks quite complicated because there's a lot of different columns on it but the one we want to look at is this column here which is the weight percent oxide and this tells us for each element...

Sue Nelson - I can see there it's sodium, magnesium, aluminium, silicon. These are all in ash?

Victoria Cullen - Yeah, these are just what we call the major and minor elements, so these are the main constituents of this ash.  There are other trace elements in there.  So you can see that the greatest composition is silicea, there are silicate materials like all volcanic rock, so silica, aluminum, sodium, iron and potassium and sometimes calcium are the main elements that we're measuring.

Sue Nelson - And what can you actually learn from analysing these bits of ash, this tephra.

Victoria Cullen - We're looking in records where we have a story already, so we might have an archaeological site which tells a story of what the population in that site had been doing, when they've been there, what sort of behaviour they've been doing and  what tools they've been making.

Or we might be working in an environmental records, so sediments accumulated in the bottom of the  lake over time which record changes in vegetation or the landscape around the lake basin. And by finding these tephra layers or the same tephra layers in different sites we can link up those records. So where we have a population in a cave and we find evidence then just below a tephra layer, if we find the same tephra layer in a lake record that tells us what the climate was doing  at the time that eruption took place we can infer that that was also the climate at the time of that that population was there.  So we're using them as marker layers to transfer climatic and environmental information between sites.



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