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 temparature 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 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.