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Is Iceland’s next volcanic eruption about to happen?

Subglacial Gjalp eruption in 1996. Páll Stefánsson, Iceland Geological Survey, CC BY

The Iceland Meteorological Office has increased the risk of an eruption at Bárðarbunga (or Bardarbunga) volcano, after hundreds of earthquakes were reported over the weekend. The risk level has been set to orange, which is the fourth-highest on a five-level scale.

Here we asked Dave McGarvie, a volcanologist at The Open University, to explain what we need to know.

Should we be worried?

We have known for some time that Bárðarbunga was going to do something – we just didn’t know what. Because it is covered in ice, we rely on instruments to reveal its behaviour.

Now it has stirred, it is giving us clues about what it is about to do. The clues from the patterns of earthquakes and earth movements reveal two clusters where magma is moving towards the surface, and if it gets there it will erupt. But whether this will be a gentle or a violent eruption is uncertain at the time of writing.

There is no way to predict when the eruption may happen, but we should get a few hours notice. The good news for air travel is that both clusters are away from the heart of the main volcano which makes it less likely that an eruption will produce the fine ash that causes disruption.

What would the eruption look like?

At the very least, magma will stall in the Earth’s crust and form an intrusion. We may never see any manifestation of this, except on instruments. But if magma does break through to the surface, then how much magma erupts and what is above it will determine the eruption style.

If it is under thick ice – that is more than 400m thick – and not much magma comes up, then a pile of volcanic rock will accumulate at the base of the glacier. This will melt a lot of water (14 times the volume of magma under ideal conditions), and we may see a depression in the ice surface. This will add water to a major river, and cause flooding downstream.

If it is under thick ice and lot of magma erupts along a fissure, then we will see a repeat of the Gjálp eruption of 1996, with erupting magma melting a pathway to the ice surface within hours and forming an eruption plume. Compared to the massive plume of Grímsvötn 2011, this will be a small plume and less problematic for air travel as the particles will not be dispersed widely.

If magma breaks to the surface outside the glacier margin, there are likely to be small but powerful local explosions as the rising magma encounters the water-bearing sediments that occupy the land in front of the glacier margin. Explosion may occur because flashing water to steam involves more than a thousand times expansion in volume. After the water has been used up, or the magma isolated from the water, then a normal fissure eruption would be expected.

I emphasise that the above are what I currently consider the most likely scenarios. The “likeliest” scenario could change at a moment’s notice. That is part of the fun and frustration of anticipating eruptions at poorly-known and remote volcanoes.

What is the worst-case scenario?

That this is the start of a major volcano-tectonic event at Bárðarbunga, which may further develop to the southwest. This is a concern because in the southwest there are fissures that have produced Iceland’s most voluminous lava flows, since the ice melted some 9,000 years ago.

These fissures are up to 100 km long, and far to the southwest they can trigger eruptions at the Torfajökull volcano. Torfajökull happens to have an abundance of sticky magma that can erupt explosively and produce lots of fine ash. The last eruption, in 1477-1480, produced just two lava flows and minor explosions. But the one before, in about 874 AD, produced an explosive eruption plume that was carried over much of Iceland.

Also to the southwest of Bárðarbunga lie the rivers which produce much of Iceland’s hydroelectric energy, and a fissure eruption in this area could cause big problems. Icelanders have long known about this possibility and have specific plans in place should this happen.

I emphasise that we don’t know yet whether this is an isolated event or the start of a more prolonged and larger volcano-tectonic episode. It may be years before we know for certain. But at some time in the future there will be a major fissure eruption to the southwest of Bárðarbunga – we just don’t know when.

Bardarbunga caldera is about 10km wide in diameter. Oddur Sigurdsson, Icelandic Met Office, CC BY

Could this be another Eyjafjallajökull?

The Eyjafjallajökull eruption caused a lot of disruption to flights in Europe. However, it is important to note that, all things being equal, if an Eyjafjallajökull-like eruption happened tomorrow then there would be far less disruption to air traffic – something less than half the flight cancellations of 2010.

There are two main reasons for this. First is that the old flight rules – avoid all ash – have been relaxed so aircraft can now fly when there is some (but not too much) ash in the sky. Second is that the Met Office revised its model that estimated ash concentrations in the atmosphere, so we now have more certainty about how much ash there is and where it is.

If something unusual were to happen and a substantial amount of magma started rising within the heart of Bárðarbunga, then there could be a large explosive ash-producing eruption. The good news is that we have a better idea of what to expect from such an explosive basalt eruption because we had one in 2011 at Grímsvötn.

To provide context, Grímsvötn was Iceland’s most powerful explosive eruption since Katla 1918, and was about 100 times more powerful than Eyjafjallajökull. Compared to Eyjafjallajökull 2010, Grímsvötn 2011 produced twice as much ash in a tenth of the time.

We were lucky with Grímsvötn 2011, because a combination of wind direction and the new flight rules meant far less disruption to air travel. Over Europe only about 900 out of 90,000 of flights were cancelled in 2011. In comparison, about 94,000 flights were cancelled during Eyjafjallajökull’s 2010 eruption.

Why was Eyjafjallajökull so bad?

Eyjafjallajökull 2010 was a “perfect volcanic storm”. It was unusually long-lived, about 39 days, whereas most explosive eruptions in Iceland last just a few days to a week. It produced an unusually high proportion of the type of fine ash that is most easily transported long distances. Dry weather meant that the ash was not “washed” out of the atmosphere, and prevailing winds carried the ash almost directly to the UK and western Europe. We had the old “ash in the sky so you don’t fly” flight rules which grounded everything. Finally, the old Met Office model slightly overestimated the concentration of ash in the sky.

No knowledgeable volcanologist worth their salt would ever suggest basing western Europe’s ash cloud mitigation plans on a repeat of Eyjafjallajökull 2010, because it is very unlikely. A short-lived but powerful injection of ash into the atmosphere like Grímsvötn 2011 is more typical.

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