AFA - Aerospace Forum Asia

Sign of the times ...recommends industry activists to review this abridged/highlighted clip from The Economist online.....it revels the history and threat to airtravel from Icelandic volcanos.( Posted 22 Apr,2010)

Back into the clouds
The threat to air travel from Icelandic volcanos is still troubling- The Economist Apr 20,2010

EUROPEAN airports have reopened for business. Eurocontrol, which co-ordinates national air-traffic controllers, expects around 75% of flights to operate in Europe on Wednesday April 21st. It will take far longer to sort out a backlog of cancelled flights; to move planes and crew in the wrong places to where they should be; and to return supply chains to normal.

Iceland has a lot of volcanoes, and it’s a rare decade where one of them doesn’t erupt. So why has the eruption of Eyjafjallajokull caused such chaos, and what does that mean for the future? The answer to the first question is that the Eyjafjallajokull eruption is peculiarly well attuned to messing with air travel; most eruptions of a similar size would do a lot less long-distance harm. The answer to the second is that less well attuned but considerably larger eruptions are all but certain in decades to come.

The earth’s volcanoes appear for the most part in three types of setting. The most familiar, and most of the most dangerous, are found where one tectonic plate overrides another, as happens in the ring of fire around the Pacific. Then there are those which sit over isolated “hotspots” of upwelling magma from deep in the earth, like the volcanoes of Hawaii. Finally there are those—a great many, but normally deep under the ocean—formed at the spreading ridges where tectonic plates pull away from each other and new crust is formed. Iceland is peculiarly volcanic because it is formed by the intersection of a hotspot and a mid-ocean ridge. The hotspot has pushed the spreading ridge up to the surface and supplemented its activities with some extra volcanism on the side. There are 33 large volcanoes on the island, or just offshore, which have erupted since the end of the last ice age, around 12,000 years ago.

As these volcanoes go, Eyjafjallajokull is not a very big one. It is, however, particularly good at producing fine-grained ash — the sort of stuff that can hang in the air for days—and it has done so at a time when weather conditions have allowed that ash to be spread to the south east, in a slightly clumpy way, through a great deal of European sky.

The fineness of the ash is, says Thorvaldur Thordarson, an Icelandic volcanologist, unusual. Ash particles are normally in the 50-100 micron (0.05 to 0.1 millimetre) range. But at a site 50km east of the eruption, 24% of the ash falling to the ground was in the form of particles 10 microns or less in size. Studies of ash captured from the air show that for every one of the largest particles (about 300 microns) there are a million or more in the 2 micron range. So though the total volume of the eruption, put at about 0.14 cubic kilometres, is low, the amount of ash capable of travelling long distances is high.

Two factors are contributing to the fineness of the ash. One is the composition of the lava. The more viscous lava is, the harder it is for gases within it to bubble out, so such lava has an explosive tendency. Eyjafjallajokull’s lava is, by Icelandic standards, quite viscous. The other factor is the presence of water. Putting molten lava into direct contact with water or ice also leads to explosions, which again lead to fine dust. A small ice cap on top of Eyjafjallajokull has promoted such shenanigans.

Last time Eyjafjallajokull erupted, from late 1821 to early 1823, it also had quite viscous lava. But that does not mean it produced fine ash continuously all the time. The activity settled into a pattern of flaring up every now and then before dying back down to a grumble. If this eruption continues for a similar length of time, it would seem fair to expect something similar. The effects of water and ice, too, are likely to be intermittent, and to trail off at least a bit as the reserves on top of the mountain are used up. There was some indication on April 19th that water has found new ways to drain away from the crater area, which is encouraging in this respect. Moreover, the wind is not always from the north-west. All this means that a repeat of 1821-1823 would not lead to a permanent pall over Europe, though it would undoubtedly cause further disruptions.

But what of the other volcanoes on the island? Much attention is currently being focused on Katla, a considerably larger volcano 25km from Eyjafjallajokull. Previous eruptions of Eyjafjallajokull seem to have acted as harbingers of a subsequent Katla eruptions. There is no geophysical evidence for a causal relationship between such eruptions — no indication that the two volcanoes share the same plumbing — and since Katla erupts more often than Eyjafjallajokull the effect could just be coincidence. Only two or three (depending on who’s counting) of the 23 eruptions of Katla over historical times (which in Iceland means the past 1,200 years or so) have been preceded by eruptions of Eyjafjallajokull, says Dr Thordarson. Katla is monitored with seismometers, which pick up little earthquakes, and with Global Positioning System stations, which pick up deformations in the crust (an early indicator of the Eyjafjallajokull eruption was that GPS monitors on opposite sides of the volcano moved six centimetres closer together). Neither system is currently showing anything untoward.

That said, Katla does seem to erupt on a semi-regular basis, with typical periods between eruptions of between 30 and 80 years. The last eruption was in 1918, which makes the next overdue. There is some evidence that the longer Katla waits before relieving itself, the more spectacular the results are. When it comes to lava output, its eruptions can be 100 times larger than what is going on at Eyjafjallajokull. This would be far more disruptive for the people living in southern Iceland and could also do harm much further afield. A very large eruption, thought to be one of Katla’s, left ash all across northern Europe about 10,300 years ago. When explosive volcanic eruptions in Iceland and elsewhere in the Arctic are large enough to insert significant long-lasting hazes into the upper atmosphere they can change weather patterns around the world; there is some evidence for them weakening the flow of the Nile and disrupting monsoons.

And Katla is not the only game in town. Iceland has others capable of even greater nastiness. The eruption of Oraefajokull in 1362 may have been almost as large as that of the Philippines’ Mount Pinatubo in 1991, which was the largest eruption of the 20th century. The Laki eruption of 1783 sent poisonous gases far and wide across Europe. And there is evidence that some of the island’s volcanoes, especially those under the central ice cap, (which, other things being equal, will produce more dusty and explosive plumes if they break through) may be in cahoots, their average activity rising and falling in a cycle of about 130 years. On this analysis, the past few decades have been one of the quiet patches.