The volcano Fagradalsfjall is a tuya, formed under a glacier during the ice age. For years, it has been expected that volcanic activity would ‘soon’ start on the Reykjanes Peninsula, perhaps within the next century or so. Now it started in 2021, and it is expected that, from now on, eruptions will occur with an average interval of a few decades for the next three centuries. One thing we should try to understand is what is special about the location of the eruption. It occurs at the crossings of a deep dike and an earthquake fault.
The basic map is from the Icelandic Met Office. It shows the location of earthquakes for two days, from March 30th to April 1st 2021. I added the eruption site location, the dike, and the relevant earthquake fault. It is interesting that the tuya area can be expressed with a basic form with sides parallel to the dike formation. Not strange though, because the stress field of the tectonic plate shapes the surface in many ways. The symmetry can be explained according to the existence of symmetric convection rolls in different layers below.
This eruption is effusive, with steady flow of lava. It has a vertical flow component underground, whereas the conduit from below is found slightly NE of the eruption site. At one point of time, the magma within the dike must have penetrated to the surface due to the weakness generated by the earthquake fault, mapped on the behalf of the Icelandic Met Office and named t2-02, seen here: https://www.vedur.is/media/vedurstofan/utgafa/greinargerdir/2006/06001.pdf.
The main earthquake faults system was mapped by Vigfús Eyjólfsson, as seen here: https://www.mbl.is/media/36/11536.pdf?fbclid=IwAR32Qe3aZNEoeBlURDBRHc6aDvasvTWsSziEGVYsJJf6dVGZJ2UyMQm1PAs He noted that the eruptios of the tuya, and other tuyas as well, tended to coincide with the earthquake faults. Therefore, the mountain has clear north-south aligned topographic features.
For the moment, the eruption can be watched broadcasted live here: https://www.ruv.is/frett/2021/03/18/eldgosid-i-geldingadolum-i-beinni-utsendingu