The lower mantle is divided along the middle of the Atlantic. This division line becomes double within the intersection zone at the latitudes of Iceland.
If there was just one line, it would be easy to understand. (If it was so easy, it would probably have been found out at least 100 years ago.) It is difficult to understand, because there are actually two lines.
Tracing the calculated lines of division lines, the main central lower division lines are found:
The crossings are found at 64°N and 21°10’W in the slopes of Reykjafell mountain (meaning Steam Mountain) at the side of the town Hveragerði (meaning Hot Spring Town). That point is found directly north of the middle point of the Atlantic at equator ( 0°N and 21°10’W).
The plane of each convecting cell section shoul be parallel to the rotation plane of Earth at other latitudes too. The centrifugal force constantly affects a moving particle of the flowing and convecting mantle. That has to be kept in mind, understanding the proportions of the convection rolls extending north and south from equator. An hours lecture would clarify further what is meant by that 🙂