An article published this week reveals the volume, grainsize and eruption rate characteristics of the tephra (volcanic ash, pumice and other materials) erupted during the eruption of Eyjafjallajökull in 2010. This information is important because these are the inputs needed by computer simulations to predict where ash from an eruption is likely to be dispersed. It is also interesting because the volume of ash that caused all that travel chaos in Europe turns out to be surprisingly small.
Ten swimming pools?
The group of scientists, led by the University of Iceland, combined measurements of tephra deposited on the ground, meteorological data, satellite data and theoretical models of ash dispersion to work out how much tephra was produced at the volcano at different times during the eruption, and to where it was dispersed. I helped out with measurements of ash deposition across Europe. The results were published as open access, so you can download the article and read them for yourself here.
The total mass of erupted tephra was 480 million tonnes. Most of this landed in Iceland, however, and only a tiny fraction (0.02%) of this made it as far as mainland Europe. This ash consisted of tiny grains of pulverised rock between 1 and 50 millionths of a metre (microns) across. For comparison, an individual red blood cell is about 6-8 microns in diameter.
Now, a cubic metre of dense Eyjafjallajökull magma would have a mass of about 2.6 tonnes. It would look like rocky grey washing machine, but with no door. So if you compacted all the ash in Europe back into a single lump, it would have a volume of 36,000 cubic metres. This would form a cube with 33 metre sides. It would look like a rocky grey 11-storey building, but with no windows.
To continue in the Olympic spirit, we can calculate how many swimming pools this would fill. The standard competition pool is 50 x 25 x 2 m, but the one in London was actually 3 m deep, so that waves would be reduced and the swimmers could go faster. The volume is therefore 3750 cubic metres.
This means that, theoretically, all the travel chaos of Eyjafjallajökull was caused by magma with a volume of only ten Olympic-sized swimming pools.
This seems unbelievable, but remember that huge volumes of air get sucked through jet engines, so even low ash concentrations can quickly add up to trouble. Airlines now have to take special measures if the concentration exceeds two thousandths of a gram per cubic metre, so to keep aeroplanes out of your swimming pool needs only 7.5 grams of tephra. That’s about a quarter of a teaspoon.
EDIT (30 Aug 2012): Of course, the chaos wasn’t caused by the ash itself, but by the rules that stopped planes from flying where the ash might be. These were changed as the Eyjafjallajökull 2010 eruption was ongoing, and the 2 milligrams per cubic metre limit was introduced. This means that you can now have a much more powerful eruption in Iceland, but with much less disruption.