Author: Steingrimur Thorbjarnarson

The large convection rolls, spanning 30° from east to west, rule the main trend of tectonic drift. But the smaller rolls above, spanning 1.5° from east to west, make it more complicated. The Great Rift Valley of Africa is located above a main division line of up-welling within the lower mantle. At equator, the difference between convergent and divergent small scale rolls can be examined.

The red lines show the different points, one local diverging point, one large scale diverging point which is at the same time a local converging point. This seems controversial, but is an inherent factor of the convection rolls system.

Scandinavia is marked by one convection roll of lower mantle. The coast of Norway is the down-welling side, and the up-welling is found east of the White Sea and Finland.

As Iceland is one of the Scandinavian countries, the system of a pair of rolls, with down-welling in the middle, more and less marks the area.

For those really interested, the middle of the Scandinavian Peninsula, and all Iceland, are found within an intersection zone of two different convection rolls, although closely related to each other. The rolls reaching from equator meet the rolls extending to the North Pole.

But for the sake of simplicity, this is the picture we should comprehend at first. Also, please note that this is found within a 3D globe, but this is an over-simplified 2D drawing 🙂

The distance between W-Greenland and W-Norway along the 64th parallel shows the location of large convection rolls below 410km depth.

This is the same result as along equator, where the distance of 60° is found between Africa and S-America.

The pair of convection rolls under the continental shelf of Norway along 66°N is shown here:

You can go further and visualize the next convection roll to the east, under the Norwegian mountains, as they span more or less the width of one convection roll.

The symmetry around the coastal point of Iceland at 66°N, 30° to East and West, is coherent with the analytical result that the span of one convection roll of lower mantle is 30° along each latitude. Still, the West Fjords Peninsula is not located over the division line of large convection rolls of lower mantle, but the tectonic drift has resulted in this symmetry.

The coast of Norway is 6° farther to the East (the span of 2 upper mantle convection rolls), where the main division line between lower mantle convection rolls is found. These are down-welling lines closest to the tectonic plate at 120 km depth below Earth’s surface. It is therefore to be understood that at those three points, the down-welling division lines also mark the division lines between topographic and bathymetric features.