What if Katla erupted?

Myrdalsjökull glacier and the Katla Volcano. Photograph by Chris 73.

First of all, I should clearly state that there are no current signs that an eruption at Katla is about to happen in the near future.

Instead, the reason is that I was asked by one of our readers, Patricio Oliver, what would happen if the volcano erupted, especially for the inhabited areas.

This is a very good question; we rarely write about eruptions from the perspective of what the effects would be on the local population. First, we need context.


The Icelandic LIP

Conjecture – All Icelandic volcanism is a function of strain caused by the spreading of the MAR and magma intruding from the Mantleplume and the Mantle.


Katla 1918 eruption.

Iceland is a true geologic marvel, but few people realise that for the last 14 million years it has been running the tectonic show for the entire Northern Hemisphere. We know this because that before the Icelandic Mantleplume was born 16 million years ago, directly under the MAR, Northern America was moving in the opposite direction.

As the mantleplume started Northern America switched trajectory in a geologic instant. One good thing is that the oceanic crust above the mantleplume was thin, otherwise we would have seen volcanoes that was epic in size and scale of eruption.

Instead, the crust was too thin for ‘super-eruptions’, and we got numerous slightly more manageable eruptions to deal with.

On average Iceland is spreading at a rate of 2.8 centimetres per year, but at the point central of the LIP it can move apart at express speed during larger events. The largest known spreading event occurred at Eldgjá as the local rate of spread was 150 metres in a year. Don’t worry, it somehow averages out into 2.8 centimetres again over time and distance through processes that we still do not fully understand.

Now, let us look at the chain of other large volcanoes along this portion of the Mid Atlantic Rift (MAR).


The Local MAR

Overview of the Mid Atlantic Rift in Iceland. I stole this image from Alamy that stole it from a paper that I could not find online. If anyone can find a better image with more details of the various parts, I would be much oblliged to switch it out.

The Mid Atlantic Rift is more complex as it goes through Iceland than what I am describing here, I am after all concentrating on Katla now.

Katla belongs to a chain of large central volcanoes that stretches all the way from the easy to pronounce Þeistareykjarbunga down to Eldfell on Heimaey. From north to south these large volcanoes are, and yes there are more small volcanoes there, Þeistareykjarbunga, Krafla, Askja, Bárðarbunga, Grimsvötn, Þórðarhyrna (it just flows off the tongue), Torfajökull, Hekla, Vatnafjöll, Eyjafjallajökull, Katla and Eldfell.

These are all fed by plume-derived magma at various grades, and are subject to the ripping apart of the MAR. There are though a couple of features more local to Katla that is also interacting with it.


The Local Group

Katla is affected by no less than 3 different regions of the Icelandic Portion of the Mid Atlantic Rift. The first one of these is the East Volcanic Zone (EVZ) that roughly runs from Grimsvötn down to Katla.

South of Katla you have the Vestmannaeyjar Volcanic Belt (VVB), this is where the Mid Atlantic Rift is desperately trying to find a new and shorter route through Iceland. Over time this one will take over as the new MAR-route. Over time the VVB will connect the islands into a peninsula that makes landfall south of Katla.

To the west you have the South Icelandic Fracture Zone (SIFZ), this feature is mainly not volcanic, with the glaring and obvious exception named Hekla that is the Easternmost part of the SIFZ.

By now most people would feel that this was a complex enough setting for any volcano on the planet. Nope, this is where it starts to get really funky.


The Dead Zone

Eldgjá, the unzipped crack of Katla. Photograph by Andreas Tille.

In the movie Stalker by Andrej Tarkovsky, they enter a place called The Zone, a place where physical laws and causality are suspended. The Dead Zone is similar in many ways.

Obviously, the laws of physics and causality are not suspended in the Dead Zone, but we do not understand what is happening enough to yet understand what is going on in there fully.

The Dead Zone is an intensely aseismic area located roughly inside an area that is bordered by Katla, Vatnafjöll, Torfajökull, Þórðarhyrna, and back to Katla. The margins of the Dead Zone can at times be extremely seismically active, but inside the area very few earthquakes occur, and they are very small when they do happen.

It is believed that the region is made up of ductile hot crustal material that is more akin to rubber than rock, and that this causes the aseismicity.

We also know that the area is prone to suffer from the largest known effusive eruptions on Earth, and that it during those eruptions suffers from the fastest tectonic movements on Earth. What we do not know is how it happens, what is causing it, and why it is happening at this spot and at no place else.

It is the beating heart of the LIP, and I will come back to this feature in an upcoming article about Vatnafjöll.


The Katla Central Volcano

Katla is one of the Big 3 volcanoes in Iceland if you look at the combined ability of explosive and effusive eruptions. Yes, Grimsvötn have caused larger explosive eruptions, but the average explosive eruptions are smaller out of Grimsvötn, and yes Bárðarbunga has caused larger effusive eruptions. But Katla is on average as good as Bárðarbunga and Grimsvötn at producing the greatest shows on Earth.

It is the ability of causing on average large explosive eruptions that set Katla apart. Only one confirmed eruption at VEI-3 has happened in historical times. The average size is in the large VEI-4 range bordering to VEI-5, and a VEI-6 is never out of the question from this volcano.

It is also able to produce effusive eruptions in the near 20 cubic kilometre range out in the Dead Zone, this last happened in 934AD at Eldgjá.

The last confirmed eruption at Katla happened on the 12th of October 1918 and it was a borderline VEI-5. The current hiatus is unusually long, but not unheard of.

Like most other volcanoes a prolonged hiatus will often end up with a larger than average eruption, so when the eruption happens next time, it is likely to be in the VEI-5 range but that is far from a certainty.


The risks of Katla

A volcano like Katla comes with a diverse set of risks depending on the size of eruption and where it happens. I will here go through the risks in order of likelihood to cause problems.

Jökulhlaups – Katla is situated under the Myrdalsjökull Glacier. The glacier has completely filled in the caldera with Ice, and during an eruption the geothermal heat caused by the eruption will melt large amounts of the ice causing massive jökulhlaups.

The Jökulhlaup caused by the 1918 eruption was large enough to create 5 square kilometres of new land on the beaches near Katla due to the amount of tephra and ash deposited by the water.

The Jökulhlaup of 1755 had a peak discharge rate of 200 000 – 400 000 cubic metres per second. More than the combined output of the Amazon, Mississippi, Nile and Yangtze River combined. Not something you wish to be in the way of.

Ashfall – Even though this is not deadly in and of itself, it will in large amounts cause roofs to collapse and damage building and infrastructure. If the prevailing wind is southerly during an eruption the local villages will be impacted.

Southerly winds would also cause problems for airlines in the same way as happened during Eyjafjallajökull in 2010. Obviously, the problem would be even greater since the amount of airborne ash would be much larger.

Volcanic bombs – The world record of Lava Bomb killing is set by Hekla with a 12kg lava bomb being hurled 32 kilometres before decapitating a farmer, Katla is amply able to hurl lava bombs quite a distance. The safety zone here during a larger eruption would be around 30 kilometres.

Pyroclastic Base Surges – This would be counted as an uncommon risk and would only be a factor during a VEI-6 eruption. It is when an eruptive ash column collapses and hot ash and gasses fall down and come running down the sides of the volcano. If that would happen nothing within 50 kilometres would be safe.

Rifting Fissure Eruption – Having 10 to 20 cubic kilometres of lava gushing forth within a few months would be bad mojo indeed. Do not around be the safety tip here due to the ample amount of volcanic gases.

The good news is though that the volcano will give off ample warning signs prior to erupting, so evacuating the locals will not be a problem. And also, the locals are well aware of what they have to do and are well prepared to do so.


The Fallout of Katla X

The Village of Vík. Photograph by Efrainlarrea.

So, in a few years Katla will suffer from the hypothetical eruption X. It turns out to be exactly like expected, it was a medium sized VEI-5, it caused a large jökulhlaup peaking at 50 000 cubic kilometres, the beaches got extended with yet another 5km.

There was only one death that happened during the eruption, it was caused by a French volcanic tour guide who smuggled in tourists through the safety checkpoints. One of the tourists tried to steal drugs from a village pharmacy and succumbed to volcanic gases.

The Bridge across the Road 1 was washed away, and the road was closed for a week after the eruption before the Icelandic authorities had it replaced.

In Hólt and Vik several roofs caved in due to the weight of ash, but the houses was rebuilt in short order. Some houses at the outskirts of Vik were destroyed by the Jökulhlaup and was also rebuilt.

After two years all was back to normal in Iceland, and everyone was waiting for the next large eruption.

The eruption caused the SAS Airline to default due to volcanic ash and bad food. It was missed by nobody.



Why is Iceland so uniquely able to withstand large eruptions compared to other areas in the world?

The first thing to remember is that Iceland is sparsely populated, and there are not that many people living near the biggest volcanoes in Iceland.

It is also important to acknowledge that the Icelandic Met Office is among the best volcanic authorities in the world, they will be able to forecast an eruption and evacuate the locals with ample time to spare. Well, not perhaps in regards of Hekla, that one just has to be special…

Also, the knowledge and preparedness of the Icelandic people in regards of volcanic eruptions is second to none. They know what to do, they are ready to do it, and they will do it when needed.

If Katla erupted anywhere else on the planet it would be an unmitigated disaster, but in Iceland it will be a nuisance of temporary nature before the locals go back to eating the national dish, hamburgers.


754 thoughts on “What if Katla erupted?

  1. So strange now there is a swarm La Palma but most earthquakes have been below M2.0 .

    There have already been 65 earthquakes since midnight and they are happening within minutes of each other nearly as many Earthquakes already as the whole 24 hrs from yesterday .

    • You should ask why there have not been any quakes below M2.0 reported during the entire eruption. I suspect that the answer is that it hasn’t been possible to detect them since they have been drowned in tremor and noise from the eruption, or that there has simply been too many quakes to process and anything below 2.0 has been considered insignificant and thus ignored. The fact that they are now reported could be seen as a sign that things are slowing down.

      • That is a reasonable and likely explanation too.

    • The eruption seems to have changed. The activity during the previous two days appears to be of a paroxysm nature, like Etna’s SE Crater, Pu’u’o’o’s first three years, or the later part of the Fagradalsfjall eruption. I’m not entirely sure yet however, but if more paroxysm-like episodes keep happening then the eruption will have definitely evolved.

      What is a paroxysm exactly? When a conduit widens the speed of the rising magma increases and this drains rapidly the volcano. It starts switching between two states. First, rapid rise of magma with gas bubble growth and volume increase, which is the paroxysm. Second, a recharge stage where magma rises slowly building up pressure for the next paroxysm. It should be a cyclic repeating process. It is typical of stratovolcanoes, but some fissure eruptions can also evolve into this state of activity.

      Why the small earthquakes? As far as we know there could be many reasons, but my take is that they are caused by pressure changes in the conduit and in the storage. Yesterday’s paroxysm which was highly violent probably decreased pressure in the volcano. Now pressure is building up again. Pressure changes cause rock to break and make small magnitude earthquakes. I think that’s what may be happening. Up to now earthquakes had been deep and mostly related to the rising magma, a different mechanism.

      • Yes, agree. It is an indication that the pressure in the magma chamber is no longer sufficient for continuous eruption, and the eruption is winding down. Finally some relief for the poor people. In addition to what Hector notes, once the bubble stop a layer of colder magma develops at the top which acts as a lid. The new paroxysm has to break through that. The eruption ends when it no longer can. At least, it ends in this location. In La Palma it is possible that the eruption briefly resumes elsewhere.

        • I disagree on the last part. “In La Palma it is possible that the eruption briefly resumes elsewhere.”
          If the pressure is to low for it to erupt in a fairly open conduit, it is not by a longshot enough for a new eruption to form in a new spot that is far thicker and harder to break through.
          For that to happen pressure would need to build up sufficiently, and if that happens we would most likely have a repeat eruption at the same spot, and that would sort of point towards us having a new central volcano on our hands in La Palma.

        • I’m not sure if this new style should be attributed to the decreased pressure or to widening of the conduit.

          I haven’t read about many basanitic fissure eruptions, and they aren’t too common either, so don’t know if this is how they end or not. Tholeiitic basalts can keep this state of activity for a prolonged time though.

        • For example Fagradalsfjall was in intermittent eruption for almost three months. So this change may not necessarily be due to weakening of the eruption, even if the eruption is nearing its end, which is hard to know for sure.

          • The actual situation

            Since 21:00 yesterday, there is no volcanic tremor in Cumbre Vieja. This does not imply a termination of the eruption, as its cessation has sometimes been followed by a further increase in activity. This is the longest tremor-free interval since the eruption began.

            The activity continue, meanwhile need wait about more info.

    2021/12/14 10:15:12

  3. Kilauea has started erupting again, it looks like it is becoming episodic too.

  4. Has anybody ever seen this report about of irresponsable scaremongering journalism?:

    “A seismic swarm in a volcanic region like the Canary Islands is perfectly normal at the Volcanic Institute of the Canary Islands
    David Calvo
    That didn’t stop Express, The Sun, Daily Mail and the US edition of Newsweek from claiming the seismic swarm – which no one in the scientific community has associated with a potential volcanic eruption of Teide – as an “imminent“ risk to British tourists.”

    • Link not working too well. Here’s the image (you can work your way to the source via IMO’s hydrology pages).

    • Conductivity of the water in microSiemens.
      The flow has normalized, but it will take longer time before the usual type of (melt)water is dominating. Conductivity measures the amount of positive ions in the water.

      • Exactly 🙂

        It is also good to remember that there is not a lot of fresh meltwater being produced in December, and it will take som time before all of the water is out of the tunnels. Even though the hlaup itself is over, most of the discharge into the river is still hlaupwater.
        I think it will be spring before we see completely normal values.

  5. Thank you all three, Albert, Carl and Héctor for your very scientific discussion about the volcano.
    I think it is gone forever, or at least becoming very extremely weak.
    Tremor is missing for an extremely long time until now.

    I too suspect the tremor of a healthy La Palma volcano to exceed the small M2.0 quakes, and as such, as soon as the volcano is going down and becoming deleted, the small earthquakes having always been present are now becoming visible.

    I also think that the conduit may crust over soon, and then no pressure currently still available will be able to break through, effectively finally deleting the whole volcano.

    Yes, will be extreme relieve for the locals 😀

    • Forever would surprise me a lot… 😉

      I think it is rapidly diminishing, and that after the end of the eruption we will see at least 4 decades of intermission before the next eruption, probably longer judging from the size of the eruption.
      But, regardless it will be a blink in geological times before it goes off hurling again. So, no forevermore here 🙂

  6. The La Palma eruption seems to be over following the second paroxysmal episode of eruption. I guess the eruption can go two ways now: the conduit doesn’t reactivate and it ends, or the eruption reactivates and enters a mode of episodic paroxysmal episodes.

    • It may be time to celebrate. An eruption that will be long remembered. A thing of awe, beauty and terror. And let’s not forget the suffering.

      • Absolute truth.
        This eruption was so intriguing to me I had it run all night when sleeping!
        It was so much over FAF…

      • I will never forget the house that barfed lava out of the front door… Surreal doesn’t even counter what happened there.

        It has indeed been the eruption of the ages for La Palma.
        It will be remembered as The Great Eruption of 2021 even in a hundred years time.

        I feel a lot for the locals that now will have to try to dig out their houses (where possible), or build anew on top of the lava. And getting rid of all that ash that will be flying about for years… Lot of work…

        • Yes. Volcanoes can be devastating and this eruption is worse than any VC has covered in details, I think. Back in September we commented that it could last until December. I regret that we were correct.

          • No fatalities though. They must be thankful for this. Just economic loss. And loss of a cemetery and a church, I think.
            Thank you for posting the first picture of that cemetary on All Saints Day.
            Pico Viejo on Tenerife went on for three months in 1796, but I cannot find any useful information about it.

            When will they be able to walk on the terrain and when can they build new streets? Two years or more?

          • I would state that the eruption of Kelud was worse.
            It was though very short.

        • I also saw some untouched houses on a drone video of GutnTog. Problem is that they can’t get there. They are completely surrounded by lava, no street.

      • Just wait until Mauna Loa sends a large As lava flow into Hilo or When a large flank cinder cone opens up in Catania.. it will be stuff That will Dwarf La Palmas destruction. Or when Brennsteinsfjöll sends tube feed pahoehoe lava flows into Reykjavik

        Luckly will perhaps never happen in our lifetimes, althrough Hilo coud dissapear tomorrow

        • Mauna Loa probably wont erupt big enough to threaten Hilo, eruptions that get so far are very rare. 1881 was actually the only eruption to reach Hilo since human settlement of the islands if I recall correctly.

  7. Explosive activity resumes at Etna with heavy ash emission, Aviation Color Code raised to Red, Italy

    Link removed due to leading to a clickbait site. Please use links to relevant authorities in the future.

  8. Whats the chance that Mount Erebus lava lake overflows and it starts strange Phonolitic shield building with very very Viscous taffy green gray phonolite pahoehoes?

    I guess that lava lake degasses Mount Erebus effectivly preventing that. Shield Building also requires a constant high magma supply.

    Still Mount Erebus is a very effective open conduit for now. And probaly is the longest lived historical lava lake , some data says its been that way for the last 17 000 years. Erebus viscous magma Only allows gas slugs and Strombolian explosions in its lava lake

    Erebus have had immense pyroclastic eruptions before that have collapsed the whole cone, so clearly a very difftent behaviour now with an open conduit. Erebus haves the infilled summit calderas of Etna, But it also lacks a valle del bove scar. Still the two volcanoes looks remarkable similar, althrough the geologic setting and compositon is difftent.

    Erebus basicaly erupts a very hot, very very alkaline andesite. And is the only ”higher viscosity” lava lake on the planet, requires alot of energy to heat an Intermediate composition to near 1100 C

    • Erebus phonolitic magma viscosity in its lava lake is about 100 times more viscous than basalt Halema’uma’u in Hawaii If anyone is curious

  9. OT Rant.

    First, I am quite pissed off, so take this with a grain of salt… hell, used the whole salt lick if you need to.

    Yesterday, on the way home, I encountered a rather significant traffic oriented “constipation.” I had just come from a service call that was performed in a secure area on a military facility. The issue with that, is that no personal electronics were allowed in the area and I was under full time escort. The escort bit was no problem, I get that every time I go “behind the wire” in a state prison and I fully appreciate it. What irked me, was that all of my service documentation is contained on my tablet. Being electronic, not allowed. What I hate more than anything is doing technical work with no references to use. Being a secure area in itself was not the problem, two separate tours as a Naval instructor were in such compounds. I understand the reasoning and agree with it. Luck being what it is, I have a tendency to be able to “pull shot out of my ass” and come up with a solution. In this case, I had to rely on my general knowledge from doing this sort of job for 16 years, and having 40+ years of general electronics experience under my belt. Just because I am well suited to deal with it, doesn’t mean I like it. I was successful, and just want to go home. Then I encountered the interstate “constipation” I called my wife to see if she knew anything about it, Turns out that 5 or 6 vehicles had tangled up on the Escambia Bay bridge. Okay, fine. I diverted to Ward Basin road and made my way to Hwy 90. One of the precursors to the interstate system that would get me across the bay and home. Upon reaching hwy 90…. the other diverts had beat me to it and that road was literally socked with traffic. Since it passed through Pace Florida… a very dangerous traffic area… understandable. With little sunlight left, I opted for a little known 3rd crossing point to get into my home county. Hamilton Bridge The problem with that route, is that you have to wind through a myriad of twisty back roads in heavy residential areas. My other option was to drive 45 miles north to Jay Florida and cross on Hwy 4 into Century.

    Now here is why I’m pissed. In another chat area of the internet, I ran across a guy bragging about “Brake checking” anyone that got too close to him from behind. This sort of action is PROBABLY what caused my additional 3 hr drive to get home. Ya see, the Escambia Bay bridge is brand new. It’s 3 lanes wide, and was build as such because of roadwork plans laid down several years ago… When the existing bridge was destroyed by Hurricane Ivan in 2004, they just rebuilt the bridge as 3 lane since they were going to replace it anyway to handle the sizable traffic volume between Milton Florida and Pensacola. Forr the most part, the original 3 lane plan has been completed, and the traffic flows quite well. HOWEVER…. at the bay bridge, many drivers treat it as a segment of Talladega speedway. Always darting in and through traffic jockeying for position. Utter shitheads. Today, I traversed the same section of road and had not incidents to deal with. I did note a new skid mark near the crest of the bridge that was about 40 feet long, and drifted into the emergency breakdown lane next to the bridge railing. What caught my eye, is that it was MUCH wider than a normal tire on a vehicle of tractor trailer rig. In fact, it was about as wide as the front tires on a concrete truck. This means that whoever laid that skid mark came upon a traffic incident that required him (or her) to lock up the tires on the truck. Concrete trucks do NOT have low mass. Whoever was driving it came damn close to going over the side.

    If it was a “break check” incident. Who ever did that should be drawn and quartered.

    I’ll give you a hint. At sea, there is an UNWRITTEN LAW. That is the law of GROSS TONNAGE. Sure, you may have the right of way as a sail boat… but a 100,000 tonne cargo vessel MAY NOT BE PHYSICALLY ABLE to yield to your legal right of way. The same applies to vehicles. If it is BIGGER than you…. you may not fare too well.

    My point? The interstate isn’t TALLADEGA… and you aren’t Dale Earnhardt.

    • Addendum.

      Today, I was able to have my service manual crutch with me. But I had the wrong manuals. Since I had ample experience with this particular brand and model of unit, I was able to effect the repair by the seat of my pants. Still wasn’t happy about the lack of reference documentation… but it was my fault.

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