Wednesday, 11 July 2012

Eyjafjallajökull VS Hekla.

Ask someone to name an Icelandic volcano and most of the time they'll make a good attempt at tryingto pronounce Eyjafjallajökull. After all, this is the volcano which 2 years ago brought chaos to flights across Europe and is estimated to have cost airlines around $1.7 billion dollars in cancelled flights. On top of this, the photos and impressive film-footage of the eruption and ash cloud made headline news stories for several days. I always find it interesting when talking to pupils at school to hear how the eruption affected their lives; trips to Florida and Australia were disrupted through cancelled flights and delays. A truly global event!

How much ash did each eruption produce?

The Eyjafjallajökull eruption was fairly small by Icelandic standards chucking out around 500 tonnes of tephra per second. Although this sounds a significant amount, it is worth bearing in mind that the Grímsvötn eruption of 2011 threw out around 10,000 tonnes of tephra per second. Quite a bit more. John wrote a good comparison on the two eruptions on his blog last year.

What has become apparent to me over the last three weeks whilst digging sample pits and looking at ash samples from across Iceland, is really how little ash and tephra Eyjafjallajökull put out in comparison to other eruptions, most noticeably the Hekla 3 (1000 BC) and Hekla 4 (2300BC) eruptions that we have been focusing on during the trip.

For example, the photo below is taken on the main road just outside of the Þorvaldseyri farm, approximately 10km from Eyjafjallajökull. You can see the Eyjafjallajökull 2010 eruption ash around 1cm down in the soil profile. It's a black grey fine ash that is approximately 1.5 cm in thickness. The ash colour comes from the composition of the magma that was thrown out in the eruption which was an andesite-like composition. The Hekla ash is of a rhyolitic composition so is white in colour.

Cut away of the soil outside of Þorvaldseyri farm, 9 km south of Eyjafjallajökull. Photo: Alasdair Monteith, 2012

Now, compare the photo above with the photo below. This sample pit was dug close to a towncalled Egilsstadir in Iceland's eastern fjords. You can see the layer of white ash from a Hekla eruption that is, again, approximately 1.5 cm in height. As the Hekla 3 eruption took place around 3000 years ago there has been time for soil and organic matter to accumulate on top of it.

Suspected Hekla 3 ash (in white) from a sample pit near Egilsstadir. Similar thickness, but what about the distance? Photo: John Stevenson, 2012

This photograph was taken 300 km from Hekla!

Hekla: a much bigger eruption

Weather patterns, type of eruption and magma composition (amongst other things) will obviously effect how far tephra travels from the sourceof eruption. But the comparison of the two samples gives you an ideaof how big the Hekla eruptions must have been. Hekla 3, for example, resulted in cooler temperatures in the northern hemisphere for a few years afterwards and has even been held as a cause of famines during Ramasses III's reign in Egypt.

So, how much tephra is there from the Hekla 3 eruption 9km from Hekla? The photo below says it all really! Except this time it's a mix of white to pink pumice. The sample site is a similar distance to the one below Eyjafjallajökull, both sites were downwind of the original eruptions.


Anyone order two metres of Hekla 3 tephra? Photo: Alasdair Monteith, 2012

Pumice, 2-3cm in size, deposited 9km from Hekla. Photo: Alasdair Monteith, 2012
So, although Eyjafjallajökull 2010 caused a considerable amount of disruption, it's effects could be shadowed by future Icelandic eruptions, particularly those on a scale with Hekla 3 or the Laki fissure eruptions of 1783-84.


Following the Eyjafjallajökull eruption in 2010, the UK government updated the country's risk register to include the dangers from explosive eruptions- such as Eyjafjallajökull 2010, and effusive eruptions - such as the Laki fissure eruptions of 1783-84. You can view the 2012 register here, the section on volcanoes starts on page 17.


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