August 12, 2011
UK Ash Cloud Likely ‘Once In A Lifetime’ Event
The 2010 ash cloud from Iceland's Eyjafjallajokull volcano that disrupted air travel in the UK and cost European businesses more than $2.6 billion was likely a once in a lifetime event, according to scientists, who have analyzed a record of past similar events stretching back into prehistory across northern Europe.
Their analysis showed that past ash clouds like the one seen in 2010 occurred on an average of once every 56 years. A report of the findings has been published in the journal Geology.
While witnesses to some ash clouds have written records of such events throughout history, no such evidence exists from before 1600 AD.
Despite this, experts say a detailed 7,000-year record is preserved in peat bogs and lake beds in the form of microscopic layers of volcanic material, including ash -- known as tephra.
BBC News reports that a team of scientists has compiled data from both written history and sediment records from the UK, Ireland, Germany, Scandinavia and the Faroe Islands to show how common these events are.
A computer model was used to establish that ash deposits occurred at about the rate of a 16 percent chance every decade over the past 1,000 years. Over the 7,000-year period analyzed, the team found 38 tephra layers in Scandinavia, 33 in Ireland, 14 in the UK and 11 in Germany.
The pattern of decreasing ash deposits with the more southerly latitudes can be explained when the source of the ash is considered.
"The vast majority are Icelandic," co-author Gill Plunkett of Queen's University in Belfast told BBC's Hamish Pritchard.
Analysis has shown that nearly a third of all ash deposits came from the Icelandic volcano Hekla. The prevailing westerly winds over the Atlantic are most likely the main source of carrying the ash clouds to Scandinavia.
However, when high pressure builds over Europe, northerly winds are common over the Atlantic, bringing ash to Ireland and the UK.
Not all eruptions produce far-reaching ash clouds, however. Icelandic eruptions have been far more common than ash deposits, averaging 20 to 25 per century for the past 1,000 years.
While last year's ash cloud cause significant disruptions in air travel, past eruptions have shown little human impact. Plunkett sees little evidence of alarm in past eruptions.
"I'm skeptical that there was very much human impact - I looked to see if periods of volcanic activity correspond with cultural change in Ireland," Plunkett told BBC news.
"We can look at particular eruptions and try to find out if there were changes in climate or changes in agricultural practices at the time, but I haven't found any correlation. I think we're pretty resilient," she added.
Image Caption: Ash plume from Eyjafjallajökull volcano over the North Atlantic, April 15, 2010. Credit: NASA / MODIS Rapid Response Team
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