Activity at Hekla and The Dead Zone

The Eldgjá fissure in all it’s 934 glory. Photograph by Andreas Tille, Wikimedia Commons.

While we are waiting for Öraefajökull to drop a Christmas present and Grimsvötn to hatch an Easter egg, we instead might get a gift from Hekla. And at the horizon suddenly, a far darker bird looms.

So, once more we must ask and answer the age-old volcanic question; what gives in Iceland?


The two earthquakes swarms of Hekla and The Dead Zone. Image from the Icelandic Met Office.

Many people have noticed that Hekla has been quite busy in the last few months. There have been no less than 3 different types of earthquakes there. Or, to be more exact, 3 different locations indicating 3 different things going on at the same time.

Let us start at the top, then go to the bottom, and then cover the in-between.

So far roughly half of the earthquakes have occurred at a depth of no more than 0.1 kilometre. It is easy to think that these earthquakes are dangerous as they are occurring at the bottom of the edifice.

Morphology of Hekla, image by Gaz Dale based on data from the Icelandic Met Office.

In reality, they are just a sign that one of the most common forces in physics is at work. You might even say that it is evidence that our Universe is quietly doing business as usual.

The bedrock under Hekla is slowly pulling apart making the foundation of the volcano fragile. And on top of that fragile cleavage, you have an edifice that has grown rapidly over the last 5000 years (the edifice is not older than that).

At every eruption, Hekla gains quite a bit of height. But, as soon as the show is over and the volcanologists have taken their GPS reading on the top, gravitation goes into overdrive and the entire edifice will start to drop down like a stone into an ocean of lard.

Plot of earthquakes at Hekla since 2011. Image by the Icelandic Met Office.

And it is that downward force that causes those earthquakes at 0.1 kilometre as the rock under the heavy edifice breaks Under Pressure. One can think of Hekla as the perfect Queen metaphor, during eruptions it is singing “We will rock you”, and directly after it switches to “Under Pressure”.

If we go down into the deep bowels instead, we find earthquakes associated with upward movement of magma. These occur at depth around 20 kilometres, or even deeper. As magma ascends towards the edifice it puts the conduit under pressure and the walls crack and groan. These deep earthquakes are among the most important earthquakes in Iceland to keep track of, since they normally herald future activity.

A few weeks ago we saw that kind of activity at Hekla, and it is not surprising therefore that it has become a tad more lively.

Now, if we instead look at the medium level earthquakes we can divide them into two separate categories. The first one would be caused by a pressure increase in and around the magma reservoir of Hekla as the rock surrounding it start to crack. This is also a potential sign of an upcoming activity.

The other type of intermediate depth earthquakes are instead tectonic. The volcano Hekla is a rifting fissure volcano that has formed a polygenetic cone on top of it reminiscent of a stratovolcano. I have dubbed this volcanic class of one as Stratofissure. It is an apt description since the initial phase of an eruption behaves like a classic explosive stratovolcano on steroids, and then it quickly starts to behave more like a large effusive fissure eruption.

Normal Icelandic tectonic activity rarely has any volcanic ramifications, but here these tectonic earthquakes are a sign that the fissure is being pulled apart. And pulling apart the roof of a magma reservoir at a steady pace is not good for magma containment.

Here there is a case of the hen and the egg. Is it the increasing magma pressure that is pushing the fissure apart? Or is it the pulling apart of large tectonic forces that causes magma to ascend upwards?

In this case, we get both hen and eggs to eat. By now this has become a self-sustaining system being pulled apart at the same time as it is being pushed apart. It explains quite a lot of Hekla’s behaviour and characteristics.

In the end, all we can say is that it seems like Hekla is closing in on doing the fandango. But, what exactly the time frame is, well… that is a completely different question. In a geological timeframe, it is probably just moments away. So, it is 30 minutes away at any given time, and those 30 minutes can happily extend into years.

The Dead Zone: Revenge of Elgjá

Image with the major fissure swarms in the Dead Zone, complete with the recent earthquakes. Image by Gaz Dale based on data from the Icelandic Met Office.

Roughly East northeast of Hekla is a zone that is aseismic. It is a bit of an oddity that it is aseismic since it quite literally is the spot where the planet is being pulled apart. Lurking and I dubbed it The Dead Zone, and in the end, we even came up with a good explanation for how it can be so quiet for such a long time before all hell breaks loose.

The area is filled with some of the largest volcanic features on the planet, massive rifting fissure swarms coming out 3 different large volcanic systems. Bardarbunga has the largest rifting fissure swarm, it is sometimes called the Veidivötn system.

Grimsvötn has two different fissure swarms in the area, the most famous is called Lakí. And Katla has one called Eldgjá.

All of these rifting fissure swarms are true giants able to erupt between 15 and 35 cubic kilometres during their rare eruptions.

Since humans settled in Iceland, 3 large eruptions have occurred. The first one was the 934AD 18 cubic kilometre eruption of Eldgjá. In 1477 Veidivötn had a VEI-6 eruption creating 10 cubic kilometre of tephra and 5 cubic kilometres of lava flow.

The last big hit was Lakí in 1783, known as the deadliest eruption to ever happen to humanity. Out of the two million dead, very few died from ash-fall or lava. Instead, it was the ensuing famine caused by a rapid cold onset that killed most people, the second largest killer was the released gas destroying lung tissue all over northern Europe.

Morphology of Eldjá and Katla, image by Gaz Dale based on data from the Icelandic Met Office.

The tally of Lakí was 12 cubic kilometres of tephra judged from distal ash fall found in bogs and ice cores all over the northern hemisphere. The oddity is that very little ash is found in Iceland, probably due to the heat sucking the ash high up and out over the ocean.

It also produced prodigious amounts of lava, more than 15 cubic kilometres.

The large Icelandic rifting fissure eruptions are cyclic in nature occurring roughly 210 – 250 years apart. As some of you may have noticed, sometimes the cycle skips a beat. But, in the end, during one of the cycle peaks, another large eruption occurs.

And it does not take a large amount of mathematical skill to realise that the last eruption occurred 235 years ago.

Iceland is often called The Land of Ice and Fire, but it should be called the land of volcanic cycles. Almost every volcanic area of Iceland has its own volcanic cycle, and all of them are ruled in part of the mightiest of all volcanic cycles, and that is the mantleplume cycle.

Roughly every 170 years apart the mantleplume has a pulse pushing up deep magma from the mantle, injecting fresh material into many Icelandic volcanic systems causing a marked uptick in eruption frequency. A bit arbitrarily we say that the current pulse started in 1996 and that it is peaking about now.

This is probably a bit of conjecture because the deep magmatic earthquakes started after that in the new millennium. This is in the end just a bit of nitpicking, we know that the mantleplume pulse by now is in full swing.

This combination of Iceland’s two largest volcanic cycles increases the probability that a large rifting fissure eruption will occur in the Dead Zone.

Now, let us get into the tectonic nitty-gritty side of things. We know that Iceland is being pulled (or pushed) apart at a fairly steady rate of 2.8 centimetres a year. 2 of those centimetres are taken up in The Dead Zone and up through Vatnajökull.

This does not account for local variations, we are after all talking about continental plate movement averages. So, any given year the in this context small area of The Dead Zone quietly is pulled apart 2 centimetres. We can see that this motion is steady and clear on all GPS stations in Iceland. Locally it can differ a bit, but not that much.

This changes quite a bit during a rifting fissure eruption. All of a sudden a large line extending through 2/3rds of the length of The Dead Zone can pull apart 150 metres in just a few months, leaving immense scars in the scenery.

This difference between large-scale plate tectonic motion and local area tectonic motion is caused by the combinative forces of plate tectonic pull and rapid injection of vast amounts of magma pushing violently apart the land.

Another effect of this is that the least likely candidate to suffer a rifting fissure eruption is the system that last erupted since the tension is all spent in that area. Now, remember that the pent-up tension is not the same as the amount it will pull apart in the end.

In 934 Eldgjá happily rifted 150 meters, but since then the strain has built up with roughly 11 metres. One would then assume that it could not rift on a large scale again, but that is a fallacy since it does not consider the pushing side of the push-pull nature of The Dead Zone.

In fact, the pull part of the equation is the weaker one, since deglaciation The Dead Zone has only pulled apart 240 metres, but the total amount of fissure width combined is believed to be 1500 metres. Quite a difference, on the local scale magma push, is the giant and on the continental-scale, the gravitational pull is the winner. Always remember the scale at hand.

Now, let us just assume that the cycle will not skip a beat, after all, we have two cycles at the same time. Which volcano of our 3 usual suspects would be The One?

Lakí is a bit of a long shot. First of all, Grimsvötn is mainly a major central volcano erupter. Out of the roughly 100 known eruptions, only 3 has come out of the fissure swarms of Grimsvötn. Also, it is also the most recent to suffer a large rifting fissure eruption. The upside is that Grimsvötn is the world record holder of magmatic influx, so it has by now amply received more magma than it expelled as lava during the 1783 eruption. In the end, I still judge this as the least attractive candidate, it seems quite content with causing ever larger explosive eruptions at the moment.

Bardarbunga is the champion of rifting fissure eruptions in Iceland. 14 of the 20 known rifting fissure eruptions in the Dead Zone has stormed out of Bardarbunga, and that includes the 3 largest. The problem here is that it erupted in 1477, so the tension is perhaps not enough. Also, the Holuhraun rifting fissure eruption in 2014-2015 removed a lot of pent-up lava from the system. I would say that we are at least a decade away before Bardarbunga could get into the fray due to low systemic pressure.

Katla, on the other hand, has had a long repose time since it’s last eruption, on the 12 of October Katla celebrated its centennial since the last known eruption. At the same time, we know that Katla has had 3 major intrusive phases in the last 3 decades, and there are signs that a fourth intrusive episode is underway as evidenced by renewed earthquake activity below the 20 kilometre mark.

Combining this with it being the volcano that has gone the longest without a rifting fissure eruption places Katla and Eldgjá in the pole position until another one bites the dust.

Well, don’t stop me now. As I mentioned above, The Dead Zone is quite aseismic. Every little earthquake out there will have the Conoscenti raise their eyebrows hoping for a bit of I want to break free from one of the fat bottomed girls.

I also mentioned above the importance of deep intrusive earthquakes.

So, what got me writing today is that a deep intrusive earthquake episode started at 21 km depth and faulted up to 16 km depth in a direction heading towards Eldgjá. The event started at 09.28,10 Icelandic time and lasted 9 minutes.

At 09.56,57 Icelandic time, the first of 3 earthquakes (so far) occurred at Eldgjá. As such it is not such a stretch of the imagination to think that the initial upwards moving intrusion injected directly into the fissure swarm. If this piece of conjecture holds up to scrutiny we should in the next few days to weeks see more earthquakes out on the Eldgjá fissure swarm.

A bonus volcano

And as I was writing this I also noticed that another of the usual suspects has changed its behaviour. Obviously, it had to be Grimsvötn. Since the last eruption in 2011, it has been quite a bit in the doldrums as it quietly inflated towards the next eruption.

Morphology of Grimsvötn, image by Gaz Dale based on data from the Icelandic Met Office.

This happy little swarm is happening right south of Grimsvötn proper in the direction towards the sister volcano Háabunga. This is in an area that has caused repeated trajectory changes at the GFUM GPS-station in the last few years.

Grimsvötn Cumulative Seismic Moment Plot. Image by the Icelandic Met Office.

It has though not had an awful lot of earthquake activity in the form of Cumulative Seismic Moment energy release. CSM is historically a very good tool to predict when Grimsvötn is ready to erupt, so a sharp increase in Cumulative Seismic Moment energy release is a good sign that Grimsvötn is once more rapidly cooking up another batch of Bohemian Rhapsody.

Grimsvötn 15 day earthquake plot. Image by the Icelandic Met Office.

I will guess that the volcanic version of Montserrat Caballe is not more than a couple of years away from erupting, and as usual, it will be noticed even in Barcelona.



218 thoughts on “Activity at Hekla and The Dead Zone

  1. My top moments living 5 years in Iceland:
    – sleeping in my jeep 40km away from Hokuhraun and then my jeep broke down in the middle of Iceland
    – flying above Hokuhraun a week later
    – hiking atop Oraefajokull with glacier ropes
    – returning home (from 100 meters away) crawling in the floor due to a snowstorm with hurricane winds
    -walking in Fimmvorduhals 2 months after the eruption and having my rubber of my left boot partly melted (yes I was scared)
    – kayaking in the West fjords againt the tidal flow
    – watching clouds as dark silhouettes as Jupiter casted my shadow on the ground
    – northern light storm levels for 4 days in row in 2011
    -camping for 10 days in Sprengisandur and nearby
    – feeling a M4.5 earthquake and two days going camping in the nearby mountain which was the epicentre
    – snorkelling in Thingsvellir cracks in ice cold water
    -swimming in Arctic sea in spring. Promised to never do it again.
    – seeing 3 tractors breaking down in 20 minutes due to elves and seeing the stare in the face of everyone
    – deciding not to camp by Katla the night before the glacial flood in 2011, how lucky I was! Tourists got evacuated by helicopter that day!

    I was scared by most of the above. Some just a bit, some a lot!

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