After several days of heavy shaking, the new eruption started today. We know very little and are mainly enjoying the view. That there was an eruption is not a surprise, the location perhaps was.
The earthquakes showed an intrusion in the general area of Fagradalsfjall. They were located a little east of last year’s dike, running from the mountains to east of Keilir. Soon, a large range of the Reykjanes fault zone joined in leaving only the western and eastern edges of the peninsula excluded (poor things). However, the plot above shows that the kink (the intrusion) was clearly at Fagradalsfjall. The other regions just responded to the pressure from the centre.
The intrusion had started with some weaker earthquakes at a depth of around 10 km. Here is the original events, from Saturday 30 July showing these weak ones, which were shortly followed with larger shallower events. The rocks below 7 km or so are different from the ones above and tend not to show much activity. The weak quakes signify a more significant event. We saw the magma moving up, in a way we had not seen in last year’s eruption.
After this things moved quickly and by yesterday the magma was reported to be just 1 km below the surface. The rest is history.
We don’t know whether the intrusion infiltrated the original dike from last year. The largish earthquakes suggest it founds it own way up. The eruption started close to the location of most of the activity. It is on the side of the hill next to the northern edge of the Meradalir lava field.
One may wonder whether the eruption actually started earlier. Below the thick crust, much of the Meradalir lava will still be liquid. Magma could have been seeping into it before. But the current fissure seems to be above the old lava field and is therefore a new one. This also shows that the new dike followed the same line as the old one, but offset to the east. The location could have been a lot worse. Lava here will largely cover empty ground in the heart of the peninsula, and of course will provide a new layer on an already existing field. Tourists will have to walk much further though than before. I would not recommend walking on the old lava field before knowing what is going on below you.
Last year’s eruption started very slowly, moved a bit, and became larger after weeks when focussing on a single cone. It always kept a low flow rate, and this helped to keep the lava field contained within the old mountains. The current eruption has been developing faster, perhaps because there was already magma available at the magma collection points some 7 km deep, from where the dikes intrude. However, the limitations which caused a low flow rate last year will still be there. The magma pressure is distributed over a larger area, so it would be reasonable to again expect a slow eruption. The eruption could also be short-lived if the intruded amount is small – certainly we have not seen that much inflation. The Krysuvik GPS showed an intrusion closer to Keilir and INSAR showed a clear dike, but the movement was much less than last year. However, it the eruption taps the existing reservoir, it could be more extensive. At the moment, we can only guess.
It is again notable that the eruption does not make use of old conduits. Every eruption here seems to create its own fissure (later to become a cone). The region is unpredictable. In fact, if ten years ago you would have predicted something here, you would have gone for the volcanic centres on either side. Fagradalsfjall, lacking hydrothermal activity or any holocene eruption, would be not be high on the list.
What is clear now is that the race for the next eruption has had a winner. After 800 years of solitude, Reykjanes has won twice in two years. We can declare a new era of Reykjanes fires.
And now we are waiting to see what Grimsvotn will do. After all, it also is on a yellow warning.
Albert, August 2022
update: fissure map fixed, now with a more correct (but probably not yet perfect) location