When I was a child I grew up with Krafla. Not so odd perhaps since it erupted for nine years. In my childhood mind it seemed like Krafla divided Iceland in two parts, and I dreamt up weird bridges spanning the fire fountains that in my dreams spanned the entire length of the island. Come to think about it, no wonder I became both a volcanoholic and Icelandophiliac when I grew up.
Kraflas diverse mechanics
Krafla is considered to be a caldera volcano, and a complicated one to boot. It is actually a double caldera volcano where the outer caldera is visible, and the inner one is filled up with lava-fields. The calderas are in turn dissected in a north-south direction by the Eastern Icelandic Rifting Zone and in the east-west direction by a transverse Graben. On top of that there is a northern fissure zone extending out to the ocean, and a southern extending to Lake Mývatn. So, it is quite easy to discuss activity as being in one of the quarters of Krafla. The latest eruption was in the northwest quarter.
Already one starts to understand that this is a very odd volcano. It is actually rather hard to say what type of volcano it is. It is a rifting fissure volcano, but with a magma chamber that is large enough to actually be able to building up enough to collapse. It can have central caldera eruptions like the latest, and it can also have fissure eruptions. It is also able to have crater and cone formative eruptions, as it did 1300 when it had a crater eruption south of Viti.
Krafla normally erupts alternating to the north or south of the transverse Graben. Of course this does not go like a pendulum, but in general it never erupts both to the north and the south. The transverse Graben also has the effect that any tremoring in the north half will only show up on the north SIL-station, and activity in the South will show on the south SIL only. In this capacity the Graben functions as an imperfect sound-shield.
Normally Krafla has rather unexplosive eruptions, but there have been a couple of confirmed VEI-4 eruptions, and the Viti formation was probably a bit messy. Another feature of Krafla is that it normally erupts in intervals that are loosely based around 230 year cycles. This is associated with increases in the rifting in the North Icelandic Rift Zone. Sometimes it seems to jump over 1 or more of the rift cycles, and sometimes it seems to disregard the rifting totally.
According to IMO Krafla will not erupt again during this current cycle of high rifting in Iceland. There is a bit of a debate if the current cycle started with the 1960 eruption of Askja, or with the eruptive sequence of Krafla. Be that as it may, we are definitely closing in on the high-point of the current rifting cycle.
Kraflas latest eruption
The rifting of Iceland goes in intervals where the rifting is sometimes slower than the annual average 2.5cm, and sometimes faster, even up to 5cm annually. But on average it rifts at that speed. These episodes of high rifting normally correspond to Kraflas eruptive cycles.
1975 Krafla started her latest eruptive cycle, up until 1984 she had no less than 6 eruptions. The eruptive fissures started to the west and ran in roughly north/south direction, and then each new fissure eruption moved towards the north and east. Each new fissure generally erupted more material than the previous.
The site of the eruptions was trending from Leihrnjúkur to the north. There is still activity at Leihrnjúkur with intermittent small earthquake swarms and occasional harmonic tremoring. So the area is in no way really dormant. Leihrnjúkur is situated roughly 1.6 kilometers NNW of the Krafla Geothermal Power plant which was being constructed during the initial eruption. One can easily understand that they were a bit nervous about that back then. Especially when a likely fissure opened close to the bottom of one of the boreholes and fire started to sprout forth out of the tube sticking up out of the ground. This was probably the first and only manmade volcanic vent in history.
Today there is a very low probability of Krafla erupting, even though one should never say never about any Icelandic volcano. Remember the earthquake swarms, and the occasional little spasm of harmonic tremor in the northwest quarter.
During the Krafla eruption something odd was noticed, and that is that Askja deflated prior to the eruption of Krafla. This deflation of Askja continued up until just a few years ago, and when Askja started to inflate, Krafla in turn started to deflate. There is probably no direct magmatic link in between them. But there might be a deeper relationship at the feeder level down at the MOHO.
A fun fact is that during the eruption the IMO used one of the simplest tools to measure a volcano ever deployed. They used a water filled device to measure how the water level was rising and sinking at the ends. By using what is in practice a very long aquarium they got a very good inclinometer showing how the angle of the volcano shifted. A lot of data was gleaned through this very simple device. Never ever short sell simple things; they are often giving the best and most reliable data. Well, as long as you remember to pour in new water from time to time.
Ah… The title. I should perhaps explain that. I am of course talking about the divorce between the American plate and the Eurasian plate. As the plates move apart, the poor child of Krafla gets cranky.