Before I start I would like to point towards the previous 3 parts. It might be a good read for those who missed them when I first published them.
The pre Skaftár Fires eruption at Grimsvötn
Two weeks (15 may 1783) prior to the eruption at Skaftár a Brigg called to port at Höfn; the Captain dutifully wrote in his log that there was an ongoing eruption at Grimsvötn. There are few things we can say about the eruptions at Grimsvötn during the Skaftár Fires. What we can say about the eruption prior to the Skaftár Fires is that it most likely was a minimum of a VEI-2 and a maximum of a VEI-3. If we compare to contemporary eruptions it was most likely an ashy VEI-2. Anything smaller would not have been noticed, and might even not have been able to break through the Ice above the caldera, and a larger one would most likely have been written more about.
The same thing goes for all of the activity at Grimsvötn prior, during and after the Skaftár Fires. The eruptions from Grimsvötn back then were fairly small compared to the activity that we have seen during the last 50 years.
The Skaftár Fires Timeline
The eruptions started at the SW part of the Grimsvötn Fissure Swarm at Úlfarsdalur and extended all the way up to Sídujökull.
We know that there was a minimum of 10 eruptive fissures along this line; they are subdivided into the SW and the NE fissure rows. The dividing point is the old Lakí Mountain that is a hardened leftover from previous phases of volcanism. As such there was no erupted lava from the Lakí Mountain, even though it has a few tuff cones as sub-terranean material was blasted out.
The fissure rows contain five proven fissures each, even though there is a high probability that the fissuring continued further to the NE under the then present Glacier. But, as glaciers move they bulldoze away any proof of such a thing having happened. As we get closer to Sídujökull the evidence is more and more destroyed, and the timeline becomes a lot less certain.
I should here like to point out that there are temporal changes in the eruption as it progresses from SW to NE. From the first fissure we have tremendous output and a much higher rate of activity then what happened as the last fissures opened up. Also there is a temporal shift during the lifespan of the fissures. As they opened up they normally ejected about half of their total output within the first few days. There is also a third temporal shift at play during the fissure eruption, and that is that the fissures opened up with ever increasing intervals. One should also not that the fissures stayed open throughout the entire eruptive period, so fissure number one was open a lot longer than the last fissures.
Just to point out what we are talking about, as the first Skaftár fissure opened up it erupted about 2 cubic kilometers of lava within the first 48 hours of its existence. During the rest of the eruption it effused about 2 to 2.5 cubic kilometers of lava. That gives that the first single fissure erupted roughly 25 percent of all the lava during the Skaftár Fires, and that the same fissure erupted 12.5 percent within 48 hours of the start of the eruption. If we then use an inverted logarithmic scale to the fissures we find that the last fissure erupted far less than 0.1 cubic kilometers in total. And that explains a lot about the last fissures not even being noticed in the general mayhem, and that there is even little evidence remaining of them having taken place.
So, the beginning we know a lot about, but the end is a bit shady.
15 – 29 May: Small but noticeable earthquakes. It is hard to say if these had anything to do with the upcoming Skaftár Fires, or if this was due to the ongoing eruption at Grimsvötn. I have come to believe that they had more to do with the ongoing eruption at Grimsvötn than anything else, but this is not possible to prove even though it is likely.
29 May – 8 June: Strong earthquakes from the area of where the Skaftár Fires would soon start. They were both increasing in numbers and in strength over time. Known damage to houses and witness accounts give at hand that many earthquakes where 5M or above in strength. The residents in the vicinity had to move out into tents to not have their houses fall in over them.
8 June: Large scale explosions and phreatic ash fall over Sída. The start of the Skaftár Fires.
9 June – 7 February 1784: Fissures 1 to 10 (with most likely two more sub glacial fissures) opened up along a line of 27 kilometers. During the eruption several large scale lava surges came pouring out of what used to be river gorges. During the highest rates of effusion entire rivers stopped flowing.
Summer and autumn 1783: Large scale ash fall over all of Europe, Canada, Greenland and Spitsbergen. Ash from this period is found in drill cores from all over the northern hemisphere.
24 November 1783: A very large earthquake ranging from 6 to 7 M happened in the Skaftár district. This was probably caused as an entire section of the fault line collapsed.
The East Northeastern Fires
Contemporary witnesses talked about a second set of Fires during the Skaftár Fires eruption. This has been discredited by later researchers as a geographical mistake by the witnesses. More or less they have stated that the local witnesses did not know how the land around them looked like and that they did not know what part of Iceland was where.
In the end it seems like they knew exactly what they were talking about. During a campaign in 2010 samples was taken and analyzed from Eldgigur, Hágöngur (rubble) and East Geirvörtur that was found to be showing consistencies with general Lakí lavas. Additional testing places the samples close to the given time frame. The area of these samples are following the eastern side of Sídujökull and is situated on the eastern side of the Grimsvötn fissure swarm, whereas the Skaftár Fires took place at the western side. Since no other eruptions have been noted in the area either before or after the Skaftár Fires it seems like the old Priest knew exactly what he was talking about.
There is also reason to believe that this secondary fissure line extended via the known central volcanoes on the Grimsvötn fissure swarm up to Thordharhyrna which is the point that the witnesses said erupted.
Not only did Grimsvötn erupt prior to the Skaftár Fires, it also erupted at least 3 times during the Skaftár Fires. One of the eruptions that have previously been attributed to Grimsvötn was most likely Thordharhyrna.
After the Skaftár Fires ended Grimsvötn continued to erupt until April 1784. Grimsvötn once again had a final eruption April 4 – 26 1785. After this final eruption Grimsvötn had a 38 year long rest, probably one of the longest uneruptive periods in the last 500 years.
In the next installment I will get down to the nitty gritty details of the ash and dust of the Lakí eruption.