The Forgotten Volcano

Vatnafjöll doing it’s best to look like any place in Iceland that is not interesting. Photograph by Fjola Dogg Thorvaldsdottir.

A few days ago, I was sipping on a rum and coke as I was flying over Kilimanjaro and the great continental divide ripping East Africa away from the embrace of the rest of Africa. As I did that, I got to ponder the other major tectonic powerhouse, Iceland.

People tend to forget or underestimate the effects of the African superplume and the Icelandic Mantle-plume on global tectonics. Only these major plumes can jostle about entire continents, or split continents apart.

There are of course a few other tectonic forces driving plate tectonics but compared to these plumes they are mere afterthoughts.

How do we know that these plumes are so important? The reason is simple, we can temporally lock the birth of the Icelandic plume (14.4 to 15.0 million years ago), with continental drift trajectory changes in the North American continent. But that is something best left to another article.

As I flew over Lake Turkana I got to think about the elephant in the room. The Icelandic volcano that I have not written about, the direct sibling of Hekla. In a tectonic sense of it, this volcano might be the most important volcano on earth. In other words, I have only told half of the volcanic story of Iceland. Time to fix things and go to the continental drift centre-of-the-world.



Location and orientation of the main chock. Image from Áugústson et Al.

Unlike it’s mechanical sibling Hekla, Vatnafjöll is a highly unassuming volcano from a visual perspective. It looks like any other volcanoclastic ridgeline in Iceland. In fact, it was so unassuming that up until recently it was just seen as some sort of dead-beat sub-set of Hekla, and since it had not erupted since 350AD, it was even seen as a dead volcano.

Before we go into that, we need to look at the tectonic setting, because that one is interesting. Vatnafjöll is first of all a triple junction volcano connecting the end of the Southern Icelandic Seismic Zone (SISZ) to the Eastern Volcanic Zone (EVZ) and the beginning of the Vestmannaeyjar Volcanic Belt (VVB).

Most often Hekla get the distinction of being at the end of the SISZ, but that is not true as we will see below.

Now most of you will be slightly oogle-eyed trying to get the tectonic setting to fit with the lack of large earthquake swarms. A place like that should be rife with large tectonic and volcano-tectonic earthquake swarms.

And it is indeed highly tectonically active, it is even giving Bárðarbunga a run for its money if you tally up the Cumulative Seismic Moment over time. The difference is just that Vatnafjöll does them far bigger, and further apart.

So, according to volcanic wisdom, as we partied ourselves into the year of 1987, Vatnafjöll was a dead volcanic subset of Hekla. As per usual, nature was about to throw us a spanner into our wheel of faulty preconceptions.



Timeline of earthquakes versus Búrfell station strainmeter (BUR) changes. Image from Áugústson et Al.

Before we move further, I must mention a few things about the paper I am referencing, it contains a glaring error. It sets the MOHO at 13km depth, instead of the 24km (and counting), that we now know to be true.

The reason for this is not to be put on the highly accomplished scientists in the field that wrote the paper, it is just that new and far superior equipment has made it possible to find those tiny deep earthquakes that we nowadays love to follow.

This leads to errors in ground plasticity in their modelling and is making the depth centroid dubious. It also has effects on the size of the intrusion that I will discuss below.

That being said, the paper is still sound at the core, and it is one of the more important volcanic papers that nobody has read.


11.31 May 27, 1987 M5.8 Earthquake

Location of the earthquakes, note how the after chocks tail off towards the VVB. Image from Áugústson et Al.

On May 27 at 09.01 in the morning one of those events that will make volcanologists slobber with saliva started at the southern end of the Vatnafjöll proper. It was an M4.0 Earthquake with a tectonic signal, so far not much to write home about. But, at 09.11 the Búrfell strainmeter started to detect an anomalous strain change signal indicating that an intrusion had started somewhere.

Initially it was believed that the earthquake might have kickstarted Hekla, after all the epicentre was inside what was then believed to be the Hekla volcanic field.

As more fore chocks came rolling in at the same spot as the initial earthquake this was ruled out, partially by utilizing changes at other strainmeters.

At 11.31 the main earthquake occurred. It was a North-South strike-slip earthquake lateral dipping slightly to the right. The depth of the fault plane was 4-13 kilometres deep and the length was 10-12 kilometres long.

In the region (SISZ) large earthquakes tend to be slightly offset from the transform fault patterns that run to the NNE. In other words, small earthquakes tend to be running from SSW to NNE, but the large ones go on a more NS trajectory.

For the next couple of days after chocks followed and the strainmeter changes indicated that magma moved into the newly formed fault. In the end a 0.8-meter-wide, 10km long and 9-kilometre-deep dyke formed containing a sum of 0.072km3 of magma according to the signals detected at the time.

If we now would remove the errata up above from this calculation, we get a minimum volume of 0.16km3 of intruded magma in 2 days flat.

Now, in most cases a dyke carrying that volume residing at 4km depth would become a problem of eruptive nature. But, it obviously did not. It didn’t even result in the usual seismic swarms as the magma would try to move upwards due to natural buoyancy.

Instead the magma ended up in what I believe to be a central magma chamber. We will obviously return to that in the next part where we plot the living daylights out of Vatnafjöll. Caveat here, I have not seen the upcoming plots by Andrej Flis, so I am writing in the dark here and nature might show me wrong.

Now we have a constraint downwards on how large a seismic event is needed for Vatnafjöll to erupt. A volcano-tectonic M5.8 is just not cutting it.


May 6, 1912 M7.0 Earthquake

Strainmeter changes on all the stations caused by the intrusion and earthquake. Image from Áugústson et Al.

This earthquake had a hypocentre 10km west of the 1987 earthquake. It was NS strike-slip earthquake that also was slightly right-lateral dipping.

This earthquake extended to the surface and is as deep and extends for 30 kilometres. As such it was a tectonic heavy hitter. We can safely assume that it too was a volcano-tectonic event, because this time it produced two eruptions.

The known width of the dyke is 8 metres, so we get an intruded volume of 4.8km3.

On the 25th of April 1913 eruptions started at Lambafit and Mundafit along the earthquake surface rifts, and they lasted until the 18th of May the same year, the total length of the erupting fissures was 10 kilometres and about 1/10th of the intruded magma extruded through the vents. As such it was not that much to write home about on an Icelandic scale of eruptions.

Especially from a volcano that at numerous times during the Holocene has erupted between 5 and 15km3 of lava. For being an unassuming hyaloclastite ridge, Vatnafjöll is a major leaguer.


Constraints and effects

Stress field projection. Ever wonder why we believe that Iceland is built from micro-continental-platelets? Image from Áugústson et Al.

Now we know that an M5.8 is not large enough to cause an eruption, and that an M7.0 is enough to cause an eruption. Somewhere in between those two figures we would find the eruption threshold for Icelandic crust of similar type to suffer from a rifting fissure eruption originating bottom up, and not out of an obvious central volcano.

A volcano-tectonic event able to produce a large eruption (5 to 15) km3 of erupted magma would probably require an even larger main chock (or a series of large earthquakes). I have calculated the needed seismic force to be equal, or larger than M7.5, and since we know that large eruptions have occurred frequently here, we should probably up the largest possible magnitude for the area to around M7.5.

If we now transpose this into a similar setting with long repose times interspersed by major tectonic activity, like the Dead Zone, we get a major spanner in the flywheel for those who believe that we can have a Lakí event without major earthquakes.

To get Lakí we would need 8748 M5s, or 324 M6s, 12 M7.0s, or a single M7.5. If we average things a bit, we would most likely have a large number of earthquakes topping out at a maximum of an M7.



The conclusion is that we need to look closer at the volcano of Vatnafjöll to see if it is indeed a central volcano, and to closer study seismic trendlines. To do that we will use all of the recorded seismic data on the volcano, while trying to remove Hekla proper from the picture.




85 thoughts on “The Forgotten Volcano

    • We developed an AI based on the old Carl, it’s actually Carl 2.0-h. There was a bug in the base 2.0 release where he started drilling into the earth’s core to power his secret volcano base and his plan to take over the world, hence the hotfix. He’s slightly more stable now in his megalomaniac programming.

      • Are we ever going to know what’s been going on or do we pretend he never “disappeared”?

        • I’ll leave that for Carl to divulge, if he so wishes.

        • It was all a dream. Carl was in the shower after a marathon binge-watching old episodes of Dallas.

        • As long as he is alright. I hope it was nothing (too) serious (I won’t speculate). I hope I’ve not been intrusive by bringing it up (now and when it happened), it was just a rather odd and out-of-the-blue event when he posted “that” comment. I have ASD so my self-awareness sometimes fails me!

          • My oldest son is currently going through the ASD (or ASC) assessment. It’s been a difficult journey and he’s only four. When were you diagnosed?

          • Going through this as a parent myself. Challenging, but also uplifting in some rather beautiful and unexpected ways. I hope you always find the peace that you need with it.

          • Unfortunately not until I was 16 (2014). It has left me with several major mental health issues that possibly could have been avoided with if I had known much earlier. Of course, it affects people differently but an early diagnosis (if it is confirmed) will be a huge advantage and he won’t have to grow up thinking he is inferior and ‘faulty’ as a result of not knowing why he is ‘different’ and therefore being extremely vulnerable to bullying, especially in high school.

          • My child is listed as a “probable” after initial assessment, but is now 17 and therefore not going to get a diagnosis through the high school as he has already left.
            Much as he is too timid to consider physical defence in the face of bullying, he takes after me for size, and is well over 6ft tall, which I’m sure keeps some of them at bay.

            Whilst I wouldn’t call him “Savant”, he is remarkably intelligent in his own ASD sort of way… Wanted to learn to play accordion, an instrument we can’t afford, but his aunt gave him an old broken one we might be able to fix up for him.

            I know a young guy called Cohen Kilcoyne-Braithwaite , nominee for last year’s “Radio 2 young folk musician of the year” award, who plays (check youtube for the band Granny’s Attic) and also repairs these things… But he was on tour.

            My son got bored of waiting, so he fixed it himself. It has 80-something bass keys !

            I tried to fix it once. Took the covers off, saw literally hundreds of rods and joints, put the covers back on, sat in a darkened room sucking my thumb for a while 😀

            But he fixed it, and taught himself to play.

            Actually, I’ll stop there. This is too much and should be in the bar.

      • As the semi-stable AI of Carl 2.01 I am still drilling into the core to power my lair. 🙂

        • I’d like to know how the drilling is going. Will the coming Swedish superplume be Carl-powered? I recommend evacuation of Malmo.

          • And here I have to answer in a corporate responsible manner.

            All legal entities that I represent utilizes safe conduct while performing drilling after geothermal sources.
            In no instance are we, nor I, drilling in a manner that is jokingly referred to above.
            We also use only non-toxic fractioning fluids.

          • point taken, and I think the corporate entity will be happy with clarification.

            now about that plan to have the Carl-2.0 AI relocated to the Earth’s core..

          • And here I was more pondering how to relocate out into space…
            First a bit of uploading into a computer core, then build a spaceship around it, and off I go out into the stars as a happy little starship.

          • Space has its moments. But the Earth’s core is greatly underestimated as a location from which to take over the world.

            But back to the post. The oldest rock in Iceland is around 22 million years old, I recall. This is from memory so may be off. You put the birth of the plume ab it more recent. Is Iceland older than its hot spot? Or is Iceland younger than I think?

          • Swedish Superplume…. now dear Albert is out and walking in the fog
            The Next Superplume event is Africa and Maybe Baikal arera
            large continents that heats up the mantle insulating.

            We can also send a Carl AI IO HiRise probe to Pele Patera so I can see that 60 km long – and 27 km wide lava lake upclose
            I wants a new IO mission

  1. Good to read your words here again Carl, under whatever arrangement it may be.
    Interesting article. I have a suspicion that some sort of question or point to raise is in a gestation period somewhere in my grey matter. But it needs to evolve a little more.

    • The fun part with Vatnafjöll is that the magma is not as evolved as one might think that a prolonged gestation period should give. This might though have changed since its last main ridge eruption happened in the 350AD.

      • Thanks .
        Although the gestation period to which I referred was my own intellectual gestation of whatever question it is. Part of the narcolepsy thing for me is that it has changed the way my brain seems to be wired, and I find it much more efficient to let foggy ideas just sit untended on some sort of backburner, being given no direct attention.

        Eventually they come to the fore at a later date with much greater clarity if I trust the process to run itself.

        • Problem for me is that if I do that I will forget about it, so I write post-its and put them on my wall to remember, but when I get around to read them I do not even understand what I wrote on it. Utterly confusing.

          • People know that my memory plays up because of a neurological disorder. They very helpfully advise me on various mnemonic techniques. I often have to point out the terminal flaw in such things.

            You have to remember to use them !

  2. Would you still not dare to jump around on the summit of Hekla?? 😄😉

    • I would only do it if I A) had a laptop hooked up to FED and BUR stations, and B) a helicopter with spinning rotor a few meters away. Otherwise, not a chance in hell.

      • Hekla’s ‘warning’ reminds me of those folks who decided to build their houses in Ocean View, HI. *On* Mauna Loa’s SWRZ. I’d love to build a house there with two provisos: !. My house was worth less than 1000USD, and 2. I had a fast car in my driveway, with the keys in it, pointing downhill!

        • Bugger it. Let’s go up to the top with big amps and blast out some ac/dc! Give Miss Hekla the push she needs 😆

          • Probably not loud enough. It might work if you book Disaster Area… but they only play from orbit.

      • Nice caveats, organise it Carl and I’ll accompany you anytime! 😂😂

        • Lurking on, a feast of TOOL and Metallica should do the trick 😉

          • I’m looking forward to Friday when the Tool back catalogue finally gets streamed, the hard copies died years ago!

          • Beardy Gaz. Better late than never!! Though I think the chances of seeing them live again out-with the USA seem slim!

  3. Nice to see new terminology being coined on Volcanocafe again. “Fore chocks” will be remembered in seismology for many years to come.

  4. A few questions pop up in my head. Can you really draw conclusions about Laki opening quake sizes from a SISZ quake? The larger SISZ quakes are as you say of a N-S strike slip nature, so I guess most of the released energy would be spent on lateral ground displacement. For a Laki opening type of event the alignment matches the rift, so quakes would not be predominantly strike slip, but normal faulting and crack widening. I’m missing some deeper analysis here.

    Also, linking magma volumes to maximum quake sizes seems a bit speculative. I wonder if there are any underlying assumptions not included in the blog post? Interesting read anyway and good to have you back Carl.

    • My estimate of Laki opening quake sizes is based on witness accounts of the time and working through some of the USGS propagation models at the range to the Laki area. Both estimates are about the same.

      • What are the figures you got to start a Laki-event? Something like a dozen to a few dozen of M6 earthquakes?

      • It came up as a sequence of M5+ quakes representing the unzipping of the rift.

    • Good morning Thomas!

      Yes there is indeed an underlying assumption of two here. The first one is that the crustal composition and sheer point of the rock is roughly the same at the Dead Zone as over at Vatnafjöll.
      The second assumption is that it is sheer point and not grind that is the predominant factor in the earthquake.
      This might sound contrary to common behaviour at strike-slip and normal faulting. And yes, it would have been true if the earthquakes happened at pre-existing faults.
      1912 and Lakí (the southern part running up to Laki Mountain) was creations of new faults. This changes the moment magnitude calculations more into the sheer-modulus than the friction part.

      It is fun that nature seems to prefer to crack hard rock, instead of just pulling on the old bungychords in the dead zone.

      But obviously I am quite open to nature throwing me a spanner on this. Natural spanners are fun, they tend to be good motivation for learning new things. And learning new things is probably the best thing on earth.

      • Thanks Carl! I knew I was missing some important piece.

        Part of me wants to see a dead zone eruption providing us with all the answers to our questions. Spanners included or not. Then there is another part of me that wishes we will never see anything like that in our lifetime. You see, I like that island and its people (and horses) too much to wish something like that upon them.

        • Eruption should be survivable. Anything worse is certainly not to be wished on anyone! That makes Holuhraun ok (although uncomfortable enough in Iceland) but anything worse is to be avoided.

          • A Holuhraun sized dead zone eruption would be nice, if that is possible. Or do they only come as super sized magmageddon events?

  5. Can someone help me? I been fiddling on the website and now i got a new but looks old volcanocafe site. Its space on both sides and i cant see the comments before i go in to the articel. And the comments are unreadebul. Its not easy to follow you gays now.

    • Have you tried emptying the cache, deleting all cookies etc? To be honest, I have no idea.

    • Can you send over a screenshot so we can see what the issue is, please?

    • That sounds possibly like the sites CSS style-sheet didn’t load properly – that’s most usually a transient issue and clearing cache, as suggested, is a good idea in case there’s a broken version in there. You say you were “fiddling” so if you mean changing settings you can disable the site style-sheet in a browser so if you did anything like that then turn if back on again. In Firefox for example ALT-V > Page Style >

      I just tried no page style because I couldn’t work out why comments would be unreadable but it turns out that the smilies in comments fill the entire screen with no CSS which definitely makes the comments awkward to read!

  6. Many thanks for a fascinating article Carl. Glad to see you here.

    • Or, as the Daily Excess puts it

      Petrifying moment tourists flee as colossal volcanic eruption blocks out the sun

      Google it if you really want to read the article but I wouldn’t encourage them with page views.

      • I see that we have a new scientific power acronym to learn. PTM = Petrifying Tourist Moment, that in turn is the force needed to petrify tourists into rock. First applied at Vesuvius obviously.

          • Gorgons may have been around at that time too – depending on how old you think they were when Medusa died

  7. Is Hekla up to something? A few larger than usual (for Hekla) quakes to the SW of the edifice in the last few days.

    • Hekla is always up to something, question is if it is anything potentially eruptive.

      Most earthquakes are fairly shallow and located in such a manner that it is due to pesky gravity pushing the entire volcano downwards.
      But in this case it is firmly at the southern tip of Heklugjá, and at a depth associated with one of the magma reservoirs. So, it is a bit exiting as such.
      That being said, no oddities on the strainmeter-plots are visible, so probably just a bit of pressure changes in the reservoir.

      What is interesting is that at the same time there was a distinct swarm roughly 10 km to the west in the South Icelandic Seismic Zone, and those could presumably be for-shocks (four-chockolattes).
      But, that is very much just a fever hypothesis for now.
      If that would turn out to be correct Hekla is just mirroring a larger seismic regiment.

      That being said, I am looking with heightened interest for the next few days.

  8. For lack of on-going eruptions, the weather here has erupted. There is flooding in several directions around us although we are not under threat.

    • 🙂 for You…. 🙁 for others what is Your temperature? Best!motsfo

      • Thanks. I am about 36.8C, I think. The weather is around 20C which is typical for summer here. (August is when autumn starts though.) The exceptional rain came from a very slow moving low pressure system coming from the south. Because the Atlantic has warmed so much, these systems carry more moisture than they used to – as we found out.

      • A town just upriver from us is being evacuated because a reservoir is in danger of collapse. Who needs volcanoes when you can have English weather? But if you prefer volcanoes, Mauna Loa has had 11 quakes near the summit since yesterday. Rock breaking quakes – no tremor.

  9. Kīlauea forming a crater lake? “…Last week, geological LIDAR surveys of Kīlauea observed a small, green patch within the deepest visible area of the new summit caldera. HVO scientists are working to confirm if this may be water, and if so, its origin and significance. …”

    • LIDAR sees green..? I am slightly baffled. More likely it was the HVO scientists who saw green. The origin of water: it probably is overflow from Manchester. They do of course have two tropical storms heading for Hawaii, and if either of these hit they won’t need our water.

      • I seriously doubt they were stupid enough to mistake a green phosphor display as the color of the target.

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