Tephra is volcanic ash. It is ejected during eruptions and since it is so small and light (a typical tephra shard is about 1/20th of a millimetre long, but can be smaller than that!) it travels long distances, typically following prevailing winds. Eventually, as is the way of the world, gravity wins out and the tephra falls to the ground. Some of it, the stuff we are interested in, falls on bogs!
This might not sound so special, but it helps us in our studies for several key reasons. The first is that the geochemistry, or what elements the tephra is made up of and in what proportions, is unique to each volcano and each eruption. So by analysing the geochemistry, we can tell the difference between the tephra from different volcanoes. We can even tell the difference between tephra from two different eruptions of the same volcano.
Given its tiny size, tephra can often be rather hard to find deep down in our peat cores, so is often known as “crypto”, or hidden, tephra. But we can treat samples of the peat in a lab in such a way (burning it in a furnace then rinsing with weak acid – please do not try this at home!) that only the tephra and some other inorganic materials are left – this makes it much easier to find.
Tephrochronology is the word we use to describe the use of these tephra layers as a chronological tool – meaning a tool that helps us to date our cores. For example, say there are historically documented records of a volcanic eruption in a given year. An example we have found before is the eruption of the Icelandic volcano Askja in 1875. If we find a tephra layer in our core, analyse the geochemistry and find it is the same as that Askja eruption, we can then apply the date of 1875 to the depth that we found it.
If you read the dating methods page, you can find out more about why it is so important to our work to have the best dating we possibly can on our cores.
The final great use of tephra comes if we are comparing multiple cores from one area. Say for example we want to have a look at how the records of past wetness vary between three bogs in north-west Europe, one in the UK, one in Germany and one in Sweden. If we can find the same tephra layer in each core (matched up by its unique geochemistry) then we know that whatever depth we find it in each of our cores, which could be very different depending on how fast or slow each bog has grown, is exactly the same age. This makes comparisons between each site much easier. In this sense, we say that we use tephra layers as ‘pinning points’ between the different cores.