With the precondition that convection takes place in a regular way, with equal height and width, it is easy to show that the main discontinuities at 410 km and 670 km are found where different convection cell layers meet.
Convection rolls of Earth’s mantle
As shown above, the convection rolls fit exactly to the measured layers. The intersection between the rolls is always of the same proportion.
The lower rolls below 410 km depth are found within the larger rolls of the lower mantle. It explains why volcanic activity can follow the tectonic drift above, for instance the Cameroon line of volcanic activity in Africa.