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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:

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.

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.

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.

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.

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.

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:

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.

With GPS technology, we finally could measure tectonic drift quite accurately. Iceland is divided into two parts, and it is manifested with direct measurement. The points measured have been fixed, the satellites give signals to relevant equipment. After measuring the points twice, the vector of displacement appears clearly. This is the result:

This can be manifested from the report: https://www.lmi.is/static/files/maelingar/isnlet2004-skyrsla.pdf, and also https://www.lmi.is/static/files/eldra/2019/09/skyrsla.pdf

Both reports make it clear that these vectors are the results of measurement. All other vectors are found by changing their directions and length according to a formula. Therefore, we should adhere as much as possible to this original result of measuring the tectonic drift, to remain closer to the truth, being able to know about reality, and be aware of the facts we should base our understanding of the world on.