After writing the Woolly Mammoth Guide to Icelandic Volcanism I vowed to not write about Iceland for a while. That comment bit me in the posterior rather quickly.
Since the cessation of the Holuhraun eruption the central volcano Bárðarbunga has been highly seismically active. Many people have fervently believed that this has been due to inflation and the volcano being close to an eruption, but up until now that was not simply true. But let us bone things out together.
As a large Icelandic volcano near the hotspot stops its eruption it will still be receiving fresh magma at an astonishing rate. For Bárðarbunga the amount is between 0.04 and 0.8 cubic kilometres per year on average.
Let us now put that magmatic influx into perspective, of all known so called supervolcanoes and what is supposed to become supervolcanoes have a far smaller magmatic influx. The current leader of that pack is the nascent supervolcano of Cerro Uturunku on the Altiplano-Puna Volcanic Complex (that houses other supervolcanoes).
The normal accumulation rate for all of Altiplano-Puna Volcanic Centre is estimated at 0.001 cubic kilometres per year, but since 1996 the area containing Cerro Uturunku has had a magmatic influx of 0.01 cubic kilometres per year. So, it is perhaps not surprising that Iceland is erupted on such a scale as it does and with the frequency it does.
After that detour we can get back to the nitty gritty of Bárðarbunga. During Holuhraun 1.2 cubic kilometres of lava was erupted which equates in cooled and degassed form to 0.6 cubic kilometres of magma. Based on that figure alone the volcano should be ready to erupt after 18 months or less.
There is just one problem with that equation and that is that most magma is not ending up in Bárðarbunga as such; most of it ends up as harmless intrusions along the fissure swarm, or ends up staying in the large accumulation zone under the Veidivötn Fissure Swarm that Bárðarbunga is a part of. Normally about one tenth or less of all emplaced magma erupts. So, instead of one eruption per 1.5 years we should be seeing one every 15 years or even more seldom.
Let us get back to all that juicy seismicity that has had a lot of people going “oh” and “ah” about. In the end they were of tectonic nature and not magmatic. The cause for them was the lowered pressure in the upper magma reservoir allowing the caldera floor to drop downwards.
As such that does not mean that an eruption will occur, but it helps with raising the pressure inside the magma reservoir in a more rapid fashion. It also weakens the ring-fault that run all around the caldera so that it becomes easier for magma tendrils to move upwards.
Up until a week ago this behavior of tectonic earthquakes dominated the activity at Bárðarbunga.
The current activity
In the months prior to the last eruptions there was a spattering of deep earthquakes at depths larger than 20 kilometers indicating that fresh magma was moving upwards into the Veiðivötn volcanic system, it was those earthquakes among other things that I used to predict the Holuhraun eruption.
After the eruption those had ceased so we knew that there was no high influx of magma, but a few weeks ago those started to crop up again in the same manner as prior to Holuhraun. These earthquakes are widely distributed and are per se not a good way to predict exactly what volcano will erupt, but they are an indicator of a possibility for a future eruption in that particular region of Iceland.
Prior to the eruption they clustered around the Kistufell central volcano and Trölladyngja on the Veiðivötn fissure swarm. Out of those deep earthquakes an earthquake swarm formed clustered around Kistufell where an intrusion occurred that ended up in what was discovered to be a magma chamber. For various reasons that intrusion could not penetrate the roof of the magma chamber and instead diverted via a dyke into Bárðarbunga proper and the hubbub commenced.
It was at the time the inter-volcanic dyke formed I became convinced that we would have a Bárðarbunga eruption, but most in here remember that fruitful volcanic prediction.
Right now we have those deep earthquakes under Kistufell and Trölladyngja and I am rather convinced that this once again is a sign of rapidly moving magma pushing upwards like a soft pillow hitting the underside of a table. As of now no intrusion earthquake swarm has started to move upwards at Kistufell, nor has one started at Bárðarbunga. If we judge solely from that we are still a bit of ways from another eruption.
Let us now jump back to the tectonic seismicity that has plagued Bárðarbunga since cessation of the Holuhraun eruption. About a week ago we started to see a few earthquakes of a type that could mean that magma was moving. And early Wednesday morning a small but intense earthquake swarm started with a M2.9 earthquake that rapidly turned into a series of earthquakes of a type indicating moving magma.
The epicentre of that swarm was on the NE corner of the caldera and it is my opinion that a tendril of magma moved upwards through cracks along the NE part of the ring-fault and that if it happens a few more times it could lead to an eruption.
What would an eruption be like?
There are still not enough signs around to believe that yet another large eruption would take place at Bárðarbunga at this time. Even though the prior dyke that lead to Holuhraun is mainly filled with warm magma it is not a likely place for an eruption since the area has already rifted apart.
It is therefore far more likely that a large eruption would occur down to the south. But, remember that a large magmatic intrusion has not yet occurred into Bárðarbunga, so therefore another large eruption is unlikely right now.
Instead it is far more likely that a small caldera eruption would occur as magma leaks upwards along the NE corner of the ring-fault. The last proven eruption from the caldera occurred in 1996 during the Gjálp eruption (seen by IMO-staff). That eruption was a minor VEI-1 eruption and caused no problems at all.
And if we look at the historical record of eruptions from the Bárðarbunga caldera we see that a majority of the eruptions have been VEI-1 or VEI2 eruptions with a small sprinkle of VEI-3 eruptions. No larger eruptions are known to have taken place at the caldera in the Holocene.
So in short, do not be overly surprised if there is a small eruption at the caldera in the next few days to months. And my bet is that it would occur in the NE part of the caldera. And I do not expect anything big to happen. Unless of course we get yet another large intrusion, but then it is far more likely that the side SW wall of the upper magma reservoir breaches and something squirts out down at the southern side of Veidivötn.