I had never seen the Manchester sky so blue. The usual milky white which goes by the name ‘Manchester sunny day’ was gone, transformed into an azure experienced mainly during distant holidays. Great Britain of course has a bit of a reputation. Already the Romans wrote that “the atmosphere in this region is always gloomy”.
But Manchester isn’t always that bad. At least, blue skies do occasionally happen, in between the rainy days. But that was before the modern days of air travel. Nowadays, a day here may start out clear, but quickly lines of white cirrus form, and expand until they cover most of the sky. Aircraft contrails have changed our world: 10% of cloud cover over the North Atlantic is now due to us. But suddenly, the BA cirrus was gone and the sky showed us its pre-flight colours.
The cause was out in the North Atlantic. Precisely on the flight path between Europe and the US, a volcano had begun to act up. After a period of stutterings, Eyjafjallajökull exploded on 14 April 2010. The next day, the ash cloud became dangerous to plane engines, and for five days the air space was closed – and the Manchester skies turned blue.
Eyjafjallajökull is a fairly minor volcano, at least for Icelandic standards. But it erupted explosively, and the power of volcanoes should not be underestimated. A VEI-4, which Eyjafjallajökull was, can eject ash to 8-10km altitude where jet stream winds can spread the ash around. It did, and the ash rapidly affected northern Europe. Economically, Eyjafjallajökull became a disaster. The flight ban caused an estimated cost of 1.8 to 2.4 billion dollar (incidentally, it also made this the first carbon-neutral volcanic eruption). It showed us the vulnerability of the transatlantic links.
But how common is such an eruption? This was the only such event in Iceland in a century of flight. Were we lucky? Or was this a once-in-a-life-time event? Let’s do some digging and clear the dust from the ash.
No one likes ash. Lava, bright red and pitch black, impresses us; especially at night it has a beauty that belies its destructiveness. Ash, in contrast, gives a deadly grey hue to the landscape. It covers everything, falling like snow in thick flakes. Vegetation is killed and houses collapse under the weight. Water will collect it, and a torrent of wet ash can form lahars. Ash has an ugly side.
In large eruptions, the ash clouds can become fully opaque. Within ten kilometer or more from the volcano, day turns to night as the light goes out. It is not like ordinary cloud. Sunlight has no problem getting through clouds: the water droplets scatter the light but do not absorb it. That is why you can still see on a cloudy day and why mist seems luminous. I once was in a snowstorm with lightning. Every time the lightning came, the swirling snow would shine with the light coming from everywhere. It was quite an experience. But mix a bit of dust in with the cloud and things grow dark. As I write this England is preparing its annual experiment with this. Bonfire night, November 5, is the traditional time in the UK for firework displays. Initially, the fireworks are brightly visible, but after a while the smoke from the displays begins to obscure the view. It can take a full day for the smoke to be fully dispersed. And when the smoke mingles with fog (this is the UK, after all), visibility goes to zero. It is best to stay off the roads during bonfire night: when people drive at normal speed into a fog bank and suddenly visibility turns out to be less than a meter, accidents can happen. Go on, and you hit what ever is in front of you. Stop, and the car behind you slams into you. Smoke is bad news. After all these years, Guy Fawkes still kills.
The impenetrable darkness is a common feature of major eruptions. Here is an example from Krakatoa, a description from captain Watson of the British ship Charles Bal:
“At 11.15 there was a fearful explosion in the direction of Krakatoa, then over thirty miles distant. [..] At the same time the sky rapidly covered in; the wind came out strong from S. W. to S., and by 11.30 A. M. we were inclosed in a darkness that might almost be felt; and then commenced a downpour of mud, sand, and I know not what, the ship going N. E. by N. seven knots per hour under three lower topsails. We set the side lights, placed two men on the lookout forward, the mate and second mate on either quarter, and one man washing the mud from the binnacle glass. We had seen two vessels to the N. and N. W. of us before the sky closed in, which added not a little to the anxiety of our position. At noon the darkness was so intense that we had to grope our way about the decks, and although speaking to each other on the poop, yet we could not see each other. This horrible state and downpour of mud and debris continued until 1.30 P.M., the roaring and lightning from the volcano being something fearful. By two P.M. we could see some of the yards aloft, and the fall of mud ceased; by five P.M. the horizon showed out to the northward and eastward, and we saw West Island bearing E. by N., just visible. Up to midnight the sky hung dark and heavy, a little sand falling at times, and the roaring of the volcano very distinct, although we were fully seventy-five miles from Krakatoa. Such darkness and such a time in general, few would conceive, and many, I dare say, would disbelieve. The ship from truck to water-line was as if cemented; spars, sails, blocks, and ropes were in a horrible state; but, thank God, no one was hurt, nor was the ship damaged. But think of Anjer, Merak, and other little villages on the Java coast!”
The ash deposit near a major eruption can become meters thick. The larger particles do not travel very far as they are too heavy to be carried by wind. Smaller particles go further and these are actually more opaque. They still can’t travel as far as the gas, but a large eruption can deposit centimeters of ash hundred of kilometers away. Toba, in Indonesia, managed to deposit 9 meters in India, although this was after water had washed the ash into valleys and thickened the layers; the original layer was probably more like tens of centimeters.
But ash also has a good side. It kills vegetation, but also fertilizes the ground. Over time, the land will recover and plants will grow with more vigour than before, on an enriched soil. From the green and colourful peat of central Iceland to the highlands of central Africa, volcanic soil brings life. Coffee is a well-known beneficiary. The best coffee (Arabica) grows where the volcanoes are. Ethiopia, Guatemala or Kona: this demanding plant lives on ashes.
Hither and tither
Ash follows the wind. The initial explosion can give a narrow ash trail pointing down-wind. After some time, the winds change and the ash is blown in a different direction. Different altitudes also have different wind speeds and directions. This causes the plume to bulge into a spreading cloud. But it remains loath to travel against the wind. Even Toba, which to the west devastated an area the size of a continent, left much less destruction to the east.
Iceland is not known for its stable weather. It seemingly changes every 5 minutes. Still there are patterns. South of Iceland is a highway for low-pressure systems, on the way to Scandinavia. It gives rise to something called the ‘Icelandic low’: a semipermanent region of low pressure, really a succession of one such system after the other, where the average pressure is 10-15mbar below that of standard sea level pressure.
This highway is more pronounced in winter than in summer but it remains a common feature. Because of it, the dominant wind direction in Iceland is east to northeast. Icelandic ash will therefore at first often travel towards America. While doing this, it also tends to wrap around the Icelandic low and once it finds the right place, will be carried towards Europe. Iceland is well placed to cause maximum havoc. But during the Eyjafjallajökull eruption, the wind was from the north or northwest. This slightly unusual situation made the impact on Europe much worse. Rather than cutting transatlantic routes, the ash cloud went directly for the intra-European air space. And it did so during the Easter holiday season.
Ash and engines
The part of an airplane that is affected by volcanic ash is fairly important: it is the engine. The silica particles have a melting temperature that is well below the temperature found inside the engine. The engine sucks in the air, heats it, and expels it from the back. That is not good thing to do when there is ash. The ash partially melts, sticks to the turbo blades, and re-solidifies. Larger particles (1 mm or more) are more prone to doing this. The ash cover increases the compressor discharge pressure, which may cause the engine to surge and loose thrust. This has happened on occasion. As the engine turns off, it cools, and when it cools below the glass transition temperature, the frozen ash becomes brittle. After a while it becomes possible to turn the engine on again as some of the ash fragments and breaks off. Hopefully by that time the airplane is still at a safe altitude.
Eyjafjallajökull spewed its ash mainly to 4-6 kilometer height, with occasional bursts to 9 kilometer. That limited the area that was affected as it made the ash drop down faster. Planes should have been able to find routes around and over it. But we lacked the technology to see where the ash cloud was. It consisted mainly of glassy silica, a material that is transparent to radar (radomes tend to be made out of fibreglass for that reason). Thick clouds can be seen in satellite images, but thin ash clouds may not be.
Because of the major impact of the Eyjafjallajökull eruption, the rules on flying through ash clouds have been relaxed since 2010. Charts now identify three levels of ash concentration: cyan (low contamination), grey (medium contamination) and red (high contamination). It is now allowed to fly through zones of low contamination. However, even though the aircraft may not be endangered, the engines can still suffer damage. Ash affects different engines differently, but without dismantling the engine, it is impossible to know how badly it has suffered.
Ash in bogs
Ash does more than cut our light and damage our planes. We have already mentioned how it fertilizes distant places. An eruption can give a dusting of ash across a continent. Some of it falls in places where plants can’t get to it: icecaps are one such place, but lakes, swamps and peat bogs also qualify. The ash comes down and becomes included in the sediment layers or the annual growth. Later, you can dig down into a peat bog and find layers with ash in it, a safely stored record of past eruptions.
There is a second advantage to the bog-loving scientist. Neither lava nor ash can be carbon dated. It lacks suitable carbon. Lava flows can only be dated by digging out any vegetation it may have covered and carbon dating that. But often there is no such vegetation. Find the ash in a peat bog, and finding carbon will not be a problem. The peat is full of it.
And finally, the composition of a tephra ash shard shows its origin. Every volcano has slightly different ratios of various elements (and to make it more complex, this can change over time). This makes it possible to identify which volcano caused which tephra layer, and to show which tephra layers in different locations came from the same eruption. What is there not to like about bogs?
Tephra layers are sometimes visible by eye. But more often, the fragments are too small and few to see: this is called cryptotephra. A detailed analysis takes time consuming research. A common method is to cut the peat into small segments, burn it for several hours at high temperature, wash the remains, centrifuge them for 10 minutes, extract the bottom layer and view through a microscope. Now start counting the shards. Science can require patient work.
This brings us back to the topic of the post. If we can find out how many tephra layers there are in the peats of northern Europe, we know how often an ashy eruption similar to Eyjafjallajökull happens. This is more difficult than it sounds. It means looking through many different bogs and lakes, since ash disperses into different directions. Tephra from the Askja 1875 eruption, for instance, ended up in Sweden while tephra from Öræfajökull 1362 is found in Northern Ireland. Hekla 1104 is found all over Ireland and the Faroe Islands. The so-called Glen Garry tephra, of unknown origin and dated to around 100 BC, is found in various locations in Scotland, Cumbria, and Northern Germany. Sampling one location just won’t do.
Lawson and collaborators (2012, published in Quaternary Science Reviews 41) created a list of tephra layers that are present in at least three different locations. The most recent one in their list is Hekla 1947, and the oldest one is from around 5000 BC. Of the 22 layers that they find, 8 come from Hekla. Surprisingly, 3 are from Torfajökull. One of the layers is not from Iceland: it is attributed to Jan Mayen. The plot below shows where the tephra is found. The size of the circles indicates how many of the 22 layers are found at each site. Ireland is doing particularly well. The Faroe islands also catch many of the eruptions. Elsewhere in Europe the coverage is more patchy.
This study was followed by a paper by Watson et al., published in 2017 in the journal Earth and Planetary Science Letters, Volume 460. It is largely the same team that presented the previous work. Here, they greatly extend the number of sites in Europe searched for tephra layers, and also added in eruptions with documented ash fall outside Iceland but where tephra has not been recovered from peat storage.
Watson et al. find that over the last 1000 years, 24 ash falls have occurred in Europe or the Faroe. Of these, four are known only from historically documents but lack tephra grains (1755, 1660, 1625 and 1619). The list below is a shortened version of their list where I have removes ones that may be double counted. That leaves me with a minimum of 21 events over the past 1000 years.
2011 Grímsvötn ashfall
2010 Eyjafjallajökull ashfall
1947 Hekla Dacitic-Andesitic
1875 Askja Rhyolitic
1800 Basaltic (1728-1880) Laki?
1755 Katla ashfall
1700 unknown (1650-1750) Dacitic
1693 Hekla Dacitic-Andesitic
1660 Katla ashfall
1650 unknown (1600-1700) Rhyolitic
1625 Katla ashfall
1619 Grímsvötn ashfall
1510 Hekla Dacitic-Andesitic
1477 Veidivötn Basaltic
1400 Jan Mayen (date uncertain; Azores?) Trachyte
1362 Öræfajökull Rhyolitic
1250 (Rinjani?) (date uncertain) Rhyolitic
1158 Hekla Dacitic
1157 unknown Rhyolitic
1104 Hekla Rhyolitic
1000 unknown (date uncertain) Rhyolitic-Dacitic
900 unknown (possibly several eruptions) Rhyolitic
871 Veidivotn Basaltic
860 Mount Churchill Rhyolitic
721 Snaefellness Trachy-Dacite
(The Greenland ice cores add a bit more. It has fewer tephra layers (but more sulphate layers) but it includes some grains from much larger distances, such as Rinjani and the 1453 eruption. Tephra from Laki, Eldgja and the so-called settlement layer from 870 AD are found in Greenland.)
Over the past 7000 years, a total of 84 ash clouds in northern Europe are identified by Watson et al. The originating volcano is identified for 46 of these, 15 of which are Hekla. Six events appear not to be Icelandic, based on the composition: four are from Jan Mayen, one from the Azores and one is from the 853 AD eruption of Mount Churchill in Alaska.
The plot shows the various origins. The vast majority of the events come from Iceland’s eastern volcanic zone. Of the remainder, and perhaps surprisingly, Snaefellnes is the most productive. Hekla has been the most prodiguous and consistent producer. Katla has also been active throughout the period studied. Grimsvotn is a major producer over recent years but there is only one older tephra layer known from it. Of course, it can be difficult to assign tephra to a volcano if that volcano rapidly changes its magma composition. The ‘unknowns’ may hide some of the knowns.
There are obvious absentees. No European tephra has been found that is attributed to Laki or Eldgja. (There is one find of basaltic tephra with a possible date range that overlaps with Laki.) Basaltic tephra is in fact very rare in Europe. The reason is clear: it takes an explosion to lift ash high enough to reach Europe, and effusive eruptions don’t normally manage this. It is easier for ash to reach Greenland, which is closer and in the direction of the prevailing winds, and this is why grains from Laki and Eldgja are found there. The dead zone is effusive and therefore unlikely to cause European ash. Bardarbunga is completely missing. It erupts through fissures and this takes the pressure off – it tends to avoid explosions and keeps Europe clean. Katla is a different beast: it does do fissures but also happily covers Europe in ash through explosions. The Vedde ash, which was not included in the study by Lawson because of its age, is the most widespread tephra event in Europe; it probably came from Katla.
Comparing the plot above with the VC mammoth map of Iceland shows that most tephra comes from the southwest side of the dead zone. For tephra, southwest is best. This is the region where Iceland explodes most frequently. (Incidentally, it is also the region with the highest snow fall.)
How large does an eruption need to be to cause ash in Europe? Over the past 1000 years, Iceland has had 36 eruptions of VEI 4 or VEI 5. Not all of these were explosive: a peculiarity of the VEI scale is that it uses volume rather than energy to measure explosiveness, and this means effusive eruptions intrude into the list, often in the top position. If we exclude all the mafic (basaltic) eruptions, there are 18 eruptions left. Of these, 13 have caused ash in Europe. In contrast, not a single VEI 3 eruption has been identified in the tephra record. (A disclaimer here is needed, as the origin of some of the tephra deposits remains unknown.) And the VEI 5 eruption of Hekla in 1104 left tephra in no fewer than 27 different sites. Size matters.
Watson et al. conclude that a silicic eruption of VEI 4 or (rare) larger has a 73% chance of causing an ash cloud over Europe. More precisely, their data suggests that to affect Europe requires an Icelandic explosion of 0.2km3 or more.
What about the exceptions? Three of these stand out: Hekla in 1766, 1597 and 1300. All were large enough to reach Europe: 0.3-0.5 km3. In 1766 and 1300, the dominant air flow was to the north and if the eruption was brief, Europe may have been saved by wind. But in 1597, the ash did go southward and it is not clear why no tephra from this eruption has been identified in Europe. It is just one of those things.
And what about the unidentified layers? Each of these tends to be found only within a single region. It may be that these do trace smaller, VEI 3, eruptions. We need a scientist with a lot of patience to analyze more bogs!
The question that we really would like to answer: what is the chance of this happening again? The next time we fly across the Atlantic, what is the risk of not being able to go back due to an uncooperative volcano? The relaxed flying criteria since 2010 means that an eruption will not have as bad an impact as Eyjafjallajökull had. But a lesser impact would still be costly.
If we assume that it takes a silicic VEI 4 for this to happen, we can expect European travel chaos to happen once every 55 years. If only 73% of these do so, we may have to wait a bit longer, 75 years on average. If, on the other hand, each tephra layer indicates a flight disruption event, it would happen as often as every 45 years. Pick your choice!
Ash in waiting
But the signs at the moment are that we won’t have to wait that long. Hekla is quiet – but elsewhere in Iceland, Öræfajökull is building up to an eruption. It is still early days. The earthquakes only started during 2017. But the signs are there: an increasing number of earthquakes (at the rate of this week, November 2018 would top the previous record, set in August), inflation, and increasing heat at the summit. Öræfajökull erupts infrequently but when it does, it can really go for it. An eruption will be rhyolitic, ashy because of the ice cover, and it may well reach VEI 4 in strength. The build up to a volcanic eruption tends not to be regular. Swarms can be intermittent. The Öræfajökull earthquakes may well die down again, at least for a while. But once the process is started and a conduit begins to open, it is hard to stop an eruption. Somewhere in the next decade, Öræfajökull is likely to explode spreading its dust in the wind. And the planes will stop flying.
Albert, November 2018
Dust If You Must (Rose Milligan)
Dust if you must, but wouldn’t it be better
To paint a picture, or write a letter,
Bake a cake, or plant a seed;
Ponder the difference between want and need?
Dust if you must, but there’s not much time,
With rivers to swim, and mountains to climb;
Music to hear, and books to read;
Friends to cherish, and life to lead.
Dust if you must, but the world’s out there
With the sun in your eyes, and the wind in your hair;
A flutter of snow, a shower of rain,
This day will not come around again.
Dust if you must, but bear in mind,
Old age will come and it’s not kind.
And when you go (and go you must)
You, yourself, will make more dust.