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Volcanic Zones of Iceland and the Length of the Flank Zones

The volcanic zones of Iceland replace the Reykjanes Ridge in the south and Kolbeinsey Ridge in the north. This interval, found approximately between 63°N and 67°N, is as complex as the ridges look simple.

The volcanic zones of Iceland.

Here is a list of the zones: A = Snæfellsnes Volcanic Zone B = Reykjanes Peninsula Rift C= Western Volcanic Zone D = Central Iceland Volcanic Zone E = South Iceland Volcanic Zone F = East Volcanic Zone G = North Volcanic Zone H = Öræfajökull Volcanic Zone = I = Grimsey Oblique Rift J = Skagafjordur Volcanic Zone (extict). The red arrows at the bottom indicate mantle convection flow vectors (convective layer 1) just below 120 km depth. Names derived mainly from https://jardvis.hi.is/sites/jardvis.hi.is/files/Pdf_skjol/Jokull58_pdf/jokull58-einarsson.pdf

The drawing is superimposed on a map from the report from National Land Survey of Iceland: https://www.lmi.is/static/files/maelingar/isnlet2004-skyrsla.pdf. The map includes tectonic drift vectors showing shift of position of relevant points was measured with GPS technology within the time interval of 1993-2004.

Here the relationship between individual zones and a relevant pair of convection rolls is considered:

A – Snæfellsnes Volcanic Zone covers a pair of convection rolls, according to the model.

B – Reykjanes Peninsula Obique Rift Zone is not very well defined at its western end, because mapping is terminated at the coastline. It is not a proper geological border, but according to this model the system extends out to the beginning of the regularly aligned Reykjanes Ridge, before it starts bending eastwards towards the peninsula.

C – The Western Volcanic Zone, as defined, extends to the third convection roll in the north, but within that polygon the faults start bending northwards, in the same way as the rifts of D and F.

D – Central Iceland Volcanic Zone is mainly confined to only one polygon, except a part trending slightly west of north. It can be explained according to this model that the convection rolls of second layer affect the surface along the downwelling line.

E – South Iceland Volcanic Zone is itself very divided into the Eyjafjallajökull-Katla part and the Westman Islands part. Those two parts clearly cover a pair of rolls.

F – The East Volcanic Zone spans four polygons. But the central two polygons have a special status, being the only two clearly belonging to it. The southernmost polygon is divided into two parts, as a transition over to the South Iceland VZ (marked E). The northernmost polygon plays the role of transition over to the North Volcanic Zone, and has similar features as the polygon of the West Volcanic Zone at the same latitude.

G – The North Volcanic Zone, as defined here, extends over a pair of convection rolls, of convective layers 3 and 4. If we would define the NVZ as extending farther south (to Kverkfjöll Volcano), the zone would be seen covering two rolls of layer 1 and 2. A question of definition is met at the northern end, because it is submerged and not as clearly mapped as other parts.

H – Öræfajökull Volcanic Zone is a flank zone, clearly extending over a pair of convection rolls of the 4th layer, that is closest to the 410 km discontinuity.

I – The Grímsey Oblique Rift Volcanic Zone is hardly ever mentioned as a volcanic zone, because it is found under the ocean. Geologically, it has similar properties as the Reykjanes Peninsula. The upper most convection rolls below 120 km depth are found to mark the ends of this zone.

J – The extinct volcanic zone of Skagafjordur is most often omitted on simplified geological maps. The last eruption took place there less than 800.000 years ago, so the main features of Iceland were the same as today. The area is still geothermally active. Therefore, I tend to include this as a volcanic zone, specially because some rifting took place making Skagafjordur wider (perhaps 10 km wider) during a million years period of rifting process. https://orkustofnun.is/gogn/Greinar-starfsmanna/Arni-Hjartarson-2003-PhD/AH-04-Skagafjordur-Zone-an-ephemeral-Rift-Zone.pdf. The zone also seems to have covered a pair of convection rolls, of layer 4.

The volcanic zones, seem to fit quite well into sections of pairs of volcanic rolls. The model might therefore prove useful for a more accurate mapping and definitions of the outer limits of the zones, besides combining in the south with the Central Iceland VZ (D).

The four convective layers mentioned are found in between 120 and 410 km depth, the uppermost one labelled as convection layer 1.

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The Basic Unit of Earth’s Inner Structure – a Pair of Convection Rolls

You can look at the basics of Earth’s inner structure in two ways, either mantle plumes or convection cells. The idea about mantle plumes (Morgan 1971: https://www.nature.com/articles/230042a0) is tempting, simply because of the geological settings of Hawaii and some other places. The other way of trying to understand the basics of Earth´s inner structure is looking at the Mid-Ocean Ridges, reasoning that convection rolls are found at each side of them, pulling the ocean floor apart. The starting point seafloor spreading (Harry Hess 1962: https://www.semanticscholar.org/paper/The-history-of-ocean-basins-Hess/acb4469c428cb5ad2ea9d70d2dd9424102f14bae) was clearly of this category.

Sadly, the idea about a mantle plume was easier to mentalize, and even the geology of Iceland was suddenly being analyzed according to the assumption that a mantle plume should be found under it. I have been trying to reverse this trend, pointing out that the Reykjanes Ridge must be the result of convection rolls pulling at each side. Trying to drag out branches from an imagenary mantle plume to explain things does not work at all.

On the other hand, a convection roll system actually contains an upwelling of mantle, in a similar way as many geoscientists visualize a mantle plume. Let us have a look at a section of convection rolls:

The basic unit of Earth´s inner structure – a pair of convection rolls.

A diagram showing the temperature with depth is inserted (from https://en.wikipedia.org/wiki/Geothermal_gradient

The upwelling within a specific area can be measured with tomographical, petrological and other scientific methods, such as measuring gravity anomalies. Under Iceland, many studies have shown that the upwelling is of the order of a few cubic kilometers of mantle material per year. At school, we were taught that about one cubic kilometer is added to the crust of Iceland per year. The speed of the flow is also supposed to be a few centimeters a year, in harmony with the speed of tectonic drift. So the ‘plumists’ and ‘rollists’ are not that far apart from each other. In between the convection rolls, a very long ‘plume’ is found according to that model. Then the layers above act like a buffer, making everyone confused. Scientists start asking: ‘Are the hot spots originated from Core Mantle Boundary (CMB) at 2,900 km depth, or within a transition zone at 410 km depth?’ The answer is both, except that we should talk about convection rolls, and then the picture of Earth’s inner structure will become much clearer.

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Canary Islands E-W and N-S Alignment

The Canary Islands are a bit of a mistery. They become progressively older from west to east, but the line does not follow the drift vector, as it points to the NE. It seems as the oldest islands found as seamounts south of the EW-oriented chain, were formed when the Atlantic Ocean was still in its infancy. The same is true for the northernmost seamounts. The eastern limit of the islands is characterized by the location of the main lower mantle division line.

Convection division lines and the Canary Islands.

The map base used above is from: https://en.wikipedia.org/wiki/Canary_Islands_Seamount_Province

The activity seems to be associated with the combined effect of the two convective layers between 120 and 410 km depth, found at the 32nd latitude. An older NE-SW oriented chain extends from the western and eastern end, respecteviely, of the present Canaries. This probable, and double, nature of the origini of the Canaries is of course confusing. Comparing the geology of the Canaries with the Convection Rolls Model leads to this preliminary conclusion.

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Canary Islands Convection Rolls System

The Cumbre Vieja volcanic eruption on La Palma is a reason for looking into the system of convection rolls according to the relevant model underneath the Canary Islands. Mainly four convection rolls, and thereby four division lines are responsible for the activity of this chain of volcanic islands.

The convection rolls below the Canary Islands.

Narrow red lines show upwelling, black lines show downwelling. The lines oriented NNW show division between uppermost convection rolls at 120 km depth, directly affecting the tectonic plate’s ductile part. Because the tectonic drift carries the islands eastwards, the youngest islands are the westernmost ones. The islands appear at first close to division line between rolls 1 and 2, and go through a stage of building up a shield volcano. The activity diminshes within the realms of convection roll 3. Rejuvinated volcanism does, according to this model, occur due to the fact that the islands drift from one convection roll to the other. Relatively recent eruptions have occurred above convection roll 4.

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A Model of Mantle under Iceland

A simple 3D model can be made to understand better the arrangement of mantle currents under Iceland. It has three 2D components, surface, E-W vertical section along 64°N and N-S vertical section along 21°10’W. It looks like this:

Convection rolls arrangement model.

The easy part to understand is the E-W arrangement, as the convection rolls of asthenosphere are always 1.5° wide along each latitude. The latitude chosen here is 64°N, so when you follow it you will always meet with intersection between rolls at the interval corresponding to 1.5° of longitude. This can be comprehended rater easily, and it is clearly drawn above.

A more difficult part is the N-S section, showing the sides of the convection rolls. There are two rolls found in between 120 and 410 km, usually, but here two systems intersect each other, so the rolls become 4! our mind is not trained for this, so we become tired when confronted with this puzzle. Still, this is very simple compared with the most complex things most of us need to learn one day or another.

The surface part might be most difficult, because the different lines separating the individual pairs of convection rolls are drawn there, showing a pattern of all four layers. It is very easy to imagine a line between two convection rolls, but many convection rolls side by side are more difficult, and four different layers add considerably to the complexity.

Therefore, this simple model, based on the fact that hot material rises up, flowing to the sides while cooling, then sinking and being heated again, becomes too complicated for us to understand without a considerable effort of study.

You can make this model by printing the picture, then cutting out the three parts separately and combine the dotted lines of the vertical parts, gluing with surface at the crossings of 64°N and 21°10’W.

You see why 64°N is chosen as E-W section. It is the only latitude where all the upper rolls have the same dimension and are regularly arranged. If you understand the model, you might be able to make similar sections at other latitudes.