The Five Large Volcanoes of Iceland
Iceland holds five volcanoes that in historical time shave had eruptions transcending the 10 cubic kilometer mark during post-glacial times. The most obvious are of course Grimsvötn, Bardarbunga and Katla who share the same eruptive grounds for their large scale fissure eruptions in the area of rifting running roughly from Vatnajökull down towards Katla. There we find the Eldgja/Laki/Veidivötn-fissure systems. Among those Veidivötns Thjorsarhraun is the largest lava flow after postglaciation.
These 3 volcanoes are of the rifting fissure type (with that I mean that they have massive fissure swarms that rift, not that they are driven by rifting processes per see), have massive central volcanoes with huge calderas and massive systems of magma-chambers. Normally they have frequent small scale to medium sized eruptions, and only have their large eruptions when the SIFZ (south Icelandic fissure zone) endures a rifting episode. Two of these are powered pretty much directly from the hotspot mantle plume under Iceland. The current location of the hotspot is believed to be close to Bardarbunga.
The two triple junction Behemoths of Iceland
The other two are different from the first 3 in many respects. Both of them are triple-junction volcanoes, driven almost entirely by rifting processes, are mainly fissure volcanoes with fairly unknown internal systems. Both of them are located fairly far away from the hotspot, and both are centers of unusually strong seismic activity for being located in Iceland. Both of them erupt in large scale when erupting, before going back to long periods of dormancy.
In southwestern Iceland we find Hengill, a large scale fissure volcano located next to Lake Thingvellir. The other is the northernmost of Iceland’s large sub-aerial volcanoes, just north of Krafla volcano. Theistareykjarbunga lies in the junction where the Tjörnes Fracture Zone (TFZ), the Grimsöy Oblique Rift (GOR), the Husavik Flatöy Fault (HFF), either directly intersects, or where they mechanically interact in a sense of it. Southwards from this the Northern Volcanic Zone (NVZ) comes running from Bardarbunga. Another thing to keep in mind is that Theistareykjarbunga is the northernmost of the main band central volcanoes of Iceland, and that it might have been responsible for the sub-aquatic eruptions out in Tjörnes area at the tip of its northern fissure swarm.
This of course creates tremendous tectonic strain that from time to time is released as massive earthquakes in the region. The last regional massive earthquake episode was in 1872 when 2 earthquakes larger than M 6.5 hit the Tjörnes Fracture Zone. During the last decades small but persistant quake-swarms have plagues the area.
Due to this a separate dense network of GPS stations where put up in 2006 for continuous measurement to enhance ability of calculating the pent up strain in TFZ and adjacent fault zones. It was emplaced and booted up in 2006. As a point of reference a GPS was placed on the believed to be dormant, or even dead, Theistareykjarbunga Central Shield Volcano, since this should be fairly stationary since it is almost dead center on the triple junction.
And for the first year that actually worked well. Then a continuous uplift started at Theistareykjarbunga with a maximum uplift of 30mm per year. This uplift caused concern, and the researchers then added additional GPS equipment directed at the volcano to get a better picture. Also other observational techniques were employed. Premier among the additional methods was the Envisat Interferograms that confirmed a circular uplift directly under Theistareykjarbunga Shield volcano. It confirmed inflation in a massive magma-reservoir with a depth of about 6.5 kilometers down, and covering an area of more than 70 kilometers making it into one of the largest on Iceland. And it is only logical that a Volcano of this eruptive ability should have a magma-reservoir on a large scale. Data and information taken from the article referenced in the end of the article.
Theistareykjarbunga as we know it came into existence as a shield volcano in one majestic continuous eruption of Hawaiian type. The amount of magma ejected is sourced by Global Volcanism Program to be the largest in the history of Iceland. But it is actually so that GVP also states that Thjorsahraun in Veidivötn (Bardarbunga) is the largest effusive eruption in Iceland after de-glaciation. I understand their confusion. They are so close in size that a competition is rather unnecessary, but on shear effusive volume nothing on Iceland can compete with Theistareykjarbunga. But then one should remember that it was not an explosive event, and Thjorsahraun had explosive components, and was a lava flow, not a central volcano building event.
According to GVP there have been only 3 eruptions at Theistareykjarbunga, but that is most likely not true, there are more large lava-fields belonging to both the southern and northern fissure swarms than they give credit to. But, it is understandable that they miss some, this is a very poorly researched volcano. A moderate number would be five large lava-field producing eruptions scaling in around 5 – 10 cubic kilometers, and the initial eruption scaling in on a total ejecta volume of 35 to 40 cubic kilometer if one combines the fissure eruption and the lava fields that was produced, and the part of the eruption that created the 30 cubic kilometer shield volcano. The last large eruption is well known, it was the 2700 year old Theistarekjahraun eruption.
Likelihood of Eruption
Theistareykjarbunga will most likely erupt. Big surprise. This is after all Iceland. Almost all of the active volcanoes will erupt many times in the coming geological timescale. So, just saying that it will erupt doesn’t mean that much really.
So what points towards an eruption? First of all the inflation points to an increase in eruptive risk, but there is also tectonic activity under the shield volcano that has a magmatic signature. And of course the sharp harmonic tremoring episodes that happened in October and November.
If one take into account these 3 signs it will give a certain relevance to assuming that the volcano is waking up from the 2700 year old slumber. It might of course still go back to sleep for a thousand years or more, one should always remember that.
But, if it continues to show signs, the inflation keeps on being steady, when would it then go? Well, at a minimum it would need another decade. But that is probably way to short time span. We would need long periods of uplift, ever increasing quake swarms, continuing movement of the adjacent micro-plate. Regarding the uplift, we would most likely need to wait for meters of uplift to happen due to the immense size of the magma-reservoir before it achieves critical pressure. Yes, we could have missed earlier large inflation periods down through the last 2700 years. But we should remember that Theistareykjarbunga deflated during the Krafla-fires, and that we still do not see enough quake activity for there being high pressure in the system. So I still would say that we need between a meter and five meters before anything happens. And that gives a time frame spanning between 30 to 150 years and current rate of inflation. But, the inflation could pick up speed at any time too.
Time will tell.
How would an eruption look like?
Here we are leaving science totally. I admit being on skimpy ground when guessing when Theistareykjarbunga could erupt, but here I am putting on my psychic hat full on.
The known eruption was effusive only, or almost effusive only. There could have been a bit of explosivity due to local hydrodynamic circumstances, but that would have been it. So if nothing has changed in the chemical composition in the magma-reservoir, hydrodynamics, or in the chemical composition of the new magma that has been entering the magma reservoir, then it should be a Hawaiian type of eruption with lava fountaining, probably a fissure eruption either on the flank of the shield volcano, or out into the southern fissure swarm (signs from the harmonic tremoring make the southern part a bigger risk), it would be ranging from the 5 cubic kilometer range and upwards.
Sadly we do not know the amount of gas that would be released during an eruption. We quite simply just do not know enough today to guess about that.
Is there a risk that the system has changed? Could it be explosive? Let us start with the hydrodynamics. I do not think that the amount of water in the system have increased a lot since the last eruption, my guess would actually be that it has decreased due to land uplift after de-glaciation, and a general drying out of that part of Iceland. On that reason I would say no. Could the chemical composition of the magma in the magma chamber have changed? Most likely. As time goes by magma changes chemically as it cools off and mixes with newly infused magma, and cools off again. Often this intermingling of evolved and unevolved magmas can produce magma types that can be fairly explosive. The magmas that was erupted before was low in Rare Earth Minerals, so they were not of the same hotspot origin as the Bardarbunga type that is massive in REM-content. And since the hotspot has not moved that much since the last eruption there is not much talking for it being another type of magma entering now, but this is just guess-work.
So yes, there is a very slight increase in risk that it will be a bit of explosive component during an eruption. But the average would be mainly effusive only. The scale of eruption though means that parts of the eruption probably would go as a VEI-2, or even a small VEI-3. Not much really if one count the scale of a likely eruption. But, the risk would be high gas content, and of course that so little is known about the volcano. Icelandic Met Office (IMO) will most likely keep an eye on things and expand the network around the volcano well in advance.
Edit: The end of Bob?
This could very well be the end of Bob as we learned to love her. The tremoring just went away earlier today. And the earthquake list for the last ten days also speak of diminshing pressure. It is still a bit to early to say that the eruption is over, but it looks like there is a possibility. If it is just a blockage of the conduit, we should during the coming days start to see a marked increase in earthquakes, both in number and intensity. If we do not see that it is probably over for this time. Sleep well little Bob.
As noted, it might be to fast to bump of poor Bob. Gas emissions are keaping steady, or even showing a slight increase during the last hours. Still the big one to keep an eye on is the earthquake list. But as noted by some of the ever vigilant readers and writers on this blog, the tremoring on La Gomera did not lower when CHIE stoped showing tremoring. So, little Bob might still be able to surprise IGN.