I am a geologist, graduated from the University of Iceland, and taught geology for a few years. I have gained some knowledge about Earth's inner structure, so I provide this website as my contribution to answer one of the greatest questions remaining within the realm of geoscience. Experiments show that the mantle should form convection rolls when close to the melting point. I took this literally, and calculated the dimensions and shape of these mantle convection rolls. Then I compare that model with the surface. This makes it possible to provide many interesting examples about geology found on my blog.
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.
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.
Magic Magma 