Author: Steingrimur Thorbjarnarson

Scientists have already mentioned that Surtsey and Geldingadalir eruptions have something in common. Both have the characteristics of effusive eruptions (shield volcanoes), and the flow rate of basaltic lava is similar. Looking at the convection rolls model, the two locations of ascending magma are exactly parallel to each other. Comparing the framework of the two polygons of Reykjanes and Westman Islands, they are alomost copy-paste of each other. In turn, Surtsey has the same location within Westman Islands Polygon as has the ascending magma where it flows into the dike of Geldingadalir eruption, close to Keilir Volcano.

The narrow, black line, parallel to the NW-SE trending convection rolls division line (blue), does cross the ascending spots of magma for Geldingadalir and Surtsey. The Geldingadalir eruption is fed by a dike, but the flow into the dike is not directly below the eruption site, but closer to Keilir Volcano, farther NE. Considering that the lower inflow of magma was more or less directly below Surtsey, the spots are identical for the polygon of Reykjanes and polygon of Westman Islands.

Scientists have reasoned that Surtsey eruption and Heimaey eruption in 1973 were actually connected events, meaning that magma from Surtsey was connected with Heimaey. Therefore we can draw a line in between Surtsey and Heimaey. That line is parallel to the dike formed at the Geldingadalir eruption site. It is tempting to consider that the dike did also extend to the SW from Surtsey, towards the downwelling division line (blue color).

Both eruptions have the same distance from the theoretical upwelling division line between convection rolls, which should be the original feeders of magma for those eruptions. The process is complicated, because the hot magma from underneath does melt material within the crust, which then erupts and forms lava on the surface. Some mixing takes place, as described in this article about the three magmatic components in the 1973 eruption of Eldfell volcano, Iceland: https://www.researchgate.net/publication/232395907_Three_magmatic_components_in_the_1973_eruption_of_Eldfell_volcano_Iceland_Evidence_from_plagioclase_crystal_size_distribution_CSD_and_geochemistry

The differentiation of the magma entering the eruption site of Geldingadalir in Iceland tells a story. The magma did become more primitive quite rapidly as this graph shows:

This tells us that the lava ascending at first to the surface must have had time for differentiation, whereas what follows has not been altered in the same way. This makes sense if the magma did flow some distance horizontally before entering the dike and eventually the eruption site. This is one argument for speculating about the existence of another, lower dike, being aligned perpendicularly to the upper dike.

The eruption is developing stepwise, and now higher fountains of lava are measured to have reached over 200 meters hight.

The Convection Rolls Model, used here to explain various geological phenomena, can be used to trace the flow of magma within the tectonic plate. The flow below the brittle part of the cryst can nor be monitored, because it does not cause earthquakes. Also, it does not cause any alterations on the surface, so we can just make a wild guess.

Therefore, I use the model. and speculate how the eruption could occur within the model. Model is not reality, just a tool to eneavour to find something out about the real world. A guess based on a model can then be a used to make further research and investigations.

First, let have a look at the three dimensional drawing:

Then we can have a closer look on a map, showing the convection rolls division lines from above.

To see how the model works, the two lower dikes have to be drawn in 3D.

This drawing shows how magma enters the ductile part of the tectonic plate, and the lowermost part of the brittle crust, still not causing earthquakes or uplifting of surface. The hidden dike, probably around 10 km below the surface, of unknown height and width, channels magma towards the inner part of the polygon, until it reaches the central part near Keilir, where it ascends into the brittle crust, forming the dike responsible for the eruption at Fagradalsfjall. Out of 20 cubic meters per second, five are lost due to the eruption, but the remaining 15 proceed to the next lower crust dike, flowing all the way to the other side of the convection roll, where downelling occurs.

This might explain how the flow can be maintained for a relatively long time. The extra 15 cubic meters per second maintain the temperature and flow paths within the upper dike. The standard 5 cubic meters flow of the eruption are then related to the width of the dikes, and the upwelling through the crust originated from the convection rolls division lines, where the original melting should have taken place.

This can not be proved with any measurements at hand, so this is just an analyzis of what should happen according to the model, and these ‘lower dikes’ might not exist at all. Take care to know the line between the known and the unknown 🙂

For some real data, pleas check: https://www.facebook.com/jardvis/photos/a.934315269916271/4477778478903248/?type=3

The approximately 5 cubic meters per second of lava, flowing on the ground of Reykjanes these days, represent huge forces of the Mid-Atlantic Ridge. The ascending magma is originated from convection rolls, which on the time scale of 100 million years have divided the continents and shaped the Atlantic Ocean, as can be learned about here: https://en.wikipedia.org/wiki/Mid-Atlantic_Ridge

The convection rolls responsible for this huge formation of the Atlantic Ocean, is also driving the mechanism behind the eruption at Fagradalsfjall on the Reykjanes Peninsula in Iceland. The outer limits of the convection rolls are marked here (approximately) with red lines.

The eruption site at Fagradalsfjall is pointed out. The ridge coincides with the lower mantle division line along the Reykjanes Ridge close to Iceland, and also at 32°N and equator.

Some people like to go to see what is going on. Here is all about that: https://www.nationalgeographic.com/travel/article/is-volcano-tourism-safe

The river Ölfusá flows in a remarkable zigzag pattern, either directly to the south or to the west. It can be explained by the fact that it flows through an earthquakes zone, which is aligned from east to west, and affected by individual earthquake faults, perpendicular to the comprehensive zone, and therefore oriented from north to south. Besides this, two volcanoes affect the flow, namely Hestfjall and Ingólfsfjall. Those volcanoes are located outside the West Volcanic Zone (WVZ), but appeared there all the same. Hestfjall is a shield volcano, and therefore the lava ascended through the weakness provided by a N-S trending earthquake faults. The same applies to the tuya of Ingólfsfjall. Shield volcanoes are formed in the same way as tuyas, but during ice-free periods, whereas a tuya forms under ice or water.

Here is a map, showing the river flow directions, exaggerated by superimposed blue forms:

The South Iceland Seismic Zone shapes the area, both with the seismic activity itself, and by providing weakness for magma to flow to the surface at certain locations. The shield volcano of Skálafell is also shown. The silhouette of Skálafell resembles a house seen from the east, and it is said that Ingólfur, the first settler of Iceland, once built a house there, and therefore the mountain got this name. These relatively young volcanoes are all found along the 64th parallel, shown on the map, which is also the theoretic main division line between tectonic plates. The line then bends southwards when prolonged to the west along the Reykjanes Oblique Rift, and the current eruption at Fagradalsfjall on Reykjanes Peninsula is also related to that division line.

Some relevant information can be found here:

Hestjall: https://de.zxc.wiki/wiki/Hestfjall_(Gr%C3%ADmsnes_og_Grafningur)

SISZ: https://en.wikipedia.org/wiki/Geological_deformation_of_Iceland

Ölfusá: https://en.wikipedia.org/wiki/%C3%96lfus%C3%A1

To clarify where the area is, please find it on this map below. The small, black square is found in between the South Iceland Seismic Zone (SISZ) and the Reykjanes Oblique Rift (RORZ).