Now that Fagradallsfjall has finally erupted it is clear that a new eruptive period has initiated in the Reykjanes Peninsula, we can expect many more eruptions to come in the following few centuries, but what should we expect? and what about the eruption that has already started?
The last time the Reykjanes Peninsula was active was during a series of eruptions, roughly in the 950-1240 year period. Although there were people around, there was of course no one to take scientific notes, so the information is very sketchy. In an attempt to fill the lost history I have looked at a couple of lava flows in Google Earth and tried to interpretate their origin. And how do I do that?
Volcanic landforms reflect the kind of eruptions that created them, for example the height of a lava fountain is about the same as the diameter of the top of the cone that it creates. There are also various types of lava flows, pahoehoe sheets, aa sheets, tube-fed pahoehoe, toothpaste lavas, raised lava channels and ponds, all of this together, and with the relationship between the various features, the volume and complexity of the lava flow, it can give clues as to how the eruption happened.
Before getting into the individual eruptions I should mention the two general progressions that fissure eruptions generally follow.
Two types of fissure eruptions
We are used to used to seeing Piton de la Fournaise, Kilauea, Galapagos, or Bardarbunga erupt, their fissure eruptions follow one general pattern. First, the curtain of fire, when a long fissure bursts into a chain of lava fountains and for a short time the highest eruption rates are reached. Then the eruption focuses into one or a few vents and stays there, the effusion rate falls exponentially until it stops, or it enters a prolonged phase of slow activity at just a few cubic meters per second. From a curtain of fire to a single vent is the typical sequence of a fissure eruption.
However there is another important end member, this is a somewhat rare type and not many examples come into my mind, but there is a big one, the Timanfaya eruption of 1730-36 on Lanzarote, in the Canary Islands. This is a very dramatic case, but that’s why it’s a good end-member example.
Timanfaya started from one single vent initially, but then over the following 6 years, the focus of activity kept shifting from one vent to another, countless times. By the time it was over there were many vents lined up in a row, so that it looked like a fissure, but no one would have thought that when it started. There was no clear reduction in activity either, it just kept going, a terrible deluge of ashes and lava with no end in sight.
It’s not easy to tell apart the two types from looking at moss covered lava flows from ages ago, however, I do get the impression eruptions of Brennisteinsfjöll and Hengill may follow the Timanfaya type, whilst eruptions of Svartsengi follow the curtain of fire type, I’m not sure about the others.
And now, with that explained, we should start where the Reykjanes Fires started, with Brennisteinsfjöll.
Brennisteinsfjöll eruptions. ~950 AD.
There are four different historic lava flows in the Brennisteinsfjöll fissure swarm. All of them formed in slow eruptions, from fissures that were less than 500 meters long with one to a few active vents, and fountains that only reached up to 100 meters high at most. From what I’ve seen each of the flows must have been erupted in a period longer than a month.
Hellnahraun is an example of a very slow eruption. Lava moved inside lava tubes, it inflated from within and formed many small breakouts of glassy pahoehoe lavas. It was the sort of activity that was typical in the Pu’u’o’o eruption of Kilauea. Lava reached what is now the small coastal town of Hafnarfjörður, on the outer edge of Greater Reykjavik. Taking into account the volume of around 46 million cubic meters, and effusion rates that must have been in the range of lava tube eruptions, 1-10 m3/s, I estimate the eruption most likely lasted around a few months.
Another nearby flow, Húsfellsbruni, was possibly the longest and most voluminous eruption of the Reykjanes Fires. It forms an extensive apron of lava flows that is extremely complex. It was hard to understand its origin story because at places there are as many as 4 or 5 flow sheets that seem to lie on top of each other. The eruption involved mostly channelized flows and aa sheet flows, but there were also lava tubes, ponds, inflation plateaus, and toothpaste lavas.
Toothpaste lavas, also called rubbly or spiny pahoehoe, are a higher viscosity form of pahoehoe. Although Reykjanes lava flows have a very low viscosity the cooling lavas within the core of aa flows can erupt again from breakouts and form fields of toothpaste lavas, these look a bit like a mix of pahoehoe and aa.
The volume of the Húsfellsbruni lava flow is hard to estimate because of how complex it is, however I wouldn’t be surprised if it came close to or exceeded 1 cubic kilometre. This eruption I’d say most likely lasted one to a few years. There is also the question of whether the historic lava flows of Brennisteinsfjöll represent separate eruptions, or rather just stages in a long eruption with a shifting focus of activity.
Svínahraunsbruni. ~1000 AD.
This eruption happened between Brennisteinsfjöll and Hengill in an unnamed fissure swarm. There are three lava flows that are considered historic, according to the geologic map I’m following. They lie close to each other with vents that line up.
Each of the flows issues from a fissure that is only 250-500 meters long. One fissure is unnamed and seems to have had only weak spattering and a very small flow. Two bigger flows issue from fissure vents called Nyrðri and Syðri respectively, and both had lava fountains that probably reached 50-80 meters high at some point, issuing mainly from 1 or 2 vents. Syðri erupted on top of Nyrðri.
It is hard to know if they formed in one or multiple eruptions. If we follow the Timanfaya model it could have been an eruption with three stages in which the vent focus shifted, but it can’t be ruled out that they were different eruptions altogether.
I shall describe Nyrðri, the biggest flow. Most of the time the lava erupted from a single circular spatter cone, bubbling up or making low fountains that spilled into a lava channel 30 meters wide. Lava streamed down this channel, raised above the ground with flanks covered in shiny pahoehoe. The channel shifted its path a few times throughout the eruption. The lava was channelized for 2.5 kilometres, then it was distributed over a branching system of blocky aa lobes and toothpaste lava outbreaks.
Considering the three lava flows as stages of one eruption then the volume is 110 million m3. I thought eruption rates of 10-20 m3/sec were reasonable for its level of activity, that gives a duration of 65-130 days. This is a guess. The flows may have formed in separate eruptions, but still most likely the duration would be in the range of one to a few months.
Eruptions of Trölladyngja and Krýsuvík. 1151-1188
Fire is reported near Trölladyngja mountain in 1151 and again in 1188. These reports probably refer to three historic age lava flows that exist in this area. Of these, Ögmundarhraun and Kapelluhraun were large eruptions. Each formed large lava channels running into the ocean in opposite sides of the Peninisula. The other smaller flow was short lived and may have been part of the Ögmundarhraun eruption; their fissure systems line up too.
These lava flows, as well as many other prehistoric eruptions of both Trölladyngja and Krýsuvík, are stronger than those of Brennisteinsfjöll and Hengill: they erupt curtains of fire with tens of lava fountains bursting at once over a length sometimes exceeding 1 kilometre, and very rarely some fountains can reach up to 200 meters height.
In the eruption of Ögmundarhraun intermittent fissures opened over a length of 9 kilometres, I don’t know if there was an initial phase of fire curtains affecting the entire length or if the various segments were activated at different times. The vents opened along the valley of Mohalsadalur, flooding the entire floor. Lava poured in rapids where the valley was narrow, and then dived below the smooth brilliant crust of lava lakes that collected where the valley was flat and wide. Lava entered the ocean in massive flows of aa lavas. It seems the system of winding channels extending towards the coast reorganized multiple times, so it was probably a lasting feature and a significant volume of lava must have ended up underwater.
In the late stages of the eruption activity had focused in a 500 meter long fissure, on the order of 10 fountains were playing to low heights, or some perhaps up to 50 meters, their streams merged into a singular mighty river slowly flowing towards the ocean 6 kilometres away. The flow broke into two branches, each half a kilometre wide, and entered the ocean 2 km from each other, amidst what must have been boiling seawater, dead fish, and towering columns of snow-white steam.
It seems difficult to make any guesses about the volume or duration of Ögmundarhraun, other than it must have lasted more than a few days, but could have been much more, and that the volume must be more than 100 million cubic metres.
Eruptions of Svartsengi and Reykjanes. 1210-1240 AD.
We arrive to the last volcanoes of the Reykjanes Peninsula as we travel westward together with the migrating activity of the Reykjanes Fires. These two fissure swarms produce intense eruptions with curtains of fire similar to those of Trölladyngja and Krýsuvík, or even more intense. For example, one prehistoric eruption included a kilometre long unbroken curtain of fountains shooting to heights of 100-200 meters.
Svartsengi produced three lava flows, Arnarseturshraun, Illahraun and Eldvarpahraun, I will focus on the first two, which I think are parts of the same eruption.
Illahraun erupted from a 200 meters long fissure, but the eruption was very intense. An area of 8 km2 was rapidly flooded by a sheet of molten pahoehoe lavas, including the present location of the Blue Lagoon. The surface flowed as a mass of crustal plates carried by the molten rock below, as it moved the slabs clashed against each other lifting into broken ridges.
The cones that fed the eruption barely have any prominence and the flow is one simple sheet, this is all probably because the outbreak was very short lived. The total volume was 38 million cubic metres and eruption rates must have been on the order of hundreds of cubic meters per second, so that the effusion can’t have lasted much more than a few days. Illahraun however may have taken place concurrently with the opening of fissures along the same line to the northeast which fed the longer lived Arnarseturshraun flow.
Arnarseturshraun must have opened with a line of fountains and a rapid outpouring of pahoehoe lavas, although this initial stage is largely buried under later activity. The eruption rate declined progressively. For some time a raised lava channel kept supplying lava towards the north which fed a massive slowly advancing wall of aa lava, crustal plates from the initial lava flood as large as hundreds of meters across were rafted downstream and collided with each other raising up meters high ridges of rubbly scoria, much like tectonic plates that collide to form mountain ranges. Activity kept decreasing and some more lava channels were formed to the north, however activity focused more and more around the vents.
Because the ground is mostly flat, lava had ponded around the fountains of the Arnarseturshraun lavas, this evolved into three lava lakes raised slightly above the ground by overflows, the largest with a triangular shape and 200 meters wide. Small fountains and dome fountains probably played in the lakes and distributed lava over an intricate system of channel and lava tubes leading up to small lava flows nearby. It is clear that this must have been long lasting, more than a few weeks, but it is hard to know how long.
Lastly the historical records mention some submarine eruptions offshore Reykjanes, or by the coastline. An eruption in 1211 formed new islands, called Eldeyjar, meaning islands of fire. An eruption in the winter of 1226-27 is said to have produced darkness at midday, it must have been an explosive phreatomagmatic eruption with ashfall. New eruptions happened in 1231, 1238 and 1240. This last event was the final of the Reykjanes Fires and then the whole region entered a prolonged dormancy that was only broken now that Fagradallsfjall has erupted.
The story of older eruptions can help understand what is to come. I can now make some general predictions. For example, if Reykjanes of any offshore volcano erupts it might be explosive phreatomagmatic and ashy, if Svartsengi erupts it will start with a very intense eruption with curtains of fire and be followed by an exponential drop in effusion rates, but if Brennisteinsfjöll or Hengill erupt then expect slow eruptions at near constant rates.
Problem is that the current eruption doesn’t belong to any of these volcanoes, in fact it opened in a very strange location and it turns out the magma is coming straight from the mantle. According to scientists this is quite extraordinary, and extraordinary magmas bring fantasies of extraordinary eruptions.
There is one very rare type of Icelandic eruption known as a lava shield, or a dyngja. These eruptions are thought to feed from primitive magma from great depths, and they are not associated with the shallow magma chambers of central volcanoes that feed the large fissure eruptions. Many commenters have already been discussing that we may be witnessing the birth of a shield volcano, Irpsit I think was the first to mention the possibility. The reason to think this is that the magma is from the mantle and the vent has opened outside the fissure swarms. A shield is a huge eruption that produces multiple cubic kilometres and lasts decades, it is a slow tourist-friendly eruption with lava tubes and inflated pahoehoe flows. The magma coming from the mantle is certainly a good sign, the last time an eruption came from the mantle was in Mayotte… 5 km3 or more, and still going. But while it’s true that shields erupt magma that is very primitive, it is not true that every eruption that brings out primitive stuff is going to be a shield.
So far the Fagradallsfjall eruption has followed a similar pattern to those of Brennisteinsfjöll and Hengill, a short fissure eruption at a slow steady rate. If it is similar in the other respects, the eruption should probably last on the order of some months, but it could also last years if it is a large eruption like Húsfellsbruni, or even decades if it becomes a dyngja. If it is Timanfaya-like then dyke activity may keep going and new vents could open elsewhere as much as months or years from now.
We shall see what comes to be of Fagradallsfjall, but in the meantime we can understand the eruption from a much more full perspective knowing about previous fissure eruptions of the Reykjanes Peninsula in their rich variety.