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 🙂
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 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.
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.
The Golden Circle Tour is by far the most popular day tour in Iceland. The scenic areas are mainly Thingvellir, The Golden Waterfall and Geysir hotspring area. The travellers enjoy nice view on the way, such as of the mountain Ingólfsfjall, and probably regard it as extra scenery. But the mountain is a very interesting tuya, and should be inspected more closely. You can read about it here: https://en.wikipedia.org/wiki/Ing%C3%B3lfsfjall
The mountain has a NS-structure, just like other tuyas in Iceland. It has been shown that eruptions take place through NS-aligned earthquake faults, explaining the formations. Looking at the mountain from above, the resulting square form can be identified easily. The eruption at Fagradalsfjall of Reykjanes is of that category, so this has rewaken our interest in this type of volcanic activity, as shaped Ingólfsfjall.
At the eastern side of the mountain, two rivers meet, namely Sog and Hvítá. The Sog River flows from Lake Thingvallavatn at the olld National Assembly Site of Iceland, and Hvítá River has its origin in Langjökull Glacier, and does posess the Golden Waterfall. The name of the river, formed by the combination of those two, is Ölfusá. It can be noticed that Sog and Ölfusá together flow directly from north to south, due to the NS earthquake faults found in the area. Hvítá, on the other hand, follows the alignment of the volcanic zones at the meeting point.
Driving along the eastern slopes of Ingólfsfjall is therefore more interesting than may would think at first, remebering that Ingólfsfjall acts as a monument for the first settler of Iceland, Ingólfur Arnarson. The story says that his grave is found on the mountain. It is interesting that a similar story is found about Fagradalsfjall, the other tuya now erupting at Reykjanes, as the grave of Isólfur, the settler of that area more than a thousand years ago, is found on top of that mountain.
The latitude of 64°N is pointed out, as it is the central axis of the South Iceland Seismic Zone. The zone is aligned EW, about 70 km long, combined of around 100 NS-oriented parallel earthquake faults. Such faults mark the western and eastern sides of the mountain, direct the rivers, and are sources of geothermal activity in the area. The mountain is 4.5 kilometers wide from east to west, shaped altogether by seven parallel earthquake faults.
For orientation, Ingólfsfjall is here pointed out on a map of Iceland.
The mountain Ingólfsfjall is found between the town Hveragerði, famous for geothermal activity, greenhouses and flowers, and Selfoss, the largest town within the Southern Lowlands area.