Serendipity is one of those words that sometimes seem to rule my world. Pure oddity and fluke would be others. As Erik Klemetti pointed out, Saturday was the two hundred and thirtieth anniversary of the onset of the Skaftár Fires. I do not really feel that bright that I missed that, even though I just wrote the dates down as I started the Lakí series.
As you will notice if you have read the previous part(s) I have changed the order from part 2 to part 3 in the series. It was just more logical to do the central volcanoes on the Grimsvötn Fissure before the compiled timeline of the events. So, unless you have recently read part 1 and the prequel, I suggest that you do so first.
Grimsvötn Fissure System
The Grimsvötn fissure system is foremost a magmatic production system. It produces magma at the boundary between the crust and the mantle, quite often in pulses that seem to be dictated by heightened activity from the hotspot and mantle plume residing under Iceland.
It is also governed by the strain as the MAR is driven apart. As we saw in part one the strain will sometimes be released during a rifting fissure eruption as the Skaftár Fires. Normally it will though mainly produce mild to medium sized volcanism in the central volcanoes that lays en echelon down the fissure system.
The correct way to understand the fissure system is that the central volcanoes are a part of the fissure swarm that have permanent magmatic systems that gives them capacity to erupt without rifting occurring. A normal eruption will not facilitate any rifting along the fissure system. And as a rifting fissure eruption of the Skaftár Fires type happens they may erupt at the same time due to them being permanent weaknesses on the fissure swarm. They will though not be the driving force behind the rifting fissure eruption in and of themselves, instead they will be a part of the larger picture.
Grimsvötn Central Volcano
Grimsvötn is a highly active volcano with a magmatic system of limited capacity. It has 3 adjacent calderas forming a larger caldera system. Inside of the calderas lie a large lake filled with glacial water permanently heated by internal hydrothermal activity.
The hydrothermal activity is fairly steady and gives off more heat than is necessary to maintain status quo. In another word, more water is melted so the water level slowly rises until the water lifts the glacial ice above it and a river is formed under the ice. The phenomenon is called a Jökulhlaup and often occurs without any eruption taking place, but they do often occur in or slightly after an eruption at Grimsvötn.
The sub glacial lake has an immediate effect on the eruptive pattern of Grimsvötn. Any eruption taking place inside the caldera will be quenched by the water if it is not of a sufficient size to actually blast the water aside. That is the reason why few eruptions from Grimsvötn take place inside the caldera. Most eruption takes place along the edges or in short local fissures like the 1996 Gjálp eruption.
Grimsvötn have had 3 caldera formation events. The last one of them occurred 10 200 years ago during the end of the last glacial. It formed due to a large eruption next to the sub glacial lake erupting sufficiently large amounts of magma to weaken the roof over the magmatic chamber. As the roof over the chamber collapsed the sub glacial lake fell down into the remaining magma and a very large steam explosion occurred. It is in a way a variant on the Krakatau scenario.
During the eruption the magma chamber was destroyed and it took quite some time for a new chamber to start forming. Most likely it took several hundred years or more before Grimsvötn once again had a magmatic system sufficient enough to form an eruption. And for the next 10 000 years Grimsvötn had predominantly small eruptions ranging from VEI-0 to VEI-2.
Global Volcanism Program erroneously attributes all known ash and explosive activity from the Skaftár Fires eruption to Grimsvötn. That is though not true. Yes, Grimsvötn erupted before, during, and after the Skaftár Fires eruption, but those where mainly VEI-0 to VEI-2 eruptions.
In reality the first larger eruption of Grimsvötn occurred in the year 1873 when it had its first VEI-4 eruption. And after that the VEI-4s started to come more often, a sure sign of a maturing magma chamber that is starting to enlarge at a more rapid pace. From 1873 to now Grimsvötn has had VEI-4 eruptions in 1902 and 2011. Notable is that each of those have been bigger than the previous VEI-4 eruption. Also notable is that the eruptions of 1996, 1998 and 2004 was rated as VEI-3s. A considerable increase in eruptive power if one takes the 2011 VEI-4 into account.
In short, the Grimsvötn that we know is much more powerful than the Grimsvötn around the time of the Skaftár Fires.
Grimsvötn during the Skaftár Fires
First we need to kill off a very persistant error. There are no distinguishable tephra from the period around the Skaftár Fires, so we know that the eruptions were small from Grimsvötn.
As I have written this series I have used the general timeline in the Thordarson & Self (1993) groundbreaking paper on the Skaftár Fires together with the diary of Jón Steingrímsson. There are though a couple of things in the Thordarson & Self paper that in retrospect has shown to not fully stand up to the test of time. To the defense of Thordarson & Self they did not have the data that we have today. I deviated a bit from their paper in the previous post, and that was mainly in regards of adding the gas as driving force for the explosive power at the onset of the fissure openings.
What I do take a bit of amiss is how they treat the written documentation from the steadfast priest Jón Steingrímsson. During the eruption he wrote about the East-northeastern fires. Thordarson & Self makes a rather haphazard correction of the direction to make it fit into the direction of Grimsvötn who was known to erupt before the onset of the eruption of the Skaftár Fires. Steingrímsson thought there was an additional row of fissures, something we know that there was not. But, I would be hard pressed to believe that he did not fully well know in what direction he was seeing an eruption. If we just draw a line in Thorgrímssons given direction we happen in exactly the spot of another large central volcano on the Grimsvötn Fissure System, Thordharhyrna.
After that, let us now get back to the Grimsvötn eruption that we can prove the location of. And that is the already ongoing eruption that was duly noted in the log book by the captain on a brig visiting the port of Höfn on the 15th of May 1783.
Previous records and papers state that Grimsvötn erupted five times during 1783, two times in 1784, and once in 1785. But, the only two we can without the shadow of a doubt attribute is the one witnessed by the ship crew in Höfn, and the last one in 1785. The others could as well have been the result of eruptions from either of Háabunga, Thordharhyrna, East Geirvörtur, Hágöngur and Eldgigur.
It is though likely that Grimsvötn had several eruptions during the Skaftár Fires.
Not much is known of the volcano of Háabunga, it is alternatively marked as a cone of Grimsvötn, and respectively as a dome volcano of its own. I chose to mark it up as a separate volcano here due to lack of proper petrochemical analysis. The volcano is not known to have erupted in modern times, and if it has done so it has been mixed up with the Grimsvötn central volcano. There is no evidence that it should have erupted during the Skaftár Fires period.
Thordharhyrna is an active central volcano on the Grimsvötn Fissure System that has erupted several times during settlement. If we take Jón Steingrímssons written record accord into account we can assume that Thordharhyrna had at least one eruption during the time of the Skaftár Fires. There is though no evidence in the form of tephra, and no drilling has been done down into the volcano to test the lavas.
4 tested samples show consistency with late stage Skaftár Fires lava. The Ur/Th ratio is slightly higher than from the Skaftár Fires, this is a result of the volcano being further “upstream” of the fissure system in regards of the Hotspot/Mantle plume.
Hágöngur volcano samples are inconclusive, due to a high degree of “pollution” from the glacier. It has clearly erupted fairly recently, and the rock samples show that it has erupted more than once. Only surface sampling exists due to the geographical impossibility to drive a drill rig up the mountain. There is though an interesting layer of blast debris from an eruption from Hágöngur inside the adjacent glacier, but there is no way to correctly date the debris.
The Eldgigur volcano erupted a small amount of lava on top of what is considered to be older material. The lava is consistent with the Skaftár Fires late stage (heavily vesicularised tholeiitic olivine).
Sadly a lot of data have been destroyed by the glacier, or is simply not available due to the glacier covering up any possibility to take samples. We though know what there is not and that is a large layer of ashes and tephras from Grimsvötn. Instead the ash and tephras found in Vatnajökull are from the Skaftár Fires.
We also see that the East-northeastern fires of Jón Steingrímsson existed after examining geochemical evidence, but not in the form Jón thought. Instead we find a series of en echelon eruptions among the central volcanoes of the Grimsvötn Fissure System. There is also a bit of evidence that the fissure of the Skaftár Fires could have continued in under the ice, but that I will return to in the next installment.