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.
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
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
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
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.