Originally Posted By: Bill S.
Your example of the cubes works because you have established an arbitrary division “in the right place” between them. Without such a boundary the temperature/pressure gradient, and therefore the density/viscosity would vary gradually, facilitating local inversions.

Rubbish.

Earth formed from a collision
www.preearth.net

Plate-tectonics is wrong
www.preearth.net/plate.html