The Happy Dyke of Fagradalsfjall

Dalssel at Fagradalsfjall. Image from


The good part about volcanology is that nature will sooner or later test both your theories and your scientific models. In this case what was tested was the original model of the available pent up seismic strain in the currently active area of Reykjanes Peninsula in Iceland.

It turned out that the amount of pent-up strain was higher than the models had given at hand. Science loves when oddballs like this comes around the corner, then you get the opportunity to see if something is wrong or missing with your theories, or if there is something wrong with the models used. I love the smell of science in the morning.

As everyone has noticed by now, well at least everyone interested in Icelandic geology, the seismic activity continued well beyond what could be expected according to the model for the area. Since the start of the seismic crisis an average of 20 000 earthquakes has occurred per week, and the size of the larger earthquakes are comparatively in the same range without any sign of them abating.

During the first week of seismic activity, we mainly saw tectonic earthquakes along the portion where the Mid Atlantic Rift enters the Reykjanes Peninsula in a WSW/ENE-direction spanning most of the peninsula.

After that the activity started to concentrate in an area between Keilid and Fagradalsfjall, with a migrating trend towards the southern part of Fagradalsfjall.

At this point in time this is the largest earthquake swarm recorded instrumentally in Iceland. It is in fact so numerous in earthquakes that it stands for a large portion of all earthquakes ever measured in Iceland. It even dwarfs the famously noisy eruption of Holuhraun in 2014-2015.


Tectonic or Magmatic?

Dykemap, courtesy of the Icelandic Met Office.

One of the most irrelevant questions in Iceland is probably if an intense earthquake swarm is tectonic or volcanic. The reason I am saying this is that sooner or later an initially purely tectonic swarm will turn into a magmatic swarm as intrusions sets in.

Here I am talking about earthquake swarms that are not directly in or around the well known large central volcanoes, they operate a bit differently. I am here keeping to areas like the Reykjanes Peninsula.

The reason for this is simple fluid dynamics. The main tectonic regiment in Iceland is caused by the rifting apart of the Eurasian and North American continents. This is by no means a constant process, instead you get increased strain locally, and that strain is later released as an earthquake swarm (or as a large earthquake, but now we are talking about swarms).

The swarm in turn is in the beginning tectonic as things are pulled apart, and here it is where fluid dynamics comes into action. I will here invoke a rule, nature abhors a vacuum. So, if a void is formed in the crust as it is pulled apart two things will happen.

Scenario number one is the purely tectonic one, and the rift will snap back together giving no dilation of the rift. In other words, the vacuum problem is solved by the sides of the rift coming back together (most often not exactly in the same way).

I am obviously simplifying things quite a bit here, but the principle is correct. In these cases, you might get lateral slip or vertical slip of one edge, but you do not see dilation. This is quite easy to see a few days later on the GPS-network.

The second option happens if the swarm is ongoing for some time and the crust is fractured enough that magma can enter from below. In this scenario you will instead be having fresh magma being sucked into the void, filling it up.

Now we have dilation that will be visible on the GPS system if we are in the happy situation that we have a GPS-station on either side of the rift as the earthquake(s) strike. It is visible since the stations will move apart, and then they stay apart.

The important part here is that it is quite common that this happens in Iceland, it usually happens a couple of times per year, or even more. After all, most intrusions will fail before an eruption occurs since the intruding magma is just enough to fill up that pesky void in the crust.


The Happy Dilating Dyke

Cumulative seismic release over 11 days. Image by Gaz Dale based on data from the Icelandic Met Office.

Now things will become a tad technical, so I need to explain a few expressions as I go. The first one is obviously the word dyke. It is a form of volcanic conduit that is extending from the mantle upwards like a hanging sheet of magma, sometimes spanning a considerable horizontal distance.

It forms like I explained above as the rift is pulled apart by the continental drift, the ensuing void will be filled by magma. If the magma moves far enough upwards it will start to exert force in two ways.

The first way is through buoyancy, due to being hotter than the surrounding bedrock it will buoyant and will strive to move upwards like a hot balloon. If the magma is shallow an eruption might happen since there is not enough time for the magma to solidify in the dyke.

This is a fairly slow process, and it can be followed since the earthquakes will be slowly migrating upwards.

The other force will kick in if the magma comes close enough to the surface. All magma contains volatile compounds like water and gasses. In Iceland there is comparatively little water in the mixture, but there is quite a bit of gas that can nucleate out of the magma.

Cumulative number of earthquakes over the last 11 days. Do note that the numbers of earthquakes do not correspond to the energy release. The numbers show a decrease, but the energy is increasing. Image by Gaz Dale based on data from the Icelandic Met Office.

The amount of gas becomes higher the closer you come to the mantleplume that is residing near Bárdarbunga below Vatnajökull. In Reykjanes the magma is poorer in gas, so the force is less.

As the magma nears the surface the pressure from the crustal over-burden will decrease, and the gasses that are locked into the magma will start to come out.

Think of it as a warm soda can that has been shaken. If the lid is on (over-burden pressure) the carbon dioxide will stay dormant in the soda, but as soon as you pop the lid you will get covered in sticky sugary soda.

As and when this start to happen the magma will start to expand and that increases the pressure, and this in turn will push apart the dyke even more, and the gasses will push mainly upwards since they are a lot lighter than the magma it is originating from.

By now we know that a dyke has formed at the old (formerly) dormant Fagradalsfjall volcanic system. From GPS measurements and InSAR we know that the dyke is 7km long (horizontal extension) and that it is 5km deep (vertical extension), and that it is 1.2 to 1.5 metres wide (dilation).

Depth of earthquakes over the last 11 days. Image by Gaz Dale from data by the Icelandic Met Office.

We know that in volcanic rift systems dykes prefer to extend horizontally so that they become longer as magma continues to pour into the expanding rift. This is due to the mass of the magma creating a disproportionate pressure lengthwise.

This will continue until the expanding rift encounters harder colder bedrock at the ends. When this happens, the systemic pressure will jump up a notch.

In a system like Fagradalsfjall there is a lot of pent-up strain, up to 22 meters worth of it. This means that the lava can push the sides of the dyke apart until all strain has been accounted for.

What now, 22 meters? Well, that is the maximum figure of the accumulated motion of the MAR at Fagradalsfjall since the last eruption in the region. From this we obviously need to deduct all swarms that have formed dykes since the last eruption in the region.

To this question we do not have an answer. So, the available pent-up strain might be another half a metre, or several meters.

Latitude of earthquake locations for the last 11 days. Image by Gaz Dale over data from the Icelandic Met Office.

And this is the interesting part, because as soon as that is done the pressure will notch up again, and the dyke will start to propagate upwards, plus the buoyancy effect, plus the expansion of the magma as it nucleates out volatiles… By this point there is literally no return and an eruption will occur.

Now we just need two more figures of interest. The first is how much magma is entering the system, and that is roughly 20 cubic metres per second on average. This in turn equates to 50 cubic meters per second of lava in its fully nucleated fizzy state if an eruption would occur. At least if we assume a steady state of magma arrival to eruption rate.

Now we just need to know how much magma has been emplaced at this moment. It is so far a measly 0.05 cubic kilometres and counting.

This equates to what is called a tourist eruption in Iceland, complete with nice lava fountains that may come from one or several vents along the rift. There will obviously be little, or no ash if the eruption does not end up in the ocean. I will soon return to the ocean issue.


The miscalculation

Longitude of the earthquakes during the last 11 days. Image by Gaz Dale using data from the Icelandic Met Office.

This section could also be called: When nature kicked Carl’s model in the teeth. I can though comfort myself with being in good company on this.

My initial modelling was based on historic instrumental data. And this gave at hand that the pent-up strain was significantly less than it was.

This made me assume that the maximum possible seismic release would be akin to the famously noisy eruption at Holuhraun, and that when about as much seismic energy had been expended an eruption would occur. Oh boy was I wrong.

Back when I did my research on the Lakí eruption I modelled that it had by necessity had a very high seismic activity, including several earthquakes above M6. This also seemed to be evidenced by written sources and collapsed houses.

Back then nobody agreed with my estimates, but after seeing this I think that it was correct after seeing this utter melee of earthquakes. After all, the strain potential in the area where Lakí happened is much higher compared to Reykjanes.

I do think that Reykjanes is proving my original point. Alas, I digress…

What I had done was to take the historic instrumental data and just extend it backwards for Reykjanes all the way back to the last eruption.

In hindsight it is easy to see where this model was wrong, and I should really have caught my mistake. After an eruption, the strain build-up will be aseismic. This means that for quite some time there will be no earthquakes releasing the strain, and this creates a bias in that you would assume that there is more released strain compared to the reality of nature.

Second mistake was that I assumed that the crust was more plastic compared to what it turned out to be. This in turn meant that you get more, and larger, earthquakes compared to the size of the dyke since it takes more power to crack the rock.

Therefore, my original estimate of the needed time until an eruption would start was off. Originally my original estimate was 5 to 12 days (starting the clock from the beginning of week two of the swarm).


Current estimate

The Icelandic word for a dyke is Kvikugangur. This is a pretty good graphic of the dyke at Fagradalsfjall. Think of it as a hanging sheet of magma candy… Image from Iceland Review.

I am still convinced that an eruption will occur, and for the same reasons as back when I started to see this as a runup phase.

That being said, pinpointing exactly when and where is something completely different. Let us start with guesstimating the when (well, at least if nature does not throw something new into the works that I have not yet figured out).

Let us start with the length part. I cannot for the life of me see that the dyke will continue much further south, at least not longer than to near the coast. There is just no evidence that Fagradalsfjall has erupted previously that far south.

This means that the crust is becoming ever harder and more resistant the further south the dyke extends. As and when this happens the dyke can extend a bit more to the north, and then the same cold and hard crust will happen again.

After that, the question is how much the dyke can dilate as the magma pushes it apart. This is obviously the big unknown part. We do know that it is less than 22 meters, and significantly less so, but how much? I have a problem seeing more than another 1.5 to 3 meters, but here is where I was very wrong previously.

Let us say that we have another 0-5 days of horizontal extension, and 0 to 20 days of dilation. After that there is less than 2 days of vertical extension until the magma reaches the surface.

Now, is there any sign that the pressure is increasing that we can look for? Well, yes there is.

Currently the magmatic intrusion has been mostly evidenced as horizontal displacement and only locally have upwards motion been detected. As and when the horizontal displacement dies off and the vertical motion picks up pace an eruption could happen anytime.

So, what is the verdict on that? Yes, there is increasing vertical uplift, not by much but enough to take notice of.

So, the guesstimate is that an eruption is probable to occur within 2 to 25 days from now, unless something happens that is stopping things up, and I see that as ever less likely. I also believe that the eruption will occur in the southern part of the rift.


486 thoughts on “The Happy Dyke of Fagradalsfjall

  1. The location mentioned is Geldingadalur, midway between Fagradalsfjall and Stóri Hrútur.

    • If I understand correctly, that is a small valley that would confine the lava nicely. If it escapes, it could quickly cut the main road and reach the sea but that depends on where exactly it is. It is looking good. (And it might even flow north?) Of course gas emission is less easily confined.

  2. Its really funny how that went. First endless quakes and constant will it go up here? or there? Then relaxation and subsiding of the land compared to before and everyone relaxed again. And just as we relaxed the eruption came.

    • Yes, I think it was mentioned the other day that many eruptions like this in Iceland often happen as the swarm dies down. I think it is pretty clear now that for rifting eruptions like this the main swarm is associated with lateral movement of the dike, vertical movement might not make a lot of noise.

      At least we now know an eruptive event in this bit of Iceland is not going to blast out of the ground in a day from the deep mantle, so we know what to look for in future. It is still quite crazy that a month ago Fagradalsfjall would qualify as an extinct volcano, at least assuming it isnt a part of Krysuvik-Trolladyngja. Extinct to erupting in a month.

    • Just seen a helicopter flying over the eruption area… Right on cue.
      Eruption appears to be not far beyond the ridge line, just left of the largest peak.

  3. hi, we live in grundarfjordur and we would like to come and see the lava. where can we leave the care and from witch point can we walk. How long is it ? thank you ,)

    • We would not recommend this. The eruption is not explosive and there is little or no ash but the cone is on a ridge and is already build up a fair amount: it could collapse which could fling debris around. The gas emission is probably the most dangerous part. I believe that the main roads in the area are closed. Your bets bet is to ask IMO and follow their advice. They will tell you to watch it on the cameras.

        • Bjarki commented that there is now advice on how to see the eruption safely. There are a fair number of tourists now on the live feed! I am quoting his comment:

          “Authorities are recommending that people who want to hike to the eruption, start at the Blue Lagoon, as you’ll have the wind at your back, so that dangers from gas are minimized. You should be well equipped for the hike good boots/clothes, as there’s wind and rain/sleet atm, it’s a multihour hike. Preferably be in groups, bring a packed lunch, and be aware that in case of an emergency that cellphone coverage in the area isn’t great.

          When getting closer to the eruption site, people should be aware that new cracks in the ground can occur without warning.

          The S&R team on duty have already encountered illprepared volcano tourists, some only wearing jeans and sneakers, some attempting to go with bikes, etc.

          It will also do the job of S&R easier if they know the area the hikers will be in beforehand, instead of them being spread out over a wide area.”

          The bit about coming from up-wind is quite important. Do check the weather forecast. And always: the safety advice should come from the local authorities.

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