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When tracing the origin of the eruption, Kolbeinsey Ridge and the relevant convection rolls are actually the most relevant cause. The Arctic rolls are the upper most of the system, in direct contact with the tectonic plate at around 120 km depth. This could even be tested with petrological relationship between the Kolbeinsey Ridge and the primitive form of magma finding its way up to the surface at Reykjanes. Here is a drawing showing the upper most roll and its relationship with Reykjanes, with the upwelling division line passing the west coast of the Reykjanes Peninsula.

Red line is for upwelling, blue for downwelling. The convection roll is of course a part of a comprehensive system of rolls, extending mathematically to the pole itself. The convection roll shaping the Reykjanes Ridge is actually subducted by this Arctic roll and its eastern counterpart at the latitudes of Reykjanes. Therefore, tensional direction ruling dike formation should be induced by the Kolbeinsey roll, not the Reykjanes roll. At 64°N, the two rolls have exactly the same alignment, but the difference can be recognized at the latitude of the eruption at the southern half of Reykjanes Peninsula (slightly south of 64°N), where the direction is close to N44°E.

Reykjanes Peninsula is closely related to the Reykjanes Ridge, that in turn can be looked at as a huge elongated volcano. https://en.wikipedia.org/wiki/Mid-Atlantic_Ridge

Keeping this in mind, the volcanic eruption at Geldingadalir is a small event in the large context of the Reykjanes Ridge. It has been found that this time the magma is very primitive, close to what is found on Mid-Ocean Ridges. https://grapevine.is/news/2021/03/23/lava-from-geldingadalur-is-the-most-primitive-iceland-has-seen-in-7000-years/

The ridge is about 900 km long and takes over a large part of the ocean floor. It is then subducted by the Icelandic Plateau, but the division line found on the top of the Reykjanes Ridge can be traced through Iceland. The red square is therefore both affected by the special conditions found in Iceland and the Reykjanes Ridge. This time, the magma must be originated relatively directly from the division of the Reykjanes Ridge, close to the so called MORB type of basalt. https://en.wikipedia.org/wiki/Basalt

Geophysics give us some insight into what is happening at Reykjanes, now when the eruption at Geldingadalir has started. Force and Stress is the key to understanding the situation. You can learn about it here: http://kurd-brave.weebly.com/uploads/5/3/8/0/53800263/3130122022015_lec._2.pdf. Then this map can be studied:

If you look at this carefully, you see symmetric vectors, marked with black arrows. The northern half of the red polygon drifts rather freely along with the North American Tectonic Plate, but the southern half is more trapped between the drift effect from the Reykjanes Ridge and the division line between the two tectonic plate of the Reykjanes Oblique Rift Zone (RORZ). That is where the eruption is taking place.

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

Fagradalsfjall is a tuya with NS structure due to earthquake faults having that alignment. Besides that, the volcanic systems have left NE-SW oriented slopes, as can be expected in that area. Thirdly, perpendicular lines can be detected shaping the other sides pointing NW-SE. That is a bit unexpected, except keeping the convection rolls underneath in mind. The result looks like this, when sharpened with inserted lines:

This diamond shape is quite apparent, when pointed out. The NS structure is also exaggerated with inserted lines. The red line indicates the site of eruption.

For Fagradalsfjall on wikipedia: https://en.wikipedia.org/wiki/Fagradalsfjall