Since the eruption in Iceland started, some of us have been wondering about other past volcanic events that we could compare it to, in such a search I came up with an Icelandic eruption that I believe might by a good analogue, and no, it’s not the Krafla Fires.
Currently some of the characteristics of the ongoing eruption in Fagradalsfjall are that it is slow, both in intrusion and eruption, and that new vents keep popping up a considerable time after it started.
The Krafla Fires may seem like an adequate comparison to the Reykjanes Fires, and the current eruption of Fragradallsfjall, given that they all are episodic fissure eruptions, although even Laki was episodic, two-year-long and punctuated by short intense surges, there are thus many ways an eruption may be episodic. Rekyjanes and Krafla also have in common that their lavas are very fluid, some of the most fluid basalts in Iceland, they easily take on the smooth pahoehoe texture, even at high rates, and with very low explosivity, fountains not exceeding a height of 200 meters. But otherwise their similarities end there. Although it is not how the situation in Fragradallsfjall will turn out the Krafla Fires is one of the most fascinating volcanic events that I know of, so I will delve into them.
The Krafla Fires
Krafla is a basaltic caldera system, with just a bit of rhyolite. Although somewhat unappreciable in the landscape, its vast caldera collapse is 10 kilometres wide, up there with Grimsvotn, Bardarbunga, or Katla. The top of its magma storage lies only 2 kilometres from the surface, the magma was accidentally drilled into during a geothermal exploration in 2009, really shallow compared to Reykjanes where the magma comes from near the mantle-crust boundary 15-20 kilometres down!
The Krafla Fires opened in 1975 with a massive magma intrusion underground, a megadyke, that rifted a length of 70 kilometres. This triggered a rapid influx of magma into the shallow magma chamber under the caldera to restore the enormous amount it lost during the first intrusion. The caldera inflated rapidly until the rift snapped again, a dyke propagated into the fissure swarm, and then the caldera deflated sharply as magma drained into the growing fractures. This inflation followed by sharp deflation pattern repeated over and over again. A total of 18 dyke intrusions happened from 1975 to 1984, initially they stayed underground or had small eruptions that were almost accidental, where the dykes grazed the surface here and there. They filled the rift progressively so that the intrusions became shorter, closer to the caldera, a few of the last ones did have significant eruptions when the filling was mostly done,
Laki wasn’t so different, a series of dyke intrusions that breached the surface producing magma surges over a period of 2 years, 1783-85 , what sets them apart is that Grimsvötn had enough magma available to fill the rift multiple times over, so every dyke vastly overflowed, and the repressurization must have been much faster because the fissure swarm snapped again while the previous dykes were still erupting.
Eruptions of Krafla followed the typical sequence of fissure eruptions, they open with a curtain of fire, a flood of pahoehoe, then effusion rates drop exponentially, later it can focus into one vent and go on at a lower intensity for some time. One of its largest eruptions was in November 1981, the dyke travelled in one hour from the magma chamber to the surface, within another two hours a curtain of fire 8 kilometres long was already shooting from rows fountains and sending forth floods of fluid lavas. During the initial peak hours of the strongest eruptions in 1984 and 1981 effusion rates of 800 m3/s were probably reached, if not surpassed, and this was similar to the rate at which the previous intrusive dykes expanded.
Krafla vs Reykjanes
I have been inspecting some of the old lava flows of the Reyjanes Peninsula in Google Earth, which I was talking about in my last article, it turns out there is some variety.
Hengill, Krýsuvík-Trölladyngja and Svartsengi erupt with great intensity, a curtain of fire feeds a flood of pahoehoe lavas of similar proportions to Krafla’s strongest, the eruption rate decays exponentially and may last for a total of a few weeks or so.
Brennisteinsfjöll is the exception, its eruptions are slow, long, complex, they don’t follow any general trend of decrease or increase in intensity. Fagradalsfjall, although shoulder to shoulder with volcanoes that are into lava floods, seems to have actually turned out the Brennisteinsfjöll way. In hindsight the rate at which the dyke grew, a mere 15-20 m3/s, a snail’s pace in the world of racing dykes, already pointed towards the eruption being a slow one. There is evidently a lot of magma in the mantle below Fagradalsfjall that is buoyant or pressurized and could readily erupt, 800 years of melting building up below the region, and if it could come out faster it would. If the dyke grew out so slowly it is probably because that is all that the conduit system that brings magma from the mantle into the dyke is capable of transporting. Presumably the space in the conduit somewhere down there is very little, so that the small opening and friction against the sides does not allow ascent to be any greater than that. As such, it is unlikely the eruption rates will exceed much more than 20 m3/s at any moment in the future.
The idea of a lava flood may sound attractive, however one should realize that this is something that happens very quickly, the peak eruption in a matter of a few hours, should it happen in the middle of a blizzard then perhaps the best part might go unseen entirely by everyone. Instead Fagradalsfjall seems to be putting a slow, long lasting, show that keeps changing.
I’ve got the impression from inspecting the lava flows of Brennisteinsfjöll that these probably formed in multistage eruptions where the active vent shifted location multiple times, that may have went on for multiple years and erupted more than 1 km3. Fagradallsfjall seems to be intent on shifting the eruption location, it has already opened three different fissures, this has surprised some scientists who, following the model where the dyke solidifies and output focusses into stable pipes, didn’t thought it should be possible for it to happen so far into an eruption. If we look into the recent past there are however a number of examples of eruptions that simply kept moving their vents around.
I think that the eruption of Surtsey might be a good analogue to the current eruption of Fagradalsfjall, as long as we ignore the difference that one was partly submarine whilst the other is subaerial. Fagradalsfjall is like a Surtsey out of the water. The eruption rates of Surtsey were low, around 5 m3/s when converted to lava, it lasted a total of 3.5 years, 1963-67.
Surtsey was a wandering eruption, from multiple vents along a 3 kilometre-long line. The eruption started from the vent Surtur I, which was joined 1.5 months later by Surtla that erupted for several days, Surtur I went on for about ~2.5 months. The activity moved to the vent Surtur II when Surtur I had waned, Surtur II erupted for about ~16 months. After Surtur II, a new vent, Syrtlingur, opened and erupted for ~5 months. After a brief break in activity, yet another vent, Jólnir, became active for ~10 months. Later effusion shifted to the first vent, Surtur I, for the 10 final months of the eruption.
The way Surtsey kept shifting the action around defies the typical assumption that after initial fissure opening, the first hours or days, then the eruption settles into a single or a few nearby vents and stays there, this is because the rest of the dyke solidifies so output concentrates into small pipes, which is how it happens almost every time. However, there are several examples of eruptions that do not obey this law, during the eruption of 1730-36 eruption of Timanfaya, in Lanzarote Island, the focus of activity kept shifting, the last outbreaks were as strong as the first ones, the activity being overall rather slow. During the 1991-93 eruption near Nyamuragira Volcano, a new vent would open every 2 weeks on average, a few or one of them being active at any given time, they did not open through a single line but sporadically over an oval area.
Why do new fissures keep opening up?
Fagradalsfjall has opened 6 fissures already, it does seem to be walking down the path of Surtsey, or Timanfaya, or Brennisteinsfjöll. I do think it is likely that new vents may keep opening until the eruption stops because it is how this kind of volcanic event seems to typically progress, a fissure eruption that does not have a proper curtain of fire, that does not have a exponential reduction in effusion rate.
How is it possible that new vents would open up even when the dyke should have solidified already? It doesn’t seem likely the magma would come for the initial intrusion which should be solid by now, and not able to flow any more. However there are pipes filled with hot new magma rising from depth into the vents. Geldingadalir, or Smeagol, or Bob, or was it the Twins? however it was called, this vent was fed with hot magma from depth, its magma pipe could have broken at some location down below and fed a lateral magma-filled fracture that opened fissures 2 and 3, a small dyke branch. This does seem like a plausible reason.
Some ground cracks did form between fissure 1 and the site of the incipient fissure 2. Ground cracks are possibly the best way of tracking the way magma moves underground, because at this point intrusions are small they may not generate detectable earthquakes or ground deformation, but they will probably open cracks. It is difficult however to monitor the opening of tiny cracks on the ground.
Often we may underestimate the complexity of a situation, for example we know that the dyke feeding the current eruption extends down to a depth of 5 kilometres, and yet we also know that the magma comes directly from a depth greater than 15 kilometres, with no intermediate magma chambers, so what happens in the 5-15 km depth range? Certainly the magma must have found a way through, not a very effective connection given that the intrusion and the eruption receive magma att such slow rates, however the exact way it happened remains unclear. When did magma start to rise upward? In January 2020 there was an intrusion, a sill, below Mount Thorbjörn, this is quite close to Fagradalsfjall, was magma already rising from the mantle then? Perhaps the intrusion was already growing long before it is assumed and it continues to grow now, as shown by the new fissures that opened up recently, and will probably keep doing so. How does the intrusion look like? a system of interconnected sills, dykes, with molten pipes transporting magma towards the vents and growing dyke branches? I guess this is up to our imagination.
The eruption in Fagradalsfjall is the subaerial equivalent of a Surtsey-like eruption, it seems also similar to the historical eruptions of the Brennisteinsfjöll Fissure Swarm, which happened around the 11th century, they were of significant volume and long-lasting.
New fissures will probably open along the dyke axis that runs from the eruption site to Keilir, at some point, days, weeks, or months from now. A decrease in surface volcanic activity may indicate that magma is moving into fractures towards a new eruption location, same is true for the formation of ground cracks. Old vents can be abandoned if the flow is redirected towards a new fissure. The eruption will probably end up lasting multiple months or years because the transport of magma from source to surface is slow.
This eruption is turning out to be really interesting, can’t wait to see what volcanic surprises await us!