The Tuya Fagradalsfjall in Iceland Erupting

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.

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


Reykjanes Geldingadalir Eruption – a Model of Magma Flow

The magma flow into the eruption site of Geldingadalir does not show any trace of origin in the form of subduction. Therefore, no magma chamber is visible. The magma ascends directly from the mantle. The path can be traced according to the convection rolls model and a simplified drawing of the basic structure of the tectonic plate.

The path of magma into Geldingadalir eruption site (vertical section as imagined from south).

What is confusing is the interplay between convection rolls of the two uppermost layers. The layer interacting directly with the tectonic plate extends from the Reykjanes Ridge (and the Kolbeinsey Ridge as well). Magma ascends from the division line into the ductile part of tectonic plate, until it reaches the brittle part (or the crust itself). Usually, backflow of Munroe Effect would take all the magma back to the lower circulation process. What can happen, is that the lower convection rolls get coupled with the upper one, leading to tension, and thereby opening paths from below, enabling magma to flow horizontally below the brittle crust. The crust has been extended for a long period of time due to the effect of the uppermost roll of the area, opposing the main drift direction of the North American Tectonic Plate.

It has also been pointed out by Vigfús Eyjólfsson, that the eruption site is found where the dike crosses an earthquake fault of the area, creating the weakness within the crust for enabling lava to flow on the surface.


The Structure of Fagradalsfjall and the Surroundings of Geldingadalir Eruption Site

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.

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


Geldingadalir Eruption on the Reykjanes Peninsula

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.

The position of eruption is marked here: https://www.mbl.is/frettir/innlent/2021/03/19/upptok_gossins_eru_i_geldingadal/


Dyke Formation at Reykjanes in 2021

As magma is entering the brittle crust with a flow rate of perhaps about 20 cubic meters per second, a dyke is forming in the southern half of the Reykjanes Peninsula. Geoscientists are therefore in the difficult position of explaining what is going on, and at the same time trying to predict what will happen. No one can foresee an eruption, but different scenarios are studied. Here I will show you a simple map with the main features. Convection rolls model divisions are marked with red lines. The tensional vectors are parallel to those lines. A red arrow indicates the present scope of the dyke being formed. Information from the Icelandic Met Office is found here: https://www.vedur.is/um-vi/frettir/skjalfti-m57-a-reykjanesi

Dyke formation at Reykjanes Peninsula in 2021

We are aware that, where the dyke has already formed, the magma might make its way upwards. The triggering effect might be extra heat entering the dyke, and that might originate from the convection rolls division lines found south-west of it. As the dyke propagates farther in that direction, the more likely it is that an eruption will occur according to this model. In 2014, the Holuhraun eruption started under similar circumstances.