Statistics and volcanism does not exactly go hand in hand, especially since it tends to end up with idiots claiming that volcanoes are “overdue”. A volcano is never overdue, they may shift patterns of their eruptions, go into extended periods of dormancy, or pretty much do anything that they jolly well pleases.
It is always far better to study their current behavior if you wish to predict what they will do in the near future.
It might though be good to have knowledge on what the volcanoes have been up to in the past century or so, especially if you wish to ponder upon whom amongst a set of volcanoes are the usual suspects.
Before we go, it is good to keep in mind that Icelandic volcanism is very much about black swans. Or in other words, unexpected volcanism in not so commonly erupting places. For all points and purposes pretty much any spot along the spreading parts of Iceland can erupt, and have done so previously, and will do it again.
I will start in the north and go south in this exposé. There are two places that I will not mention, and that is the eruptions that has occurred north of Iceland in the Tjörnes zone and I will not go into the ones that have occurred out in the Reykjanes Ridge. There have been eruptions there in the last century, but among other things those small eruption was more likely to be Mid-Atlantic Rift eruptions than Icelandic, and we do not know exactly when they happened.
This volcano is one of those cyclic volcanoes in Iceland that is mainly driven by rifting episodes. The length of the cycles are approximately 200 years give or take, sometimes Krafla jumps a cycle or more, but it then tends to get back on track. It rarely if ever has eruptions between rifting episodes.
Between 1975 and 1984 the volcano suffered 6 separate rifting eruptions of diminishing size and prior to that it erupted 1724 to 1746 in six separate rifting eruptions.
A good bet would be that we will not see an eruption at Krafla until somewhere between 2170 and 2230, if it does not skip a cycle again. An eruption now would be a true black swan event. It may though have a few intrusive episodes and seismic crisis in between without any eruption occurring.
Before the large 1875 rifting eruption of Askja very little was known about this large volcano. In the Global Volcanism it is cited as having had an eruption in 1797 at Holuhraun, something that we now know was not true, that was an eruption from Bárdarbunga and not Askja. It is quite likely that the volcano erupted between the recorded eruptions of 1300 and 1875, but that the eruptions was to small to be noticed.
After the massive VEI-5 eruption in 1875 we come to the timeframe we are talking about in this article. The period between 1916 and present.
Between 1919 and 1961 Askja had a series of 8 eruptions, they are probably part of a larger eruptive cycle that started in 1875. All of these 8 eruptions have been small scale ranging between VEI-0 and VEI-2, and even as effusive events they were pretty small for happening in Iceland.
Askja has had several episodes of inflation and deflation in the period after 1961 and has also had several seismic episodes with earthquake swarms. Even though Askja is one of the larger Icelandic volcanoes and has a proven track record of large eruptions it has the record of being the least well monitored volcano on Iceland.
Currently there is nothing pointing towards an eruption occurring soon at Askja.
This is one of the 3 largest volcanoes in Iceland. It is situated on a large fissure swarm that feeds directly from the mantleplume under Iceland. Historically it has produced some of the largest effusive eruptions since deglaciation, only rivaled by a few other Icelandic eruptions. It has also had numerous other eruptions and up until 1910 it was the most frequently erupting volcano on Iceland. And then, nothing.
For some unknown reason Bárdarbunga switched eruption frequency completely after the minor eruption of 1910. There have been a few uncertain eruptions that might have happened, or not, out in Loki-Fögrufjöll.
In 1996 during the final phase of the Grimsvötn Gjálp eruption Bárdarbunga came back to life during a small VEI-1 eruption that created a 4 kilometer high ash column. The eruption was very brief and would probably not have been spotted unless it had been witnessed by volcanologist studying the far larger eruption at Gjálp. This eruption happened inside the caldera.
I am not going to go into Holuhraun here in detail, I am just going to state that this rifting fissure eruption was a purple swan among a heard of black swans. In the aftermath we have had to rewrite entire chapters on how Icelandic volcanism can operate. It was the largest effusive eruption in the last 100 years.
After the majestic Saksunarvátn tephras 8230 BC, this the largest of Icelandic volcanic systems have suffered mainly from small frequent eruptions, lulling many into believing that it was not able to operate big explosive eruptions.
With the exception of the 4550BC Botnahraun (Laki) and the Skaftár Fires (Laki) there has mainly been small VEI-2 eruptions emanating from Grimsvötn. The only exception to this was the 1873 VEI-4 eruption. All other larger eruptions have been emanating from Thordarhyrna, and it is possible that the Laki eruptions originated from that particular volcano and not Grimsvötn.
Between 1919 and present 12 confirmed eruptions have originated out of Grimsvötn giving an average between eruptions of roughly 7,5 years between eruptions.
During this period something changed inside of Grimsvötn and it started to suffer from larger eruptions. In 1996 the Gjálp fissure eruption happened and that was rated as a VEI-3 and was the largest effusive eruption up until Holuhraun during the last one hundred years. It erupted again in VEI-3 eruptions in 1998 and 2004.
In 2011 Grimsvötn kicked off in what was the largest explosive eruption in Iceland in the last 100 years. The massive VEI-4 eruption surpassed the 2010 Eyjafjallajökull eruption within hours and before being done it had surpassed both the 1947 Hekla eruption and the 1918 Katla eruption. To date it is one of the 3 largest explosive eruptions in this millennium on the planet.
Why Grimsvötn has changed into more energetic eruptions will be covered in an upcoming two-part series of articles in here.
I am here breaking stride a bit. There has been no confirmed eruption out of Thordarhyrna since the 1902 VEI-4 eruption. I am though briefly going to mention it since it has historically put in rather large and frequent appearances.
Thordarhyrna is the second largest of the central volcanoes on the Grimsvötn fissure swarm and this is one of the longer repose periods for this volcano. It has caused large rifting eruptions and the bulk of the larger explosive eruptions out of the Grimsvötn fissure swarm.
This volcano is rather likely to put in an appearance sooner or later, I am very much looking forward to that since there is so much that we do not know about it.
Hekla holds the record of being the oddest volcano on the planet. For many reasons this volcano should not exist in the realm of what is physically possible. First of all, it erupts magma that should not exist in Iceland.
The magma is bi-modal, during eruptions it first erupts calc-alkali andesite that is extremely rich in volatiles, it then in the blink of an eye changes to calc-alkali basalt that is low in volatiles. This creates an extremely explosive initial phase that can hurl large lava bombs 40 kilometers or longer. Then it effuses large amounts of basalt unexplosively.
It is also a very young volcano; it roared into life 5150BC and has since then built a majestic edifice shaped as an upside down boat hull. It exists on a local fissure that is basically as long as the volcano and this fissure is not connected to the larger tectonic rifts in Iceland.
Unlike all other volcanoes on the planet this volcano does not do run up sequences prior to eruption. It does not inflate in any great respect, and it does not do prolonged earthquake swarms. Instead it has a brief smattering of small earthquakes running for about 30 to 80 minutes before it rips open. No other well instrumented volcano on the planet is known to have the ability to erupt without giving off signs first, especially after a decade long repose period.
After Hekla came into existence it had 6000 years of doing only large scale eruptions. Up until the 1104 VEI-5 H1-Tephra eruption it had only had 9 eruptions, the tenth being the 1104. All of these eruptions were large, ranging from borderline VEI-6 to very large VEI-4 eruptions.
Then it changed pattern, enormously. At the same time the adjacent Vatnafjöll system shut down. Between 1104 and 1845 Hekla suffered from 11 eruptions.
Then, in 1947 Hekla kicked in an even higher gear with. From 1947 to 2000 the volcano had 6 eruptions, one per decade on average.
This points towards why it is almost impossible to use any kind of statistics on a volcano. They can change their pattern in the blink of an eye and go into a much higher frequency for a while, and then go dormant for a thousand years or so.
Problem with Hekla is that we do not know what it will do now. Will it go dormant for a prolonged time? Will it continue erupting about every decade? Will it start to erupt even more frequently? The jury is out on this and the only answer to all these questions lie within that rather nasty piece of work that is Hekla.
When Eyjafjallajökull innocuously came into life at Fimmvörduhals it was in the form of a gently strombolian action. Where the lava come from is still debated, either it came via a feeder system of its own, or it was fed from the Gódabunga cryptodome that coincidentally is not a part of the adjacent Katla volcano.
As Fimmvörduhals shut down a finger of lava worked its way into Eyjafjallajökull proper where it found a source of old rhyolitic mush and an unexpectedly explosive eruption followed. The eruption was not particularly energetic, nor was it that big. But it ran for quite some time until it had barely achieved the VEI-4 status.
Pretty much this was a small volcano having a maximum sized eruption. But, this eruption will forever be famous for having shut down the European airspace stranding a lot of passengers all over Europe, including me.
Historically this volcano has had the highest eruption frequency in Iceland, and to top it off, the largest eruptions on average together with Hekla.
This means that Katla on average belts out one VEI-4 every 40 years, and this has been going on for 8000 years.
This makes the latest lull into somewhat of an oddity even though it is still not entirely unheard of. The only real eruption in the last 100 years was a VEI-4 back in 1918. It was of comparative size to the 1947 Hekla eruption and the 2011 Grimsvötn eruption.
First of all, let me state this. Katla will erupt sooner or later and when it happens it will most likely be on the slightly larger scale. VEI-4 would most likely be the smallest size, with a medium sized VEI-5 in the upper end. When will it happen? Well, there are no signs as of now of an upcoming eruption.
The Vestmannaeyjar sits on a formative rifting faultline that most likely will take over as the new landfall point of the Mid-Atlantic Rift in the near geological future. Eruptions here do not form central volcanoes as in the rest of Iceland, at least not yet.
It was a tremendous black swan event when the Surtsey eruption started in 1963, nobody really thought it would happen and it was also the first island formation that scientists got to study up close.
As the eruption wound down nobody thought that they would see another eruption in the Vestmannaeyjar, and if it happened it would be yet another island formation. Nobody thought that an eruption would occur close to a previous eruption site.
In 1973 the next black swan occurred and landed on Heimaey. The eruption started without a lot in the way of a run up sequence. The locals felt a string of earthquakes during the hours prior to the onset of eruption. It is possible that there had been small earthquakes that was not felt prior to that and we will never know since there was no instrument record of the start of the eruption.
What is interesting with the Vestmannaeyjar eruptions is that they might represent a new stage in Icelandic volcanism with more frequent eruptions taking place out there. In the end this black swan may turn into a white swan.
One thing to remember with Iceland is that it suffers more black swans than this article gives credence to. Odd rifts open up now and then, some volcanoes erupt very seldom, and often eruptions take forms that are unexpected.
In the end there may have been more black swans in Iceland than white ones. Currently the most likely next eruption is Grimsvötn, but it could equally well be something unexpected, and that is what makes Icelandic volcanism so addictive.