A couple of years ago I was asked on a radio show which volcano in Iceland I wanted least to erupt. I quickly answered Öraefajökull. It confused everyone, and I got bogged down in explaining why an unknown (to the layman) volcano would be that bad.
It is though true, if there is a single volcano in Iceland that I do not wish to see erupt, it is by a longshot Öraefajökull. And that for a great many reasons that I will try to explain down below.
At the same time, it is also a great challenge for volcanologists to decipher, also something that I will get back to down below. But first, let us get into the nitty-gritty of this severely understudied volcano.
The background of Öraefajökull
Most people interested in Icelandic volcanoes know that Öraefajökull is the tallest volcano in Iceland with it’s 2110 meters above the ocean. The actual top of the volcano is called Hvannadalshnúkur. The volcano has a 14 square kilometres caldera that is five by four kilometres across.
It is the second largest volcano by bulk in Europe, only surpassed by Etna, and it is the largest volcano in Iceland. But it is not the size that is the real problem here.
But, before we get into the problems, let us instead talk about age. For being an Icelandic volcano Öraefajökull is really old. We know this since there are two layers that have reverse polar magnetization. The upper reversed layer is fairly weak, and this places it as more than 41 400 years old. In other words, we have stumbled upon the “Laschamp event” – a failed magnetic pole reversal that only spanned 440 years.
Above and below that reverse magnetic layer we find normal magnetic polarization. But, as we go deep down into the bottom layers of the volcano we find that they are strongly reversed in magnetic polarization. This put’s the origin of the volcano prior to the Brunhes-Matuyama Reversal 781 000 years ago. (And no, a polar reversal is not an extinction event.)
So, even if we do not know the exact age of the volcano, we do know that it is by far the oldest active volcano in Iceland. But, first we must talk about the location.
Öraefajökull is situated outside the ordinary mainline volcanic belts in Iceland. Instead it forms its own volcanic belt called the Öraefajökull-Snaefell Volcanic Zone (ÖSVZ), alternately also called Öraefajökull Volcanic Belt (ÖVB). This belt contains 3 old volcanoes. From north to south we find Snaefell (not the same as the famous Snaefellsjökull), and Esjufjöll.
Snaefell is most likely extinct, or heading that way. No known Holocene eruption has occurred from this volcano. It is also seismically inactive. Esjufjöll is an active volcano that may have erupted as recently as September 1927 when a large jökulhlaups came gushing out from the volcano, and there was a light sprinkling of ash on the glacier. It is also seismically active with infrequent earthquake swarms.
As Öraefajökull Volcanic Belt was pushed towards the East from the main volcanic line in Iceland it started to have more infrequent eruptions. In the beginning the volcano was constructed of thick layers of regular Icelandic basalt, but as time moved on it started to produce ever more evolved magmas richer in silica, transcending through andesite, dacite into the rare realm of erupting mainly rhyolite.
The rhyolites at Öraefajökull contain between 70 and 75 percent silica, depending upon if the sample is taken from the tephra falls, or from extruded domes or spines on the top of the volcano. Normally this would be rather sticky, but due to a neat trick of nature the rhyolite at Öraefajökull is both rich in gasses and unusually hot. This is caused by repeated intrusions from depth into the system of basalt that delivers the usual for Iceland high content of volatile gases, and also functions as an ample heat source keeping the rhyolite merrily boiling.
Sometime during the last interglacial the volcano suffered from a major eruption in the form of a large VEI-6 eruption that gouged out the caldera. During the end stage of that eruption several rhyolite spines and domes were extruded, among them Hvannadalshnúkur.
After that the magma system revamped itself and the bottom of the caldera started to suffer from resurging dome building in the middle and eruptions resumed.
During the Holocene there have been no less than 5 known eruptions. The first 3 are badly constrained in time, all we know is that they happened after the last ice age, and prior to colonization.
One of those was yet another VEI-4 explosive event from the caldera, and the other two were flank events with rhyolite lava flows and dome extrusions.
Thanks to the crafty Icelander’s there are an abundance of written records about the coastal area below Öraefajökull prior to the devastating eruption. We know that more than 30 farmsteads existed in the region, and that there where 4 main churches. We also know that there where more than 220 people of age living there. So, with the normal abundance of children we can safely say that there were between 400 – 500 men, women, children and priests (these where not counted in the same journal as the farmers).
The eruption caused a major Jökulhlaup with a maximum discharge estimated to be between 100 000 and 250 000 cubic meters of water per second. That amounts to between 1 and 2.5 times what the Amazon can produce as a maximum. Roughly two thirds of the farms and three churches was unceremoniously picked up and dumped into the ocean complete with all hands and animals. The other farmsteads were eradicated by between 1 and 2 meters of un-compacted tephra.
A distance of 160 kilometres of the coastline was completely destroyed or massively changed. Most likely not a single soul survived the ordeal, but there are anecdotal tales of either an old woman and her mare, or a priest, surviving. If a priest had survived we can safely bet he would have written down an account of his ordeal, that did not happen, so we can strike a surviving priest. There may have been a woman on her mare, but in all likelihood, she was out travelling and did not witness the destruction of an entire Icelandic region. As such this was the deadliest eruption in Icelandic history.
Several sources state that there was a smaller Jökulhlaup coming out of the mountain in 1331, so we can safely assume that this means that there was a hydrothermal event back then, or even a minor sub-glacial eruption. There are though no mentions of earthquakes or tephra from that year, so whatever it was, it was minor compared to the later event.
My interpretation is that there was an intrusion from depth occurring in 1331, quite possibly like the one we are seeing now. And that heated up a geothermal field enough to melt a considerable amount of water that leaked out as a Jökulhlaup.
This eruption changed the coastline, an area of 50-meter-deep ocean was after the eruption covered with debris and tephra and became a new piece of land extending the Icelandic coast quite a bit outwards.
The total amount of ejected tephra was between 2 and 5 cubic kilometres, placing the eruptions as a solid VEI-5. Even though both Lakí and Veiðivötn ejected more ash and tephra, those eruptions were not per see explosive events. Therefore the 1362 eruption is counted as the largest explosive Icelandic eruption since settlement.
In the annals the eruption is said to have lasted for almost a year, but the later stages were said to have been low level activity compared to the main phase of the eruption in the beginning.
Even though this was a far smaller eruption rated as a mid VEI-4, it was still almost as deadly as the first one for the residents that had picked up farming below the mountain.
Once more a major (but smaller) Jökulhlaup roared forth and swept away farms, churches and assorted people and animal. And once more ash and tephra took its toll. It is estimated that a full two-thirds of the residents perished this time. At least one priest survived and wrote a highly chilling account of the devastation.
This eruption was also long lasting, and did not end until well into 1728.
Timeline of the current events
During the summer seismic activity under Öraefajökull picked up and transcended normal background levels.
It all began with the for Iceland usual deep (20 to 30km) earthquakes that indicates that magma is being pushed up from the depth. These earthquakes occurred to the south-west of Öraefajökull, but trended as they progressed upwards in under the caldera.
The origin of the upwards mobile magma was either from a deep magma reservoir connected to the MOHO via an open conduit, or came directly up from the MOHO. The earthquake data seems to support a deep elongated magma reservoir (wedge), but more earthquakes are necessary to say that for certain.
After this initial intrusion there was a month-long hiatus in activity before the main ruckus began. This time shallower earthquakes were the main ingredient of the seismic swarm indicating rising magma.
What is apparent is that there is a silent zone between the deep earthquakes and the later upper earthquakes. This indicates a slightly off-centre magma chamber roughly the size of 20 to 40 cubic kilometres at about 17 to 14 kilometres depth.
It is quite likely that there are smaller chambers higher up filled with extremely evolved rhyolite, but those would not be visible on these types of plots, and would need far more earthquakes to find and accurately reproduce.
All of this indicated that the volcano was reactivating, but nothing thus far pointed towards an impending eruption. It is after all quite common for Icelandic volcanoes to have several large intrusions prior to an eruption.
Yesterday a pilot found an astonishingly large cauldron inside the caldera of Öraefajökull. A cauldron forms as ice is melted from below by increased geothermal energy, and as the water leaks out in the form of Jökulhlaup the ice above sags down. A cauldron is easy to mistake for a caldera by laypeople, but they are not the same.
In Iceland cauldrons are often a sign of either a large geothermal event, or a small sub-glacial eruption. As of writing this, we do not know which of the two types of events have happened, but it was most likely a very large geothermal energy increase and not an eruption since there is a lack of seismic signals associated with an eruption.
The Jökulhlaup is rather slow and small compared to previous Jökulhlaups, but according to locals the water is reeking with the smell of sulphuric compounds.
The size of the cauldron is prodigious and should have caused a larger hlaup, so it is entirely possible that one will follow and that the water is trapped somewhere on the road down the mountain.
All of this lead the Icelandic authorities to declare a phase of uncertainty for Öraefajökull, and later on they also declared a Yellow alert for all flights in the area.
It is though important to point out that there as of now are no signs of an impending eruption, and that the seismic levels are currently low.
As Andrej Flis churned out his exquisite plots of the seismicity two previous events stood out. It is with perfect hindsight easy to see that two previous intrusions happened. These originated from the main magma chamber and rapidly went upwards. During those two, no new magma seems to have arrived from near MOHO-reservoir. This indicate that the system has been active down through the years, unclear how long.
Conclusion and thoughts
It is important to once more state that volcanoes can have large intrusions that does not cause eruptions.
But, as I said in the beginning, this is a volcano that I do not want to see erupt. Because this is as far as we know the most dangerous volcano in Iceland for the local inhabitants. I am though certain that the competent Icelanders can solve the problems with eventual evacuations of both people and animals before an eruption occurs. It would still though be an economic disaster for the country.
The reason for it being so dangerous is that the magma type it erupt is highly prone to be explosive, and we do know that it is full of volatile gases and that it is hotter than usual. This is a bad recipe, nobody likes turbo-charged rhyolite.
The second problem is the lack of knowledge and historical records of how the volcano behave prior to an eruption. The small few things we do know indicates that the current behaviour is how the volcano behaves prior to an eruption, but that there can be decades long waiting time from reactivation to the final eruption.
And due to our technical knowhow, we do not only face a potential disaster stretching 160 or more kilometres in south eastern Iceland, we also risk losing the ability to fly for weeks or even months. Because this would be a very ashy history indeed.
One way to try to comprehend Öraefajökull is to go beyond the scope of Iceland, because it more resembles a large subduction resurging caldera, than the typical Icelandic volcano. Yes, it is not going to super-erupt, but it will produce a similar eruption on a smaller scale with similar characteristics.
I still hope that the volcano will calm down, but if not, I wish all the people in the region the best possible outcome. I know that the Icelandic authorities are the best in a crisis, so they are in good hands. Remember, better to be safe than sorry for eternity.