Words by

James Ashworth

In the last week or so, there’s been a lot of speculation and rumour flying around about the potential for an eruption of Grímsvötn. Firstly, I should say that this is still a distinct possibility. Secondly, I should say that in the field of volcanology, nothing is ever certain, so you have to take any news with a pinch of salt. But my biggest issue is with a lot of utterly uninformed reporting, quoting all sorts of old bollocks, not just about Grímsvötn, but Icelandic volcanoes in general. And frankly, it’s time a couple of these misconceptions were laid to rest. So, allow me to take your mind for a wander through some real science – or at least, science we think is correct at the moment – and we’ll see what we can do about that.

Question one: So, Eyjafjallajökull screwed up all our airspace. Another Icelandic eruption means another messed up holiday season, right?

No. No it does not. The Eyjafjallajökull eruption involved a glorious mix of factors which all came together to produce what we saw. The ash that went so high, so far and caused so much disruption was unusually fine, caused by a combination of eruption type and location, magma type (predominantly andesite), the presence of ice (ice melts and becomes liquid water, which then interacts with the hot lava to explosively fragment it into tiny pieces) and the overlying ice thickness (not thick enough to largely contain the eruption beneath it). The amount of erupted lava was also quite high, most likely due to the time since the last eruption there in 1821.

So what about Grímsvötn? Well, Grímsvötn generally erupts a different type of lava (basalt), less prone to highly explosive eruptions than what was erupted this spring. Additionally eruptions here are usually much smaller than the recent Eyjafjallajökull eruption, due to the believed size of the magma chamber below the surface (there is a lot of debate about this, though). The ice above the eruptions here often takes a few hours to melt, also, often containing much of the most powerful beginning of the eruption.

During Grímsvötn’s 2004 eruption, there was some ash/tephra fallout towards northeast Iceland, but nothing major. There was some minor disruption of airspace over Iceland, but again, this was short-lived. Heck, the eruption only lasted a couple of days. The previous eruption in 1998 was quite similar. Basically, given the information we have, there is no real reason to believe that any eruption resulting from the current jökulhlaup (or shortly thereafter) would be much different.

Question 2: The flood at Grímsvötn is due to new volcanic activity?

Grímsvötn is a complicated system that we’re still trying to fully understand. However, we know there is a lake sitting above the central volcano (see Q 3), and we know that this grows in periods between eruptions. It’s there because of geothermal activity from the volcano, but not related to an actual eruption – simply, the volcano is just hot because there is magma inside it, even if it’s not so close to the surface. This heat melts the ice of the glacier and forms a lake beneath the ice surface.

We think that this lake acts like a cork in the top of a bottle of volcanic champagne. The pressure exerted downwards onto the volcano act against the growing magma pressure acting upwards. Then, when the lake reaches its maximum level, the ice dam holding it in place has to give way. A flood begins, and the water drains away. This release of pressure is what is thought to have triggered the 2004 eruption at Grímsvötn, and probably the 1998 event as well. Going back to the champagne analogy, the removal of this water is like popping the cork – with little downwards pressure left, the magma explodes upwards towards the top of the volcano, resulting in an eruption.

Question  3: Going back a bit, Katla and Eyjafjallajökull are the same volcano, right?

No. There is thought to be some linkage between the two, but they are not the same.

Volcanic activity in Iceland generally takes the form of ‘volcanic systems’. In the middle of each of these you have a ‘central volcano’ – usually a quite well defined mass. There is a lot of eruptive activity here, and they all take place close together, so they build up what you might think of as a typical volcano. The second phase of the Eyjafjallajökull eruption, under the glacier, would probably be classed as a central volcano eruption. Extending out from the central volcano, usually in a northeast-southwest direction, if a ‘fissure swarm’. This is an extended area where fissure eruptions, usually less explosive events, occur. Typically a fissure will erupt once, and then another one will open up somewhere else in the area for the following fissure eruption. Because the activity it spread out, you don’t tend to get large landforms here, just rows of small craters.

It just so happens that the Katla and Eyjafjallajökull central volcanoes are situated very close to each other, and the magma sources for each system lie beneath their respective central volcano. Note that these are separate sources, and do not mix. Lavas erupted from each one are chemically very different. But it is thought that pressure changes in Eyjafjallajökull, caused by magma moving around inside it, might somehow cause pressure changes within Katla (like a poke to the ribs, perhaps) which could then push it towards an eruption.

Note that although we’ve now witnessed eruptions of both central volcanoes in the past 100 years, we’ve never seen the two acting together – this suspected link is just derived from historical records. So until we actually observe it happen, with scientific instruments in place, much of this is educated speculation and best-guessing.

Question 4: So what are you saying, exactly? We’re all going to die?!

We are not all going to die. Probably. Volcanology is a modern, rapidly advancing science. Something that was considered fact ten years ago may now be laughably out of date to someone in the field. We’re always trying to get a better understanding of what’s going on, but it’s incredibly hard.

We think an eruption of Grímsvötn is possible because we have seen the jökulhlaup (glacial outburst flood), and in past occasions this has prefaced an eruption there. And we have data to back that up. But we can’t be sure. Equally, we think there is some link between Eyjafjallajökull and Katla, but we don’t really understand it. So all we can say is that there is some possibility of something happening in the future, because…

To offer some final comfort, look back through history. There have been many eruptions in Iceland over the past century, even, and very few of them have been particularly large. There’s always the potential for a big eruption, but the next one could also be very small. So to anyone worried about Icelandic volcanoes… well, stop. There are plenty of other more pressing issues to deal with!

See more Eruption Iceland stories.