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The formation of a dyke near the town of Grindavik in Iceland was quite sudden. First, a sill did form at the depth of approximately 5 km. Then magma started to make its way upwards, forming a dyke reminding us of what has happened in the vicinity of Fagradalsfjall three times during the last three years.

The dyke can be compared to the convection rolls model, and two lines cross the center of the dyke. The lines are blue, standing for down-welling. Down-welling can provide magma just as up-welling. The southern part of the dyke is aligned parallel to the convection rolls pattern, but the northern half is more affected by the earthquake zone, where NS oriented earthquake faults are a dominant factor within the brittle part of the crust.

The inserted maps are from ISOR (for volcanic systems) and the Icelandic Met Office (dyke formation indicated by earthquake epicenters). The main drawing is superimposed on a map from the National Land Survey. The main divison line is based on present tectonic drift vectors.

A section of the layers, represented by red and blue lines, is shown here below. This was prepared to show the location of the Fagradalsfjall eruptions, but they were so close to the site of the formation of the dyke presently ascending, so it is also used here as a representation:

The basic elliptical form of the ocean topography can be used to elaborate further on its characteristics. First, we should have a look at the map http://www.data.menntanet.is/kennslupakkar/umhverfiislandsmida/umhverfi_islandsmida_rafbokin.pdf:

In general: The 200 meters depth margin of the bathymetry of Iceland (ocean topography) is elliptical. Here it is exaggerated by encircling the area with a rather thick black elliptical line. Then, numbers from 1-8 are added in the relevant sections, fitting the mathematics of the ellipse. Let us discuss the sections one by one.

1. The Reykjanes Ridge extends from this section. The shallow sea margin of about 200 meters depth is therefore both found extending from Iceland itself and the Reykjanes Ridge extendint to the SW from it. If we could subtract the direct addition caused by the Ridge, the remaining shallow area should probably be found within the elliptical form.
2. The western most border coincides with the ellipse, and south of that the margin seeks to combine with the Reykjanes Ridge.
3. This section is formed in a fairly accurate way, lacking a small fraction to be regarded perfectly fitting into the basic elliptical form. What is interesting about this part, is that it coincides with the Greenland-Iceland Ridge, It is a bit of a coincidence, but the same applies to section 7, where the connection between the elliptical shallow sea plateau connects with the Iceland-Faroe Ridge.
4. The light-coloured outer part of this section coincides quite well with our imaginary elliptical form. The N-S axis at its eastern most side coincides with the effect of Kolbeinsey Ridge.
5. This part differs from the elliptical form due to the effect of Kolbeinsey Ridge, as on both sides of it, the ocean becomes deeper than 200 meters. At the eastern part, where the effect of the Kolbeinsey Ridge is negligible , the margin reaches slightly farther out than the imaginary elliptical form.
6. This part follows the ellipse fairly closely, reaching a little farther out, just about to the same extent as its counterpart No. 3 does not reach the elliptical limit.
7. As pointed out before, this section coincides with the Faroe-Icelandic Ridge. It fits almost perfectly to the elliptical form, along with part 8.
8. This part follows our imaginary elliptical form almost perfectly. Only its southern most part seem to lack a bit. As said before, parts 7 and 8 make up the most perfect quarter of the ellipse.

In this way, the discrepancy between the theoretical elliptical form and real mapping of the Icelandic ocean topography is partly explained. Without explanation, the resemblance between the mathematical form and real mapping is quite apparent, though. So why is this pointed out here and not by someone else? The elliptical form is a really interesting fact to mention. The reason is probably due to the fact that this would just be for the fun of it, if resemblance to the combined function of the two ocean Ridges, Reykjanes Ridge and Kolbeinsey Ridge, along with the volcanic zones, was not found to play a role. It has been dealt with in former posts.

It is now accepted that the two eruptions of Fagradalsfjall in 2021 and 2022 took place where an active dike crossed NS oriented earthquake faults.

Several NS earthquake faults have been mapped, and it turns out that the interval between them is quite regular. Therefore, by extrapolating the mapped part of the earthquake zone, the position of other earthquake faults can be guessed theoretically, as shown here below. Thick NS oriented lines show the already mapped faults, and thinner blue lines show the theoretical ones, numbered 1-8, along with the nineth which is theoretically connected with Keilir volcano. (The locations of those lines are not meant to be exact, only indicative according to the so-called bookshelf theory of parallel eartquake faults. See for instance: https://opinvisindi.is/bitstream/handle/20.500.11815/4328/Einarsson_2008.pdf?sequence=1&isAllowed=y.)

The drawing is superimposed on a map from the Icelandic Met Office, plotting recent earthquakes in the area. The dike covers the intervals of 8 or 9 earthquake faults, and if an eruption occurs within the area, the magma should make use of the weakness caused by one of those earthquake faults to reach the surface.

The map inserted, with mapped earthquake NS faults, is from the essay written by Vigfús Eyjólfsson and Páll Einarsson: https://www.mbl.is/media/36/11536.pdf. Earthquake faults mapped by Sigurlaug Hjaltadóttir and Kristín S. Vogfjörð are also added within that map: (https://www.vedur.is/media/vedurstofan/utgafa/greinargerdir/2006/06001.pdf).

The third eruption at Fagradalsfjall of the Reykjanes Peninsula in Iceland is now considered likely to occur, according the news. https://www.ruv.is/frettir/innlent/2023-07-05-ovissustigi-lyst-yfir-vegna-jardskjalftahrinu-gos-mogulegt-innan-farra-daga-386976.

The dyke feeding the eruptions is active again, causing earthquakes. Land has been rising considerably according to measurements as shown here: https://www.vedur.is/um-vi/frettir/landris-hefur-maelst-vid-fagradalsfjall-sidan-i-april.

Let us draw a theoretical map showing the basic geological framework of the area. The volcanic systems are not included, except for inidcating the location of the active dyke with red elliptical form. The Reykjanes Seismic Zone is shown theoretically, along with earthquake faults around Fagradalsfjall. The apparent shape of Fagradalsfjall is shown with a blue square.

Having inserted these features, it appears that Fagradalsfjall is formed by the interaction between the volcanic and seismic activity of the peninsula. It is predicted that an eventual eruption will occur at an intersection point between the dyke and an earthquake fault. The earthquake faults are oriented N-S, wheras the dyke is oriented NE-SW.

The South Iceland Seismic Zone and Reykjanes Seismic Zone are here shown as connected, according to the Convection Rolls Model.

The main volcanic zones of Iceland can be divided into two different parts as shown on the map below. The lines representing the convection rolls model are superimposed on the map found on Wikipedia.

The best known volcanic zones are probably the West Volcanic Zone (WVZ) and the East Volcanic Zone (EVZ). The EVZ is often seen as covering five polygons, but more advanced analysis includes a distinct South Iceland Volcanic Belt (SIVB), where proper rifting process does not occur. The Westman Island (WI) are formed within a single polygon, extending from the SIVB. The Reykjanes Oblique Rift Zone (RORZ) is mainly associated with the Reykjanes Peninsula, but here the extension on the seafloor is seen as the second half of the volcanic zone. The Öræfajökull Volcanic Belt extends over two polygons in a very decisive way, in between the two volcanic sites of Öræfajöull itself at the southern end and Snæfell at the northern end. Then we still have to account for the Snæfellsnes Volcanic Belt (SVB), which can not be defined as clearly as the other volcanic zones within the context of polygons. On the other hand, Snæfellsjökull Volcano at the western end of that zone is exactly on the same latitude as Snæfell of the ÖVB, found in context with a division line in a mirrored way. Therefore, the similarities between SVB and ÖVB become quite suspicious. The Grímsey Oblique Belt is poorly known in the geological literature, but it resembles the RORZ very well, in has functional similarities. The Mid Iceland Belt (MIB) is found within one polygon, but it was connected with the now volcanically extinct Skagafjörður Volcanic Belt (SKVB). The North Volcanic Zone is found within a tighter network of convection rollls division lines, so the outer limits are not clearly defineable. but can be roughly divided into two sections, with the Mývatn area found in between. The Reykjanes Ridge then extends along many polygons to the south-west. https://en.wikipedia.org/wiki/Volcanism_of_Iceland