Someone once said that Iceland is one big volcano. Obviously, this is not true. In fact, it is not even one big volcanic system. Parts of Iceland are mainly powered by the Icelandic mantleplume, with just a bit of power coming from the spreading of the Mid Atlantic Ridge (MAR). Then there is a group that is more evenly powered by the mantleplume and the MAR, and the third group is powered by the spreading MAR only.
But, in the hinterlands of Iceland lurks volcanoes that defy all of this, volcanoes that seemingly does not draw power from any of these two driving forces. In the east, we find the Öræfajökull Volcanic Belt (Ö–VB) and in the west we find the Snæfellsjökull Volcanic Belt (SVB).
Whereas we do know that the origins of the Ö–VB lies in a long-gone history as a main part of the MAR that over time has gotten shoved to the side, the SVB is shrouded in mystery.
One would think that this would be enough for a single volcanic country, but alas no. Out beyond the volcanic hinterlands there are more unknown volcanoes to be found. And as we will learn, almost all we know about them is turning out to be wrong due to new data emerging from an ever more accurate sensor-network. Let us go for a walkabout into the unexpected.
About a month ago a highly unusual and slow earthquake swarm started in one of the few spots in Iceland without known volcanism and sprungur (earthquake fissures). At least that we knew about. So, I started digging and got the help from Andrej Flis in my volcanic spelunking.
As he compiled all of the earthquakes that have happened in the area he found not one, but 3 unknown sprungur that are unusually distinct, and prolonged in time. And, to my surprise I found a nicely preserved little shield volcano with a crater lake called Skotmannsfell.
By the looks of it Skotmannsfell has not erupted since the end of the last glacial period in Iceland. We know this since the volcano is so low that the ice age glacier would have ground down the very distinct crater, filled in the crater lake and turned the entire volcano into an indistinct mound.
By using gravimetric and seismic velocity anomaly maps we should be able to discern if it is a central volcano or not. The size, depth and consistency of seismic activity tells us that there should be an active magma feeding system leading up from the bottom. But, here comes the surprise, there is no magma reservoir lurking under Skotmannsfell.
Instead there is a residual trail leading towards another volcano, making Skotmannsfell into a monogenetic flank volcano that once was fed by a dyke from its parent volcano.
Even though a few of the earthquakes seems to have had fluid components it is most likely water or low temperature hydrothermal fluids and not magma that is at play. Skotmannsfell is so far a bad bet for the next eruption.
The name is derived from a nautical term, Skotmann is the title of a sailor that handled the ropes to the sails in old Norse. Now, let us go and visit the parent of Skotmannsfell.
If Skotmannsfell had a nautical origin, Ok is more of a farmers name since it means yoke. And there is a joke hidden in this volcano. The classic explanation of Ok is that it is a polygenetic shield that erupted during the last glacial period forming a distinct intermediary between a shield volcano and a Tuya.
The idea is that the volcano Prestahnúkur under Langjökull was the central volcano that injected magma via a dyke leading to Prestahnúkur, and thusly causing the eruptions at Ok.
But, as is quite evident in the gravimetric and seismic velocity maps there is a strong anomaly indicating a sizeable magma reservoir under Ok, and that it is discontinuous from Prestahnúkur. This is further supported in the earthquake plots made by Andrej.
Will Ok erupt again in the future? Currently there is nothing pointing towards an eruption being upcoming, but the level of seismic activity leads to the conclusion that it should still be counted as dormant volcano able to become active again.
Now, the anomaly maps that Andrej gave me for this article left yet another trail of breadcrumbs for me to track down. And that trail led me all the way into the capital of Iceland.
“It was the best of cities, and it was the worst of cities”
It often surprises tourists that there is an open steam pipe coming out of the ground in the town square in Reykjavik. In most other cities, this would indicate that a heating pipe has burst in the ground, but in Iceland that steam is smelling of nicely rotten eggs. This of course indicates that there is a not too long distant volcanic past in Reykjavik.
Just a few decades ago there was still springs hot enough to clean white linens in, but they are now gone. Previously it was believed that a field of monogenetic cones east of Reykjavik had caused an intrusion running in under the city, and that the cooling magma from that intrusion was heating the springs.
But, there is an equally plausible explanation for those springs, and that is a far more majestic volcanic feature called Esjan.
It is a volcanic fissure line that started erupting 3.2 million years ago at the beginning of the ice age, and that it stopped erupting 1.8 million years ago. During the interglacial periods the eruptions left flood basalt layers and during glacial periods thick layers of tephra was deposited.
The classic view is that Esjan was fed by massive dykes leading from central volcanoes in Kjalarnes and Stardal. The only problem with this idea is that it is ludicrous when evidence is set towards belief.
First of all, if the classic view was correct, we would get the same picture as between Skotmannsfell and Ok. The area directly under Esjan should be free from a magma reservoir, unless it was a central volcano in its own right.
Only problem with that idea, is that by now the magma reservoir would be so cold that any gravimetric and seismic anomaly would be gone. Now for the big surprise. Esjan has one of the largest anomalies in Iceland.
This is a bit of a stumper. The only way there could be an anomaly is if intrusions from depth had come up repeatedly for 1.8 million years without causing an eruption.
What has most likely happened is that after the last large eruption at Esjan a very large magma reservoir remained under the volcano. And that at a slow but highly persistent frequency just enough fresh hot magma intruded to keep the large anomaly alive.
By now all of that magma should have turned into Iceland’s largest reserve of rhyolite, but thankfully it is most likely non-eruptible.
It always amazes me how many volcanic features there are in Iceland to study. And in the end the lesson is that Iceland can truly sprout a volcano almost anywhere at any time. It is just more likely that it will happen over the spreading Mid Atlantic Rift, or in the vicinity of the mantleplume.
Of the 3 volcanoes I have written about here only Ok is likely to erupt sometime in the deep future, even though I am highly intrigued by what is happening next door at Skotmannsfell. Esjan is of course only of academic interest, but I must admit to having been amazed and awed of finding such a large reservoir of magma near the capital.
Now, there is just one last thing to clear up. I have mentioned that Skotmannsfell is of nautic origin and that Ok is a highly agricultural volcano. So, what about Esjan? Let us once more turn to old norse. The word still lingers on in modern Swedish with the spelling Ässja (it is pronounced the same). If it is accurately translated into English it means The Forge and nothing else. And what could be a better name for an old majestic volcano?