Long-range transport of volcanic ash was in the news last week, thanks to a recently published study by an international team of scientists, led by Britta Jensen and Sean Pyne-O’Donnell from Queen’s University in Belfast.They showed that volcanic ash grains found in Ireland had come from an ancient volcanic eruption 7000 km away in Alaska. The study is a really nice example of scientific detective work, where different pieces of evidence are pieced together to support the conclusion.This post explains what they found and what it means.
Needless to say, the grains are extremely small (typically 20-125 microns).
Such deposits are called ‘cryptotephra’ (Greek for ‘hidden ashes’; the word ‘tephra’ applies to everything ejected by an explosive volcanic eruption) and are useful as time markers in sediments and lake deposits.
If you know that age of the eruption that produced a particular layer, you know the age of the sediment in which you found it.
Cryptotephra are extracted by roasting samples, or digesting them in strong acid, to remove the organic material, then separating the cryptotephra from other sediments using magnets or by floating in them liquids of carefully chosen densities.
The grains can then be analysed under a microscope.
Databases such as Tephrabase record the layers that have been found.The layer in question, known as AD860B, had been recognised in Northern Ireland, Germany and even cores from the Greenland ice sheet. Structurally, volcanic ash is a type of glass; all the atoms that were jiggling round freely in the molten magma were suddenly frozen in place when it quenched in the atmosphere.An electron microprobe fires a beam of electrons into the glass, exciting the atoms to give off x-rays.The x-rays from each element in the sample (O, Si, Al, K, Na, Fe etc.) have their own characteristic frequency so, by recording the energy released at each one, you can calculate how much of each element is present.Jensen’s team used electron microprobes to measure the chemical composition of the ash, looking for the clues about the source volcano.Most European cryptotephras come from Iceland, but the compositions of AD860B didn’t match any known Icelandic eruptions, especially when comparing the aluminium and iron content.