Reykjanes Ridge and the Volcanic Site of Geldingadalir.
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:
Reykjanes stress vectors.
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 location of Geldingadalir eruption.
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.
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:
The shape of Fagradalsfjall area.
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.
The eruption that started March 19th ein the Geldingadalir valley at Fagradalsfjall on the Reykjanes Peninsula of Iceland is the first to occur within the area since the year 1240. No one can predict for how long it will proceed, but it has been found that the lava is rather hot, or between 1180-1190°C, as can be seen here: https://www.mbl.is/frettir/innlent/2021/03/22/likist_dyngjugosbergi/
The lava is therefore of the category found in shield volcanoes. Therefore the possibility that this eruption will go on for some time can not be ruled out. The crater of Thrainsskjoldur (Þráinsskjöldur) is found nearby, a large shield volcano covering the central parts of the peninsula. Now we consider this as the smallest eruptions we have seen in Iceland. If this is really an effusive eruption of ‘shield volcano category’, it surely comes as a surprise.
The dyke of Geldinigadalir eruption.
The dyke extends about 8 km over the theoretical area of the seismic zone as extended from South Iceland Seismic Zone over the Reykjanes Peninsula, bending towards the Reykjanes Ridge. It is assumed according to the model that the main part of magma is originated from NW, and when the dyke propagates to the SW, it encounters the division line of the polygon.
The different composition of this lava compared with the lavas erupted around 1000 years ago, can be explained according to this model with influx through the ductile mantle (below the brittle crust) from the NW division line of the Reykjanes polygon, reaching underneath the dyke into a conduit leading to the eruption site at Geldingadalir.
The material of the dyke might have propagated from the NW, and outflow could be found into the mantle at the SW end of the dyke. At least, no clue of either depletion or accumulation of magma within the earth is being measured for the moment.
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