Laki deconstructed II. Anatomy of an Eruption

Image from the Open University

We continue Carl’s story of Laki.

I highly recommend anyone who has not read ‘Central volcanoes of Vatnajökull’ to do so carefully, otherwise you will most likely be lost fairly quickly, see it as the prequel to this multi-part article.

Photograph by Mavur

This image from the 1981 fissure eruption of Krafla shows how ludicrous it is to compare. The scientists on the image were caught unawares as a fissures opened up. If that had happened during an opening of a Skaftár Fire fissure opening they would have been instantly pulverized by the explosive force.

Previous Lakí research

Up until 1993 research into the Skaftár Fires was based upon a model using the Krafla Fires as a type case. This has proven to be a large mistake. Sadly the image of a series of fissures opening up and closing one at a time, in turn drawing magma from a shallow central magma chamber resulting in fire curtaining and mild strombolian eruption characteristics was highly false. It in turn has caused climatologists to gravely misjudge the factors behind the climatic effects of a large rifting fissure eruption.

I will return to the climatic driving factors and the magma behind the eruptions. I will also go through the timeline for the Skaftár Fires eruptions.

First I would though like to talk about the name of Lakí; it is after all very misleading since Lakí never erupted. Lakí is an old laccolite, a geolithic artifact, that is a remnant of old quarternary volcanic activity. The latest eruption was before glaciation. During the Lakí eruption Lakí was too hard for the opening fissures to penetrate, something that in the end had a bit of consequence for how the eruption unfolded. So, I prefer the Icelandic term of Skaftár Fires. The main reason is that the Icelandic name is nicely plural.

I will come back to this in detail in the next part, but the Skaftár Fires consisted of at least 10 fissure eruptions and at least 5 other eruptions.

Anatomy of an eruption

All of the fissure eruptions are sufficiently alike each other that I can start off with describing one and just note the few differences afterwards. I have for practical reasons chosen the first eruption as my type case.

For those of you who remember the Krafla Fires eruption, strike that image out of your brains, as we will see the Skaftár fires was a much more violent affair going through several more stages.

At the time of the onset of the Skaftár Fires the tectonic rift zone running between Myrdalsjökull (Katla) up to and including the western part of Vatnajökull had not suffered a fissure eruption since the 1477 Veidivötnahraun eruption. In the zone there are 3 large fissure swarms that have had large fissure eruptions.

Basically one can say that the tectonic rift zone is being pulled apart with a speed averaging at 28mm per year. During non-eruptive times the spread rate is lower, about 17mm annually as shown by GPS measurements. One can look about the area as a large rubber band that is pulled apart until it snaps, and as it recoils the rubber band moves much further than the pent up strain. At the sites of the fissures the movement was much larger than the pent up 8.6 meters. It is the phrase “on average” that bites us here. The actual fissure width was between 50 and 150 meters at the surface, and that is not even half the truth as we will see later.

In the beginning

What we do not know is what set off the eruptive phase, and we might never really know. It could just have been the rubber band snapping, but at the same time we know that the hotspot under Iceland is cyclic so it could have been a cyclic high (a pulse) that set off the rifting episode. Be that as it may, the rest we can infer from physical evidence and witness reports.

Early in the day on the 15th of May 1783 tremors was noticed in Skaftártunga. It was not anything unusual about that, the local priest though made a dutiful notation about it in his daily journal. The perceived tremor continued up until the early afternoon on the 29th of May. Not even the duration of the tremoring was really that noteworthy for a hardened Icelander.

Nowadays we know what was happening. It was a large scale earthquake swarm running thousands of small earthquakes per day ranging up to 3M. As an earthquake swarm it was fairly large, but nothing that does not happen every ten years or so in Iceland.

In the end it was not a normal earthquake swarm, or even a normal pre-eruptive earthquake swarm. It was the tectonic strain closing in on the point of a critical fail. And that critical fail started on the early afternoon on the 29th of May as the earthquakes suddenly got much bigger. Due to the distance of the witness reports we know that the earthquakes had a minimum of 5M, probably up towards 6M, since we know that people had to live outside in tents due to damages to the houses. During the next ten days the earthquakes opened up a wedge shaped fissure from the bottom of the crust, as extremely gas rich magma started to travel upwards the wedge widened at the bottom and more magma poured up. By now the eruption was a self sustaining machine rapidly hammering its way up towards the surface.

As the magma rapidly rose upwards at an average speed of at least 3 km per day it started to degas at an ever increasing rate, and since the volume was very large and the magma gas content was so high a bow wake of gas preceded the magmatic ascent upwards. I will return to how we know about that gas front wave later.

At 09.00 on the 8th of June the bedrock above the rising magma and gas become too thin to be able to withstand the extreme pressure of the rising magma and gas. To compound matters even more the top layer had a water table ranging from 150 to 300 meters depth, mainly consisting of bogs and small lakes.

As the compressed superheated sulphuric and fluorine gas punched through the bedrock the combined force of the hydrothermal explosion, the phreatic detonation and kinetic gas release pulverized the bedrock with tremendous force in an explosion that lasted for about two days. The larger debris and medium sized rubble rained down over the surrounding area, and the coarser sands fell over Skaftártunga and the stoic priest noted that another of Iceland’s eruptions had started. The force was though energetic enough to literally pulverize most of the overlaying bedrock into exceedingly fine dust. The length of the first fissure was 1.6km.

On the 10th of June magma was spotted for the first time as it came flowing out of Skaftárfljót. This first magma was not in an overly thick layer and came out of a fire curtain 1400 meters high. As the fissure continued to widen the height successively lowered as pressure dropped in the system, but the magma had barely started to arrive. The main bulk arrived during the night of the 11th of June, it quickly filled up the river gorge of the Skaftár river gorge and then moved onwards at a speed of up to 3 km per hour.

At that time a second fissure had already opened ejecting a fire curtain and a third was about to open up. For every opening fissure the pressure dropped at our initial fissure and after a while the main activity from it was quiet effusion of a steady stream of lava and strombolian activity at the cones that had formed. The activity continued like this with diminishing activity as every new fissure opened up, but the effusive activity at the fissure did not stop until the 7th of February 1784, and the lava flow did not halt until April of the same year.

As the eruption continued the lava changed that came out of the fissure and eruptive layer evidence shows that the temperature of the melt increases as time went by. The magma that came up after the fissure surge tended toward olivine, in the end almost pure forsterite, an extremely high melt point olivine member, was erupted. The bottom layer is almost exclusively blasted bedrock, but over that you have an intricate layering between ashes, lavas, finer grained bedrock as the fissures opened further and further away, and strombolian artifacts. One interesting feature is that some of the fissures produced large amounts of Pele’s hair and Pele’s tears.

At least two fissure openings had a minimum columnar height of 12 kilometers. We know that due to eyewitnesses stating that they could see the columns from a great distance. Problem is just that the previous calculations pretty much did not feel confident with stating higher columns. As we will see this was wrong.

I previously mentioned that the magma was very gas rich. We know this from the amount of vesicularisation (fancy word for bubbles of gas in the rock) of the lava. The lava that ended up at the outer of the lava floods cooled rapidly as it contacted with cool rock and retained the high vesicularisation. The lava that cooled slower lost all of its vesicularisation (bubble free). This caused the lava to lose half of its volume, and then it had lost a lot more already during the ascension due to degassing.

One should note that the eruption created a series of Grabens along the entire stretch of fissures. A Graben is created as the area around a large fissure eruption settles due to the area below have emptied out all available magma. You could in a way say that it is a non round caldera formation through subsidence. Most of the Graben line was covered by the magma, but in many places the Graben structures are evident. We should remember that not only did Lakí erupt 15.1 cubic kilometers of lava (DRE), it also erupted tephra (0.8 to 1.9 cubic kilometers DRE) and blasted a far larger amount of rock out. To further compound problems we have all that area evacuated as the wedge formed from the mantle upwards to the surface. So, in reality the entire magmatic emplacement was more into the hundreds of cubic kilometers if we count all the magma going up into the crust. No wonder we got a Graben formation when all that magma cooled and shrunk.

No evidence has been found of more evolved magmas in either the ashes, tephras, or in the lavas. Instead all of the samples point to unevolved fresh magma arriving at high temperature directly from the mantle/crust boundary, also the high gas content points to such an origin. There is no sample pointing to magma having either encountered previous melted material, or rested in a magmatic chamber of any sort. This disqualifies any central volcano having taken any part in the Skaftár Fires. In the next episode we will discuss briefly the eruptions at Grimsvötn and the other central volcanoes on the fissure swarm during the time of the Skaftár Fires.

CARL

24 thoughts on “Laki deconstructed II. Anatomy of an Eruption

  1. Thank you Albert! Very informative. I wish I’d been there to see the event… I wonder if we will ever see something like this in our lifetimes? I have a feeling we shall not.

    • None of this is mine..it is Carl speaking. But it is hard to recognize what are the important events of our own lifetime. We have no Laki, Tambora, or Toba. Instead we will be remembered as the generation when the carbon budget of humanity vastly outgrew that of animals. Wild life has suddenly (within decades) been reduced to just 5% of the total. This is the era when the term ‘the mass of humanity’ first meant ‘everything’.

      • I agree. I would have agreed 20 or even 30 years ago. However despite that was the time we could have done something, we didn’t. Now its too late and we still won’t. The effect of a small epidemic that barely kills 0.5% of the world will have in the future is immense, yet we worry about a few hundred thousand old people dying a bit early but care not about the hundreds of millions who will die in the future due to the waste of our limited resources. I will be dead by then, but I can see it now. Its never any use saying ‘we should have’.

        • The best way to do something is by supporting education. Population growth is directly related to the level of education of women: where women get schooling, they have fewer children (the same is not true for men!). If you support charities, perhaps there is one that support or trains teachers. Our problems are solvable. But too many people prefer denial. Perhaps even VC does a bit of good.

          • A society that allows women to access education is more likely to allow them a livable wage and access to birth control and some autonomy as to wether to marry or not. It is not the education of the woman herself but the education of the men in her society that frees her to chose the number of children she wishes to bear. Here in the USA women are educated, but a political party dominated by men wishes to limit women’s control of their bodies and access to birth control so you see it is not the education of the woman but rather the males that would control her reproduction.The Supreme Court is due to argue tomorrow if employers can deny birth control/as health care to women if her employer doesn’t believe women should have control of their reproduction. So I must disagree that just educating a woman will mean she is free to bear fewer children.

          • Sure, point taken. It is a very important issue. But it has little impact on the world population growth. Studies have shown that the best correlation is with female education levels. Wherever that goes up, regardless of political system, reproduction rate goes down. The studies do not show the same level of correlation with male education levels. (If I were to comment on your point: most politicians, regardless of gender, have probably been educated to the limit of their ability..) And reversely: the sudden increase in birth rates in central Africa correlates with the wars 20 years ago when the education systems collapsed. Iran, Bangladesh, China, all have seen birth rates plummet. China has been trying to reverse the trend but the population is not cooperating, and now China is predicted to hit peak population within a decade. I am quite convinced that the solution lies in widening education. And especially in Africa where the bulk of the population increase for the next decades is going to happen. Everywhere else, it is being brought under control.

        • 0.5% is high enough but note that a few small Italian small towns/villages have already lost more than more than 1% of their population in excess deaths. Antibody tests put the positive rate at about 60% in the worst hit areas so far tested.

          Many of even the so called asymptomatic have CT scan lung damage and lower blood oxygen levels. Some patients may be producing non-optimal antibodies that actually make things worse. Long term effects to be seen.

  2. Thank you.

    Gives a new dimension to phrase, ‘ Let Rip’…

    I suppose while Afar’s Erta Ale vent stays open, and spreading rate remains but a fraction of Atlantic’s, we may not see Laki-grade apocalyptica…

  3. Looks like Thorbjörn read my comment on the last page, realized the mistake and made a u-turn. GPS trajectories show inflation again.

    I note that the post was written before the Holuhraun eruption, otherwise there would have been a comparison between Skaftár fires and Holuhraun, together with an explanation why the eruption mechanism must be different between the two.

      • I’m looking at this one. Maybe I was a bit too quick. If you look at the combination of N-S, E-W and U-D components, there is a clear deflation pattern between April 20 and the beginning of May. After that, I thought in my mind that a couple of samples indicated inflation. With a few more samples added after that I’m not so sure anymore. From the looks of it, it could just as well be measurement noise.

        http://brunnur.vedur.is/gps/thorbjorn.html

        • I would say that since April 10 there has been no detectable change. The VC eruption prevention has clearly worked..

          • We all thought it was an April Fool’s joke.

            Did you really have to prevent the eruption from going forward? It would have rescued us from volcanic boredom in these times of self isolation…

  4. More quakes in the Loihi region than usual as well as the Pahala region.

    7 days all magnitude.

  5. Hi Albert!
    Yes Iceland is amazing… Iceland is togther with Hawaii the worlds most powerful oceanic hotspots

    Iceland is probaly 15 times larger influx than the reuinion hotspot?
    Iceland is a very powerful magma source

    Iceland is 10 times stronger magma source than Canary islands and Reunion?

    • I am not going in to top trumps. Iceland and Hawaii each contribute about 20% of the total magma production underneath land (excluding the oceanic rifts which produce much more). In lava production Hawaii does better as there is no rift to fill. But in terms of heat flow for different hot spots, Iceland actually ranks pretty low, at least in the numbers I have seen.

      • Icelands thick oceanic crust over the hotspot
        Masks the heat flow

        But Iceland haves a 10 to 15 times larger magma influx than example Reunion and Canary Islands?

        • Iceland has almost 10 times the lava production of Reunion, but take into account that a large fraction of the melt production is due to tectonic spreading and glacial retreat (yes even now, about a half if I remember right), not just the mantle plume.

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