The Magnetic Anomalies of Iceland Showing the Background Story of Reykjanes Eruption

Volcanic eruptions have formed Iceland along with the tectonic drift. Magnetic anomalies have been mapped and analyzed, telling the story of how Iceland has evolved for millions of years. Grétar Ívarsson and others have shown this step by step. This topic is dealt with in the paper ‘Geology and geodynamics of Iceland’ by, R.G. Trønnes (

The first map shows the conditions prevalent, where you find Iceland today, 8 million years ago. By then, the structure of Iceland was simpler than today. The mid-ocean ridges were rather directly connected, and the magnetic anomalies were aligned regularly parallel to the mid-axis of the ridge.

Magnetic anomalies – 8 million years old axis.

The square with dotted lines indicates where the West Rift Zone was going to form. A NS-axis could be defined, similar to the one found to day within the North Volcanic Zone of Iceland.

Then the West Rift Zone of Iceland started to emerge. The West Rift Zone is a term combining the areas often referred to as the West Volcanic Zone and the Reykjanes Oblique Rift.

Magnetic anomalies – 6 million years old axis.

About 6 million years ago, the rift axis stopped spreading north of the 64th latitude. Rifting started farther to the east and the West Volcanic Zone started to form, offset from the Reykjanes Ridge. The rift zone cuts through older parallel anomalies east of the old axis from the NE, as the older ones were oriented NS. Therefore, the pattern of anomalies becomes more complex. At the same time, a new NS-axis started replacing the old one.

Magnetic anomalies – 4 million years old axis.

Now the West Rift Zone (WRZ) had developed withn the framed part. The new NS-axis had shifted eastwards, as it adds constantly material symmetrically to both sides.

Magnetic anomalies – 2 million years old axis.

Now, the new East Volcanic Zone starts to propagate to the south-west from the North Volcanic Zone.

Magnetic anomalies – as measured today.

Today, we see this pattern of magnetic anomalies in Iceland. As expected, they show the same characteristics of geological features as other types of mapping. Pointing out the site of the eruption taking place in Geldingadalir and Meradalir on the Reykjanes Peninsula of Iceland today, it can be easily understood that it is an important volcanic area. If you trace the maps one by one, it can be argued that this eruption is actually found at the spot from where all the development of the volcanic zones of Iceland started 8 million years ago, leading to the situation we learn about today!


Fagradalsfjall and the Earthquake Zone of Reykjanes

The structure of the volcanic area now active in Iceland is quite fascinating. The base of Fagradalsfjall is diamond-shaped, and is formed within the earthquake zone of the Reykjanes Oblique Rift. The zone is bent, because it connects two different systems of the Reykjanes Ridge (aligned NE-SW) and South Iceland Seismic Zone (aligned E-W). The earthquake faults are oriented NS, all the same, because the pressure from NE and SW is undisrupted by the swayed structure of the plate boundary and the related tension and rifting process.

The geological settings around the volcanic sites of Geldingadalir. Map base from ISOR.

TThis map can be explained further: 1) Blue frame indicates the base of the tuya Fagradalsfjall. The sides are oriented N44°E and N44°W, because of the convection rolls underneath shaping the surface in various ways. 2) Thick black lines indicate the earthquake zone, connecting the South Iceland Seismic Zone and the Reykjanes Ridge. The middle line is in fact the division between the two tectonic plates of N-America and Eurasia. 3) Thinner NS oriented lines indicate location of earthquake faults within the zone. They have been mapped by many scientists for the last decades, for instance Páll Einarsson and Vigfús Eyjólfsson who I have referred to in other posts. The general layout here is extrapolated from their findings. 4) The red line shows the approximate location of the dike. It has the same orientation as the slopes of Fagradalsfjall, which in turn has the same orientation as the Reykjanes polygon of the convection rolls model. 5) The red ellipse indicates the sites of eruption. The erupting locations coincide with crossings between earthquake faults and the dike, as the magma can most easily make its way to the surface through the earthquake faults.

The earthquake faults form a pattern of dominantly NE-SW oriented fractures with en echelon arrangement on the surface, Farther down, they tend to become aligned closer to NS, as indicated on the map.

The end of the dike seems to be at the border line of the earthquake zone. All this harmony of nature becomes quite obvious once the eruption has started. Why do we not notice those things unless something happens, bringing more attention to the area?

Map base of ISOR is found here:

Mapping of earthquake faults: and…/utgafa/greinargerdir/2006/06001.pdf

A map with inserted square of the map above, indicating location of Fagradalsfjall dike formation.

The Kolbeinsey Ridge in Context with the Geldingadalir Eruption

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.

Kolbeinsey Ridge and Eruption Site of Geldingadalir.

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.


The Reykjanes Ridge in Context with the Geldingadalir Eruption

Reykjanes Peninsula is closely related to the Reykjanes Ridge, that in turn can be looked at as a huge elongated volcano.

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.

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.


The Tectonic Force and Stress at Reykjanes Peninsula

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: 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.